Minimization of excess sludge production in the treatment of municipal wastewaters by using membrane bioreactor-ultrasound combined systems

Abstract

The treatment and disposal of excess sludge has become the most challenging problem in wastewater treatment plants due to economic, environmental and legislative factors. Therefore, to develop strategies and technologies in order to minimize excess sludge production has become unavoidable. The purpose of the study was to investigate the effect of ultrasonic sludge disintegration on the minimization of excess sludge production in the treatment of municipal wastewater by using membrane bioreactors (MBRs). In the first stage of the study, in order to evaluate the effect of sonication on sludge properties, a series of batch experiments were performed. Ultrasonic disintegration increased the solubility of the organic matter and so the soluble chemical oxygen demand (SCOD), disintegration degree (DDCOD) and supernatant turbidity in sludge while it deteriorated the dewaterability and the rheological properties of sludge. In the second stage, membrane bioreactor-ultrasound (MBR-US) combined systems with different sludge sonication conditions were studied. In order to minimize the production of excess sludge in the MBR, ultrasonic disintegration was applied to a part of excess sludge and this sludge returned back into the MBR. When sonication was applied to a part of excess sludge daily, the effluent quality of MBR-US system was deteriorated. However, it was observed that the MBR-US system can tolerate the negative effects of returned disintegrated excess sludge when the sonication time interval increases. The results indicated that minimization of excess sludge production can be achieved in MBR-US system without any deterioration in the effluent quality. Furthermore the enhancement of nutrient removal is possible in MBR-US systems.