Ultrasound, thermal and alkali treatments affect extracellular polymeric substances (EPSs) and improve waste activated sludge dewatering

The present study investigates the effect of ultrasound, thermal and alkali treatments on the characteristics of the extracellular polymeric substances (EPSs) in activated sludge. EPSs were fractionated through centrifugation and ultrasound approaches to generate two different EPS fractions: loosely bound EPSs (LB-EPSs) and tightly bound EPSs (TB-EPSs), where the latter is the innermost fraction. An analysis of the untreated sludge revealed that the proteins, followed by the humic acids and polysaccharides, were the major constituents of the EPSs. Each of these components was primarily observed in the TB-EPS fraction. The treatments, particularly the alkali treatment, thoroughly solubilised the EPS, as indicated by the increased LB-EPS content. As a result, the viscosity of the sludge decreased, while the capillary suction time (CST) increased. The molecular weight distributions of the EPS fractions were determined through gel permeation chromatography, which revealed that the thermal treatment (80 °C) denatured the high-molecular-weight proteins. Despite increasing the CST, the three treatments improved the sludge dewatering by releasing the interstitial water trapped within the EPSs. This improvement was more important in the case of the alkali treatment, which solubilised the highest portion of EPSs and resulted in the highest reduction in the absolute value of zeta potential.