Balancing energy recovery and direct greenhouse gas emissions in wastewater treatment

Achieving net-zero emissions is a critical goal for the water industry. This study provides a comprehensive evaluation of energy recovery and direct greenhouse gas (GHG) emissions from a full-scale wastewater treatment plant (WWTP), highlighting the important balance between carbon capture and emissions reduction. Long-term monitoring results revealed that upstream carbon capture, while recovering significant energy for carbon offset (40% of total emission), stimulated downstream nitrous oxide (N 2 O) emissions, a major contributor to Scope 1 emissions. In response, integrated mitigation strategies were developed using mechanistic modelling, incorporating process optimizations (adjusting split ratios, DO setpoints, and mixing ratio) and retrofitting solution (raw wastewater diversion). The identified strategies reduced N 2 O emissions by 50% and the overall carbon footprint by 40%, despite a 31% decrease in energy recovery, compared with the baseline case (N 2 O emission factor: 1.31% of TKN load, net emissions: 354.29 kg CO 2 -e/ML, and energy recovery: 386.02 kg CO 2 -e/ML). The findings demonstrated the need for a holistic assessment of carbon capture, energy recovery, and GHG emissions across the entire treatment process. The outcome offers actionable insights for improving WWTP operations towards net-zero emissions.