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

Floc breakup exposes the EPS present inside the floc structure as well as increasing
the EPS level in bulk solution, which causing seriously membrane fouling (Chang and Lee,
2001). The floc breakup also leads to a loss of biological activity (Brockmann and
Seyfried, 1996; Ghyoot, et al., 1999; Chang and Lee, 2001), change in microorganism
population (Rosenberg, et al., 1999) and decreasing settleability (Cicek, et al., 1999).
Extracellular Polymeric Substances (EPS)
The EPS production is a general property of microorganisms in natural environments
and occurs in bacteria, algae, yeast, and fungi (Flemming and Wingender, 2001). They are
construction materials for microbial aggregates such as biofilm, floc, and sludge.
An activated sludge floc is a microbial entity which is formed by different species
of biomass. The components of the floc are embedded in a polymeric network of EPS. A
significant barrier to permeate flow in the MBR is due to EPS providing a highly hydrated
gel matrix in which microorganisms are embedded. Microbial EPS are high molecularweight
mucous secretions from microbial cells. They are important for floc formation in
activated sludge liquors (Sanin and Vesilind, 2000; Liao, et al., 2001). The EPS matrix is
very heterogeneous, with polymeric materials which includes polysaccharides, proteins,
lipids, and nucleic acids (Bura, et al., 1998; Nielson and Jahn, 1999)
Many MBR studies have identified EPS as the most significant biological factor
responsible for membrane fouling. Chang and Lee (1998) found there to be a linear
relationship between membrane fouling and EPS levels. Nagaoka, et al. (1996, 1999)
found that increase in hydraulic resistance and viscosity of the mixed liquor was due to the
accumulated EPS in the system and also on the membrane. There was a linear relationship
between the hydraulic resistance and viscosity of the mixed liquor, which caused rapid
attachment of the suspended EPS. Huang, et al. (2001) found soluble organic substances
with high molecular weights, mostly attributable to metabolic products, to accumulate in
the bioreactor. These had an indirect proportionality with the membrane permeability.
Accumulation of 50 mgTOC/L resulted in 70% decrease in flux. The fouling proneness
due to specific EPS components has also been studied. Shin, et al. (1999) ascribed 90% of
the cake resistance to EPS and found resistance varied with the ratio of carbohydrate and
protein in the EPS, thereby influencing permeated flux during ultrafiltration. The permeate
flux decreased with an increasing protein content (Mukai, et al., 2000). Kim, et al. (1998)
found that the addition of powdered activated carbon to the MBR was shown to increase
permeability by reducing dissolved EPS levels from 121-196 mg/gVSS to 90-127
mg/gVSS.
Most studies on the effect of EPS on membrane fouling rely on EPS extraction from
the sludge flocs. However, relatively large amounts of EPS can originate from
unmetabolized wastewater components and bacterial products arising either from cell-lysis
of cell-structural polymeric components (Dignac, et al., 1998). Thus, the quantitative
expression of flux as a function of EPS concentration has an inherent limitation.
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