Role of extracellular polymeric substances in bioflocculation of activated sludge microorganisms under glucose-controlled conditions

Extracellular polymeric substances (EPS) secreted by suspended cultures of microorganisms from an activated sludge plant in the presence of glucose were characterized in detail using colorimetry, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. EPS produced by the multi-species community were similar to literature reports of pure cultures in terms of functionalities with respect to C and O but differed subtly in terms of N and P. Hence, it appears that EPS produced by different microorganisms maybe homologous in major chemical constituents but may differ in minor components such as lipids and phosphodiesters. The role of specific EPS constituents on microbial aggregation was also determined. The weak tendency of microorganisms to bioflocculate during the exponential growth phase was attributed to electrostatic repulsion when EPS concentration was low and acidic in nature (higher fraction of uronic acids to total EPS) as well as reduced polymer bridging. However, during the stationary phase, polymeric interactions overwhelmed electrostatic interactions (lower fraction of uronic acids to total EPS) resulting in improved bioflocculation. More specifically, microorganisms appeared to aggregate in the presence of protein secondary structures including aggregated strands, beta-sheets, alpha- and 3-turn helical structures. Bioflocculation was also favored by increasing O-acetylated carbohydrates and overall C-(O,N) and O=C-OH+O=C-OR functionalities.