Microbial Flocs In Wastewater Treatment

Floc formation and associated chemical and biological reactions are an important part of many industrial processes.16-18 Floc formation provides a means of separating solids from the liquid phase in process streams. In wastewater treatment that utilizes the activated sludge process, the quality of the effluent is highly dependent on the efficiency of the solid-liquid separation processes. Flocs play a key role in this regard, and an understanding of floc properties and behavior has led to significant advances in biological C, N, and P removal and in the sludge dewatering process. Floc properties control the efficiency of phase separation.

Floc properties are controlled by the nature of the wastewater. The chemistry of the wastewater controls the types of microorganisms that grow in the system as well as the quality and quantity of EPS elaborated by the microbial community. EPS composition and form depend on process conditions. EPS in activated sludge can be found in two forms: directly associated with microbial cell biomass or physically separated from the biomass as free colloids or slimes.19,20 Floc properties and behavior directly reflect EPS properties. Bulking and foaming reduce the efficiency of the separation process. Bulking occurs when aggregates do not form dense, compact flocs — a direct consequence of EPS production and composition by the microorganisms in the system. Recently, Sponza21 provided evidence that EPS composition depends more on waste stream composition than on process operating conditions. Thus, an understanding of microbial EPS production offers new opportunities to control the flocculation process and overall efficiency of wastewater treatment. This chapter will focus on the fraction of EPS comprised of extracellular enzymes.

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