Wastewater treatment plants often face the problems of sludge settling mainly due to sludge
bulking. Generally, synthetic organic polymer and/or inorganic coagulants (ferric chloride,
alum and quick lime) are used for sludge settling. These chemicals are very expensive and
further pollute the environment. Whereas, the bioflocculants are environment friendly and
may be used to flocculate the sludge. Extracellular polymeric substances (EPS) produced by
sludge microorganisms play a definite role in sludge flocculation. In this study, 25 EPS
producing strains were isolated from municipal wastewater treatment plant. Microorganisms
were selected based on EPS production properties on solid agar medium. Three types of
EPS (slime, capsular and bacterial broth mixture of both slime and capsular) were harvested
and their characteristics were studied. EPS concentration (dry weight), viscosity and their
charge (using a Zetaphoremeter) were also measured. Bioflocculability of obtained EPS was
evaluated by measuring the kaolin clay flocculation activity. Six bacterial strains (BS2, BS8,
BS9, BS11, BS15 and BS25) were selected based on the kaolin clay flocculation. The slime EPS
was better for bioflocculation than capsular EPS and bacterial broth. Therefore, extracted
slime EPS (partially purified) from six bacterial strains was studied in terms of sludge settling
[sludge volume index (SVI)] and dewatering [capillary suction time (CST)]. Biopolymers
produced by individual strains substantially improved dewaterability. The extracted slime
EPS from six different strains were partially characterized.