Balancing energy recovery and direct greenhouse gas emissions in wastewater treatment

Abstract

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<inf>2</inf>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<inf>2</inf>O emissions by 50 % and the overall carbon footprint by 40 %, despite a 31 % decrease in energy recovery, compared with the baseline case (N<inf>2</inf>O emission factor: 1.31 % of TKN load, net emissions: 354.29 kg CO<inf>2</inf>-e/ML, and energy recovery: 386.02 kg CO<inf>2</inf>-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.