4. Effect of sludge characteristics on membrane fouling in MBRs
4.2 Effect of SRT on sludge characteristics and membrane fouling
4.2.3 SMP characteristics
membrane fouling in MBRs, SMP was separated into parts of different MWCO by use of a series of ultrafiltration membranes. Moreover, SMP was also categorized into different parts according to its hydrophobicity by use of DAX-8 and XAD-4 resins.
SMP in mixed liquor and effluent was analyzed and compared to elucidate its effect on membrane fouling.
Figure 4.12 shows the molecular weight distribution of the SMP in mixed liquor and effluent at SRT 10 and SRT 60 days. Regardless of SRT, both mixed liquor samples shows a bimodal pattern of SMP molecular weight distribution with the majority of SMP having a molecular weight >30 kDa and <5 kDa, which is in agreement with other studies (Liang et al., 2007; Huang et al., 2008)
. Molecular weight distribution of SMP was found to shift from larger molecular weight to smaller molecular weight when changing SRT 10 to 60 days. This phenomenon might be due to the decomposition of larger molecular-weight SMP by microorganisms (Huang et al., 2000; Shin &
Kang, 2003)
. In contrast to the finding reported by Liang et al (2007), membrane sieving did work for SMP accumulation in MBRs. Around 20% of the SMP larger than 30 kDa was retained in the mixed liquor for both SRT 10 and 60 days, and around 4 % of the SMP between 30 and 10 kDa was retained for SRT 60 days. However, microfiltration membranes (0.4 µm) used in this study was much larger compared with the molecular weight of SMP, and, therefore, membrane sieving could not provide the explanation.
The retention of larger SMP might be owing to the formation of self-forming dynamic membrane on membrane surface. As the membrane filtration reaches a steady state a dynamic membrane will have been formed on the membrane surface, which acts as a barrier to protect the membrane surface and pores from being fouled (Lee et al., 2001)
. The self-forming dynamic membrane implies that the rate of particle convection to the membrane surface is balanced by the rate of back transport. Figure 4.13 shows that TOC was greatly reduced to a lower value within 10 to 60 minutes of the filtration at both SRT 10 and 60 days. This means that the dynamic membranes have been stably formed on the membranes. The dynamic membranes can reject small components in the mixed liquor such as SMP. However, in the beginning of the filtration, an increase in TMP was not significantly observed, which means that the dynamic membrane would not result in apparent membrane fouling but improve membrane rejection instead.
Figure 4.14 shows that the hydrophilic fraction of SMP was the most abundant fraction in the MBR at SRT 10, which was not in agreement with the observation obtained by Liang et al (2007). This contradiction may be due to the difference in feed characteristic and operational condition. Figure 4.14 also shows that 54% of hydrophilic fraction was rejected in the mixed liquor and 36 % of hydrophobic fraction was rejected in the mixed liquor. The result implied that hydrophilic fraction had significant effects on SMP accumulation in the MBR. Hydrophobic interaction is generally considered important mechanism regarding to fouling (Madaeni et al., 1999)
. However, in this study only 36% of hydrophobic fraction was rejected in the mixed liquor, less than the 54% of hydrophilic fraction. Therefore, hydrophobic interaction seemed not to be the major fouling mechanisms of SMP in this study. SMP is mainly composed of carbohydrates and proteins. Proteins are more hydrophobic than carbohydrates (Shin & Kang, 2003)
. Figure 4.15 illustrates that carbohydrates represented hydrophilic characteristics and the hydrophilic fraction of carbohydrates accumulated in the mixed liquor. Therefore, the hydrophilic fraction of carbohydrates in SMP was most likely the main foulants in the MBR.
Molecular weight (kDa)
>30 30-10 10-5 <5
Percentage (%)
0 10 20 30 40 50 60 70
Supernatant Permeate
Molecular Weight (kDa)
>30 30-10 10-5 <5
Percentage (%)
0 10 20 30 40 50 60 70
Supernatant Permeate
Figure 4.12. Molecular weight distribution of the SMP in mixed liquor and effluents:
(a) SRT 10 days and (b) SRT 60 days.
(a)
(b)
Elapsed time (min)
0 50 100 150 200 250 300
TOC (mg/L)
0 5 10 15 20 25 30 35
Elapsed time (min)
0 10 20 30 40 50 100 150 200
TOC (mg/L)
0 2 4 6 8 10 12 14
Figure 4.13. Changes of TOC with elapsed time: (a) SRT 10 days and (b) SRT 60 days.
(a)
(b)
Mixed liquor Permeate
TOC (mg)
0 2000 4000 6000 8000
HPI HPOA TPIA
Figure 4.14. Hydrophilicity of SMP in the mixed liquor and effluent at SRT 10. (HPI:
hydrophilic fraction; HPOA: hydrophobic acids; TPIA: transphilic acids) 54%
36%
Miixed liqupr permeate
Polysaccharides (mg as glucose)
0 200 400 600 800
HPI HPOA TPIA
Figure 4.15. Hydrophobicity of polysaccharides in the mixed liquor and effluent at SRT 10.
Chapter 5
Fouling mitigation in MBRs by TiO
2-composite membrane
5.1 Characterization of TiO2 and TiO2 composite membranes
To characterize the synthesized TiO2 particles and TiO2 composite membranes, TEM, XRD, XPS, and contact angle goniometer were used.
5.1.1 Particle size and crystal structure of synthesized TiO2
The structures of the TiO2 particles synthesized in neutral and acidic colloidal sol were directly observed through TEM. As shown in Figure 5.1, the black spots displayed in these two pictures are the synthesized TiO2 in neutral and acidic sol. It is noted that all TiO2 particles were smaller than 10 nm regardless of the synthetic methods. Further use of the dynamic light scattering particle size distribution analyzer also confirmed that most TiO2 particles synthesized in neutral sol were less than 10 nm (as shown in Figure 5.2).
TiO2 exists in three crystalline phases, anatase, rutile, and brookite, among which rutile is thermodynamically stable, while the other two are metastable. The photocatalytic activity of TiO2 depends on its phase structure, crystallite size, specific surface area and pore structure. Many researchers have claimed that TiO2 in anatase form is an excellent photocatalytic material for air purification, water disinfection, hazardous waste remediation and water purification (Hoffmann et al., 1995; Pekakis et al., 2006)
. The XRD diffraction patterns of the two TiO2 nanoparticles are shown in Figure 5.3, in which the 2θ of the eminent peaks are 25.24° for anatase and 27.46° for rutile.
The TiO2 nanoparticles synthesized in acidic suspension contain both anatase and rutile phases. The result was different from others’ reports (Kwak et al., 2001; Kim et al., 2003; Luo et al., 2005)
, in which the synthesized TiO2 particles were composed entirely of anatase, suggesting that the acidic method may result in more than one mineral phase. On the other hand, the TiO2 nanoparticles synthesized in neutral sol are composed entirely of anatase, which promises the highest photoreactivity and the best efficiency in anti-bio and anti-organic fouling when the UV light is introduced in future application.
Figure 5.1. TEM micrographs of the TiO2 particles: (a) TiO2 nanoparticles in neutral sol and (b) TiO2 nanoparticles in acidic sol.
(a)
(b)
Diameter (nm)
0.1 1 10 100 1000
Volume (%)
0 5 10 15 20 25
Figure 5.2. Particle size distribution of TiO2 synthesized in neutral sol.
2 theta degree
10 20 30 40 50 60 70
Intensity (a.u.)
A
A
A A
A A
A A
A A
R
(a)
(b)
R
A: anatase R: rutile
Figure 5.3. XRD patterns of the synthesized TiO2: (a) TiO2 nanoparticles in acidic sol and (b) TiO2 nanoparticles in neutral sol.
5.1.2 Surface characterization of the TiO2 composite membrane
XPS was conducted to confirm the coating of TiO2 nanoparticles on the surface of the composite membrane. Figure 5.4 (a) is the full survey on the surface of the TiO2
composite-CA membrane. The major constituents of the TiO2 composite membranes are hydrogen, carbon, oxygen and titanium. Because XPS is insensitive to hydrogen, only the XPS analysis of C, O and Ti were shown. As shown in Figure 5.4 (b), the binding energies of Ti 2p core levels were 458.2 eV and 464.1 eV for Ti 2p3/2 and Ti 2p1/2, respectively, which suggested that the Ti was mostly as Ti4+. The binding energy of O 1s core level shown in Figure 5.4 (c) was 530.1 eV, which suggested that the O was mostly as O2-. The XPS spectra of the TiO2 composite membrane confirmed that TiO2 was indeed successfully coated on the membrane through the dip-coating method.
Contact angle was measured to evaluate the changes of hydrophilicity after coating TiO2 on membranes. Contact angle of the virgin membrane and the TiO2
composite membranes are summarized in Table 5.1. The contact angle of the virgin membrane is 89.13. After the membrane was coated one-time and three-time with TiO2 particles, the contact angles of the TiO2 composite membranes decreased to 80.72∘and 21.18, respectively. The hydrophilicity of the membranes was increased by the immobilization of TiO2 nanoparticles on the membrane surface. The reason of decreasing contact angle may be due to the aggregation of a great amount of TiO2
nanoparticles on the membrane surface. This aggregation results in a large number of three-dimensional tiny voids between nanoparticles. Therefore, when a droplet is dropped on membrane surface, the droplet would spread instantly due to the capillary effects of the three-dimensional tiny voids and the hydrophilic effect of anatase TiO2
nanoparticles in nature (Song et al., 2008)
. As a result, the coating of TiO2 may reduce membrane fouling by increased hydrophilicity (Luo et al., 2005; Jung et al., 2006)
.
Binding energy (eV)
Figure 5.4. XPS reports of (a) full survey of TiO2 composite-CA membrane; (b) TiO2
2p and (c) O 1s core level of TiO2 composite-CA membrane.
O 1s
Ti 2p
C 1s (a)
(b) (c)
Table 5.1. Contact angle of the virgin membrane and the TiO2 CA-composite membranes
Contact angle (°)
Virgin membrane 89.13
Composite-1* 80.72
Composite-2** 21.18
*Composite membrane with one-time coating of neutral TiO2 sol
**Composite membrane with three-time coating of neutral TiO2 sol
5.2 Effect of TiO2 composite membranes on membrane fouling 5.2.1 Fouling mitigation of the composite membranes
Two MF membranes (CA and MCE) were made into TiO2 composite membranes by coating with neutral TiO2 sol to evaluate the antifouling ability of membrane modification. The filtration resistance of the CA membrane and the TiO2
composite-CA membrane are depicted in Figure 5.5 (a) and the filtration resistance of the MCE membrane and the TiO2 composite-MCE membrane are shown in Figure 5.5 b. Both results indicate the improvement in fouling control by coating TiO2 particles on membrane surface. Regardless of membrane materials membrane fouling can be reduced by coating TiO2 on membrane surface. Hydrophilic surface can reduce hydrophobic adsorption between sludge and membrane because sludge cake formed on hydrophilic surface can be readily removed by shear stress (Parsmore et al., 2002; Maximous et al., 2009)
.
Flux declines of the virgin CA membrane and the TiO2 composite-CA membranes dip-coated in acidic TiO2 suspension are shown in Figure 5.6 for comparison. As shown in Figure 5.6, fouling reduction was also observed, which was consistent with others’ results (Bae & Tak, 2005c; Luo et al., 2005; Bae et al., 2006; Choi et al., 2007)
. Results from Figure 5.5 and Figure 5.6 suggest that membrane modification can be a simple and effective way to reduce membrane fouling. To avoid the potential hazard of acidic TiO2 suspension on membrane, TiO2 composite membranes dip-coated in neutral sol would be more membrane-friendly, which could be applied for more pH-sensitive membranes.
The membrane was dip-coated in TiO2 sol for various times to determine the optimal amount of TiO2 particles on membrane surface for best fouling mitigation.
The filtration resistance for different coating is illustrated in Figure 5.7. Although the 2-time coating further improved the filtration, the 3-time coating, on the other hand, reversed the effect. It is clear that increasing the amount of TiO2 particles on membrane surface by increasing coating times ameliorated membrane fouling before a critical amount was reached. The SEM micrographs of the surface topography of the virgin membrane and the TiO2 composite membranes, as shown in Figure 5.8, strongly suggest that the higher filtration resistance at 3-time coating is due to the blocking of the membrane surface. The surface of the TiO2 composite CA membrane
water flux and the permeability of the virgin membrane and the TiO2 composite membranes, as shown in Figure 5.9. Therefore, the amount of TiO2 on membrane surface must be accurately controlled to obtain optimal antifouling effect.
Filtrate volume (ml)
0 20 40 60 80 100
Filtration resistance (1/m)
0 5e+11 1e+12 1e+12 2e+12 3e+12 3e+12
Composite membrane Virgin membrane
Filtrate volume (ml)
0 20 40 60 80 100
Resistance (1/m)
0.0 5.0e+10 1.0e+11 1.5e+11 2.0e+11 2.5e+11 3.0e+11 3.5e+11
Virgin membrnae MCE-composite membrane
Figure 5.5. Fouling mitigation patterns of the virgin and the TiO2 composite membrane: (a) composite-CA membrane (0.2µm) and (b) composite-MCE membrane (0.45µm). (coating with neutral TiO2 sol)
(a)
(b)
Filtrate volume (ml)
0 50 100 150 200 250 300 350
Nominal flux
0.0 0.2 0.4 0.6 0.8
1.0 Virgin membrane
Composite membrane
Figure 5.6. Flux decline of the virgin and the TiO2 composite-CA membrane (0.2µm).
(coating with acidic TiO2 sol)
Time (s)
0 100 200 300 400 500
Filtration resistance (1/m)
0 2e+11 4e+11 6e+11 8e+11 1e+12
Control Composite-1 Composite-2 Composite-3
Figure 5.7. Antifouling ability of TiO2 composite-CA membranes (0.45µm) with different dip-coating times in neutral TiO2 sol.
Figure 5.8. SEM micrographs of (a) virgin CA membrane, (b) composite-CA membrane coated with one-time coating of neutral TiO2 sol, and (c) composite-CA membrane with three-time coating of neutral TiO2 sol.
(a) (b)
(c)
Virgin membrane Composite-1 Composite-2 Composite-3 Flux (m3 /m2 s)
0 2e-5 4e-5 6e-5 8e-5 1e-4
Permeability (m/Pa s)
2.5e-8 3.0e-8 3.5e-8 4.0e-8 4.5e-8 5.0e-8 5.5e-8 Clean water flux
Permeability
Figure 5.9. Clean water flux and permeability of the virgin membrane and the TiO2
composite membranes.
5.2.2 Fixation of TiO2 particles on composite membranes
In order to evaluate the reliability of the TiO2 composite membranes for long time operation, these composite membranes were washed in an ultrasonic bath with a frequency of 40 kHz and a nominal power of 400 W. Table 5.2 summarizes the relative atomic concentrations of elements remaining on the membrane surface after various ultrasonic washing. It is noted that after ultrasonic washing for three minutes, the relative atomic concentration of titanium element decreased from 52.58 to 27.44
%. No significant reduction of Ti was observed at longer washing. The result indicates that the loosely attached TiO2 particles were lost in the first couple minutes of ultrasonic washing. Most TiO2 particles were tightly bound on the membrane even after vigorous membrane cleaning by ultrasonic washing. Therefore, the firm attachment of TiO2 nanoparticles on membrane surface implies that the composite membranes are reliable for operation. The antifouling propensity would not significantly decrease when operating under high shear stress or performing physical or chemical cleaning.
Table 5.2. Relative atomic concentration of elements on the TiO2 composite membrane surface under ultrasonic washing.
Relative atomic concentration (%) Samplea
C O Ti
1 8.34 39.08 52.58
2 15.28 57.28 27.44
3 14.71 55.08 30.22
4 16.45 61.94 21.60
a Analysis were performed for the TiO2 composite membrane: (1) freshly prepared, (2) after ultrasonic washing for 3 min, (3) after ultrasonic washing for 30 min and (4) after ultrasonic washing for 1 h.
Chapter 6
Conclusions and recommendations
6.1 Conclusions
The following conclusions have been made based on the results of this study:
(1) Even under severe sludge bulking, the MBR can still maintain excellent effluent.
The removal rates of TOC and ammonia nitrogen were as high as 98% and 99%, respectively, regardless of the changes in sludge characteristics.
(2) Particle size distribution has no direct connection with the serious fouling by bulking sludge. The higher membrane fouling caused by bulking sludge can be contributed to the SMP released from filamentous bacteria.
(3) CST correlated well with SMP and can be a potential fouling indicator.
(4) Soluble polysaccharides and proteins, especially the former, rather than bound EPS are responsible for membrane fouling in MBR caused by filamentous bacteria.
(5) Membrane fouling in MBR under subcritical flux operation is caused by smaller particles such as colloids and solutes in the mixed liquor.
(6) In the range of SRT studied, SRT has significant impact on membrane fouling through the alteration of sludge characteristics particularly the concentration of SMP. The shorter the SRT, the severer the fouling.
(7) Longer SRT results in smaller molecular weight of SMP due to the decomposition by microorganisms. Longer SRT brings negligible fouling due to the rejection of small components by the rapidly formed dynamic membrane on the membrane surface.
(8) Hydrophilic fraction dominates in SMP, which is largely accumulated in the MLSS of the bioreactor. Hydrophilic carbohydrates are most likely the major foulant at SRT 10 days.
(9) TiO2 coating on CA and MCE membrane can reduce membrane fouling by enhancing the hydrophilicity of the membrane surface. Both acidic TiO2
suspension and neutral TiO2 sol are effective in membrane modification for fouling reduction.
(10) Optimal amount of TiO2 coating must be justified to avoid blocking membrane1
while reducing membrane fouling. Dip-coating of membrane can provide strong fixation of TiO2 particles on membrane surface.
6.2 Recommendations
(1) Further investigation is needed to verify the findings in the real world situation.
(2) Temperature may have impact on sludge characteristics and therefore, the membrane fouling. It may be interesting to see if temperature affect any of the result.
(3) More studies on fouling mitigation by TiO2 composite membranes can be done, such as applying UV on the membrane to further enhance its hydrophilicity and resistant to biofouling. More tests can also be performed to involve more types of membrane to search for more efficient TiO2 composite membranes for fouling mitigation.
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