Thesis - faculty.ait.ac.th - Asian Institute of Technology

Though, the obtained COD removal efficiency of both systems was slightly different,
the majority of organic concentrations in both effluents were in the lower molecular weight
range indicating that yeast and bacteria were effective in degrading high molecular weight
organics. The high molecular weight organics may be highly refractory organics (Hosomi,
et al., 1989). However, the effluent from both systems still consists of the medium
molecular weight organics such as fulvic acid which are unaffected by biological
treatment. It could be further treated with post treatment such as ozonation, increasing the
biodegradable organics or even elevating the water quality of the final effluent.
4.5.3 Sludge Properties
In the membrane coupled biological treatment systems, complete separation of
microorganisms is possible; thus, high microbial concentration as well as excellent effluent
quality (Kim, et al., 1998) can be achieved. The membrane biofouling could be largely
affected by physico-chemical characteristics and the physiology of the activated sludge as
well as the membrane materials (Sato and Ishii, 1991; Pouet and Grasmick, 1995; Chang,
et al., 1996) Therefore, the sludge properties of the membrane bioreactors are important in
terms of membrane fouling and sludge dewaterability. Dewaterability is usually measured
in terms of Capillary Suction Time (CST) for evaluating the performance of sludge
dewatering. Sludge Volume Index (SVI) is another indicator used to measure the
settleability of the sludge. The bacterial sludge showed a better dewatering quality
compared to that of the yeast system as shown in Table 4.15. As suspended solids also
affect the sludge properties, the MLSS was also measured. Higher viscosity and
dewaterability could be attributed to the difference between MLSS of mixed bacteria
sludge and mixed yeast sludge. But, the difference in the MLSS was not found to be large.
Table 4.15 Sludge Properties in the YMBR and BMBR Systems
Sample Reactor DSVI (ml/gSS) Viscosity (cP) CST (s/g SS) SS (mg/L)
1
YMBR
BMBR
not settle
not settle
6.24
13.00
-
-
-
-
2
YMBR
BMBR
not settle
79
6.30
9.78
128
12
13,267
14,133
3
YMBR
BMBR
not settle
60
-
-
126
10
13,367
13,233
Though the bacterial sludge showed a better dewaterability, the viscosity of the
bacterial system was found to be more than that of the yeast system. The content of microfloc
components, such as EPS might have an influence on the permeability (Kim, et al.,
1998). This could be one of the reasons for a frequent membrane fouling in the bacterial
system compared to that of the yeast system. Along with the MLSS, MLVSS of the sludge
was also measured as given in Table 4.16.
When MLVSS/MLSS was measured, it was found that the bacterial sludge had a
lower degradability (0.6) compared to that of the yeast sludge (0.7). However, the
difference between the degradability of the bacterial sludge and yeast sludge was not much.
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