The trade-off between N2O emission and energy saving through aeration control based on dynamic simulation of full-scale WWTP

Abstract

This study investigated the trade-off between energy saving and N2O emission reduction of WWTP under the precise control of dissolved oxygen (DO) concentration through model simulation. A long-term dynamic model for full-scale WWTP GHG emissions was established and calibrated with monitored year-round hourly water quality data to quantify the annual GHG emissions from WWTP. Results showed that N2O dominated the direct emission, up to 76.1%, and the variability of N2O generation could better be revealed by dynamic simulation. Furthermore, GHG emissions of the WWTP were mainly contributed by electric energy, among which the blower consumes the most electricity. To reduce the electricity consumption of blowers, improve mechanical efficiency and reduce DO concentration should be considered. DO setting played a significant role in the N2O and CH4 emission, electricity consumption and effluent quality, which was challenging to balance. The ultralow-oxygen (0–1/0.2–1 mg/L) and low oxygen (1–2 mg/L) control strategies were proposed, and their effects on total GHG emissions and effluent water quality were discussed. If the anaerobic environment (DO<0.2 mg/L)could be avoided, the control frequency (high and low) of the DO set-point did not have a significant effect on the emissions of N2O and CH4 and the effluent quality. The ultralow-oxygen strategy (0.2–1 mg/L) with a high-frequency control strategy achieved the lowest GHG emissions under the current energy mix. However, by 2050, as the energy supply gets cleaner, the total GHG emissions of WWTPs with ultralow-oxygen aeration (0.2–1 mg/L) will exceed low-oxygen aeration by 3.6%-4.2%, as N2O dominates 61.6%. Therefore, considering the trade-off between N2O emission and energy saving in WWTP, ultralow-oxygen aeration is a transition scheme to cleaner energy.