Impacts of dissolved oxygen control on different greenhouse gas emission sources in wastewater treatment process

Activated sludge wastewater treatment plant (WWTP) has been recognized as one of the main contributors of greenhouse gas (GHG). Several technologies have attempted to control GHG in WWTP but have resulted in limited impacts and caused secondary pollution. Recently, often calculating total GHG as an evaluation indicator, the GHG emissions from different GHG sources have rarely been investigated. In this study, focusing on the automation of WWTP process, an alternative solution is proposed to explore the effects of control implement on GHG reduction. Firstly, simulating with the influent data of 14-days stormy weather, based on the recognized benchmark simulation model No 1 (BSM1) that accounts for the reaction kinetics and material balances of microorganism, the on-site GHG emission models of four sub-processes in WWTP, including CO2 from endogenous decay, biochemical oxygen demand removal, nitrification, and N2O from denitrification, are developed, and the off-site CO2 and CH4 emissions caused by the sludge disposal, CO2 from energy for aeration, pumping and chemical usage, are also estimated. The simulation results show that the overall averaged GHG of 844.01 kg CO2-eq/h are emitted, where 25.83% emits from the on-site sub-processes and 74.17% discharges from the off-site sources. Subsequently, the traditional proportional-integral (PI) control strategy is applied to control the concentrations of dissolved oxygen (DO) in three aerated tanks. With the control actions, the DO-PI control systems manage to reduce the GHG emissions by 15.53 kg CO2-eq/h averagely, and the GHG reduction mainly comes from the sources of aeration and nitrification, which are reduced by 11.67 kg CO2-eq/h and 4.64 kg CO2-eq/h, respectively. But, there are few influences on the other sources. Summarily, the impacts of DO-PI control on the GHG emissions from different sources of WWTP are investigated, revealing that the automatic control system can be a practical and viable approach to achieve the GHG reduction in WWTP, which caters to the sustainable development of WWTP.