Response of Activated Sludge to Long-Term Nanosilver Input and Changes in Extracellular Polymeric Substances (EPS)

This study investigates the response of activated sludge to continuous nanosilver (AgNPs) input in a large concentration range. In addition to substrate removal, changes taking place in the different fractions of Extracellular Polymeric Substances (EPS), namely, very loosely bound-EPS (VLB-EPS), loosely bound-EPS (LB-EPS), and tightly bound-EPS (TB-EPS), as well as the protein and carbohydrate content of each fraction were examined. At relatively low AgNP concentrations (5 mg/L), production of loosely bound fractions became higher. At AgNP >= 12 mg/L, also TB-EPS production was promoted. Seemingly, bacteria were protected from inhibitory effects by regulation of EPS production. Moreover, new proteins emerged in EPS. Respiration tests indicated no effect on substrate removal even at high AgNP concentrations because the nanomaterial lost its effectiveness upon continuous aeration. In WWTPs, AgNP concentrations are normally below 0.8 mg/L. Therefore, no negative impact on substrate removal and no visible changes in EPS production would be expected.