The optimal usage of antiscalants and their effect on fouling of reverse osmosis membranes
Thesis submitted in partial fulfillment of the requirements for the degree of “DOCTOR OF PHILOSOPHY”
By
Amer Sweity
Submitted to The senate of Ben-Gurion University of the Negev
March 2015
Beer-Sheba
The optimal usage of antiscalants and their effect on fouling of reverse osmosis membranes
Thesis submitted in partial fulfillment of the requerments for the degree of “DOCTOR OF PHILOSOPHY”
By
Amer Sweity
Submitted to the senate of Ben-Gurion University of the Negev
Approved by the advisors Prof. ______Moshe Herzberg ______Prof. Zeev Ronen Approved by the Dean of Kreitman School of Advanced Graduate studies ______
March 2015
Beer-Sheba
This work was carried out under the supervision of Prof. Moshe Herzberg1 and Prof. Zeev Ronen2 1) Department of Desalination and Water Treatment 2) Department of Environmental Hydrology and Microbiology Zuckerberg Institute for Water Research, Jacob Blaustien Institutes for Desert Research, Ben Gurion University of the Negev, Sde Boqer campus.
Research-Student’s Affidavit when Submitting The Doctoral Thesis for Judgment
I Amer Sweity, whose signature appears below, hereby declare that (Please mark the appropriate statements):
_X_ I have written this Thesis by myself, except for the help and guidance offered by my Thesis Advisors.
_X_ The scientific materials included in this Thesis are products of my own research, culled from the period during which I was a research student.
__ This Thesis incorporates research materials produced in cooperation with others, excluding the technical help commonly received during experimental work. Therefore, I am attaching another affidavit stating the contributions made by myself and the other participants in this research, which has been approved by them and submitted with their approval.
Date:______18-3-2015 Student’s name: __Amer Sweity__ Signature:______
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ACKNOWLEDGMENTS Foremost, I would like to express my sincere gratitude to my supervisors, Prof. Moshe
Herzberg and Prof. Zeev Ronen. Thank you for your invaluable guidance, encouragement, comments, criticism, support, and suggestions. Your incredible help was the key to my success in my Ph.D. research. With no doubt, this research hasn’t had to be done without your enthusiasm and you being patient with me. I am so grateful to The
Jacob Blaustein Institutes for Desert Research for giving me the opportunity to conduct my Ph.D. study.
Also, many thanks extended to all the faculty members of the Zuckerberg Institute for
Water Research especially the staff of Department of Desalination and Water Treatment.
My sincere appreciation goes to Prof. Yoram Oren, Prof. Jack Gilron and Prof. Amit
Gross for their spiritual and professional support. I would like to thank and appreciate the help and the support that I got from the technical staff of the ZIWR people, special thanks to Dr. Anna Mamontov, Dr Naphtali Daltrophe and Itzik Lutvak for you being phenomenal.
I would like sincerely to acknowledge my former and current lab colleagues at the
Zuckerberg Institute for Water Research, who shared their knowledge, perceptive ideas and experiences with me. (Dr. Wang Ying, Oded Orgad, Nune Vanoyan, Tesfalem
Rezene, Diana Ferrando, Adi Avni, Yael Shabtai, Eli Assa, Nofar Assa, Dr. Jenia
Gutman and Dr. Chris Ziemba). Thanks also to my friends and neighbors in Sede Boqer campus, for their warmth and support specially Ashraf Alshhab, Omar Bawab, Suleiman
Halssah, Bihter Bayramoglu, Ani Vardanyan and Hadeel Majid .
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Last, I want to applaud my family indeed, the ultimate keystone of my success and accomplishments. I want to express my thanks to my brother, my sisters, my nieces and my nephews for their support and being wonderful with me. I don’t know how to thank my parents. "My Lord! Bestow on them Your Mercy as they did bring me up when I was small." Thanks to my father who was my inspiration and my leader for his unwavering encouragement, patience and understanding, thanks for believing in me. I am so grateful to my mom, where all the words in the world stuck to describe how supportive she was and still, I am grateful to the mom who prayed days and nights at every stage in my life wishing me success and fortune where ever I go.
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TABLE OF CONTENTS………………………...iv ACKNOWLEDGMENTS……………………...... ii LIST OF FIGURES and TABLES……………...v ABBREVIATIONS……………………………....vii ABSTRACT……………………………………….ix LITERATURE REVIEW ………………………..1 HYPOTHESIS and AIMS…………….………...14 THE DISSIRTATION STRUCTURE………….15 PUBLISHED PAPERS………………………….17 PAPER 1……………………………………….....17 PAPER 2……………………………………….....27 PAPER 3………………………………………….35 DISCUSSION AND CONCLUSIONS………....51 SUPPLEMENTRY MATERIALS……………..59 REFRENCES…………………………………….68 HEBREW ABSTRACT…………………………76
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LIST OF FIGURES AND TABLES Comment: This section includes only figures and tables that were not described in the published articles.
Figure 1 Daily global water production by desalination currently and projection in the near future……………………………………………………………………………….3 Figure 2 SWRO desalination plant showing the various stages—seawater intake, pretreatment, reverse osmosis, post-treatment, and brine discharge—and their interactions with the environment adopted from Elimelech (2011)………………………………….4 Figure 3 Schematic illustration of scale formation schemes………………………….…9 Figure 4 Schematic illustration of antiscalant inhibition mechanisms…………………11 Figure 5 Chemical structure of some antiscalants used in RO desalination: (A) sodium hexametaphosphate (SHMP); (B) potassium pyrophosphate; (C) 2-Hydroxy phosphono acetic-acid; (D) polyacrylic-acid; (E) polyphosphonate; and (F) carboxylated dendrimeric polymer………………………………………………………………………………….13 Figure S1 paper 1 XPS analysis of carbon binding energy spectra on membrane surface: (A) virgin membrane, (B) membrane treated with seawater only, (C) membrane treated with seawater supplemented with carboxylic acid based antiscalants and (D) membrane treated with seawater supplemented with polyphosphonates based antiscalants………………….
Figure S2 paper 1 XPS analysis of nitrogen binding scan of 4 samples (A) virgin membrane, (B) membrane treated with seawater only, (C) membrane treated with seawater supplemented with carboxylic acid based antiscalants and (D) membrane treated with seawater supplemented with polyphosphonates based antiscalants reveals the presence of two types of nitrogen binding scans…………………………………………………………………
Figure S3 XPS paper 1 O1s scan of 4 samples (A) virgin membrane, (B) membrane treated with seawater only, (C) membrane treated with seawater supplemented with carboxylic acid based antiscalants and (D) membrane treated with seawater supplemented with polyphosphonates based antiscalants reveals the presence of two type of Nitrogen……..
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Figure S4 paper 1 XPS analysis for the binding energy BE curve of P2p scan of 4 samples (A) virgin membrane, (B) membrane treated with seawater only, (C) membrane treated with seawater supplemented with carboxylic acid based antiscalants and (D) membrane treated with seawater supplemented with polyphosphonates based antiscalants reveals the presence of phosphorous…………………………………………………………………
Table S1 paper 1 Chemical analysis of antiscalant solutions used in the study, antiscalant was prepared by dilution the antiscalant in double distilled water to final concentration of 10 ppm (v/v)………………………………………………………………………………
Figure S1 paper 3 Growth of bacterial biofilms with and without antiscalants in fixed bed plug flow bioreactors. The bioreactors were of 50 mL volume, packed with 0.5 mm glass beads, and operated continuously at hydraulic retention time of 5 hrs and influent flow rate of 0.17 mL / min.
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ABBREVIATIONS
AFM atomic force microscopy ATP adenosine triphosphate AS antiscalants BSA Bovine serum albumin BWRO brackish water reverse osmosis desalination CA cellulose acetate CLSM confocal laser scan microscope CML carboxylated modified latex DOC dissolved organic carbon ED electro-dialysis EDTA ethylenediaminetetraacetic acid EPS extracellular polymeric substances FTIR fourier transform infrared spectroscopy gfp green florescent protein LPS lipopolysaccharides MATH microbial adhesions to hydrocarbon MED multi-effect distillation MF microfiltration MSF multi-stage flash distillation NF nanofiltration NOM natural organic matter OD optical density PA polyamide QCM-D quartz crystal microbalance with dissipation RO reverse osmosis SEM scanning electron microscope SHMP sodium hexametaphosphate SWRO seawater reverse osmosis desalination TDS total dissolved solid
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TFC thin film composite TN total nitrogen TOC total organic carbon TP total phosphorus UF ultrafiltration XPS X-ray photoelectron spectroscopy
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Abstract
Antiscalants are surface active polyelectrolyte compounds commonly used in water treatment processes in general and in reverse osmosis (RO) desalination processes in particular to avoid membrane scaling. In spite of the significant roles of antiscalants in preventing membrane scaling, their potential side effects were scarcely investigated. The main finding of this study is that antiscalants are prone to enhance biofilm growth on RO membranes by either altering membrane surface properties or by serving as nutritional source for microorganisms. In the first chapter of the current research, the contribution of antiscalants to membrane biofouling in seawater desalination was investigated. The effects of two commonly used antiscalants, polyphosphonate- and polyacrylate-based, were studied. The effects of RO membrane (DOW-Filmtec SW30 HRLE-400) exposure to feed solution supplemented with these antiscalants on the membrane physico-chemical properties were studied, including the consequent effects on initial deposition and growth of the sessile microorganisms on the RO membrane surface. The effects of antiscalants on membrane physico-chemical properties were investigated by filtration of seawater supplemented with the antiscalants through flat-sheet RO membrane and changes in surface characteristic zeta potential and hydrophobicity were delineated. Adsorption of antiscalants to polyamide surfaces simulating RO membrane's polyamide layer and their effects on the consequent bacterial adhesion was tested using a quartz crystal microbalance with dissipation monitoring (QCM-D) technology and direct fluorescent microscopy. A significant increase in biofilm formation rate on RO membranes surface was observed in the presence of both types of antiscalants. Polyacrylate-based antiscalant was shown to enhance initial attachment of bacteria by ~50% in the feed seawater using Vibrio fischeri as a model bacterium observed with parallel plate flow cell and confirmed with QCM-D. This enhanced attachment was due to rendering the polyamide surface more hydrophobic. Polyphosphonate-based antiscalants also increased biofilm formation rate, most likely by serving as an additional source of phosphorous to the seawater microbial population. More microbial biofilm was formed on the RO membrane when the polyacrylate-based antiscalant was used: cells amount and their associated EPS was elevated 5-fold and 50%, respectively. It should be mentioned that enhanced biofilm growth experiments were conducted with a relatively high concentration of antiscalants
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(100 mg/L) in order to achieve biofilm growth within a reasonable experimental period and enable the required comparison. The second chapter of the research was aimed to delineate the effect of exposure RO membranes to different kind of antiscalants on consequent organic fouling during seawater desalination. Membrane surface properties (hydrophobicity and zeta potential) were altered upon the conditioning of the membrane with antiscalants during the desalination process. For all antiscalants used, polyphosphonate, polyacrylate, or dendrimeric carboxylated based antiscalants, membrane zeta potential became less negatively charged over pH range between 3 to 10. Furthermore, the membrane became significantly more hydrophobic when dendrimeric carboxylated and polyacrylate based antiscalant were used and only minor effects were observed for the polyphosphonate based antiscalants: Surface hydrophicities detected for the polyacrylate and carboxylated dendrimeric based antiscalant were 54.3±5.4° and 57±4.7°, respectively. Surface hydrophicities detected for the the polyphosphonate based antiscalant and seawater without antiscalant were, 42±4.3° and 32±1.5°, respectively. The membrane organic fouling process, tested with different model organic foulants (like alginate or BSA), was significantly enhanced in the presence of polyacrylate or carboxylated dendrimeric based antiscalants, which were used to condition the membrane surface. These changes in fouling behavior are likely attributed to the antiscalants effects on RO membrane hydrophobicity and zeta potential after exposure and adsorption to the RO membrane. Indeed, force curve measurements using atomic force microscopy (AFM) on membranes conditioned with polyacrylate, as well as with carboxylated dendrimeric based antiscalant, showed higher adhesion forces to carboxylated modified latex (CML) particle in comparison to membranes conditioned with seawater only or with polyphosphonate based antiscalants. Comparing the effect of the different antiscalants on the organic foulant adsorption kinetics to polyamide surface was carried out in a QCM-D: similar induced adsorption of the model organic foulants by polyacrylate and carboxylated dendrimeric based antiscalant was observed. Clearly, according to these findings, a wise selection of antiscalant for scaling control should take into account their contribution to membrane organic fouling propensity. In the third chapter, the combination of the findings from the first two chapters
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was used to investigate the biofouling potential of antiscalants including Polyacrylate- and Polyphosphonate-based antiscalants in both laboratory and industrial scale RO treatment systems during desalination of brackish water. Both the physico-chemical effects of the antiscalants on the initial attachment of bacterial cells to RO membrane surfaces, as well as their nutritional contribution to biofilm growth were investigated in real brackish water of the Negev desert of Israel. At the lab scale, bacterial deposition experiment on RO membrane was conducted using Pseudomonas fluorescence model bacterium, feed brackish water, and two antiscalants (polyphosphonate- and polyacrylate- based antiscalants). The nutritional contribution of these antiscalants to biofilm growth was investigated by growing biofilms in packed-bed biofilm reactors in the presence and absence of the antiscalants. Eventually, the impact of the antiscalant-induced biofilm growth on the RO membrane, on the performance of the membrane was investigated in a crossflow RO filtration unit under typical brackish water desalination conditions. The results showed a significant membrane biofouling in the presence of the antiscalants comparing to the case where no antiscalant was used accompanied by higher flux decline (30% difference) and salt passage (increased from 4% to 6%). The polyacrylate-based antiscalant enhanced the deposition of the bacterial cells to the RO membrane by altering membrane hydrophilicity. On the other hand, it is suggested that the polyphosphonate- based antiscalant enhanced biofilm growth by serving as a source of phosphorus, a limiting nutrient in the brackish water used. These observations were further supported by examining RO membrane modules from industrial and pilot desalination plants with polyacrylate- and polyphosphonate-based antiscalants. Similar behavior was observed in both scales of membrane operation, where polyacrylate was shown to induce biofilm growth. In general, antiscalants, mostly polyelectrolytic compounds, can adsorb to the active polyamide layer of RO membranes and alter its surface charge density, and / or hydrophobicity. Upon exposure to antiscalants, bacterial surface properties can change and therefore, the antiscalants enhance fouling of RO membrane surfaces by altering membrane and/or bacteria surface characteristics and promote the initial attachment and deposition of bacteria and organic materials. Thus, the total amount of biofilm growth and/or formation of organic fouling layer on the membrane surface increases. In
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addition, antiscalants can serve as nutrients for microbial growth, especially in oligotrophic environments such as in seawater desalination systems where the dissolved organic carbon concentration can be less than 1 mg/L. Antiscalants are added at a concentration of several mg/L in the feed water of the RO process and therefore, increase the dissolved organic carbon concentration. Notably, polyphosphonates and polyphosphates increase the available phosphorous for microbial growth, especially in brackish- and seawater where phosphorous is a limiting nutrient for microbial proliferation. Interestingly, different biofilm formation stages on RO membrane surfaces are shown to be affected by antiscalants in different ways: while polyacrylates increase bacterial initial attachment by altering membrane physico-chemical properties, polyphosphonates increase biofilm growth under phosphorous limiting conditions. Thus, the selection of the type and dosage of antiscalant should take into account the associated contribution to membrane biofouling propensity.
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Literature review:
1.1 Water scarcity Approximately three quarters of the Earth's surface is covered with water, which is among the most abundant and vital resources on the planet. Around 96.5% of Earth's water is located in seas and oceans and its concentration varying between 3.5-4.5% of salt. Around 1.7% of Earth's water is located in the ice caps mountain glaciers and only a small fraction of the Earth’s water is considered to be fresh and usable for human purposes: it is around 0.8% of the total water allocation, mainly trapped in aquifers. The remaining percentage is made up of brackish water, slightly salty water found as surface water in estuaries and as groundwater in salty aquifers [1]. Around 1.2 billion people in the world live in areas of physical water scarcity, and 500 million people are approaching this situation. Water scarcity is one of the major problems faced by communities in arid regions, and is already affecting all continents throughout the globe. Many countries in the Middle East, Southern Europe, North and mid Africa, Australia as well as many states of America (including California, Florida, and New Mexico) are already suffering from this acute problem. Water scarcity is both a natural and a human-caused phenomenon [2]. It is considered a major political and economic problem, which has arisen from the increasing demand for water supply generated by rapid population growth, urbanization, industrialization, increasing standards of living, irrigation needed to fulfill food demand and daily basic needs. Also, the uneven distribution of freshwater resources, water being wasted, polluted and unsustainably managed, has worsened this problem. This has lead to a need for new and unconventional water resources, such as water reuse and salt water desalination that can be recognized as significant and reliable options to feed the growing demand for water and provide the keys for sustaining future generations across the globe [3].
1.2 Desalination: the real solution for water scarcity Desalination, which involves the extraction of pure water from highly or moderately concentrated salty or brackish water, is a rapidly growing source of potable water in the
1 world [3]. Seawater desalination offers an unlimited, steady supply of high-quality water, without harming natural freshwater ecosystems [4]. The idea of extracting salt from seawater, which turns into fresh water has been developed and used for centuries. Fresh water contains less than 1000 mg/L of salts or total dissolved solids (TDS) [5]. Sea and brackish water desalination has become an increasingly strategic water management option and presents a promising solution to the growing pressure and the stress on water resources for fulfilling the increasing water demands [6]. According to the 22nd DesalData IDA Worldwide Desalting Plant Inventory in 2009, there are 14,451 desalination plants with a combined capacity of 59.9 million cubic meters per day (m3/d). This represents an annual increase of 12.3% of total desalination capacity. In addition, a further 244 plants with a capacity of 9.1 million m3 /d were announced to be under contract [7]. In 2016, the global water production by desalination is projected to be double the amount which was produced in 2008, reaching around 40 billion m3 of water produced by desalination [8]. Figure 1 shows the daily production of fresh water by desalination and the expected production in the coming years. Sea and brackish water desalination processes can by divided into thermal desalination processes and membrane desalination process. Thermal desalination separates salt from water by evaporation and condensation, whereas membrane processes use semi- permeable membranes and driving forces like pressure to separate salts from water [5]. Thermal desalination has been used for hundreds of years to produce fresh water, with several large scale plants built during the 1950s [1]. The Middle East countries were among the pioneers of adopting thermal desalination, by introducing a process called multi-effect distillation (MED) and later using a process called multi-stage flash (MSF) distillation [9]. MSF is among the most applied technology in the Middle East, especially in the Arab Gulf countries with a contribution of 50% of the world's desalination capacity due to the low cost of fossil fuel in this region and the poor water quality of the local feed water [10]. Thermal desalination is significantly more energy demanding than membrane- based desalination, but it can deal with more saline water and produce even higher permeate quality [11].
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Fig.1. Daily global water production by desalination currently and projection in the near future [8] ( http://www.nature.com/news/2008/080319/full/452260a.html )
1.3 Desalination by Reverse Osmosis RO Increasing water scarcity and economical considerations have attracted scientists to develop and adopt advanced membrane technologies based on processes such as reverse osmosis (RO), nanofiltration (NF), electrodialysis (ED), ultrafiltration (UF), and microfiltration (MF) for desalination, among which RO is the most accepted and well introduced technology. RO technology is among the most versatile desalination method for water purification in the present time. The number of desalination plants that are based on RO and NF membranes has dramatically increased during the last 40 years due to the improvement in RO technology, especially in arid regions where natural water
3 resources are already scarce. RO technology has become the major choice for new desalination facilities [5] due to its robustness and relatively low energy consumption. It has rapidly gained popularity and is established as a strong proportion of the desalination market [10-11]. Figure 2 shows a sketch of a seawater reverse osmosis (SWRO) desalination plant with its various stages.
Fig.2. SWRO desalination plant showing the various stages—seawater intake, pretreatment, reverse osmosis, post-treatment, and brine discharge—and their interactions with the environment adopted from Elimelech (2011) [4].
1.3.1 History of the modern RO membranes The early development of RO technology started in 1958 by Reid and Breton. They conducted experiments at the University of Florida by using cellulose acetate (CA) film to separate salt from water which later was commercialized. They used a hand-cast thin symmetrical cellulose acetate (CA) membrane and they achieved 98% rejection of salt. Their main failure was the low permeate flux, which was in the order of 10 mL/m2·h1. In 4
1960, Loeb and Sourirajan developed the first asymmetric membrane material from cellulose acetate, which effectively separated salt from water based on the earlier finding of Reid and Breton. The development of the Loeb and Sourirajan membrane model was the breakthrough of commercializing the RO process. Based on their technique, membrane formation processes, including interfacial polymerization and multilayer composite casting and coating were developed and high performance membranes with high salt rejection and better permeate flux were produced. Using these processes, membranes with selective thin layers of ~0.2 μm were being produced by a number of companies. RO asymmetric membrane materials, composite membranes with better and durable performance supplemented with polyamide material were introduced in the 80’s. Better salt rejection, productivity and durability were the main goals of membrane production during that period, by which the cost of RO membrane processes for producing freshwater was dropped [6]. Advances in chemical engineering contributed enormously to the desalination process by efficient membranes and made it more feasible and affordable for implementation in the water industry of the current century.
1.3.2 Commercial RO membranes RO is based on a property of certain polymers termed semi-permeability. While these polymers are very permeable for water, their permeability for dissolved substances and colloids is very limited [11]. RO membranes are predominantly thin film composite (TFC) polyamide (PA) membranes [12]. TFC can be defined as a multilayer membrane in which an ultrathin semi-permeable membrane layer is deposited on a preformed, fine microporous support structure. The composite PA membrane consists of three layers: a dense and ultrathin selective polyamide layer with a thickness varying to as low as 200 angstroms, a porous to micro-porous polysulfone support layer, and a non-woven fabric layer for mechanical strength and support. The simple principle of RO desalination is water diffusing through a non-porous membrane with high applied pressure to overcome the osmotic pressure in the feed. The applied pressure can range between 60-80 bars in case of seawater desalination [10, 13] and around 15 bars in case of brackish water desalination. The applied pressure forces the water contained in the feed to permeate through the membrane. Also, salts and all
5 colloidal or dissolved matter from aqueous solutions in the feed are almost completely retained, generating a concentrated brine and permeate, which consists of nearly fresh and pure water [14].
1.3.3 RO fouling As any of membrane-based processes in water treatment, certain challenges have been recognized to affect the optimal and the efficient operation of RO desalination plants [14]. One of the foremost challenges arises from membrane fouling. Fouling refers to the attachment or adsorption of undesired materials (foulants) onto the membrane surface and/or within the membrane pores, which results in a decrease in permeate water flux and quality. Most of fouling layers on RO membranes consist of a combination of colloidal and particulate materials, (organic and microbial deposits such as microbial biofilms) as well as inorganic precipitates (contributed by sparingly soluble salts), all of which have been rejected by the membrane [15]. Different types of fouling can occur simultaneously, influencing each other [16]. All of these foulants can significantly affect RO performance and the economical values of the plant's operation.
1.3.3.1 RO biofouling Biofouling is attributed to living microorganisms and their activities, mainly bacteria accompanied with a lower degree of fungi and eukaryote microorganisms [17-18]. Biofouling starts when bacterial activities take place on membrane surface, which include bacterial deposition, attachment, growth, and accumulation of cell clustering and microcolonies. Moreover, colonization of cells is followed by secretion of metabolic products such as extracellular polymeric substances (EPS). After cell deposition and irreversible attachment, the cells proliferate and form microcolonies, followed by growth of the sessile community to mature and cohesive biofilms that self-produce and secrete EPS. EPS affect the immediate heterogeneous microenvironment of living cells, producing changes in porosity, density, water content, charge, sorption properties hydrophobicity, and mechanical stability [19]. Biofilm formation process takes place on the membrane surface that eventually provides a good environment for microbial colonization where nutrients are accessible [18].
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1.3.3.2 Inorganic fouling (scaling): Membrane scaling is caused by the precipitation of sparingly soluble inorganic salts, i.e. divalent and multivalent ions exceeding their solubility level. Common scaling salts include calcium carbonate (CaCO3), calcium sulphate (CaSO4·xH2O), barium sulphate
(BaSO4), strontium sulphate (SrSO4), silicates, calcium phosphate (Ca3(PO)4) and aluminosilicates. Therefore, the concentration of salts in the feed side of the membrane increase, and with increasing recovery, the RO membrane is easy to be scaled. Nevertheless, solubility levels only define the minimum concentration levels at which scaling may occur. In RO systems with higher feed salt concentrations, scaling may not occur due to the long induction times of the crystallization process. However, it is recommended not to exceed the solubility limits [11]. Super-saturated or sparingly soluble salts can precipitate on the membrane surface, building a thin layer or a boundary layer, which hinders water transport through the membrane. This layer is formed due to a concentration gradient that occurs as product water (permeate) continuously passes through the membrane, leaving behind an ever-increasing level of dissolved and suspended solids in what is called a concentration polarization phenomenon. The most susceptible place for scaling is the downstream part of the RO process where concentration in the feed solution is the highest [11]. Moreover, membrane scaling is also enhanced as permeate water recovery increases. Scaling by inorganic salts reduces permeate flux, increases feed pressure, decreases product quality and eventually shortens membrane life [20-21]. Therefore, membrane scaling increases the operational costs of RO facilities by increasing (i) energy consumption, (ii) system failure by time, (iii) necessary membrane area, and (iv) construction, labor, time, and material costs for washing and cleaning procedures [22]. Several studies have suggested that membrane scaling is dependent on several factors including, but not limited to, membrane characteristics, module geometry, feed solution characteristics and operating conditions [23-25].
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1.3.3.3 Scale formation mechanisms: Scale formation is a complex process in which both crystallization and hydrodynamic transport mechanisms are involved. Three stages were suggested to explain the crystallization process on RO membranes [20, 26-27]: surface (heterogeneous) crystallization, bulk (homogeneous) crystallization and the growth of salt crystals. In surface crystallization, the ion clusters begin to form nuclei, characterized by more orderly association and aligning of ions. Flux decline results from surface blockage mechanisms of the membrane surface by lateral growth of the scale deposit. Assuming the areas occupied by crystals are completely impermeable, the flux in the absence of the cake formation could be expressed as follows:
P A J free R A m t ……………………………………………………………..(1) Where ΔP is the trans-membrane pressure, π is the osmotic pressure, η is the permeate viscosity, Rm is the membrane resistance, A is the total membrane area and Afree is the membrane area unoccupied by surface (heterogeneous) crystallization. In the bulk crystallization, precipitation occurs when ions of opposite charge associate and begin to cluster together in large groups (>1000 atoms). Crystals formed in the bulk solution precipitate on the membrane surface and lead to a flux decline. Therefore, the flux decline increases with the accumulation of the porous layer of precipitate and could be described by a resistance-in-series model: P J R R m c ……………………………………………………………… (2)
Where Rc is the resistance due to cake formation. Fig. 3 illustrates the two different schemes of scale formations in RO systems. The third and final stage is the growth of salt crystals on the formed nuclei (seed crystals). While the first two stages are reversible, the third stage is irreversible and will continue to occur until the ion concentrations decrease to reach the solubility limit. Heterogeneous precipitation can also occur, where nuclei or ion clusters precipitates associate with suspended or colloidal particles in the feed
8 solution. Moreover, metals such as magnesium, barium, and strontium often co- precipitate when salts such as calcium carbonate precipitate [28].
Fig. 3. Schematic illustration of scale formation schemes [29].
1.4 Scale control in RO system: Solubility and the chemical compositions of the sparingly soluble salts in the feed water of RO system are of concern. Thus, to avoid such scaling problems and prolong the working-life of RO membranes, the production and development of pretreatment chemicals in RO membrane processes is needed, especially scaling inhibitors, named antiscalants. Scale formation in RO desalination systems can be successfully prevented or avoided following the various pretreatment methods and approaches [30]: (i) operating RO systems at low recovery, (ii) removing di- and tri-valent ions by either ion exchange or lime softening methods, (iii) using acid feed (sulfuric acid) for reduction of alkalinity, and (iv) using antiscalants to control the precipitation and deposition of scale forming minerals.
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1.4.1 System Recovery: In RO systems, membrane scaling can be avoided and controlled by operating the system at low recovery. Under these conditions, the solubility of the scale-forming mineral will not be exceeded. However, this technique is usually not valid due to the decreased efficiency of the RO system [30].
1.4.2 Softening: Lime softening and sodium cycle cation exchange can be applied to avoid scaling by removing hardness related ions from RO feed water. Sodium, which can replace the hardness ion is rarely a scale forming ion, and therefore can be tolerated and used [31]. Acid treatment: The addition of acids in water industry is among the oldest treatment techniques to control calcium carbonate scaling. Acids such as sulfuric or hydrochloric acid can be introduced to the feed in order to reduce the alkalinity and to prevent appearance of calcium carbonate precipitate species as followed:
Ca (HCO3)2+ H2SO4→ CaSO4 +2CO2 ↑+ H2O Sulfuric acid is used more frequently than hydrochloric acid because it is relatively inexpensive and easy to be dosed. Nonetheless, it could be another source of scaling by increasing the potential of sulfate scale (such as calcium sulfate, strontium sulfate, or barium sulfate) [30].
1.4.3 Metal Ion Stabilization: The stabilization of metal ions (i.e., iron, manganese, copper, zinc) can be done through the addition of chelating agents to keep the metal ions in a soluble form. Among the commercial chelating agents are gluconic acid, citric acid, ethylenediaminetetraacetic acid (EDTA), and polymeric dispersants such as polyacrylic acid, or acrylic acid, and maleic acid-based copolymers [30].
1.4.4 Antiscalants addition: The most commonly used method for controlling scale formation involves the addition of scale inhibitors, antiscalants, to the feed water [30]. Antiscalants are polyelectrolyte
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chemical compounds [32], with reported optimal molecular weights in the range of 1000– 3500 g/mol [33-34], typically consisting of one or a combination of polyacrylic acid, carboxylic acid, or phosphonates. Antiscalants have a superior effect in preventing the precipitation of scale forming salts even at low dosage levels, by preventing the formation of crystals larger than the critical size (preventing nucleation) and by surface modification of the crystals that do form (Figure 4). In oversaturated salt solutions supplemented with antiscalants, a significant delay in the induction time needed for precipitation is usually observed and the quality of the feed water does not have an effect [10, 31, 35-38]. Generally, antiscalants play crucial roles in maintaining efficient RO plant operations at the highest possible recovery rate by circumventing the need to replace expensive membranes prematurely, eliminating or reducing the use of hazardous acids, reducing water consumption by safely operating at high recovery rates, using fewer chemicals, producing less concentrate and allowing for better environmental acceptance of that concentrate, and reducing energy costs, as well as reducing the downtime caused by frequent membrane cleanings [10, 31, 38-39].
Fig. 4. Schematic illustration of antiscalant inhibition mechanisms [40]
1.4.4.1 Antiscalant forms: Commercially available antiscalants can be classified into four major categories, phosphates, phosphonates, polycarboxylates and dendrimeric polymers. Polyphosphate
11 based antiscalants, which carry the O-P-(O)3 functional group, were among the first commercially used in membrane desalination [41]. The most commonly used polyphosphate based antiscalants are sodium hexametaphosphate (SHMP) or sodium hex and potassium pyrophosphate [(NaPO3)6]. SHMP is relatively inexpensive compared to other scale inhibitors and has been commonly used in conjunction with a mineral acid for membrane scale inhibition. SHMP has gained exceptional fame in the membrane desalination industry with its successful history as scale inhibitor, but it has been showed that SHMP has certain insufficiency [42]. One of its main problems is its susceptibility to hydrolysis in the dosing feed tank, leading to cleavage of the O-P-O group, and the formation of orthophosphate. This weakness of SHMP has led it to be replaced by more efficient and stable inhibitors, organophosphonates. Organophosphonates are another form of phosphonates, which are the salts and esters of phosphonic acid HPO(OH)2with a superior effects over SHMP. They are more resistant to hydrolysis due to their more stable C-P bonds and offer scale inhibition and dispersion potential similar to that of SHMP, but they are more expensive [42]. The stability of the phosphonate increases as the number of phosphonic acid groups increase in the complex. Polyphosphonates exhibit superior performance in comparison to polyphosphates, especially for the inhibition of
CaCO3, Mg(OH)2 and BaSO4 scales [43]. Also, the inhibitory action of polyphosphonates towards CaCO3 was significantly stronger compared to CaSO4 scale [30, 41]. Polycarboxylates are characterized by functional –COOH groups. Among the most used inhibitors in this category are polyacrylic acids (PAA) [CH2CHCOOH]n. Polycarboxylates are anionic, low molecular weight polyelectrolyte polymers [42]. They are mainly homopolymers and copolymers of acrylic acid and maleic acid, but polymers such as polymethacrylic acid and polyasparates are also available [29]. The expansion of RO technology applications and the benefit that this industry has gained from the scale inhibitor industry, has led to the development of dendrimeric carboxylated polymers, which are highly branched with a three-dimensional structure, some of which have been referred to as environmentally friendly antiscalants [44-45]. Figure 5 shows chemical schematic structure of the major antiscalants used in RO desalination processes.
12
A B C
D E F
Fig. 5. Chemical structure of some antiscalants used in RO desalination: (A) sodium hexametaphosphate (SHMP); (B) potassium pyrophosphate; (C) 2-Hydroxy phosphono acetic-acid; (D) polyacrylic-acid; (E) polyphosphonate; and (F) carboxylated dendrimeric polymer.
1.4.4.2 Antiscalant problems: As mentioned before, antiscalants play crucial roles in preventing membrane scaling. However, in spite of their significant capacity to do so, they have been reported to enhance membrane fouling, which is another setback for the optimum operation of RO desalination processes [10, 35, 39, 46-54]. It is very important to use and determine the optimum dosage of antiscalants, otherwise they can function as foulants at higher concentrations [55]. Antiscalants were shown to induce biofilm formation in RO systems by increasing the microbial growth potential up to 10 times their normal growth rate [56- 57]. Pretreatment chemicals carried over from other processes may react with antiscalants and form foulants or decrease antiscalant efficiency. Also, it has been shown that some cationic flocculants used for pretreatment can particularly react with some types of antiscalants, forming a sticky and gel like fouling layer [58-59]. Moreover, the eutrophicating properties of some antiscalants and associated disposal problems in the marine environment are another problem which led to a substantial reduction in the use of these kind of chemicals in the RO industry [11]. Some phosphonate antiscalants are also
13
suspected to form calcium phosphate deposits [60-62]. The protonation of chemical moieties on the phosphonate based antiscalants led to a decrease in the pH of the RO feed water, in turn leading to the increased precipitation rate of gypsum salts, resulting in deterioration of the effectiveness of phosphonate based antiscalants [63].
2. Hypotheses and Aims 2.1 Hypotheses: Antiscalants, mostly polyelectrolytic compounds, can adsorb to the active polyamide layer of RO membranes and alter its surface charge density, and / or hydrophobicity. Upon exposure to antiscalants, bacterial surface properties may also be changed. Therefore, the first mechanism we propose by which antiscalants might enhance fouling of RO membrane surfaces, is by altering membrane and/or bacteria surface characteristics and promoting the initial attachment and deposition of bacteria and organic materials. Thus, a higher amount of bacteria and/or organic matter will be attached and eventually the total amount of biofilm growth and/or formation of organic fouling layer on the membrane surface will increase. Our second working hypothesis is that antiscalants can serve as nutrients for microbial growth, especially in oligotrophic environments such as in seawater desalination systems where the dissolved organic carbon concentration can be less than 1 mg/L. Antiscalants are added at a concentration of several mg/L in the feed water of the RO process and may significantly increase the dissolved organic carbon concentration. In addition polyphosphonates and polyphosphates may significantly increase the available phosphorous for microbial growth, especially in brackish- and seawater where phosphorous is a limiting nutrient for microbial proliferation. It was reported that microbial growth in the Mediterranean Sea is most likely limited by the availability of soluble phosphate [64,65]. 2.2 General aims: For the successful operation of any RO process, the key is to ensure the compatibility of the pretreatment chemicals with both the polyamide RO membrane and RO process additives. To achieve an excellent RO operation it is necessary to find a combination
14 between the optimum dosing of pretreatment chemicals with both the polyamide RO membrane, RO process additives and the feed water. To date, the literature is lacking complete knowledge regarding the direct effect of antiscalants and their role in fouling of RO membranes. In this project, the overall goal is to investigate the effect of antiscalants commonly used in seawater and brackish water desalination processes on membrane fouling in general, and organic and biological fouling specifically.
Specific aims: The influence of antiscalants on biofouling of RO membranes in seawater desalination: o To explain the effects of antiscalants on the initial attachment of bacterial cells on RO membrane surface. We investigated the effect of antiscalants on the physical- chemical properties of bacterial cells and RO membranes. o To study the effect of antiscalants on surface physical-chemical properties of RO membranes, such as surface charge and hydrophobicity, in order to further delineate the related consequences on organic fouling. o To investigate the biofouling potential of antiscalants during desalination of brackish water using RO membrane technology, at both lab and industrial scale levels. Both the potential of antiscalants to nutritionally supplement microbial biofilm growth as well as conditioning and providing a preferred surface for microbial attachment are presented.
15
3. The dissertation structure o The first paper explains the effects of antiscalants on the initial attachment of bacterial cells on RO membrane surface. We investigated the effect of antiscalants on the physical-chemical properties of bacterial cells and RO membranes.
o The second paper is focused on studying the effect of antiscalants on surface physical-chemical properties of RO membrane, such as surface charge and hydrophobicity, in order to further delineate the related consequences on organic fouling.
o The third paper integrate the fundamental information gathered in the first two papers and investigates the biofouling potential of antiscalants during desalination of brackish water using RO membrane technology, in both lab and industrial scale levels. Both the potential of antiscalants to nutritionally supplement microbial biofilm growth as well as conditioning and providing a preferred surface for microbial attachment are presented.
16
water research 47 (2013) 3389e3398
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
The influence of antiscalants on biofouling of RO membranes in seawater desalination
Amer Sweity a, Yoram Oren a, Zeev Ronen b, Moshe Herzberg a,* a Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research, Albert Katz International School for Desert Studies, Ben Gurion University of the Negev, Sede-Boqer Campus 84990, Israel b Department of Environmental Hydrology and Microbiology, Zuckerberg Institute for Water Research, Albert Katz International School for Desert Studies, Ben Gurion University of the Negev, Sede-Boqer Campus 84990, Israel article info abstract
Article history: Antiscalants are surface active polyelectrolyte compounds commonly used in reverse Received 4 January 2013 osmosis (RO) desalination processes to avoid membrane scaling. In spite of the significant Received in revised form roles of antiscalants in preventing membrane scaling, they are prone to enhance biofilm 16 March 2013 growth on RO membranes by either altering membrane surface properties or by serving as Accepted 19 March 2013 nutritional source for microorganisms. In this study, the contribution of antiscalants to Available online 29 March 2013 membrane biofouling in seawater desalination was investigated. The effects of two commonly used antiscalants, polyphosphonate- and polyacrylate-based, were tested. The Keywords: effects of RO membrane (DOW-Filmtec SW30 HRLE-400) exposure to antiscalants on its Reverse osmosis physico-chemical properties were studied, including the consequent effects on initial Antiscalant deposition and growth of the sessile microorganisms on the RO membrane surface. The Biofouling effects of antiscalants on membrane physico-chemical properties were investigated by Fouling filtration of seawater supplemented with the antiscalants through flat-sheet RO membrane Scaling and changes in surface zeta potential and hydrophobicity were delineated. Adsorption of Biofilm antiscalants to polyamide surfaces simulating RO membrane’s polyamide layer and their effects on the consequent bacterial adhesion was tested using a quartz crystal microbal- ance with dissipation monitoring technology (QCM-D) and direct fluorescent microscopy. A significant increase in biofilm formation rate on RO membranes surface was observed in the presence of both types of antiscalants. Polyacrylate-based antiscalant was shown to enhance initial cell attachment as observed with the QCM-D and a parallel plate flow cell, due to rendering the polyamide surface more hydrophobic. Polyphosphonate-based anti- scalants also increased biofilm formation rate, most likely by serving as an additional source of phosphorous to the seawater microbial population. A thicker biofilm layer was formed on the RO membrane when the polyacrylate-based antiscalant was used. Following these results, a wise selection of antiscalants for scaling control should take into account their contribution to membrane biofouling propensity. ª 2013 Elsevier Ltd. All rights reserved.
1. Introduction of 1000e3500 g/mol, typically consisting of one component or a combination of polyphosphates, polyphosphonates, poly- Antiscalants (AS) are scale inhibitors polyelectrolytes (Smith, acrylates, and dendrimeric polymers (Farahbakhsh et al., 1967) with reported optimal molecular weights in the range 2004; Shih et al., 2006). AS have a superior effect avoiding
* Corresponding author. Tel.: þ972 8 6563520; fax: þ972 8 6563503. E-mail address: [email protected] (M. Herzberg). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.03.042 17 3390 water research 47 (2013) 3389e3398
precipitation of scale forming salts even at low dosage by work, a broad perspective including both biological and physico- preventing formation of crystal larger than a critical size chemical effects of AS on biofouling of RO membranes were (preventing nucleation) as well as by modifying the surface of investigated. One important AS effect includes the alteration of larger crystals (Drak et al., 2000). In supersaturated salt solu- physico-chemical properties of either the membrane or bacte- tions supplemented with AS, a significant delay in the in- rial surface due to AS adsorption, which consequently increases duction time needed for precipitation is usually observed and attachment of bacterial cells to the membranes’ surface. the quality of the feed water is not affected (Fritzmann et al., 2007; Greenlee et al., 2009; Hasson et al., 1998, 2003; Gloede and Melin, 2006; Darton, 2000). In general, AS play crucial 2. Materials and methods roles in maintaining efficient RO plant operations at the highest possible recovery by preventing the need to replace 2.1. Model bacterial strain and media expensive membranes prematurely; eliminating or reducing the use of hazardous acids; producing less concentrate, Vibrio fischeri, a well-known marine bacterium used in this reducing energy costs, as well as the downtime caused by study, was obtained from Stritch School of Medicine in Loyola frequent membrane cleaning (Greenlee et al., 2009; Gloede University, Chicago (http://www.meddean.luc.edu/lumen/ and Melin, 2006; Darton, 2000; Vrouwenvelder et al., 2000). deptwebs/microbio/kv/kvmain.php). This bacterium, a green Despite the significant roles of AS, they are reported to florescent protein ( gfp) chromosomally tagged mutant enhance membrane fouling, which is another setback for KV2682, was grown on LB agar medium supplemented with the optimum operation of any RO desalination process double the amount of NaCl (20 g/L), Trizma base (2-amino-2- (Fritzmann et al., 2007; Greenlee et al., 2009; Vrouwenvelder (hydroxymethyl)-1,3-propanediol) THAM 6.057 g/L, and et al., 2000, 2008; Baker and Dudley, 1998; Subramani and 2.5 mg/L chloramphenicol. The strain was incubated and Hoek, 2008; Sadr Ghayeni et al., 1998; Kochkodan et al., 2008; grown at 25 C and 250 rpm to final OD600 nm of 1. Then, the Abd El Aleem et al., 1998; Fletcher, 1994; Ouazzani and late exponential cells (after 5e6 h of incubation) were centri- Bentama, 2008; Chong et al., 2008). Accordingly, it is very fuged (4 C, 2500 g, 20 min), washed three times, and re- important to use the minimum possible dosage of AS in order suspended to final OD600 nm of 0.1 with either filtered to avoid fouling enhancement at higher concentrations (Al- seawater or with seawater supplemented with 20 mg/L of Shammiri et al., 2000). AS were shown to induce biofilm for- each of the AS. Polyphosphonate- and polyacrylate-based AS mation in RO systems by increasing the microbial growth analysis is shown the supplementary material (Table S1). potential up to 10 times of their normal growth rate (Vrouwenvelder et al., 2000, 2010). 2.2. Membrane preparation Chemicals used in water pretreatment in RO operations can promotes instability of colloids, particles and bacteria that can A laboratory scale RO unit (Fig. 1) comprised a membrane increase fouling rate (Winters, 1997). Biofilm formation is a cross-flow cell, high-pressure pump, feed water reservoir, progressive and developmental process initiated in bacterial chiller equipped with a temperature control system and PID deposition and irreversible adhesion to surfaces, followed by pH controller for dosing CO2 gas and a data acquisition sys- formation of micro-colonies encased in self-produced extra- tem. Permeate flow rate, conductivity, and pH were monitored cellular microbial matrix (also termed as extracellular polymeric in each experiment. A high flux RO flat-sheet membrane SW30 substances e EPS), maturation, and finally, dispersion of the HRLE 400 (Dow-Filmtec, USA) was compacted with deionized bacterial cells back to their planktonic stage (O’Toole et al., 2000). water (DW) at a pressure of 60 bar before each experiment Deposition and attachment of the bacterial cells onto surfaces is until the permeate flux attained a constant value, after 24 h. a critical step in the overall process of biofilm formation, which A pressure of 60 bar and a temperature of 25 C were kept is mediated by biological, physical, and chemical factors constant during all the experiments. After the compaction (Donlan, 2002). These factors includes substratum properties stage completed, pretreated seawater sampled from Palma- such as roughness, charge density and hydrophobicity (Diaz chim desalination plant (Palmachim, Israel) (after floccula- et al., 2010; Park et al., 2005); a “conditioning film” of macro- tion, coagulation, and sand and micronic filtration), with or molecules on the surface; hydrodynamics forces (Eshed et al., without AS (20 mg/L) were filtered through the membrane for 2008; Purevdorj et al., 2002); solution chemistry such as ionic 24 h in the RO desalination lab unit. Then, the conditioned strength, pH, and the presence of multivalent cations (Chen membranes were kept in 4 C, in similar seawater used for et al., 2009; Chen and Walker, 2007; Rijnaarts et al., 1999); and conditioning the membranes until further analysis of biofilm bacterial cell surface properties such as hydrophobicity, formation or surface properties characterization has been expression of flagella and pili, lipopolysaccharides (LPS), and carried out. Usually, AS dosage in real RO installations varies EPS. The reversible attachment of bacteria involves weak forces between 2 and 8 mg/L. The reason for choosing 20 mg/L can be such as van der Waals, electrostatic and hydrophobic in- justified due to buildup of AS concentration at the tail end of teractions between the bacterial cell and the substratum the RO cascade due to recovery. (Vanloosdrecht et al., 1987). Vrouwenvelder et al. showed how AS can enhance biofouling in RO membrane applications 2.3. Membrane surface properties (Vrouwenvelder et al., 2000, 2010) by serving as phosphorous and carbon source of nutrients under limited nutritional con- Prior to each biofilm formation experiment, after conditioning ditions. While Vrouwenvelder et al. were focused on the nutri- the membranes with seawater in the presence or absence of tional effects of different AS on biofouling phenomena, in this AS, membrane surface properties, i.e., surface zeta potential
18 water research 47 (2013) 3389e3398 3391
Fig. 1 e Cross-flow, flat-sheet, RO desalination unit used for conditioning RO membranes with AS during desalination of seawater. and hydrophopicity, were measured. Membrane surface zeta 10 mM NaCl with and without antiscalant (20 mg/L). The potential was measured using a streaming potential analyzer relative hydrophobicity of the cells was determined by the (SurPass Elektrokinetic Analyzer) at 10 mM NaCl solution with “adhesion of microbial cells to hydrocarbon droplets” test or without pretreatment with AS. For each solution, mea- (MATH test) with n-dodecane (Pembrey et al., 1999). surements were done twice. During each measurement, each run of the electrolyte solution flow proceeded in two di- 2.5. Monitoring bacterial attachment with quartz rections (right to left and then left to right). The zeta potential crystal microbalance with dissipation (QCM-D) of the RO membranes was calculated from the streaming potentials using the HelmholtzeSmoluchowski equation with Polyamide sensors mimicking RO membrane surfaces were the Fairbrother and Mastin substitution (Benavente and used in E4 module QCM-D (Q-Sense, SWEDEN) for analyzing Jonsson, 2000; Deshmukh and Childress, 2001). The hydro- the effect of AS on bacterial deposition and attachment. All phobicity of the RO membrane was deduced from the contact QCM-D experiments were performed at flow-through condi- angle analysis method, whereby it is determined by the tions using a digital peristaltic pump (IsmaTec Peristaltic captive bubble method (OCA, Data Physics). A droplet size of Pump, IDEX) operating in a pushing mode. The flow rate of the air water with diameter of 0.4e0.5 mm was introduced to the working solution in the QCM-D flow cell was 150 mL/min. The RO membrane surface after the antiscalant treatment stage in following solutions were injected sequentially to the QCM-D the RO unit. Duplicated experiments, with five different system: (i) double distilled water baseline for 20 min; (ii) measurements of contact angle, were carried out for each of 0.2 mm filtered seawater for 20 min; (iii) seawater supple- the treated membranes to obtain at least 10 measurements of mented with 20 mg/L AS; (iv) bacterial suspension in 0.2 mm contact angle for each set of conditions. In addition, X-ray filtered seawater with or without AS for 20 min; and (v) finally, photoelectron spectroscopy (XPS) was used to verify the effect these steps were repeated in a reverse order. When no AS was of AS addition on the elemental composition of the polyamide supplemented, step (ii) was injected for 40 min. The adsorp- surface membrane and properties after treating the mem- tion kinetic curves were made by Q-Tools software (Q-SENSE, brane surface with seawater supplemented with or without Sweden). This software adjusts the incoming and outgoing AS. Elemental composition XPS analysis was deduced from electrical currents, regulates the amplitude of the oscillation deconvoluted spectra of carbon, nitrogen, oxygen, and phos- and controls the temperature according to the temperature phorous binding energies on membrane surface presented in set. The variations of frequency shift (Df, Hz) and dissipation the Supplementary material. Figures S1eS4, for pristine factor (DD) were measured for five overtones (n ¼ 3, 5, 7, 9 and membrane as well as membranes treated with seawater, and 11) and the 7th overtone is presented. seawater supplemented with either polyacrylate- or poly- phosphonate-based AS, respectively. 2.6. Bacterial deposition experiments using parallel plate flow cell 2.4. Bacterial physico-chemical properties After treating both the RO membranes and the bacterial cells The electrophoretic mobility of the bacterial cells was with AS, three sets of duplicated deposition experiments were measured by zeta potential analyzer (ZetaPlus 1994, Broo- carried out to characterize the effect of AS treatment on the khaven instruments Co., Holtsville, NY). Cells were washed in deposition and attachment of the bacterial cells on the RO
19 3392 water research 47 (2013) 3389e3398
membrane surface. AS concentration used to treat bacterial growth experiments according to the antiscalant being cells and RO membranes was 20 mg/L. In each set of experi- analyzed. At the end of each experiment, the membrane was ments, characterization of the initial bacterial deposition to collected for confocal laser scanning microscope (CLSM) the RO membrane surface was done by placing a piece of each analysis. Prior to the CLSM, pieces of the membranes of the treated RO membranes in a parallel flow cell (FC81, (5 mm 5 mm) from the flow cell were cut from the middle of Biosurface Technologies, Bozeman, Montana). Then, the the membrane coupon and fixed immediately for all runs, treated bacterial suspension was injected into the flow cell without drying the membranes. The fixations of the fouled (140.6 mm long 12.7 mm wide 0.20 mm deep) at a velocity membranes were done by adding 0.05 M sodium cacodylate of 8.5 cm/s. The accumulated bacteria on the membrane buffer supplemented with 2% glutaraldehyde for 1 h. Then the surface were monitored with a fluorescent microscope (Zeiss fouled membranes were stained with concanavalin A (ConA) AXIO Imager) with magnification of 10X and every image was conjugated to Alexa Fluor 633 (Invitrogen, Israel) and with taken at fixed spot of the membrane. The monitored spots on propidium iodide (PI) for probing EPS and cells, respectively. the membrane coupon were visualized every 20 min and ac- Microscopic observation and image acquisition were per- quired with a CCD camera. At least 5 pictures were taken formed using Zeiss-Meta 510, a CLSM equipped with Zeiss dry every 20 min, and the fluorescent signals of the bacteria were objective LCI Plan-NeoFluar (25X magnification and numerical manually counted on a rectangular viewing area of aperture of 0.8). CLSM images were generated using the Zeiss 861 650 mm, which was recorded by the CCD camera at a LSM Image Browser. Gray scale images were analyzed, and the magnification of 10 . Before snapping the pictures, the same specific biovolume (mm3/mm2) was determined with IMARIS background solution as in the experiment (without bacteria) v7.5 software (IMARIS Bitplane, Zurich, Switzerland). Table 1 was injected into the flow cell for 20 s for washing the sus- summarizes the types of bacterial cultures used in the pended bacteria in order to visualize only the attached cells. different deposition and biofouling experiments. This way, each set of experiment was carried out for three hours and the gradual increase in the number of deposited bacteria was recorded and the number of deposited bacteria 2 per cm of the observed RO membrane surface was calculated. 3. Results and discussion As mentioned, at each time point, 5 different spots on the membrane were visualized and the number of the deposited 3.1. Effect of antiscalants on membrane surface cells was averaged, calculated per unit of area and normalized properties to the initial cell concentration in each experiment. The deposition coefficient was calculated as the number of The effect of AS on the surface physico-chemical properties of deposited cells per min per cm2 and normalized to the injected the Filmtec SW30 RO membranes was determined by filtration bacterial cell concentration (Vanoyan et al., 2010). of Palmachim desalination plant feed seawater with or without AS through the membranes for 24 h (Fig. 1). 2.7. Biofilm growth experiments on RO membranes in a parallel plate flow cell 3.1.1. Zeta potential effect For all cases, at 10 mM NaCl, membranes’ surface zeta po- The conditioned RO membranes from the RO lab desalination tential was negative for pH values above 3.8 (Fig. 2A). At lower unit (Fig. 1) were used for biofilm growth experiments, which pH, both AS increased membrane positive charge: a neutral were conducted in a parallel plate flow cell (FC81, Biosurface zeta potential was observed for polyacrylate-based AS at pH Technologies, Bozeman, Montana). Seawater with or without below w3.5 and slightly higher positive zeta potential was AS (100 mg/L) were injected into the flow cell, which was observed for the membrane treated with polyphosphonate- occupied with SW30 membrane. It should be mentioned that based AS. Clearly, when the membrane was not exposed to enhanced biofilm growth experiments were conducted with a any of the AS, the membrane was more negatively charged for relatively high concentration of AS in order to compare be- the entire pH range. tween the AS being used and to achieve biofilm growth within a reasonable experimental period. The flow cell dimensions were 140.6 mm long 12.7 mm wide 0.20 mm deep and a Table 1 e Types of bacterial cultures used in this research flow velocity of 8.5 cm/s was kept constant for ten days period study. of biofilm growth. The AS concentration (100 mg/L) in the Experiment Type of culture inoculum was relatively high, in order to boost the bacterial growth during the relatively short experimental period. The Bacterial physico-chemical V. fischeri pure culture properties microbial inoculum used for this experiment, was bacterial Bacterial deposition and V. fischeri pure culture culture cultivated for four months prepared as following: attachment using QCM-D sterile flasks (250 mL) with 150 mL of Palmachim desalination Bacterial deposition and V. fischeri pure culture plant treated seawater supplemented with AS were incubated attachment experiments for 4 months at 30 C and 250 rpm. A weekly replacement of using parallel plate flow cell 140 mL of seawater with or without antiscalant was carried Biofilm growth experiments Natural microbial throughout the 4 months period in order to create a selective on RO membranes in a consortium isolated parallel plate flow cell from a fouled RO pressure for the growth of the antiscalant-degrading bacteria. membrane Each of the cultures was used as inoculum for the biofilm 20 water research 47 (2013) 3389e3398 3393
3.1.2. Hydrophopicity effect spectra, five different types of carbon bonds, two different Using captive bubble contact angle method (Zhang and types of nitrogen bonds, three types of oxygen bonds, and two Hallstro¨ m, 1990) enabled us to deduce the effect of types of phosphorous bonds were revealed. The different polyacrylate-based and polyphosphonate-based AS under peaks of the binding energies (BE), the related bonds found in most realistic conditions: upon exposing to seawater in the previous studies (Ferjani et al., 2000; Williard, 2007; Boussu absence of AS, the membrane surface was relatively hydro- et al., 2007; Liu et al., 2006; Tang et al., 2007, 2009), and our philic (Fig. 2B) with a contact angle of 32 1.5 . Interestingly, suggested origin of the peaks (membrane or both membrane when exposed to seawater in the presence of AS, a significant and AS) are summarized in Table 2. As it was impossible to increase in hydrophobicity of the RO membrane surface was distinguish between bonds originated from AS and the observed with contact angle measurements of 54.3 5.4 , and membrane due to similarity of the bonds being detected, ev- 42 4.3 for the membranes treated with polyphosphonate- idences for the AS adsorption to the membrane were absence e e e based and polyacrylate-based AS, respectively (Fig. 2B). of the aromatic C C/C H peak of 285 eV and the CH2 NCO While polyacrylates are linear with amphiphilic structure, the peak of 288 eV, only for the membrane treated with poly- positive shift in zeta potential and the increased hydropho- phosphonate-based AS (Table 2). In addition, for the mem- bicity of the membrane exposed to polyacrylate-based AS, brane treated with carboxylic acid-based AS, the nitrogen implies that carboxylic moieties on this AS interact with the peak that is attributed to amine group (with binding energy BE RO membrane, probably chemically cross-bridged with diva- between 400.76 and 400.85) was not detectable, probably due þ lent cations as Ca 2 with the hydrophobic tail of this AS to interaction of carboxylic acids with a surface amine group. exposed to aqueous surrounding environment. It should be For the oxygen peaks, as presence of the three main types of mentioned that zeta potential analysis of the membranes was oxygen was detected on all membranes, and therefore oxygen conducted in 10 mM NaCl after treating the membrane with bonds were not useful to determine the presence of adsorbed seawater with or without AS. The results imply insignificant antiscalant. Still, the absence of O 1s peak at BE around 529, possible washout of the AS from the membranes during the rules out the presence of any metal oxides on the membrane streaming potential analysis. Since polyphosphonate and surface. Regarding the phosphorous bonds, the virgin mem- aminophosphonates, which mainly comprise the poly- brane had the least amount of phosphorous: the atomic per- phosphonate-based AS, are smaller in their size and hydro- centage of phosphorous on the membranes were 0.19%, 0.3%, philic than polyacrylates, almost no effect on membrane 0.26% and 0.54% for the virgin membrane, membrane treated hydrophobicity was observed. However, interactions of both with seawater, membrane treated with seawater and poly- of carboxylic and phosphate moieties of the polyphosphonate acrylate-based AS and membrane treated with seawater and and the membrane surface interaction will be facilitated by polyphosphonates-based AS, respectively (Table 2). As ex- þ Ca 2 cations (Guo and Severtson, 2004; Petit-Agnely et al., pected, the membrane treated with polyphosphonate-based 2000; Skwarczynski et al., 2010; Tsiourvas et al., 1997). AS showed the highest percentage of phosphorous.
3.1.3. XPS analysis 3.2. Effect of antiscalants on bacterial physico-chemical The SW30 HRLE 400 polyamide membranes treated with properties seawater supplemented with AS were characterized using XPS to monitor the presence of AS fingerprinting on the 3.2.1. Hydrophopicity effect membrane surface as shown in Table 2. XPS is a sensitive The effect of AS on the relative hydrophobicity of the V. fischeri surface technique providing analysis of surface chemical KV2682 bacterial cells was analyzed. Interestingly, when groups in a depth range of about 5e10 nm (Ferjani et al., 2000; treated with 20 mg/L polyphosphonate-based AS (PP), these Williard, 2007). From the deconvoluted binding energy bacteria showed lower hydrophobicity compared to the
10 Blank 0 CA 60 PP -10
-20 40
-30 Zeta potential, mV
Contact angle ,Degrees 20 -40 246810 Blank CA PP pH Membranes condtioned
Fig. 2 e The effect of antiscalant treatment (20mg/L) on membrane surface charge and hydrophobicity: polyphosphonate- based (PP) and polyacrylate-based (CA) AS were tested. (A) Filmtech-SW30 RO membranes surface zeta potential plotted as a function of the pH in a background solution of 10 mM NaCl; (B) Captive air bubble contact angle on the surface of Filmtech- SW30 RO membrane. 21 3394
Table 2 e Elemental compositions were computed based on C (1s) N (1s), O (1s), and peaks, which are centered around 532, 399, and 284 eV, respectively. BE refers to the peak binding energy and % At refers to the elemental percentage on the surface of the membrane for 4 different membranes. Samples are pristine membrane, membrane treated with seawater only, membrane treated with seawater supplemented with carboxylic acid-based AS and a membrane treated with seawater supplemented with polyphosphonate-based AS. Suggested bond Pristine membrane Seawater w/o AS Seawater þ CA Seawater þ PP
Peak BE At %. Suggested origin Peak BE At %. Suggested Peak BE At %. Suggested origin Peak BE At %. Suggested origin 3389e (2013) 47 research water origin
C1S scan Aliphatic CeC/CeH 284.65 52.51 Membrane 284.71 49.39 Membrane 284.62 49.48 Membrane or AS 284.59 46.32 Membrane or AS CeO/CeN 286.31 22.12 Membrane 286.45 25.55 Membrane 286.38 22.07 Membrane 286.13 39.82 Membrane
COO orCH2eNCO 288.14 10.07 Membrane 288.4 11.26 Membrane 288.43 11.78 Membrane or AS Non-detectable: covered by layer of AS Aromatic CeC/ 285.41 10.39 Membrane 285.49 9.94 Membrane 285.43 11.57 Membrane Non-detectable: covered by layer of AS CeH stretch O]CeN 287.33 4.9 Membrane 287.6 3.86 Membrane 287.44 5.09 Membrane 287.86 13.86 Membrane (shifted) N1S scan O]CeN 399.79 67.08 Membrane 400.04 92.69 Membrane 400.04 100 399.74 65 Membrane Amine 400.85 32.92 Membrane 401.49 7.31 Membrane Non-detectable: interactions of COO 400.76 35 Membrane (shifted) or with ammonium amine groups 3398 O1S scan e no metal oxides (absence of peaks at BE ¼ 529) COO or OH 532.59 42.3 Membrane 532.54 58.78 Membrane 532.38 41.5 Membrane or AS 532.47 62.51 Membrane or AS
H2O 533.43 31.8 Moisture 533.69 22.67 Membrane 533.45 30.52 Moisture 533.76 17.33 Moisture O]CeN 531.53 25.9 Membrane 531.41 18.55 Membrane 531.42 27.98 Membrane or AS 531.17 20.16 Membrane or AS P2p scan CeP or PO 133.57 0.19 Membrane 133.74 0.30 Membrane 133.62 0.26 Membrane and 132.74 0.54 Membrane and and seawater seawater seawater þ AS
22 water research 47 (2013) 3389e3398 3395
polyacrylate-based AS as well as to the case without AS sensor is shown later in this study, to be positively related to (Fig. 3A). The values for the percent of partitioning between the amount of attached/detached bacteria to/from the sensor the aqueous and the organic phase were 17% 0.62, in the as confirmed by direct fluorescent microscopy. The results in absence of the AS; 14% 0.42 in the presence of Fig. 4 show the AS effects on the adherence of the bacterium V. polyphosphonate-based AS; and 28% 0.54, in the presence of fischeri KV2682. In the presence of the polyacrylate-based AS, polyacrylate-based AS (Fig. 3A). the bacteria cells attached to the sensor at higher extent compared to their attachment in the presence of the 3.2.2. Zeta potential effect polyphosphonate-based AS, since a higher decrease of the The electrophoretic mobility of the V. fischeri KV2682 cells frequency was observed for the case using polyacrylate-based treated with different AS is shown in Fig. 3B. The results show AS. These results can be related to the combination of higher no significant effect on the zeta potential of this strain by the hydrophobicities of the bacterial cells and polyamide surface presence of both AS, with zeta potential values for all cases in treated with polyacrylate-based AS as well as to the more the range between 35 and 38 mV. Similar to the membrane positive zeta potential of the AS treated membrane (Figs. 2 and surface, changes in bacterial cell hydrophobicity in the pres- 3). Yet, we do not have an explanation for the lower adhesion ence of polyacrylate- and polyphosphonate-based AS are also of bacteria after treatment with polyphosphonate-based AS, affected by the chemical nature of these compounds and the where in fact, the bacteria were excluded from the poly- effects are similar. Since bacterial cells were washed with phosphonate treated surface, while attracted to the poly- þ 10 mM NaCl and in the absence of Ca 2, prior to their zeta acrylate treated one. Possible reason may be the lower potential measurements with the different AS, no change in hydrophobicity of the bacteria treated with polyphosphonate- zeta potential was observed, in contrast to the membrane based AS as shown in Fig. 3A. Careful interpretation of the surface, significantly affected by prior conditioning filtration frequency shift acquired during bacterial deposition on QCM- step of seawater in the RO lab unit. D sensors should be taken into account, as the frequency can be influenced by cell surface morphology with some cases, 3.3. Antiscalants effects on bacterial deposition where frequency shift is not positively related to the increase of bacterial adhesion (Marcus et al., 2012). Therefore a positive In order to study bacterial deposition and the related physical relation between deposition of bacterial cells and frequency and chemical interactions, model bacterium with defined shift was established: similar trend for the antiscalant effect properties must be used. Clearly, a pure strain does not on the bacterial cells attached to the RO membrane surfaces represent the actual behavior of natural microbial consortium was observed using direct fluorescent microscopy with this in the environment but it could be the guidance for such bacterium as shown in the inset graph of Fig. 4. phenomena. The selected model strain, V. fischeri KV2682, is a representative marine bacterium and its expression of a green 3.4. The effect of antiscalants on biofilm formation florescent protein ( gfp) allows precise microscopic tracking. Recently, Naidu et al. (2013) used V. fischeri to investigate the Biofilm growth experiments conducted using parallel plate role of microbial activity in biofilter used as a pretreatment flow cell were conducted and the effect of the polyacrylate- stage for seawater desalination. and polyphosphonate-based AS on biofilm formation by nat- Here, the effects of AS on bacterial attachment were con- ural microbial consortium was delineated. As already ducted on a polyamide coated sensor in a QCM-D flow cell. mentioned, prior to the injection of the seawater, with or First, a baseline is achieved with double distilled water (DDW) without the AS, inoculums from the incubated seawater were for 20 min to assure that the polyamide coated sensors is supplemented to the flow cell in order to boost the biofilm absolutely clean. Then, filtered seawaters were injected as a growth process. CLSM analysis of membrane samples taken background solution, followed by injection of seawater with or from the parallel plate flow cell shows that addition of anti- without antiscalants used to condition the polyamide surface. scalant promoted biofilm growth on the surface of the RO After conditioning the surface with seawater in the presence membrane (Fig. 5). Specific biovolume analysis of the biofilms or absence of antiscalant, bacterial suspension was injected using IMARIS software (Bitplane, Switzerland) on the mem- under similar aquatic conditions. Frequency shift of the branes showed that polyacrylate treated membrane had more
Fig. 3 e The effect of antiscalant treatment (20 mg/L) of polyphosphonate (PP) and polyacrylate (CA) based AS on the relative hydrophobicity (A) and the zeta potential (B) of Vibrio fischeri KV2682 bacterial strain in 10 mM NaCl at ambient pH of 6.2. 23 3396 water research 47 (2013) 3389e3398
DDW SW SW+AS Bacteria SW+AS SW DDW 3.0x10 0 2.5x10
2.0x10 -6 -5 1.5x10
1.0x10 -10 5.0x10 0.0 -15 SW CA PP Treatment
-20 Frequency, Hz X10 PP -25 CA
-30 Blank
0 20 40 60 80 100 120 140 Time, Minutes
Fig. 4 e The effect of antiscalant addition on the attachment of bacterial cells to polyamide coated QCM-D sensor as measured by change in frequency of oscillation of the 7th overtone. Inset graph describes the deposition coefficient for GFP tagged Vibrio fischeri on SW30 RO membrane surfaces during cross-flow of 2 mL/min in a parallel plate flow cell (FC81 flow cell, Biosurface Technologies, Bozeman, Montana). biomass in comparison to the case of polyphosphonate phosphorous limiting growth conditions in seawater (unde- treated membranes and the membrane without antiscalant tectable level of total P). Note that in real RO installations, a (Fig. 5). While both polyacrylate- and polyphosphonate-based lower concentration of AS will be used, but still may reach AS increased biofilm formation, the increased bacterial close to the levels used in this study, due to seawater desali- attachment analyzed for polyacrylate-based AS induced more nation recovery of around 50%. Likely, the lower AS concen- effectively biofilm formation process. Polyphosphonate, on tration being used, but still efficient in avoiding membrane the other hand, had an opposite effect on bacterial attach- scaling, the lower biofouling side effects will rise. In should be ment that could not explain the moderate elevation in biofilm mentioned that more study need to be done in order to un- formation as presented for this case in Fig. 5. The moderate derstand the contribution of each antiscalant to biofilm increase in biofilm formation in the presence of poly- growth. This can be achieved by measuring continuously phosphonates could be attributed to the supplement of a other biofilm growth-related parameters, including the pres- limiting nutritional element for microbial growth under ence of adenosine triphosphate (ATP) in the biofilms, and
Fig. 5 e Left panel represents three-dimensional reconstructed images acquire from CLSM using Imaris Bitplane software (the red colour represents biomass and the green colour represents EPS) of the fouled SW30 RO membrane surface after injecting seawater without AS (A); seawater supplemented with polyacrylate AS (B); and seawater supplemented with polyphosphonate AS (C). The resolution of the perspective images is 450 3 450 mm. Right panel (D) represents the amount of biomass per unit area of attached bacterial cells and adsorbed EPS in the biofilm layers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
24 water research 47 (2013) 3389e3398 3397
phosphorous and total organic carbon (TOC) concentration in Chen, G., Walker, S.L., 2007. Role of solution chemistry and ion the feed and effluent water of the parallel plate flow cell. valence on the adhesion kinetics of groundwater and marine bacteria. Langmuir 23 (13), 7162e7169. Chen, G., Beving, D.E., Bedi, R.S., Yan, Y.S., Walker, S.L., 2009. Initial bacterial deposition on bare and zeolite-coated 4. Concluding remarks aluminum alloy and stainless steel. Langmuir 25 (3), 1620e1626. While scale formation in RO desalination systems can be suc- Chong, T.H., Wong, F.S., Fane, A.G., 2008. The effect of imposed cessfully prevented by AS, they can enhance biofilm formation flux on biofouling in reverse osmosis: role of concentration on RO membranes during seawater desalination in several polarisation and biofilm enhanced osmotic pressure phenomena. Journal of Membrane Science 325 (2), 840e850. ways, at different stages of biofilm growth. Polyacrylate-based Darton, E.G., 2000. Membrane chemical research: centuries apart. AS were shown to enhance biofilm formation, most likely by Desalination 132 (1e3), 121e131. altering the physico-chemical properties of the RO membranes Deshmukh, S.S., Childress, A.E., 2001. Zeta potential of such as hydrophobicity and surface charge, which in turn commercial RO membranes: influence of source water type promote the initial deposition and attachment of bacterial and chemistry. Desalination 140 (1), 87e95. cells. Polyphosphonate-based AS were shown to contribute Diaz, C., Salvarezza, R.C., Fernandez Lorenzo de Mele, M.A., membrane biofouling probably by acting as a phosphorous Schilardi, P.L., 2010. Organization of Pseudomonas fluorescens on chemically different nano/microstructured surfaces. ACS source of nutrients under the phosphorous limitation condi- Applied Materials & Interfaces 2 (9), 2530e2539. tions prevailing in seawater. Comparing these two effects Donlan, R.M., 2002. Biofilms: microbial life on surfaces. 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Journal of Environmental Engineering and Science 3 (4), 237e253. This research study was funded by the Israel Governmental Ferjani, E., Mejdoub, M., Roudesli, M.S., Chehimi, M.M., Picard, D., Authority for Water and Sewage and by Albert Katz Interna- Delamar, M., 2000. XPS characterization of tional School for Desert Studies (AKIS) in Ben-Gurion Univer- poly(methylhydrosiloxane)-modified cellulose diacetate e sity of the Negev, which partially supported Amer Sweity’s membranes. Journal of Membrane Science 165 (1), 125 133. PhD fellowship. Fletcher, M., 1994. Bacterial biofilms and biofouling. Current Opinion in Biotechnology 5 (3), 302e306. Fritzmann, C., Lo¨wenberg, J., Wintgens, T., Melin, T., 2007. State- of-the-art of reverse osmosis desalination. Desalination 216 Appendix A. Supplementary data (1e3), 1e76. Gloede, M., Melin, T., 2006. Potentials and limitations of Supplementary data related to this article can be found at molecular modelling approaches for scaling and scale e e http://dx.doi.org/10.1016/j.watres.2013.03.042. inhibiting mechanisms. Desalination 199 (1 3), 26 28. Greenlee, L.F., Lawler, D.F., Freeman, B.D., Marrot, B., Moulin, P., 2009. Reverse osmosis desalination: water sources, references technology, and today’s challenges. Water Research 43 (9), 2317e2348. Guo, J., Severtson, S.J., 2004. Inhibition of calcium carbonate nucleation with amino phosphonates at high temperature, pH Abd El Aleem, F.A., Al-Sugair, K.A., Alahmad, M.I., 1998. and ionic strength. Industrial & Engineering Chemistry Biofouling problems in membrane processes for water Research 43 (17), 5411e5417. desalination and reuse in Saudi Arabia. International Hasson, D., Semiat, R., Bramson, D., Busch, M., Limoni-Relis, B., e Biodeterioration & Biodegradation 41 (1), 19 23. 1998. Suppression of CaCO3 scale deposition by antiscalants. Al-Shammiri, M., Safar, M., Al-Dawas, M., 2000. Evaluation of two Desalination 118 (1e3), 285e296. different antiscalants in real operation at the Doha research Hasson, D., Drak, A., Semiat, R., 2003. Induction times induced in e plant. Desalination 128 (1), 1 16. an RO system by antiscalants delaying CaSO4 precipitation. Baker, J.S., Dudley, L.Y., 1998. Biofouling in membrane systems e Desalination 157 (1e3), 193e207. a review. Desalination 118 (1e3), 81e89. Kochkodan, V., Tsarenko, S., Potapchenko, N., Kosinova, V., Benavente, J., Jonsson, G., 2000. Electrokinetic characterization of Goncharuk, V., 2008. Adhesion of microorganisms to polymer composite membranes: estimation of different electrical membranes: a photobactericidal effect of surface treatment e e contributions in pressure induced potential measured across with TiO2. Desalination 220 (1 3), 380 385. reverse osmosis membranes. Journal of Membrane Science Liu, L.-F., Yu, S.-C., Zhou, Y., Gao, C.-J., 2006. Study on a novel 172 (1e2), 189e197. polyamide-urea reverse osmosis composite membrane Boussu, K., De Baerdemaeker, J., Dauwe, C., Weber, M., Lynn, K.G., (ICICeMPD): I. Preparation and characterization of ICICeMPD Depla, D., Aldea, S., Vankelecom, I.F.J., Vandecasteele, C., Van membrane. Journal of Membrane Science 281 (1e2), 88e94. der Bruggen, B., 2007. Physico-chemical characterization of Marcus, I.M., Herzberg, M., Walker, S.L., Freger, V., 2012. nanofiltration membranes. ChemPhysChem 8 (3), 370e379. Pseudomonas aeruginosa attachment on QCM-D sensors: the
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26 Desalination 352 (2014) 158–165
Contents lists available at ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal
Induced organic fouling with antiscalants in seawater desalination
Amer Sweity, Zeev Ronen, Moshe Herzberg ⁎
Albert Katz International School for Desert Studies, Jacob Blaustein Institutes for Desert Research, Zuckerberg Institute for Water Research, Ben-Gurion University of the Negev, 84990, Israel
HIGHLIGHTS
• Antiscalants were shown to affect membrane zeta-potential and hydrophobicity. • Polyacrylate and carboxylated dendrimeric antiscalants enhanced organic fouling. • Polyphosphonate antiscalant had no effect on organic fouling. • QCM-D and AFM were used to study antiscalant effects on organic fouling.
article info abstract
Article history: The effect of exposure reverse osmosis (RO) membrane to antiscalants (AS) on consequent organic fouling during Received 12 June 2014 seawater desalination was analyzed. Membrane surface properties (hydrophobicity and zeta potential) were Received in revised form 18 August 2014 altered upon the conditioning of the membrane with AS during the desalination process. For all AS used, Accepted 19 August 2014 polyphosphonate, polyacrylate, or dendrimeric carboxylated based AS, membrane zeta-potential became less Available online 14 September 2014 negatively charged over pH range between 3 and 10. Furthermore, the membrane became significantly more hydrophobic when dendrimeric carboxylated and polyacrylate based AS were used and only minor effects Keywords: Desalination were observed for the polyphosphonate based AS. The membrane organic fouling process, tested with different Reverse osmosis model organic foulants (alginate and BSA), was significantly enhanced in the presence of polyacrylate or carbox- Antiscalants ylated dendrimeric based AS, which were used to condition the membrane surface. These changes in fouling be- Organic fouling havior are likely attributed to the AS effects on RO membrane hydrophobicity and zeta potential after exposure and adsorption to the RO membrane. Force curve measurements using atomic force microscopy (AFM) on mem- branes and adsorption of the organic foulants on sensors of quartz crystal microbalance with dissipation (QCM-D) monitoring were used to explain the induced adsorption of the model organic foulants by polyacrylate and carboxylated dendrimeric based AS. © 2014 Elsevier B.V. All rights reserved.
1. Introduction carbonate (CaCO3), calcium sulfate (CaSO4·H2O), barium sulfate (BaSO4), strontium sulfate (SrSO4), silicates, calcium phosphate Fouling of reverse osmosis (RO) membranes used for seawater and (Ca3(PO4)2) and alumino-silicates (Al2SiO5). Scaling of RO membranes wastewater desalination is considered as a main obstacle, which chal- has a proportional relation with plant recovery; increasing the recovery lenges the widespread application of this technology. RO membrane elevates the concentration of salts in the feed side and therefore, the RO fouling is caused by different agents including inorganic (scaling), col- membrane will be more prone to scaling [8]. Different studies suggested loidal, or dissolved organic matters, [1–3] as well as by microbial biofilm that membrane scaling is dependent on several factors including, but formation (biofouling) [4,5]. Scaling of RO membranes used for seawa- not limited to, membrane characteristics, module geometry, feed solu- ter desalination is the most serious problem, which could impair plant tion characteristics and operating conditions [9–11]. One of the most performance and reduce permeate quality. Scaling reduces permeate commonly used method for controlling scale formation involves adding flux, increases feed pressure and eventually shortens membrane life antiscalants (AS) to the RO feed water [12]. AS are poly-electrolyte poly- [6,7]. Membrane scaling is caused by the precipitation and the accumu- mers used in a multitude of traditional applications. AS have a tremen- lation of sparingly soluble inorganic salts present in the feed water, dous importance, which increased their usage in water applications associated with divalent and multivalent ions, which exceed their solu- such as cooling and boiling water systems, seawater and wastewater bility level, on the membrane surfaces. Major scaling salts are calcium treatment facilities, and oil field operations to prevent inorganic fouling (scaling) [13]. AS play a vital role in keeping the efficiency of the RO ⁎ Corresponding author. Tel.: +972 8 6563520; fax: +972 8 6563503. plant operation at the highest possible recovery rate; avoiding the E-mail address: [email protected] (M. Herzberg). usage of hazardous acids; reducing water consumption by safely
http://dx.doi.org/10.1016/j.desal.2014.08.018 0011-9164/© 2014 Elsevier B.V. All rights reserved. 27 A. Sweity et al. / Desalination 352 (2014) 158–165 159
Fig. 1. Cross-flow flat-sheet RO desalination unit used for conditioning RO membranes with AS during desalination of seawater. operating at high permeate recovery; using fewer chemicals; producing surfaces [28]. Controversially, it was reported that bovine serum albu- less concentrate; reducing energy costs, and reducing the downtime min (BSA) fouling could be greatly reduced by using AS in desalination caused by frequent membrane cleanings [14,15]. Moreover, AS have of brackish water [29]. Also, it was shown that organic fouling may be low molecular weight, in the range of 1000–3500 g/mol [16]. All the mitigated under appropriate conditions such as optimal AS dosage previous advantages placed AS in a high rank to be recommended as a [30]. best pretreatment choice in comparison to different chemicals includ- The overall goal of this work was to investigate AS effects on RO ing acids. Commercially available AS fall into four major groups: organic fouling in seawater. In this work, it was shown that AS were polyphosphates, polyphosphonates, polyacrylates and carboxylated adsorbed to the polyamide RO membrane surface active layer and dendrimeric polymers [13]. One of the main disadvantages of AS was therefore, RO membrane surface properties were altered. RO permeate reported to enhance membrane fouling [17–19]. Also, it has been flux decline was tested when two model organic foulants (alginate shown that some cationic flocculants used for the pretreatment stages and BSA) were added to the feed seawater with different AS including can particularly react with some types of negatively charge AS and polyphosphonates, polyacrylates and carboxylated dendrimeric based form adhesive gel like fouling layer. By dosing a small amount of AS, AS. Moreover, results from quartz crystal microbalance with dissipation the chemical composition of the feed water modifies due to the (QCM-D) and atomic force microscopy (AFM) provided explanation for polyelectrical effect of these chemicals. Previously, we showed that AS the cases when organic fouling was enhanced. adsorption to RO membrane surface strongly affects subsequent fouling behavior by altering the physico-chemical surface properties of the 2. Materials and methods membrane and serving as a conditioning film for further organic- or bio-fouling [20]. Fine colloidal and dissolved organic compounds, in- 2.1. Model organic foulants cluding proteins, polysaccharides, and natural organic matter (NOM) found in natural water can pass all the pretreatment stages of the desa- Alginate and BSA have been selected as model organic foulants. Algi- lination process, interact with other pretreatment chemicals including nate represents polysaccharides with a hydrophilic nature, while BSA is AS, and subsequently foul the RO membranes. a hydrophobic foulant. The molecular weights of BSA and alginate were Organic fouling mechanisms of RO membranes involve the initial 66.5 kDa and 10–60 kDa, respectively. The model foulants were deposition of organic foulants on the membrane surface (foulant– purchased from Sigma-Aldrich (St. Louis, MO) and were received in a membrane interaction) followed by subsequent growth of a dense foul- powder form. For the alginate and the BSA, both were dissolved in ing layer (foulant–foulant interaction) that adversely influences mem- seawater and a stock solution of 2 g·l−1 was prepared for further use. brane performance and efficiency [21]. Organic fouling is influenced Prior to their use, the organic foulant stock solutions were filtered by (i) membrane characteristics (structure and chemical properties through 0.45 μmhydrophilicfilters (Millipore, Billerica, MA). including surface charge and hydrophobicity); (ii) feed solution chem- istry (divalent cations, ionic strength, and solution pH); (iii) foulant 2.2. RO unit and membrane preparations composition and properties (molecular weight and polarity); and (iv) operating conditions at the membrane surface including hydrodynam- The RO laboratory unit (Fig. 1) comprised a membrane cross-flow ics and permeate flux [22,23]. Accordingly, organic fouling is induced cell, high-pressure pump, feed water reservoir of 10 l, chiller equipped by a variety of parameters, among others are the presence of divalent cations, elevated ionic strength, low pH, hydrophobic fouling com- Table 1 Organic carbon (DOC), nitrogen, phosphorous and ammonium content (mg/l) in an AS so- pounds, elevated surface charge (away from the membrane isoelectric lution of 10 mg (liquid) AS per liter of deionized water. point), concentration polarization, elevated surface roughness, and + increase in permeate flux [24–27]. Type of AS DOC Total N Total P NH4 Up to date, relations between organic foulants and AS in seawater Neutralized polyacrylate 2.19 0.1 3.8 0.1 desalination were not investigated. Some AS and dispersants enhanced Neutralized polyphosphonate 1.46 1.4 7.3 0.2 Polycarboxylated dendrimer 5.6 0.3 0.3 0.1 organic fouling through adsorption of humic acids onto the membrane
28 160 A. Sweity et al. / Desalination 352 (2014) 158–165
with a temperature control system, PID pH controller for dosing CO2 CML particle and the membrane surface. The cantilever sensitivity was gas, and a data acquisition system. Permeate flow rate, conductivity, measured on an unconditioned membrane surface prior to each mea- and pH were acquired for relevant experiments. A high flux RO flat- surement and was found relatively constant during the measurement. sheet membrane SW30 HRLE 400 (Dow Filmtec, USA) was compacted The tip velocity was kept at 1 μm·s−1. Raw data were converted from before each experiment with deionized water (DW) at a pressure of cantilever deflection and z-piezo position into force-separation curves. 60 bars until the permeate flux attained a constant value, usually after 120 force measurements were carried out, on eight different locations 18–24 h. The rectangular, crossflow, channel membrane unit (without of each sample to minimize inherent variability in the force data. spacer) dimensions were 7.7 cm × 2.6 cm with a channel height of Mean averages and one standard deviation of the force measurements 0.3 cm feed flow rate 75 l·h−1 corresponding to cross flow velocity of are presented. For a given system, the force normalized by the radius 26.8 cm·s−1 and shear rate of 536 s−1. A pressure of 60 bars and a of the CML particle, F/R, yields the interaction energy and serves as an temperature of 25 °C were kept constant during all of the experiments. indicator for the membrane fouling potential [31]. Following the compaction stage of the RO membrane with DW, feed of seawater was collected from the Palmachim desalination plant (Palmachim, Israel) after passing through all the pretreatment process, 2.4. Extracting of the organic foulants from the fouled membranes except for AS addition. In all the experiments, seawater supplemented with 20 mg·l−1 AS (weight of liquid AS per liter of seawater) was desa- After each run, pieces of each membrane from different fouling ex- linated and conditioned the RO membranes for 24 h, prior to the fouling periments were divided and placed in 20 ml scintillation vials. Then, experiments using seawater supplemented with model foulants (con- 5 ml of 0.2% sodium pyrophosphate in mineral salts buffer (10 mM trol experiment was conducted without AS). General chemical analysis NaCl) was added to enhance desorption of organic material from the of the commercial AS used in this study is presented in Table 1. membrane surface. The vials were sonicated 10 min to extract foulants, After the conditioning of the RO membranes in each experiment, a and the extracted suspensions were collected for protein and carbohy- model organic foulant was added to the feed seawater (10 l) of the RO drate analysis. Total protein concentration in the extracted suspension unit. The initial foulant (BSA or alginate) concentration at the beginning was analyzed using the colorimetric quantitative protein determination of the fouling experiment was 20 mg·l−1. For the control experiment, with the Bio-Rad© protein assay according to Bradford [32]. BSA in the the feed seawater used for conditioning the membrane did not include extraction buffer was used as a standard. Polysaccharide content was AS. The whole period of the organic fouling experiments was four days. determined according to Dubois et al. [33] using alginate as a standard. Permeate flux decline was measured every 20 min and pH was kept Masses extracted from all the membranes were normalized to the constant in the range of 6.8 ± 0.2. Also during the fouling experiments membrane surface area. (similar to the membrane conditioning step) seawater was supple- mented with the different types of AS, at concentration of 20 mg·l−1 (weight of liquid AS per liter of seawater). Polysaccharides and proteins 2.5. Membrane surface properties analysis of the feed seawater without AS or model foulants showed lower concentration than 0.1 and 0.9 ± 0.2 mg/l, respectively. Membrane surface zeta potential was measured using a streaming potential analyzer (SurPass Elektrokinetic Analyzer) at 10 mM NaCl 2.3. Force interaction analysis between carboxylated modified latex (CML) solution with or without pre-treatment with AS. For each solution, mea- particles and the membrane surface using AFM surements were done twice. During each measurement, each run of the electrolyte solution flow proceeded in two directions (right to left and Force curves of the RO membranes after conditioning period (desa- then left to right). The zeta potential of the RO membranes was calculat- lination of pretreated seawater with or without AS for 24 h) were ac- ed from the streaming potentials using the Helmholtz–Smoluchowski quired with AFM using a Nanoscope IIID MultiMode AFM microscope equation with the Fairbrother and Mastin substitution [34,35]. The hy- (Veeco-DI, Santa Clara, CA). The scans were carried out using aqueous drophobicity of the RO membrane was deduced from the contact solution of 100 mM NaCl at pH 6.8 and the presence or absence of the angle analysis method, whereby it is determined by the captive bubble AS. The temperatures of the samples were monitored during the method (OCA, Data Physics). A droplet size of double distilled water scans. For each experimental condition, the sample was allowed with a diameter of 0.4–0.5 mm was introduced onto the RO membrane to equilibrate with the solution for at least 20 min. The force curve surface after the antiscalant treatment stage in the RO unit. Duplicated measurements were carried out using CML particle (1 μm diameter) experiments, with five different measurements of contact angle, were attached to triangular cantilever with spring constant of 0.06 N/m carried out for each of the treated membranes to obtain at least 10 (Novascan Ames, IA) to measure the adhesion forces between the measurements of contact angle for each set of conditions.
10 A Seawater B Polyacrylate 60 0 Polyphosphonate Dendrimer4) -10 45 -20
-30 30 Zeta potential, mV Contact angle, degrees -40 234567891011 Blank CA PP Den pH Membrane conditioned
Fig. 2. The effect of AS treatment (20 mg/l) on membrane zeta potential and hydrophobicity: polyphosphonate-based (PP), polyacrylate-based (CA) and carboxylated dendrimer (Den) AS were tested. (A) Filmtech-SW30 RO membranes surface zeta-potential plotted as a function of the pH in a background solution of 10 mM NaCl; (B) Captive air bubble contact angle on the surface of Filmtech-SW30 RO membrane. The results present average of five measurements with one standard error, and are significantly different with p-value b 0.05.
29 A. Sweity et al. / Desalination 352 (2014) 158–165 161
2.6. QCM-D measurements the SW30 HRLE 400 (Dow Filmtec, USA) membrane, which in turn, likely affect the initial adsorption of organic foulants (Fig. 2) [20]. Polyamide coated sensors mimicking RO membrane surfaces were Polyphosphonate-based AS is shown to alter, to smaller extent, mem- employed in QCM-D, E4 type (Q-Sense, Sweden), for analyzing the brane hydrophobicity in comparison to the polyacrylate based AS, effect of AS on organic fouling. All QCM-D experiments were performed though, the effect of exposure a third polycarboxylated dendrimeric under flow-through conditions using a digital peristaltic pump based AS on membrane hydrophobicity was similar to the case of expo- (IsmaTec Peristaltic Pump, IDEX) operating in a pushing mode. The sure of the membrane to the linear polyacrylate. Higher surface flow rate of the working solution in the QCM-D flow cell was hydrophicities were detected for the polyacrylate (54.3 ± 5.4°) and car- 150 μl·min−1. The following solutions were injected sequentially to boxylated dendrimeric based AS (57 ± 4.7°) than the polyphosphonate the QCM-D flow cell: (i) double distilled water baseline for 20 min; based AS (42 ± 4.3°), and seawater without AS (32 ± 1.5°). A similar ef- (ii) filtered seawater for 20 min; (iii) filtered (0.2 μm) seawater supple- fect on membrane surface charge was observed for all AS, in which mented with 20 mg·l−1 AS for 1 h; (iv) filtered (0.2 μm) organic foulant membrane surface zeta potential was elevating to the positive sign, for model solution with a final concentration of 20 mg·l−1 of either BSA all cases (Fig. 2). Most likely, an important driving force for adsorption or alginate, supplemented with or without AS for 30 min; (v) finally, of the polyacrylate and carboxylated dendrimeric based AS to the RO steps iii, ii, and i were repeated in a reverse order. When no AS was membrane surface is mediated by specific interactions between carbox- supplemented, the duration of step (ii) was 80 min. A replicated set yl groups of the polyamide surface and backbone carboxyl groups of of experiments was made for each model organic foulant with the polyacylate and carboxylated dendrimeric based AS bridged polyphosphonate, polyacrylate and carboxylated dendrimer based AS, with divalent cations, interactions which are accompanied with an which was separately injected to the QCM-D. The adsorption kinetic entropic gain during the adsorption event [38].Carboxylgroups, curves were made by Q-Tools software (Q-SENSE, Sweden). This soft- abundant in the polyacrylate and carboxylated dendrimeric AS, are like- ware adjusts the incoming and outgoing electrical currents, regulates ly de-protonated at the common working pH range (above 6). De- the altitude of the oscillation and controls the temperature according protonation of polyacrylates to large extent can lead to a high swelling to the temperature set. The variations of frequency shift (Δf, Hz) and capacity [39]. In conclusion, our results suggest that adsorption of dissipation factor (ΔD) were measured for five overtones (n = 3, 5, 7, polyacrylates and carboxylated dendrimers to the membrane, mediated 9 and 11) and the 7th overtone is presented in the results section. The by multivalent ions, can lead to a complexed, swollen, and cross-linked adsorbed mass of AS to the polyamide sensors was calculated to using fouling deposit with a high fouling potential. the Sauerbrey [36] equation, which describes the linear relationship Δ ¼ − 1 Δ Δ between frequency and mass: m C n f where m is the change 3.2. Effects of AS conditioning on force interaction with CML particles in mass per area unit (ng·cm−2), C = 17.7 ng·s·cm−2, n is the overtone number and Δf is the frequency shift (s−1). Retraction force curves analyzed with AFM between RO membranes conditioned with AS (20 mg·l−1) were conducted (Fig. 3). The adhesion 3. Results and discussion forces between CML particle (1 μm diameter) and the RO membrane after conditioning with different AS were measured and the distribution 3.1. Effects of AS on membrane surface properties of the adhesion forces of the retraction curves (maximum values) is presented in Fig. 3. By analyzing the force interactions between the RO Both surface charge and hydrophobicity are of major importance membrane and a highly charged surface of CML particle with dense to the propensity of the membrane to organic fouling [37].Both layer of carboxyl groups, we try to simulate the effect of conditioning polyphosphonate and polyacrylate based AS were shown in our previ- RO membranes with AS on the initial organic fouling process ous study to alter both the hydrophobicity and surface charge of (representing foulant–membrane interaction). A significant difference
Fig. 3. Frequency distribution of the adhesion forces between the AFM CML particle probe and the conditioned RO membranes, which mimic foulant–membrane interaction. The back- ground solution was 100 mM NaCl at pH 6.8. Force measurements were performed at 8 different locations on the membrane, with 20 measurements at each location.
30 162 A. Sweity et al. / Desalination 352 (2014) 158–165 between the force distribution of the membranes conditioned with alginate fouling through permeate flux decline due to fouling with these polyacrylate and carboxylated dendrimeric based AS versus the mem- model foulants accompanied with each of the AS being tested. For both brane conditioned without AS or with polyphosphonate is presented the foulants, alginate and BSA, highest rate of permeate flux decline was in Fig. 3. The membranes conditioned with polyacrylate and carboxylat- observed for the fouling experiments accompanied with the ed dendrimeric based AS showed a higher adhesion to the CML particle polyacrylate and carboxylated dendrimeric based AS. BSA fouling rate with average forces of 1.2 mN/m ± 0.43 and 1.06 mN/m ± 0.36, respec- was 1.2 and 1.02 l m−2 h−1 for polyacrylate and carboxylated tively versus values of 0.09 mN/m ± 0.08 and 0.16 mN/m ± 0.11 for the dendrimeric based AS, respectively, and 0.63 and 0.48 l m−2 h−1 for membranes conditioned with seawater and polyphosphonate based AS, polyphosphonate based AS and seawater, respectively. For the fouling respectively. Commonly, fundamental approach for delineating organic experiments with BSA, the elevated fouling with polyacrylate and car- fouling mechanisms has been carried out in a variety of publications by boxylated dendrimeric based AS, is probably due to the higher hydro- analyzing force interactions between CML particles and RO membranes phobicity induced by AS conditioning the membrane surface as shown with AFM [31,40]. Hence, the adhesion force normalized by the radius of in Fig. 2B. The slight increase of fouling by BSA with polyphosphonate the particle (F/R) was found as a good indicator for RO membrane foul- may be related to lower repulsion of the membrane conditioned with ing potential with humic acids. F/R describes the energy per unit area re- polyphosphonate, due to its lower zeta potential, also induced by quired to separate the particle and the membrane surface to an infinite polyphosphonate conditioning (Fig. 2A). For the fouling with alginate, distance [41].Probably,theincreaseinmembranehydrophobicitydue arapidflux decline is observed for all cases, with, or without AS, at to conditioning with either polyacrylate or carboxylated dendrimeric the beginning (1st 50 h) of the fouling experiment. The similar and based AS (Fig. 2 B) is the main cause for the elevated adhesion analyzed fast permeate flux decline rate ~7.5 ± 0.5 l m−2 h−1 during the first with AFM. This suggestion is supported later by QCM-D experiments 50 h of fouling with sodium alginate is likely because of the formation and flux decline measurements in the RO system. of alginate gel on the membrane surface by the abundant concentration of calcium cations from the seawater used as a feed solution [37]. For al- 3.3. Effects of AS conditioning on fouling of RO membrane with BSA and ginate fouling, a different pattern of permeate flux decline was observed alginate in the presence of polyacrylate and carboxylated dendrimeric based AS, where the rate of flux decline was relatively constant throughout the The effect of AS on membrane performance, in the presence of 150 h experiment, while either for the polyphosphonate AS or in the model organic foulants, BSA and alginate was tested at AS concentration absence of AS, flux decline leveled off after ~50 h. The typical alginate of 20 mg·l−1. Enhanced fouling experiments were performed with gel layers formed on the membranes conditioned with polyacrylate pretreated seawater under controlled pH (6.8–7.0) conditions by a con- and carboxylated dendrimeric based AS showed a more obvious, thicker fl trolled dosage of CO2(g). Fig. 4AandBshowstheeffectofASonBSAand layer observed later. Intriguingly, permeate ux decline results correlate
A B
C D
Fig. 4. Effect of AS on organic fouling of RO membrane: The effect of conditioning the RO membrane with AS on RO membrane fouling propensity induced by BSA (A) andalginate(B).The effect of AS on salt rejection of the membrane in the presence of BSA (C) and alginate (D). Initial permeate flux for all experiments was 33.9 ± 1.1 l m–2 h–1. The rejection results present average of five measurements with one standard error, and are significantly different with p-value b 0.05.
31 A. Sweity et al. / Desalination 352 (2014) 158–165 163 to the higher adhesion forces between the RO membrane and CML 14.3 μg·cm−2 versus 196 ± 9.9 and 166.6 ± 21.1 μg·cm−2, respective- probe analyzed with the AFM for these membranes, which were condi- ly (Fig. 5). Similar correlation was obtained between permeate flux tioned with these AS. Different complexes between the BSA and either decline and the amount of deposited alginate on the membrane: the the polyacrylate and carboxylated dendrimeric based AS may have alginate concentration dissolved from the membranes that were condi- been formed and may also induced membrane fouling [23,42],inaddi- tioned with polyacrylate and carboxylated dendrimeric based AS tion to the aforementioned effects of gel layer formation by adsorbed AS was higher than the membranes that were conditioned with either to the membrane surface [43]. The faster fouling in the presence of polyphosphonate or seawater without AS: 640.4 ± 48.4 and 633.5 ± polyacrylate and carboxylated dendrimeric based AS, mediated by the 32.2 μg·cm−2 versus 415.4 ± 66.2 and 357.8 ± 48.4 μg·cm−2, respec- presence of divalent cations, could be explained by their high density tively (Fig. 5). of carboxyl functional groups fully de-protonated at pH 6.8–7 [44], which contribute to high density of possible chemical bridging between the membrane, the AS, and the foulants. Opposite effect when AS re- 3.4. The effect of AS conditioning on BSA and alginate adsorption to duced fouling of BSA and humic acid (HA) was shown by Yang et al. polyamide: QCM-D analysis [29,30] with a negative impact when AS was overdosed. In a Yang et al. study, when the AS was used (polyaspartic acid – PASP), the charge The effect of conditioning polyamide surface with AS on BSA and al- of the fouling colloids became more negative and their size decreased ginate adsorption was elucidated in a QCM-D flow cell using polyamide due to formation of a water-soluble complex: BSA–Ca–PASP and HA– coated quartz crystals. Fig. (6A&B) shows the frequency shift during the Ca–PASP via Ca2+ bridging. It should be mentioned that Yang et al. entire QCM-D run, changes in the chemical species in the water as well conducted their experiments under significantly lower ionic strength as changes of the adsorbed mass on the sensor surface were reflected (~10 mM), while our experiments were done with real seawater at by the shifts in the resonance frequencies of the quartz crystal sensor ionic strength of ca. 600 mM. coated with polyamide. In order to provide accurate measurement, Significant effect of the induced fouling by AS on salt rejection was after acquiring a stable baseline with double distilled water step (1), fil- observed only for the polyacrylate and carboxylated dendrimeric tered seawaters were injected as a background solution in step (2). The based AS, with both foulants, BSA and alginate (Fig. 4C and D). The decrease in the frequency during the injection of the seawater as back- results showed lower salt rejection for the membranes that were condi- ground solution in step (2) is due to viscosity changes of the solution at tioned with polyacrylate and carboxylated dendrimeric based AS and the interface as well as due to the adsorption of cations that exist in sea- fouled with either BSA or alginate. Since permeate flux decline was water to the polyamide sensor surface. In step (3), seawater with or also the highest for these AS and since organic fouling has been reported without different AS (20 mg·l−1) was injected to condition the polyam- to have minor or almost no contribution to the “cake enhanced osmotic ide surface. In this step, a minor shift in the frequency was observed pressure” (CEOP) phenomenon [40,45], the decrease in salt rejection is when the seawater was supplemented with polyacrylate (CA) and probably because of “concentration effect” of the solutes passage in a carboxylated dendrimer (Den) due to adsorption of AS, while no smaller permeate volume. changes were observed when the seawater supplemented with In order to provide further analysis on the effect of membrane expo- polyphosphonate (PP) AS or seawater alone has been used for this sure to AS on organic fouling, in addition to membrane performance, type of analysis. Changes in the mass on the sensor in step (3), which the associated amount of adsorbed fouling layer was analyzed. The are likely attributed to the addition of AS to the seawater, according to adsorbed BSA and alginate were dissolved into 0.2% sodium pyrophos- Sauerbrey equation, were the highest for the sensors that were exposed phate buffer solution (see Section 2) and the total protein and polysac- to polyacrylate (CA) and carboxylated dendrimer (Den): 4.2 ± 0.6 and charide content was colorimetrically analyzed [32,33].Asexpectedand 4.9 ± 0.8 ng·cm−2 respectively. The changes in mass on the sensors correlated to the flux decline results (Fig. 4C), for the membranes condi- that were exposed to seawater alone (SW) and polyphosphonate (PP) tioned with polyacrylate and carboxylated dendrimeric based AS, a were 0.43 ± 0.1 and 2.8 ± 0.4 ng·cm−2, respectively. Consequently, higher adsorbed BSA was detected compared to the cases of using either the surfaces conditioned with polyacrylate (CA) and carboxylated den- polyphosphonate or seawater without AS: 262.3 ± 18.6 and 256.3 ± drimer (Den), were preferable for adsorption of BSA and alginate during step (4) of injection BSA or sodium alginate (20 mg·l−1) dissolved in similar pre-filtered seawater. Significantly higher decrease of the frequency shifts was observed for both cases of BSA and alginate adsorption (Fig. 6A&B), when the sensors were conditioned with polyacrylate (CA) and carboxylated dendrimer (Den) compared to conditioning with polyphosphonate (PP) and seawater alone (SW). Step (5) shows the washing step of the organic fouling layer by injecting the same background solution that was used in steps (3) and (4) (with AS but in the absence of BSA or alginate). In step (5), loosely adsorbed BSA and alginate molecules were desorbed and a stable fouling layer remained on the surface. Later, steps (6) and (7) included washing of the fouling layers with seawater followed by double distilled water. The last two steps show significantly higher desorption of both BSA and alginate fouling layers from surfaces conditioned with polyphosphonate (PP) and seawater alone (SW), observed by the higher increase in the associated frequency shifts compared to condi- tioning with polyacrylate (CA) and carboxylated dendrimer (Den). The QCM-D results suggest that the presence of polyacrylate (CA) and carboxylated dendrimer (Den) causes a higher extent of irreversible fouling compared to the presence of the polyphosphonate (PP) and seawater alone (SW). The QCM-D results corroborate with both the Fig. 5. Foulant concentrations on the membrane analyzed at the end of the fouling AFM and the RO flux decline results, providing important evidence for experiments with BSA (white bars) and alginate (dashed bars). The results present average of five measurements with one standard error, and are significantly different the side effects of polyacrylate (CA) and carboxylated dendrimer with p-value b 0.05. (Den) based AS on organic fouling.
32 164 A. Sweity et al. / Desalination 352 (2014) 158–165
A B
Fig. 6. The effect of conditioning polyamide surface with antiscalant on the consequent adsorption of BSA (A) and alginate (B) as analyzed on QCM-D sensor and measured by frequency shift of oscillation of the 7th overtone. QCM-D sensors were conditioned with seawater only (SW) and with seawater supplemented with 20 mg·l–1 of polyacrylate (CA), carboxylated dendrimer (Den), and polyphosphonate (PP) based AS.
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34 Journal of Membrane Science 481 (2015) 172–187
Contents lists available at ScienceDirect
Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
Side effects of antiscalants on biofouling of reverse osmosis membranes in brackish water desalination
Amer Sweity a, Tesfalem Rezene Zere a, Inbal David b, Sarit Bason b, Yoram Oren a, Zeev Ronen a, Moshe Herzberg a,n a The Jacob Blaustein Institutes for Desert Research, Zuckerberg Institute for Water Research, The Albert Katz International School of Desert Studies, Ben Gurion University of the Negev, Sede Boqer Campus, 84990 Midreshet Ben Gurion, Israel b Mekorot Water Co. Ltd., Tel Aviv, Israel article info abstract
Article history: In this study, we investigated the contribution of antiscalants to biofouling of reverse osmosis (RO) Received 19 December 2014 membranes in brackish water desalination process. Both the physico-chemical effects of the antiscalants Received in revised form on the initial attachment of bacterial cells to RO membrane surfaces, as well as their nutritional 3 February 2015 contribution to biofilm growth were analyzed. A bacterial deposition experiment on ESPA-2 RO Accepted 4 February 2015 membrane was conducted using Pseudomonas fluorescens, and the two polyphosphonate- and Available online 13 February 2015 polyacrylate-based antiscalants. Both the model bacterium and the ESPA-2 RO membranes were treated Keywords: with feed brackish water with and without antiscalants. The nutritional contribution of these Biofouling antiscalants to biofilm growth was investigated by growing biofilms in packed-bed biofilm reactors in Antiscalants the presence and absence of the antiscalants. In the presence of both antiscalants, biofilm development Reverse osmosis was enhanced in comparison to the control. Eventually, the impact of the antiscalant-induced biofilm Desalination fl fi Brackish water growth on the performance of RO membranes was investigated in a cross ow RO ltration unit under typical brackish water desalination conditions. Significant membrane biofouling was detected in the presence of the antiscalants compared to the case where no antiscalant was used, accompanied by higher flux decline and salt passage. RO membrane modules were also utilized in an industrial and pilot desalination plants to test the effect of the polyacrylate-based antiscalant. Similar behavior was observed on both scales of membrane operation, with polyacrylate inducing biofilm growth. We assert that the two types of antiscalant induce biofilm formation on RO membrane surfaces through two different mechanisms: while polyacrylates increase bacterial initial attachment by altering membrane physico- chemical properties, mainly increasing membrane hydrophobicity, polyphosphonates increase biofilm growth under phosphorous limiting conditions by providing the cells with the phosphorous they lack. Thus, the selection of the type and dosage of antiscalant should take into account the associated contribution to membrane biofouling propensity. & 2015 Elsevier B.V. All rights reserved.
1. Introduction [1,3,9]. The viability of RO membrane desalination of brackish water depends on maximizing the water production and reducing In arid and semi-arid regions, scarcity of potable water is the cost of brine management, which can be achieved by increas- recognized as a present and future threat for human endurance ing recovery [6,10–12]. Increasing permeate recovery provides a [1–3]. Desalination of inland brackish water (brackish ground- great benefit in brackish water desalination, reducing costs and water and agricultural drainage water) by reverse osmosis (RO) brine discharge [13,14]. However, high permeate recovery is membrane technology (BWRO) has become a widely accepted limited due to inorganic fouling (mineral scaling); under such technology to alleviate water shortages and scarcity in these areas conditions the precipitation propensity of sparingly water soluble
[4–8]. Also other water resources, such as desalinated seawater mineral salts, such as gypsum (CaSO4 2H2O), calcium carbonate and wastewater effluents have emerged as a key to sustain fresh (CaCO3), barite (BaSO4), and silica (SiO2) is enhanced [15–17]. Thus, water resources for future generations throughout the globe to overcome these scaling problems and prolong the operational lifespan of RO membranes, the addition of certain chemicals has become a necessity; in order to influence the characteristics of the
n Corresponding author. Tel.: þ972 8 6563520; fax: þ972 8 6563503. feed water and minimize the risk of working at high permeate E-mail address: [email protected] (M. Herzberg). recovery. The chemicals used to evade scaling in RO desalination http://dx.doi.org/10.1016/j.memsci.2015.02.003 0376-7388/& 2015 Elsevier B.V. All rights reserved.
35 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187 173 processes include acids for pH adjustment of the feed water (mainly co.il/ktziot-bwro-desalination-plant), located in the Negev highlands, to prevent calcium carbonate precipitation) [18,19],andmore Israel. commonly used are polyelectrolytic compounds, known as anti- scalants. Though the exact mechanism for the inhibition of salt precipitation by antiscalants is not clear, a significant delay in the 2.2. The effect of antiscalants on the initial attachment of bacterial induction time needed for precipitation is usually observed in over- cells to RO membrane surfaces saturated salt solutions supplemented with antiscalants [20–22]. Most commonly used antiscalants include polyphosphonates, poly- 2.2.1. Bacterial culture preparation fl acrylic acids, and polyphosphates that are added to the feed water An overnight culture of the P. uorescens F113 (rrnBP1: gfp- n R [23–25]. Hence, these antiscalants generally play a crucial role in mut3 , Kan ) culture was diluted 1:1000 in 500 mL of Luria maintaining efficient RO plant operation at the highest possible Bertani (LB) medium supplemented with 25 mg/L kanamycin. 1 recovery by preventing the need to replace expensive membranes The culture was cultivated at 30 C with shaking at 250 rpm for – fi 9 prematurely, eliminating or reducing the use of hazardous acids, 5 6 h to a nal OD600 of 1 ( 10 cells/mL). Then, the late 1 using fewer chemicals, producing less concentrate and allowing for exponential cells were centrifuged (4 C, 2500 g, 20 min), washed 8 better environmental acceptance of that concentrate, reducing twice, and diluted to an OD600 of 0.1 ( 10 cells/mL) (Lambda – energy costs and reducing the downtime caused by frequent mem- EZ201 UV vis Spectrophotometer, Perkin Elmer) with either brane cleaning cycles [3,20,26,27]. However, despite their impor- brackish water (for the control), or brackish water supplemented tance, antiscalants have been reported to enhance membrane with 5 mg/L of each of the two antiscalants. These conditions biofouling, which is another severe operational problem in RO des- enabled counting of the irreversibly deposited bacteria during the alination processes [27,28,30–32]. Once microbial biofilms are course of the three hours deposition experiment. formed on membrane surfaces, biofouling can occur, followed by severe operational problems in RO/NF desalination processes. 2.2.2. Bacteria and membrane Biofouling effects include a decline in membrane water flux, an Bacterial deposition experiments were carried out with the increase in salt passage and elevated energy requirements, the above mentioned GFP expressing P. fluorescens F113 model strain, latter caused by the increase in the transmembrane pressure as well and the high flux RO flat-sheet membrane ESPA2 (Hydranautics as the feed channel pressure. Besides, membrane biofouling Co.) as the receiving surface. Both the bacterial cells and the RO requires frequent chemical cleaning, which ultimately shortens membranes were treated using brackish water with and with- the operational period of the RO membrane modules and conse- out antiscalants under conditions mimicking brackish water quently imposes a huge economic burden on RO desalination desalination. processes [1,3,33,34]. Thus, when analyzing the role of antiscalants in preventing membrane scaling it is important to analyze the biofouling potential of these additives. Yet, quantitative data on the 2.2.3. ESPA-2 RO membrane preparation fi contribution of anticalants to bio lm growth potential and their Circular pieces of ESPA-2 RO membranes ( 40 mm diameter) consequent effects on biofouling of RO membrane installations is were treated in a dead end filtration cell at operating pressure of limited [27,30]. The main goal of this work is to investigate the 10 bar with 50 mL brackish water, supplemented with or without biofouling potential of antiscalants during their usage in RO antiscalant. The antiscalants were used at a high concentration of brackish water desalination processes, both on lab and industrial 50 mg/L for a short duration ( 1 h), to substitute for the long- scale levels. term exposure at low antiscalant concentrations (2–4 mg/L), which are typical working conditions at the Ktziot plant.
2. Materials and methods 2.2.4. Bacterial deposition experiment 2.1. General approach After treating the RO membranes and the bacterial cells with or without antiscalants (similar to those used in the full scale plants), The effect of antiscalants on the initial deposition of bacterial cells sets of duplicated deposition experiments were conducted to on RO membrane surfaces was explored using a model bacterial characterize the deposition and initial attachment of the bacterial strain expressing green fluorescent protein (GFP), Pseudomonas fluor- cells to the RO membrane surfaces. In the set of control experi- escens F113 (rrnBP1: gfp-mut3n,KanR) [35] and ESPA-2 RO mem- ments, bacterial cells and the RO membranes were treated with brane. Constitutive GFP expression by this bacterium enabled real brackish water only. The characterization of the initial bacterial time microscopic visualization of cell deposition. For analyzing the attachment to the RO membrane surfaces was done by mounting a nutritional contribution of antiscalants to indigenous biofilm growth, small piece of each of the treated RO membranes on a modified packed-bed bioreactors were operated after inoculation with cultures coupon of a FC270 flow cell (Biosurface Technologies, Bozeman, grown in the presence of the different antiscalants (Supplementary Montana). Then, the treated bacterial suspension was injected into material – Fig. S1). The effect of the antiscalant-induced biofouling on the flow cell (140.6 mm long 12.7 mm wide 0.20 mm deep) at the performance and efficiency of the ESPA-2 RO membrane was a velocity of 8.5 cm/s. The membrane coupon was excited at explored using a cross-flow lab RO filtration unit. Ultimately, 8″ RO 480 nm and was observed under a Zeiss ZX10 epi-fluorescent membrane modules from the “Granot” (Mekorot, Israel) desalination microscope equipped with the proper filter set at magnification of plant were analyzed to compare the fouling propensity of different 40 . Ten different points on the membrane coupon were visua- antiscalants used in the plant. These membrane modules were lized after every 30 min and were acquired with a CCD camera. autopsied and analyzed in order to study the effects of antiscalants Before snapping the pictures, the same background solution as in on fouling of the RO membranes. All the lab scale works were done the experiment (without bacteria) was injected into the flow cell under typical brackish water desalination conditions using two for one minute in order to wash the loosely bound and suspended antiscalants, one polyacrylate-based (CA) and one polyphosphonate- bacteria and visualize only the attached cells. This way, each based (PP), which both are general purpose antiscalants. The brackish experiment was carried out for about 3 h and the gradual increase waters used in lab scale part of this study, were taken from the feed in the number of deposited bacteria per square centimeter of the water of “Ktziot” brackish water desalination plant (http://www.ges. observed RO membrane surface was enumerated.
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2.3. The effect of antiscalants on physico-chemical properties of 2.4.1. Biological activity during biofilm growth in packed-bed bacterial cells and RO membrane surfaces reactors Fixed bed plug-flow bioreactors (empty volume of 50 mL) were 2.3.1. Bacterial surface zeta potential packed with 0.5 mm glass beads used as substratum for the Bacterial cells were cultured and further treated exactly as growth of the bacterial biofilms, which were inoculated with prepared for the deposition experiment. Then, the electrophoretic cultures cultivated under selective pressure for degradation of mobility of the bacterial cells was measured using a zeta potential antiscalants. One reactor was used as a control to which the analyzer (ZetaPlus 1994, Broohaven instruments Co., Holtsville, culture cultivated with brackish water was injected. The second NY) in brackish water with and without antiscalant used in the reactor was injected with polyphosphonate-based antiscalant deposition experiment. degrading culture, and the third reactor was injected with polyacrylate-based antiscalant degrading culture. Each of the 2.3.2. RO membrane zeta potential reactors was supplied continuously with brackish water with RO membranes conditioned by filtration of brackish water (5 mg/L) or without antiscalant from their respective 1 L tanks – supplemented with and without antiscalant (50 mg/L) at pressure (Supplementary material Fig. S1). Every three days, the feed fi of 10 bar. The RO membranes were then equilibrated with 10 mM tanks were lled with the respective new batch of brackish water NaCl for at least 30 min before measurement. Then, membrane with and without antiscalant. The reactors were run under zeta potential was measured using a streaming potential analyzer continuous mode of operation with a hydraulic retention time of fl (SurPass Elektrokinetic Analyzer) in 10 mM NaCl with and without 5 h and a ow rate of 0.17 mL/min for a time period of four antiscalants at pH value of 7.4. During measurement, the flow of months. Over this time period, samples were taken every three days from the influent of the reactors for total nitrogen (TN), total the solution proceeded in two directions (right to left and then left þ to right). For each solution, the runs were repeated twice. The zeta phosphorus (TP), and ammonium (NH4 ) analyses. Similarly, fl potential of the RO membranes was calculated from the streaming samples were also taken from the ef uent of the reactors for – monitoring the nutritional uptake of the biofilm by analyzing the potentials using the Helmholtz Smoluchowski equation with the þ Fairbrother and Mastin substitution [36]. decline in the TP, TN, and NH4 concentration. Parallel to this analysis, the dissolved oxygen uptake, temperature, and pH of both the influent and effluent of the three reactors were mon- 2.3.3. Measurement of bacterial surface hydrophobicity itored using a multi-parameter meter (CyberComm 6000) in real The adhesion of microbial cells to hydrocarbon droplets time. Dissolved organic carbon (DOC) concentration was analyzed (MATH) method was adopted to evaluate the relative hydropho- using Teledyne Tekmar Apollo 9000 TOC analyzer (Mason, OH). bicity of the bacterial strain used in the deposition experiments [3,37]. Bacterial cells were treated exactly as in the deposition experiment before the assay. Then, 4 mL of the treated bacterial 2.4.2. CLSM analysis of antiscalant-degrading biofilms suspension were transferred into 15 mL falcon test tubes and At the end of the biological activity experiments using the incubated for 10 min with 1 mL of n-dodecane (Sigma-Aldrich, packed-bed bioreactors, confocal laser scanning microscope Israel). Then, the tube was vortexed for two minutes. Lastly, (CLSM) analyses of the sessile cultures grown on the walls of the separation of the organic and the aqueous phase took place for tubing of the reactors were conducted. CLSM imaging of the 15 min. The loss in the absorbance of the aqueous phase relative to biofilm grown on the glass spheres was impossible due to fragile the initial absorbance value in the aqueous phase represents the biofilm cultures, which were destructed during sample processing. amount of cells adhering to n-dodecane. Accordingly, hydropho- Biofilm samples on the plastic tubing were stained with concana- bicity is expressed as the percentage of the cell partitioning in n- valin A conjugated to Alexa Fluor 633 (ConA), propidium iodide dodecane [38]. (PI), and SYTO 9. ConA (prepared by diluting the 5 mg/mL stock solution 1:100 in 150 mM sodium chloride) was used for staining 2.3.4. RO membrane hydrophobicity the extracellular polymeric substances (EPS) while the PI and the The hydrophobicity of the RO membrane was deduced from the SYTO 9 (prepared by taking 1.5 mL from a stock solution of 20 mM contact angle analysis method (OCA, Data Physics). A droplet size of PI and 3.34 mM of SYTO 9 in 1 mL of 150 mM sodium chloride) of double distilled water with a diameter of 0.4–0.5 mm was were used for staining the live and the dead microorganisms, introduced onto the RO membrane surface after the antiscalant respectively. Excess electrolyte solution was carefully drawn off treatment stage in the RO unit. Duplicated experiments, with five from the specimens by an absorbent paper tissue (Kimwipe). Then, different measurements of contact angle, were carried out for each 100 mL of the staining solutions were added to cover the biofilm of the treated membranes to obtain at least 10 measurements of samples, which were then incubated in the dark at room tem- contact angle for each set of conditions. perature for 20 min. Unbound stain was drawn off the specimens using a three-step wash of 150 mM sodium chloride solution. The 2.4. Batch growth experiments of indigenous cultures in the presence unbound stain solutions and the washing solutions were carefully of antiscalants removed by gently touching the edge of the specimen with an absorbing paper. Microscopic observation and image acquisition Firstly, bacterial cultures were cultivated in sterile flasks were performed using a laser scanning confocal microscope (Zeiss (250 mL) with the brackish water supplemented with and without LSM 510, META), equipped with Zeiss dry objective Plan – Neo- antiscalant. Each flask was filled with 100 mL brackish water with Fluar (10 magnification and numerical aperture of 0.3) [39]. The or without antiscalant (5 mg/L) and incubated for six weeks at CLSM was equipped with detectors and filter sets for monitoring 30 1C and shaking at 250 rpm. A daily replacement of 50 mL of PI, SYTO 9 stained cells and Alexa Fluor 633 (excitation wave- brackish water with or without antiscalant was carried out lengths of 488 nm for both the SYTO 9 and the PI, and 633 for the throughout the six week period in order to create a selective Alexa Fluor 633). CLSM images were generated using the Zeiss pressure for the growth of the antiscalant-degrading bacteria. LSM image browser. Gray scale images were analyzed, and the Then, each of the cultures were used as inoculum for the biological specific biovolume (mm3/mm2) in the biofouling layer was deter- activity experiments conducted for investigating the nutritional mined by Imaris 3D imaging software (Bitplane, Zurich, Switzer- contribution of the antiscalants to bacterial biofilm growth. land). The Threshold was fixed for all image stacks.
37 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187 175
Table 1 Feed water analysis of “Ktziot” and “Granot” BWRO desalination plants.
Parameter “Ktziot”“Granot”
Ca2 þ , mg/L 258 102 Mg2þ , mg/L 120 78.5 Naþ , mg/L 1441 340 K þ , mg/L 35 6 Sr2þ , mg/L 9.8–10.8 2.1 Ba2 þ , mg/L 0.05 0.18 þ – NH4 , mg/L 0.8 3.5 0.1 HCO3 , mg/L 30–60 350 2 – SO4 , mg/L 700 750 80 Cl , mg/L 2450 645 NO3 , mg/L 5 57 B, mg/L 1.5–2.2 0.5
SiO2, mg/L 21 28 Fig. 1. Crossflow RO laboratory desalination unit for analysis of the effect of þ Al3 , mg/L – 21 antiscalants on membrane biofouling [44,45]. Polyphosphonate- and polyacry- þ Fe3 , mg/L 10–30 45 late-based antiscalants were used. RO membrane used was ESPA-2 TDS, mg/L 4600–5100 1691.4 (Hydranautics Co.). pH 6.9–7.0 6.8 Temperature, 1C28–30 22–24 2.5. Antiscalant-induced biofouling of RO membrane in a lab-scale Turbidity, NTU – o0.5 cross-flow desalination system
2.5.1. RO unit preparation measuring the gradual decline of the permeate water flux and salt The effects of antiscalants on membrane biofouling in a laboratory rejection of the RO unit. The salt rejection of the system was scale cross-flow desalination unit were measured using a flat sheet monitored by measuring the conductivity of both permeate and RO membrane flow cell under conditions that mimic a brackish retentate using a multipurpose meter (CyberComm 6000) in water desalination process (Fig. 1). Before and after every biofouling real time. experiment and prior to inserting the RO membrane coupon to the flowcell, the RO unit was disinfected and thoroughly cleansed to 2.5.4. Characterizing the biological activity of the antiscalant- remove trace organic impurities and microorganisms by applying the induced biofouling layer following steps: (1) recirculation of 0.02% sodium hypochlorite for þ Analysis of nutrient uptake (TP, TN, NH conducted according 20 min, (2) cleaning trace organic matter by recirculation of 2 mM 4 to standard methods [42], and DO) of the fouling layer was done EDTA (pH 11) for 30 min, (3) rinsing the unit twice by recirculation of on samples every three days for the entire duration of the tap water for 10 min, and (4) rinsing the unit two times with DI experiment (ca. four months). Dissolved oxygen concentrations water and then inserting the ESPA-2 RO membrane coupon (which of the permeate and retentate were monitored using a multi- was stored at 4 1Cindistilledwater)[39]. purpose meter (CyberComm 6000) in real time, using polaro- 2.5.2. Membrane compaction and biofouling experiment graphic dissolved oxygen electrode with self-stirring mechanism Following the sterilization/cleaning protocol, the membrane (EC620SSP, Eutech instruments). Furthermore, the fouling layer was compacted with DI water at a pressure of 20 bar until the was characterized by CLSM. At the end of each experiment, the permeate flux attained a constant value (usually after 12–18 h). membrane coupon was carefully removed and cut with sterilized Following compaction of the membrane, a 1 h baseline perfor- scissors into pieces for staining with ConA, PI, and SYTO 9 for mance with DI water at 15 bar and 25 1C was conducted with this probing the EPS, dead cells, and live cells, respectively, as pressure and temperature being maintained during all experi- explained previously. ments. After attaining a stable flux with DI water, 10 L of the feed “ ” brackish water to Ktziot desalination plant, with or without 2.5.5. SEM analysis of the antiscalant-induced biofouling layer 5 mg/L of the antiscalants, was added to the feed reservoir and the SEM (FEI Company, Philips XL30) was used in a high vacuum system was equilibrated for 5 h. Water chemistry of the feed mode for imaging of the biofouling layers on the RO membranes. “ ” brackish water to Ktziot desalination plant is presented in Prior to measurement, the fouled membrane layers were fixed, fi Table 1.After ve hours, sessile culture inoculums from the dehydrated, and coated with a layer of carbon approximately 10– reactors of the same treatment were added to the system. 15 nm thick. The fixation method involved the following steps Throughout the entire time period of the experiment (5 weeks), [43]: (1) excess electrolyte solution was carefully removed with a the brackish water solution supplemented with and without filter paper from the specimens (fouled membrane pieces of antiscalant was changed with a new batch every three days. around 5 mm 5 mm); (2) the fouled membrane specimens were Before and after three days of every batch of the solution, samples incubated in 0.05 M sodium cacodylate buffer supplemented with were taken for monitoring the bioactivity of the antiscalant- 2% glutaraldehyde (Electron Microscopy Sciences) for one hour; (3) induced biofouling layer formed on the RO membrane surfaces the specimens were incubated for 10 min and rinsed three times – [39 41]. Furthermore, the impact of the antiscalant-induced with 0.05 M sodium cacodylate buffer; (4) a second fixation step fi bio lm growth on the RO membrane performance was monitored was performed by incubating the specimens in 0.05 M sodium fl daily by analyzing the gradual decline in membrane permeate ux cacodylate buffer supplemented with 1% osmium tetroxide for one and salt rejection over time period of 33 days. hour (Electron Microscopy Sciences); (5) excess amounts of osmium tetroxide were removed according to the same procedure 2.5.3. Analyzing antiscalant-induced biofouling – permeate water followed in step 3; and (6) specimens were dehydrated for 20 min flux and salt rejection in ethanol/water solutions with increasing ethanol concentrations The impact of the antiscalant-induced biofouling on the per- (25%, 50%, 75%, 95%, and 100%) and dried overnight in a chemical formance (efficiency) of the RO membrane was monitored by hood at room temperature.
38 176 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187
emission spectrometry (ICP-AES) for determining cation content; SEM and CLSM. Also, EPS were extracted from the fouled membrane.
2.6.3. ATR-FTIR analysis Attenuated total reflection Fourier transform infrared (ATR- FTIR) spectroscopy was used to provide insight into the chemical nature of deposits on the fouled RO membranes. A duplicate of approximately 2 cm2 for each of the fouled membranes of the 4 modules were dried in a vacuum oven and taken to the FTIR. Spectra (range 400–4000 cm 1) of the fouled membranes were obtained using a VERTEX 70/80 standard system spectrophot- ometer, using a germanium crystal (BRUKER Optiks, Ettlingen, Germany).
2.6.4. EPS extraction EPS extraction was performed from the RO membranes that were cut from the fouled RO modules. The EPS extraction steps were carried out according to Liu and Fang [49]. First, pieces of the fouled membranes were cut and suspended in 20 mL of 0.1 M NaCl solution in a 50 mL polypropylene tube, and vortexed for 45 min to ensure that the biofilm is totally suspended. Then, 120 μLof37% formaldehyde (Sigma-Aldrich, Israel) were added to the solution and incubated with gentle mixing for 1 h in a Vortex Genie 2s (Scientific Industries, Bohemia, NY) at 4 1C. Sodium hydroxide (1 M, 8 mL) was added and the solution was incubated at 4 1C Fig. 2. BWRO desalination plants: (A) Granot commercial plant and the selected RO for another 3 h, in order to facilitate dissociation of the acidic membrane modules from the first stage – 1st and 7th modules were selected from groups from the EPS to the solution. Next, the suspension was the same train and (B) Granot pilot BWRO desalination unit and the selected RO centrifuged (35,000 rpm, 30 min, 4 1C) and the supernatant was fi – membrane modules from the rst stage 1st and 7th modules were selected from filtered through a 0.2 μm hydrophilic nylon filter (Millipore Co.). the same train. Asterisks show the locations where membrane sheets were fi sampled in the selected module, for fouling layer analysis[46]. After ltration, the supernatant was dialyzed against deionized water, using a 3500 Da cutoff membrane (Spectra/Por) for several days until all salts were completely removed. Extracellular protein 2.6. Analysis of fouled membranes from commercial and pilot of the extracted EPS was analyzed using the colorimetric quanti- desalination plants (“Granot”): the effect of antiscalants on fouling of tative protein determination with the Bradford Protein Assay (Bio- BWRO membranes Rad©), according to Bradford [50]. Polysaccharide content was determined according to DuBois et al. [51], using alginic acid as the The effect of antiscalants on biofouling of BWRO membranes standard. Extracted EPS was expressed as DOC concentration was examined in both commercial and pilot desalination plants measured using the Teledyne Tekmar Apollo 9000 TOC Analyzer. named “Granot” (Mekorot Water Company, Israel). In this part, four different reverse osmosis (RO) membrane modules were 2.6.5. Microscopy taken to examine the effect of different antiscalants used on CLSM imaging was conducted as mentioned in Section 2.3. SEM fouling the RO modules. imaging was conducted as mentioned in Section 2.4 and according to Herzberg et al. [52]. 2.6.1. Sampling of fouled RO membranes from “Granot” plants The first and seventh membrane modules were taken from the 3. Results and discussion first stage pressure vessel of “Granot” commercial brackish water desalination plant (Fig. 2A – each pressure vessel accommodates 3.1. Physico-chemical effects of antiscalants seven modules). Sheets of fouled membranes were cut from the middle of each module, close to the permeate collecting tube. 3.1.1. The effect of antiscalants on bacterial zeta potential and Membrane sheets were kept at 4 1C for further analysis of the hydrophobicity fouling layer no more than 3 days after cutting the samples. From The electrophoretic mobility of the P. fluorescens F113 bacteria “Granot” pilot plant (Fig. 2 B), the first and the seventh modules in was analyzed on a cell suspension of 107–108 cells/mL (OD ¼0.1). the pressure vessel at the first desalination stage were taken for 600 The bacteria were treated with brackish water with and without sampling. Membrane sheets from the first module of the pilot antiscalants (see Section 2). The presence of both antiscalants (5 mg/ plant were cut from the feed entrance and from the middle L) did not significantly affect the calculated surface zeta potential of locations of the module. From the 7th module of the pilot plant, the P. fluorescens F113 bacteria (Table 2). Bacterial zeta potential membrane sheets were cut from the middle and from the end under all conditions was in the range of 14.1 to 14.9 mV with location of the module (Fig. 2 B). and without antiscalants. The effect of the antiscalants on the surface hydrophobicity of the P. fluorescens F113 bacteria was analyzed by the
2.6.2. Analysis of fouled RO membranes from “Granot” plants MATH test (OD600 ¼0.3). According to this analysis, both antiscalants Membrane autopsies of the fouled modules were carried out on (polyphosphonate and polyacrylate) did not have any significant the fouled membranes as described elsewhere [47,48]. The fouled effect on the surface hydrophobicity of the bacteria (Table 2). In all layer was investigated and studied using Fourier transform infra- cases i.e., in the presence or absence of both antiscalants, the bacteria red spectroscopy (FTIR); inductively coupled plasma atomic were found to be relatively hydrophilic. The values for the percent of
39 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187 177
Table 2 Effects of antiscalant treatment on the hydrophobicity and zeta potential of P. fluorescens F113 bacteria on ESPA-2 RO membrane in brackish water. The exposed concentration of antiscalant was 5 and 50 mg/L, for bacteria and membrane, respectively.
Physicochemical effects Control Polyacrylate Polyphosphonate
Bacterial P. fluorescens zeta potential (mV) in brackish water 14.1 72.4 14.973.2 14.972.6 Bacterial P. fluorescens hydrophobicity (% partitioning) 13.773.2 1274.2 12.572.1 ESPA-2 membrane contact angle (deg) 21.2 75.8 48.574.9 49.571.3 ESPA-2 membrane streaming potential (mV) in 10 mM KCl 2771.5 2672.8 14 73.4
partitioning between the liquid and the n-dodecane organic phase bacteria. This mechanism was investigated by growing biofilms in were in the range of 12.0–13.7%. packed-bed bioreactors in the presence and absence of the antiscalants. At the beginning of the experiment each of the three 3.1.2. Physico-chemical effects of antiscalants on RO membrane bioreactors (w/o antiscalant, with polyacrylate, and with polypho- surfaces (hydrophobicity and zeta potential) sphonate) were inoculated with bacterial cultures previously The effect of both antiscalants on the surface hydrophobicity of incubated for six weeks (see Section 2). The nutritional contribu- the ESPA-2 RO membrane was deduced from contact angle tion of the antiscalants to biofilm growth was assessed according þ measurements. In the absence of both antiscalants, the pristine to the TN, NO3 ; NH4 and TP concentrations in samples taken ESPA-2 RO membrane was hydrophilic, with a deionized water from the influent and effluents of each of the three reactors every drop contact angle of 21.275.8, while upon exposure the mem- three days over a period of 140 days. Dissolved oxygen uptake, brane to both antiscalants (50 mg/L), the hydrophobicity of the temperature, and pH of both the influent and the effluent of the membrane increased significantly and interestingly, both antisca- three reactors were also monitored in real time using a multi- lants showed similar effects: deionized water contact angles on parameter meter (CyberComm 6000). membranes previously conditioned with polyphosphonate-based and polyacrylate-based antiscalants were 49.571.31, and 4874.91, respectively. In contrast to the effect of both antiscalants 3.2.1. Dissolved oxygen (DO) on the membrane surface hydrophobicity, no significant effect on No significant variation was observed in the influent DO con- the membrane zeta potential was observed for polyacrylate centration to all the three reactors throughout the entire time period (Table 2). With the polyphosphonate based antiscalant, membrane of the experiment. The average DO concentration in all the three zeta potential became slightly less negative. For the pristine reactors was 2 mg/L, meaning that aerobic conditions prevailed in all membrane, the zeta potential value was 27.071.5 mV with the reactors. However, a significant decline in the DO concentration 10 mM KCl, while zeta potential values of 14 7 3.4 mV and was observed in the effluents of all the three reactors. A stronger 26.172.8 mV were observed in the presence of the polypho- decline in was observed in the antiscalant supplemented reactors, sphonate and polyacrylate, respectively (Table 2). It has been even more so in the reactor supplemented with the previously reported that antiscalants can adsorb to the RO mem- polyphosphonate-based antiscalant, in comparison to the control brane, hence membrane surface properties could be affected [30]. reactor (with no antiscalant) (Fig. 4A). During the first three months of operation, a relatively higher oxygen uptake was observed in the 3.1.3. Initial attachment of bacterial cells on RO membrane surfaces polyacrylate supplemented reactor compared to the reactor that was in the presence and absence of antiscalants not supplemented with an antiscalant. However, after three months, The effect of both antiscalants (polyphosphonate-based and the a relatively similar and constant DO concentration was observed in polyacrylate-based) on the initial attachment of the bacterial cells (P. both reactors. The gradual decline in the DO concentration in the fl uorescens F113) to the surface of ESPA-2 RO membrane was inves- effluents of the antiscalant-supplemented reactors and in particular fl fl tigated in a ow cell using a uorescent microscope. Fig. 3 shows a in that supplemented with polyphosphonate, indicates a positive higher initial attachment rate of bacteria the ESPA-2 RO membrane contribution of polyphosphonates and polyacrylates to the oxidative surfaces in the presence of both antiscalants. In the presence of the activity of the biofilms. polyacrylate-based antiscalant, after 90 min, bacteria deposited onto theROmembranesurfacetoamuchlargerextentthanonthecontrol membrane: bacteria deposition elevated by 150%: A corresponding deposition coefficient (deposition rate, cell/s, normalized to cell 3.2.2. Dissolved organic carbon (DOC) concentration and area) of 3.53 10 976.84 10 10 m/s was calcu- Unfortunately, the DOC originated from the addition of the lated for the polyacrylate based antiscalant, compared to 1.87 antiscalants and its fate could not be analyzed due to accumula- 10 974.01 10 10 m/s for the polyphosphonate based antiscalant tion and secretion of dissolved microbial products in the reactors. and 1.34 10 974.88 10 10 m/s for the pristine membrane Under the conditions operated in this study, we could not (Fig. 3B). Representative images showing the gradual increase in the differentiate between DOC originated from biological activity and number of deposited P. fluorescens F113 bacteria on the ESPA-2 RO DOC originated from the antiscalants. DOC concentration in the membrane surface in the presence and absence of the antiscalants are original brackish water used in these experiments was lower than shown in Fig. 3A. The effect of the antiscalants on membrane 0.5 mg/L and was below the minimum limit of detection. Taking hydrophobicity (Table 2), likely contributes to the high extent of into account both sources of DOC (microbial activity and 5 mg/L of bacterial deposition, particularly in the presence of the polyacryl- antiscalants added), the DOC in the feed and effluent in the ates (Fig. 3). experiment with polyacrylate antiscalant were 1.470.19 and 1.8870.75, respectively, during the entire operation. The DOC in 3.2. Nutritional contribution of the antiscalants to biofilm formation the feed and effluent in the experiment with polyphosphonate antiscalant were 1.470.39 and 1.670.52, respectively. Hence, we Antiscalants might enhance membrane biofouling in brackish could not withdraw any conclusion on the nutritional contribution water desalination processes by serving as a source of nutrients for of each of the antiscalants based on DOC analysis.
40 178 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187
Control Control
4.00E-009
3.20E-009
2.40E-009 Polyphosphonate Polyphosphonate
1.60E-009
8.00E-010
0.00E+000 Polyacrylate Polyphosphonate Control
Polyacrylate Polyacrylate m/s coefficient, deposition Bacterial Treatment
Fig. 3. Effect of antiscalants, polyphosphonate and polyacrylate, on the initial deposition of P. fluorescens F113 bacteria to the RO membrane surface, after 90 min. Field of view is 219 μm 164 μm (A). Deposition coefficient for P. fluorescens F113 parallel flow (8.5 cm/s) on polyamide ESPA2 membrane surface (B). Deposition coefficient was determined as the rate of cell deposition normalized to cell concentration and the membrane surface area observed [39].
3.2.3. Total nitrogen (TN) Similar to the TN uptake rate also for ammonium, integrating the The TN concentration in the influents of all the three reactors brackish water flowrate and the decrease in ammonium concentra- ranged approximately between 3 and 3.25 mg/L (Fig. 4B). This tion can provide the ammonium uptake during the experiment. As þ verifies that neither antiscalants has a significant TN contribution expected, the NH4 uptake rate was in the same range as the total N to the feed water, nor may enhance membrane biofouling by uptake rate: 0.01 mg/min during the first week of the experiment, serving as sources of nitrogen for the bacterial biofilms. As reaching 0.204 mg/min at the end of the experiment (Fig. 4C). Only a þ observed for the DO, a gradual decline in the TN concentration minor decline in the effluent NH4 concentration was observed was observed in all the three reactor effluents and the most after two months in the other reactors. significant decline was observed in the reactor supplemented with polyphosphonate (Fig. 4B). Integrating the brackish water flowrate 3.2.5. Total phosphorus (TP) and the decrease in TN concentration can provide the TN uptake No detectable TP was found in the influents of the two reactors during the experiment: during the first week of the experiment, without the polyphosphonate-based antiscalant. As expected, 0.075– the TN uptake rate in the polyphosphonate-supplemented reactor 0.1 mg/L of TP was found in the influent of the reactor supplemented was 0.01 mg/min while after about four months, it reached with the polyphosphonate-based antiscalant (Fig. 4B), likely provid- 0.174 mg/min (as N). In contrast, a noticeable and similar trend in ing a source of phosphorus for biofilm growth in the reactor. In this the decline of the TN concentration was observed only after about reactor,agradualdeclineintheTPconcentrationintheeffluent was two months in the other reactors. Therefore, the TN analysis observed over the four months and the trend of the TP uptake indicates that nitrogen was not a limiting nutrient for the biofilm þ corroborates with those observed for the DO, TN, and NH growth in all the three reactors. The higher uptake rate of nitrogen 4 analyses. TP uptake rate increased from 0.002 mg/min, during the in the presence of polyphosphonate was probably due to the first week of the experiment, to 0.011 mg/min towards the end of the higher biomass growth under these conditions. experiment. þ The most significant decline in DO, TN, and NH4 concen- þ trations was found in the reactor supplemented with the 3.2.4. Ammonium (NH ) 4 polyphosphonate-based antiscalant. Since phosphorous is AmmoniumwasfoundtobethemainN-sourceastotal þ the limiting nutrient in the brackish water being used, supple- N–NH4 concentrations were similar to those of the total N; and mentation of the reactor with a polyphosphonate-based anti- no nitrate (NO ) was detected in all of the three reactors. Not 3 scalant enhanced biofilm growth, as well as uptake of oxygen surprisingly, as was observed for TN, ammonium concentration in and nitrogen. the influent was similar in all the three reactors, ranging between 3.25 and 3.5 mg/L over the entire time period of the experiment (ca. four months). In the polyphosphonate supplemented reactor, a 3.2.6. CLSM analysis of the fouling layer on reactors’ tubing trend similar to the TN and DO analyses was observed for the Fig. 5 shows a representative three-dimensional reconstruction þ effluent NH4 concentration. A sharp decline in the effluent amm- image and quantitative biovolume analysis of the biofouling layer onium concentration of the reactor was observed after three weeks. covering the reactor tubes, performed with Imaris software 41 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187 179
3.4
2.0 3.2
3.0 1.6
2.8 1.2 2.6
0.8 2.4 Influent wo/AS Influent wo/AS Effluent wo/AS Effluent wo/AS Influent polyacrylate Influent polyacrylate Dissolved Oxygen, mg/L Total nitrogen TN, mg/L 2.2 Effluent polyacrylate 0.4 Effluent polyacrylate Influent Polyphosphonate Influent polyphosphonate Effluent Polyphosphonate 2.0 Effluent polyphosphonate 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Operating time, days Operating time, days
3.6 0.09 3.4 0.08 3.2 0.07 3.0 0.06 2.8 0.05 2.6 Influent wo/AS 0.04 Effluent wo/AS 2.4 0.03 Influent polyacrylate Influent polyphosphonate TP concentration, mg/L NH4+ Concentration, mg/L Effluent polyacrylate Effluent polyphosphonate 2.2 Influent polyphosphonate 0.02 Effluent polyphosphonate 2.0 0.01 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Operating time, days Operating time, days
Fig. 4. The effect of polyphosphonate- and polyacrylate-based antiscalants (5 mg/L) on the bioactivity of the packed-bed reactors fed with brackish water. Dissolved O2 (A), þ total N (B), NH4 (C), and total P (D) were continuously analyzed in the influent and effluent streams.
(Bitplane, Zurich, Switzerland). In this figure, blue, green, and red 3.3.1. Effect of antiscalant-induced biofouling on permeate flux and colors represent EPS, live cells, and dead cells respectively, as well salt rejection as the specific biovolumes (mm3/mm2) of the different biofilm Both antiscalants (i.e., polyphosphonate and polyacrylate) had a components. Interestingly, in the presence of both the antiscalants, significant effect on permeate water flux of the RO membrane comparable volumes of biofilm components (live and dead cells as (Fig. 6A). At the beginning of the experiment, the permeate water well as EPS) were detected, which were significantly higher than flux was 46.1 L m 2 h 1 in the case of the polyacrylate- and the volumes formed in the absence of antiscalant. The high 44.3 L m 2 h 1 in the case of the polyphosphonate-based antisca- volume of biofilm formed in the polyacrylate-supplemented reac- lant. However, at the end of the experiment (after ca. five weeks), tor, despite the undetectable concentration of TP in the influent of the permeate flux declined to about 31% and 44.6% of its initial this reactor, could be possibly explained due to the enhanced value in the case of the polyacrylate and polyphosphonate, respec- bacterial attachment to the surface (see Fig. 3),which brought with tively. The flux decline was stronger in the presence of the it trace, undetectable amounts of phosphorous. polyacrylate than the polyphosphonate-based antiscalant, in line with the higher cell attachment rate rather than the oxygen, nitrogen, and phosphorous uptake rates, which were higher in the presence of the polyphosphonate. Note that during the first and 3.3. Impact of antiscalant-induced biofouling on the performance of the last two weeks of the experiment, the flux decline in presence ESPA-2 RO membrane of polyacrylate was much stronger in comparison to the polypho- sphonate. In contrast, during the third week of the experiment, the The effect of the antiscalant-induced membrane biofouling on the permeate flux was approximately constant and similar in both cases performance of ESPA-2 RO membrane was investigated in a cross but decreased significantly later only with polyacrylate, during the fl fi fl ow RO ltration unit using a laboratory plate and frame at sheet 4th and 5th weeks of the experiment (Fig. 6A). Fouling behavior was fl RO membrane ow cell under typical brackish water desalination also observed in the control experiment, where no antiscalant was conditions. The analysis was done by measuring the permeate water used. In the case of the control, the reason for the marked decline in flux and the salt rejection of the ESPA-2 RO membrane. Oxygen fl þ the permeate water ux could be the attachment of particulate and uptake rate, TP, TN, NH4 ,andNO3 analyses were done to monitor colloidal impurities from the feed brackish water as well as some activity of the biofouling layer on the membrane surface. Moreover, growth of microbial biofilm. Moreover, a decrease in salt rejection CLSM analysis was conducted at the end of each experiment to was also observed in the RO units. A strong effect was observed in quantify the biofouling layer components. the presence of the polyacrylate-based antiscalant, in which the salt 42 180 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187
Control Polyacrylate
Polyphosphonate
100 Poly acrylate Polyphosphonate 2 Control 80 / 3 60
40
20
Biovolume 0 EPS Dead cells Live cells Biofilm components
Fig. 5. The effect of polyphosphonate- and polyacrylate-based antiscalants (5 mg/L) on biofilm formation in the packed-bed reactors fed with brackish water. The three- dimensional images were reconstructed from the CLSM image stacks using Imaris software. The CLSM image stacks were taken from the biofouling layer formed on the reactor tubes. Green, red, and blue spots represent live cells, dead cells, and EPS, respectively. The field of view for each figure is a perspective of 1273 μm 1273 μm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
98.0 1.0 97.5 0.9 97.0 0.8 96.5 0.7 96.0 0.6 95.5 95.0 0.5 Control Salt rejection, % 94.5 0.4 Polyacrylate Control Polyacrylate Polyphosphonate 94.0 0.3 Polyphosphonate 93.5 Normalized permeate flux, % 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Operating time, days Operating time, days
Fig. 6. Normalized flux (A) and salt rejection (B) during desalination of brackish water in a cross flow RO filtration unit using in the presence and absence of polyacrylate- and polyphosphonate-based antiscalants. rejection of the ESPA-2 RO membrane declined from 97.3% to 93.9%. with the faster flux decline when antiscalants were added to the RO A decrease in salt rejection, to lower extent, was also observed in feed water, SEM analysis clearly shows that bacterial biofilms grew the presence of the polyphosphonate-based antiscalant and in the on the RO membrane in the presence of polyacrylate-based or control experiment (Fig. 6B). The decline in salt rejection in these polyphosphonate-based antiscalants (Fig. 7B and C, respectively), latter cases, could be attributed to the permeate flux–salt rejection while we could not detect microbial growth in the absence of mass transport relation of RO membrane separation process. In line antiscalant (Fig. 7A).
43 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187 181
Fig. 7. SEM images of the biofouling layers formed on RO membrane, Control (A) polyacrylate treated (B), polyphosphonate treated (C) and pristine ESPA-2 membrane (D).
3.3.2. Oxygen uptake rate of the biofouling layer respectively (Fig. 8C and D). These results corroborate with the Oxygen uptake rate was calculated as the product of the results obtained in the packed-bed reactors, where the feed subtraction of dissolved oxygen concentration in the permeate concentration ranged from 3.00 to 3.25 mg/L for TN, of which and in the retentate by the flow rate of the permeate (assuming the ammonium consisted the majority. Hence, neither in the minor oxygen transport in parallel flow compared to cross flow packed-bed reactors nor the crossflow desalination experiments, through the biofilm). At the beginning of the experiment, the did the antiscalants contribute a nitrogen source for biofilm oxygen uptake rate was 0.153 and 0.148 mg/min in the case of the growth. Moreover, the TP analysis results in the cross-flow filtra- polyacrylate- and polyphosphonate-based antiscalants, respec- tion experiments were similar to those obtained in the packed-bed tively (Fig. 8A). At the end of the experiment (after five weeks), reactors: In both cases, no detectable TP was found in the presence the oxygen uptake rate increased to 0.58 and 0.41 mg/min in the of polyacrylate-based antiscalant or in the control experiment. In presence of the polyacrylate- and polyphosphonate-based anti- contrast, a TP concentration ranging from 0.087 to 0.096 mg/L in scalants, respectively (Fig. 8A). A gradual increase in the oxygen the cross-flow desalination experiment (Fig. 8B) and 0.075– uptake rate was observed in the presence of both antiscalants, and 0.1 mg/L in the packed-bed reactors was found for the case with was significantly higher for the polyacrylate-based antiscalant, a polyphosphonate-based antiscalant. thus indicating the extent of activity of the biofouling layer on the Similar to the results obtained from the effluent of the packed- membrane surface. Hence, the formation of the biofouling layers bed reactors, in the cross-flow filtration experiments a gradual þ and the consequent trends observed in the oxygen uptake rates, in decline was also found in the effluent TN, NH4 , and TP concen- the presence of each of the antiscalants, corroborate with the trations in the presence of the polyphosphonate-based and poly- membrane permeate flux decline. acrylate based antiscalants, with only a slight decline for the control brackish water. In the beginning of the experiment where polyphosphonate was used, the nutrient uptake (percent of the 3.3.3. Biological degradation of total nitrogen and phosphorous in initial concentration) was 0.81% for the TN and 28.7% for the TP. the RO unit After five weeks, the nutrient uptake increased to 30.8% TN and Water samples from the RO system were taken every three 80.7% TP. This gradual increase in the nutrient uptake indicates days before (“Effluent”) and immediately after changing the batch elevated microbial biological activity in the system in the presence (“Feed”) of brackish water in the desalination experiments. The TN of the polyphosphonate-based antiscalant. It should be mentioned þ and NH4 concentrations in the feed side of the cross flow that polyphosphonate antiscalant contributes phosphorus, which filtration experiments, both in the presence and absence of the is the limiting nutrient in the RO unit and therefore, the elevated antiscalants, range from 2.9 to 3.1 mg/L and 3.2 to 3.4 mg/L, biological activity (as indicated by the sharp decline in the effluent
44 182 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187
0.0007 0.10 n i m
/ 0.0006 g 0.08 m
, 0.0005 e t a
r 0.06
e 0.0004 k a t
p 0.0003 0.04 u n e 0.0002 Control g TP feed Polyacrylate y TP concentration, mg/L 0.02 TP effluent x Polyphosphonate
O 0.0001 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 Time, days Time, days
3.2 3.4 3.0 3.2 , mg/L )
2.8 +
4 3.0 NH
2.6 ( 2.8 Feed Control Feed control Effluent Control 2.4 Effluent control 2.6 Feed Polyphosphonate Feed Polyphosphonate Effluent Polyphosphonate 2.2 Effluent Polyphosphonate Feed Polyacrylate Feed Polyacrylate 2.4
Total Nitrogen (TN), mg/L Effluent Polyacrylate Effluent Polyacrylate Ammonium 2.0 2.2 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Time, Days Time, days
Fig. 8. The effect of polyphosphonate- and polyacrylate-based antiscalants (5 mg/L) on the bioactivity of the RO laboratory desalination unit fed with brackish water: Oxygen þ uptake rate (A), total P (B), total N (C), and NH4 (D) were analyzed in the samples taken every three days before (“Effluent”) and immediately after changing the batch (“Feed”) of brackish water in the RO unit. The analysis was done in the presence and absence of polyacrylate-based and polyphosphonate-based antiscalants.
þ TN, NH4 and TP concentrations) reflects the activity in the RO to the DOC results shown for the bioreactors part of this study, also unit. The biological activity in the RO unit is likely contributed by for the biofouling experiments, we could not differentiate between the biofilm growth on the membrane in addition to the growth of DOC originated from biological activity of biomass accumulated in suspended biomass in the RO unit. Unlike the results obtained the RO units and DOC originated from the antiscalants. The DOC in from the packed-bed reactors, in the crossflow filtration experi- the feed and effluent in the biofouling experiment with polyacry- ments enhanced activity was observed in the presence of the late antiscalant experiment were 2.570.13 and 1.870.24, respec- polyacrylate-based antiscalant (as indicated by the gradual decline tively, and the DOC in the feed and effluent in the experiment with þ in the effluent TN and NH4 concentrations) (Fig. 8C and D). polyphosphonate antiscalant were 1.670.14 and 1.470.16, According to the TN uptake by the RO biofouling layers, microbial respectively, during their entire operation. Hence, also for this activity was lower in the presence of polyacrylate antiscalant case, we could not withdraw any conclusion on the contribution of compared to the polyphosphonate antiscalant (Fig. 8C). Probably, each of the antiscalants based on DOC analysis. the nutritional contribution of the polyacrylate-based antiscalant to biofilm growth was significantly lower than that of the polypho- sphonate. On the other hand, the higher oxygen uptake rate of the 3.4. The effect of antiscalants on fouling of BWRO pilot and biofilm in the RO unit in the absence of a P source can be explained commercial Granot desalination plant by concentration polarization effects on the RO membrane, increasing the local concentration of salts and dissolved nutri- This part of the work was done in order to examine and outline ents, including non-detectable P source substances, for biofilm the technical issues of the suspected membrane fouling (biofouling þ growth on the membrane. The results for TN and NH4 uptake or inorganic-fouling) and the interrelated effects of using different do not corroborate with permeate flux decline, though oxygen antiscalants. Membrane autopsies were taken only from the first uptake rate of the biofouling layer and permeate flux decline desalination stage (Fig. 2) during the plants’ operation and the follows similar trends. It is possible that different microbial effect of the different antiscalants on fouling behavior of the RO communities develop in the presence of each of the antiscalants, modules was tested. The feed water for Granot desalination plants with different characteristics of EPS production as well as (both commercial and pilot) is a blend from five local ground oxygen uptake rate. brackish water wells. Pre-treatment before the RO process includes pH correction (to 6.8) by HCl addition, rough filtration (80 mesh), cartridge filtration (5 mm) and antiscalant injection. The commercial 3.3.4. DOC in the RO unit desalination plant has been working continuously since 2004, while þ Similar to TN, NH4 , and TP analysis, DOC analysis was carried the pilot plant has been working since 2009. Chemical cleaning is out in the water samples taken every three days from the conducted once or twice a year in both the commercial and pilot desalination experiments in the RO unit. Unfortunately and similar plants. The water chemistry of the feed water is shown Table 1.
45 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187 183
Antiscalants used in the commercial and the pilot desalination plants were polyphosphonate and polyacrylate based, respectively. The nominal feed flow rate to Granot commercial and pilot plants was 475 m3 h 1 and 20 m3 h 1, respectively, and the working pressure was set at between 12 and 15 bar. It should be mentioned that the relevancy of the delineated biofouling mechanisms found so far, under laboratory controlled conditions, are highlighted with respect to the severe biofouling observed when polyacrylate-based antiscalant is being used in the pilot plant facility. The effects of antiscalant type as well as the location of membrane sampling on the analysis of membrane autopsy are thoroughly discussed in the following sections.
3.4.1. FTIR analysis FTIR spectra of the fouled membranes are shown in Fig. 9. The main absorption bands in Fig. 9A and B were in the vicinity of 1631 cm 1 (CQO stretching of amide I, quinone, and ketones), 1563 cm 1 (N–H deformation, C–N stretching of amide II and symmetric stretching of COO–), and 1078 cm 1 (C–O stretching of polysaccharides). The band in the vicinity of 1400 cm 1 could be due to aliphatic C–H deformation, C–O stretching or O–H deforma- tion of phenol. The band in the range 600–800 cm 1 could be representative of aromatic compounds. These results suggest that the constituents of the membrane fouling matter included pro- teins, polysaccharides, and aliphatic and aromatic compounds derived from organic or microbial matter. Fig. 9C, which represents the pilot plant module from the 7th element showed a weaker shift than the previous membranes at the vicinity of 1078 cm 1 (C–O stretching of polysaccharides) and at 1563 cm 1 (N–H deformationþC–N stretching of amide II) protein contents. As expected, the samples collected from the first module (and from the entrance to the module, in the case of the pilot plant autopsies) suffered the most from organic and biological foulants. The FTIR results indicate that the first module acted as a “pre- treatment” stage for the entire train and therefore it was the most fouled in the commercial and in the pilot plants.
3.4.2. EPS analysis EPS components were extracted from the surface of the fouled membranes taken from same locations as described above. Fig. 10 (A–C) shows different degrees of EPS, correlative for biofouling. The highest amount of EPS as analyzed by concentrations of polysaccharides, proteins and TOC, was extracted from the first membrane module of the pilot plant, where samples were cut from the entrance and from the middle. Polysaccharide concen- trations were 33.4671.41 (PS) mg/cm2 and 33.271.23 mg/cm2 for the 1st module cut from the entrance and from the middle, respectively. These results indicate that this module suffered from severe fouling. These results correspond with the FTIR analysis and provide strong evidence for the most severe fouling of the first element module. Interestingly, the first module contained the largest amount of biological fouling components. This observation could lead to a conclusion that the pre-treatment process was insufficient for decreasing the organic contents from the feed water. In addition, since the pilot plant feed water was supple- Fig. 9. FTIR spectra of fouled ESPA1 membranes taken from the first stage of the mented with polyacrylate-based antiscalant, enhanced adsorption commercial and pilot plants: (A) commercial plant autopsies – red color – 1st of colloidal and organic material could induce consequent biofilm element module cut from the middle, blue color – 7th element module cut from the growth. Hence, lower levels of EPS components were detected – – middle; (B) Pilot plant 1st element module autopsies blue color membrane cut when the commercial desalination plant was operated for a longer from the entrance, red color – membrane cut from the middle; (C) Pilot plant 7th element module autopsies – red color – membrane cut from the middle, blue color time with polyphosphonate-based antiscalant receiving similar – membrane cut from the exit. Black color spectra in all panels represent a pristine feed water, probably indicative of a lower rate of biofilm growth. ESPA1 RO membrane. (For interpretation of the references to color in this figure Another possible explanation for higher amount of EPS observed legend, the reader is referred to the web version of this article.) in the pilot plant (supplemented with polyacrylate antiscalant),
46 184 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187
fouled by microorganism components, with variations in the amount of adsorbed biofilm and its components. The highest biovolume values of cells and EPS were observed on the pilot plant module (1st element, which was cut from the entrance and from the middle of the module (Fig. 11G)). The total biovolume of the adsorbed dead cells and EPS was 354.3736.16 mm3/mm2 and 120.26711.35 mm3/mm2, respectively. These total specific biovolumes of biofilms on RO membrane are prone to reduce permeate flux by more than 50% as observed in our previous studies [53,54].
3.4.4. SEM analysis Fig. 12A and B represents the SEM micrograph of the first and seventh module elements, respectively cut from the commercial membrane. Biofouling is clearly evident at a magnification of 10,000 . Due to cell cluster and EPS matrix, it became clear from the SEM images that the fouling layers on the membrane surfaces consisted of particulate matter embedded in an apparently amor- phous matrix in nature. The images also show that the fouling pattern on the RO membrane surfaces from the commercial plant is a compact layer with uneven fouling material throughout the membrane surface. It is also clear from Fig. 12A that in the commercial plant, the 1st module element in the train suffered from the highest amount of biofilm and organic matter deposition. This is in line with the CLSM as well as the EPS component analyses. Fig. 13A and B represents the SEM micrograph of the mem- brane surfaces taken from the 1st module element of the pilot plant, cut from the entrance and the middle respectively, at a magnification of 10,000 . The images indicate a thick layer of foulants due to biofilm formation on the surface of the mem- branes. These images are in line with the CLSM and the EPS composition analyses. At the pilot facility, the polyacrylate-based antiscalant could enhance the growth of a thicker biofilm layer, in comparison with that formed in the presence of the polyphosphonate-based antiscalants that were used in the com- mercial plant. In addition, the first module element probably served as a pre-treatment stage for the rest of the pressure vessel, thus the 1st module was covered with organic matter. Fig. 13C and Fig. 10. EPS analysis of the fouling layer extracted from fouled ESPA1 membranes D presents the SEM micrograph of the 7th module element of the taken from the first stage of the commercial and pilot plants: (A) total polysacchar- ides; (B) total proteins; (C) TOC. pilot plant, cut from the middle and the end of the module, at a magnification 10,000 . Corroborating with our supposition, the images indicate lower amount of biofilm on the membrane surface can be a greater nutritional stress in the absence of P source, in these two locations. Likely, a lower biofouling rate is attained for which could induce EPS production. the 7th membrane element in comparison to the 1st element. CLSM analysis also corroborates with the SEM micrographs, showing lower biofilm density on the surface of the 7th module 3.4.3. CLSM analysis element compared to the entrance of the pressure vessel, on the The fouling layers of each membrane were analyzed using surface of the 1st module element (Fig. 11). CLSM. Fig. 11(A–F) shows three-dimensional reconstructed images of the fouling layers acquired from the CLSM image stacks using Imaris Bitplane software (the red color represents biomass and the blue color represents EPS). As observed in Fig. 11, the 1st element 4. Concluding remarks module of the pilot plant that was cut from the entrance and the middle had the highest amount of cells and EPS adsorbed to the It can be concluded from the data presented herein, that in membrane surface. These results correspond to the EPS analysis as desalination of brackish water, depending on its chemical well as to the FTIR spectra (comparing the 1st module to the 7th nature, antiscalants can enhance RO membrane biofouling in module of the pilot plant). Clearly, a significantly lower amount of several different ways at different stages of the biofilm growth. biofilm was observed on the surface of the module at the end of Polyacrylate-based antiscalants were shown to enhance mem- the pressure vessel (7th element) in comparison to the 1st module brane biofouling by altering the surface physico-chemical located at the entrance of the pressure vessel. As already men- properties of the membranes, which in turn promotes the tioned, this implies that the first membrane in the pressure vessel initial attachment and deposition of the bacterial cells. functioned as a pretreatment module for the 7th membrane, Polyphosphonate-based antiscalants contribute to the mem- located at the end of the pressure vessel. Specific biovolumes of brane biofouling by serving as sources of nutrients for the the biofilm components (cells and EPS) were calculated using bacterial cells. Comparing the two effects, the results in this IMARIS software. It is clear from Fig. 11G that all the modules were work showed that the former effect of the antiscalants (i.e., the
47 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187 185
Fig. 11. Biofilm formation on the RO membranes – three-dimensional reconstructed images acquired from CLSM using Imaris Bitplane software scale: the red color represents biomass and the blue color represents EPS. (A) commercial module 1st element cut from the middle, (B) commercial module 7th element cut from the middle, (C) Pilot module 1st element cut from the entrance, (D) Pilot module 1st element cut from the middle, (E) Pilot module 7th element cut from the middle and (F) Pilot module 7th element cut from the exit, (G) Total biomass as measured in specific biovolume (mm3/mm2) of the adsorbed biofilm (dead cells and EPS). The field of view for each figure is a perspective of 200 micrometer 200 micrometer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
physico-chemical effect) causes more severe RO membrane processes, they can significantly contribute to the biofouling of biofouling. It should be mentioned that a very severe membrane the RO membrane in several ways as illustrated in this study. biofouling might also result from coexistence of the two effects Hence, based on the findings of this work, we recommend that with certain antiscalant or combination of two or more. There- antiscalants should be screened for their biofouling enhance- fore, even though antiscalants play significant roles in pre- ment potential in addition to their antiscaling activity, prior to venting RO membrane scaling in brackish water desalination their use.
48 186 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187
Fig. 12. SEM images of the fouled membrane layers from the commercial plant under a magnification of 10,000 : (A) 1st element membrane cut from the middle; (B) 7th element membrane cut from the middle; (C) Pristine ESPA-1 membrane. Bar¼5mm.
Fig. 13. SEM images of the fouling layers on the pilot membranes (magnification of 10,000 ): (A) 1st module element cut from the entrance; (B) 1st module element cut from the middle; (C) 7th module element cut from the middle; (D) 7th module element cut from the exit.
49 A. Sweity et al. / Journal of Membrane Science 481 (2015) 172–187 187
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50 5. Discussion and conclusions:
Desalination of seawater using reverse osmosis (RO) membranes is considered one of the most promising technologies to overcome water stress and scarcity, which are prevalent throughout the globe. Clearly, preventing a decline in treatment efficiency due to scaling is essential. Antiscalants are surface active polyelectrolytic compounds that are commonly used in RO desalination processes to prevent membrane scaling. However, in spite of their significant impact in preventing membrane scaling at the highest possible recovery rate of the desalination plant, antiscalants are prone to enhance biofilm growth on RO membranes and induce organic fouling.
In the first paper of this dissertation we show the contribution of antiscalants to membrane biofouling in seawater desalination processes. A laboratory scale RO test unit (see first paper - Fig. 1) was used to examine the effect of antiscalants on biofouling of the RO membrane. The biofouling potential was examined with two commercially available antiscalants commonly used in desalination processes (polyphosphonate and polyacrylate based). These antiscalants were tested in pretreated seawater (by coagulation and sand filtration), collected from the Palmachim desalination plant, Israel. We found that the membrane's physical-chemical properties including surface charge and hydrophobicity are important parameters which should to be considered for knowledgeable selection of antiscalants, which affect these properties and eventually the RO desalination process.
Consequently, we moved on to study how membrane surface characteristics affect membrane interactions with different foulants. It has been reported that hydrophobic membranes exhibit higher fouling potential than hydrophilic ones [66-68]. Hydrophilic membranes are less prone to adsorption of hydrophobic foulants [69], attaining higher specific flux and better salt rejection [70]. Moreover, surface roughness also affects colloidal fouling [71-73] and microbial adhesion [73-75]. However, in the present work we did not find that antiscalants affect membrane roughness. We calculated the zeta potential of the RO membranes from the streaming potentials using the Helmholtz-Smoluchowski equation with the Fairbrother and Mastin substitution [76]. Our findings show an increase of the membrane positive charge at pH values below ~3.5 (pH of a neutral zeta potential) for polyacrylate-based antiscalants, while a slightly higher positive zeta potential was observed for the membrane treated with polyphosphonate based antiscalants (See
51 first paper, Figure 2A). The effect of polyacrylate-based and polyphosphonate-based antiscalants on the hydrophobicity of RO membranes was measured using the captive bubble contact angle method, which mimics realistic aquatic conditions. The RO membrane surface remained relatively hydrophilic when no antiscalants were used, while it became more hydrophobic when polyacrylate-based and polyphosphonate-based were supplemented to the feed seawater (see first paper – Figure 2B).
In the first paper we also described the effect of antiscalants on bacterial surface properties (surface charge and hydrophobicity) and their related effects on bacterial deposition. Increased hydrophobicity of the bacterium Vibrio fischeri KV2682 was observed when treated with a polyacrylate based antiscalant compared to the treatment with a polyphosphonate based antiscalant or in the absence of antiscalant (see first paper, Figure 3A). The electrophoretic mobility of the V. fischeri did not differ as a result of the antiscalant treatment, and zeta potential values for all of the treatments were also similar, ranging between −35 and −38 mV at an ionic strength of 10 mM adjusted with NaCl at ambient pH of 6.2 (see first paper, Figure 3B).
We studied the effect of antiscalants on bacterial deposition and the involved physical and chemical interactions using QCM-D sensor and parallel plate flow cell (from which we calculated the bacterial deposition coefficient). In the presence of the polyacrylate-based antiscalants, a higher bacterial cell attachment rate to the QCM-D sensor was observed compared to their attachment in the presence of the polyphosphonate-based antiscalants. These results can be related to the combination of both stronger hydrophobicities of the bacterial cells, and the related effects of polyacrylate based antiscalants on the properties of the polyamide surface (see first paper, Figure 4). As expected, the deposition coefficient for this model bacterium was higher when the polyacrylate-based antiscalant was used.
The effect of polyacrylate- and polyphosphonate-based antiscalants on biofilm formation by a natural microbial consortium was also investigated. CLSM analysis showed that addition of polyacrylate or polyphosphonate-based antiscalants promoted biofilm growth on the surface of the RO membrane compared to seawater without antiscalants (see first paper, Figure 5). Specific biovolume analysis of the biofilms on the membranes (visualized using CLSM) showed that polyacrylate treated membranes had more biomass in comparison to the polyphosphonate treated membrane and to the non-treated membrane. Polyphosphonates could enhance biofilm formation
52 by serving as a source of phosphorous, which is a limiting nutritional element for microbial growth in seawater.
Overall we concluded in the first paper that the usage of antiscalants could enhance biofilm formation on RO membrane surfaces. The use of the polyacrylate-based antiscalant enhanced the formation of biofilm by changing the physico-chemical properties of the RO membrane including the hydrophobicity as well as the surface charge that stimulated the deposition and attachment of the bacterial cells. Moreover, polyphosphate-based antiscalants enhanced the biofouling potential of the RO membrane by serving as a phosphorous source. Therefore, prior to their usage, antiscalants should be screened for their biofouling potential and the associated enhancing mechanisms.
In the second paper we investigated the effect of exposure of RO membranes to antiscalants on possible consequent organic fouling during seawater desalination. Another commonly used antiscalant, carboxylated dendrimeric polymer, was used in this paper. We found that antiscalants adsorbed to the RO membrane polyamide active layer and strongly affected subsequent organic fouling behavior by altering the physico-chemical surface properties of the membrane (See second paper, Figure 2) and serving as a conditioning film for organic fouling.
Colloidal foulants and dissolved organic compounds (proteins, polysaccharides, and natural organic matter (NOM)) exist in natural water and can pass all the conventional pretreatment stages of the desalination process. These substances interact with other pretreatment chemicals including antiscalants and consequently foul the RO membranes. Our results suggest that adsorption of polyacrylates and carboxylated dendrimer antiscalants to the membrane can be mediated by multivalent ions, and can cause a swollen and cross-linked fouling deposit with a high fouling potential. Adhesion force measurements conducted by AFM, between a carboxylated modified latex (CML) particle, mimicking organic model foulants, and the RO membrane conditioned with different antiscalants were conducted and the distribution of the adhesion forces of the retraction curves was presented (see second paper, Figure 3). Membranes conditioned with polyacrylate and carboxylated dendrimeric based antiscalants had a high surface affinity to the CML particle, probably due to the increased hydrophobicity of these membranes (see second paper, Figure 2). In contrast, membranes that were conditioned with seawater alone or with seawater supplemented with polyphosphonate based antiscalant exhibited
53 weaker adhesion forces to the CML particles (see second paper, Figure 3). The effect of antiscalants on membrane performance during an organic fouling experiment with alginate and BSA as model foulants was also tested. A significant drop in RO permeate flux due to organic fouling was noted in the presence of polyacrylate or carboxylated dendrimeric based antiscalants in the feed seawater (see second paper, Figures 4A and 4B). The permeate flux decline was related to the aforementioned higher adhesion forces between the RO membrane and carboxylic groups in the organic foulant, as analyzed with a CML probe by AFM.
Lower salt rejection rates for the membranes that were conditioned with polyacrylate and carboxylated dendrimeric based antiscalants and fouled with either BSA or alginate were observed due to the “concentration effect” of the solutes passage in a smaller permeate volume (see second paper, Figures 4C and 4D). Furthermore, we observed a higher BSA adsorption rate to the RO membrane surfaces that were conditioned with polyacrylate and carboxylated dendrimeric based antiscalants compared to the membrane conditioned with seawater supplemented with polyphosphonate or seawater without antiscalants (see second paper, Figure 5). The same trend was observed when the RO membrane was fouled with alginate: more alginate was adsorbed to the RO membranes that were conditioned with polyacrylate and carboxylated dendrimeric based antiscalants in comparison to those that were conditioned with either seawater supplemented with polyphosphonate or seawater without antiscalants (see second paper, Figure 5).
A correlation between the permeate flux decline results (second paper, Figures 4A and 4B) and the amount of deposited BSA and alginate on the membrane surfaces (see second paper, Figure 5) was observed. The effect of conditioning polyamide surfaces with antiscalants on BSA and alginate adsorption was tested on the QCM-D sensors. A stronger decrease was found in the resonance frequency of the polyamide coated sensor due to BSA and alginate adsorption when polyacrylate and carboxylated dendrimer antiscalants were used to condition the polyamide coating layer of the sensor (see second paper, Figure 6), in comparison to seawater and seawater supplemented with polyphosphonate antiscalant.
It was concluded in the second paper that the RO membrane surface properties including hydrophobicity and zeta potential are affected by exposure to polyacrylate and carboxylated dendrimer antiscalants and their adsorption, which consequently induced organic fouling by BSA
54 and alginate. It is suggested that further research and development of antiscalant chemistry (molecular weight and chemical composition) is important, in order to decrease fouling side effects of RO membranes. Additionally, novel pretreatment methods for scale prevention need to be considered in order to minimize the usage of antiscalants.
In the third paper we amalgamate the findings from the first two papers to investigate the biofouling potential of polyacrylate- and polyphosphonate-based antiscalants during desalination of brackish water using RO membrane technology, at both laboratory and industrial scale levels. In agreement with the finding from the first paper, the hydrophobicity of the membrane surface during brackish water desalination process was affected by antiscalant usage (see third paper, Table 2). The initial attachment rate of bacteria to the RO membrane surfaces was elevated in the presence of both polyacrylate and polyphosphonate based antiscalants. The polyacrylate-based antiscalant induced bacterial deposition onto the RO membrane surface to an extent of ~150 times higher than on the control membrane (no antiscalant conditioning) and the membrane conditioned with polyphosphonate based antiscalants (see third paper, Figure 3A). The deposition coefficient for the polyacrylate based antiscalant was the highest compared to the polyphosphonate based antiscalant and the control membrane (see third paper, Figure 3B).
Nutritional contribution of antiscalants to biological activity was shown to induce biofilm + formation as investigated in packed-bed reactors (see third paper, section 2.4). TN, DO, NH4 and TP analysis of the influent and the effluent of the packed bed bioreactors was conducted to trace the effect of the antiscalants. DO concentration of the influent of all the three reactors throughout the entire time period of the experiment was constant (~2 mg/L), which means that aerobic conditions prevailed in the bioreactors (see third paper, Figure 4A). Although a decline in the DO concentration of the reactor effluent was observed after 3 months for all the reactors, this decline was most significant in the reactor that was supplemented with polyphosphonate antiscalants. These results show a contribution of the polyphosphonate and polyacrylate antiscalants to the oxidative activity of the biofilms.
+ TN and NH4 analyses indicate that nitrogen was not a limiting nutrient for the biofilm growth in all the three reactors. The higher uptake rate of nitrogen in the presence of polyphosphonate was likely a result of the higher biomass growth rate under these conditions (see third paper – Figures 4B and 4C). TP was found in the influent of the reactor supplemented with the
55 polyphosphonate-based antiscalant (see third paper, Figure 4D) and a sharp decline in the TP concentration of the effluent was observed. This decline in TP is likely because phosphorous is the limiting nutrient for microorganisms in the brackish water being used [57]. We couldn’t analyze changes in the DOC concentrations in the reactor effluent and influent due to the accumulation and secretion of dissolved microbial products from the accumulated biofilm as suspended biomass.
The main finding in this paper (Figure 5 – third paper) was that polyacrylate based antiscalants enhanced bacterial attachment (see third paper - Figure 3), whereas polyphosphonate antiscalants served as a phosphorous source for the fouling biofilm bacteria. Moreover, antiscalants were shown to induce biofouling; permeate flux decline and reduced salt rejection were observed in the RO laboratory unit operated for 35 days (see third paper, Figure 6A). A significant decline in permeate flux was observed in the presence of the polyacrylate based antiscalant compared to the polyphosphonate based antiscalant, due to elevated bacterial attachment. Furthermore, deterioration in salt rejection was observed in the case of polyacrylate antiscalants due to elevated biofilm growth on the membrane surface, which led to a drastic increase in salt concentration in close proximity to the membrane surface, allowing for the biofilm enhanced osmotic pressure phenomenon [77] to take place, as well as reduced permeate flux and consequent elevation of salt concentration in the permeate (third paper, Figure 6B).
Bioactivity was also influenced from the presence of antiscalants; the polyphosphonate- and polyacrylate-based antiscalants induced a higher microbial respiration rate in the RO laboratory desalination unit fed with brackish water compared to unconditioned membranes (third paper, Figure 8A). We also observed a higher degradation rate of TN and ammonium uptake in the antiscalant treated RO unit, compared to the case without antiscalants (third paper, Figures 8C and 8D). The results from the RO laboratory desalination unit (Figure 8) confirmed our earlier finding (third paper, Figure 4) regarding the bioactivity of antiscalants and their role in the biofilm formation process.
The last section of the third paper addressed the effects of antiscalants on fouling of the BWRO pilot and desalination plants. Antiscalants used in commercial and pilot desalination plants were polyphosphonate and polyacrylate based, respectively. Membrane autopsies were conducted on the fouled RO membranes. FTIR analysis of the fouled RO membranes showed a higher extent
56 of microbial biofilm formation in the pilot plant where polyacrylate was used compared to the commercial plant, where mostly a polyphosphonate based antiscalant was used (third paper, Figure 9). Moreover, concentrations of EPS components were relatively higher in the biofouling layers of the pilot plant, likely due to polyacrylate usage, which as mentioned earlier - enhanced bacterial attachment (third paper, Figure 10).
CLSM and SEM microscopy analysis (third paper Figures 11-13) for the same fouled membranes strengthened the findings from the FTIR and the EPS component analyses, also demonstrating a higher amount of microbial biofilms in the pilot plant, likely because of exposure to polyacrylate. We concluded from the results of the third paper that antiscalants can enhance membrane biofouling in brackish water desalination processes by influencing different stages of the biofilm growth. Polyacrylate based antiscalants enhance bacterial attachment due to their adsorption onto the RO membranes and their altering of membrane surface properties, while polyphosphonate based antiscalants can serve as a source of phosphorous for growth of bacterial cells.
In general, antiscalants, mostly polyelectrolytic compounds, can adsorb to the active polyamide layer of RO membranes and alter its surface charge density, and / or hydrophobicity. Upon exposure to antiscalants, bacterial surface properties can change as well and therefore, the antiscalants enhance fouling of RO membrane surfaces by altering membrane and/or bacteria surface characteristics and promote the initial attachment and deposition of bacteria and organic materials. Thus, the total extent of biofilm growth, and/or formation of an organic fouling layer on the membrane surface, increases.
Antiscalants can also serve as nutrients for microbial growth, especially in oligotrophic environments such as in seawater desalination systems where the dissolved organic carbon concentration can be less than 1 mg/L. Antiscalants are added at a concentration of several mg/L in the feed water of the RO process and therefore, increase the dissolved organic carbon concentration. Notably, polyphosphonates and polyphosphates increase the available phosphorous for microbial growth, especially in brackish- and seawater where phosphorous is a limiting nutrient for microbial proliferation. Interestingly, different biofilm formation stages on RO membrane surfaces are shown to be affected by antiscalants in different ways: while polyacrylates increase bacterial initial attachment by altering membrane physico-chemical
57 properties, polyphosphonates increase biofilm growth under phosphorous limiting conditions. Thus, the selection of the type and dosage of antiscalant should take into account the associated contribution to membrane biofouling propensity.
More extensive research efforts need to be conducted on applications of green additives, environmental friendly and sustainable pretreatment technologies. The general aim should focus on minimization of the amount of additives, taking into account both the quality of the feed water and the need to improve the RO membrane performance. Revisiting conventional pretreatment technologies as an alternative to the use of antiscalants should be considered and can include hardness removal and alkalinity reduction. In addition, non-conventional RO pretreatment based on nanofiltration can be adopted to separate di- and tri-valent cations in order to minimize scaling [78]. Economical optimization of the RO desalination process should be assessed to evaluate the cost of chemical additives and their overall consequent effects, in order to achieve low operational and maintenance cost.
58
The influence of antiscalants on biofouling of RO membranes in seawater desalination
Amer Sweity1, Yoram Oren1, Zeev Ronen2 and Moshe Herzberg1*
Supplementary Material
1 Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research,
Albert Katz International School for Desert Studies, Jacob Blaustein Institute for Desert
Research, Ben Gurion University of the Negev, Israel 84990
2 Department of Environmental Hydrology and Microbiology
Zuckerberg Institute for Water Research, Ben Gurion University of the Negev, ISRAEL 84990
Submitted to Water Research December 30, 2012
* Corresponding author: Moshe Herzberg, Department of Desalination and Water Treatment,
Zuckerberg Institute for Water Research, Ben Gurion University of the Negev, Israel 84990,
Tel: +972-8-6563520, Fax: +972-8-6563503, Email: [email protected]
59
XPS analysis
XPS is highly surface sensitive technique and gives highly sensitive analysis of surface chemical groups. It is best in the range of about 5-10 nm of the surface layer [77-78].
A B
C D
Figure S1 XPS analysis of carbon binding energy spectra on membrane surface: (A) virgin membrane, (B) membrane treated with seawater only, (C) membrane treated with seawater supplemented with carboxylic acid based antiscalants and (D) membrane treated with seawater supplemented with polyphosphonates based antiscalants.
60
Carbon analysis
From the deconvoluted binding energy spectra, five different types of carbon bonds are revealed. In virgin sample the peaks at BE of in range of 284 belongs to the main carbon C-C peak and also there can be the contribution C-H peak at this range. The C- contribution can also be from the Phosphonate C-H groups used as antiscalants. The peaks at binding energy in the range of 286 eV can be attributed to the contribution from the C-O / C-N bond which is most likely originated from the polyamide active layer [81-82]. The peak at BE of 287 range is due to the amide group ( O=C-N) contribution in the polyamide layer. In addition, there is the peak at BE range of 285 which can be attributed to the aromatic C-C /C-H stretch, which is also from the polyamide layer [81]. The peak at 288 range can be attributed to carboxylic group or to CH2- NCO group in membrane C. There is significant shift in the BE of this peaks up to 287.86 eV for phosphonate treated samples because of the interaction /adsorption of phosphonates with the surface amine/amide groups in membrane D. Looking at C1S scans of all the samples, the Peak at BE of 285 (Aromatic C-C/C-H) stretch is absent where we have used phosphonates as antiscalants but the peak at 284 (C-C/C-H aliphatic) is present. According to all the scans, adsorption of phosphonate on the membrane surface obscured the C-C /C-H aromatic peak, as phosphonate might be-forming the thick layer on the surface of membrane. This is further supported by the fact that the peak for CH2-NCO is absent in the sample treated with phosphonate antiscalants which somehow forms a very thick layer on the surface.
Nitrogen analysis
The peak in BE range of 399.74-400.04 are attributed to the nitrogen contribution from the amide peak. The second peak in BE range from the 400.76 and 400.85 is due to the amine nitrogen contribution [81]. The peak for amine nitrogen has been found to have significant shift for the sample treated with the sea water. The Peak was found at 401.49 can be attributed to nitrogen in the form of ammonium ion. No individual amine peak is found in the spectra treated with the carboxylic acid antiscalant. This could be due to some interaction of carboxylic acids with surface amine group and hence the amine peak could be well merged with the contribution of the main amide peak at 400.04 eV.
61
B A
C D
Figure S2 XPS analysis of nitrogen binding scan of 4 samples (A) virgin membrane, (B) membrane treated with seawater only, (C) membrane treated with seawater supplemented with carboxylic acid based antiscalants and (D) membrane treated with seawater supplemented with polyphosphonates based antiscalants reveals the presence of two types of nitrogen binding scans.
Oxygen Analysis
The oxygen 1s scan reveals the presence of three main types of oxygen binding scans. The peak at 532 BE range is attributed to surface carboxylic or hydroxylic groups. The peaks at BE in
62 range of 533 BE are due to atmospheric moisture/water. The peaks at BE range of 531 are due to amide group oxygen. The absence of O 1s peak at BE around 529 rules out the presence of any metal oxides on the surface. This means that the metals like, Na, Mg etc are not present as there oxides or chlorides on our membrane surface, which is treated with antiscalants, which might be in form of carboxylates and phosphonates.
A
C D
Figure S3 XPS O1s scan of 4 samples (A) virgin membrane, (B) membrane treated with seawater only, (C) membrane treated with seawater supplemented with carboxylic acid based antiscalants and (D) membrane treated with seawater supplemented with polyphosphonates based antiscalants reveals the presence of two type of Nitrogen.
63
Phosphorous analysis
A B
C D
Figure S4 XPS analysis for the binding energy BE curve of P2p scan of 4 samples (A) virgin membrane, (B) membrane treated with seawater only, (C) membrane treated with seawater supplemented with carboxylic acid based antiscalants and (D) membrane treated with seawater supplemented with polyphosphonates based antiscalants reveals the presence of phosphorous.
64
Figure S4 shows the XPS binding energy curve of the phosphorus P2p scan of 4 membranes treated and filtered using the RO system with seawater with and without antiscalants. P2p peaks revealed three phosphorus components. The three components were phosphates, pyrophosphates and metaphospates. The binding energy of P2p line was in the region between 132.7+0.81 to 133.7+1.17 eV. Under the previous BE range, the main constitutes of the P2p peaks are attributed to pentavalent tetra coordinated phosphorus (PO4 tetrahedra) surrounded by different chemical environment (phosphate-like structure) [83-84]. It is also well known that the C-P fall in the previous range. We assumed that the virgin membrane has very little amount of P which is appeared from the P % from XPS results. The atomic % of P on the membranes were (0.19%,0.3%, 0.26 % and 0.54%) for the virgin membrane, membrane treated with seawater only, membrane treated with seawater supplemented with 20 ppm carboxylic acid based antiscalant and membrane treated with seawater supplemented with 20 ppm polyphosphonate based antiscalants, respectively. It should be noted that the membrane treated with polyphosphonate based antiscalants showed the highest percentage of elemental P. This could confirm the previous XPS findings of the C1s scans (scans D and E), which reflect the absence of aromatic C—C, C—H, and NH-C=O bonds due to the adsorption of phosphonate based antiscalants.
Table S1 Chemical analysis of antiscalant solutions used in the study, antiscalant was prepared by dilution the antiscalant in double distilled water to final concentration of 10 ppm (v/v).
TP Concentration of AS 10 ppm TOC ppm TN ppm ppm NH4 ppm
Neutralized carboxylic acid CA Polyacrylates 2.19 0.1 3.8 0.1
Neutralized phosphonic acid PP polyphosphonates 1.46 1.4 7.3 0.2
65
Side Effects of Antiscalants on Biofouling of Reverse Osmosis
Membranes in Brackish Water Desalination
Amer Sweity1, Tesfalem Rezene Zere1, Inbal David2, Sarit Bason2, Yoram Oren1,
Zeev Ronen1 and Moshe Herzberg1*
SUPPLEMENTARY INFORMATION
FOR Research Paper Submitted to Journal of Membrane Science
rd Revised version submitted on February 3 , 2015
1 The Jacob Blaustein Institutes for Desert Research, Zuckerberg Institute for Water Research, the Albert Katz International School of Desert Studies, Ben Gurion University of the Negev,
Sede Boqer Campus, Israel 84990
2 Mekorot Water Co. Ltd., Tel Aviv, Israel
Corresponding author: Phone + 972 8 6563520; Fax + 972 8 6563503; E-mail: [email protected]
66
Figure S1: Growth of bacterial biofilms with and without antiscalants in fixed bed plug flow bioreactors. The bioreactors were of 50 mL volume, packed with 0.5 mm glass beads, and operated continuously at hydraulic retention time of 5 hrs and influent flow rate of 0.17 mL / min.
67 References:
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תקציר על מנת לאפשר פעולה תקינה, רציפה וכלכלית של מתקני התפלת מי ים ומים מליחים בשיטת אוסמוזה הפוכה יש להימנע משיקוע של אבנית )אילוח מינראלי( ומגידול מיקרוביאלי )אילוח ביולוגי( על פני שטח הממברנה. על מנת לקבל יחסי השבה של כ 05% וכ 50% בתהליכי התפלת מי ים ומי קולחין, בהתאמה, השימוש באנטיסקאלנטים, מעכבי אבנית, הינו בלתי נמנע, עקב עליית ריכוז המלח במי הרכז הגבוה מערך מכפלת המסיסות ועלייה בפוטנציאל השיקוע. ברוב המקרים, אנטיסקאלנטים הינם פוליאלקטרוליטים מסוג פולי-אקרילאטים, פולי-קרבוקסילאטים, פולי- פוספונאטים, פולי-פוספאטים ואמינו-פוספונאטים אשר מונעים היווצרות אבנית על פני הממברנות. שימוש מושכל באנטיסקלנטים במערכות אוסמוזה הפוכה הינו הכרח אך יש להשתמש בהם כך שמצד אחד ימנעו תהליכי שיקוע על הממברנות ומצד שני לא יעודדו אילוח ביולוגי, קרי גידול שכבת ביופילם מיקרוביאלי על פני שטח הממברנה. אנטיסקלנטים יכולים לעודד יצירת ביופילמים במנגנוני פעולה שונים: )1( על ידי שינוי פיזיקו-כימי של תכונות השטח של הממברנה והמיקרואורגניזמים, תוך הגדלת הזיקה להצמדות בין פני שטח הממברנה לפני שטח המיקרואורגניזמים ו )2( אנטיסקלנטים יכולים לשמש כמקור לנוטריאנטים )פחמן, זרחן ולעיתים חנקן( עבור מיקרואורגניזמים אשר עתידים ליצור ביופילם מיקרוביאלי על פני הממברנה.
במחקר זה בחנו את השפעת נוכחות ריכוזים נמוכים )מג"ל בודדים( של אנטיסקלנטים במערכות אוסמוזה הפוכה על התכונות הפיזיקו-כימיות של פני השטח שכללו הידרופיליות/הידרופוביות ומטען פני השטח תחת תנאי הזרימה אופיניים להתפלה. שינויים פיזיקו-כימיים אלו נבחנו בהקשר הצמדות חיידקים וחומר אורגני. בנוסף, מכיוון שאנטיסקלנטים מכילים חומר אורגני אשר יכול לשמש כמקור פחמן ו/או אנרגיה לחיידקים ומיקרואורגניזמים אחרים, ישנו סיכוי סביר שבריכוזים בהם ריכוז הפחמן האורגני נמוך כבמי ים (mg/l 1>) וריכוז הזרחן גם הוא לרוב בלתי ניתן למדידה (mg/l 0.1>) תהייה השפעה ניכרת של ריכוז האנטיסקלנט המוסף על גידול מיקרוביאלי על פני שטח הממברנה. תופעה זו מואצת במיוחד עקב עלייה בריכוז המומסים והאנטיסקלנטים קרוב לפני השטח, עקב קיטוב ריכוזים. פוליפוספונאטים ופולי פוספאטים בהיותם מכילי זרחן יכולים לספק יסוד זה לביומסה הגודלת. במקרים רבים זרחן הוא גורם המגביל את קצב גידול הביופילם המיקרוביאלי ולאספקת זרחן ממקור נוסף השפעה משמעותית. השאלה היא באיזו מידה אנטיסקלנטים שונים מעודדים גידול מיקרוביאלי ובאילו תנאים אנטיסקלנטים שונים יהוו גורם מגביל לקצב הגידול? במהלך המחקר התקבל )1( שינוי בתכונות פני שטח חשובות של ממברנות אוסמוזה הפוכה מסוגים שונים )של מי ים ומים מליחים( והן זווית המגע )הידרופוביות( ופוטנציאל זטה )מטען חשמלי(, כתוצאה מחשיפת הממברנה לאנטיסקאלנטים שונים: אנטיסאלנטים מסוג פוליאקרילאטים ודנדרימרים קרבוקסילאטים העלו את ההידרופוביות וגרמו למשטח הממברנה להיות מעט פחות טעון שלילית בטווחי PH בין 3 ל 15 )2( נבחנה ההשפעה של תכונות אלו על אילוח אורגני אשר במקרים רבים מהווה שכבת ציפוי אורגנית ראשונית המעודדת היצמדותם של מיקרואורגניזמים (”conditioning film“) וגידול מואץ של ביופילם: התקבלה עלייה משמעותי בהצמדות ראשונית של חיידקים וחומר אורגני כתוצאה מחשיפת הממברנה לאנטיסאלנטים מסוג פוליאקרילאטים ודנדרימרים קרבוקסילאטים, )3( התבצעו
67
ניסויים מבוקרים של גידול ביופילם טבעי הניזון מחומרי הזנה טבעיים שקיימים במי ים ובמים מליחים ומאנטיסקאלנטים שונים, ו )4( נעשו ניסיונות שדה עם מתקן חלוץ ובהשוואה למתקן תעשייתי של התפלת מים מליחים )בשיתוף עם חברת מקורות( ואנטיסקאלנטים עם וללא זרחן. התקבל שלאורך זמן, אנטיסקאלנטים המכילים זרחן )פוליפוספונאטים( יכולים לתמוך ביעילות רבה יותר בגידול ביופילמי בסביבות בהן מקור הזרחן מגביל. באופן פרטני התקבלו התוצאות הבאות: )1( השימוש באנטיסקאלנטים על בסיס פוליאקרילאטים ודנדרימרים מגביר אילוח אורגני גם עם BSA וגם אם אלגינאט כמולקולות מודל לאילוח ממברנות אוסמוזה הפוכה. מסקנה זו נבחנה גם במערכת ה QCM-D בה הוכחה עלייה בספיחת ה BSA והאלגינאט כמו גם במערכת ה RO בה נצפתה ירידה בשטף מי התוצר תוך שימוש באלגינאט ו BSA. העלייה בשני סוגי האילוח ,אורגני וביולוגי, נגרמת ע"י ספיחת האנטיסקאלנטים לפני שטח הממברנה הגורמים לעלייה בהידרופוביות ולהקטנת המטען השלילי ע"פ השטח המעלים את קצב ספיחת המאלחים, )2( השימוש עם דנדרימרים קרבוקסילאטים הינו בעייתי, באופן משמעותי, לעומת שימוש בפוליאקרילאטים כחומרי בסיס לאנטיסקאלנטים, גם באילוח אורגני וגם באילוח ביולוגי, )3( השימוש עם פוליאקרילאטים לא מעודד גידול ביופילמי ע"י תוספת נוטריאנטים היות ואינו מכיל זרחן. אך על מנת שלא יגרמו תופעות המעודדות ספיחה של חומר אורגני וקולואידלי יש לטפל במי ההזנה טיפול מקדים אשר ירחיק קולואידים וחומר אורגני. מסקנה זו הוכחה גם בניסיונות מעבדה עם ממברנות קטנות וגם במתקן פיילוט ומתקן תעשייתי של מקורות. להמשך המחקר אנו ממליצים: )1( להמשיך לבצע ניסיונות אילוח ביולוגי עם תרבית טבעית של מי ים ואנטיסקאלנטים שונים במתקני חלוץ עם מודולים ספירליים, כפי שמתבצע כיום בחברת מקורות עם מים מליחים, )2( בהיבט הסביבתי, יש לבודד את האוכלוסיות אשר הינן בעלות יכולת פירוק ביולוגי של אנטיסקאלנטים ולחקור את הפריקות הביולוגית של אנטיסקאלנטים מסוגים שונים.
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הצהרת תלמיד המחקר עם הגשת עבודת הדוקטור לשיפוט
אני החתום מטה מצהיר/ה בזאת: )אנא סמן(:
_X_ חיברתי את חיבורי בעצמי, להוציא עזרת ההדרכה שקיבלתי מאת מנחה/ים.
_X_ החומר המדעי הנכלל בעבודה זו הינו פרי מחקרי מתקופת היותי תלמיד/ת מחקר.
___ בעבודה נכלל חומר מחקרי שהוא פרי שיתוף עם אחרים, למעט עזרה טכנית הנהוגה בעבודה ניסיונית. לפי כך מצורפת בזאת הצהרה על תרומתי ותרומת שותפי למחקר, שאושרה על ידם ומוגשת בהסכמתם.
תאריך _18-3-5112_ שם התלמיד/ה _עאמר סווטי_ חתימה
העבודה נעשתה בהדרכת
פרופ' משה הרצברג פרופ' זאב רונן
במחלקה מכון המים עייש צוקרברג
בפקולטה מכונים לחקר המדבר עייש יעקב בלאושטיין
שימוש מושכל במעכבי שיקוע אבנית והשפעתם על אילוח אורגני וביולוגי של ממברנות אוסמוזה הפוכה
מחקר לשם מילוי חלקי של הדרישות לקבלת תואר "דוקטור לפילוסופיה"
מאת
עאמר סווטי
הוגש לסינאט אוניברסיטת בן גוריון בנגב
אישור המנחה פרופ' משה הרצברג______פרופ' זאב רונן ______
אישור דיקן בית הספר ללימודי מחקר מתקדמים ע"ש קרייטמן ______
כ״ו בַּאֲדָ ר תשע״ה מרץ 5102
באר שבע
שימוש מושכל במעכבי שיקוע אבנית והשפעתם על אילוח אורגני וביולוגי של ממברנות אוסמוזה הפוכה
מחקר לשם מילוי חלקי של הדרישות לקבלת תואר "דוקטור לפילוסופיה"
מאת
עאמר סווטי
הוגש לסינאט אוניברסיטת בן גוריון בנגב
כ״ו באדר תשע״ה מרץ 5102
באר שבע