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Colloidal organic matter fouling of UF membranes: role of NOM composition & size
Desalination 220 (2008) 200–213
Colloidal organic matter fouling of UF membranes: role of NOM composition & size Maria D. Kennedya*, Joe Kamanyia, Bas G.J. Heijmanb,c, Gary Amya a
UNESCO-IHE, Institute for Water Education, P.O. Box 3015, 2601 DA Delft, The Netherlands email: [email protected] b Kiwa Water Research, Groningenhaven 7, 3430 BB Nieuwegein, The Netherlands c Delft University of Technology, The Netherlands Received 11 April 2007; accepted 15 May 2007
Abstract The aim of this study was to fractionate pre-filtered surface water using a 3.5 and a 10 kDa dialysis membrane, and to compare the rate of fouling and the fouling reversibility/irreversibility of the NOM fractions. Trial dialyses (3.5 and 10 kDa) were carried out for 6 and 21 days with pre-filtered surface water using synthetic surface water as dialysate. The aim of the trials was to optimize the dialysis process for NOM fractionation. DOC, Ca2+, Mg2+, soluble silica and bacteria were monitored at intervals during the dialysis process. Thereafter, the various NOM fractions (with low and high Ca2+) were fed to a miniature UF system operated at a constant flux of 138.5 L/m2 h, filtration cycle times of 31.5 min and backwash duration of 1.75 min. A PES/PSV hollow fiber UF membrane (MWCO 100 kDa) with a surface area of 0.0125 m2 was employed for the filtration tests (X-Flow). Transmembrane pressure (TMP) and UF feed and permeate (LC-OCD) were monitored at regular intervals. For a dialysate recirculation of 95 L/h, sample to dialysate ratio of 5.2:80 L and a dialysate change frequency of 3 times per 24 h, the shortest duration of dialysis was about 6–7 days for both 3.5 and 10 kDa dialyses membranes. The removal of organic carbon (OC) increased with dialysis duration and MWCO of the bags. The biopolymer fraction increased from 120% to 240% when the duration of dialysis was increased from 6 days (1.1 mg DOC/L, 151 mg Ca/L) to 21 days (0.82 mg DOC/L, 133 mg Ca/L) with the 10 kDa dialysis membrane. The increased biopolymer fraction in the NOM sample that was dialyzed for 21 days resulted in a doubling of the fouling rate from 3.5 to 6.6 mbar/min per mg DOC/L. The other NOM fractions (humics and building blocks) and the Ca/DOC ratio was more or less the same in both NOM samples suggesting that biopolymers were the major cause of UF fouling. Keywords: Colloidal organic matter; Dialysis; UF; Fouling; NOM fractionation
*Corresponding author. Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Society and Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007. 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.05.025
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
1. Introduction The main limitation of UF in drinking water treatment is membrane fouling developed over long-term operation. Undesirable consequences of fouling include membrane failure in the worst case or increase in energy costs a decline in water production. One remedy for fouling is membrane cleaning. Frequent chemical cleaning however leads to increased production downtime, energy usage and chemical consumption, waste generation, and reduction of membrane life. This translates into further costs increments. More so, if membrane cleaning is poorly done, the membranes may have to be replaced to maintain the water production, which further increases costs. Several studies have shown that natural organic matter (NOM) is a major UF membrane foulant, and that different components in NOM cause different forms of fouling [1–4]. Other studies have shown that increased concentration of Ca2+ and Mg2+ exacerbate NOM fouling [5–7]. Based on the analysis of the extracted solution during chemical cleaning, [8] found that the majority of soluble organic foulants were of low molecular weight, and Calcium was the major inorganic element in the foulants. These studies did not differentiate between reversible and irreversible fouling. Kimura et al. [9], however, differentiated between the fouling and concluded that neither aluminum nor calcium contributed to irreversible fouling. Although several researches have tried to identify the components of NOM that cause fouling of UF membranes, the results are confusing and conflicting. Weisner et al. (1992) identified four NOM categories that are strong foulants: proteins, amino-sugars, polysaccharides and polyoxyaromatics. Makdissy et al. [3] partially agreed and also showed that the organic colloidal fraction (bacterial peptidoglycan — bacterial cell wall residue rich in sugars and amino-sugars) caused the most significant fouling. However,
201
Cho and Amy [10] and Kimura et al. [9] identified polysaccharides as dominant foulants. Speth et al. [11] revealed that the NOM components of conventionally treated feed-water fouled membranes in the order of polyhydroxyaromatics, proteins, polysaccharides and amino-sugars. Jucker and Clark [4] concluded that humic acids (the largest and most hydrophobic fraction of the humic material) caused irreversible fouling while fulvic acids caused less fouling than humic acids, later on, Aoustin et al. [2] found that fulvic acids were responsible for reversible fouling of UFs via concentration polarization. In other studies, Nilson and DiGiano [12] reported most fouling was caused by hydrophobic NOM components. However, Carroll et al. [13] found neutral hydrophilic NOM components were the major foulants, while Cho et al. [10] and Fan et al. [14] reported NOM components as the major foulants in the order neutral hydrophilics > hydrophobic acids > transphilic acids > charged hydrophilics. Due to these conflicting opinions, it is still necessary to study further the causes of UF membrane fouling and to find a relationship between NOM composition in feed water and fouling. The aim of this study is to investigate the fouling due to various NOM fractions in the presence of Ca2+/Mg2+. 2. Methods 2.1. Sample water The sample water was surface water from Westvest canal near UNESCO-IHE in Delft (Fig. 1). A portion of the unfiltered sample was taken for the determination of pH, conductivity/ temperature, turbidity, total suspended solid (TSS) and total organic carbon (TOC). The remaining sample was pre-filtered with 0.45 μm pore size disc membrane filters and preserved in the dark at a temperature of 4°C. After pre-filtration, a portion of the filtered sample was taken for the determination of pH,
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M.D. Kennedy et al. / Desalination 220 (2008) 200–213
Fig. 1. Sample point — Westvest canal near UNESCO-IHE, Delft.
conductivity/temperature, turbidity, dissolved organic carbon (DOC), monomeric/soluble silica − (SiO2), Ca2+/Mg2+, Na+/K+, HCO3− and SO2− 4 /Cl and finally a LC-OCD analysis was performed. These ions are the most common in natural water, according to Solt and Shirley (1991).
(Fig. 3). M5 contains glycerine for pore protection and bisulfite for prevention of microbiological growth. The characteristics of this hydrophilic membrane module are shown in Table 1.
2.2. Pre-filtration and dialysis membranes
3.1. Equipment for characterization of water samples
Pre-filtration was done using 0.45 μm flat disc membrane filters (47 mm, diameter) made of cellulose acetate. For separation of colloidal and non-colloidal NOM, dialysis membrane bags were used. These were Spectra/Por Regenerated Cellulose Membranes (Fig. 2) — specifically the Spectra/Por 3 Ready-to-Use Dialysis Sacks (MWCO 3.5 kDa) — and Spectra/Por 6 Dialysis Membranes (MWCO 10 kDa) from Spectrum Laboratories. 2.3. Hollow-fibre pen modules The hollow fibre filter pen module (UFC M5) manufactured by X-Flow (Norit) and supplied by Filtrix in Utrecht — The Netherlands was used
3. Equipment
3.1.1. Dissolved organic carbon/total organic carbon The dissolved organic carbon (DOC) and total organic carbon (TOC) were measured using a total organic carbon (TOC) analyzer (O × I Corporation Model 700). For DOC, the samples were pre-filtered with cleaned cellulose acetate 0.45 μm filters. 3.1.2. pH, turbidity, conductivity/ temperature The pH of samples was measured using a Metrohm 691 pH-meter while the conductivity
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
203
Fig. 3. X-Flow UFC M5 hollow fibre pen module.
probe measures the trans-membrane pressures and sends the signals to a computer that records the data (every 20 to 22 sec) directly in a Microsoft Excel worksheet commanded by a Microsoft Visual Basic program.
Fig. 2. Spectra/Por 3 Ready-to-Use Dialysis Sacks (MWCO 3.5 kDa).
Table 1 Characteristics of the X-Flow UFC M5 hollow fibre pen module
and temperature were measured with a WTW cond 330i conductivity meter. Turbidity was measured using a DrLange Trubüngsphotometer LTP 4 turbidity meter.
Property
Value
Supplier
Filtrix, Utrecht, The Netherlands PP and PE Epoxy 200 mm 24 mm 50 Polyethersulfone/ polyvinylpyrrolidone 0.8 mm
3.1.3. Filtration equipment The filtration equipment was a UF dead-end mini system (Fig. 4) consisting of a syringe pump, a pen UF membrane module, a pressure probe, a 4-way valve and an air purge valve. The equipment operates at constant flux with a piston pump that works with two syringes, one sucking water from the feed container and the other one pushing with a piston sample at constant flow rate into the membrane. The pressure
Material (plastic parts) Potting material Overall length Outside diameter Number of hollow fibres Membrane material Internal hollow fibre diameter Membrane surface area — complete membrane Molecular weight cut-off (on dextrans) Filtration mode
0.00025 m2 × 50 = 0.0125 m2 100 kDa Inside to outside
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M.D. Kennedy et al. / Desalination 220 (2008) 200–213
Fig. 4. Mini UF equipment consisting of pump, UF pen module, pressure probe, 2-way valve, air valve and computer for logging trans-membrane pressures.
after 6 days of dialysis and after 21 days of dialysis. They were analyzed and the results are shown in Fig. 5. From Fig. 5 (Left), it can be seen that the DOC reduced from about 15.5 mg/L to about 2 mg/L (87% reduction) for the 10 kDa dialysis, and to about 4.5 mg/L (71% reduction) for the 3.5 kDa dialysis in a period of 7 days. Beyond this period, the rate of DOC reduction was significantly lowered due to a reduced concentration
4. Results 4.1. Optimisation of dialysis — final dialyses (3.5 and 10 kDa)
DOC (mg/L)
To determine the optimum duration of dialysis for the production of 2.6 L of colloidal NOM, 21 day dialysis tests were performed. The dialysate was changed 3 times in a day. Samples (including LC-OCD samples) of colloidal NOM were taken before dialysis, during the dialyses, 16 14 12 10 8 6 4 2 0
DOC (mg/L)
Final dialysis 1 (3.5 kDa, 21 day) Final dialysis 2 (10 kDa, 21 day)
0
2
4
6
8
10 12 14 Time (days)
16
18
20
Before dialysis
15.5
% of initial DOC removed
0%
After 3.5 kDa, 21-day dialysis
2.8
% of initial DOC removed
81.9%
After 10 kDa, 21-day dialysis
1.6
% of initial DOC removed
89.7%
22
Fig. 5. Variation of DOC during 21-day final dialyses against synthetic surface water.
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
gradient between the sample in the dialysis bags and dialysate. By the end of the 21 days, about 82% and 90% of the initial DOC was removed in the 3.5 and 10 kDa dialysis bags respectively (Fig. 5 — Right). Therefore, for a sample volume of 5.2 L per 80 L of dialysate, a dialysate recirculation of 95 L/h, a dialysate change frequency of 3 times per 24 h, and for the aquarium construction, the optimum dialysis duration was found to be about 7 days for both the 3.5 and 10 kDa MWCO.
4.2. Effect of dialysis on NOM composition of pre-filtered canal water Dialysis results showed a general increase in organic carbon (OC) removed (or accumulation) as the dialysis duration increased. The OC removal of DOC increased from 71% to 82% of the initial value when the dialysis (3.5 kDa) duration was raised from 6 to 21 days (Fig. 7). Removal of humics, building blocks, neutrals and hydrophobics also increased with an increase in dialysis duration as shown in Figs. 6, 7 and Table 2. An increase in dialysis duration allowed more time for more of the NOM to diffuse out or adsorb onto the bags. It should be noted that the amount of
OC relative signal response
NOM composition of the various isolates 20
Humics Building blocks
15
LMW acids and humics 10 Biopolymers
5
LMW neutrals
0 0
30
60 Retention time (min)
90
120
UF1 feed pre-filtered canal water (No dialysis) UF3 feed pre-filtered canal water after 6-day dialysis (3.5 kDa) UF7 feed pre-filtered canal water after 21-day dialysis (3.5 kDa) UF5 feed pre-filtered canal water after 6-day dialysis (10 kDa) UF9 feed pre-filtered canal water after 21-day dialysis (10 kDa)
Fig. 6. Variation of NOM composition of the prefiltered canal water during dialysis.
205
hydrophobics was not measured directly. It was estimated as the difference between the chromatographic (or hydrophilic) DOC and the total DOC. The hydrophobic part is retained by the column and is therefore not shown on the chromatographs. The OC removal also increased with an increase in MWCO of the dialysis bags (Fig. 6). For 6-day dialysis, OC removal increased as follows: DOC – 71% to 92%, hydrophobics – 91% to 93%, humics – 67% to 95%, building blocks – 85% to 96% and neutrals – 77% to 92%, when the MWCO of the dialysis bags was increased from 3.5 to 10 kDa respectively (Fig. 7 and Table 2). Increases were also shown for the 21-day dialysis. The increase in OC removal with increase in MWCO was probably due to the fact that larger MWCO (10 kDa) bags allowed more of the larger sized NOM to diffuse out of the bags compared to the smaller MWCO (3.5 kDa) bags. Hydrophobic NOM and building blocks were best removed, averaging about 92% of their initial respective concentrations. An average of 84% of the hydrophilic NOM was removed by dialysis. Among the hydrophilic NOM, Fig. 7 and Table 2 show that OC removal was generally in the order building blocks > neutrals > humics > biopolymers. Fig. 7 and Table 2 also show that there was no significant difference in the removal of hydrophobics for the different MWCOs and dialysis durations. The increase in removal of OC with increase in dialysis duration and MWCO was lowest for the hydrophobics. The maximum increase in removal of hydrophobics was only 91% to 93%, which occurred when the MWCO increased from 3.5 to 10 kDa for 6 days of dialysis, and also when the dialysis duration was increased from 6 to 21 days using 3.5 kDa dialysis membranes. This however was not the same for the other NOM components, which suggests that the mechanism of removal of hydrophobics could be different from that of the other NOM components, and that size exclusion was not the main mechanism of removal for hydrophobic NOM.
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
93 93
92
94
100
93
93
93
92
88 100
77
75
85
91 71
71
92
92
86
82
82
96
95
67
–20
50 25 0
–17
–25
–123
–50 –75 –100 –125 –150 –175 –200 –225
21 days, 10 kDa 6 days, 10 kDa
–214
Neutrals
Humics
Biopolymers
Hydrophilics
Hydrophobics
DOC
6 days, 3.5 kDa
Building blocks
21 days, 3.5 kDa
OC removal (% of initial NOM component concentration)
206
NOM component 6 days, 3.5 kDa
21 days, 3.5 kDa
6 days, 10 kDa
21 days, 10 kDa
Fig. 7. Removals of the various NOM components by dialysis.
They probably adsorbed onto the walls of the regenerated cellulose (dialysis) membrane, and therefore the MWCO was probably not as important (for hydrophobics) as the material from which the bags were made. However, not all NOM components were removed during dialysis. Biopolymers were not removed at all. In fact they increased by 17% and 20% in the 6-day dialysis, and by 214% and 123% in the 21-day dialysis using 3.5 and 10 kDa MWCO respectively (Fig. 7 and Table 2). In addition, there was an increase in bacteria during dialysis. The increase in biopolymers was probably due to bacterial activity, and the release of extra-cellular polymeric substances (EPS) by the bacteria. In aquatic environments, many microorganisms (e.g. bacteria) secrete EPS
(a biopolymer consisting of proteins, polysaccharides and amino-sugars) into the surrounding environment during the endogenous phase of their growth (Underwood et al., 1995). During the endogenous growth phase, the bacteria death rate is higher than reproduction rate, the substrate is very limited and microorganisms metabolize their own protoplasm. Also old cells die and lyse, and nutrients (e.g. proteins) are released into surrounding environment (Citadel, 2006). According to Leenheer et al. (2000), organic colloids may represent up to 20% to 30% of the DOC of surface waters, are hydrophilic in character, and consist mostly of bacterial cell wall residues (mucopolysaccharide type structure-EPS). It is therefore logical that an increase in the bacterial count in the dialysis bags most likely
Average
21 days
10 kDa 6 days
21 days
3.5 kDa 6 days
OC removed (ppb) % of initial OC removed
OC removed (ppb) % of initial OC removed OC removed (ppb) % of initial OC removed
OC removed (ppb) % of initial OC removed OC removed (ppb) % of initial OC removed
– –
12,148 92.2% 12,236 92.8%
9418 71.4% 10,788 81.8%
13,182 0%
Before dialysis
Initial OC (ppb) % of initial OC removed
DOC
Description
Table 2 OC removed before and after the various dialyses
10,864 84.4%
11,863 92.1% 11,950 92.8%
9139 71.0% 10,504 81.6%
12,875 0%
Hydrophilics
283 92.3%
284 92.5% 286 93.2%
279 90.9% 284 92.5%
307 0%
Hydrophobics
– –
−52 −20.2% −317 −123.3%
−44 −17.1% −549 −213.6%
257 0%
Biopolymers
4570 87.1%
7004 95.2% 0 100%
4946 67.2% 6328 86.0%
7357 0%
Humics
2055 91.9%
2140 95.7% 2107 94.3%
1910 85.5% 2061 92.2%
2235 0%
Building blocks
2647 87.4%
2789 92.1% 2806 92.7%
2329 76.9% 2665 88.0%
3027 0%
Neutrals
– –
– – – –
– – – –
– –
Acids
M.D. Kennedy et al. / Desalination 220 (2008) 200–213 207
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M.D. Kennedy et al. / Desalination 220 (2008) 200–213
resulted in an increase in EPS, and therefore an increase in biopolymers. The regenerated cellulose membrane was probably a substrate for the bacteria. Some strains of pseudoalteromonas bacteria isolates have been found to produce unusually large polymers (EPS) up to 5700 kDa MWCO (Nichols et al., 2004). Fig. 7 and Table 2 show a higher increase of biopolymers in the 21-day dialysis (214%) than in the 6-day dialysis (17%) using 3.5 kDa membranes. Another increase of biopolymers was observed when 10 kDa membranes were used (20% for 6-day and 123% for 21-day dialysis). The possible reason is that the higher cumulative count of bacteria after 21 days (570,000 No./mL) than after 6 days (450,000 No./mL) of dialysis resulted in a higher amount of EPS released after 21 days of dialysis than after 6 days of dialysis. However, for dialysis duration of 21 days, dialysis with 10 kDa membranes resulted in a lower increase in biopolymers than dialysis with 3.5 kDa membranes (i.e. 123% for 10 kDa and 214% for 3.5 kDa). This was due to the size difference in the membranes used. Some of the EPS (or biopolymers) could have passed through the pores of the 10 kDa membrane, resulting into lower retention of biopolymers by the 10 kDa than the 3.5 kDa membranes. Another possible explanation for the strange behaviour of the biopolymers could be the difference in the pH and electrical conductivity (EC). The pH increased from 7.9 to 8.4 while the EC reduced from 1708 µS/cm to 1672 µS/cm when the dialysis duration increased from 6 days to 21 days (Table 3). This could have caused the agglomeration or shift of humics, building blocks and neutrals to larger apparent molecular sizes. A similar observation was reported by Braghetta et al. (1997) who found that pH and ionic strength affected the molecular size distribution of NOM. Lower pH and higher ionic strength caused Suwanee River organic matter (SROM) macromolecules to shift to a smaller apparent molecular size, and vice versa. Organic molecules are most
Table 3 Comparing pH and EC of the pre-filtered canal water with colloidal NOM after 6-day and 21-day dialyses using 3.5 and 10 kDa MWCO dialysis membranes Description
pH
EC (μS/cm)
No dialysis 3.5 kDa 6 days 21 days
8.1
1304
7.9 8.4
1708 1672
8.0 8.2
1695 1643
10 kDa 6 days 21 days
often negatively charged in the aqueous solution at neutral to high pH, an effect that is usually attributed to an abundance of ionised carboxylic (–COOH) and phenolic (–OH) functional groups (Braghetta et al., 1997). Gosh and Schnitzer (1980) proposed two hypothetical end points for NOM macromolecular configurations: (1) a flexible linear macromolecule (apparently larger) at very low ionic strength, high pH and low solution concentration, when intramolecular charge repulsion is high and repulsive and, (2) a rigid compact (apparently smaller), spherocolloidal macromolecule at high ionic strength, low pH, and high solution concentration, when intramolecular charge shielding acts to neutralise functional groups. Several other researchers reported increases in size of NOM molecules with decreasing ionic strength and increasing pH [4] (Shaw et al., 1994; Xi et al., 2002). Larger molecules were also observed in the 10 kDa dialysis, and may be attributed to the same reasons outlined above (pH and EC differences). However, due to the larger MWCO (10 versus 3.5 kDa), more of these large molecules diffused out of the dialysis bags in the 10 kDa case than in the 3.5 kDa case. This resulted in a lower biopolymer peak in the 10 kDa than in the 3.5 kDa dialysis. Table 4 shows the amounts of inorganic colloids before and after the dialyses. The level of
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
209
Table 4 Level of inorganic colloids compared to humic colloids before and after the dialyses Sample description
Inorganic colloids UVA254 (m−1)
% Removal of inorganic colloids
SUVA (L/mg*m)
DOC (mg/L)
Organic colloids UVA254 (m−1) (calculated)
Pre-filtered canal water After 6-day dialysis (3.5 kDa) After 6-day dialysis (10 kDa) After 2-day dialysis (3.5 kDa) After 2-day dialysis (10 kDa)
0.12 0.05 0.06 0.05 0.02
– 58.3% 50.0% 58.3% 83.0%
3.03 3.38 1.54 1.72 0.74
13.182 3.764 1.034 2.394 0.946
39.94 12.72 1.59 4.12 0.70
inorganic colloids in all the samples before and after dialysis was much lower than the level of organic colloids. For example, the UVA254 of inorganic colloids in the pre-filtered canal water was 0.12 m−1, which is much lower than the UVA254 of about 40 m−1 for the organic colloids. This indicates that the majority of the colloids were organic in nature. There was a clear reduction of inorganic colloids inside the bags in all the dialyses. About 50%–58% and 58%–83% of inorganic colloids were removed after 6 and 21 days of dialysis. The highest removal of 83% occurred after 21 days of dialysis using 10 kDa bags. These could have escaped along with the NOM during dialysis. The highest reduction is observed for the 21-day 10 kDa dialysis. This is consistent with the highest DOC reduction observed for the same 21-day 10 kDa dialysis experiment. 4.3. Ultrafiltration experiments Different feed waters were used in the UF experiments. For each UF experiment, a new membrane was used. The purposes of the UF experiments were: (1) To compare the backwashable, non-backwashable and total fouling of the various NOM fractions obtained from the dialysis process, (2) To identify the NOM components responsible for backwashable and non-backwashable fouling and, (3) To determine the effect of Ca2+ on backwashable, non-backwashable and total fouling for the various NOM fractions.
4.4. Effect of NOM size on total fouling rate Tables 2, 5 and Fig. 8 show the various UF feeds, their NOM composition and the resulting fouling behaviour when fed to UF membranes. Each of the NOM fractions exhibited different characteristics in total fouling of UF membranes due to the different size, composition and environment. The colloidal NOM content of the 3.5 kDa dialysis bags after 21 days of dialysis (i.e. 2.2 mg DOC/L). Pre-filtered canal water contained a total NOM of 7.3 mg DOC/L, out of which about 2.2 mg/L (about 30%) was therefore assumed to be colloidal NOM. The total fouling rate curves (Fig. 8 — Left) were obtained by plotting the slope of each of filtration cycles in Fig. 8 (Right) against the filtration cycle number. From Fig. 8 — Left, the total fouling rate generally increased with an increase in duration of dialysis. This was probably due to increased percentage of large sized NOM in the NOM fractions as dialysis proceeded. By comparing the fouling results for UF experiments UF1 (pre-filtered, 7.3 mg DOC/L, 142 mg Ca/L), UF3 (6 d, 3.5 kDa, 2.6 mg DOC/L, 145 mg Ca/L) and UF7 (21 d, 3.5 kDa, 2.2 mg DOC/L, 132 mg Ca/L), it can be seen that when biopolymers increased by 17.1%, while all the other NOM components decreased, and while the Ca/DOC ratio increased from 11 to 40, the average fouling rate was more than doubled (i.e. 0.4 to 1 mbar/min per mg DOC/L). This suggests that Ca2+ and
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M.D. Kennedy et al. / Desalination 220 (2008) 200–213 UFs (1, 3, 5, 7, 9) Effect of NOM size on fouling rates (mbar/min per mg DOC/L) during each cycle – UF experiments with high Ca
0.6
8 Normalised TMP per unit DOC concentration (bars per mg DOC/L)
0.5
Fouling rate (mbar/min per mg Doc/L)
7 6 5 4 3 2
0.4
0.3
0.2
0.1
1 0 1
2
3
4 5 6 7 Filtration cycle no.
8
9
10
UF1 (Prefiltered, 7.3 mg DOC/L, 142 mg Ca/L) UF3 (6 d, 3.5 kDa, 2.6 mg DOC/L, 145 mg Ca/L) UF5 (6 d, 10 kDa, 1.1 mg DOC/L, 151 mg Ca/L) UF7 (21 d, 3.5 kDa, 2.2 mg DOC/L, 132 mg Ca/L) UF9 (21 d, 10 kDa, 0.82 mg DOC/L, 133 mg Ca/L)
0 0
30
60
90
120
150 180 210 Time (min)
240
270
300
330
UF9 (21 d,10 kDa, DOC = 0.82 mg/L, Ca = 133 mg/L) UF5 (6 d,10 kDa, DOC = 1.1 mg/L, Ca = 151 mg/L) UF1 (Prefiltered, DOC = 7.3 mg/L, Ca = 142 mg/L) UF7 (21 d, 3.5 kDa, DOC = 2.2 mg/L, Ca = 132 mg/L) UF3 (6 d, 3.5 kDa, DOC = 2.6 mg/L, Ca = 145 mg/L)
Fig. 8. Right – Normalised TMP vs time for the various UF feeds; Left – Total fouling rates for the various UF feeds of different NOM size compositions (high Ca2+/Mg2+ with HB NOM).
biopolymers played an important role in total fouling. In addition, a further 200% increase in biopolymers from 117% to 314% of the initial value, and a small increase in Ca/DOC ratio from 40 to 55 tripled the fouling rate, suggesting that biopolymers were probably the main cause of total fouling. It should be noted that the Ca2+ concentration in the NOM fractions was kept as close as possible to the Ca2+ concentration in the pre-filtered canal water. Increase in total fouling rate with increase in dialysis duration could also be attributed to increased bacterial activity that led to increased degradation of the regenerated cellulose dialysis bags and production of biopolymers. Another large increase in biopolymers — UF5 (6 d, 10 kDa, 1.1 mg DOC/L, 151 mg Ca/L)-120% to UF9-223% — in the absence of humics, and a negligible increase in building blocks (4.3% to 5.7%) resulted in almost double the fouling rate from 3.5 to 6.6 mbar/min per mg DOC/L. The Ca/DOC ratio only reduced slightly from 146 to 141 during UF5 and UF9
respectively. This suggests that biopolymers were the major cause of total fouling. Similar results were reported by Fan et al. [14] who concluded that the large MW fraction of NOM (>30 kDa — most likely in colloidal state) was responsible for most of the flux decline. Fan et al. [14] and Lahoussine et al. (1990) seemed to agree when they concluded that higher SUVA NOM contained a greater amount of colloidal NOM molecules (i.e. >30 kDa) and caused greater fouling than lower SUVA NOM. Schäfer et al. (2000) found that fouling was independent of the membrane pore size and was caused by large colloids (0.25 μm) in MF and small colloids in UF. Jarusutthirak et al. (2002) found polysaccharides and/or amino-sugars from colloids in wastewater effluent to play an important role in fouling of UF membranes. Leenheer et al. (2000) demonstrated that the organic colloids may represent up to 20%–30% of the DOC or surface waters, and are hydrophilic in character, and mostly of bacterial cell wall residues (mucopolysaccharide type structure). Cho et al. [15] found that large
21 days (UF9)
10 kDa 6 days (UF5)
21 days (UF7)
3.5 kDa 6 days (UF3)
ppb % ppb %
ppb % ppb % 1034 7.8% 946 7.2%
3764 28.6% 2394 18.2%
13,182 100%
Pre-filtered canal water (No dialysis) - UF1
ppb %
DOC
Description
1012 7.9% 925 7.2%
3736 29.0% 2371 18.4%
12,875 100%
Hydrophilics
23 7.5% 21 6.8%
28 9.1% 23 7.5%
307 100%
309 120.2% 574 223.3%
301 117.1% 806 313.6%
257 100%
Hydrophobics Biopolymers
353 4.8% 0 0.0%
2411 32.8% 1029 14.0%
7357 100%
Humics
Table 5 NOM composition of the various UF feeds (UF1, 3, 5, 7, 9) and resulting average total fouling rates
95 4.3% 128 5.7%
325 14.5% 174 7.8%
2235 100%
Building blocks
238 7.9% 221 7.3%
698 23.1% 362 12.0%
3027 100%
Neutrals
141
146
55
40
11
Ca/DOC ratio
6.6
3.5
3.0
1.0
0.4
Average total fouling rate (mbar/min per mg DOC/L)
M.D. Kennedy et al. / Desalination 220 (2008) 200–213 211
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M.D. Kennedy et al. / Desalination 220 (2008) 200–213
the absence of humics and a negligible increase in building blocks (4.3% to 5.7%), resulted in almost double the fouling rate from 3.5 to 6.6 mbar/min per mg DOC/L. The Ca/DOC ratio only reduced slightly from 146 to 141 during UF5 and UF9 respectively. This suggested that the biopolymer fraction was a major cause of fouling in the filtration tests performed with colloidal NOM solutions.
sized NOM components such as amino acids, polypetides, proteins, colloidal NOM, and not the smaller sized humic substances were efficiently rejected by UF membranes. 5. Conclusions (1) The OC removal increased with an increase in MWCO (3.5 kDa to 10 kDa) of dialysis bags. For 6 day dialysis, OC removals increased as follows: DOC – 71% to 92%, hydrophobics – 91% to 93%, humics – 67% to 95%, building blocks – 85% to 96% and neutrals – 77% to 92%, when the MWCO of the dialysis bags was increased from 3.5 kDa to 10 kDa respectively. Similar trends were shown for the 21-day dialyses. (2) The OC removal increased with an increase in duration of dialysis (6 to 21 days). For the 3.5 kDa MWCO dialysis, OC removals increased as follows: DOC – 71% to 82%, hydrophobics – 91% to 93%, humics – 67% to 86%, building blocks – 85% to 92% and neutrals – 77% to 88%, when the dialysis duration was increased from 6 to 21 days respectively. Increases were also shown for the 10 kDa MWCO dialysis. (3) Among the hydrophilic NOM, building blocks were removed the most (average 92%) by dialysis, followed by neutrals (average 88%) and then humics (average 87%). Biopolymers significantly increased (17% and 20% using 3.5 and 10 kDa MWCO bags respectively) after 6 days of dialysis, and most especially after 21 days of dialysis (123% and 214% using 10 and 3.5 kDa MWCO bags respectively). This increase in biopolymers was attributed to increase in EPS released by the bacteria during the dialysis. (4) The large increase in biopolymers from 120% in UF5 (6 d, 10 kDa, 1.1 mg DOC/L, 151 mg Ca/L)-120% to 223% in UF9 (21 d, 10 kDa, 0.82 mg DOC/L, 133 mg Ca/L), in
References [1]
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[3]
[4]
[5]
[6]
[7]
[8]
[9]
N. Lee, G. Amy, H. Habarou and J.C. Schrotter, Identification and control of fouling of low-pressure (MF and UF) membranes by drinking-water natural organic matter, Water Sci. Technol. Water Supp., 3 (5–6) (2003) 217–222. E. Aoustin, A.I. Schafer, A.G. Fane and T.D. Waite, Ultrafiltration of natural organic matter, Sep. Purif. Technol., 22–23 (1–3) (2001) 63–78. G. Makdissy, J.P. Croue, H. Buisson, G. Amy and B. Legube, Organic matter fouling of ultrafiltrafiltration membranes, Water Sci. Technol. Water Supp., 3 (5–6) (2003) 175–182. C. Jucker and M.M. Clark, Adsorption of aquatic substances on hydrophobic ultrafiltration membranes, J. Membr. Sci., 97 (1994) 37–52. S. Hong and M. Elimelech, Chemical and physical aspects of natural organic matter (NOM) fouling of NF membranes, J. Membr. Sci., 132 (1997) 159–181. V.A. Yangali-Quintanilla, Colloidal and noncolloidal NOM fouling of ultrafiltration membranes: analysis of membrane fouling and cleaning, M.Sc. Thesis SE 05–013, UNESCO-IHE, Delft, The Netherlands, 2005. S. Lee, J. Cho and M. Elimelech, Combined influence of natural organic matter and colloidal properties on nanofiltration membrane fouling, J. Membr. Sci., in press. L. Mo and X. Huang, Fouling characteristics and cleaning strategies in a coagulation-microfiltration combination process for water purification, Desalination, 159 (1) (2003) 1–9. K. Kimura, Y. Hane, Y. Watanabe, G. Amy and N. Ohkuma, Irreversible membrane fouling
M.D. Kennedy et al. / Desalination 220 (2008) 200–213 during ultrafiltration of surface water, Water Res., 38 (2004) 3431–3441. [10] J. Cho and G. Amy, Interactions between natural organic matter (NOM) and membranes: rejection and fouling, Water Sci. Technol., 40 (9) (1999) 131–139. [11] T.F. Speth, R.S. Summers and A.M. Gusses, Evaluating of membrane foulants from conventionally-treated drinking waters, Proceedings of Natural Organic Matter Workshop, Poilers, France, 1996, p. 44. [12] J. Nilson and F.A. Di Giano, Influence of NOM composition on nanofiltration, JAWWA, 88 (1996) 53–66.
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[13] T. Carroll, S. King, S.R. Gray, B.A. Bolto and N.A. Booker, Fouling of microfiltration membranes by NOM after coagulation treatment, Water Res., 34 (2000) 2861–2868. [14] L. Fan, J.L. Harris, F.A. Roddick and N.A. Booker, Influence of the characteristics of natural organic matter on the fouling of microfiltration membranes, Water Res., 35 (18) (2001) 4455–4463. [15] J. Cho, G. Amy and J. Pallegrino, Membrane filtration of natural organic matter: factors and mechanisms affecting rejection and flux decline with charged ultrafiltration (UF) membrane, J. Membr. Sci., 164 (2000) 89–110.
Colloidal organic matter fouling of UF membranes: role of NOM composition & size Maria D. Kennedya*, Joe Kamanyia, Bas G.J. Heijmanb,c, Gary Amya a
UNESCO-IHE, Institute for Water Education, P.O. Box 3015, 2601 DA Delft, The Netherlands email: [email protected] b Kiwa Water Research, Groningenhaven 7, 3430 BB Nieuwegein, The Netherlands c Delft University of Technology, The Netherlands Received 11 April 2007; accepted 15 May 2007
Abstract The aim of this study was to fractionate pre-filtered surface water using a 3.5 and a 10 kDa dialysis membrane, and to compare the rate of fouling and the fouling reversibility/irreversibility of the NOM fractions. Trial dialyses (3.5 and 10 kDa) were carried out for 6 and 21 days with pre-filtered surface water using synthetic surface water as dialysate. The aim of the trials was to optimize the dialysis process for NOM fractionation. DOC, Ca2+, Mg2+, soluble silica and bacteria were monitored at intervals during the dialysis process. Thereafter, the various NOM fractions (with low and high Ca2+) were fed to a miniature UF system operated at a constant flux of 138.5 L/m2 h, filtration cycle times of 31.5 min and backwash duration of 1.75 min. A PES/PSV hollow fiber UF membrane (MWCO 100 kDa) with a surface area of 0.0125 m2 was employed for the filtration tests (X-Flow). Transmembrane pressure (TMP) and UF feed and permeate (LC-OCD) were monitored at regular intervals. For a dialysate recirculation of 95 L/h, sample to dialysate ratio of 5.2:80 L and a dialysate change frequency of 3 times per 24 h, the shortest duration of dialysis was about 6–7 days for both 3.5 and 10 kDa dialyses membranes. The removal of organic carbon (OC) increased with dialysis duration and MWCO of the bags. The biopolymer fraction increased from 120% to 240% when the duration of dialysis was increased from 6 days (1.1 mg DOC/L, 151 mg Ca/L) to 21 days (0.82 mg DOC/L, 133 mg Ca/L) with the 10 kDa dialysis membrane. The increased biopolymer fraction in the NOM sample that was dialyzed for 21 days resulted in a doubling of the fouling rate from 3.5 to 6.6 mbar/min per mg DOC/L. The other NOM fractions (humics and building blocks) and the Ca/DOC ratio was more or less the same in both NOM samples suggesting that biopolymers were the major cause of UF fouling. Keywords: Colloidal organic matter; Dialysis; UF; Fouling; NOM fractionation
*Corresponding author. Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Society and Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007. 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.05.025
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
1. Introduction The main limitation of UF in drinking water treatment is membrane fouling developed over long-term operation. Undesirable consequences of fouling include membrane failure in the worst case or increase in energy costs a decline in water production. One remedy for fouling is membrane cleaning. Frequent chemical cleaning however leads to increased production downtime, energy usage and chemical consumption, waste generation, and reduction of membrane life. This translates into further costs increments. More so, if membrane cleaning is poorly done, the membranes may have to be replaced to maintain the water production, which further increases costs. Several studies have shown that natural organic matter (NOM) is a major UF membrane foulant, and that different components in NOM cause different forms of fouling [1–4]. Other studies have shown that increased concentration of Ca2+ and Mg2+ exacerbate NOM fouling [5–7]. Based on the analysis of the extracted solution during chemical cleaning, [8] found that the majority of soluble organic foulants were of low molecular weight, and Calcium was the major inorganic element in the foulants. These studies did not differentiate between reversible and irreversible fouling. Kimura et al. [9], however, differentiated between the fouling and concluded that neither aluminum nor calcium contributed to irreversible fouling. Although several researches have tried to identify the components of NOM that cause fouling of UF membranes, the results are confusing and conflicting. Weisner et al. (1992) identified four NOM categories that are strong foulants: proteins, amino-sugars, polysaccharides and polyoxyaromatics. Makdissy et al. [3] partially agreed and also showed that the organic colloidal fraction (bacterial peptidoglycan — bacterial cell wall residue rich in sugars and amino-sugars) caused the most significant fouling. However,
201
Cho and Amy [10] and Kimura et al. [9] identified polysaccharides as dominant foulants. Speth et al. [11] revealed that the NOM components of conventionally treated feed-water fouled membranes in the order of polyhydroxyaromatics, proteins, polysaccharides and amino-sugars. Jucker and Clark [4] concluded that humic acids (the largest and most hydrophobic fraction of the humic material) caused irreversible fouling while fulvic acids caused less fouling than humic acids, later on, Aoustin et al. [2] found that fulvic acids were responsible for reversible fouling of UFs via concentration polarization. In other studies, Nilson and DiGiano [12] reported most fouling was caused by hydrophobic NOM components. However, Carroll et al. [13] found neutral hydrophilic NOM components were the major foulants, while Cho et al. [10] and Fan et al. [14] reported NOM components as the major foulants in the order neutral hydrophilics > hydrophobic acids > transphilic acids > charged hydrophilics. Due to these conflicting opinions, it is still necessary to study further the causes of UF membrane fouling and to find a relationship between NOM composition in feed water and fouling. The aim of this study is to investigate the fouling due to various NOM fractions in the presence of Ca2+/Mg2+. 2. Methods 2.1. Sample water The sample water was surface water from Westvest canal near UNESCO-IHE in Delft (Fig. 1). A portion of the unfiltered sample was taken for the determination of pH, conductivity/ temperature, turbidity, total suspended solid (TSS) and total organic carbon (TOC). The remaining sample was pre-filtered with 0.45 μm pore size disc membrane filters and preserved in the dark at a temperature of 4°C. After pre-filtration, a portion of the filtered sample was taken for the determination of pH,
202
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
Fig. 1. Sample point — Westvest canal near UNESCO-IHE, Delft.
conductivity/temperature, turbidity, dissolved organic carbon (DOC), monomeric/soluble silica − (SiO2), Ca2+/Mg2+, Na+/K+, HCO3− and SO2− 4 /Cl and finally a LC-OCD analysis was performed. These ions are the most common in natural water, according to Solt and Shirley (1991).
(Fig. 3). M5 contains glycerine for pore protection and bisulfite for prevention of microbiological growth. The characteristics of this hydrophilic membrane module are shown in Table 1.
2.2. Pre-filtration and dialysis membranes
3.1. Equipment for characterization of water samples
Pre-filtration was done using 0.45 μm flat disc membrane filters (47 mm, diameter) made of cellulose acetate. For separation of colloidal and non-colloidal NOM, dialysis membrane bags were used. These were Spectra/Por Regenerated Cellulose Membranes (Fig. 2) — specifically the Spectra/Por 3 Ready-to-Use Dialysis Sacks (MWCO 3.5 kDa) — and Spectra/Por 6 Dialysis Membranes (MWCO 10 kDa) from Spectrum Laboratories. 2.3. Hollow-fibre pen modules The hollow fibre filter pen module (UFC M5) manufactured by X-Flow (Norit) and supplied by Filtrix in Utrecht — The Netherlands was used
3. Equipment
3.1.1. Dissolved organic carbon/total organic carbon The dissolved organic carbon (DOC) and total organic carbon (TOC) were measured using a total organic carbon (TOC) analyzer (O × I Corporation Model 700). For DOC, the samples were pre-filtered with cleaned cellulose acetate 0.45 μm filters. 3.1.2. pH, turbidity, conductivity/ temperature The pH of samples was measured using a Metrohm 691 pH-meter while the conductivity
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
203
Fig. 3. X-Flow UFC M5 hollow fibre pen module.
probe measures the trans-membrane pressures and sends the signals to a computer that records the data (every 20 to 22 sec) directly in a Microsoft Excel worksheet commanded by a Microsoft Visual Basic program.
Fig. 2. Spectra/Por 3 Ready-to-Use Dialysis Sacks (MWCO 3.5 kDa).
Table 1 Characteristics of the X-Flow UFC M5 hollow fibre pen module
and temperature were measured with a WTW cond 330i conductivity meter. Turbidity was measured using a DrLange Trubüngsphotometer LTP 4 turbidity meter.
Property
Value
Supplier
Filtrix, Utrecht, The Netherlands PP and PE Epoxy 200 mm 24 mm 50 Polyethersulfone/ polyvinylpyrrolidone 0.8 mm
3.1.3. Filtration equipment The filtration equipment was a UF dead-end mini system (Fig. 4) consisting of a syringe pump, a pen UF membrane module, a pressure probe, a 4-way valve and an air purge valve. The equipment operates at constant flux with a piston pump that works with two syringes, one sucking water from the feed container and the other one pushing with a piston sample at constant flow rate into the membrane. The pressure
Material (plastic parts) Potting material Overall length Outside diameter Number of hollow fibres Membrane material Internal hollow fibre diameter Membrane surface area — complete membrane Molecular weight cut-off (on dextrans) Filtration mode
0.00025 m2 × 50 = 0.0125 m2 100 kDa Inside to outside
204
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
Fig. 4. Mini UF equipment consisting of pump, UF pen module, pressure probe, 2-way valve, air valve and computer for logging trans-membrane pressures.
after 6 days of dialysis and after 21 days of dialysis. They were analyzed and the results are shown in Fig. 5. From Fig. 5 (Left), it can be seen that the DOC reduced from about 15.5 mg/L to about 2 mg/L (87% reduction) for the 10 kDa dialysis, and to about 4.5 mg/L (71% reduction) for the 3.5 kDa dialysis in a period of 7 days. Beyond this period, the rate of DOC reduction was significantly lowered due to a reduced concentration
4. Results 4.1. Optimisation of dialysis — final dialyses (3.5 and 10 kDa)
DOC (mg/L)
To determine the optimum duration of dialysis for the production of 2.6 L of colloidal NOM, 21 day dialysis tests were performed. The dialysate was changed 3 times in a day. Samples (including LC-OCD samples) of colloidal NOM were taken before dialysis, during the dialyses, 16 14 12 10 8 6 4 2 0
DOC (mg/L)
Final dialysis 1 (3.5 kDa, 21 day) Final dialysis 2 (10 kDa, 21 day)
0
2
4
6
8
10 12 14 Time (days)
16
18
20
Before dialysis
15.5
% of initial DOC removed
0%
After 3.5 kDa, 21-day dialysis
2.8
% of initial DOC removed
81.9%
After 10 kDa, 21-day dialysis
1.6
% of initial DOC removed
89.7%
22
Fig. 5. Variation of DOC during 21-day final dialyses against synthetic surface water.
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
gradient between the sample in the dialysis bags and dialysate. By the end of the 21 days, about 82% and 90% of the initial DOC was removed in the 3.5 and 10 kDa dialysis bags respectively (Fig. 5 — Right). Therefore, for a sample volume of 5.2 L per 80 L of dialysate, a dialysate recirculation of 95 L/h, a dialysate change frequency of 3 times per 24 h, and for the aquarium construction, the optimum dialysis duration was found to be about 7 days for both the 3.5 and 10 kDa MWCO.
4.2. Effect of dialysis on NOM composition of pre-filtered canal water Dialysis results showed a general increase in organic carbon (OC) removed (or accumulation) as the dialysis duration increased. The OC removal of DOC increased from 71% to 82% of the initial value when the dialysis (3.5 kDa) duration was raised from 6 to 21 days (Fig. 7). Removal of humics, building blocks, neutrals and hydrophobics also increased with an increase in dialysis duration as shown in Figs. 6, 7 and Table 2. An increase in dialysis duration allowed more time for more of the NOM to diffuse out or adsorb onto the bags. It should be noted that the amount of
OC relative signal response
NOM composition of the various isolates 20
Humics Building blocks
15
LMW acids and humics 10 Biopolymers
5
LMW neutrals
0 0
30
60 Retention time (min)
90
120
UF1 feed pre-filtered canal water (No dialysis) UF3 feed pre-filtered canal water after 6-day dialysis (3.5 kDa) UF7 feed pre-filtered canal water after 21-day dialysis (3.5 kDa) UF5 feed pre-filtered canal water after 6-day dialysis (10 kDa) UF9 feed pre-filtered canal water after 21-day dialysis (10 kDa)
Fig. 6. Variation of NOM composition of the prefiltered canal water during dialysis.
205
hydrophobics was not measured directly. It was estimated as the difference between the chromatographic (or hydrophilic) DOC and the total DOC. The hydrophobic part is retained by the column and is therefore not shown on the chromatographs. The OC removal also increased with an increase in MWCO of the dialysis bags (Fig. 6). For 6-day dialysis, OC removal increased as follows: DOC – 71% to 92%, hydrophobics – 91% to 93%, humics – 67% to 95%, building blocks – 85% to 96% and neutrals – 77% to 92%, when the MWCO of the dialysis bags was increased from 3.5 to 10 kDa respectively (Fig. 7 and Table 2). Increases were also shown for the 21-day dialysis. The increase in OC removal with increase in MWCO was probably due to the fact that larger MWCO (10 kDa) bags allowed more of the larger sized NOM to diffuse out of the bags compared to the smaller MWCO (3.5 kDa) bags. Hydrophobic NOM and building blocks were best removed, averaging about 92% of their initial respective concentrations. An average of 84% of the hydrophilic NOM was removed by dialysis. Among the hydrophilic NOM, Fig. 7 and Table 2 show that OC removal was generally in the order building blocks > neutrals > humics > biopolymers. Fig. 7 and Table 2 also show that there was no significant difference in the removal of hydrophobics for the different MWCOs and dialysis durations. The increase in removal of OC with increase in dialysis duration and MWCO was lowest for the hydrophobics. The maximum increase in removal of hydrophobics was only 91% to 93%, which occurred when the MWCO increased from 3.5 to 10 kDa for 6 days of dialysis, and also when the dialysis duration was increased from 6 to 21 days using 3.5 kDa dialysis membranes. This however was not the same for the other NOM components, which suggests that the mechanism of removal of hydrophobics could be different from that of the other NOM components, and that size exclusion was not the main mechanism of removal for hydrophobic NOM.
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
93 93
92
94
100
93
93
93
92
88 100
77
75
85
91 71
71
92
92
86
82
82
96
95
67
–20
50 25 0
–17
–25
–123
–50 –75 –100 –125 –150 –175 –200 –225
21 days, 10 kDa 6 days, 10 kDa
–214
Neutrals
Humics
Biopolymers
Hydrophilics
Hydrophobics
DOC
6 days, 3.5 kDa
Building blocks
21 days, 3.5 kDa
OC removal (% of initial NOM component concentration)
206
NOM component 6 days, 3.5 kDa
21 days, 3.5 kDa
6 days, 10 kDa
21 days, 10 kDa
Fig. 7. Removals of the various NOM components by dialysis.
They probably adsorbed onto the walls of the regenerated cellulose (dialysis) membrane, and therefore the MWCO was probably not as important (for hydrophobics) as the material from which the bags were made. However, not all NOM components were removed during dialysis. Biopolymers were not removed at all. In fact they increased by 17% and 20% in the 6-day dialysis, and by 214% and 123% in the 21-day dialysis using 3.5 and 10 kDa MWCO respectively (Fig. 7 and Table 2). In addition, there was an increase in bacteria during dialysis. The increase in biopolymers was probably due to bacterial activity, and the release of extra-cellular polymeric substances (EPS) by the bacteria. In aquatic environments, many microorganisms (e.g. bacteria) secrete EPS
(a biopolymer consisting of proteins, polysaccharides and amino-sugars) into the surrounding environment during the endogenous phase of their growth (Underwood et al., 1995). During the endogenous growth phase, the bacteria death rate is higher than reproduction rate, the substrate is very limited and microorganisms metabolize their own protoplasm. Also old cells die and lyse, and nutrients (e.g. proteins) are released into surrounding environment (Citadel, 2006). According to Leenheer et al. (2000), organic colloids may represent up to 20% to 30% of the DOC of surface waters, are hydrophilic in character, and consist mostly of bacterial cell wall residues (mucopolysaccharide type structure-EPS). It is therefore logical that an increase in the bacterial count in the dialysis bags most likely
Average
21 days
10 kDa 6 days
21 days
3.5 kDa 6 days
OC removed (ppb) % of initial OC removed
OC removed (ppb) % of initial OC removed OC removed (ppb) % of initial OC removed
OC removed (ppb) % of initial OC removed OC removed (ppb) % of initial OC removed
– –
12,148 92.2% 12,236 92.8%
9418 71.4% 10,788 81.8%
13,182 0%
Before dialysis
Initial OC (ppb) % of initial OC removed
DOC
Description
Table 2 OC removed before and after the various dialyses
10,864 84.4%
11,863 92.1% 11,950 92.8%
9139 71.0% 10,504 81.6%
12,875 0%
Hydrophilics
283 92.3%
284 92.5% 286 93.2%
279 90.9% 284 92.5%
307 0%
Hydrophobics
– –
−52 −20.2% −317 −123.3%
−44 −17.1% −549 −213.6%
257 0%
Biopolymers
4570 87.1%
7004 95.2% 0 100%
4946 67.2% 6328 86.0%
7357 0%
Humics
2055 91.9%
2140 95.7% 2107 94.3%
1910 85.5% 2061 92.2%
2235 0%
Building blocks
2647 87.4%
2789 92.1% 2806 92.7%
2329 76.9% 2665 88.0%
3027 0%
Neutrals
– –
– – – –
– – – –
– –
Acids
M.D. Kennedy et al. / Desalination 220 (2008) 200–213 207
208
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
resulted in an increase in EPS, and therefore an increase in biopolymers. The regenerated cellulose membrane was probably a substrate for the bacteria. Some strains of pseudoalteromonas bacteria isolates have been found to produce unusually large polymers (EPS) up to 5700 kDa MWCO (Nichols et al., 2004). Fig. 7 and Table 2 show a higher increase of biopolymers in the 21-day dialysis (214%) than in the 6-day dialysis (17%) using 3.5 kDa membranes. Another increase of biopolymers was observed when 10 kDa membranes were used (20% for 6-day and 123% for 21-day dialysis). The possible reason is that the higher cumulative count of bacteria after 21 days (570,000 No./mL) than after 6 days (450,000 No./mL) of dialysis resulted in a higher amount of EPS released after 21 days of dialysis than after 6 days of dialysis. However, for dialysis duration of 21 days, dialysis with 10 kDa membranes resulted in a lower increase in biopolymers than dialysis with 3.5 kDa membranes (i.e. 123% for 10 kDa and 214% for 3.5 kDa). This was due to the size difference in the membranes used. Some of the EPS (or biopolymers) could have passed through the pores of the 10 kDa membrane, resulting into lower retention of biopolymers by the 10 kDa than the 3.5 kDa membranes. Another possible explanation for the strange behaviour of the biopolymers could be the difference in the pH and electrical conductivity (EC). The pH increased from 7.9 to 8.4 while the EC reduced from 1708 µS/cm to 1672 µS/cm when the dialysis duration increased from 6 days to 21 days (Table 3). This could have caused the agglomeration or shift of humics, building blocks and neutrals to larger apparent molecular sizes. A similar observation was reported by Braghetta et al. (1997) who found that pH and ionic strength affected the molecular size distribution of NOM. Lower pH and higher ionic strength caused Suwanee River organic matter (SROM) macromolecules to shift to a smaller apparent molecular size, and vice versa. Organic molecules are most
Table 3 Comparing pH and EC of the pre-filtered canal water with colloidal NOM after 6-day and 21-day dialyses using 3.5 and 10 kDa MWCO dialysis membranes Description
pH
EC (μS/cm)
No dialysis 3.5 kDa 6 days 21 days
8.1
1304
7.9 8.4
1708 1672
8.0 8.2
1695 1643
10 kDa 6 days 21 days
often negatively charged in the aqueous solution at neutral to high pH, an effect that is usually attributed to an abundance of ionised carboxylic (–COOH) and phenolic (–OH) functional groups (Braghetta et al., 1997). Gosh and Schnitzer (1980) proposed two hypothetical end points for NOM macromolecular configurations: (1) a flexible linear macromolecule (apparently larger) at very low ionic strength, high pH and low solution concentration, when intramolecular charge repulsion is high and repulsive and, (2) a rigid compact (apparently smaller), spherocolloidal macromolecule at high ionic strength, low pH, and high solution concentration, when intramolecular charge shielding acts to neutralise functional groups. Several other researchers reported increases in size of NOM molecules with decreasing ionic strength and increasing pH [4] (Shaw et al., 1994; Xi et al., 2002). Larger molecules were also observed in the 10 kDa dialysis, and may be attributed to the same reasons outlined above (pH and EC differences). However, due to the larger MWCO (10 versus 3.5 kDa), more of these large molecules diffused out of the dialysis bags in the 10 kDa case than in the 3.5 kDa case. This resulted in a lower biopolymer peak in the 10 kDa than in the 3.5 kDa dialysis. Table 4 shows the amounts of inorganic colloids before and after the dialyses. The level of
M.D. Kennedy et al. / Desalination 220 (2008) 200–213
209
Table 4 Level of inorganic colloids compared to humic colloids before and after the dialyses Sample description
Inorganic colloids UVA254 (m−1)
% Removal of inorganic colloids
SUVA (L/mg*m)
DOC (mg/L)
Organic colloids UVA254 (m−1) (calculated)
Pre-filtered canal water After 6-day dialysis (3.5 kDa) After 6-day dialysis (10 kDa) After 2-day dialysis (3.5 kDa) After 2-day dialysis (10 kDa)
0.12 0.05 0.06 0.05 0.02
– 58.3% 50.0% 58.3% 83.0%
3.03 3.38 1.54 1.72 0.74
13.182 3.764 1.034 2.394 0.946
39.94 12.72 1.59 4.12 0.70
inorganic colloids in all the samples before and after dialysis was much lower than the level of organic colloids. For example, the UVA254 of inorganic colloids in the pre-filtered canal water was 0.12 m−1, which is much lower than the UVA254 of about 40 m−1 for the organic colloids. This indicates that the majority of the colloids were organic in nature. There was a clear reduction of inorganic colloids inside the bags in all the dialyses. About 50%–58% and 58%–83% of inorganic colloids were removed after 6 and 21 days of dialysis. The highest removal of 83% occurred after 21 days of dialysis using 10 kDa bags. These could have escaped along with the NOM during dialysis. The highest reduction is observed for the 21-day 10 kDa dialysis. This is consistent with the highest DOC reduction observed for the same 21-day 10 kDa dialysis experiment. 4.3. Ultrafiltration experiments Different feed waters were used in the UF experiments. For each UF experiment, a new membrane was used. The purposes of the UF experiments were: (1) To compare the backwashable, non-backwashable and total fouling of the various NOM fractions obtained from the dialysis process, (2) To identify the NOM components responsible for backwashable and non-backwashable fouling and, (3) To determine the effect of Ca2+ on backwashable, non-backwashable and total fouling for the various NOM fractions.
4.4. Effect of NOM size on total fouling rate Tables 2, 5 and Fig. 8 show the various UF feeds, their NOM composition and the resulting fouling behaviour when fed to UF membranes. Each of the NOM fractions exhibited different characteristics in total fouling of UF membranes due to the different size, composition and environment. The colloidal NOM content of the 3.5 kDa dialysis bags after 21 days of dialysis (i.e. 2.2 mg DOC/L). Pre-filtered canal water contained a total NOM of 7.3 mg DOC/L, out of which about 2.2 mg/L (about 30%) was therefore assumed to be colloidal NOM. The total fouling rate curves (Fig. 8 — Left) were obtained by plotting the slope of each of filtration cycles in Fig. 8 (Right) against the filtration cycle number. From Fig. 8 — Left, the total fouling rate generally increased with an increase in duration of dialysis. This was probably due to increased percentage of large sized NOM in the NOM fractions as dialysis proceeded. By comparing the fouling results for UF experiments UF1 (pre-filtered, 7.3 mg DOC/L, 142 mg Ca/L), UF3 (6 d, 3.5 kDa, 2.6 mg DOC/L, 145 mg Ca/L) and UF7 (21 d, 3.5 kDa, 2.2 mg DOC/L, 132 mg Ca/L), it can be seen that when biopolymers increased by 17.1%, while all the other NOM components decreased, and while the Ca/DOC ratio increased from 11 to 40, the average fouling rate was more than doubled (i.e. 0.4 to 1 mbar/min per mg DOC/L). This suggests that Ca2+ and
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M.D. Kennedy et al. / Desalination 220 (2008) 200–213 UFs (1, 3, 5, 7, 9) Effect of NOM size on fouling rates (mbar/min per mg DOC/L) during each cycle – UF experiments with high Ca
0.6
8 Normalised TMP per unit DOC concentration (bars per mg DOC/L)
0.5
Fouling rate (mbar/min per mg Doc/L)
7 6 5 4 3 2
0.4
0.3
0.2
0.1
1 0 1
2
3
4 5 6 7 Filtration cycle no.
8
9
10
UF1 (Prefiltered, 7.3 mg DOC/L, 142 mg Ca/L) UF3 (6 d, 3.5 kDa, 2.6 mg DOC/L, 145 mg Ca/L) UF5 (6 d, 10 kDa, 1.1 mg DOC/L, 151 mg Ca/L) UF7 (21 d, 3.5 kDa, 2.2 mg DOC/L, 132 mg Ca/L) UF9 (21 d, 10 kDa, 0.82 mg DOC/L, 133 mg Ca/L)
0 0
30
60
90
120
150 180 210 Time (min)
240
270
300
330
UF9 (21 d,10 kDa, DOC = 0.82 mg/L, Ca = 133 mg/L) UF5 (6 d,10 kDa, DOC = 1.1 mg/L, Ca = 151 mg/L) UF1 (Prefiltered, DOC = 7.3 mg/L, Ca = 142 mg/L) UF7 (21 d, 3.5 kDa, DOC = 2.2 mg/L, Ca = 132 mg/L) UF3 (6 d, 3.5 kDa, DOC = 2.6 mg/L, Ca = 145 mg/L)
Fig. 8. Right – Normalised TMP vs time for the various UF feeds; Left – Total fouling rates for the various UF feeds of different NOM size compositions (high Ca2+/Mg2+ with HB NOM).
biopolymers played an important role in total fouling. In addition, a further 200% increase in biopolymers from 117% to 314% of the initial value, and a small increase in Ca/DOC ratio from 40 to 55 tripled the fouling rate, suggesting that biopolymers were probably the main cause of total fouling. It should be noted that the Ca2+ concentration in the NOM fractions was kept as close as possible to the Ca2+ concentration in the pre-filtered canal water. Increase in total fouling rate with increase in dialysis duration could also be attributed to increased bacterial activity that led to increased degradation of the regenerated cellulose dialysis bags and production of biopolymers. Another large increase in biopolymers — UF5 (6 d, 10 kDa, 1.1 mg DOC/L, 151 mg Ca/L)-120% to UF9-223% — in the absence of humics, and a negligible increase in building blocks (4.3% to 5.7%) resulted in almost double the fouling rate from 3.5 to 6.6 mbar/min per mg DOC/L. The Ca/DOC ratio only reduced slightly from 146 to 141 during UF5 and UF9
respectively. This suggests that biopolymers were the major cause of total fouling. Similar results were reported by Fan et al. [14] who concluded that the large MW fraction of NOM (>30 kDa — most likely in colloidal state) was responsible for most of the flux decline. Fan et al. [14] and Lahoussine et al. (1990) seemed to agree when they concluded that higher SUVA NOM contained a greater amount of colloidal NOM molecules (i.e. >30 kDa) and caused greater fouling than lower SUVA NOM. Schäfer et al. (2000) found that fouling was independent of the membrane pore size and was caused by large colloids (0.25 μm) in MF and small colloids in UF. Jarusutthirak et al. (2002) found polysaccharides and/or amino-sugars from colloids in wastewater effluent to play an important role in fouling of UF membranes. Leenheer et al. (2000) demonstrated that the organic colloids may represent up to 20%–30% of the DOC or surface waters, and are hydrophilic in character, and mostly of bacterial cell wall residues (mucopolysaccharide type structure). Cho et al. [15] found that large
21 days (UF9)
10 kDa 6 days (UF5)
21 days (UF7)
3.5 kDa 6 days (UF3)
ppb % ppb %
ppb % ppb % 1034 7.8% 946 7.2%
3764 28.6% 2394 18.2%
13,182 100%
Pre-filtered canal water (No dialysis) - UF1
ppb %
DOC
Description
1012 7.9% 925 7.2%
3736 29.0% 2371 18.4%
12,875 100%
Hydrophilics
23 7.5% 21 6.8%
28 9.1% 23 7.5%
307 100%
309 120.2% 574 223.3%
301 117.1% 806 313.6%
257 100%
Hydrophobics Biopolymers
353 4.8% 0 0.0%
2411 32.8% 1029 14.0%
7357 100%
Humics
Table 5 NOM composition of the various UF feeds (UF1, 3, 5, 7, 9) and resulting average total fouling rates
95 4.3% 128 5.7%
325 14.5% 174 7.8%
2235 100%
Building blocks
238 7.9% 221 7.3%
698 23.1% 362 12.0%
3027 100%
Neutrals
141
146
55
40
11
Ca/DOC ratio
6.6
3.5
3.0
1.0
0.4
Average total fouling rate (mbar/min per mg DOC/L)
M.D. Kennedy et al. / Desalination 220 (2008) 200–213 211
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M.D. Kennedy et al. / Desalination 220 (2008) 200–213
the absence of humics and a negligible increase in building blocks (4.3% to 5.7%), resulted in almost double the fouling rate from 3.5 to 6.6 mbar/min per mg DOC/L. The Ca/DOC ratio only reduced slightly from 146 to 141 during UF5 and UF9 respectively. This suggested that the biopolymer fraction was a major cause of fouling in the filtration tests performed with colloidal NOM solutions.
sized NOM components such as amino acids, polypetides, proteins, colloidal NOM, and not the smaller sized humic substances were efficiently rejected by UF membranes. 5. Conclusions (1) The OC removal increased with an increase in MWCO (3.5 kDa to 10 kDa) of dialysis bags. For 6 day dialysis, OC removals increased as follows: DOC – 71% to 92%, hydrophobics – 91% to 93%, humics – 67% to 95%, building blocks – 85% to 96% and neutrals – 77% to 92%, when the MWCO of the dialysis bags was increased from 3.5 kDa to 10 kDa respectively. Similar trends were shown for the 21-day dialyses. (2) The OC removal increased with an increase in duration of dialysis (6 to 21 days). For the 3.5 kDa MWCO dialysis, OC removals increased as follows: DOC – 71% to 82%, hydrophobics – 91% to 93%, humics – 67% to 86%, building blocks – 85% to 92% and neutrals – 77% to 88%, when the dialysis duration was increased from 6 to 21 days respectively. Increases were also shown for the 10 kDa MWCO dialysis. (3) Among the hydrophilic NOM, building blocks were removed the most (average 92%) by dialysis, followed by neutrals (average 88%) and then humics (average 87%). Biopolymers significantly increased (17% and 20% using 3.5 and 10 kDa MWCO bags respectively) after 6 days of dialysis, and most especially after 21 days of dialysis (123% and 214% using 10 and 3.5 kDa MWCO bags respectively). This increase in biopolymers was attributed to increase in EPS released by the bacteria during the dialysis. (4) The large increase in biopolymers from 120% in UF5 (6 d, 10 kDa, 1.1 mg DOC/L, 151 mg Ca/L)-120% to 223% in UF9 (21 d, 10 kDa, 0.82 mg DOC/L, 133 mg Ca/L), in
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