Reduced membrane fouling in a novel bio-entrapped membrane reactor for treatment of food and beverage processing wastewater

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journal homepage: www.elsevier.com/locate/watres

Reduced membrane fouling in a novel bio-entrapped membrane reactor for treatment of food and beverage processing wastewater Kok-Kwang Ng a, Cheng-Fang Lin a,*, Sri Chandana Panchangam a, Pui-Kwan Andy Hong b, Ping-Yi Yang c a

Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Rd., Taipei 106, Taiwan Department of Civil and Environmental Engineering, University of Utah, 110 South Central Campus Drive, 2068 MCE, Salt Lake City, UT 84112, USA c Department of Molecular Biosciences and Bioengineering, University of Hawaii at Manoa, Honolulu, HI 96822, USA b

article info

abstract

Article history:

A novel Bio-Entrapped Membrane Reactor (BEMR) packed with bio-ball carriers was con-

Received 24 December 2010

structed and investigated for organics removal and membrane fouling by soluble microbial

Received in revised form

products (SMP). An objective was to evaluate the stability of the filtration process in

27 May 2011

membrane bioreactors through backwashing and chemical cleaning. The novel BEMR was

Accepted 31 May 2011

compared to a conventional membrane bioreactor (CMBR) on performance, with both

Available online 7 June 2011

treating identical wastewater from a food and beverage processing plant. The new reactor has a longer sludge retention time (SRT) and lower mixed liquor suspended solids (MLSS)

Keywords:

content than does the conventional. Three different hydraulic retention times (HRTs) of

Bio-entrapped membrane reactor

6, 9, and 12 h were studied. The results show faster rise of the transmembrane pressure

Conventional membrane bioreactor

(TMP) with decreasing hydraulic retention time (HRT) in both reactors, where most

Hydraulic retention time

significant membrane fouling was associated with high SMP (consisting of carbohydrate

Sludge retention time

and protein) contents that were prevalent at the shortest HRT of 6 h. Membrane fouling

Soluble microbial products

was improved in the new reactor, which led to a longer membrane service period with the

Membrane fouling

new reactor. Rapid membrane fouling was attributed to increased production of biomass and SMP, as in the conventional reactor. SMP of 10e100 kDa from both MBRs were predominant with more than 70% of the SMP <100 kDa. Protein was the major component of SMP rather than carbohydrate in both reactors. The new reactor sustained operation at constant permeate flux that required seven times less frequent chemical cleaning than did the conventional reactor. The new BEMR offers effective organics removal while reducing membrane fouling. ª 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Membrane bioreactors (MBRs) have been widely adopted for secondary treatment of municipal wastewater in the past

decade, especially in developed countries. They have advantages over conventional activated sludge systems such as in high removal efficiency, stable and good effluent qualities, small footprint, short hydraulic retention time (HRT), simple

* Corresponding author. Tel.: þ886 2 3366 7427; fax: þ886 2 2392 8830. E-mail address: [email protected] (C.-F. Lin). 0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.05.031

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operation, ease in maintenance of high biomass concentrations, and enhanced nitrification (Meng et al., 2007; Malamis and Andreadakis, 2009). However, a major issue of MBRs is the rapid decline of permeate flux due to a high level of biomass in the reactor that accelerates membrane fouling (Meng et al., 2005; Chae et al., 2006). In recent years, extracellular polymeric substances (EPS) and/or soluble microbial products (SMP) have been established as a main cause of membrane fouling (Cho et al., 2005; Jarusutthirak and Amy, 2006; Liang et al., 2007; Malamis and Andreadakis, 2009; Meng et al., 2009). Bound EPS are extracellular components tightly attached to the biological flocs, whereas soluble cellular components are soluble EPS or SMP from microbial growth and decay, as well as from dissolution of bound EPS (Ramesh et al., 2006; Ng et al., 2010). EPS and SMP typically consist of polysaccharides, proteins, lipids, and nucleic acids (Le-Clech et al., 2006; Liang et al., 2007). Found in most biologically treated effluents, SMP contributes significantly to soluble organic matter and Chemical Oxygen Demand (COD) of the effluent (Barker and Stuckey, 1999; Zhou et al., 2009). In addition, polysaccharide-like and protein-like substances are found predominant in EPS and/or SMP (Rosenberger et al., 2006; Le-Clech et al., 2003; FrØlund et al., 1995; Malamis and Andreadakis, 2009), though the species of these substances have not been well defined. Depending on its nature and molecular size, SMP may form a cake layer on the membrane surface or penetrate into the membrane pores (Jarusutthirak and Amy, 2006; Rosenberger et al., 2006; Meng et al., 2009). The behavior of SMP in membrane fouling is complex because of its disparate molecular weight (MW), hydrophobicity, and zeta potential (Barker and Stuckey, 1999; Jarusutthirak and Amy, 2006; Pan et al., 2010). SMP comprises a wide range of MW ranging from <1 kDa to 0.45 mm (Barker and Stuckey, 1999; Malamis and Andreadakis, 2009; Ni et al., 2010). Further research is warranted on how MW distribution and characteristics of SMP influence membrane fouling. Studies were undertaken that investigated moving bed biofilm reactor (MBBR) coupled with membrane as an alternative to the conventional MBR (Artiga et al., 2005; Ivanonic et al., 2006; Lee et al., 2006; Leiknes and Ødegaard, 2007). Yang et al. (2009) found that moving bed membrane bioreactor (MBMBR) produced more biomass in the effluent leading to increased membrane fouling than did CMBR. SMP in MBMBR were found to be more abundant than in CMBR. The objective in these studies of MBBR was to reduce MLSS in MBR, as ours was in developing the BEMR. Reducing MLSS enables less frequent backwashing and reduces downtime to clean the membrane. Contrarily, high MLSS in MBRs may increase non-Newtonian viscosities that hamper oxygen transfer and require additional energy for pumping (Drews and Kraume, 2005; Drews et al., 2005). We have developed here a new BEMR that reduced suspended biomass and increased SRT in the reactor with the objectives to achieve high organics removal in a more facile operation with a short start-up period. As membrane fouling may differ between the new BEMR and conventional MBR, we have investigated SMP and their characteristics in membrane fouling of both reactors at various HRTs, and further evaluated membrane cleaning of reactors for comparison. An overall

study goal is to reduce membrane fouling commonly encountered in MBRs.

2.

Material and methods

2.1.

Setup of laboratory-scale BEMR and CMBR

A BEMR and a CMBR were set up in the laboratory for experiments as shown in Fig. 1. Each of the MBRs had a working volume of 50 L with a polyvinylidene fluoride (PVDF) hollow fiber UF membrane module installed in it. They were operated in parallel for about four months with configurations and operation parameters specified in Table 1. The BEMR consisted of two compartments, with the first housing the entrapped bio-balls and the second housing the membrane module. The separate compartments allowed each to be designed and operated optimally. The entrapped bio-balls, 2.5 cm in diameter, were prepared per Yang et al. (2002, 2003), and were packed in the BEMR occupying 55% of the first compartment (first compartment volume ¼ 42 L). The activated sludge immobilized in the bio-balls of the BEMR and in the CMBR was from a wastewater treatment plant of the food industry. Prior to data collection, the CMBR was operated in batch mode and the BEMR in continuous-flow for 20 days to reach a steady-state condition that attained 90% removal of COD (Yu et al., 2009). After the steady state was reached in the effluents, the membrane modules were installed into the reactors. Both BEMR and CMBR were then operated at varied hydraulic retention times (HRT) of 6, 9, or 12 h during experiments on membrane fouling. Both MBRs were allowed to run for a period longer than 10 times the test HRT prior to experimental data collection. The SRT in the BEMR was determined according to Qian et al. (2001). The average SRTs of the BEMR and the CMBR were calculated to be 500 d and 20 d, respectively. The MLSS concentration in the CMBR was maintained at 8000e9000 mg L1 by withdrawal of excess sludge from the CMBR. The hollow fiber membrane module had an effective filtration area of 0.046 m2 and nominal pore size of 0.036 mm (GE ZeeWeed-1, USA). The membrane characteristics are summarized in Table 2. The wastewater was taken from a food processing complex of the Uni-President Enterprises Corporation in Taoyuan, Taiwan which manufactured flavored and fresh milks, grain and tea beverages, fruit juices, dairy products and instant noodles. The wastewater contained 590e1350 mg L1 of COD and 77e120 mg L1 of suspended solids (SS) and was fed to both MBRs. The wastewater was introduced into the MBRs by a peristaltic pump and air into the bottom of the MBRs by an air pump to maintain an aerobic environment for the microorganisms. Dissolved oxygen (DO) concentrations were maintained at the optimal levels of 7e8 mg L1 and 2.5e3.5 mg L1 for the BEMR and the CMBR, respectively. The membrane permeate was withdrawn through the UF hollow fiber membrane by a suction pump. An average flux of 20 L m2 h1 was maintained and the increasing transmembrane pressure (TMP) was continually monitored for both MBRs performance evaluation as membrane fouling results in an increase of TMP. The water level sensor, suction pump, and backwash pump were controlled by a computer.

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Fig. 1 e Schematic of the membrane bioreactors, a) BEMR, b) CMBR.

2.2.

Membrane test configurations

Three experimental configurations, Experiments I, II, and III, were used. Experiment I was conducted at an average flux of 20 L m2 h1 through the membrane at various HRTs without backwashing. Variations of HRTs at 6, 9, and 12 h were facilitated by operating the BEMR and the CMBR with recirculation. This was studied because changes in HRT could significantly impact membrane fouling. The data was collected after 14 days of operation at each HRT to ensure establishment of the steady state in the MBRs. Once the transmembrane pressure

exceeded the maximum operating pressure, i.e., 55 kPa, the membrane modules were removed for chemical cleaning. Chemical cleaning was performed by soaking the membrane in a solution of sodium hypochlorite (NaOCl) for 30 min. After cleaning the membrane was tested by compressing air into the hollow fibers in a tank filled with distilled water; the member was then reinstalled or replaced when air bubbles were observed. In Experiment II, the two MBRs (BEMR and CMBR) were operated at the optimum HRT of 12 h while maintaining an average flux of 20 L m2 h1, the performance of the

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Table 1 e Operation parameters of the lab-scale BEMR and CMBR. Parameter Carrier diameter size (cm) Total reactor volume, (L) Void volume (L) Packing ratio (%) Dissolved oxygen (mg L1) Average SRT (d) MLSS (mg L1) Temperature ( C) PH Average permeate flux (L m2 h1)

BEMR 2.5 50 23 55 7.0e8.0 500 11000a 25  1.0 7.0e8.3 20

CMBR e 50 e e 2.5e3.5 20 8000e9000 25  1.0 7.0e8.2 20

a The biomass in the bio-carrier.

membranes was examined periodically after brief operations. Membrane conditions after 15 min of suction and 1 min of backwash, after 30 min of suction and 1 min of backwash, and after backwashing of a fouled membrane at the flow velocity of 0.1 m s1 were compared. In Experiment III, both MBRs were operated at the flux of 20 L m2 h1 with 15 min of suction and 1 min of backwash. Experiment III was carried out to determine the service duration that the MBRs could be continually operated before the TMP threshold of 55 kPa was reached when chemical cleaning became necessary.

2.3.

Analytical methods

The soluble microbial products (SMP) in the MBRs were collected by centrifugation (BOECO U-320R, Germany) of samples at 3500g for 15 min. The supernatant was filtered through a 0.22-mm filter. The filtrate thus contained the total SMP contents (total protein and total carbohydrate) in the MBRs. The SMP were then fractionated into four groups, G-1 (>100 kDa), G-2 (10e100 kDa), G-3 (1e10 kDa), and G-4 (<1 kDa), according to their apparent molecular weights (AMW) by gel filtration chromatography (GFC), as previously detailed (Lin et al., 1999; Ng et al., 2010). The total SMP and all GFC fractions were analyzed for their total protein and total carbohydrate contents, which were regarded as the most important parts of SMP materials. The total protein content

was determined using the modified Lowry method with bovine serum albumin (BSA) as the protein standard (Lowry et al., 1951; FrØlund et al., 1995; Liang et al., 2007), and the total carbohydrate content was determined using the modified Anthrone method with glucose as the standard (Raukjaer et al., 1994; FrØlund et al., 1995). Organic compounds in the MBRs, once reaching the steadystate condition, were analyzed two or three times per week throughout experimentation. Daily measurements were recorded at the start-up stage of each run. The COD was analyzed by HACH closed reflux colorimetric method with the use of a spectrophotometer (HACH DR 2800). The pH and DO values were measured with a portable dissolved oxygen/pH meter (HACH HQ20) and a HACH Conductivity Meter (secsION5), respectively. The MLSS concentration of the CMBR was measured with an MLSS meter (KRK SS-5Z, Japan). The SS concentration was determined by drying at 105  C and the residual weight.

3.

Results and discussion

3.1. Performance of BEMR (new) and CMBR (conventional) on COD removal Fig. 2 shows the influent COD ranging from 590 to 1350 mg L1 in the food processing wastewater being fed to both MBRs, the effluent COD, and the calculated COD removal in each. After the start-up period of 20 days, the removal of COD in both reactors reached a stable removal value of about 90% and from days 21 through 120 the removal was 91e98% for the CMBR and 93e98% for the BEMR. A major advantage of MBRs is their ability to achieve more than 90% removal of COD (Cho et al., 2005; Fallah et al., 2010).

3.2.

Membrane fouling in the MBRs

Both the CMBR and BEMR in Experiment I were operated at 20 L m2 h1 without backwashing. Fig. 3 illustrates the continual rise of TMP in both reactors as a function of operation time. The results further showed that the rate of rise in TMP significantly increased as HRT decreased, which could

HRT=6h

HRT=9 h

HRT=12 h 100

Membrane material Module type Membrane pore size (mm) Membrane surface area (m2) Outer/inner diameter (mm) Maximum operating temperature ( C) Operating pH range Cleaning pH range Maximum operating pressure (kPa) Operating flux (L m2 h1)

Characteristics Polyvinylidene fluoride (PVDF) Hollow fiber 0.036 0.046 1.9/0.8 40 5.0e9.5 2.0e10.5 55 18e40

1500

COD (mgL-1)

Category

Influent BEMR Effluent CMBR Effluent BEMR COD Removal CMBR COD Removal

80

60

Pseudo steady-state condition for both BEMR and CMBR

1000

40

500 20

0

0 0

20

40

60

80

100

Time (d)

Fig. 2 e COD removal by BEMR and CMBR.

120

COD Removal (%)

Table 2 e Characteristics of the PVDF ultrafiltration membrane (ZeeWeed-1).

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6 9

60

6

12

9

12

40

CMBR 6 HRT CMBR 9 HRT CMBR 12 HRT BEMR 6 HRT BEMR 9 HRT BEMR 12 HRT

20

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250

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Fig. 3 e Effects of HRT on TMP and membrane fouling of the BEMR and CMBR.

hasten membrane fouling. This was in agreement with previous studies (Cho et al., 2005; Chae et al., 2006; Meng et al., 2009). The TMP of the CMBR increased exponentially to 55 kPa within 39, 48, and 57 min at HRT of 6, 9, and 12 h, respectively, while the TMP of the BEMR reached 55 kPa in 135, 175, and

a

16

BEMR

> 100 kDa 10 - 100 kDa 1 - 10 kDa < 1 kDa

14

SMPc (mg L-1)

12

-1 Total SMPc (6 h) = 19.32 mg L -1 Total SMPc (9 h) = 12.98 mg L -1 Total SMPc (12 h) = 11.16 mg L

10 8 6

215 min at HRT of 6, 9, and 12 h, respectively. A higher MLSS content in the MBR has higher potential to cause membrane fouling because an increased amount of bound EPS will be released into the sludge flocs (Trussell et al., 2006). The BEMR sustained a longer period of operation before reaching the TMP of 55 kPa because the membrane was exposed to a much lower concentration of suspended solid (20e30 mg L1) in the separate membrane compartment. A long SRT in the BEMR allows development of slow-growing microorganisms (Ahmed et al., 2007) and specific microorganisms that could assimilate dead or inactive microorganisms (Han et al., 2005). These microorganisms are more capable of consuming macromolecules such as polysaccharides, carbohydrate, and protein as substrates and producing less biopolymers (Masse et al., 2006), key to reduce membrane fouling. Ahmed et al. (2007) also reported less membrane fouling with increasing SRT from 20 to 100 d. Besides, bacteria were found widely present on fouled membrane (Ng et al., 2006); the growth of bacteria on membrane that resulted in formation of foulants and biocake on the membrane surfaces was attributed to membrane fouling. Hijnen et al. (2009) studied the biofouling of membrane in spiral-wound membrane and the results showed biofouling of membrane occurs at very low concentrations of easily biodegradable organic compounds because of the microbial growth in the feed water.

a

> 100 kDa 10 - 100 kDa 1 - 10 kDa < 1 kDa Total SMPp (6 h) = 25.81 mg L

10

Total SMPp (9 h) = 16.65 mg L

-1 -1

-1 Total SMPp (12 h) = 15.88 mg L 8 6

4

4

2

2 0 HRT = 9 h

HRT = 12 h

HRT = 6 h

16

CMBR

> 100 kDa 10 - 100 kDa 1 - 10 kDa < 1 kDa

14 12

Total SMPc (6 h) = 25.87 mg L 10

Total SMPc (9 h) = 18.18 mg L

HRT = 9 h

HRT = 12 h

16

CMBR 14

Total SMPp (6 h) = 41.59 mg L Total SMPp (9 h) = 31.91 mg L

-1 -1

Total SMPp (12 h) = 23.90 mg L 12

-1

> 100 kDa 10 - 100 kDa 1 - 10 kDa < 1 kDa

-1 -1

-1 Total SMPc (12 h) = 13.50 mg L

8

b

6

SMPp (mg L-1)

HRT = 6 h

SMPc (mg L-1 )

BEMR

12

0

b

16 14

SMPp (mg L-1)

Trans-Membrane Pressure (kPa)

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10 8 6

4

4

2

2 0

0 HRT = 6 h

HRT = 9 h

HRT = 12 h

Fig. 4 e Carbohydrate components of SMP at various HRTs in a) BEMR, and b) CMBR.

HRT = 6 h

HRT = 9 h

HRT = 12 h

Fig. 5 e Protein components of SMP at various HRTs in a) BEMR, and b) CMBR.

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Fractionation of SMP in CMBR and BEMR

Figs. 4 and 5 illustrate the SMPc (total carbohydrate) and SMPp (total protein) contents of the two MBRs at different HRTs. The total SMP (SMPc þ SMPp) was also calculated. The CMBR produced more SMP than the BEMR by 33%, 41%, and 28% at

50

Trans-Membrane Pressure Flux

40

Backwashing -1

30

-2

30

Flux was maintained at about 20 LMH 20

20

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c) BEMR running 15 min and backwashing 1 min Trans-Membrane Pressure Flux

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30

30

Flux was maintained at about 20 LMH 20

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Flux was maintained at about 20 LMH 20

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Trans-Membrane Pressure Flux Backwashing

Flux (L m-2 h-1)

d

50

b) CMBR running 30 min and backwashing 1 min

100

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d) BEMR running 30 min and backwashing 1 min Trans-Membrane Pressure Flux

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Backwashing

Flux (L m-2 h-1)

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Time (min)

Time (min)

b

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Backwashing

30

30

Flux was maintained at about 20 LMH 20

20

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10

0

Trans-Membrane Pressure (kPa)

40

Flux (L m h )

c

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a) CMBR running 15 min and backwashing 1 min Trans-Membrane Pressure (kPa)

a

Flux (L m-2 h-1)

3.3.

HRT of 6, 9, and 12 h, respectively. In both MBRs, the SMP contents with the HRT of 6 h were higher than those with HRTs of 9 and 12 h. The total SMP contents in BEMR and in CMBR increased from 27 to 45 mg L1 and from 37 to 68 mg L1, respectively, when the HRT decreased from 12 to 6 h. At lower HRTs, a larger amount of SMP could be released by the abundant filamentous organisms in the bioreactors during their stationary and endogenous metabolism stage that could adversely impact the membrane (Choi et al., 2002). Fallah et al. (2010) compared the removal of styrene from synthetic wastewater by MBRs with HRTs of 18 h and 24 h and found that the SMP concentration increased when HRT was reduced to 18 h. The relationship between SRT and EPS and/or SMP has been previously studied relative to membrane fouling (Cho et al., 2005; Han et al., 2005; Liang et al., 2007). The bio-balls employed in the BEMR had longer SRT and good contact with supplied nutrients. This reduced cell death and production of EPS and SMP, resulting in enhanced membrane filterability (Ng et al., 2010). Meng et al. (2007) reported increased production of bound EPS and sludge viscosity when the foodto-microorganism ratio (F/M) and organic loading rate were increased. This may be due to the formation of bound EPS that was growth-related and produced in direct proportion to substrate utilization (Laspidou and Rittman, 2002).

Trans-Membrane Pressure (kPa)

Decreasing the HRT in CMBR might also decrease DO, which could lead to poor flocculation and filamentous bulking in the bioreactor (Liu and Liu, 2006). Meng et al. (2009) found that DO decreased as the HRT decreased from 10 to 12 h to 3e4 h, which increased growth of filamentous organisms in the flocs. Meng et al. (2007) found DO concentrations to be 3.8e6.5 mg L1 at HRT of 10e12 h and 0.2e1.5 mg L1 at HRT of 4e5 h. Microorganisms in bioreactors likely required high oxygen consumption to degrade organic matters and form new cellular materials. In addition, Meng et al. (2006) reported that overgrowth of filamentous bacteria could lead to a sharp increase of EPS, sludge viscosity, and sludge hydrophobicity, resulting in increased fouling. Jin et al. (2006) and Kim et al. (2006) reported more severe membrane fouling with low DO levels. Therefore, it is reasonable to conclude that higher DO and longer SRT conditions employed in the BEMR resulted in less membrane fouling (as measured by TMP).

0 0

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150

200

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300

Time (min)

Fig. 6 e The rise of TMP with different membrane filtration times: a) CMBR with 15-min filtration time, b) CMBR with 30-min filtration time, c) BEMR with 15-min filtration time, and d) BEMR with 30-min filtration time.

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obstructive to membrane pores were less in the BEMR than in the CMBR. This was consistent with our previous findings on the positive effects of membrane fouling by proteins of low molecular weights. The protein likely narrowed membrane pores via adsorption, and fouling was exacerbated by denatured protein (Ng et al., 2010). With backwashing, the membrane process of BEMR could sustain a longer operation period at a higher permeate flux than could the CMBR. Operation duration and frequency of backwashing are central design parameters of MBRs. Jiang et al. (2005) found that an MBR using a 600-s membrane filtration process with 45-s backwashing was more efficient than one using a 2000-s membrane filtration process with 45-s backwashing. Therefore, to reduce fouling for the CMBR at an average flux of 20 L m2 h1, it is conceivable that operating with 15-min suction and 1-min backwash can be less fouling than with 30-min suction and 1-min backwash. The operation protocol may be varied to achieve a longer service run with a nominal backwashing interval. Fig. 7 shows results of Experiment III that contrast the performances of the two MBRs. Both MBRs were operated at 20 L m2 h1 with a service cycle of 15-min suction and 1-min backwash. In the CMBR, TMP threshold of 55 kPa was reached much sooner, necessitating chemical cleaning every 5 days (i.e., at day 10, 16, 21.5, 27, 32.5, and 38).

a

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-2

-1

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0 10

Membrane performance of BEMR and CMBR

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Operating Time (d)

b

60

TMP Flux Chemical Cleaning

40

40

20

20

0 0

10

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30

Trans-Membrane Pressure (kPa)

BEMR 60

Flux (L m-2 h-1)

Fig. 6 compares TMP increases in CMBR and BEMR operating at two different filtration/backwash cycles (15 min or 30 min of filtration plus 1 min of backwash) and cleaning. With the cycle of 15-min filtration plus 1-min backwash, the TMP of CMBR started at 3.6 kPa at the beginning of service and reached 11.2 kPa at the end of 10 service cycles; whereas with the cycle of 30-min filtration plus 1-min backwash, the TMP reached 23 kPa at the end (Fig. 6a and b). In contrast, the TMP of BEMR started at 2.4 kPa at the beginning of service and was stable after backwashing of each cycle; it increased much more gradually and slowly reaching 3.9 kPa and 6.2 kPa at the ends of 10 service cycles with 15 and 30 filtration durations, respectively. As the operation time increased, more colloids and SMPs with different MW particles accumulated on the membrane that made it increasingly difficult to remove by backwashing. The BEMR produced less SMP and displayed less fouling than did the CMBR. SMP <100 kDa and protein that were more

CMBR 70

0

3.4.

Chemical Cleaning

70

Trans-Membrane Pressure (kPa)

TMP Flux

Flux (L m h )

Figs. 4 and 5 also show that the SMP concentration of carbohydrate and protein gradually decreased in each MW fraction when HRTs decreased, except for the SMPp at HRT of 9 h. The results reveal that the dominating MW fraction of SMP in both MBRs was G-2 (10e100 kDa) with all tested HRTs. The G-2 fraction contributed approximately 38e54% of the total SMP concentrations in the BEMR and 40e43% in the CMBR despite the effects of HRTs. These findings were supported by studies of Pan et al. (2010) and Janga et al. (2007) proved that the bulk of SMP carbohydrate and SMP protein had MW >30 kDa and 10 kDa, respectively. However, Malamis and Andreadakis (2009) and Ng et al. (2010) stated that the SMP <1 kDa was predominant. The discrepancy might have arisen from different methods of SMP separation and analysis, operation conditions, and feed characteristics being employed. Protein was found to be the major component of SMP rather than carbohydrate in BEMR and CMBR. In BEMR and CMBR, SMPp concentrations varied from 56 to 59% and from 62 to 64%, respectively, under the studied HRTs. The results were consistent with several past studies (Le-Clech et al., 2003; Yigit et al., 2008; Malamis and Andreadakis, 2009; Ng et al., 2010) that showed protein as the main component of SMP. Further, Viero et al. (2007) reported that SMPc was the major component that formed a cake layer or a gel layer on the membrane. Owing to the major occurrence of SMPp in our study, we believe protein was the main component that caused pore blocking of the membrane. Therefore, higher DO and longer SRT in the BEMR led to less SMP and reduced membrane fouling. It is important to analyze the molecular fractions larger and smaller than 100 kDa in the BEMR and CMBR. The membrane (GE ZeeWeed-1) used in this study had a pore size of 0.036 mm approximating 100 kDa. At different HRTs, the total SMP contents (SMPp and SMPc) larger than 100 kDa in the BEMR were 15e23% while in the CMBR they were 17e22%. Lin et al. (2009) reported that particles of smaller molecular weight could easily penetrate the membrane pores by adsorption in a 100-kDa membrane. Therefore, the major fouling mechanism by SMP (more than 70%) is likely caused by pore blocking, rather than by the formation of cake layer on the membrane surface.

0 40

Operating Time (d)

Fig. 7 e The rise of TMP with filtration time in a) CMBR, and b) BEMR.

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The TMP could be restored to the initial value of 3.5e4.0 kPa, which indicated complete removal of most foulants on the membrane surface and in the pores. Jeison and van Lier (2007) studied an anaerobic submerged membrane reactor over 200 days and found that external, physical cleaning was necessary because the consolidated cake could not be removed by back-flush cycles. Fallah et al. (2010) also reported that fouling by pore blocking could only be eliminated by chemical cleaning. In the BEMR, TMP was below 10 kPa for 10 days; it gradually increased till day 29 and then increased at a faster rate reaching 55 kPa at 39 days (Fig. 7(b)). The increase of TMP was attributed to colloids and soluble EPS being deposited on the membrane. The operation time of the BEMR before chemical cleaning needed was 7 times longer than that of the CMBR. This was attributed to less MLSS and less total SMP in the BEMR. Based on these results, BEMR appears to be less susceptible to membrane fouling due to reduced SMP and MLSS contents and therefore requires less-frequent cleaning.

4.

Conclusions

In this study, we have compared the new BEMR with a conventional CMBR on performance, SMP production, and service duration, and conclude the following: 1. In both MBRs, TMP arose faster as HRT decreased. The BEMR was less susceptible to fouling, and it sustained a longer service duration than did the CMBR (39 days vs. 5 days). 2. The BEMR produced less SMP than did CMBR (34e48% less protein and 16e29% less carbohydrate) due to slow-growing microorganisms with long SRT in the new bioreactor. 3. Both MBRs produced SMP of 10e100 kDa primarily of protein (59% in BEMR and 64% in CMBR), which likely caused membrane pores clogging. 4. BEMR appears promising in controlling membrane fouling, requiring less frequent chemical cleaning, and being more economical to operate.

Acknowledgments This work was partially funded by the National Science Council of the Republic of China (NSC98-2221-E-002-029-MY3 and NSC99-2811-E-002-042).

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Nomenclature AMW: apparent molecular weight BEMR: bio-entrapped membrane reactor BSA: bovine serum albumin COD: chemical oxygen demand (mg L1) CMBR: conventional membrane bioreactor DO: dissolved oxygen (mg L1) EPS: extracellular polymeric substances (mg L1) F/M: food to microorganisms ratio GFC: gel filtration chromatography HRT: hydraulic retention time (h) MBBR: moving bed biofilm reactor MBR: membrane bioreactor MBMBR: moving bed membrane bioreactor MLSS: mixed liquor suspended solid (mg L1) MW: molecular weight NaOCl: sodium hypochlorite PVDF: polyvinylidene fluoride

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SMP: soluble microbial products (mg L1) SMPc: fraction of carbohydrate contained in the sludge solution (mg L1) SMPp: fraction of protein contained in the sludge solution (mg L1)

SRT: sludge retention time (day) SS: suspended solids (mg L1) TMP: trans-membrane pressure (kPa) UF: Ultrafiltration