3. Experimental methods
3.1 Material
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 easier access to substrate, oxygen and nutrients than floc-forming bacteria owing to the long filaments, (2) The kinetic theory: filamentous and floc-forming bacteria have different maximum growth rates, (3) The accumulation/regeneration theory:
floc-forming bacteria have greater capacity of energy storage, (4) The starvation theory:
organisms with higher storage capacity are more resilient under limited substrate conditions (Dalentoft & Thulin, 1997)
. Since the majority of the nutrient compounds in the simulated feed are readily biodegradable, which are much more readily accessible to the filamentous bacteria. As a result, the filamentous bacteria became the dominant species.
To correct this problem an aerobic selector was installed. A selector is a separate mixing zone upstream of the aerobic basin in which the recycled activated sludge and influent wastewater are mixed. Three types of selectors are used in dealing with filamentous bulking: aerobic, anoxic and anaerobic. The key in preventing filamentous bulking by selector is the substrate utilization characteristics of the bacteria (Tsai & Lee, 1998)
. Filamentous bacteria have lower half-saturation constant (Ks) and maximum growth rate (µmax) than floc-forming bacteria, which therefore is the main theory of aerobic selector. In this way the sludge was successfully shifted from filamentous bacteria to flco-forming bacteria as seen in Figure 4.1 (c) and (d).
In conventional activated sludge process, sludge settleability is the key factor in maintaining effluent quality. Sludge bulking which is usually due to overgrowth of filamentous bacteria often deteriorates the performance of activated sludge. Figure 4.2
shows the removal of TOC and ammonia nitrogen. The selector was installed after the MBR was operated for 20 days. Despite the serious sludge bulking caused by overgrowth of filamentous bacteria, the effluent quality remained the same, as shown in Figure 4.2 (a) and (b). The average TOC and NH3-N in the MBR influent was 158±20.0 mg/L and 32.0±0.67 mg/L, respectively, over the entire period of operation. Nearly 98% of the organics were removed by the MBR treatment regardless of the sludge characteristics (Figure 4.1). Biological nitrification was also excellent. Almost 99% of ammonia nitrogen was nitrified during the experiment. The NH3-N of the effluent was reduced to 0.24±0.37 mg/L even when sludge bulking occurred (Figure 4.2 (b)). The result indicates that membrane bioreactor is a reliable wastewater treatment process.
The excellent pollutant removal renders MBR a promising process for wastewater reuse.
Figure 4.1. Microscopic images of sludge flocs: (a) and (b) overgrowth of filamentous bacteria without installation of the selector; (c), and (d) floc-forming bacteria after installation of the selector.
100 µm
100 µm 100 µm
100 µm
(a) (b)
(c) (d)
Time (day) bulking sludge and normal sludge.
(a)
(b)
Installing selector
4.1.2 Impact of bulking sludge on membrane fouling
Bulking sludge, on the other hand, had significant impact on membrane fouling, as illustrated in Figure 4.3 (a). In the initial period of operation, the TMP profile exhibited a typical two-stage pattern under subcritical flux operation when filamentous bacteria started to become dominant. A slow and progressive membrane fouling was observed in the initial 100 h followed by a sudden TMP increase. After the TMP reached -60 kPa, the membrane was chemically cleaned by 0.5% sodium hypochlorite for 2 hours. However, the membrane fouled right after the membrane cleaning with a fouling rate of up to 28.7 kPa/h. Therefore, frequent membrane cleaning was performed afterwards. The TMP profile of the MBR after the aerobic selector was installed was shown in Figure 4.3 (b). Membrane fouling decreased gradually and the TMP profile changed completely when floc-forming bacteria were dominant in the bioreactor. The TMP profile changed to a typical two-stage pattern in subcritical flux operation. The first stage of slow fouling rate lasted for about 200 h before the second stage of TMP jump appeared. The fouling rate was greatly reduced to 0.03 kPa/h in the progressive and slow fouling stage. After the floc-forming bacteria stabilized and became steadily dominant in the bioreactor, the fouling rate became steady and relatively slow. Meng et al (2006b) reported that the excess growth of filamentous bacteria formed a non-porous cake layer on the membrane surface which interfered with the membrane filtration. Meng et al (2006 a) and Meng & Yang (2007) further suggested that bulking sludge caused the formation of a dense cake layer on the membrane surface due to the fixation of filamentous bacteria. Chang et al (1999) also reported that bulking sludge have higher fouling tendency than normal sludge and pinpoint sludge. However, Li et al (2008) had found an opposite result that filamentous bacteria had negligible effect on membrane fouling. The contradict results might be due to the different influent wastewater and processes discussed in 2.1.2.5.
Particle size distributions of normal sludge and bulking sludge are shown in that bulking sludge caused by overgrowth of filamentous bacteria had larger particle size distribution. It contradicts the common knowledge that smaller particles are generally more easily to deteriorate membrane filtration (Chang et al., 2002; Rosenberger & Kraume, 2002)
. According to Carmen-Kozeny equation, specific cake resistance is a function of particle diameter, porosity of cake layer, and particle density. The specific cake resistance is inversely proportional to the square of the particle diameter. Thus smaller particles size will result in greater cake resistance. However, the severe fouling in
bulking sludge cannot be explained by particle size alone. There are some other important factors resulting in the severe fouling.
The distinct TMP profiles of normal sludge (floc-forming bacteria) and bulking sludge (filamentous bacteria) must be answered by the difference in sludge characteristics, which is summarized in Table 4.1. The supernatant TOC, representing SMP in mixed liquor (Liang et al., 2007)
, was about 12 times higher than that of normal sludge. The soluble EPS of bulking sludge was about 6 times higher. It strongly implies that SMP or other organic compounds in bulking sludge might be responsible for the higher fouling rate. On the contrary, the concentrations of bound EPS in normal sludge and bulking sludge were about the same. The detail will be discussed in 4.1.3. CST is commonly used to represent dewaterability of sludge. The CST of the bulking sludge was significantly larger than that of normal sludge, which echoes the findings by Wang et al (2006) and Wu et al (2007) that CST values were positively correlated to membrane fouling. Rosenberger and Kraume (2002) have also reported that soluble EPS affected the filterability of activated sludge most significantly, in agreement with our result. As a result, CST seems to be a good indicator of sludge filterability.
Operating time (h)
0 100 200 300
TMP (kPa)
-60
-50
-40
-30
-20
-10
0
Operating time (h)
0 100 200 300 400 500
TMP (kPa)
-60
-50
-40
-30
-20
-10
0
Figure 4.3. TMP profiles of different sludge properties: (a) filamentous bacteria and (b) floc-forming bacteria.
(a)
(b)
Particle size (µm)
0.1 1 10 100 1000
Volume (%)
0 2 4 6 8
Bulking sludge Normal sludge
Figure 4.4. Particle size distributions of normal sludge and bulking sludge.
Table 4.1. Comparison of sludge characteristics between normal sludge and bulking sludge
Supernatant TOC (mg/L)
CST (s)
Soluble EPSa (mg/L)
Bound EPS a (mg/g MLSS) Normal
sludge
5±2 16±2 25±16 130±13
Bulking sludge
66±9 30±28 145±37 133±20
a The concentration of EPS is expressed as the sum of proteins and polysaccharides as BSA and glucose, respectively.
4.1.3 Effect of sludge properties on EPS
Since EPS has been widely accepted as the major foulant in MBR (Nagaoka et al., 1996a;
Cho & Fane, 2002; Kimura et al., 2005; Zhang et al., 2006a)
, the EPS components in the mixed liquor were monitored and compared with the performance of the MBR operation for fouling study. Four components were monitored: soluble polysaccharides, soluble proteins, cell-bound polysaccharides and cell-bound proteins. In this study, total soluble EPS or SMP is the sum of soluble polysaccharides and soluble proteins. And the sum of cell-bound polysaccharides and cell-bound proteins represents the total bound EPS.
Figure 4.5 compares the concentration of soluble and cell-bound EPS in various sludge conditions. There was no significant difference in the production of bound EPS between bulking and normal sludge. On the other hand, much more soluble polysaccharides and soluble proteins were produced in bulking sludge, especially the soluble polysaccharides. Higher membrane fouling caused by overgrowth of filamentous bacteria seems to relate to the increased amount of SMP in bulking sludge, which echoes the observations by other researches that soluble polysaccharides or soluble proteins in SMP influence the membrane performance in MBR (Mukai et al., 2000;
Hernandez Rojas et al., 2005; Kimura et al., 2005; Rosenberger et al., 2005; Fan et al., 2006; Zhang et al., 2006b)
. However, Meng et al (2006a; 2006b; 2007) later made an observation that contradicts our results.
They concluded that severe membrane fouling caused by excessive growth of filamentous bacteria might be caused by the production of more bound EPS. The result differed from our observation, possibly because that they obtained the sludges from different MBR processes. Lately, Li et al (2008) also reported that bound EPS was the major contributor to membrane fouling. They pointed out that filamentous bacteria have no significant influence on membrane fouling. The contradictory findings could come from the difference in operation conditions or the difference in filamentous species. To verify the cause for severe fouling associated with bulking sludge, the foulants on membrane surface were identified by FTIR. FTIR spectra of fresh and fouled membranes are shown in Figure 4.6 to show the functional groups of the foulants on the membrane surface. The peaks at wave number 1647 and 1533 cm-1 are assigned to the amide-Ⅰ and amide-Ⅱ bands (Kimura et al., 2005; Jarusutthirak & Amy, 2006)
, respectively. The absorption band at 3286 cm-1 is N-H stretching. The peak at wave number 1041 cm-1 is assigned to bond vibrations of polysaccharides (Omoike & Chorover, 2004)
and the peak at wave number 2925 cm-1 is also a character of polysaccharides. The result suggests that
Protiens and polysaccharides concentration (mg/L)
0 20 40 60 80 100 120
Proteins and polysaccharides concentration (mg/g MLSS)
0 20 40 60 80 100 120 140 Soluble proteins
Soluble poolysaccharides Cell-bound proteins Cell-bound polysaccharides
Bulking sludge
Normal sludge
Bulking sludge
Normal sludge
Figure 4.5. Comparison of EPS components in filamentous bacteria and floc-forming bacteria.
Wave number (cm-1)
1000 2000
3000 4000
Absorbance
0.00 0.02 0.04 0.06 0.08 0.10 0.12
Fouled membrane Fresh membrane
1041
3286 2925
1647 1533
Figure 4.6. FTIR spectra of fresh and fouled membranes.
4.1.4 Effect of sludge fractions on membrane fouling
To provide more information concerning the contradictory results among other studies (Meng et al., 2006a; Meng et al., 2006b; Meng & Yang, 2007; Li et al., 2008)
and ours, fouling by sludge components was also investigated. Sludge was separated into three fractions by particle size: suspended solids, colloids and solutes. The experiment was first operated at two stirring rates to create different shear forces for the evaluation of the contributions of different sludge fractions on membrane fouling. Figure 4.7 illustrates the resistances of sludge fractions at different stirring rates. The difference between activated sludge and colloids + solutes represents the resistance of suspended solids.
The difference between colloids + solutes and solutes represents the resistance of colloids. In order to compare the relative contribution of sludge fraction on membrane fouling in detail, the results obtained in Figure 4.7 was summarized in Table 4.2. At low shear stress (stirring rate of 400 rpm), the major fouling contributors are colloids and solutes. The resistance contributed by colloids and solutes were 36.52% and 36.15%, respectively. When the stirring rate was increased to 1,000 rpm, the resistance contributed by suspended solids disappeared completely while the majority of resistance came from the solutes. As shown in Table 4.2, increasing the stirring rate increased the contribution of solutes to fouling. The finding of this study also proved that different operation conditions might lead to different results, which might explain the disagreement between studies (Wisniewski & Grasmick, 1998; Defrance et al., 2000; Bouhabila et al., 2001;
Lee et al., 2003; Bae and Tak, 2005a)
. Defrance et al (2000) and Bae and Tak (2005) concludes that suspended solids are the main contributor to membrane fouling because their systems were operated under relatively high flux and low cross flow. On the other hand, Wisniewski & Grasmich (1998) reported differently, that solutes were the main contributor to membrane fouling since their system was operated under high shear stress condition.
At high shear force, smaller components, namely, colloids and solutes, dictated the resistance. Back transport caused by Brownian diffusion is dominant for small particles and at low shear stress, while back transport caused by shear-induced diffusion and inertial lift increase with shear stress rate and is proportional to particles size (Belfort et al., 1994)
. As a result, the shear-induced diffusion and inertial lift of larger particles such as suspended solids and colloids keeps them away from the membrane, resulting in reduced resistance. In contrast, shear-induced diffusion and initial lift is negligible for small molecules. Back transport of small molecules is caused by Brownian diffusion. In membrane filtration when the drag force due to filtration balances the back transport, membranes are free of deposit. In subcritical flux, the back transport was equal or greater than the permeation drag, therefore, no sharp TMP increase would be observed.
Table 4.2 implies that when the membrane was operated at subcritical flux, larger
particles such as suspended solids would not deposit on the membrane to form a sludge cake. On the other hand, smaller particles such as soluble EPS and other macromolecules would be continuously attracted onto the membrane regardless of the strength of the shear force. TMP jump will be observed when local flux exceeds critical flux. The result is in agreement with the results in 4.1.3 that SMP dominated the membrane fouling in MBR, and, therefore, membrane fouling will occur eventually even though the MBR is operated under subcritical condition.
Relative filtrate volume (V/VT)
Figure 4.7. Resistances of sludge fractions at different stirring rates: (a) 400 rpm and (b) 1,000 rpm.
Table 4.2. Contribution of sludge fraction to resistance at different stirring rates Stirring rate
400 (rpm) 1,000 (rpm)
m-1 % a m-1 % a
Rssb 4.60 ± 0.04 × 1011 27.33 0 0
Rcolb
6.15 ± 0.02 × 1011 36.52 2.55 ± 0.02 × 1011 25.22 Rsolb 6.09 ± 0.14 × 1011 36.15 7.55 ± 0.09 × 1011 74.78
a Percentage in total resistance (%).
b Membrane resistance (Rmem) = 3.99±1.32 × 1010
4.2 Effect of SRT on sludge characteristics and membrane fouling 4.2.1 Fouling rate at different SRTs
As we discussed in 2.1.3.1, SRT is one of the most important operating parameters affecting membrane fouling because SRT would directly alter the characteristics of biomass. In order to investigate the effect of SRT on membrane fouling, the membrane bioreactor was operated under three SRTs, 10, 30 and 60 days.
The SRTs ranging from 10 to 60 days include most of the range of SRTs discussed in literature and are in the range of the optimum SRT (Meng et al., 2009)
. Figure 4.8 illustrates the TMP profiles of the membrane bioreactors operated at SRT 10, 30 and 60 days.
Figure 4.8 apparently shows that membrane fouling increased as SRT decreased. As the membrane bioreactor operated at SRT 10, the MBR suffered from the most serious membrane fouling. This result is in agreement with most published studies (Chang & Lee, 1998; Grelier et al., 2006; Ng et al., 2006; Zhang et al., 2006b; Ahmed et al., 2007; Holakoo et al., 2007; Liang et al., 2007;
Al-Halbouni et al., 2008; Dong & Jiang, 2009)
, though some studies showed the opposite result
(Rosenberger & Kraume, 2002; Lee et al., 2003)
. Figure 4.9 shows the membranes fouled at different SRTs. Comparing with the membranes fouled at SRT 30 and 60 days (Figure 4.9 (b) and (c)), the membrane fouled at SRT 10 days (Figure 4.9 (a)) clearly showed different fouling characteristics. Slime and transparent gel layer was observed on the membrane surface at SRT 10 days. However, sludge cakes apparently formed on the membrane surface at SRT 30 and 60 days. Thick cakes with deep red color were observed on the membrane of SRT 60. It is noted that all these pictures in Figure 4.9 were taken when TMP reached –60 kPa, which means that membrane had been seriously fouled. For subcritical operation, TMP jump is due to the cake layer formation as mentioned in 2.1.3.2. Therefore, cake layer should be observed on membranes when TMP jumps as membranes fouled at SRT 10 and 60 days. However, slime gel layer was observed at SRT 10 days instead of cake layer. These differences imply the different fouling mechanisms and foulants occurred at different SRTs.
Operation time (h)
0 50 100 150 200
TMP (kPa)
-70
-60
-50
-40
-30
-20
-10
0
SRT 10 SRT 30 SRT 60
Figure 4.8. TMP profiles at different SRTs.
Figure 4.9. Pictures of the membranes fouled at different SRTs: (a) 10 days; (b) 30 days and (c) 60 days. All these pictures were taken when the TMP reached -60 kPa.
(a) (b)
(c)
4.2.2 Sludge characteristics at different SRTs
To study the cause of difference in membrane fouling under different SRTs, sludge characteristics were investigated. Because SRT directly affects the sludge characteristics such as EPS production, particle size distribution and may subsequently result in the variation in membrane fouling. Figure 4.10 shows that the SMP (represented as TOC) in the mixed liquor decreased from around 17 mg/L to 4 mg/L when the SRT was switched from 10 days to 30 days, which was in agreement with previous studies that higher fouling potential found in shorter SRT was due to the increase of SMP or EPS in mixed liquor (Ng et al., 2006; Zhang et al., 2006b; Dong & Jiang, 2009) mixed liquor. The attachment of SMP may lead to the observation of slime gel layer which caused membrane fouling. Wang et al (2008) also reported that slime gel layer which was mainly composed of macromolecules, colloids and SMP, etc., was formed on the membrane. However, the concentration of SMP in the mixed liquor was similar in SRT 30 and 60 days, which cannot explain the difference in fouling rates of SRT 30 and 60 days.
Table 4.3 summarizes other properties of sludge at SRT 10, 30 and 60 days.
Despite the lower concentration of MLSS, shorter SRT had higher fouling propensity.
Thus MLSS would not be an important factor affecting membrane fouling in this study. In Table 4.3, it is noted that for SRT 30 and 60 days, SRT 30 days had higher bound EPS than SRT 60 days. This might result in the higher fouling propensity of SRT 30 days. After operating for a period of time, the local flux started to exceed the critical flux, resulting in initial deposition of sludge flocs on membrane surface. At this moment bound EPS had a critical effect on cake resistance. As shown in Table 4.3, higher bound EPS was found to have higher specific cake resistance. Cho et al (2005) also reported that bound EPS affected the specific cake resistance. Bound EPS and specific cake resistance had a sigmoid relationship between them. As a result, the MBR with SRT 30 days had higher fouling propensity than SRT 60 days due to higher cake resistance.
Figure 4.11 shows the particle size distribution of sludge flocs at different SRTs.
In summary, much rapid fouling rate observed at SRT 10 days is due to the SMP
In summary, much rapid fouling rate observed at SRT 10 days is due to the SMP