Sludge reduction using the oxic-settling-anoxic (OSA) process: underlying mechanisms, microbial community, and fate of trace organic contaminants

The conventional activated sludge (CAS) process is widely used for treating domestic and industrial wastewaters. However, it produces large amounts of excess sludge that requires treatment, handling, and disposal. These procedures represent a major fraction of the total operating cost of wastewater treatment plants (WWTPs). Furthermore, processed sludge called “biosolids” that are applied in agriculture may contain trace organic contaminants (TrOCs) that adversely affect the environment and human health. To reduce the cost of sludge management and the environmental risks associated with residual sludge, a strategy that will minimise sludge production must be formulated. The oxic-settling-anoxic (OSA) process is a promising approach to minimise sludge production. OSA modifies CAS by placing external anoxic reactor/s in the return activated sludge (RAS) loop. The interchange of sludge between conditions that are rich (the aeration tank) and deficient (the external anoxic reactor/s) in oxygen and substrate has been found to result in net excess sludge reduction. Due to its simple design, OSA has relatively low capital and maintenance cost. However, the promising sludge reduction rates observed in laboratory-scale OSA operated with synthetic wastewater have not been realised in pilot- or full-scale implementations. This is due to knowledge gaps in the fundamental operation of OSA. The mechanistic process of OSA, especially the factors affecting OSA performance and the role of microbial community structure on sludge reduction, are poorly understood. Moreover, the occurrence of TrOCs in residual sludge produced by OSA has not been evaluated. The overreaching goal of this study is to obtain a comprehensive understanding of the OSA process. This study aims to determine the effect of three factors – iron salt dosage, sludge interchange rate (SIR), and sludge retention time (SRT) – on sludge reduction. Applying an integrative approach that focuses on these factors will help elucidate the underlying mechanisms governing sludge reduction. Additionally, this study aims to investigate the fate of TrOCs (i.e., occurrence, sorption, and biodegradation) in OSA. In this study, a laboratory-scale OSA system consisting of a sequencing batch reactor (SBR) attached to external intermittently aerated (i.e., aerobic/anoxic) and anoxic reactors was operated. The extent of sludge reduction was assessed by comparing OSA with a control system consisting of an SBR attached to a single-pass aerobic digester. The two systems were operated in parallel using real wastewater. Using real wastewater is crucial to this study because it helped cultivate biomass with realistic growth rates and properties