A sustainable membrane operation often requires pretreatment of the feed liquor to improve its technical and economic feasibility. This paper reports the impact of pretreatment on the performance of ceramic microfiltration for several pilot studies at different locations. Four different pretreatment processes were investigated: (1) in-line coagulation (to remove high molecular weight, HMW, dissolved organic carbon, DOC); (2) ion exchange (to remove low molecular weight, LMW, DOC); (3) ozone (for disinfection, taste and odor control, and modifying the character of DOC) (4) ion exchange followed by in-line coagulation (for almost complete removal of DOC). Pretreatment in all cases was needed to control membrane fouling, to establish a technically and economically feasible process. These studies seem to show that the DOC's HMW (which includes biopolymers) and LMW fractions (the latter includes humics/acids), are primarily responsible for the TMP increase after filtration followed by backwashing (irreversible fouling). Removing one of these organic fractions often results in more stable operation. Ozonation in all cases led to better operation, but is not always economically feasible. The feasibility of ozone as pre-treatment depends largely on the initial ozone demand, and whether or not there are secondary treatment targets (e.g., higher virus removal, taste, and/or odor).

INTRODUCTION

Ceramic microfiltration

Since about 2000, there has been an increasing need to treat surface water for drinking water production and treat wastewater for reuse, for which colloid removal is a necessity. Micro- and ultra-filtration (MF and UF) are often used because they provide an absolute barrier against particles greater than the pore size. Polymeric membranes still dominate this sector of the water industry but ceramic membranes have some unique resilience properties that make them a good option. Ceramic membranes are less fragile than polymeric membranes, have a longer life, and can withstand heavy pollutant and solid loads, vigorous backwashing and a variety of chemical types and concentrations.

Membrane fouling

For membrane application, a major obstacle is the potential for fouling. Membrane operations often require pretreatment to reduce the treated water's fouling potential. Huang et al. (2009) summarized pretreatment technologies as: coagulation, adsorption, pre-oxidation, pre-filtration, dissolved air flotation, ion exchange, or some combination of them. Coagulation is much the most widely adopted pretreatment for membrane filtration. Coagulated water causes less fouling than un-coagulated water in most applications, although the opposite can happen, with coagulation leading to a higher level of irreversible fouling. The fouling mechanism seems to differ at different locations and is not completely understood. Most studies suggest that coagulation controls colloidal fouling (i.e., pore blockage) and removes the HMW fraction of natural organic matter (NOM), thus reducing NOM fouling. Gray et al. (2008) argued that coagulation's fouling control mechanism was the removal of LMW organics (with an adsorption peak at 220 nm), which ‘glue’ colloids to the membrane surface. Galjaard et al. (2005) suggested that coagulation removes HMW organics, while introducing or forming metal organic complexes. These could interact with smaller organics and the membrane, resulting in film formation on the membrane surface and irreversible fouling.

Full-scale application of other pretreatment techniques like adsorption, ion exchange and pre-oxidation is still very limited, and requires further assessment. One purpose of this study is to compare the impact of various pretreatment methods on ceramic membrane fouling.

Problem description

A better understanding of membrane fouling mechanisms is crucial when determining the optimal pre-treatment strategy for particular waters. After extensive water treatment experience, Galjaard et al. (2005) suggested that the irreversible – i.e. persistent – fouling is caused by NOM-film attachment to the membrane surface. They believe that HMW organics interact at high concentrations at the membrane surface, forming long polymers. LMW organics – e.g. carboxylic acids and humics – combine with the HMW species by electrostatic forces, this interaction accelerating film formation in the same way that organic metal complexes do. This can result in rapid, irreversible fouling, if the film and membrane have opposite charges, because the film is adsorbed by the membrane. On this basis, two solutions exist to reduce the water's fouling potential: (1) remove the humics and carboxylic acids using ion exchange, and avoid metal organic complex formation by not using coagulants; and, (2) reduce the membrane's negative surface charge or create an opposite surface charge, to promote electrostatic exclusion of the film. The wish to remove humics and carboxylic for fouling control led to the development of both a novel ion exchange technology called suspended ion exchange (SIX®) and a ceramic membrane process called CeraMac®. Water sources are, however, unique and differ in their fouling characteristics. It is valuable, therefore, to verify the fouling hypothesis for other source waters and study how different pretreatment strategies impact the feasibility of using ceramic MF.

This paper presents the results of several pilot studies using ceramic MF. At least two pretreatment strategies were investigated at each location. Membrane performance with these pretreatments is discussed in comparison to the ‘NOM film’ fouling hypothesis.

METHODS

Ceramic membrane and membrane process

The monolithic ceramic MF membrane was provided by Metawater (Japan). The nominal pore size of its separation layer is 0.1 μm, and its pore size distribution is very narrow. Two sizes of membrane elements, 0.4 and 25 m2 surface area, were used during these studies. Two different types of modules were also evaluated. The first housed one element in one module; the second, CeraMac® (see Figure 1), housed multiple elements in one vessel.
Figure 1

CeraMac® vessel system with 192 ceramic elements and a backwash tank.

Figure 1

CeraMac® vessel system with 192 ceramic elements and a backwash tank.

The CeraMac® process, developed by PWN Technologies, reduces ceramic membrane system installation costs so that they are cost competitive with polymeric membrane systems. Rather than having ceramic membrane modules in individual stainless steel casings, up to 192 modules can be housed in a single stainless steel vessel. This reduces the amount of stainless steel and number of valves significantly, while increasing productivity (i.e., all elements are backwashed simultaneously, reducing downtime during backwash [BW], from 10 minutes to a few seconds).

In all cases the ceramic microfilter is operated in dead-end mode and water is fed upwards into a vertically set module. BW occurs after a prescribed operating time. Water is forced through the membranes by air-pressure, built up in the BW tank by pumping water into it, not by a BW pump.

This creates an air-spring effect when the valve opens. The BW water rushes from the vessel and the valve closes, but the tank remains under pressure. Virtually no air volume is lost during BW, apart from that dissolved in the water. BW takes a few seconds, and forces water from the permeate- to the feed- side of the membrane. The BW water leaves the vessel through a separate backwash water port at the bottom. The flow used for BW is 3 L/m2. The process takes 3 to 5 seconds when the membrane is clean, but up to 30 seconds when it is fouled. After BW, forward flush (FF) is automatic, from the top to the bottom of the feed channels of the membrane module, and is also forced over the membrane by air (which is generated by a compressor and stored in a separate FF tank). The air pushes a fixed volume of flush water (stored on top of the modules and in the membrane feed channels) out, thus emptying the whole feed side of the membranes.

Two types of chemically enhanced backwash (EBW) were tried during these studies, one with chlorine and the other low pH/peroxide. These were run at prescribed intervals (i.e., after a fixed number of backwashes), the chlorine EBW generally being used more frequently than the low pH/peroxide EBW. A typical EBW pattern was a chlorinated EBW after every five to 15 BWs, with low pH/peroxide EBW after every five chlorinated EBWs.

The EBW flow rate was the same as for normal BW, but the chemical was added while the BW tank was filling. During the EBW, the BW tank drained for four minutes through a separate, smaller, EBW outlet. Then, while the tank was re-filling, the membrane(s) soaked in the EBW solution for approximately five minutes. The sequence ended with a standard BW and FF. The whole EBW sequence lasted approximately 10 minutes.

Locations and water sources

The pilot studies were performed at different locations treating different sources, three surface waters and one secondary effluent from a municipal wastewater treatment plant.

Andijk, Netherlands

The first pilot study was conducted on Ijssel Lake water for drinking water production in Andijk, the Netherlands. The lake is fed by the river Rhine and is the biggest fresh water lake in Holland. For the Andijk pilot work, in-line coagulation and ion exchange were used as pretreatments, with ferric chloride as the coagulant. The ion exchange process was SIX® with LanXess VPOC 1071 anion resin.

Singapore

The second pilot was at Singapore's Choa Chu Kang Waterworks (CCKWW), which has three raw water sources – the Kranji, Pandan and Western Catchments reservoirs. Here, the membrane system treated clarified water with and without pre-ozonation. The clarified water had been treated by screening, aeration, coagulation and clarification in the existing full scale plant. The target ozone contact time was as short as possible to reduce the contactor volume and dose as little ozone as possible. The ozone contactors' maximum capacity was not known when they were designed, so a conservative value of around five minutes at a maximum capacity of 110 m3/h was used before the water entered the membrane vessel. This yielded an initial ozone dose of approximately 1.3 to 1.5 mg/L, similar to the current dose at CCKWW after the existing sand filters. The target ozone concentration on the membrane surface was 0.8 to 1.1 mg/L.

Plymouth, United Kingdom

The third pilot was at South West Water's (SWW) Crownhill Water Treatment Works (WTW) in the UK. The raw water came from Burrator Reservoir, combined occasionally with water from the rivers Tamar and Tavy. The study included four pre-treatments: (1) SIX® (by PWN Technologies); (2) clarification (from Crownhill WTW); (3) clarification by the WTW followed by SIX®; and, (4) SIX® followed by pilot-scale, in-line coagulation. The anion resin used at Crownhill was LanXess S5128 (Germany).

Secondary effluent

The last pilot study was treating secondary effluent with a ceramic membrane for a confidential client. The effluent was first strained and chlorinated. Four pretreatments were evaluated, direct treatment (no pretreatment), in-line coagulation, ozonation, and ozonation followed by in-line coagulation.

The operational parameters were logged automatically in all pilot studies and included, but not limited to: time, feed water temperature, flow rate, and membrane feed- and permeate- side pressures. The transmembrane pressure (TMP) was calculated from the difference between the feed and permeate pressures. The membrane operating conditions were different for each pilot run – see below. More details of the studies can also be found in previous publications (Galjaard et al. 2013; Zheng & Galjaard 2013; Shorney-Darby et al. 2014).

DOC-characterization and particle charge

The organic matter was analyzed by size-exclusion chromatography – liquid chromatography – organic carbon detection (SEC-LC-OCD) at ‘Het Water Laboratorium HWL’ (the Netherlands). The method was developed by DOC Lab (Germany) and its principles are described by Huber et al. (2011). The zeta potential measurements were made in the University of Twente (the Netherlands), using a Malvern Zetasizer nano.

RESULTS AND DISCUSSION

Raw water organic matter analysis

Table 1 presents the TOC and DOC concentrations, and the UV Transmission at 254 nm (UVT) of the four source waters. The TOC and DOC data for the clarified surface water in Singapore were measured using USEPA's 415.1 method. The other TOC and DOC concentrations were obtained by SEC-LC-OCD. The values in the table show the range of TOC, DOC and UVT measured during the long term pilot tests, the values in brackets are their averages.

Table 1

Organic carbon concentrations in the four source waters

  TOC (mg/L) DOC (mg/L) UVT (%) 
Andijk, NL 5.5 ∼ 6.4 (5.9) 5.4 ∼ 6.3 (5.8) 72.6 ∼ 80.2 (76.3) 
CCKWW, SG 2.2 ∼ 6.7 (3.3) 1.8 ∼ 3.4 (2.4) 79.8 ∼ 89.0 (86.5) 
Plymouth, UK 1.6 ∼ 4.4 (2.4) 1.4 ∼ 4.4 (2.3) 74.9 ∼ 90.6 (82.4) 
Secondary effluent 8.5 ∼ 10.5 (9.8) 8.4 ∼ 10.3 (9.5) 66.6 ∼ 73.1 (68.7) 
  TOC (mg/L) DOC (mg/L) UVT (%) 
Andijk, NL 5.5 ∼ 6.4 (5.9) 5.4 ∼ 6.3 (5.8) 72.6 ∼ 80.2 (76.3) 
CCKWW, SG 2.2 ∼ 6.7 (3.3) 1.8 ∼ 3.4 (2.4) 79.8 ∼ 89.0 (86.5) 
Plymouth, UK 1.6 ∼ 4.4 (2.4) 1.4 ∼ 4.4 (2.3) 74.9 ∼ 90.6 (82.4) 
Secondary effluent 8.5 ∼ 10.5 (9.8) 8.4 ∼ 10.3 (9.5) 66.6 ∼ 73.1 (68.7) 

SEC-LC-OCD is a powerful tool for characterizing organic matter. SEC broadly groups the organics into five fractions: biopolymers, humics, building blocks, and LMW acids and neutrals (in order of retention time). An organic carbon detector (OCD) and an ultraviolet detector (UVD) are used to detect the organics. The OCD spectrum is used to determine the total mass of organic carbon, while the UVD spectrum counts only the UV adsorbing species (i.e., those with carbon double-bonds). Figure 2 shows the OCD signal of raw water samples representative of the three different resources – Ijssel Lake, Burrator Reservoir, and the secondary effluent wastewater. As can be seen, the secondary effluent had the highest DOC concentration, as quantified by the surface area under the graph, followed by those from Ijssel Lake and Burrator Reservoir. Compared to the others, the secondary effluent had very high concentrations of biopolymers and LMW components. These fractions could be biologically active, which matches their wastewater origin. For the two surface waters, humics were the main fraction. The DOC concentration in water from Ijssel Lake was higher than that from Burrator Reservoir, most probably because the lake is fed by the river Rhine, which is heavily polluted.
Figure 2

SEC-OCD chromatogram of three samples – Ijssel Lake, Burrator Reservoir, and a secondary effluent.

Figure 2

SEC-OCD chromatogram of three samples – Ijssel Lake, Burrator Reservoir, and a secondary effluent.

Figure 3 illustrates the UVD spectra for the same three samples. Surprisingly, the ‘biopolymer’ fraction from Burrator Reservoir has the highest UV peak, although it has the lowest concentration detected by the carbon detector. Generally, it was thought that the biopolymer fraction does not adsorb UV light at that wavelength, so the high UV adsorbing properties of the biopolymer fraction in the reservoir water suggests that the organics have other chemical/biological origins. The question remains open for future study.
Figure 3

SEC-UVD chromatogram of three samples – Ijssel Lake, Burrator Reservoir, and a secondary effluent.

Figure 3

SEC-UVD chromatogram of three samples – Ijssel Lake, Burrator Reservoir, and a secondary effluent.

Water from Ijssel Lake yields a huge humics UVD peak. In the secondary effluent, the LMW fractions show high UV adsorbing properties. Combining the observations from Figures 1 and 2 shows that the organic matter from different sources was very different, not only in quantity, but also in composition and properties. LC-OCD analyses were performed regularly and three representative samples are shown. For Ijssel Lake water and the secondary effluent, there is some variability over time according to the spectra, but the results are generally fairly consistent. The fluctuation is much more significant for water from Burrator Reservoir.

Membrane performance pilot – Andijk, the Netherlands

The DOC concentration of the coagulated and ion exchanged waters from Ijssel Lake was similar, typically between 2 and 3 mg/L, depending on the season (Galjaard et al. 2005), but the compositions are quite different. NOM analysis (Figure 4) indicated that in-line coagulation removes part of the biopolymer fraction and a small portion of humics. The ion exchange removed most of the humics and LMW fractions, but had almost no impact on biopolymers.
Figure 4

SEC-OCD chromatogram of ion exchanged and in-line coagulated Ijssel Lake water.

Figure 4

SEC-OCD chromatogram of ion exchanged and in-line coagulated Ijssel Lake water.

Figure 5 illustrates the difference in transmembrane pressure (TMP) development during a filtration cycle. TMP is plotted as a function of the volume being treated. The flux for the coagulated and ion exchange treated waters was 100 LMH. The starting TMP after BW was higher for the coagulated water. The difference is caused by a small difference in the irreversible fouling.
Figure 5

TMP development for treating coagulated and ion exchanged Ijssel Lake water; membrane feed flux 100 LMH.

Figure 5

TMP development for treating coagulated and ion exchanged Ijssel Lake water; membrane feed flux 100 LMH.

For the both pretreatments, the TMP increased relatively quickly indicating some removal of suspended matter. The biopolymers in Ijssel Lake water are difficult to coagulate, because they are totally hydrophilic and require a relatively high amount of ferric at relatively low pH to enhance coagulation (Galjaard et al. 2005). It appears that there is no significant difference in TMP build up for the two different pretreatment strategies, although long-term operation shows that there is a big difference. In-line coagulation could not control the irreversible fouling, even with EBWs. Under optimized coagulation conditions, the fouling rate was 0.5 kPa/day. In contrast, ion exchange pre-treatment enabled very stable membrane operation and almost no irreversible fouling was observed. Figure 6 illustrates TMP development over almost 12 months of continuous testing. The data were obtained at a lower flux of 68 LMH, filtration time 30 minutes and EBWs after nine BWs. The flux was limited by water availability from the SIX® pre-treatment.
Figure 6

TMP development for treating ion exchanged Ijssel Lake water; membrane feed flux 68 LMH and filtration time 30 minutes. The dashed line indicates the TMP increase treating in-line coagulated water (0.5 kPa/day TMP increase in an optimized situation).

Figure 6

TMP development for treating ion exchanged Ijssel Lake water; membrane feed flux 68 LMH and filtration time 30 minutes. The dashed line indicates the TMP increase treating in-line coagulated water (0.5 kPa/day TMP increase in an optimized situation).

Figure 6 shows fluctuations in TMP, attributed to seasonal feed water changes and operating errors (e.g., no EBWs in January). In general, however, it was very stable, with a TMP increase of 0.01 kPa/day. The first increase in TMP (June 2012) was caused by moving the 6 mg/L peroxide dosing, required for the advanced oxidation process with UV, from its initial location upstream of the membrane to a new location downstream (Galjaard et al. 2011). (Ceramic membranes can withstand high concentrations of peroxide.). This change resulted in an immediate TMP increase. Several experiments showed that dosing peroxide prior to the ceramic membranes could increase membrane permeability by around 20% (Zheng & Galjaard 2013). Ion exchange pretreatment was selected for the full scale plant, mainly because of its ability to remove nitrate as well as DOC, both of which improve membrane operation and downstream AOP processes (Martijn et al. 2010).

Membrane performance pilot – CCKWW Singapore

For the CCKWW CeraMac® demo-plant (Galjaard et al. 2013), the ceramic membrane treated first stage clarified water produced by the existing plant. DOC concentration in the clarified water was between 2 and 3 mg/L, and comparable with the DOC concentration after pre-treatment in Andijk. DOC was determined in a local lab using USEPA method 415.1. No LC-OCD analyses were carried out.

The hybrid ozone/ceramic MF process was operated by maintaining 0.8 mg/L ozone concentration at the feed side of the membrane. Ozone was always present during filtration. Figure 7 shows TMP development during two filtration cycles with a BW in between. Operating conditions are the same, with a flux of 200 LMH and filtration time 30 minutes. A big difference in TMP build up can be observed with and without ozone. Without ozone, the TMP is higher and increases during the filtration cycle. When ozone was applied, permeability increased almost immediately (Galjaard et al. 2013) resulting in a lower TMP and negligible TMP increase during a filtration cycle.
Figure 7

TMP development for clarified, and ozonated and clarified water at CCKWW; membrane feed flux 200 LMH, filtration time 30 minutes, for both feed types.

Figure 7

TMP development for clarified, and ozonated and clarified water at CCKWW; membrane feed flux 200 LMH, filtration time 30 minutes, for both feed types.

The study also showed stable operation on the clarified water without ozone at 200 LMH flux. However, both TMP and permeability fluctuation were observed, mainly caused by fluctuations in feed water quality. The hybrid ozone/ceramic MF significantly improved the system's performance, with stable operation, higher operational flux, lower TMP, higher specific flux or permeability and increased recovery. The hybrid process operated at a flux of 315 LMH. During the experiments, there was one period of 61 hours when no BW or other cleaning was performed, because the permeate storage tank was empty. This caused a minor TMP increase (Figure 8) but demonstrates the robustness of the hybrid ozone/ceramic MF process.
Figure 8

TMP development for a hybrid ozone/ceramic MF process for treating clarified water, feed flux 315 LMH, 61 hours continuous filtration without BW.

Figure 8

TMP development for a hybrid ozone/ceramic MF process for treating clarified water, feed flux 315 LMH, 61 hours continuous filtration without BW.

Membrane performance pilot – Plymouth, UK

During this pilot study many different pre-treatment options were investigated including coagulation and clarification, ion exchange, and combined ion exchange and in-line coagulation. LC-OCD analysis (Figure 9) revealed that coagulation and clarification was efficient in removing biopolymers, and ion exchange in removing humics, matching observations at Andijk. If clarification and ion exchange were combined, both biopolymer and humic organics were removed, and the DOC of the treated water became extremely low (Shorney-Darby et al. 2014). It can also be seen that the membrane does not retain DOC, which lowers the fouling potential of the feed water for it.
Figure 9

SEC-OCD chromatogram of raw, clarified, clarified followed by SIX® waters, and membrane permeate on raw feed water in Plymouth.

Figure 9

SEC-OCD chromatogram of raw, clarified, clarified followed by SIX® waters, and membrane permeate on raw feed water in Plymouth.

Figure 10 shows the TMP during 2 filtration cycles with three different pretreatments. Membrane operating conditions were the same – feed flux 150 LMH and filtration time 30 minutes. With ion exchange treatment alone, the starting TMP was low but it increased quickly, and BW could not restore the TMP completely. For the coagulated and clarified feeds, the TMP was high and showed only a slight increase. Again, however, BW could not restore the membrane completely leading, to unstable operation. Pre-treatment with ion exchange followed by in-line coagulation led to much lower TMP and its subsequent increase was minimal during filtration. In this case, the BW kept the operation stable (Figure 11), probably because of the almost complete removal of biopolymers and a large part of the humics, as well as introducing microflocs on the membrane. Besides binding the biopolymers, these microflocs protect the membrane surface and can be easily backwashed.
Figure 10

TMP development for three different pretreatments; membrane feed flux 150 LMH, filtration time 30 minutes (secondary effluent trials).

Figure 10

TMP development for three different pretreatments; membrane feed flux 150 LMH, filtration time 30 minutes (secondary effluent trials).

Figure 11

TMP development for pretreatment consisting of ion exchange and in-line coagulation; flux 100 LMH, filtration time 60 minutes.

Figure 11

TMP development for pretreatment consisting of ion exchange and in-line coagulation; flux 100 LMH, filtration time 60 minutes.

Membrane performance pilot – secondary effluent

Four pretreatment methods were tested in the secondary effluent treatment study: no pretreatment, in-line coagulation, ozonation and a combination of pre-ozonation and in-line coagulation. NOM analysis indicated that in-line coagulation worked fairly well to remove some of the biopolymers and humics. Ozonation alone had a limited impact on the total biopolymer concentration in the membrane feed water (Table 2) but did alter the character of the organic matter (according to the LC-UVD). This can also be seen in a change in the total amount of biopolymers in the permeate. Retention of these biopolymers by the membrane rose compared to in-line coagulation. Ozonation combined with in-line coagulation largely enhanced organic removal. (More details of the NOM analysis of the raw water will be published in due course).

Table 2

Charge properties and biopolymer concentration in the membrane feed and filtered streams under four different pretreatment methods (secondary effluent trials)

  no pretreatment coagulation ozonation ozonation & coagulation 
ZP membrane feed (mV) −16.9 −17.1 −18.2 −15.1 
ZP membrane filtrated (mV) −9.84 −16.5 −12.1 −14.7 
Absolute ZP change (mV) 7.1 0.6 6.1 0.4 
biopolymer concentration – feed (μg/L) 1,338 1,362 1,365 922 
biopolymer concentration – filtered (μg/L) 291 677 300 561 
biopolymer rejection (%) 78 50 78 39 
  no pretreatment coagulation ozonation ozonation & coagulation 
ZP membrane feed (mV) −16.9 −17.1 −18.2 −15.1 
ZP membrane filtrated (mV) −9.84 −16.5 −12.1 −14.7 
Absolute ZP change (mV) 7.1 0.6 6.1 0.4 
biopolymer concentration – feed (μg/L) 1,338 1,362 1,365 922 
biopolymer concentration – filtered (μg/L) 291 677 300 561 
biopolymer rejection (%) 78 50 78 39 

Table 2 shows the different biopolymer rejection performances for the various pretreatment types. The membrane rejects more biopolymer when filtering untreated and ozonated secondary effluent, associated with significant zeta potential change after membrane filtration. However, a remarkably low rejection of biopolymer was observed when filtering the in-line coagulated water, associated with minor zeta potential change.

The fluxes were different but the filtration time was changed so that each set of data is for the same amount of water per m2 of membrane surface in one filtration cycle, for the treatments shown in Figure 12. The permeability of the membrane is highest when pretreatment is with the combination of ozonation and then in-line coagulation. That also led to the most stable operation at a relatively high flux (300 LMH). Because of the high initial ozone demand, due to the high concentration of DOC, this option was not economically feasible. The process was not stable when operating either without pretreatment or with ozonation alone. In-line coagulation made the process technically and economically feasible.
Figure 12

TMP development after different pretreatments (see Table 2) when treating secondary effluent; feed flux 100 LMH, filtration time 45 minutes for no pretreatment; flux 200 LMH and filtration time 22.5 minutes for both in-line coagulation and ozonation (applied individually); flux 300 LMH and filtration time 15 minutes for ozonation followed by in-line coagulation.

Figure 12

TMP development after different pretreatments (see Table 2) when treating secondary effluent; feed flux 100 LMH, filtration time 45 minutes for no pretreatment; flux 200 LMH and filtration time 22.5 minutes for both in-line coagulation and ozonation (applied individually); flux 300 LMH and filtration time 15 minutes for ozonation followed by in-line coagulation.

Fouling discussion

The fouling of low pressure membranes is often attributed to the deposition of dissolved HMW organic carbon – e.g. polysaccharides and polyhydroxyaromatics – on the membrane surface. However, as shown in the study carried out with water from Ijssel Lake, it is not necessary to remove the HMW NOM fraction to achieve a sustainable membrane operation. In both the ion exchange and ceramic MF process, it seems that the membrane rejected biopolymer fractions. The biopolymers were not glued/linked together and could be removed easily by BW. This contributed to the pre-removal of the humics and LMW acids by ion exchange. Kim & Dempsey (2008, 2010) also found that, by removing organic acid with ion exchange, almost no fouling occurred when treating secondary effluent. How humics and LMW acids affect membrane fouling remains unclear but they might be absorbed directly onto the membrane surface. The mass percentage of this group is small (<10%), however, and the molecules are often smaller than the membrane pores, so it is unlikely to be the only reason. It seems likely that the humics and LMW acids ‘glue’ the HMW organics, to the divalent and trivalent cations. The ‘crosslink’ effect of metal ions in developing irreversible fouling has been demonstrated by Elimelech and others (Hong & Elimelech 1997; Li & Elimelech 2004) for nanofiltration, and by Li for ultrafiltration (2011).

The combination of ozonation with ceramic MF significantly improved the CCKWW system's performance. When treating the clarified water, the fouling film formed gradually from organic and inorganic species. Ozone can affect/break the fouling film by reacting with its organic and inorganic components. Once the fouling film had been loosened, it was easily washed off the membrane surface. This can be considered a scrubbing/cleaning effect (Sartor et al. 2008). After the membrane had cleared, the continued presence of ozone prevented fouling film formation.

In the Plymouth case, the biopolymer fraction can be considered the primary foulant, because the biopolymer is sticky and difficult to remove by BW. Remarkably, removal of the LMW fraction by ion exchange alone did not lead to the stable performance seen at Andijk. The nature or origin and properties of the biopolymers seem to be different. This is only evident due to the data from the LC-OCD as compared with the LC-UVD. At Plymouth, in-line coagulation for the removal of this fraction seems to be essential to control fouling.

The biopolymer concentration in the secondary effluent is relatively very high compared to that in the other sources and removal of this fraction by coagulation seems essential. Coagulation also altered the charge properties of the biopolymer and consequently led to different biopolymer rejection by the ceramic membrane. A high level of biopolymer rejection was observed for untreated effluent and that which was only ozonated (78% rejection in both cases, without coagulation), with moderate rejection in the coagulated feed (50%), and low rejection effluent ozonated followed by in-line coagulation. This could suggest that the coagulation neutralized the biopolymer change and promoted electrostatic exclusion between the membrane and the biopolymer. Because of charge exclusion, fouling was reduced.

CONCLUSIONS

Pretreatment was needed for all the study feeds, to control membrane fouling, and establish a technically and economically feasible process. In all cases, the HMW fraction of the DOC – e.g. biopolymers – and its LMW fraction (humics/acids) seem to be responsible for the increase in TMP after a filtration cycle followed by BW (i.e. irreversible fouling). Removing one of these fractions often results in more stable operation. The decision about which fraction to remove – i.e. ion exchange to remove humics or in-line coagulation to remove some of the biopolymers – is often fixed by secondary needs or other restrictions (e.g. conditioning for downstream processes or removing other species than suspended matter). Ozonation led to much improved operation in all cases, but is not always cost effective. The feasibility of ozonation as a pretreatment depends largely on the initial ozone demand, and whether or not there are secondary treatment targets (e.g. greater virus removal, taste, odor, etc). SEC-LC-OCD is a powerful tool for water analysis in combination with UV detection, because it provides quantitative information as well the organic matter's characteristics.

The slow implementation of ceramic membranes offers an opportunity to develop new pretreatment technologies or cleaning strategies, especially in combination with strong oxidants and in-line coagulation. This is possible because of the membrane's superior ability to withstand strong oxidizing agents or heavy solids loads.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the following for their contribution(s) to this study:

- PWN Water Supply Company, North-Holland, the Netherlands;

- PUB Singapore's National Water Agency (Wui Seng Ang and Mong Hoo Lim)

- Metawater;

- South West Water, United Kingdom (Chris Rocky and David Metcalfe);

- Shane Snyder Group, University of Arizona United States;

- Technical University Twente, the Netherlands;

- Het Water Laboratorium, the Netherlands.

REFERENCES

REFERENCES
Galjaard
G.
Kruithof
J. C.
Kamp
P. C.
2005
Influence of NOM and membrane surface charge on UF-membrane fouling
. In:
Proceedings AWWA Membrane Technology Conference
,
Phoenix
,
USA
.
Galjaard
G.
Martijn
B.
Koreman
E.
Bogosh
M.
Malley
J.
2011
Performance evaluation SIX-Ceramac in comparison with conventional pre-treatment techniques for surface water treatment
.
Water Practice & Technology
.
6
(
4
),
doi:10.2166/wpt.2011.0066
.
Galjaard
G.
Clement
J.
Ang
W. S.
Lim
M. H.
2013
Ceramac-19 demonstration plant ceramic microfiltration at Choa Chu Kang Waterworks
. In
Proceedings AWWA Membrane Technology Conference
,
San Antonio
,
USA
.
Gray
S. R.
Ritchie
C. B.
Tran
T.
Bolto
B. A.
Greenwood
P.
Busetti
F.
Allpike
B.
2008
Effect of membrane character and solution chemistry on microfiltration performance
.
Water Res.
42
,
743
753
.
Huang
H.
Schwab
K.
Jacangelo
J. G.
2009
Pretreatment for low pressure membranes in water treatment: a review
.
Environ. Sci. Technol.
43
(
9
),
3011
3019
.
Li
S.
2011
A new concept of ultrafiltration fouling control: backwashing with low ionic strength water
.
PhD thesis
,
Technische Universiteit Delft
.
Martijn
B. J.
Fuller
A. L.
Malley
J. P.
Kruithof
J. C.
2010
Impact of IX/UF Pretreatment on the degradation of NDMA and 1,4-Dioxane
.
Ozone Science and Engineering
.
11/2010
32
,
383
390
. doi: 10.1080/01919512.2010.515507.
Shorney-Darby
H.
Galjaard
G.
Rockey
C.
Metcalfe
D.
2014
Ceramic membrane filtration of a surface water treated with ion exchange
. In:
Proceedings of the American Membrane Technology Association Conference
,
Las Vegas, NV
March 10–14
.
Zheng
J.
Galjaard
G.
2013
Ceramac ceramic microfiltration as pretreatment technology for Ijssel Lake water
. In
Proceedings AWWA Membrane Technology Conference
,
San Antonio
,
USA
.