| Treatment and disposal of excess sludge produced in an activated sludge process has become an environmental challenge in densely populated urban areas due to difficulty to locate suitable sites for landfill and incineration. Therefore, reduction/minimization of excess sludge within activated sludge processes may be an ideal solution, This study focuses on the use of a modified activated sludge process, named the oxic-settling-anaerobic (OSA) system, to achieve an effective reduction in excess sludge production. Its key feature is to insert a sludge holding tank in the sludge return circuit to provide an anaerobic sludge zone.; The objectives of the study are to: (1) demonstrate the excess sludge reduction in an OSA system; (2) evaluate the appropriate operating conditions; and (3) study the mechanisms of excess sludge reduction, so as to maximize the excess sludge reduction efficiency of the OSA system.; In continuous-flow studies, an OSA system was operated in parallel with an activated sludge process as a reference system to investigate the effects of the oxidation-reduction potential (ORP) level and the sludge anaerobic exposure time (SAET) in the sludge holding tank on the sludge reduction. The intended level of ORP was maintained by periodical pure nitrogen purging thus preventing the pH drop by chasing the gaseous product in the sludge holding tank. SAET was regulated by changing the pumping rate of the return sludge from settling tank. In batch studies, proposed mechanisms involving soluble microbial products (SMPs) effects, microbial population effects and sludge decay effects were verified. In addition, a COD balance was carried out to supplement the understanding of the mechanisms.; The main findings of my research are: (1) the OSA system had a lower sludge yield and oxygen demand than the reference system under the same solids quantity condition, which demonstrated that an OSA system could provide a cost-effective solution for excess sludge reduction; (2) the OSA system improved COD removal efficiency and sludge settleability; (3) a low ORP in the sludge holding tank favored the excess sludge reduction; (4) low sludge growth yield, soluble microbial products (SMPs) effect and microbial predator contribution were unable to explain the excess sludge reduction; and (5) the excess sludge reduction in an OSA system was mainly due to increased sludge decay in the sludge holding tank under an anaerobic condition and the utilization of COD generated from the sludge decay through denitrification, sulfate reduction, phosphorus release and volatile organic gas production that occurred in the sludge holding tank of the OSA system, among which, the contribution of gaseous production was significant. |
