Schäfer, A.I. ; Andritsos, N. ; Karabelas, A.J. ; Hoek, E.M.V. ; Schneider, R. ; Nyström, M. (2004) Fouling in Nanofiltration, in: Nanofiltration – Principles and Applications, Schäfer A.I., Waite T.D., Fane A.G. (Eds). Elsevier, Chapter 20, 169-239. Nanofiltration – Principles and Applications Chapter 8 – Fouling in Nanofiltration 1 INTRODUCTION According toKoros et al. [1]fouling is “the process resulting in loss of performance of a membrane due to deposition of suspended or dissolved substances on its external surfaces, at its pore openings, or within its pores”. Fouling is also Chapter 8 decribed as flux decline which is irreversible and can only be removed by, for example, chemical cleaning [2]. This is different to flux decline due to solution chemistry effects or concentration polarisationwhichis describedinmore detail later inthis chapter. Those flux declines canbe reversed withcleanwaterandarehencenotconsideredasfouling. Fouling of membranes is important as it limits the competitiveness of the process due toanincrease in Fouling in Nanofiltration costs due to an increased energy demand, additional labour for maintenance and chemical costs for cleaning as well as a shorter lifetime of the membranes. Essential for effective fouling control is a proactive operationof a nanofiltration(NF) or reverse osmosis (RO) plant where anearly indicationof fouling is acted upon and a good identification of the type of fouling is carried out. Staude [3] Andrea I. Schäfer1, Nikolaos Andritsos2, Anastasios J. summarisedthepossibleoriginsoffoulingasfollows Precipitationofsubstancesthathaveexceededtheirsolubilityproduct(scaling) 2 3 4 Karabelas , Eric M.V. Hoek , René Schneider , Marianne Depositionofdispersedfinesorcolloidalmatter Nyström5 Chemical reaction of solutes at the membrane boundary layer (e.g. formation of ferrichydroxidesfromsolubleformsofiron) Chemicalreactionofsoluteswiththemembranepolymer 1Environmental Engineering, University of Wollongong, Wollongong, NSW 2522, Adsorptionoflowmolecularmasscompoundsatthemembranepolymer Australia Irreversiblegelformationofmacromolecularsubstances ph +61 2 4221 3385, fax +61 2 4221 4738, [email protected] Colonisationbybacteria(mostlyhydrophobicinteractions). 2Chemical Process Engineering Research Institute, P.O. Box 361, 57001, Thermi, This gives anindicationof the complexity of fouling andanexampleofsuchcomplexityisillustratedin Greece Figure 1 withelectronmicrographs of a membrane fouledwithsurface water without pretreatment. ph ++302310498181, fax ++302310498189 The pictures show colloids and organic matter embedded in a gel like cake layer on top of the [email protected] [email protected] membrane. InFigure 2 another surface water deposit ona NF membrane is shown, except that inthis 3Chemical & Environmental Engineering, University of California, Riverside, CA 92521, case the surface water is pretreated with ultrafiltration and fouling is dominated by inorganic precipitates. USA, ph: +011-909-787-7345, fax: +011-909-787-5696, email: [email protected] 4Departamento de Microbiologia, Instituto de Ciências Biomédicas, Universidade de Figure 1 Complex São Paulo, Brazil deposit of surface water ph +55 11 3091 7745, fax +55 11 3091 7354, [email protected] on a membrane 5Lappeenranta University of Technology, Laboratory of Membrane Technology and (adapted from Schäfer Technical PolymerChemistry, P.O.Box 20, FIN-53851 Lappeenranta, Finland [4]). ph. +358-5-621 2160, fax +358-5-621 2199, [email protected] 1 2 Schäfer, A.I. ; Andritsos, N. ; Karabelas, A.J. ; Hoek, E.M.V. ; Schneider, R. ; Nyström, M. (2004) Fouling in Nanofiltration, in: Nanofiltration – Principles and Applications, Schäfer A.I., Waite T.D., Fane A.G. (Eds). Elsevier, Chapter 20, 169-239. Nanofiltration - Principles and Applications Chapter 8 – Fouling in Nanofiltration Table 1 Fouling - Where does it occur first (adapted from Hydranautics Technical Service Bulletin TSB107 in Huiting et al. [5]) Type of Foulant Most susceptible stage of NF/RO Scaling/silica Lastmembranesinlaststage Metaloxides Firstmembranesoffirststage Colloids Firstmembranesoffirststage Organic Firstmembranesoffirststage Biofouling(rapid) Firstmembranesoffirststage Biofouling(slow) Throughoutthewholeinstallation Inorder toreduce or eliminate fouling it is necessary toidentify the foulants. This canbe achievedby a characterisationof the fouledmembrane (membrane autopsy inSection2.4)orbyfoulingstudiesinthe laboratory. Once the foulants are identifiedsuitable control strategies canbe adapted. Anoverview of foulants and appropriate control strategies are summarised in Table 2. The strategies encompass a numberofcategories[6]namely Feedpre-treatment Membrane selection (nonfouling materials/coatings, suitable surface charge, chlorinecompatibility,porosity,hydrophilicity,surfaceroughnessetc.) Moduledesign&operationmode Cleaning. Feedpre-treatment is addressedinChapter 9, Membrane materials inChapter 3, module designand Figure 2 Scanning electron micrographs (SEM) of membranes fouled during the filtration of operationinChapter4andcleaningattheendofthischapter. ultrafiltration pretreated surface water and examined in an autopsy (bar length 200 µm for all pictures; photos courtesy of Paul Buijs, GEBetz, Belgium) Table 2 Foulants and their control strategies in nanofiltration and reverse osmosis processes A number of factors contribute tofouling andare strongly interlinked. The mainfouling categories are (adapted and modified from Fane et al. [6]) organic, inorganic, particulate andbiological fouling. Metal complexes (for example Fe, Al, Si) are also Foulant Fouling Control important. While researchtraditionally focuses onone category or fouling mechanism at a time, it is General Hydrodynamics/shear,operationbelowcriticalflux, well acceptedthat inmost cases it is not one single category that canbe identified. Inmost real life chemicalcleaning applications all four types of fouling gohandinhand. The types of foulants andwhere they usually Inorganic(Scaling) Operatebelowsolubilitylimit,pre-treatment,reducepHto occurinNF/ROsystemsissummarisedinTable1. 4-6(acidaddition),lowrecovery,additives(antiscalants) Somemetalscanbeoxidisedwithoxygen Scaling and silica fouling originates in general from the concentration of inorganics exceeding the Organics Pretreatmentusingbiologicalprocesses,activatedcarbon,ion solubility limit (see Section 5 Scaling). This most often occurs in the latter membrane stages. Metal exchange(e.g.MIEX),ozone,enhancedcoagulation oxides and colloids deposit early in the process as drag forces are relatively high (see Section 6 Colloids(<0.5 µm) Pre-treatmentusingcoagulation&filtration,microfiltration, ultrafiltration Particulate andColloidal Fouling). Organic fouling remains poorly understoodandvery specific tothe Biologicalsolids Pretreatmentusingdisinfection(e.g.chlorination/ characteristics of the foulant molecules (see Section4 Organic Fouling). Organic fouling may occur at dechlorination),filtration,coagulation,microfiltration, the beginning as well as the end stages of the modules depending on the dominating mechanism. ultrafiltration Biofouling also can be found throughout all filtration stages (see Section 7 Biofouling). Rapid biofouling canbe relatedtoparticle attachment whichis foundmostly inthe first stage, whereas the Membrane fouling is the worst enemy of membrane process applications andyet fouling goes handin slow biofouling canoccur throughout all stages [5]. While inthe past bacterial depositionandfouling handwithsuccessful filtration. The searchtounderstandfoulinghasdominatedmembraneresearchfor have often been studied by using latex particles, the adhesive nature of extra-cellular polymeric some time, yet models fail to predict and adeQuately describe this complex process. Fouling often substances(EPS)makesbacteriamoreadhesiveandtheirdepositionmechanismmorecomplex[6]. reQuires freQuent cleaning of membranes andconseQuently reduces the membrane life span. Insome cases fouling causes membrane biodegradation and a loss of integrity [7]. Cleaning also reQuires chemicals, possibly an increased cleaning temperature and hence renders membrane processes less 3 4 Schäfer, A.I. ; Andritsos, N. ; Karabelas, A.J. ; Hoek, E.M.V. ; Schneider, R. ; Nyström, M. (2004) Fouling in Nanofiltration, in: Nanofiltration – Principles and Applications, Schäfer A.I., Waite T.D., Fane A.G. (Eds). Elsevier, Chapter 20, 169-239. Nanofiltration - Principles and Applications Chapter 8 – Fouling in Nanofiltration sustainable. Further, it decreases process efficiency due to the reduced flux, reQuiring either higher 2.1.2 Variation of Membrane Permeability with Solution Chemistry transmembrane pressures (andhence more energy) or larger membrane areas. Therefore, fouling is a Braghetta et al. [11]have investigated the im pact of variation in solution chemistry, namely pH and criticalparametertobeconsideredinNFprocessdesign. ionic strength on membrane permeability. At low pH and high ionic strength this permeability This chapter will offer a summary of the components of fouling, most commonfouling mechanisms decreasedwhichwas linkedtoa compactionof the membrane matrix due tocharge neutralisationand andsomecontrolstrategies. double layer compression. The authors usedthe parameter of Debye lengthtoQuantify suchchangesin membrane structure or more precisely the double layer thickness. A reducedDebye lengtheffectively 2 FOULING CHARACTERISATION increasesthecross-sectionalareaavailableforsolventtransport. 2.1 Flux Measurement and Fouling