Chemical oxygen demand and the mechanism of excess sludge reduction in an oxic-settling-anaerobic activated sludge process

A modified activated sludge process, called the oxic-settling-anaerobic (OSA) process, achieved effective reduction in excess sludge production. Its key feature is the insertion of a sludge holding tank in the sludge return circuit to provide an anaerobic sludge zone. Our previous studies suggested that such excess sludge reduction might be associated with an increased sludge decay rate and the effective consumption of organic substrates generated during the retention of the thickened sludge in the sludge holding tank under a low oxidation-reduction potential (ORP) at -250 mV. To confirm this suggestion, we analyzed the chemical oxygen demand (COD) balance in the sludge holding tank through batch experiments to simulate the sludge concentration, ORP level, and retention time in the sludge holding tank. The COD generated from the sludge reduction in the tank was utilized by organic gas (mainly CH4) production, denitrification, sulfate reduction, and phosphorus release, among which the gas production accounted for 50% of the COD utilization. We confirmed that the mechanism of the excess sludge reduction in the OSA process is through enhancement of the sludge decay rate in the anaerobic sludge zone, which increases the soluble COD level, which in turn is mainly transformed into methane gas and carbon dioxide during denitrification.