2. Literature review
2.2 Mitigation of membrane fouling in MBRs
2.2.1 Modification of membrane characteristics
2.2.1 Modification of membrane characteristics
In most cases, membranes with hydrophobic characteristics have been found more prone to membrane fouling because of the hydrophobic-hydrophobic interaction between solutes, microbial cells and membrane materials as mentioned in 2.1.1.2.
Several methods in surface modification of membrane such as plasma treatment (Yu et
al., 2005, 2008)
, graft polymerization (Yu et al., 2007)
, and TiO2 modification (Bae & Tak, 2005b; Bae &
Tak, 2005c; Bae et al., 2006)
have been proven efficient in reducing membrane fouling in MBRs. Recently, membrane surface modification by adding TiO2 onto membrane surface has attracted great attention due to its commercial availability and ease of preparation. Table 2.6 lists the studies which modified membrane surface by addition of TiO2 onto membrane surface. The idea of introduction of TiO2 onto membranes to mitigate membrane fouling was first proposed by Kwak and his coworkers (Kwak et al., 2001; Kim et al., 2003)
. They modified thin-film-composite (TFC) RO membranes by a coating of TiO2 nanoparticles. The composite membrane showed substantial prevention against microbial fouling under UV illumination. Luo et al (2005) then used the same method to prepare the composite UF membranes. They found that the composite membranes not only had antifouling properties but also perform better when filtering PEG solution. For MBRs, Bae and his coworkers performed a series of experiments investigating the antifouling ability of TiO2-entrapped membrane and TiO2-deposited membrane (Bae & Tak, 2005b, 2005c; Bae et al., 2006)
. Both membranes reduced fouling in filtration of activated sludge. The TiO2-deposited membrane had better antifouling ability than the entrapped one because larger amount of TiO2 was located on the membrane surface. More recently, several researchers have applied the similar
membrane modification technology to reduce membrane fouling under UV or non-UV illumination (Choi et al., 2007; Madaeni & Ghaemi, 2007; Yang et al., 2007; Rahimpour et al., 2008)
. No matter what the TiO2 composite membranes preparation is, all of the TiO2 composite membranes provide good antifouling ability for filtration of activated sludge, BSA, whey and non-skim milk. However, it is noted that most of these studies synthesized the composite membranes by dip-coating in acidic TiO2 solution (pH 1.5). This may have the risk of deteriorating membrane structure and shorting the lifespan of membranes.
Table 2.6. Fouling mitigation by coating TiO2 on membranes
Preparation of TiO2
Membrane and Method of
modification Details Results References
Controlling of
Dip-coating in TiO2 solution for 1h
Escherichia coli (E. coli) as a model bacterium
Antibacterial fouling was remarkably higher for the TiO2 hybrid membrane under UV
Dip-coating in TiO2 solution for 1h
Escherichia coli as a model bacterium
The TiO2 composite membrane showed substantial prevention
Dip-coating in TiO2 solution for 1h
Polyethylene glycol
Table 2.6. Fouling mitigation by coating TiO2 on membranes (continued)
Preparation of TiO2
Membrane and Method of
modification Details Results References
Commercial Degussa TiO2
PS, PVDF and PAN (UF)
membrane; addition of TiO2 into casting solution (TiO2-entrapped membrane) and dip-coating in TiO2 solution for 1 min and pressuring at 400 kPa for 2 h (TiO2-deposited membrane)
Activated sludge (MLSS: 7,000 mg/L) from a submerged MBR was used to evaluate the antifouling potential of the composite membrane
TiO2 deposited membrane showed greater fouling
mitigation than TiO2 entrapped
membrane Bae & Tak and dip-coating in TiO2 solution for 10 min
Activated sludge (MLSS: 6,900 mg/L) from a submerged MBR was used to evaluate the antifouling potential of the
hydrolysis of PES membrane (UF); sulfonattion
Activated sludge (MLSS: 7,000
mg/L) from a submerged MBR The composite membrane
Bae et al.
Table 2.6. Fouling mitigation by coating TiO2 on membranes (continued)
Preparation of TiO2
Membrane and Method of
modification Details Results References
Hydrolysis of
Dip-coating in TiO2 solution
Activated sludge was used TiO2 into casting solution
1 % bovine serum album
Whey was used to evaluate the antifouling
performance
The composite membrane acquired self-cleaning property and higher flux under UV illumination,
especially when incorporating SiO2
Madaeni &
Ghaemi (2007)
Table 2.6. Fouling mitigation by coating TiO2 on membranes (continued)
Preparation of TiO2
Membrane and Method of
modification Details Results References
Commercial Degussa TiO2
PES membrane; addition of TiO2 into casting solution (TiO2-entrapped membrane) and dip-coating in TiO2 solution for 15, 30 and 60 min
(TiO2-deposited membrane)
Non-skim milk was used to evaluate the antifouling performance at 50 psi and 4 h
Both TiO2-entrapped and TiO2-deposited membranes showed good antifouling
properties under UV illumination but TiO2-deposited membranes showed superior antifouling ability under UV illumination
Rahimpour et al.
(2008)
Chapter 3
Experimental methods
3.1 Material
3.1.1 Membrane bioreactor and operation
The experimental MBR system comprised a 30-L aerated tank as the bioreactor with a submerged flat sheet module (Kubota, Japan), which is shown in Figure 3.1.
The MF membrane is a hydrophilic polypropylene membrane with a mean pore size of 0.4 µm. The synthetic wastewater we used in this study was modified from Ng and Hermanowicz (2005). Although the synthetic wastewater is likely to be more biodegradable than the real wastewater, the difference between synthetic and real wastewater should not affect our study. Many other researchers have used synthetic wastewater in similar studies based on the same reason (Meng et al., 2006b, 2006c; Li et al., 2008)
. The composition of the synthetic wastewater is given in Table 3.1. The concentrated synthetic municipal wastewater was pumped into the bioreactor continuously at a constant rate to maintain a fixed organic load rate (1.2 kg TOC/m3.day) to the MBR.
Tap water was added to the bioreactor so that the feed flow rate matched the permeate flow rate. The concentrated sewage was diluted six-fold and the chemical oxygen demand (COD) concentration of the final feed was 400±10 mg/L. The seed for the MBR was obtained from a wastewater treatment plant in National Chiao Tung University, Taiwan. An ADAMview software and a programmable logic controller were used to adjust the flux by feedback control. A desired flow rate was first set.
When the flow rate was detected by the permeate flow meter, a signal was sent to the computer and the pump speed was adjusted accordingly to keep the flow rate constant.
The pH of the sludge suspension was adjusted to 7.0±0.2 by adding hydrate chloride and sodium hydroxide. In this study all the experiments were carried out after the MBR was acclimated for 3 SRT to ensure the stability of the sludge characteristics.
The SRTs of the MBR were maintained at 10, 30 and 60 days, respectively. After 3 SRT, the critical flux was measured by flux-step method (Le-Clech et al., 2003a)
. The step duration and step height were chosen at 15 min and 6 L/m.h in this study, respectively.
The initial flux was set at 12 L/m2.h. Each constant flux was operated for 15 min. The flux was increased by 6 L/m2.h at the end of each step until it reached 60 L/m2.h. As shown in Figure 3.2, the critical flux was around 24 L/m2.h over which TMP increased apparently. To maintain a subcritical operation, the imposed flux used in this study was set at 16 L/m2.h.
An aerobic selector of 1-L working volume was installed when sludge bulking became serious to change the sludge characteristics. The aerobic selector was set up
ahead of the MBR. The simulated sewage was fed into the aerobic selector and then, pumped to the MBR. The Membrane bioreactor system equipped with an aerobic selector is illustrated in Figure 3.3.
A dead-end stirred cell system, as shown in Figure 3.4, was used to analyze the filtration resistance contributed by individual sludge component as well as the contribution distribution. Sludge was filtered through the membrane by the pressure from a nitrogen cylinder. Permeate was continuously collected until a stable resistance was attained. The feed vessel in Figure 3.4 was replenished with clean water to maintain a fixed volume of 200 ml during the filtration. The diameter and the height of the stirred cell are 5.6 cm and 10 cm, respectively. A stirrer, which is 4.5 cm in length, is suspended above the membrane to generate shear rate by stirring. Two stirring rates, 400 rpm and 1,000 rpm, were selected in this study, to investigate the effect of shear rate on membrane fouling by sludge components. The effective area of the membrane is 24.63 cm2 and the working volume is 200 ml.
Figure 3.1. Schematic of the membrane bioreactor system.
Table 3.1. Composition of the synthesized wastewater. (adapted from Ng &
Hermanowicz, 2005)
Component Concentration (mg/L)
Sodium acetate 2527
Starch 150
Beef extract 250
NH4Cl 670
KH2PO4 154
MgSO4·7H2O 355
CaCl2 73
FeSO4·7H2O 87
CuCl2·2H2O 0.35
MnCl2·4H2O 0.63
ZnSO4·7H2O 0.66
CoCl2·6H2O 0.15
Na2MoO4·2H2O 0.08
H3BO3 0.124
KI 0.166
Time (min)
0 15 30 45 60 75 90 105 120 135 150
Flux (L/m2 .h)
0 10 20 30 40 50 60 70
TMP (kPa)
0 2 4 6 8 10 12 14 16 Flux
TMP
Figure 3.2. Critical flux determination by the flux-step method.
Figure 3.3. Schematic of the membrane bioreactor system with an aerobic selector: (1) pH meter; (2) thermometer; (3) DO meter and (4) level sensor.
Feed Tank
2
Monitor controller 3
Flow meter
Selector Pressure
meter
1 4
Permeate
Figure 3.4. Dead-end stirred cell set-up: (1) nitrogen cylinder; (2) feed vessel; (3) pressure meter; (4) stirred cell; (5) magnetic stirrer; (6) permeate vessel and (7) electronic balance.
3.1.2 Preparation of nanosized TiO2 particles
The acidic TiO2 colloidal solution was prepared from the controlled hydrolysis of titanium tetraisoproposide, Ti(OCH(CH3)2)4 (Choi et al., 1994)
. A sample of 1.25 ml Ti(OCH(CH3)2)4 (Aldrich, 97%) was mixed with 25 ml of absolute ethanol. The solution was added drop by drop to 250 ml of distilled water (4℃), followed by pH adjustment to 1.5 with nitric acid. The mixture was stirred overnight until it was clear.
3.1.3 Preparation of TiO2 composite membranes
Two microfiltration (MF) membranes, cellulose acetate (CA) and mixed cellulose ester (MCE) membranes (Advantec MFS, Inc.), were used in this study.
Both of them have nominal pore size of 0.2 µm and were cut into 24.63 cm2 to fit the experimental device. The virgin membrane was dipped in the TiO2 sol for one hour.
After that, the membrane was washed with distilled water.
3.2 Analytical methods 3.2.1 Extraction of EPS
Many methods have been proposed for EPS extraction, including heating (Li et al., 2007)
, cation exchange (Meng & Yang, 2007)
, and extraction by EDTA chelating (Sheng et al., 2005)
and formaldehyde-NaOH (Liu & Fane, 2002)
. The formaldehyde-NaOH extraction method was selected in this study since Liu and Fang (2002) have indicated that it is most effective and does not cause cell lysis. Extraction of EPS is illustrated in Figure 3.5.
Ten milliliters of mixed liquor was sampled and centrifuged for 10 min at 4,000 rpm at 4℃ (U-320R Boeco, Germany), and then the supernatant was filtered through a 0.45 µm membrane filter (Mixed cellulose ester, Adventec). The permeate contained soluble EPS. The residual pellets were resuspended to 10 ml by using Milli-Q water, followed by the addition of 0.06 ml formaldehyde (63.5%). The suspension was
further purified by dialysis through a membrane of 3,500 molecular weight cut-off at 4℃ for two days to remove the extractant.
Centrifugation of 10 ml sludge at 4,000 rpm, 4℃, 20 min
0.06 ml formaldehyde (36.5%), 4℃, 1 hr
4 ml 1N NaOH, 4℃, 3 hr
20,000 g centrifugation, 4℃, 20 min
Filtration of the supernatant with 0.2 μm membrane
Purification of the permeate with dialysis membrane (3,500 kDa), 4℃, 48 h
Decantation and filtration of supernatant with 0.45 μm membrane
Resusepnsion of precipitate to 10 ml with DI water
Soluble EPS
Figure 3.5. Extraction of EPS from sludge flocs.
3.2.2 Analysis of EPS
EPS is a complex mixture of macromolecules including polysaccharides, proteins, nucleic acids, humic acids, etc. In this study, the total EPS is defined as the sum of carbohydrates and proteins because they are the main components of EPS (Liu &
Fane, 2002)
. A phenol-sulfuric acid method (Dubois et al., 1956)
was used to quantify carbohydrates in which glucose was the standard (as shown in Figure 3.5). Protein was measured using Bradford protein assay (Bradford, Sigma) according to the manufacturer’s protocol, with Bovine Serum Albumin (BSA, Sigma) as the standard.
The measurements of polysaccharides and proteins are illustrated in Figure 3.6 and Figure 3.7, respectively.
1.0 ml sample with distilled water
Add 1 ml phenol reagent(5%)
Add 5 ml concentrated sulfuric acid (75%, vol/vol)
Mix rapidly and let stand for 10 min
Water bath at 25℃ for 15 min
Read the absorbance at 490 nm against the blank prepared without glucose
Determine the concentration of glucose from a standard curve prepared by plotting the absorbances of the standards vs. the
concentration of glucose
1.0 ml distilled water A set of glucose standards from 10 to 100 µg in 1 ml
Figure 3.6. Procedure of measurement of carbohydrates in EPS and SMP.
0.1 ml sample 0.1 ml distilled water A set of 1 ml BSA standards from 0 to 1.4 mg/ml
Add 3 ml Bradford reagent
Mixing gently and incubate at room temperature for 5 to 45 min
Read the absorbance at 595 nm against the blank prepared without BSA
Determine the concentration of protein from a standard curve prepared by plotting the absorbances of the standards vs. the
concentration of BSA
Figure 3.7. Procedure of measurement of proteins in EPS and SMP.
3.2.3 Measurement of particle size distribution of sludge
Particle size distributions of sludge were determined by Mastersizer 2000 particle size analyzer (Malvern, UK) which is based on laser diffraction scattering.
The particle size analyzer can measure particles from 0.02 to 2,000 µm which meets the requirement of this study. Each sample was measured three times with a standard deviation of less than 3%.
3.2.4 Measurement of molecular weight distribution of supernatant solutes
The apparent molecular weight distributions of supernatant solutes were determined using regenerated cellulose membranes in series with molecular weight cut-off (MWCO) of 5, 10, 30 kDa. (PXC005C50, PXC010C50 and PXC030C50, Pellion, Millipore Corp., USA). These hydrophilic membranes were used to minimize adsorption of organic matter. Samples were processed by passing aliquots through each membrane, yielding a retentate and corresponding permeate containing all molecular weight fractions below the indicated cutoff. The samples were at ambient pH and were not buffered. The actual pressure employed for a given membrane was based on the flow rate recommended by the manufacturer. The sample permeate and retentate were based on a concentration ratio of approximately 4:1. Both permeate and retentate were analyzed for TOC. The results of TOC balance generally indicated larger than 90% recovery of the introduced sample by the permeate and retentate.
3.2.5 Fractionation of supernatant solutes
Supelite DAX-8 and Amberlite XAD-4 resins were used to fractionate supernatant solutes into hydrophobic acids (HPO-A) and hydrophobic neutrals (HPO-N) adsorbing onto the DAX-8 resin, transphilic acids (TPI-A) and transphilic neutrals (TPI-N) absorbing onto the XAD-4 resin, and the hydrophilic faction which does not adsorb on either theDAX-8 or XAD-4 resin. Only the hydrophobic acids and the transphilic acids fractions were used in this study. However, the separation of the different organic matter fractions with XAD resins is not sharp, instead the fractions overlap to a certain degree (Aiken & Leenheer, 1993)
. It should be mentioned that the so-called hydrophobic fraction (adsorbing onto the DAX-8 resin) do not exhibit a truly hydrophobic character in chemistry. The organic matter found in the hydrophobic fraction merely exhibits a more hydrophobic character in comparison to
there was no interaction among these three parts and the total resistance was the sum of the three resistances. Sludge was centrifuged (4,000 rpm) at 4℃ and the supernatant was considered to contain colloids and solutes. The supernatant was then filtered through a 0.45 µm membrane (Mixed cellulose ester, Advantech) to obtain the solutes. The resistance of individual sludge component was determined by the is resistance caused by membrane itself (1/m), Ras is resistance by sludge (1/m), Rss is resistance by suspended solids (1/m), Rcol is resistance by colloids (1/m), Rsol is resistance by solutes (1/m), Jiw is stable flux by filtering Milli-Q water (clean water flux), Jas is flux by filtering sludge, Jsup is flux by filtering supernatant and Jsol is flux by filtering solutes.
First, the Milli-Q water was filtered through the membrane to determine Rm by using equation 2. Sludge was then filtered to determine Ras by using equation 4 after a stable flux was reached. Supernatant and solutes were filtered through and the Rcol+sol and Rsol were determined by using equation 5 and 6, respectively. The difference between Rcol+sol and Rsol was Rcol (Subtract equation 6 from equation 5). Once Rsol and Rcol are known, Rss can be calculated by equation 3.
3.2.7 Specific cake resistance
Constant pressure filtration using the dead-end cell system under unstirred condition was used to calculate specific cake resistance (α). Plotting t/V vs. V, knowing other parameters, α can be calculated as follows:
PV membrane (m2), C is concentration of MLSS and α is specific cake resistance (m/kg).
t/V versus V plot is depicted linearly and the slope can be obtained by linear regression analysis. Then the specific resistance is calculated from the slop value.
3.2.8 Fourier-transform infrared spectrometer
Attenuated total reflectance-FTIR (ATR-FTIR) (Bomem DA8.3, Canada) was used to characterize foulant on the membrane surface. Samples were prepared in 2 cm
× 2 cm rectangles and dried at a vacuum box overnight. Samples were examined to a resolution of 4 1/cm.
3.2.9 Characterization of nanosized TiO2 particles
The crystal structure of TiO2 particles was characterized by X-ray diffraction (XRD) with a Mac Science MXP-18 X-ray diffractometer using Cu Kα (voltage: 30 kV; current: 20mA; λ = 0.154056 nm) radiation. The particle size of TiO2 was determined by a Philip transmission electron microscope (TEM, Philip CM-200 TWIN) at 200 kV. For TEM observation, TiO2 suspension was dropped on a carbon-coated grid and then dried at room temperature. The particle size distribution of TiO2 particles was also measured by a dynamic light scattering particle size distribution analyzer (Zetasizer Nano ZS, Malvern, UK).
3.2.10 Characterization of morphology and chemical composition of membrane surface
The surface topography of the TiO2 composite membrane was observed by JEOL JSM-6700F field emission scanning electron microscopy (FE-SEM). For SEM observation, the membrane samples were cut into appropriate size and the surfaces were coated with gold by a sputter coating machine.
X-ray photoelectron spectroscopy (XPS) was conducted to determine the
The contact angle goniometer (MagicDroplet model 100, Future digital scientific, USA) was used to characterize the hydrophilicity of the composite membranes by sessile drop method. The contact angles were determined by taking the average of three measurements.
3.2.11 Fouling test of composite membranes
The membrane fouling test of the composites membranes were conducted using a stirred cell system, as shown in Figure 3.4. The activated sludge used as the feed of the fouling test was taken from a 30-L submerged MBR system with synthetic influent.
The samples in the filtration cell were stirred at a constant stirring rate over the entire experiment and all the data were automatically logged in a computer. All the experiments were carried out at 0.5 bar constant pressure by using a nitrogen cylinder.
Resistance-in-series model was used to assess the degree of membrane fouling:
Rt J PT
µ
= ∆ (8)
where J is the permeate flux (m3/m2.s), △PT the trans-membrane pressure (Pa), µ the viscosity of the permeate (Pa.s), and Rt the total filtration resistance (1/m).
3.2.12 Ultrasonic wash of the TiO2 composite membrane
To evaluate the fixation of TiO2 coating on membrane, ultrasonic washing (40 KHz) was applied. The relative atomic concentrations of elements on the membrane surface were quantified by XPS. The relative atomic concentrations of the individual elements can be calculated:
where Ai is the photoelectron peak area of the element i, Si is the sensitivity factor for the element i, and m is the number of the elements in the sample.
3.2.13 Other analytical methods
MLSS was measured following the standard method (APHA, 1998)
. TOC was measured using a TOC analyzer (TOC-5000A, Shimadzu). Each TOC sample was measured at least two times with a standard deviation of less than 5%. Ammonia nitrogen was measured using a spectrophotometer (DR/4000U, HACH) according to salicylate method (method 10031). All the samples for TOC and ammonia nitrogen measurements were filtered through a 0.45 µm membrane filter first (Mixed cellulose ester, Adventec). Capillary suction time (CST) was determined to evaluate the
filterability of sludge. Five milliliters of sludge was sampled from the bioreactor and the CST (304B CST, Triton) was measured immediately. Each CST measurement was performed at least three times with a standard deviation of less than 5%.
Chapter 4
Effect of sludge characteristics on membrane fouling in MBRs
4.1 Performance and fouling characteristics of membrane bioreactor under different sludge characteristics
MBRs with different sludge characteristics, bulking sludge and normal sludge, were investigated in this study to evaluate their effects on membrane performance.
4.1.1 Performance of membrane bioreactor treatment under different sludge conditions
In the beginning of the MBR operation, sludge bulking due to overgrowth of filamentous bacteria was observed. The excessive growth of filamentous bacteria when sludge bulking became serious is clearly shown in Figure 4.1 (a) and (b). Ideally, the sludge contains both filamentous bacteria and floc-forming bacteria. When the two are in balance, the filamentous bacteria act as the backbone of activated sludge flocs without causing sludge bulking (Jenkins et al., 1993)
. The theories of the overgrowth of filamentous bacteria are: (1) The surface/volume theory: filamentous bacteria have
. The theories of the overgrowth of filamentous bacteria are: (1) The surface/volume theory: filamentous bacteria have