Unveiling the mysteries: a deep dive into extracellular polymeric substances in activated sludge systems of wastewater treatment plants

Activated sludge is a complex wastewater purification process involving microorganisms, extracellular polymeric substances (EPS), and organic matter. Although this process has been utilized for over a century, our understanding of the relationships between the components of activated sludge and wastewater treatment plant (WWTP) metrics is still evolving. Polysaccharides play significant roles in water retention within loose-bound EPS and soluble EPS, negatively impacting dewaterability and flocculation. Conversely, proteins, primarily found in tightly bound EPS, are sensitive to pH and contribute positively to settleability and flocculation. Studies on eDNA are limited but suggest it has key functions in stabilizing the matrix and interacting with polymers. There is considerable information on how proteins and sugars affect sludge volume index, capillary suction time, and flocculation. Additionally, some studies investigated the biochemistry and molecular biology of activated sludge across temporal, geographical, and laboratory settings. However, there are few studies identifying trends between specific sugars, proteins, bacterial taxa, and WWTP metrics. This review explores the intermolecular forces binding these components together and the roles of proteins, polysaccharides, eDNA, and bacterial taxa in the sludge. It also examines the relationships between these sludge components and settleability, flocculation, dewaterability, type of WWTP, and environmental trends. The review provides a holistic and integrative perspective on EPS in activated sludge systems, emphasizing the significance of different EPS fractions (TB EPS and LB EPS) and their contributions to key WWTP metrics such as dewaterability, settleability, and flocculation.