Nanofiltration Membrane Fouling by Conventionally Treated Surface Water

Nanoﬁltration membrane fouling by conventionally treated Water Science and Technology: Supply surface water

R. Liikanen*, H. Kiuru*, T. Tuhkanen and M. Nyström*** * Laboratory of Environmental Engineering, Helsinki University of Technology, PO Box 5200, FIN-02015 HUT, Finland (E-mail: riina.liikanen@hut.fi; heikki.kiuru@hut.fi) ** Environmental Engineering and Biotechnology, Tampere University of Technology, FIN 33101, Finland (E-mail: tuula.tuhkanen@tut.fi) *** Laboratory of Membrane Technology and Technical Polymer Chemistry, Lappeenranta University of Technology, FIN 53581, Finland (E-mail: marianne.nystrom@lut.fi)

Abstract Nanoﬁltration is a very effective technique for improving the removal of trace organics after a conventional chemical water treatment train. However, the fouling of the membranes decreases the

applicability of the process, and thus, an understanding and control of membrane fouling are crucial for a Vol 3 No 5–6 pp 183–190 more widespread use of nanofiltration in water treatment. The fouling of different nanofiltration membranes by pre-treated surface waters was investigated in a laboratory-scale filtration unit in this study. The results indicate that the traditional chemical treatment does not remove membrane foulants from the surface water. No correlation was found between the feed water constituents and nanofiltration performance, but most feed water components are expected to interact in membrane fouling. Actually, the performance of the nanofiltration process was more related to membrane than to feed water characteristics. Keywords Fouling; nanofiltration; surface water treatment © IWA Publishing 2003 Introduction Nanofiltration (NF) is one of the most promising techniques for improving the removal of trace organics in drinking water treatment. However, the fouling of the membranes decreases the applicability of the process. Thus, an understanding and the control of membrane fouling are crucial for a more widespread use of NF in water treatment. Efficient control of NF membrane fouling generally requires pre-treatment to lower the fouling potential of the feed water. In surface water applications, microfiltration or ultra- filtration (UF) has proved to be a very efficient pre-treatment for NF processes (Chellam et al., 1997), but a conventional chemical treatment also improves the NF performance (Ventresque et al., 2000). In an existing surface water treatment plant the present chemical water treatment process is a presumable option for pre-treatment. A wide spectrum of feed water constituents contributes to membrane fouling. These include dissolved and macromolecular organic compounds, sparingly soluble inorganic compounds, colloidal and suspended particles and micro-organisms. Especially dissolved naturally occurring organic substances (natural organic matter, NOM) are considered important foulants in membrane filtration of natural waters. NF membrane fouling is controlled by an interaction between permeation drag, electrostatic repulsion and hydrophobic attraction. For solution chemistries typical of natural source waters or pre-treated surface waters, permeation drag under normal operating conditions plays a more significant role than membrane foulant electrostatic interaction, and may ultimately control the rate and extent of colloidal fouling (Zhu and Elimeleh, 1997). Thus, the optimisation of the process parameters is a crucial factor in fouling mitigation. The higher the permeation, the more fouling material is in contact with the membrane surface, and the higher is the flux decline. 183 The fouling of different NF membranes by pre-treated surface waters was investigated in a laboratory-scale filtration unit in this study. The objective was to identify the components or characteristics in the chemically treated feed waters that foul the membranes, and to determine the characteristics of the foulants. In addition, the effects of fouling on the membrane characteristics were evaluated.

R. Liikanen Materials and methods Pilot plant and membranes The study was carried out in a laboratory-scale membrane filtration unit. Three cross-flow et al. flat-sheet modules were run parallel using the same feed water. The membrane area in each module was 46.0 cm2. The membranes were Desal-5 DL, NF255 and NF270. The surface layer of all the tested membranes is made of polypiperazine amide and all the membranes are categorised as tight NF membranes with cut off values around 300 g/mol.

Feed waters Six different feed waters for the pilot process were collected from five surface water treatment plants and one artificial ground water treatment plant. Ninety litres of each sample was collected after a traditional water treatment train, but before post-treatment. The details of the treatment process in each studied plant are summarised in Table 1.

Operation Virgin membranes were rinsed with reverse osmosis filtered and ion-exchanged water (RO-water). Then the membranes were pressurised at 20 bar for 15 minutes to wet them thoroughly before measuring the initial pure water flux (PWF). The PWF was measured under the actual process operation conditions. Then the membranes were pre-cleaned (0.1% Ultrasil 10, 30 minutes), rinsed with RO-water, and the PWF was measured again. The pilot process was operated in a retentate circulation mode until 60 litres of permeate was produced as a sum of all modules. Fresh feed water was added to the feed stream to compensate the permeate removal from the process. The temperature of the feed water was kept constant at 20°C. Cross-flow velocity (CFV) was constant at 0.65 m/s and the net driving pressure (NDP) was 10 bar. The NDP, feed water temperature, retentate flow and permeate flows were monitored and the data was collected continuously by a computer. At the end of the run, the membranes were rinsed with RO-water for 15 minutes. Then the PWF of the fouled membranes was measured.

Table 1 Pre-treatment trains of the studied feed waters

Feedwater Pretreatment train

Espoo Coagulation with ferric sulfide, coagulation pH adjusted with lime, flocculation, flotation, sand filtration Kotka Artificial groundwater Pietarsaari Coagulation with ferric chloride sulfite, coagulation pH adjusted with lime, flocculation, flotation, pH adjustment with lime, sand filtration Raisio-Naantali Coagulation with polyaluminium chloride, coagulation pH adjusted with lime, cationic coagulation aid, flocculation, sedimentation, pH adjustment with lime, sand filtration

Tampere Alkalinity adjustment with CO2 and pH adjustment with lime, coagulation with alum, flocculation, flotation, disinfection with ClO2, activated carbon filtration Turku Alkalinity adjustment with CO2 and pH adjustment with lime, coagulation with ferric chloride sulfite, flocculation, sedimentation, addition of powdered activated carbon and lime, coagulation with ferric chloride sulfite, coagulation pH adjusted with NaOH,

cationic coagulation aid, flocculation, flotation, disinfection with a combination of Cl2, ClO , and NaClO , sand filtration 184 2 2 Evaluation of the performance of the feed waters The flux of the membranes was evaluated with RO-water as PWF and with the tested feed waters as operational flux (FWF) during the run. The fluxes were normalised to standard operating conditions (T = 20°C and NDP = 10 bar). The percentage changes in the membrane flux (flux change = FC) at different phases of the tests were calculated as described in Figure 1. The following parameters were rated in order of superiority to rank the different R. Liikanen feed waters: FCfouled, FCfeed, FCoper. The ranks were summed separately for all the tested membranes and finally as a whole for the overall rating of the feed waters.

Water analysis et al. The feed and permeate waters were characterised by pH, conductivity, alkalinity, hardness, total organic carbon (TOC), UV254, anions, cations and the molar mass distribution of organic matter (excluding anions and cations for permeates). TOC was measured by the Shimatzu TOC-5000A analysator by combustion – non-dispersive infrared gas analysis method. The organic matter hydrophobicity was evaluated by the SUVA value (Edzwald and Tobiason, 1998). The organic matter molar mass distributions were analysed by high- performance size exclusion chromatography (Hewlett-Packard HPLC 1100). Anions were analysed by ion chromatography and cations by inductively coupled plasma atomic emission spectrometry, or by mass spectrometry.

Membrane and foulant analysis The hydrophobicity, functional groups, elementary composition and visual appearance of the virgin, pre-cleaned and fouled membranes were characterised. The membrane hydrophobicity was measured as contact angle (CA) by the drop method. The functional groups were analysed using the Fourier transform infrared (FTIR) (Perkin-Elmer 2000) apparatus. Scanning electron microscopy (SEM) – elementary analysis (EDS) (JEOL JSM- 5800) were used to analyse the membrane elementary composition and visual appearance. The elementary composition of the NF270 membranes was also studied using an X-ray photoelectron spectroscopy (XPS) analysator (AXIS 165).

FCin target phase = ( Fluxtarget phase - Fluxcompared phase) / Fluxcompared phase * 100%

Flux FCpc

FCfouled

FCfeed PWFv FCoper PWFpc Average feed water FWF flux after 5 hours of PWFf operation

5 h 10 l of Run permeate time filtrated = Figure 1 Schematic presentation of the flux change (FC) calculations of the tests. PWFv virgin membrane = = = PWF, PWFpc PWF of pre-cleaned membrane, PWFf PWF of fouled membrane. FCpc PWFpc in rela- = = tion to PWFv, FCfeed average feed water flux in relation to PWFpc, FCoper feed water flux after permeate = production of 10 l in relation to initial feed water flux and FCfouled PWFf in relation to PWFpc 185 Results and discussion Membrane and feed water performance The membrane fluxes varied remarkably between different test runs (Table 2). The NF270 membrane had a higher flux than the other membranes, even if the membrane material, charge, hydrophobicity and functional groups were basically the same for all the membranes. Only the visual appearance of the virgin NF270 membrane differed from the

R. Liikanen other membranes by being scratchy and not as smooth. It may be concluded that the higher flux of the NF270 membrane was most probably due to membrane morphology favouring permeation. et al. The FCfeed, FCoper and FCfouled values are presented for the tested feed waters and membranes in Table 3. In addition to high flux, NF270 experienced most fouling. This may be partly due to more contact with the fouling material as the flux was also higher. Desal-5, which was the tightest membrane according to retention characteristics, fouled the least. On the basis of the overall rating, Kotka feed water performed the best with the NF270 membrane, Pietarsaari, Raisio-Naantali and Tampere with the NF255 membrane, and Turku with the Desal-5 membrane. Tampere feed water suited the worst for filtration with the NF270 and Desal-5 membranes. Turku feed water resulted in the worst performance with the NF255 membrane. Tampere feed water clearly ranked worst when all the membranes were considered. The differences between the other feed waters were rather small in the overall ranking, but even so, Kotka feed water can be said to rank the best. According to statistical ANOVA analysis on the fouling and flux parameters, the membrane was critical for both flux and fouling. This finding is supported by the study of Zander and Curry (2001), where membrane surface chemistry was found to be the most important factor affecting the rate of flux decline. Several studies also suggest that membrane surface roughness correlates well with fouling (Zhu and Elimeleh, 1997; Hobbs et al., 2001). The feed water seemed to correlate only weakly with flux and not at all with fouling. Habarou et al. (2001) could not establish any clear relationship between the general struc- tural characteristic of the feed water NOM and UF membrane fouling properties either. Speth et al. (2000), in turn, did not find a correlation between the feed water levels of indi-

Table 2 Average FWF, virgin membrane PWFv and pre-cleaned membrane PWFpc of different membranes during test runs. T = 20°C, NDP = 10 bar, CFV = 0.65 m/s

2 2 2 FWF, l/m .h PWFv, l/m .h PWFpc, l/m .h NF270 NF255 Desal 5 NF270 NF255 Desal 5 NF270 NF255 Desal 5

Espoo 120 72 73 183 87 68 200 101 86 Kotka 155 81 70 188 99 65 219 111 86 Pietarsaari 131 61 65 170 58 62 225 66 81 Raisio-Naantali 156 89 89 221 108 59 249 120 117 Tampere 107 64 63 206 70 62 231 79 80 Turku 143 53 70 202 59 59 227 74 76

Table 3 Flux changes in the different phases of the process. Best performances indicated by bold, worst performances by italics

FCfeed, % FCoper, % FCfouled, % NF270 NF255 Desal 5 NF270 NF255 Desal 5 NF270 NF255 Desal 5

Espoo –40 –28 –15 –13 –14 –7 –23 –2 –3 Kotka –29 –27 –18 –7 –11 –4 –25 –24 –13 Pietarsaari –42 –8 –19 –11 –19 –6 –29 –0 –2 Raisio-Naantali –37 –25 –24 –14 –9 –9 –31 –22 –10 Tampere –54 –19 –21 –20 –12 –12 –34 –13 –14 Turku –37 –29 –8 –12 –18 –2 –31 –26 5 186 vidual inorganic species and the differences in NF membrane flux decline. In conclusion, natural waters are such complex mixtures of different materials that it is difficult to determine any single component responsible for fouling, but it could be assumed that most feed water constituents interact in membrane fouling.

Feed water quality and membrane fouling

The water quality parameters of the original feed waters are presented in Table 4. The feed R. Liikanen waters became concentrated during the run as the permeate was discharged from the process. By the end of the test run conductivity was increased by 70–125% and TOC by

150–280% depending on the feed water. No trends in feed water characteristics could be et al. found according to their treatment process. All the feed waters contained mostly organic matter of molar mass 1,000–4,000 g/mol. Kotka had highest (>5,000 g/mol) and Turku lowest molar mass organics (<1,000 g/mol). SUVA values below 2 also indicate that organic matter consists mostly of low molar mass non-humic material with low hydrophobicity (Edzwald and Tobiason, 1998). The observed organic matter characteristics are not linked with the most fouling fraction of NOM (Childress and Elimeleh, 1996; Nilson and DiGiano, 1996; Hong and Elimelech, 1997; Schäfer et al., 1998; Fan et al., 2001). No unambiguous correlation could be found between membrane fouling and single or combined feed water parameters for the tested membranes in statistical analysis.

Membrane foulants SEM images revealed that all the membranes were fouled with precipitates with sizes up to 50 µm. In addition to precipitates, flocculent dirt could be seen on all the membranes. The ratio of precipitates and flocculent material, precipitate size and precipitate appearance varied from membrane to membrane and from feed water to feed water. The shape of the precipitates ranged from blocks to star-shaped calcium crystals, grape-like clusters and donuts. Microbes were also found on the membranes fed with activated carbon filtered Tampere feed water. In many cases rinsing had removed part of the fouling layer, and clean and fouled membrane surfaces were seen on the same sample. Elementary analysis by SEM-EDS indicated that the membranes were fouled with aluminium, calcium, iron, sodium, magnesium and silica precipitates (Table 5). XPS analysis

Table 4 Feed water characteristics in the beginning of the test

Espoo Kotka Pietarsaari Raisio-Naantali Tampere Turku pH 5.7 6.8 9.3 7.1 7.6 7.9 Conductivity, mS/m 15.5 9.2 20.5 17.4 14.1 24.3 Hardness, mmol/l 0.49 0.33 0.77 0.58 0.55 0.91 TOC, mg/l 2.2 3.4 4.2 3.2 2.9 2.0 SUVA 2.0 1.7 1.6 1.5 1.4 1.1 Al, µg/l 92.5 42.1 26.2 22.9 38.9 11.0 Ca, mg/l 14.3 9.4 25.2 14.2 17.5 28.5 Fe, mg/l <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 K, mg/l 1.1 1.4 2.4 2.9 1.8 2.5 Mg, mg/l 1.4 1.3 2.4 4.5 1.9 3.9 Mn, µg/l 26.5 0.2 2.6 36.4 1.3 36.9 Na, mg/l 6.7 3.0 3.3 5.5 3.0 4.5 Si, mg/l 2.6 3.9 2.1 2.8 0.6 2.2 Cl, mg/l 10.5 4.0 15.0 19.1 4.3 19.2 F, mg/l 0.1 1.7 0.2 0.2 <0.1 <0.1

NO3, mg/l 1.5 3.3 1.5 6.1 0.7 5.1 SO , mg/l 41.2 8.2 40.4 15.3 24.0 41.2 4 187 Table 5 Analysis of the membrane fouling material

SEM-EDS SEM-EDS SEM-EDS XPS NF270 NF255 Desal 5 NF270

Espoo Al, Mg Al, Fe, Ca Al, Fe, Ca, K, Si O, P, Al, Fe, C-O, C-N Kotka Al, Ca Al Al, Fe, Ca, Mg O, S, Al, P, F, Ca, Si, C-O, C-N Pietarsaari Al, Fe, Ca, Si Fe, Ca Fe, Ca O, Al, P, F, Ca, C-O, C-N R. Liikanen Raisio-Naantali Al, Fe, Si Al Al, Ca O, Al, F,C-O, C-N Tampere Al, Fe, Si Al Al, Fe O, Al, P, F, Ca, C-O, C-N Turku Al, Fe, Si, K Al, Ca, Si Al, Fe, Ca, Si O, Al, P, F, Ca, Si, Fe, C-O, C-N et al.

revealed even more foulant components on the NF270 membranes (Table 5). The increase in oxygen and C-O and C-N bonds indicates the presence of organic fouling. No correlation was found between the feed water characteristics and the membrane foulants. The membrane foulants were not membrane specific either. FTIR spectroscopy analysis revealed that amide and polysaccharide were found on the fouled membranes. Differences were observed in the fouling layer characteristics with different feed waters, as can be seen from the FTIR spectra of the NF270 membrane in Figure 2. On the NF270 membrane Espoo, Kotka, Pietarsaari and Raisio-Naantali feed waters resulted in a peak indicating amide fouling, and Kotka, Pietarsaari, Raisio-Naantali and Turku in a peak indicating polysaccharide fouling. On the NF255 membrane the amide peak was seen with the Espoo, Kotka and Raisio-Naantali feed waters, and the polysaccharide peak was seen only with Espoo and Turku feed waters. The amide peak was seen with all the other feed waters besides Turku, and the polysaccharide peak with all the other feed waters besides Espoo on the Desal-5 membranes. Similar FTIR peaks were noticed on the NF200 membrane fouled by chemically pre-treated river water (Her et al., 2001), and also Cho et al. (1998), Speth et al. (2000) and Kaiya et al. (2000) found similar organic foulants on their NOM fouled membranes. Turku feed water resulted in most suppression of the FTIR peaks related to the membrane material of all the membranes, indicating that the Turku feed water caused most

foulant accumulation on the rinsed membranes. However, according to FCfouled values, Turku feed water did not result in most membrane fouling, thus indicating the importance of foulant layer characteristics on the flux decline. Denser fouling layers have been noticed to cause a higher flux decline than higher deposition of looser foulant material (Schäfer et al., 1998).

Change of membrane properties in pre-cleaning and fouling All membranes became more hydrophobic as the membranes were pre-cleaned: the CA was 39° for NF270, 41° for Desal-5 DL and 36° for NF255 after pre-cleaning. Fouling increased the membrane hydrophobicity further to values ranging from 37° to 51°. Consequently, fouling material can be expected to be mainly hydrophobic in nature. This is consistent with the suggestion that the hydrophobic fraction of organic matter generally fouls membranes more than the hydrophilic fraction (Nilson and DiGiano, 1996; Schäfer et al., 1998; Fan et al., 2001). Raisio feed water led to the most hydrophobic membrane surface. SEM pictures of the virgin and pre-cleaned membranes revealed that organic dirt and small precipitates had already accumulated on the membrane surfaces during pre-cleaning. According to SEM-EDS, the pre-cleaning did not change the elementary composition of the membranes, but XPS analysis on the NF270 membrane indicated that the pre-cleaning 188 removed sodium from the membrane surface. Turku

Tampere

Raisio-Naantali R. Liikanen

Pietarsaari et al.

Kotka

Espoo

Pre-cleaned Amide Amide Polysaccharide 950 1750 1650 1550 1450 1350 1250 1150 1050 Figure 2 FTIR spectra of the pre-cleaned and fouled NF270 membranes

Pre-cleaning improved the PWF from 16 to 40% (Table 2). The flux improvements are suggested to be due to membrane charge modifications in alkaline environments and the ability of the cleaning chemicals to remove some constituents from the membrane struc- ture, which makes the membranes more open.

Conclusions 1. Coagulation pre-treatment or moorland filtration (artificial groundwater) did not remove the surface water fouling capacity of NF membranes. However, the membrane fouling capacity of the different feed waters with the different membranes varied clearly. Consequently, the NF process should be tailored individually for each treatment plant. 2. NF performance was more related to membrane characteristics than to feed water, especially membrane morphology seemed to be important. 3. Aluminium, calcium and iron were the main inorganic foulants. All these metals are found in natural waters at the concentrations found in the studied feed waters. In addition, they are integral components of traditional water treatment chemicals. Amides and polysaccharides, the main organic foulants, can originate from several sources and they are also found in natural waters. We assume that the foulant layers found on the membranes cannot originate from the rather dilute and pure feed waters, but are mainly a consequence of biological growth in the feed waters during the test runs. As a conclusion, it is difficult, if not impossible, to remove the fouling components from the NF feed water to low enough levels. 4. No correlation was found between feed water characteristics and NF performance. Most feed water components are expected to interact in membrane fouling. 5. Pre-cleaning improved the membrane flux by opening the pores, even if the membrane hydrophobicity increased and fouling material accumulated on the membrane.

Acknowledgements We would like to express our gratitude to the personnel of the Laboratory of Membrane 189 Technology and Technical Polymer Chemistry for their assistance and advice during the experiments. We are also sincerely grateful to Mrs. Aino Peltola and the personnel of the water works for their assistance with the analysis. For the financial support of this study we acknowledge the National Technology Agency of Finland, Kemira Chemicals Ltd, Soil and Water Ltd, and the co-operating waterworks. We thank DOW Finland and Osmonics for providing the membranes. R. Liikanen

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