Effect of ultrasound and microwave irradiation pretreatments on aerobic and anaerobic digestion and EPS structure of wastewater sludges

Abstract

The treatment and disposal of excess sludge is one of the critical issues facing modern society, due to the increase in sludge production. The research for producing less sludge is therefore necessary. Recently, many different studies were conducted in order to minimize the sludge during the wastewater treatment line or prior to sludge stabilization. This study investigates the effect of ultrasonic and microwave pretreatment applications prior to aerobic and anaerobic digestion of waste activated sludge in terms of stabilization efficiency, pathogen removal and biogas production. Sludge samples were collected from four different domestic and urban wastewater treatment plants. Ultrasonication and microwave irradiation disintegration methods were applied to specific portion of the sludge samples. In order to evaluate the digestion efficiency, one control reactor was operated. Furthermore, the change in extracellular polymeric substances (EPS) structure and dewatering properties of the sludge with the specified pretreatments were examined. Both pretreatment applications showed significant enhancement of stabilization for each wastewater sample. In aerobic digestion, pretreated reactors showed better organic reduction with an average of 45% and 42.5% volatile solids (VS) reduction for microwave irradiation and ultrasonication application, respectively while the VS reduction of the unpretreated samples was 31.5% on average. Chemical oxygen demand (COD) removals also increased with disintegration: an average of 31.5%, 33.5% and 21.75% for microwave irradiation, ultrasonication application and unpretreated samples, respectively. The applied disintegration methods resulted in the solubilization of the EPS of the sludge. Therefore the dewaterability of the sludge deteriorated right after the ultrasonication and irradiation applications. However, as the EPS removal was enhanced, dewaterability of the sludge increased during the digestion. Biogas production was increased significantly with both disintegrations. For all different wastewater samples, cumulative biogas productions of microwave irradiated samples were the highest ones. In addition, both aerobic and anaerobic digestion processes pathogen reduction was successfully achieved.