Performance of an integrated granular media – Ultrafiltration membrane process for drinking water treatment

Journal of Membrane Science 492 (2015) 164–172

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Performance of an integrated granular media – Ultrafiltration membrane process for drinking water treatment Wenzheng Yu n, Nigel J.D. Graham Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK

art ic l e i nf o

a b s t r a c t

Article history: Received 18 March 2015 Received in revised form 17 May 2015 Accepted 18 May 2015 Available online 6 June 2015

The accumulation of residual coagulant flocs and microbial substances on the surface of ultrafiltration (UF) membranes are major contributors to membrane fouling that reduces process performance. Previous approaches to reduce fouling (e.g. addition of an oxidant or disinfectant) have been only partly successful in reducing the formation of a cake layer and material deposits within membrane pores. In this study the performance of an integrated granular media – UF membrane process has been evaluated in which a hollow-fibre UF module was embedded within a sand layer in order to prevent fouling material reaching the UF surface (forming a cake layer). The evaluation involved comparing two laboratory-scale UF systems, operated in parallel for 74 days, with one incorporating the sand layer (CSUF, coagulation-sand layer filtration-ultrafiltration), and the other without (CUF, coagulationultrafiltration), serving as a reference conventional process. The results showed that the incorporation of the sand layer successfully prevented the formation of any significant cake layer on the membrane surface and substantially reduced inner membrane fouling, which lead to a much reduced transmembrane pressure (TMP) increasing rate. The difference in performance was principally attributed to microbial growth and the release of extracellular polymeric substances (EPS) which was much greater in the conventional CUF system. Thus, in the CUF system, the deposition of coagulation flocs (consisting of precipitated nano-scale primary particles) and bacteria on, and within, the membrane produced substantial reversible and irreversible fouling. In contrast, the deposited material (flocs) in the sand layer of the CSUF system was easy to be washed away, resulting in fewer bacteria in the sand layer, and a much reduced production of biopolymer and other EPS, and their accumulation by the UF membrane. & 2015 Elsevier B.V. All rights reserved.

Keywords: Ultrafiltration Sand layer Membrane fouling Coagulation Water treatment

1. Introduction Membrane technology has been developed and applied extensively in drinking water treatment, but difficulties still remain in its use and particularly with regard to membrane fouling. Pretreatment involving chemical coagulation or coagulation–hydraulic flocculation has been shown to be an effective and low-cost approach, not only to improve general water quality, but also to control membrane fouling [1,2], such as by the use of polyaluminum chloride [3]. However, unseparated flocs, or those formed after the coagulation process, will approach the surface of the membrane and form a cake layer, and many experimental studies and practical operations have indicated that cake layer formation is the main cause of membrane fouling [4–6]. As well as the direct physico-chemical effect of flocs, the presence of bacteria on the surface of the flocs or

n

Corresponding author. Tel.: þ 44 2075946121; fax: þ 44 2075945934. E-mail addresses: [email protected] (W. Yu), [email protected] (N.J.D. Graham). http://dx.doi.org/10.1016/j.memsci.2015.05.032 0376-7388/& 2015 Elsevier B.V. All rights reserved.

near the membrane surface, may induce membrane fouling after a long period of operation through the release and accumulation of extracellular polymeric substances (EPS) in the cake layer or membrane pores. It has been shown that the increased adsorption of EPS onto a membrane results in a significant decrease in permeate flux, and EPS production may influence sludge deposition (and the attachment rate) and subsequently affect the biofouling propensity of the membrane [7,8]. It has also been observed that the accumulation of biopolymer clusters in the sludge mixture of a membrane bioreactor (MBR) facilitates the formation of a sludge fouling layer on the membrane surface, thus causing serious fouling [9]. Furthermore, as the thickness of the bio-cake grows, an anoxic and endogenous environment may develop in the lower parts of the bio-cake layer, ultimately leading to cell lysis and release of polysaccharides [10,11]. Therefore, the removal of flocs more effectively before they reach and attach to the membrane surface, and/or the avoidance of a cake layer developing, should mitigate membrane fouling. Several methods/treatments prior to membrane filtration have been investigated and reported in the literature recently. One method involved changing the characteristics of the membrane

W. Yu, N.J.D. Graham / Journal of Membrane Science 492 (2015) 164–172

surface, such as through modification of the membrane surface with anti-adhesion and anti-bacteria properties [12–14], as well as by use of a quorum quenching enzyme (acylase) and fungal inoculation on the membrane surface [15,16]. A second approach was to change the characteristics of the flocs, such as by adding anionic coagulant aids [17], and polyelectrolytes [5] and flocculants [18,19], as well as through controlling residence times [20,21]. A third approach is that of removing established cake layers, such as by adding suspended carriers [22,23], moving media [24] and support media [25]. In addition, providing a sufficient supply of oxygen [10,26], gas bubbling [27] and the use of ultrasound [28] have been proposed as alternative methods. To-date there have been very few investigations of the benefits of UF membrane pre-treatment by filtration, and the findings of these are contradictory. One recent study reported that prefiltration alone (without pre-coagulation) improved ultrafiltration performance, as indicated by a flux increase and reduction of fouling rate [29], whereas an earlier study [30], also without precoagulation, found that pre-filtration (1.5 μm pores glass-fibre filters) had little effect on ultrafiltration. A further study showed that either slow sand filtration, or coagulation, before ultrafiltration could mitigate membrane fouling [31]. A novel alternative process involving the combination of coagulation and ultrafiltration within a sand bed has not been considered previously and this approach is the subject of the study reported here. In this arrangement the conventionally separate stages of floc sedimentation, deep bed sand filtration and UF membrane can be combined into a single treatment unit process, thereby saving land area and capital cost. In addition, the direct contact between the sand layer and the UF membrane can fundamentally change the nature of the external and internal accumulation of deposits that cause membrane fouling. Thus, this paper summarizes some preliminary results of a laboratory study of the performance of a UF membrane placed within a sand layer receiving coagulated model water, particularly in terms of reducing external and internal membrane fouling. The results have shown that the novel arrangement, while simple, can substantially improve the UF membrane performance. The sand layer appears to concentrate coagulant flocs, EPS and other pollutants before the membrane, thereby reducing the rate of membrane fouling.

2. Materials and methods 2.1. Synthetic raw water and coagulant A synthetic raw water was used for the tests in order to simplify the study and provide sample consistency and reproducibility. A small quantity of settled waste water was added to the local (London, United Kingdom) tap water with a volumetric ratio of 1:50, as well as 5 mg/L humic acid (International Humic Acid Substance Society, USA), to represent a micro-polluted raw water.

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The settled waste water and humic acid can represent types of organic matter which are typically difficult and easy to be removed by coagulation, respectively, and the waste water provides bacteria representative of surface water contaminated by microorganisms from effluent discharges. Prior to mixing with the waste water and humic acid solution, the tap water was left over night to ensure the complete decay of residual chlorine. The characteristics of the synthetic raw water are listed in Table 1. During the course of the experimental program the temperature of the water was maintained at 2072 1C (room temperature). 2.2. The UF treatment systems A schematic illustration of the experimental set-up involving the coagulation-UF processes with and without a sand layer (CSUF/CUF) around the submerged membrane module (dead end mode), operated in parallel, is given in Fig. S1 (Supporting information). Synthetic raw water was fed into a constant-level tank to maintain the water head for the membrane tanks. An optimal dose of Al2(SO4)3 coagulant (0.15 mM, calculated as Al) was continuously added into the rapid mixing units; the alum dose corresponded to near zero zeta potential of resulting flocs. The rapid mix speed was 200 rpm (184 s  1) in the mixing units with a hydraulic retention time (HRT) of 1 min, which then reduced to 50 rpm (23 s  1) in the three flocculation tanks, each having a HRT of 5 min. After the flocculation tanks the flow passed directly into the membrane tanks. Each tank contained a submerged polyvinylidene fluoride (PVDF) hollow-fiber UF membrane module (Tianjin Motimo Membrane Technology Co. Ltd, China) with a nominal pore size of 0.03 μm and a surface area of 0.025 m2 (inner diameter¼0.7 mm, and outer diameter¼ 1.1 mm). For the CSUF tank, the membrane module was placed within a sand layer (sand size is around 0.5 mm), retained by a stainless steel mesh (pore size is 0.256 mm), such that the flow passed through the sand (approximately 1 cm deep) before ultrafiltration. The UF permeate was continuously collected by a suction pump at a constant flux of 20 L/(m2 h), operated in a cycle of 30 min filtration and 1 min backwash (40 L m  2 h  1). For each backwash, air was supplied to each reactor immediately below the membrane modules at 100 L/h (air: water ¼200:1). The HRT of the membrane tanks was maintained at 0.5 h and accumulated sludge was released every 3 days. The integrity of the sand layer was unaffected by the routine backwashing throughout the experimental period as the backwash rate was very low, and the aeration only contacted the outer surface of the sand layer, held by the stainless steel mesh. During the 74 days of the CUF/CSUF operation, the CUF membrane was taken out and washed by sponge on two occasions, at day 35 and day 59; while for the CSUF process only one cleaning operation was required (day 59), which involved removing the UF membrane from the sand layer and subsequent washing of the sand (similar to the backwashing of sand filters in

Table 1 Water qualitya of raw water and UF influentsb/filtrates. Parameter

Raw water

CUF influent

CSUF influent

CUF filtrate

CSUF filtrate

UV254 (cm  1) DOC (mg/L) Turbidity (NTU) SS (mg/L) Al (mg/L) NH4þ –N (mg/L) NO3 –N (mg/L) pH

0.117 70.018 4.805 70.423 6.13 70.56 4.2 70.8 0.009 70.003 0.282 70.103 5.43 70.22 8.15 70.06

0.053 7 0.016 3.1797 0.325 2.05 7 0.18 47.2 7 3.2 0.083 7 0.012 0.0777 0.018 5.65 7 0.17 7.79 7 0.09

0.050 70.010 2.929 70.283 1.98 70.21 36.8 7 2.2 0.037 70.011 0.064 70.019 5.71 70.13 7.7770.04

0.048 7 0.006 3.0577 0.246 0.0770.03 – 0.0727 0.022 0.0427 0.019 5.687 0.23 7.78 7 0.05

0.0457 0.008 2.792 7 0.198 0.067 0.03 – 0.032 7 0.008 0.034 7 0.022 5.747 0.26 7.80 7 0.04

a b

The values shown in table are averages for all the measurements taken every 7 days (10 times). Influent – within membrane tank, immediately after flocculation units.

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conventional water treatment plants); more details of the cleaning are provided in the supporting information. 2.3. Extraction and measurements of EPS from cake layer and sludge At the end of the UF operation, the fouled membrane modules were taken out from the membrane tanks. The foulant materials on the membrane surface (cake layer) or in the sand layer were carefully scraped off with a plastic sheet, and analyzed by the following methods to characterize the contents of external membrane fouling. The flocs/sludge in the sand layer was washed and separated by 1 min ultrasound. The extraction of internal fouling is described later. A heating and extraction method [32] was modified to extract the loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) from the cake layer, sand layer and sludge, and to make sure the EPS was not released from bacterial cells. Sludge suspension and cake layers were first dewatered by centrifugation (Model 5417C, Eppendorf, Germany) in a 20-mL tube at 3000g for 5 min. The concentrate liquor was recovered for water quality analysis. The sludge pellet in the tube was re-suspended in 10 mL phosphate buffer saline (PBS) solution, and the sludge suspension was then ultrasonically treated (Nusonics, USA) for 1 min (0.8 W/cm2). Without any delay, the suspension was then sheared by a vortex mixer (Vortex- Genies 2, Mo Bio laboratories, Inc., USA) for 15 min, followed by centrifugation at 3000g for 10 min. The organic matter in the supernatant was readily extractable EPS, and was regarded as the LB-EPS of the biomass. For extracting the TB-EPS, the sludge pellet left in the centrifuge tube was resuspended in PBS solution to its original volume (10 mL), ultrasonically treated for 3 min, and heated to 80 1C in a water bath for 30 min. Then the mixture was centrifuged at 10,000g for 15 min. After the membrane surface was wiped with a sponge, 0.01 mol/L NaOH was used for extraction of internal foulants and the fibers were soaked for 24 h at 20 1C in the alkaline solution according to the method described by Kimura et al. [20] and Yu et al. [33]. The extracted organic matter was then subjected to the following chemical analyses. The absolute polysaccharide content in the bound EPS (LB-EPS plus TB-EPS) was measured by the phenol–sulfuric acid method with glucose as the standard [34]. A modification of the Bradford method [35] called the Coomassie procedure (Pierce Chemical), was used to quantify the absolute concentration of proteins, with bovine serum albumin (Sigma) as the standard. Polysaccharide and protein concentration were measured at least three times. 2.4. EEM and SEC EPS extracted from the cake layer and sludge was also measured by three-dimensional excitation-emission matrix (EEM) fluorescence spectrometry and size exclusion chromatography (SEC). Fluorescence measurements were conducted using a spectrofluorometer (PerkinElmer, LS 55, USA) at an ambient temperature of 25 1C. Further details of the method can be found in previous research reported elsewhere [36,37]. As suggested by Myat et al. [38], SEC-UV254 might be used to identify aggregated biopolymers, combined with other characterization techniques. SEC was carried out to determine the apparent molecular weight (MW) distribution of UV-active substances in the waters from the two systems, as well as the organic matter (EPS) extracted from the cake layers and sludges. SEC was performed using a BIOSEP-SEC-S3000 column (Phenomenex, UK) (7.8 mm  300 mm) as well as the Security Guard column fixed with a GFC-3000 disc 4 mm (ID). A solution of 10 mM sodium acetate (Aldrich, USA) was used as the mobile phase. Analysis using High Performance Size Exclusion Chromatography (HPSEC)

was achieved on the HPLC system (Perkin-Elmer, USA) using the following instrumentation: Series 200 pump, UV/vis detector operated at a wavelength λ ¼ 254 nm and autosampler. The flow rate was set at 1 mL/min, and the injection volume of water samples was 100 μL. Prior to operation, the mobile phase was purged at a volumetric flow rate of 2 ml/min in order to clear any residual and wash out the column of any contaminants. Polystyrene sulfonate (PSS) standards (American Polymer Standard Corp., U.S.) of molecular weights 33,500, 14,900, 6530, and 1100 Da. were employed to calibrate the relationship between the MW and the retention time. 2.5. Other analytical methods Fouled membrane fibers were cut from the two membrane modules, and the foulant layer attached on the membrane surface was retained on the membrane surface. The fouled membrane samples were then platinum-coated by a sputter and observed under high resolution field emission gun scanning electron microscope (FEGSEM, LEO Gemini 1525, Germany). Also, the nature of the materials on the fouled membranes was analyzed by Fourier Transform infrared spectroscopy (FTIR, Spectrum 400, PerkinElmer, USA) with Quest ATR Accessory (SPECAC Ltd, UK). The UV absorbance at 254 nm, UV254, of 0.45 μm filtered solutions was determined by an ultraviolet/visible spectrophotometer (U-3010, Hitachi High Technologies Co., Japan). Dissolved organic carbon (DOC) was determined with a total organic carbon (TOC) analyzer (TOC-VCPH, Shimadzu, Japan). Residual aluminum after 0.45 μm membrane filtration was measured by inductivity coupled plasma optical emission spectrometer (ICP-OES, Optima 7300 DV, PerkinElmer, USA). Residual turbidity measurements (Hach 2100, USA) were made for samples in the two membrane tanks. The concentrations of NH4þ –N and NO3 –N were determined by the APHA standard colorimetric/spectrometry methods [39], and the concentrations of bacteria were determined as the Heterotrophic Plate Count (HPC) by the recommended method involving the use of yeast extract agar [40].

3. Results and discussion 3.1. TMP developments in CUF and CSUF systems As the membrane flux was fixed at a constant value (20 L/ m2 h), membrane fouling was indicated by the increase in TMP. The comparative increase in TMP for the CUF and CSUF streams are shown in Fig. 1. There was no detectible, additional pressure loss caused by the 1 cm sand layer in the CSUF system at the start of process operation, as expected at such a low filtration rate. Comparing the two types of pretreatment, the additional prefiltration by the sand layer after coagulation produced a membrane fouling rate that was substantially lower than that of only coagulation pre-treatment. In particular, the initial (first 15 days) increase in TMP of the CUF membrane was rapid (1–5 kPa), while there was no measurable increase for the CSUF membrane. Subsequently, although there was an increase of TMP in the CSUF system, the increase was dramatically slower than that of the CUF system. Thus, before the CUF membrane required washing (at day 35), the rate of TMP increase of the CUF membrane was nearly five times that of the CSUF membrane, indicating the significance of membrane protection (floc capture) by the 1 cm sand layer. After physical cleaning of the CUF membranes (membrane was taken out and cleaned by high pressure tap water) at day 35 and day 59, a much greater initial TMP was found (6.5 kPa and 7.5 kPa), which may be related to the difficulty of removing the cake layer on the CUF membrane, and other organic contaminants.

W. Yu, N.J.D. Graham / Journal of Membrane Science 492 (2015) 164–172

167

22 14 21

TMP (kPa)

10

20

o

Wash

2.0

UV254 adsorbance (mV)

Wash

Temperature ( C)

12

8

19

CUF CSUF Temperature

6

18

Sand Wash

4

17 16

2

1.5

Raw water CUF effluent CSUF effluent

Humic acid

100 80

Building blocks

Biopolymer

60

1.0

40

Low MW organic matter

20 0

0.5

1000000

100000

10000

15

0 0

10

20

30

40

50

60

70

80

1000000

Fig. 1. Variation of TMP with different pretreatment conditions over a period of approximately 74 days (20 L m  2 h  1) (In the UF filtration experiments, the membrane flux of the CUF and CSUF streams were both set at a constant value of 20 L m  2 h  1 in a cycle of 30 min filtration/1 min water backwash (40 L m  2 h  1); CUF membrane was washed by sponge at day 35 and day 59; the sand layer around the CSUF membrane was washed at day 59, but there was no sponge wash for the membrane).

As established in our previous research results, a low amount of precipitate solids of coagulants cause little membrane fouling [41]. Instead, the activities of bacteria and the influence of organic matter are believed to be the main factors of fouling, including the role of EPS. The EPS in the cake layer may transfer to the membrane pores after a long operation time, as well as from the feed water. Size exclusion chromatograms (molecular weight) of the corresponding samples were produced to further characterize the dissolved organic matter (DOM) (Fig. 2a). The MW distributions obtained from the chromatograms displayed significant differences in the feed and effluent samples (there was little difference between tank samples and effluent samples), particularly in the range of large molecules identified as biopolymers. As previously reported, part of the EPS can be removed by the coagulation process [42]. However, the residual EPS was higher in the CUF tank than that in CSUF tank. Some similar results indicated by EEM analysis can be found in the supporting information (Fig. S2). The EPS properties can be linked to the microbial community and its activity [43], and thus the higher EPS concentration in the CUF

10000

1000

100

10

2000

-1

3.2. Organic matter and bacteria concentration in membrane tanks

100000

Molecular Weight (Dalton)

Bacteria concentration (mL )

Furthermore, in addition to some residual cake layer on the surface of the membrane, there may have been blockage of the pores by other materials, causing the greater degree of irreversible membrane fouling. In marked contrast, the increase in TMP of the CSUF membrane over the 59 days of operation was only 2.2 kPa. Washing of the sand layer at day 59 resulted in a much lower TMP (1.7 kPa). In this case it was assumed that the low membrane fouling was mainly caused by the residual flocs in the sand layer after the long operation time, and with only minor internal membrane fouling. Throughout the period of operation the water temperature increased no more than 2 1C, which was not significant to the process performance. In summary, it was evident that the internal and external membrane fouling in the CUF process was much greater than the CSUF process. In view of this enhanced performance arising from the pre-filtration with the sand layer, further detailed investigation was undertaken concerning the nature of the flocs in the sand layer and cake layer, and the extent of bacterial activity in the two membrane systems.

A

0.0

Time (Day)

1500

1000

500

0 Raw water

Tank

Tank

Effluent

Effluent

CUF

CSUF

CUF

CSUF

Fig. 2. MW distributions of DOM (a), and bacteria concentrations (b), in the two membrane systems at day 50.

indicated that the activity of bacteria in the CUF was greater than in the CSUF. The presence of viable bacteria (HPC) was investigated for the two membrane systems and was found to increase in both membrane tanks as the period of operation increased (Fig. 2b). Although the total bacteria concentration increased in both membrane tanks, it was much lower in the CSUF tank. The result showed that the flocs were difficult to adhere in the sand layer than onto the membrane surfaces, which caused fewer bacteria to be present on the surface of particles (flocs) in the CSUF membrane tank as the backwash process can remove retained flocs in each backwash cycle. These results clearly indicated that a higher concentration of EPS was produced by greater bacterial activity in the CUF tank. As drinking water sources are often polluted by NH4þ , such as in China [44], its removal efficiency was also discussed here. Although there were some differences between the bacteria concentration in the two systems, there were little differences in NH4þ transformation between them (Table 1). Nearly all NH4þ appeared to be converted to NO3 in both membrane tanks, with very low concentrations of NH4þ in the effluent water, which indicated that the bacterial concentration was sufficiently high to transfer NH4þ to NO3 in the two systems. Therefore, the existence of the sand layer did not change the transformation efficiency of NH4þ to NO3 (Table 1), and both processes were able to treat the NH4þ effectively.

W. Yu, N.J.D. Graham / Journal of Membrane Science 492 (2015) 164–172

The information from SEC analyses was also used to characterize the EPS of the sludge/flocs from the CSUF sand layer and the CUF cake layer (Fig. 3), and different peaks were analyzed according to the approach used by Huber et al. [45]. It can be seen that the LBEPS biopolymers (105–104 Da) in the CUF cake layer were much greater than the flocs/sludge from the sand layer (Fig. 3a). These results suggest that there were low levels of bacteria present in the sand layer flocs, which in turn produced low concentrations of biopolymer. In contrast, greater bacteria concentrations developed in the CUF cake layer as the process operation proceeded, leading to greater quantities of EPS. The results were consistent with the impact of the regular removal of flocs/sludge from the CSUF sand layer during the back wash process, where the solids were easier to be removed than the cake layer on the CUF membrane surface. Also the results given in Fig. 3b confirmed that the concentration of TB-EPS with MW larger than 500 Da was much higher in the cake layer from the CUF system than the sand layer flocs of the CSUF system, especially for the MW range between 104 and 105. These results indicate again that a higher concentration of EPS was produced by greater bacterial activity in the CUF cake layer. Therefore, the existence of high concentrations of EPS may result in their escape from the cake layer and entry into the membrane pores, causing both external and internal membrane fouling. In addition to the SEC method for EPS measurement, the absolute concentration of EPS was also measured in the cake layer and sludge (protein and polysaccharides). Comparing the EPS content in the sludge and cake layer/sand layer of the two systems, the amount of total protein in the CUF cake layer was substantially higher (approximately twice) than that in the CUF sludge and the CSUF sludge and sand layer (Fig. 4), indicating the accumulation of proteins within the membrane cake throughout the operating period (note that sludge was removed from the membrane tanks every 3 days). The variation of polysaccharide quantities extracted from the cake layer, sand layer flocs and sludges in the two membrane systems was very similar to that of protein, with the greatest concentration present in the CUF cake layer. Overall, the results further confirmed the correspondence between the higher EPS concentration (proteins and polysaccharides) in the CUF cake layer and the greater extent of external membrane fouling.

section 2.3, fouling material from within the membranes were extracted by NaOH and analyzed by EEM and SEC. In addition, the membranes after washing by sponge were examined by FTIR spectroscopy. The characteristics of EEM fluorescence spectra of internal membrane foulants were slightly different from those of the external foulants, as indicated in Fig. 5(a) and (b). Peaks A and C represent humic acid type materials, which is consistent with the

0.010

Protein concentration (g/g SS)

3.3. EPS in the cake layer and sand layer

0.004

0.002

CUF sludge

0.08

0.06

0.04

0.02

0.00

30

0.7

Humic acid

0.6 0.5

Building blocks Low MW organic matter

0.4 Biopolymer

0.2 0.1

UV254 adsorbance (mV)

CUF cake layer CSUF flocs in sand layer

0.8

CUF sludge

Humic acid

25

100000

10000

1000

100

Molecular Weight (Dalton)

10

CSUF flocs in sand layer

CSUF sludge

Building blocks Low MW organic matter

20 15 10 Biopolymer 5

CUF cake layer CSUFflocs in sand layer

0.0 -0.1 1000000

CSUF sludge

Fig. 4. The concentration of protein (a) and polysaccharide (b) in the cake layer (CUF), sand layer (CSUF), and sludges of the membrane tanks.

1.0 0.9

CSUF flocs in sand layer

0.10

CUF cake layer

As well as the existence of external fouling, the development of internal fouling was also investigated in this study. As described in

UV254 adsorbance (mV)

0.006

CUF cake layer

3.4. Internal membrane fouling

0.3

0.008

0.000

Polysaccharides concentration (g/g SS)

168

0 1000000 100000 10000

1000

100

10

1

Molecular Weight (Dalton)

Fig. 3. MW distributions of (a) LB-EPS and (b) TB-EPS extracted from the cake layer of CUF or flocs retained in the sand layer of CSUF (methods can be found in Section 2.3).

W. Yu, N.J.D. Graham / Journal of Membrane Science 492 (2015) 164–172

450

400

2

3 5 4 6 4 5 48 6

350

3 5 2

300 1

3 1

76

4

C 10 B 14 11

13 A 12

15

15

T1 2 1

8

15

5 6

7

3

250

400

350

400

450

500

237 65 8

350 4

6 5

1

9

8

12

C

7

3

B11

A

10

11

2

300

T1

1

5 4

10

250

550

250

1.4 CUF CSUF

1.2

12 Building blocks

10 8 Biopolymer

Low MW organic matter

4

Absorbance

14

300

350

400

450

500

550

Emission Wavelength (nm)

Humic acid

16 UV254 adsorbance (mV)

4 2 6 7 45

200 300

18

new membrane CSUF membrane CUF membrane

1.0 0.8 0.6 0.4 0.2

2

0.0

0 1000000

5 4 6 5 7

Emission Wavelength (nm)

6

2

3

3 2 22 4 2

Excitation Wavlength (nm)

Excitation Wavelength (nm)

450

200 250

169

100000

10000

1000

100

10

Molecular Weight (Dalton)

-0.2 2000 1800 1600 1400 1200 1000

800

600

-1

Wavelength (cm )

Fig. 5. EEM fluorescence spectra of inner membrane fouling for CUF (a) and CSUF (b) (peak values shown – expressed in absorbance units); MW distributions of DOM (c) and FTIR spectra (d) of the two membranes.

humic acid present in the raw water [33]. From the EEM fluorescence spectra of the two fouled membranes (Fig. 5a and b), there was a very low intensity of peak T1 (protein-like material) evident from the inner membrane fouling material compared to the much greater intensities for the humic-like substance peaks (C and A). The occurrence of peak B is believed to be from the overlapping of peaks A and C as was evident in the material of both membranes. The intensity of all three principal peaks (A, C and T1) in the CSUF membrane pores was much lower than that from the CUF membrane, which was consistent with the accumulation of less humic-like materials and EPS in the CSUF membrane. The SEC results also clearly indicated that less organic matter was retained or adsorbed in the CSUF membrane pores, compared to the CUF system, with a lower concentration of biopolymer and humic-like materials (Fig. 5c). The higher concentration of EPS, as well as humic-like materials, in the CUF was consistent with the higher internal fouling resistance, indicated by the greater TMP development of the CUF system observed (Fig. 1). These results confirmed that the presence of the sand layer in the CSUF system lead to a much reduced cake layer, and consequently significantly less EPS and humic acid was adsorbed within the membrane pores. The results also indicated that the organic matter in the membrane pores most likely originated from the cake layer, both from the effect of bacterial activity and desorption. The nature of the internal membrane fouling was further examined by FTIR analyses. As illustrated in Fig. 5d, the peaks at

1640 and 1542 cm  1 may correspond to CQO and –N–H bonds [46], and although they increased for both membranes, the increase level was much lower for the coagulation-sand layer filtration pretreatment (CSUF). The organic matter contained CQO bonds, such as carboxylic acid, may complex with Al (–OH) and adsorbed in the membrane pores. Also the –N–H peak may be ascribed to proteinaceous material adsorbed in the membrane pores, produced by bacteria (within the EPS). In contrast, the intensities of PVDF absorption peaks at 763, 870, 1016, 1174, and 1400 cm  1 corresponding to CF2 and CH2 chemical bonds are reduced in the two membrane systems, most likely through adsorption of organic substrates [47]. It is evident from Fig. 5d that the decrease of the peaks for the coagulation-sand layer filtration pretreatment was much less than for only coagulation pretreatment, which meant that more organic substrate was adsorbed on the CUF membrane. These results further show the benefit of the sand filtration process in reducing the adsorption and accumulation of organic contaminants within the membrane pores (internal fouling). 3.5. SEM images of fouled membrane structure In order to support the results indicating that membrane fouling can be mitigated by preventing flocs accumulating on the surface of the UF membrane, and forming a cake layer, SEM images were obtained and can be seen in Fig. 6. The SEM images showed

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350 300

Number

250

CUF CSUF

200 150 100 50 0 <10

10~15

15~20

20~25

25~30

>30

Size (nm) Fig. 6. SEM images of membranes with different pretreatment: CUF membrane without (a) and with (c) wash, CSUF membrane without (b) and with (d) wash, and of sand grains (CSUF) before water backwash (e); and open pore size distributions of fouled membranes in CUF and CSUF (f).

that there was a thick cake layer on the surface of the CUF membrane, which is composed of nano-scale primary particles, as well as some EPS-like materials (Fig. 6a). In contrast, there was little indication of a cake layer on the surface of the CSUF membrane, and only a limited number of small flocs were evident (Fig. 6b), confirming that suspended flocs in the membrane tank were prevented by the sand layer from reaching the surface of membrane. The SEM images also showed the presence of flocs on the surface of the sand (sand layer) before water backwash (Fig. 6e). Although in conventional UF operation the applied shear stress during backwashing is sufficient to remove substantial proportions of the cake layer and moderate membrane fouling, the thickness of cake layer progressively increases, leading in turn to increasing membrane fouling after a long period of membrane

operation. In contrast, the presence of the 1 cm sand layer in the CSUF was able to retain flocs in the sand layer and prevent them reaching the surface of the membrane, thus the application of fluid shear stress by backwashing is not needed. According to the EPS concentration in the sand layer, the bacteria on the flocs were also prevented by the sand layer, which was much easier to be backwashed away, causing little EPS reaching the surface of the CSUF membrane. After the cake layer was washed and the membranes were cleaned by sponge, the internal fouling of the membrane was mainly determined by the blockage of pores. As seen from the SEM images (Fig. 6c and d), there were few primary particles resident on the surface of both membranes, but more pores were blocked by the organic matter in the case of the CUF membrane. This was

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evident visually and from the statistical number of open pores of the two fouled membranes, as quantified by the use of Scion Image software (Fig. 6f). Thus, the results in Fig. 6f indicated more pores of a wide range of sizes remained open during the treatment period in the case of the CSUF membrane, compared to the CUF membrane, which is consistent with the accumulation of more organic matter in the CUF membrane pores (Fig. 4).

4. Conclusions The potential benefits of enclosing UF membranes within a sand layer, following coagulation pretreatment, have been investigated with a principal focus on reducing membrane fouling effects. 1. Compared to conventional coagulation pretreatment (CUF), the novel arrangement of pre-filtration by a sand layer after coagulation (CSUF) produced a membrane fouling rate that was substantially lower, potentially offering a more efficient and compact process, as it merge together three separate treatment steps – sedimentation, sand filtration and ultrafiltration. 2. SEM images showed that there was a thick cake layer on the surface of the CUF membrane, which is mainly composed of nano-scale primary particles. In contrast, there was little cake layer evident on the surface of CSUF membrane. 3. The presence of EPS/biopolymers in the CUF cake layer was much greater than in the flocs from the sand layer. This corresponded to the greater concentrations of bacteria present in the CUF cake layer as the membrane operation proceeded. 4. Analysis by different methods of internal membrane foulant material clearly showed that substantially less organic matter (EPS) was present in the CSUF membrane, compared to the CUF membrane, with a lower concentration of biopolymers and humic-like materials.

Acknowledgments This research was supported by a Marie Curie International Incoming Fellowship (FP7-PEOPLE-2012-IIF-328867) within the 7th European Community Framework Programme for Dr Wenzheng Yu. The authors wish to acknowledge the assistance of Thames Water Utilities Ltd (wastewater samples) and colleagues in Imperial College London.

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