Influences of extracellular polymeric substances (EPS) on flocculation, settling, and dewatering of activated sludge

Results in over 200 publications related to constituents, quantities, and characteristics of extracellular polymeric substances (EPS) in activated sludge and their influences on sludge flocculation, settling, and dewatering are compiled and analyzed. Constituents and quantities of EPS are strongly dependent on the extraction process. Most results show that EPS in activated sludge comprise substantially more proteins than carbohydrates. EPS and sludge flocs carry negative zeta potentials and negative charge. Sludge hydrophobicity increases with protent content of the EPS, and an increased hydrophobicity generally leads to a better flocculation. Most data show that sludge settles less well, as measured by sludge volume index, with increased amount of EPS. Sludge settling and dewatering characteristics are affected by the zeta potential, surface charge, and hydrophobicity, as well as the presence of multivalent mineral cations. Process conditions of activated sludge such as sludge retention time, dissolved oxygen, and nutrient levels affect the EPS properties and, thus, the sludge flocculation and settling characteristics. The effects of EPS quantities, ratio of protein and carbohydrate in EPS, sludge digestion, mineral cations, chelating reagent, nutrient level, and pretreatment on sludge dewaterability are discussed.