The fate of Transparent Exopolymer Particles (TEP) in integrated membrane systems: Removal through pre-treatment processes and deposition on reverse osmosis membranes

water research 43 (2009) 5039–5052

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The fate of Transparent Exopolymer Particles (TEP) in integrated membrane systems: Removal through pre-treatment processes and deposition on reverse osmosis membranes Loreen O. Villacorte a,*, Maria D. Kennedy a, Gary L. Amy a,b, Jan C. Schippers a a b

UNESCO-IHE, Institute for Water Education, Westvest 7, 2611 AX Delft, The Netherlands Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands

article info

abstract

Article history:

The abundance of Transparent Exopolymer Particles (TEP) in surface waters has been

Received 7 June 2009

unnoticed for many years until recently as a potential foulant in reverse osmosis systems.

Received in revised form

Recent studies indicate that TEP may cause organic and biological fouling and may

13 August 2009

enhance particulate/colloidal fouling in reverse osmosis membranes. The presence of TEP

Accepted 18 August 2009

was measured in the raw water, the pre-treatment processes and reverse osmosis (RO)

Available online 28 August 2009

systems of 6 integrated membrane installations. A spectrophotometric method was used to measure TEP in the particulate size range (>0.40 mm) and was extended to measure TEP

Keywords:

in the colloidal size range (0.05–0.40 mm). Ultrafiltration pre-treatment applied in 4 plants,

Transparent Exopolymer Particles

totally removed particulate TEP while microfiltration systems (2 plants) and coagulation/

Reverse osmosis

sedimentation/rapid sand filtration systems (3 plants) partially removed this fraction. None

Pre-treatment

of the pre-treatment systems investigated totally removed colloidal TEP. Biopolymer

Integrated membrane systems

analysis using LC–OCD showed consistency between colloidal TEP and polysaccharide

Biological fouling

removal by UF pre-treatment and further verified the presence of TEP in the RO feedwater.

Organic fouling

TEP deposition in the RO system was determined after measuring total TEP concentrations in the RO feed and concentrate. The TEP deposition factors and specific deposition rates indicate that TEP accumulation had occurred in all plants investigated. This observation was verified by an autopsy of RO modules from two RO plants. Further improvement and verification of the (modified) TEP method, in particular the calibration, is necessary so that it can be employed to investigate the role of TEP in the fouling of RO systems. ª 2009 Published by Elsevier Ltd.

1.

Introduction

In spite of the presence of pre-treatment, the occurrence of organic and biological fouling remains a major challenge in reverse osmosis systems for seawater and freshwater treatment (Flemming et al., 1997; Baker and Dudley, 1998; Kruithof

et al., 1998; van Agtmaal et al., 2007). Organic fouling occurs when natural organic matter (NOM) accumulates on the membrane, resulting in a decrease of normalized flux in RO systems. The deposited organic substances may further initiate biological fouling when microorganisms start to colonize the organic layer then multiply by feeding on

* Corresponding author. Tel.: þ31 15 2151715; fax: þ31 15 2122921. E-mail addresses: [email protected] (L.O. Villacorte), [email protected] (M.D. Kennedy), [email protected] (G.L. Amy), [email protected] (J.C. Schippers). 0043-1354/$ – see front matter ª 2009 Published by Elsevier Ltd. doi:10.1016/j.watres.2009.08.030

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Nomenclature AB b CSF CBP CPR CPS c-TEP

Alcian Blue [–] deposition factor [–] coagulation–sedimentation–filtration [–] biopolymer concentration [mg-C L1] protein concentration [mg-C L1] polysaccharide concentration [mg-C L1] colloidal TEP concentration, mg Xanthan equivalent per liter [mg Xeq L1]

biodegradable nutrients (C, P, N) from the feedwater and excreting more organic substances, leading to a significant build-up of biofilm. Moreover, the deposited sticky organic substances may enhance the deposition of more colloidal particles to the membrane and further aggravate the fouling. Most biofilm studies agree on the importance of extracellular polymeric substances (EPS) in the cohesion of cells and other particles, as well as biofilm adhesion to surfaces. EPS are microbial polymers which comprise a wide variety of polysaccharides, proteins, glycoproteins and glycolipids, but usually dominated by polysaccharides (40–95%) (Flemming and Wingender, 2001). Biofilm cohesiveness was found to have a strong correlation with polysaccharide rather than protein concentrations (Ahimou et al., 2007), substantiating previous knowledge of polysaccharides as a ‘‘strong and sticky framework of biofilms’’ (Sutherland, 2001). The physical structure of a biofilm found in membrane systems was described as either ‘‘compact and gel-like’’ or ‘‘slimy and adhesive’’ (Baker and Dudley, 1998; Schneider et al., 2005a,b), an indication of the abundance of polysaccharides. Whether the accumulation of polysacchariderich EPS on RO membranes is mainly due to local production by biofilm microorganisms or by gradual deposition of EPS from the RO feedwater, is still largely unknown. Over the last decade, an abundant form of EPS called Transparent Exopolymer particles (TEP) has been described as a major agent in the aggregation of particles in aquatic systems. The presence of transparent marine substances like TEP has been reported way back in the 1970’s (Gordon, 1970; Emery et al., 1984). However, the transparent character of TEP had complicated earlier studies to investigate its abundance in aquatic systems. In 1993, Alice Alldredge and co-workers developed a technique to visualise and identify TEP by applying Alcian Blue on filterretained particles (Alldredge et al., 1993). Alcian Blue (copperphtalocyanin with four methylene-tetramethylcisothiouronium-chloride sidechains) is a hydrophilic cationic dye which specifically stains negatively charged carboxylated and sulfated acidic polysaccharides (Passow and Alldredge, 1994). With the staining technique, TEP can be distinguished from other EPS and other organic substances based on its reaction with Alcian Blue and with the assumption that TEP consist mainly of acidic polysaccharides. Moreover, microscopic studies revealed that most TEP exist as discrete particles rather than as surface coatings or dissolved slimes often described to an EPS (Alldredge et al., 1993). TEP have been characterised as transparent, sticky and amorphous substances which may appear in different forms (e.g. strings, disks, sheets, fibers) and sizes (up to 100s of mm long). Several studies have reported its abundance in most fresh and

dissolved organic carbon [mg C L1] calibration factor, mg Xanthan equivalent per absorbance units [mg Xeq] LC–OCD liquid chromatography–organic carbon detection [–] p-TEP particulate TEP concentration [mg Xeq L1] R RO recovery [%] RSF rapid sand filtration [–] SDR specific deposition rate [mg Xeq m2 h1] TEP Transparent Exopolymer Particles [–] DOC fx

marine waters (Alldredge et al., 1993; Berman and Viner-Mozzini, 2001; Passow, 2002a) and recently in wastewater (de la Torre et al., 2008; Kennedy et al., 2009). In aquatic systems, TEP mainly originate from polysaccharides released by phytoplankton and bacterioplankton (Passow, 2002a,b) as well as from biological detritus of higher organisms such as macroalgae (Thornton, 2004) and some oysters (McKee et al., 2005). The majority of TEP may have formed abiotically from colloidal polysaccharides of about 1–3 nm in diameter by hundreds of nanometers long (Santschi et al., 1998), and some of which are flexible enough to pass through 8 kDa pore size membranes (Passow, 2000). Hence, in integrated membrane systems (IMS), MF/UF pre-treatment may not provide a complete barrier for these colloidal precursors of TEP from potentially fouling the RO system downstream. Berman and Holenberg (2005) were the first to describe the potential link of TEP with membrane fouling. They described TEP as ‘‘major initiators’’ of biofilm formation in reverse osmosis systems, which could potentially lead to biofouling. TEP are highly sticky and can easily accumulate on RO membranes, providing favourable surfaces for bacterial colonization and may thereby initiate biofilm development in the system. Moreover, a number of studies reported that significant percentages (2–68%) of the total bacterial population found in seawater were attached to TEP (Alldredge et al., 1993; Passow and Alldredge, 1994). As TEP can be a potential carrier of bacteria to the RO system, it may not only serve as an initiator but also play a vital role in enhancing microbiological growth in the system. Monitoring the presence of TEP in the feedwater of reverse osmosis plants is necessary to better understand their contribution to membrane fouling. Most biofouling studies had focused on the availability of nutrients such as assimilable organic carbon (AOC) and biomass parameters such as ATP, cell count and heterotrophic plate count as indicators of the biofouling potential of feedwater for RO/NF systems (Schneider et al., 2005a,b; Vrouwenvelder et al., 2008; Hijnen et al., 2009). However, none of these parameters are indicative of the presence of TEP entering the RO system via the feedwater. Currently, the common anti-biofouling strategies are to limit the influx of nutrients to the RO system by pre-treatment or by applying a biocide in order to control build-up of biofilm. Nonetheless, limiting biological activity in the system may not be enough to prevent organic fouling, considering that TEP may still accumulate on the RO membrane. TEP, as a very sticky substance, may act like a ‘‘natural glue’’ that can entrap or bind organic and inorganic colloids from the feed stream onto the membrane surface. In this way, TEP may not only cause biological or organic fouling but may also enhance colloidal/ particulate fouling as well.

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TEP has been widely studied for many years in the fields of oceanography and limnology but very limited information is available on its relevance as a potential foulant in RO systems. A number of methods are already available to quantify and monitor these previously elusive substances. These methods may either involve microscopic counting (Alldredge et al., 1993) or spectrophotometric measurements (Passow and Alldredge, 1995; Arruda-Fatibello et al., 2004; Thornton et al., 2007), all of which are based on staining with Alcian Blue. Although most of these methods have been around for some time now, TEP measurement for water treatment monitoring is still in its initial stage (Liberman and Berman, 2006; de la Torre et al., 2008; BarZeev et al., 2009; Kennedy et al., 2009; Villacorte et al., 2009). The main objectives of this study were as follows: 1. To apply the current spectrophotometric method to measure TEP, in which water samples are filtered through 0.40 mm pore size filters, and expanding the method to determine TEP fractions smaller than 0.4 mm; 2. To assess the presence of TEP in the raw water and its removal by different pre-treatments; 3. To evaluate the deposition of TEP in reverse osmosis systems.

2.

Materials and methods

2.1.

Water samples, collection and storage

Water samples were collected in the period between March and July 2008 from 6 IMS installations. Sample volumes of 0.5–1 L were collected in acid-washed bottles. The samples were tested within 6 h after sampling, otherwise kept in storage at a temperature of about 4  C for less than a week.

2.2.

Integrated membrane systems (IMS)

were treating different water sources with various types of pre-treatment (Table 1). To assess the presence of TEP, water samples were collected from selected points within each plant including the raw water, MF/UF feedwater, MF/UF filtrate, RO permeate and concentrate. Removal efficiencies of pre-treatments were evaluated based on the TEP results.

2.3.

TEP measurements were based on the spectrophotometric method introduced by Passow and Alldredge in 1995. In this method, the samples are filtered through polycarbonate filters with pore size of 0.4 mm. The accumulated TEP on the filters were subsequently stained with Alcian Blue. TEP were semiquantified spectrophotometrically based on a calibration with the standard polysaccharide Gum Xanthan. In this study, the existing method was expanded by filtering the filtrate samples through a series of filters having pores of 0.2, 0.1 and 0.05 mm, respectively. These modifications were developed in order to include measurements of the smaller size fractions of TEP (<0.4 mm).

2.3.1.

Staining solution

TEP were visualised and measured by applying a cationic dye Alcian Blue. The staining solution was prepared with 0.02% of Alcian Blue 8 GX (Standard Fluka, C.I.N. 74240) in acetic acid buffer solution, maintained at pH 2.5. Each batch of staining solution was stored at temperature of 4  C for not more than 4 weeks, as it coagulates over time and a significant reduction of Alcian Blue concentration is likely after pre-filtration (Passow and Alldredge, 1995). The Alcian Blue concentrations were monitored by measuring the copper content of the staining solution using an atomic absorption spectrometer (Perkin Elmer AAnalyst 200). The AB concentration was computed based on the mass proportion of copper in each Alcian Blue molecule (C56H68Cl4CuN16S4): CAB ¼

This study was conducted in one pilot and 5 full-scale IMS plants located in The Netherlands and Belgium. The 6 plants

TEP measurement

  mg AB L1

CCu 0:0384

where: CCu is the mass concentration of copper in the staining solution (mg Cu L1).

Table 1 – Overview of the 6 integrated membrane systems (IMS) investigated in this study. Plant A B C D E F

Type Full-scale Full-scale Full-scale Pilot Full-scale Full-scale

Source water River Estuarineg Lake Estuarineg Canalg River

Product water Industrial Industrial Drinking Industrial Industrial Industrial

Capacitya (m3/h) 1200 63 2300 100 225 155

Pre-treatment b

Coag1 þ RSF þ Coag2c þ UF þ AS Coagd þ MBF þ UF þ AS MS þ CSFe þ BACF þ UF þ AS MF þ AS MF þ AS Coagf þ UF þ AS

AS ¼ Antiscalant; CSF ¼ coagulation–sedimentation–filtration; Coag ¼ coagulation; MBF = moving bed filter MF ¼ microfiltration; MS ¼ microstrainer; RSF ¼ rapid sand filter; UF ¼ ultrafiltration. a Production capacity of the RO system (first pass). b Coagulant dose ¼ 8 ml L1 PACl. c Coagulant dose ¼ 3 mg Fe3þ L1. d Coagulant dose ¼ 6 mg Fe3þ L1. e Coagulant dose ¼ 15 mg Fe3þ L1. f Coagulant dose ¼ 0.3 mg Fe3þ L1. g Brackish water source.

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2.3.2.

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Sample measurement

To measure particulate (>0.40 mm) and colloidal (<0.40 mm) TEP, water samples (40–200 ml) were filtered through a series of polycarbonate filters (0.40, 0.20, 0.10 and 0.05 mm pore sizes; 47 mmØ, Whatman Nuclepore) in a glass filter holder (Sartorius). An adjustable pump was installed to maintain a vacuum of 0.2 bars. The accumulated TEP on the filter were stained with 1 ml of pre-filtered (0.05 mm polycarbonate filter) Alcian Blue solution. The applied dye was allowed to react with TEP for about 10 s. Excess stain was removed by applying low vacuum (0.2 bar) through the filter and then rinsed by filtering 1 ml of ultra-pure water (milli-Q). The filter was transferred to a 50 ml beaker and then soaked in 6 ml of 80% H2SO4 solution for 2 h to elute Alcian Blue that were bound to TEP as well as those adsorbed by the filter. The beaker was gently swirled 3–5 times within this period. After 2 h, the absorbance of the acid solution was measured using a UV–Vis spectrophotometer (Shimadzu UV-2501PC). Absorbance was measured at 787 nm wavelength using a 1-cm cuvette and ultra-pure water as reference. Two to four replicates were preformed for each water sample. Absorbance corrections due to stain adsorption on filter media and in some cases interference due to high turbidity were also determined. Filter media adsorbs significant amount of Alcian Blue during TEP staining while some suspended solids in water samples could not be totally oxidised by sulphuric acid and may subsequently interfere with the absorbance measurements. The filter blank was prepared by staining a clean filter with Alcian Blue. For turbidity correction, suspended solids in turbid water samples (same sample volume in TEP measurement) were retained in 0.4 mm pore size filters without subsequent staining. Both filter blank and turbidity filters were soaked in sulphuric acid (80% H2SO4) for 2 h, following the previously mentioned procedure until absorbance measurements. Typical ranges of filter blank absorbance were between 0.08 and 0.11 for 0.4 mm polycarbonate filters and 0.09 and 0.12 for 0.05 mm polycarbonate filters. Turbidity correction was not necessary for most of the samples but it was significant in some cases, especially in raw water samples. Turbidity correction is normally below 0.20; otherwise, the filtered sample volume was reduced to a level that would minimise retention of solid particles. To compute the net absorbance of stain eluted from TEP, corrections due to turbidity (T787) and filter blank (B787) were subtracted from the sample absorbance (A787). TEP concentrations were then computed in terms of Xanthan equivalent per liter by multiplying the net absorbance with a calibration factor ( fx) following the equation: TEP ¼

fx ðA787  B787  T787 Þ Vf

  mg Xeq L1

where A787 is the absorbance of the stain eluted from TEP and the filter; B787 is the average absorbance of stain eluted from blank filters; T787 is the absorbance correction due to turbidity; fx is the calibration factor in mg Xanthan equivalent (Xeq) per unit absorbance at 787 nm (based on Passow and Alldredge, 1995); and Vf is the filtered volume of the sample in liters. Concentrations of TEP fractions were based on serial filtration using different pore size filters. Particulate TEP or p-TEP refers to TEP retained on 0.4 mm polycarbonate filters

while colloidal TEP or c-TEP refers to TEP that passed through 0.4 mm polycarbonate filters but retained on 0.05 mm polycarbonate filters. Passow (2000) reported part of c-TEP as dissolved TEP precursors (<0.2 mm) while Verdugo et al. (2004) considered it as hydrogels. In order to be consistent with the IUPAC definition of colloidal substances (0.001–1 mm), this fraction was referred to in this study as ‘‘colloidal’’ rather than ‘‘dissolved’’. The staining capacity of an Alcian Blue solution depends on its concentration (Passow and Alldredge, 1995). Stain concentration may vary between batches of stain even with slight procedural inconsistencies in preparing and filtering Alcian Blue solutions. The differences in concentration may lead to variations in the quantities of stain that react with TEP. For these reasons, Passow and Alldredge (1995) proposed calibrating TEP results in terms of a standard polysaccharide (Gum Xanthan). A calibration factor determined for each batch of staining solution corrects the effect of variations in staining capacity. However this calibration is vulnerable to several inaccuracies such as weighing very small quantities of Gum Xanthan on polycarbonate filters and preparing solutions of Gum Xanthan (suspended/colloidal) with uniform properties. These were the reasons why the calibration procedure was not applied in this study. A partially different approach has been followed and was aiming at verifying whether the Alcian Blue solutions had sufficient staining capacity. To monitor the efficiency of TEP removal through the treatment processes, TEP were expressed relative to the TEP results of the raw water. In this case, TEP in all water samples collected within each plant were measured using the same batch of staining solution for reliable comparison.

2.4.

LC–OCD biopolymer analyses

Selected water samples were sent to the Het Waterlaboratorium (Harleem) for analysis using liquid chromatography – organic carbon detection (LC–OCD). LC–OCD was developed by Stefan Huber of Karlsruhe in early 1990s as a high sensitivity carbon detector which can measure organic carbon in the low ppb-range (Huber and Frimmel, 1991; Huber and Gluschke, 1998). This technology has been used to characterise natural organic matter (NOM) in water and to identify its constituents that causes organic fouling in membrane systems (Huber, 1998; Kennedy et al., 2005, 2008). Each LC–OCD system has an online organic carbon detector (OCD), UV detector (UVD) and organic nitrogen detector (OND) to measure the relative signal response of organic carbon, UV and organic nitrogen, respectively at different retention times. The chromatogram results produced by the system were processed using FIFIKUS software (DOC-LABOR, Karlsruhe) to compute organic carbon concentrations of biopolymers, humic substances, building blocks, low molecular weight acids and neutrals by area integration of the fractional peaks. Calibration of the signals was based on the humic peak of the ‘‘Suwannee River’’ standard IHSS-FA and IHSS-HA (DOCLABOR, Karlsruhe). Biopolymers are high molecular weight (>20,000 Da) biologically derived polymers that are present in surface waters. These organic substances often consist mostly of

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polysaccharides but may also contain significant amounts of proteins. The organic carbon concentration of protein in biopolymers can be estimated based on its organic nitrogen content. According to the ROEMPP chemical encyclopaedia, protein compounds normally contain 15–18% nitrogen and 50–52% carbon. Assuming that all organic nitrogen in biopolymers originate from proteins, the C:N ratio of the latter can be estimated as 3:1. Polysaccharide concentrations (CPS) were computed by subtracting protein (CPR) from biopolymers (CBP) in terms of organic carbon concentrations following these equations: CPS ¼ CBP  CPR CPR ¼ 3NBP

½mg-C=L

2.5.

nitrogen

content

of

biopolymers

TEP deposition analyses in RO systems

Particle transport in cross-flow reverse osmosis membranes involves three process streams: the feed, permeate and concentrate. Unlike in dead-end filtration, cake formation is limited by back diffusion in cross-flow filtration, where most of the rejected particles remain in suspension towards the concentrate stream. A simple TEP balance can be formulated based on this principle in order to estimate the rate of TEP deposition on RO membranes (Scheme 1) (Huber, 1998).

Scheme 1.

Qf ¼ Qc þ Qp

Qf TEPf ¼ Qc TEPc þ Qp TEPp þ

  dm dt membrane

where: Qf ¼ feed flow rate [L h1] Qc ¼ concentrate flow rate [L h1] Qp ¼ permeate flow rate [L h1] TEPf ¼ TEP concentration in the feed [mg Xeq L1] TEPc ¼ TEP concentration in the concentrate [mg Xeq L1] TEPp ¼ TEP concentration in the permeate [mg Xeq L1] 1 dm dt ¼ deposition rate of TEP in the RO system [mg Xeq h ] The TEP deposition factor (b) elucidates what fraction of TEP from the recovered portion (RQf) of the feedwater had accumulated in the RO system, assuming a 100% rejection of TEP. This assumption fits well for this case, since TEP could not be detected in the RO permeate and is assumed to be totally rejected by RO. The deposition rate of TEP can be expressed as a function of b such as follows:

h

1

mg Xeq h

i

where R is the recovery of the RO system. Considering that TEPp ¼ 0 mg Xeq L1, the mass balance relationship is: Qf TEPf ¼ Qc TEPc þ bRQf TEPf Based on this relationship, the deposition factor b can be derived as a function of RO recovery (R) and TEP concentrations (Schippers and Kostense, 1980). b¼

½mg-C=L

where NBP ¼ organic (mg-N L1).

dm ¼ bRQf TEPf dt

  1 TEPc 1 1 þ R TEPf R

if b < 0: TEP depletion/scouring from the membrane if b > 0: TEP deposition on the membrane. To quantitatively assess the deposition of TEP in RO systems, the specific deposition rate (SDR) of TEP was also calculated assuming a uniform TEP deposition throughout the RO membrane area. This was done by dividing the deposition rate of TEP with the total membrane area (Am) of the RO system. SDR ¼

  dm 1 dt Am

2.6.

RO membrane autopsy



  mg Xeq =m2 h

Samples of the lead and tail elements of the first and second RO stages were taken out from two IMS installations (Plant C and D). The membrane elements were packed in plastic bags and stored at 4  C. A membrane autopsy was performed within 96 h after the membranes were taken out from the plant. The Spiral-wound elements were opened lengthwise by cutting the glass fibre casing and then carefully unrolled for sampling. About 3 sections of the membrane and spacer were cut-off along the length of a leaf of the spiral-wound membrane with relatively intact deposits. The samples were prepared in squares of about 20 mm in width, placed on separate 50 mmØ petri-dishes and then stained with 2 ml of Alcian Blue. After allowing the stain to react for 5 mins, the excess stain was removed by submerging the samples in ultra-pure water and then discarding the liquid afterwards. The rinsed membrane and spacer samples were placed on slides and then viewed and photographed using an Olympus BX51 microscope (magnification ¼ 200).

3.

Results and discussion

Using a current and expanded spectrophotometric method, the presence of different fractions of TEP in raw water sources, after pre-treatment and in RO system of 6 IMS, was measured and their deposition rates on RO membranes were evaluated.

3.1. TEP measurement using smaller pore size filters (<0.40 mm) TEP were semi-quantified based on the absorbance of Alcian Blue which reacted with acidic polysaccharides retained on

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polycarbonate filters. In order to assure full reaction with TEP, an excess volume of dye was applied to the filter and the non-reacting stain was subsequently removed by vacuum filtration. The stain should be dissolved enough to pass through the filter pores and does not remain on the filters to avoid overestimation of results. For this reason, the staining solution was pre-filtered through a filter with a maximum pore size similar to the filter used for TEP retention. This aspect must be considered in order to retain and stain TEP in smaller pore size filters (<0.4 mm). However, significant reduction of Alcian Blue concentration might occur when prefiltering through filters with very small pores. The original protocol developed by Passow and Alldredge (1995) requires pre-filtering the staining solution through 0.2 mm filters, since 0.4 mm filters were used to retain TEP. To measure colloidal TEP (<0.4 mm), samples should be filtered through filters with pore size smaller than 0.40 mm. Commercially available polycarbonate filters have pore sizes ranging from 12 mm down to 0.015 mm, while colloidal TEP’s can be as small as 1 nm in diameter. To determine the appropriate filter pore size for colloidal TEP measurements, tests were performed by filtering a stock solution of Alcian Blue (w200 mg AB L1) through 0.2, 0.05 and 0.015 mm filters. The results showed that the filtered stain concentrations through 0.2 mm pore size filters were reduced by 50% (n ¼ 10), 56% (n ¼ 10) using 0.05 mm filters and 96% (n ¼ 2) using 0.015 mm filters. Filtering stains through filters with pore size smaller than 0.05 mm was not feasible as most of the Alcian Blue stains were removed and their staining capacity was significantly reduced. Therefore, 0.05 mm was selected as the minimum filter pore size for colloidal TEP measurements.

3.1.1.

Variations in staining capacity

To test variations of staining capacity, 10 different batches of stain with variable concentrations of Alcian Blue were filtered through 0.2 and 0.05 mm pore size filters. Alcian Blue concentrations were measured in both prepared and filtered solutions. Filtered stain concentrations were found linear in relation to prepared stain concentrations (Fig. 1a). Stain

a

concentrations through 0.05 mm filters were about 10% less than the stain concentration through 0.20 mm filters. The staining capacity of each batch of Alcian Blue stain can be evaluated as the amount of stain bound to a certain amount of substrate (e.g. Xanthan, TEP). The effect of the reduction of stain concentration to staining capacity was investigated by staining a specific amount of Gum Xanthan using different concentrations of stain. A Gum Xanthan solution was prepared by dissolving 20 mg of Xanthan in 200 ml of water, and then homogenised using a manual tissue grinder. Two millilitres of the homogenised solution were filtered through 0.4 mm polycarbonate filters and then the filters were separately stained using different concentrations of Alcian Blue solutions. Absorbance measurement of the stain bound to Xanthan was performed following the TEP protocol. Reduction in staining capacity was observed when prepared stain concentration was reduced to w150 mg AB L1 (0.78 in Fig. 1b) from the standard protocol of 200 mg AB L1 (1.00 in Fig. 1b). The staining capacity appear to arrive at the maximum level at concentrations higher than 200 mg AB L1, an indication that the protocol concentration was enough to stain all TEP, even after pre-filtering through 0.05 mm filters (Fig. 1b). Based on the results, under staining of TEP is unlikely with the current and expanded protocol. Since Alcian Blue solutions are known to coagulate over longer time, two batches of staining solutions prepared according to the protocol were monitored within a 4-week period. Each batch of stain was filtered through 0.05 mm filters 7–8 times within 30 days and stain concentrations were measured. Prepared stain concentrations of the two batches of stains were 217 and 210 mg AB L1, respectively. Stain coagulation did not occur within 4 weeks as there was no significant decrease in pre-filtered stain concentrations (Fig. 2). The standard deviation for batch 1 was 9% (n ¼ 7) while it was 4% (n ¼ 8) for batch 2. It was also demonstrated in the results that even the procedures and specifications indicated in the protocol were consistently followed, it is still unlikely for two batches of stain to have the same

b

std. protocol concentration

Fig. 1 – Stain concentration and staining capacity using smaller pore size filters for TEP measurement: (a) prepared versus filtered concentration of stain using 0.20 and 0.05 mm filters; best fit lines are for stains filtered through 0.2 mm and 0.05 mm filters, respectively; (b) staining capacity in terms of relative absorbance for stains (with different concentrations) pre-filtered through 0.20 and 0.05 mm filters.

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Fig. 2 – Pre-filtered (0.05 mm filters) stain concentrations with respect to age and batch of prepared staining solution. Dashed lines are average for each batch of staining solution.

concentration after pre-filtering through 0.05 mm filters. Batch 1 had 46% average reduction of stain concentration while Batch 2 was reduced by 57%. However, pre-filtered stain concentrations (90 and 116 mg AB L1) were still higher than the pre-filtered stain concentration of the protocol (w85 mg AB L1; Fig. 1b). Thus, the staining capacity of the two batches of stain were likely similar.

3.2.

TEP in different water sources

TEP measurements were performed for water samples collected from coastal seawater, estuarine, river, canal and lake water sources (Fig. 3). All water samples were collected and analysed within 4 weeks using the same batch of staining

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solution. In order to have relative comparison with published TEP concentrations, a calibration factor was adopted based on existing literature. As shown in Section 3.1.1, in spite of modifying the original protocol, stain concentrations were apparently within the same range using the original method. Hence, we adopted an arbitrary calibration factor ( fx) of 0.114 mg Xeq, which is the average of the two calibration factors (0.088 and 0.139 mg Xeq) reported by Passow and Alldredge (1995). Since fx was adopted from literature, the TEP concentrations are considered ‘‘relative’’. Among the four sources of water sampled, the highest total TEP concentration was recorded in coastal seawater (1.62 mg Xeq L1) while the lowest concentration was measured in river water (0.27 mg Xeq L1). Total TEP concentrations in estuarine and freshwater samples were relatively low in the range of 0.27–0.61 mg Xeq L1, less than half the concentration found in coastal seawater. This was found to be consistent to an earlier study by Villacorte et al. (2009). If indeed TEP causes fouling in membrane systems, it might be more pronounced in seawater RO than in fresh or brackish water RO plants. However, this may not always be the case considering that a number of publications reported TEP concentrations in freshwater that were comparable or sometimes higher than what was reported in seawater (Grossart et al., 1997; Worm and Søndergaard, 1998; Berman and VinerMozzini, 2001). Particulate TEP (>0.4 mm) in seawater samples were generally comparable to what was reported by Passow (2002a). However, a direct comparison with existing studies were not possible for freshwater samples, as most of these studies were using methods other than the spectrophotometric method by Passow and Alldredge (1995). So far, fewer studies are dealing with TEP in freshwater than in seawater, and most of these results were derived from microscopic enumerations (Berman and Viner-Mozzini, 2001; Grossart et al., 1997). Moreover, comparison with the results from previous research were only

Fig. 3 – TEP concentrations measured in 6 different water sources. From left to right: River Meuse in Limburg, Netherlands (July 2008); Lake IJssel near Andijk, Netherlands (June 2008); Gent-Terneuzen canal, Belgium (July 2008); River Schelde estuary, Belgium (July 2008); River IJ estuary, Netherlands (June 2008) and Wadden Sea near Eemshaven coast, Netherlands (June 2008).

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possible for p-TEP (>0.40 mm) but not for c-TEP (<0.40 mm). Our results were all based on the extended/modified method, which includes both p-TEP and c-TEP measurements. All of the p-TEP (>0.4 mm) results were within the range (0.02–11 mg Xeq L1) of previously reported TEP concentrations for brackish and seawater (Passow, 2002a). Interestingly, our results revealed considerable concentrations of TEP in the colloidal size range down to 0.05 mm. Previous studies had

demonstrated the presence of TEP precursors (acidic polysaccharides) in the colloidal size range but its relative abundance at a specific size range was not clear (Passow, 2000; Thornton et al., 2007). Based on the results of this study, c-TEP between 0.05 and 0.40 mm contributed around 60–90% of total TEP, suggesting the importance of measuring this fraction as it is likely abundant in aquatic systems, and with its size, might be more persistent against pre-treatment (see Fig. 4).

a

b

c

d

e

f

Fig. 4 – TEP concentration of water samples collected through the treatment processes of 6 IMS. (a) Plant A: raw river water, after coagulation D RSF, after in-line coagulation D UF, RO feed and RO permeate; (b) Plant B: raw estuarine water, after heat exchanger (HE), after coagulation D RSF, UF permeate, after buffer tank (RO feed) and RO permeate; (c) Plant C: raw lake water, after CFS, after RSF, after BACF D 60 km transport (UF feed), UF permeate, after the buffer tank (RO feed) and RO permeate; (d) Plant D: raw estuarine water, after MF, after the buffer tank (RO feed) and RO permeate; (e) Plant E: raw canal water, after MF and RO permeate; (f) Plant F: raw impounded river water, after in-line coagulation D UF and RO permeate. TEP concentrations were presented relative to the total TEP concentration in the raw water. Error bars are representative of the average of 3 different samplings.

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3.3.

Removal of TEP by pre-treatment

Passow and Alldredge (1995) operationally defined TEP as acidic polysaccharides retained on 0.40 mm filters, while those not retained on these filters were considered as dissolved TEP precursors (Passow, 2000) or hydrogels (Verdugo et al., 2004). In this study the definition of IUPAC in categorizing particles was more or less followed, namely: particulate (suspended), >1 mm; colloidal, 0.001–1 mm; and dissolved, <0.001 mm. Unlike most particles, TEP are highly flexible and can squeeze through pores smaller than its apparent size, especially if under pressure or at high filtration rate. Presumably, colloidal TEP not retained by 0.40 mm filters have the same characteristics as particulate TEP. The smaller the TEP, the more likely it is to pass through MF/UF filters. These colloidal substances may eventually form p-TEP under certain conditions (e.g. shear forces (Passow, 2000; Li and Logan, 1997)), which likely exist in the treatment processes. In general, TEP may undergo a series of change depending on the treatment conditions (e.g. shear force, coagulation, retention times, temperature, etc.) in the plant. Thus, understanding the behaviour and removal of these substances is potentially important in selecting effective pre-treatment for reverse osmosis systems. To measure TEP removal by pre-treatment, p-TEP and c-TEP concentrations were measured over the pre-treatment processes of 6 IMS installations between March and July 2008. Results are shown in Fig. 4. The four UF systems totally removed p-TEP, while the two MF systems just partially removed this fraction. A total removal of p-TEP by UF was also reported in the recent studies by Kennedy et al. (2009) and Villacorte et al. (2009). The difference between the TEP removal of MF and UF can be attributed to the difference in pore size of membranes used. The UF membranes have nominal pore size of w30 nm while the MF membranes have nominal pore size of w100 nm. Although MF membranes have nominal pore size much smaller than the particulate cut-off (<0.40 mm) of TEP, significant amount of p-TEP were still able to pass through the membrane pores. The flexibility of TEP particles is possibly the main reason for this occurrence. However, membrane porosity and its pore size distribution may also be a contributing factor. The three coagulation/sedimentation/rapid sand filtration systems just partially removed p-TEP. However one of these systems (Plant C) removed more than 90% of total TEP, which can be attributed to the relatively high coagulant dose (w15 mg Fe3þ L1) applied to the system. None of the pretreatment systems investigated totally removed c-TEP. The special nature of TEP that is highly flexible in shape and size, might explain why complete removal of c-TEP with UF membranes had not been observed.

3.4.

5047

Biopolymer removal

To verify and compare the presence of TEP through the treatment processes, LC–OCD biopolymer analysis was performed on water samples collected from Plant B in June 2008 (Fig. 5). The dissolved organic carbon (DOC) of the raw water was about 4.8 mg-C L1 which consisted of around 63% humics, 18% low molecular weight neutrals, 11% biopolymers, 6% building blocks and 2% hydrophobic organic compounds. Biopolymer removal by moving bed filtration with pre-

Fig. 5 – LC–OCD chromatograms of water samples collected along the pre-treatment process of Plant B. Retention time is an indication of the molecular weight (MW) of the NOM fraction – the higher the retention time, the smaller the MW of the fraction. A significant removal of biopolymers is shown in the signal response after UF.

coagulation (6 mg Fe3þ L1) was about 20% while 70% removal was recorded by the subsequent UF treatment. In order to compare LC–OCD and TEP results, the concentration of proteins in the biopolymer fraction was estimated to calculate concentrations of polysaccharides which is a main component of TEP (Fig. 6). Comparisons were only based on cTEP (0.05–0.40 mm) results since water samples for LC–OCD were pre-filtered through 0.45 mm filters. Polysaccharide removal after coagulation and moving bed filtration was only 16% compared with 43% removal of c-TEP by the same treatment. However, comparable removal was observed in UF treatment, where 62% of polysaccharides and 69% of c-TEP were removed. This reaffirms our findings that polysaccharides, specifically TEP, can pass through UF treatment and were present in the feedwater of RO system.

3.5.

TEP accumulation on RO membranes

In spite of the presence of MF/UF pre-treatment, results presented in this study indicate that c-TEP were present in the feedwater of all RO plants and p-TEP in three of the plants investigated. Consequently, analysis was carried out in the RO systems to determine the extent of TEP deposition on RO membranes. The computed deposition factors of TEP can provide insights on the degree of TEP deposition in the RO system. Fig. 7 shows the theoretical total TEP concentration in the RO concentrate at different recovery when b ¼ 0 (no deposition) and b ¼ 1 (100% deposition). Also shown are the

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Fig. 6 – Comparison between DOC, LC–OCD and c-TEP results of samples collected through the treatment processes of Plant B. A consistency in terms of c-TEP and polysaccharide removal by UF pre-treatment is apparent in the figure.

measured concentrate total TEP concentrations of 4 IMS plants normalised relative to TEPf ¼ 1.0. The calculated b based on actual TEP results from the 4 plants range from 0.3 to 0.8 (Table 2). Only one result indicated possible depletion of TEP from RO membranes while another one indicated zero deposition. Majority of the b were between 0.3 and 0.7, suggesting that around 30–70% of TEP from the feedwater may have accumulated on the RO membranes. This was within the range of polysaccharide deposition efficiencies reported by Huber (1998) and Herzberg et al. (2009) in cross-flow RO/NF systems. The specific deposition rates (SDR) of total TEP in the RO systems of 4 plants were between 0.53 mg Xeq/m2 h and

4.71 mg Xeq/m2 h (Table 2). Possible depletion of TEP from the RO membrane to the concentrate was observed in one case. However, in most cases SDR indicate the deposition of TEP in the RO system. For now, it is still not clear of up to what extent of TEP deposition will the plant start to experience fouling problems. Nevertheless, it is likely case specific due to differences in RO operation and feedwater characteristics (e.g. availability of nutrients, presence of other foulants, etc.) of the plants. In Table 2, cleaning frequencies are given as a reference only, since no correlation is expected with deposition rate/factor due to differences in cleaning criteria used. Moreover, different forms of fouling might have occurred in the RO

Fig. 7 – The theoretical TEP concentration at the RO concentrate at different system recoveries if b [ 0 (no deposition) and b [ 1 (100% deposition), and the measured concentrate total TEP concentration in 4 RO plants, normalised relative to the feedwater concentration.

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Table 2 – The deposition factor (b) and specific deposition rate (SDR) of total TEP in the RO system of 4 IMS plants. Plant

Capacitya (m3/h)

Recovery (%)

Total areab (m2)

A

1200

85

24 600

B

63

75

1409

C D

2300 140

82 75

38 536 5712

Sampling date

15-04-08 28-05-08 16-07-08 03-03-08 28-05-08 19-06-08 04-06-08 23-05-08 23-06-08 10-07-08

TEP (mg Xeq L1)c TEPf

TEPc

0.04 0.14 0.07 0.06 0.14 0.09 0.04 0.16 0.21 0.27

0.35 0.29 0.33 0.12 0.40 0.32 0.20 0.47 0.56 0.72

Deposition factor (b)

SDR (mg Xeq/m2 h)

CIP (#/ month)

0.31 0.81 0.32 0.67 0.35 0.18 0.00 0.34 0.45 0.45

0.53 4.71 0.90 1.35 1.60 0.54 0.01 0.71 1.25 1.61

0.5 2.0 2.0 0.5 1.0 0.6 0.2 0.1 0.1 0.1

CIP ¼ cleaning-in-place. a RO first pass clean water production. b Total membrane area of the RO system. c Total TEP (calibration factor fx ¼ 114 Xeq L1; average value from Passow and Alldredge (1995)).

system and its effects may have manifested differently in each plant (Hoek et al., 2008). Further detailed studies are therefore necessary in these aspects to better understand the role of TEP in the fouling.

3.6.

Autopsy of RO membranes

To find evidence of TEP accumulation on RO membranes, a membrane autopsy was performed for some membrane elements taken from Plants C and D. Analysis using an optical microscope revealed that significant amounts of acidic polysaccharides (Alcian Blue stainable substances) had accumulated

on the RO membranes. A few of these substances were found attached to the corners of the membrane feed spacer mesh. According to Vrouwenvelder et al. (2009a,b,c), foulant accumulation in membrane spacers is considered critical in RO/NF operations since pressure drop in the system is mainly due losses within the feed spacer channels. Moreover, some fibrils and blobs of different shapes and sizes were found on the fouled membrane samples (see Fig. 8), mostly exhibited the physical characteristics of TEP. These TEP-like substances appeared to have entrapped and bound some colloidal and particulate matter, forming a heterogeneous layer of slime (McDonogh et al., 1994). Incidentally, this was consistent with the description

Fig. 8 – Photographs of TEP-like (green to blue) substances on RO membrane samples after staining with Alcian Blue: (a) element 1, stage 1, Plant C; (b) element 7, stage 2, Plant C; (c) element 1, stage 1, Plant D; (d) element 7, stage 2, Plant D (for interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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reported in a number of RO/NF membrane autopsy studies (Tran et al., 2007; Ning and Troyer, 2007). Previous studies had also identified polysaccharide substances as a major component of the foulant layers extracted from fouled NF/RO membranes (Park et al., 2006; Karime et al., 2008). However, not all of the polysaccharide substances found on RO membranes may have originated from the feedwater as TEP because some biofilm bacteria can produce considerable amount of polysaccharides at different phases of their growth as cell coatings, release or detritus lysed from cells of dead bacteria (Flemming, 2002; Xu and Chellam, 2005). A recent study by Bar-Zeev et al. (2009) demonstrated the possible manner of how TEP can accumulate on RO membranes using glass slides. It was also found in the same study that most of the accumulated TEP originated from the RO feedwater and were not locally produced by biofilm bacteria. Considering that a significant amount of acidic polysaccharide substances were found on the membrane samples we tested, it is likely that TEP which originated from the feedwater may have provided the base layer and contributed to the subsequent build-up of biofilm on the RO membrane. In Plant D, TEP-like substances were found more abundant in the tail element of the second stage as compared to the lead element in the first stage of the RO train. This seems to be the same scenario for Plant C, although the difference in accumulation appeared to be marginal. This trend is expected as TEP are more concentrated in the second stage than in the first stage. Another notable difference was that more TEP-like substances were found on the RO membrane of Plant D compared to Plant C. A similar observation was reported in a recent autopsy study, in which polysaccharide accumulation on RO membranes was reported lower after UF pre-treatment than after MF pre-treatment (Kim et al., 2008).

4.

Conclusions

1. The currently applied method to measure particulate TEP (>0.4 mm) was extended/modified to allow measurements of smaller colloidal TEP down to 0.05 mm. Based on the results, the colloidal TEP fraction was more abundant than particulate TEP (>0.40 mm) in a number of water sources (fresh and saline). 2. Total TEP concentration in coastal seawater was significantly higher (2–6 times) compared to fresh and estuarine water sources investigated. 3. Ultrafiltration pre-treatment proved to be most effective in removing particulate TEP (>0.4 mm) than any other types of pre-treatment investigated (microfiltration, conventional treatment), but neither low pressure membranes (MF/UF) nor conventional pre-treatments (CSF) were absolute barriers against colloidal TEP (<0.4 mm) from entering the RO system. 4. Biopolymer analysis using LC–OCD showed consistency between the removal of c-TEP and polysaccharides by UF pre-treatment. The analysis also verified the presence of TEP in the RO feedwater. 5. Calculations of the deposition factors (b) and specific deposition rates (SDR) of TEP indicated that TEP had accumulated in the RO system of all the plants investigated.

Based on these calculations, around 30–70% of TEP from the feedwater were deposited in the RO system. 6. Membrane autopsies found evidence of TEP-like substances accumulated on RO membranes. However, some of these substances may have been produced locally by biofilm bacteria and not from the feedwater. 7. Further improvement and verification of the current and extended/modified TEP method, particularly the calibration, is necessary so that TEP can be employed as a parameter for fouling studies in RO systems.

Acknowledgements This research was conducted under the financial support of Delft Cluster-TSA. The authors would like to thank Paul Buijs (Global Membrains) and the plant staff of the 6 IMS plants studied for their valuable assistance. Also thanks to Sergio Salinas-Rodriguez (UNESCO-IHE) for processing the LC–OCD chromatogram results from Het Waterlaboratorium. Dr. Uta Passow (Alfred Wegener Institute for Polar and Marine Research) and Prof. Tom Berman (Israel Oceanographic and Limnological Research) are also acknowledged for their support and for sharing useful information about the TEP method.

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