Carbon Footprints of Wastewater Treatment Plants: A Comprehensive Analysis of Emission Sources and Quantification for Sequencing Batch Reactor System

Wastewater treatment plants (WWTPs) are significant contributors to anthropogenic greenhouse gas (GHG) emissions through both direct biological processes generating methane (CH4), nitrous oxide (N2O), and biogenic carbon dioxide (CO2) and indirect energy consumption. This comprehensive research paper synthesizes findings from 30 peer-reviewed studies to present a holistic analysis of carbon footprints in wastewater treatment, with a specific quantitative assessment of a sequencing batch reactor (SBR) facility processing 5000 m3/day. The SBR operates with anoxic–aerobic cycles (fill–anoxic react–aerobic react–settle–decant–idle). The analysis reveals that N2O emissions can constitute up to 75% of a plant’s carbon footprint, while aeration accounts for 40–75% of total energy consumption. For the SBR facility, the baseline carbon footprint is 1648 tCO2e/year [95% CI: 1420–1910] (0.90 kg CO2e/m3) under conservative assumptions, with CH4 yield of 0.03 kg CH4/kg COD removed and N2O yield of 0.008 kg N2O-N/kg TN removed. A realistic baseline using median literature values gives 0.52 kg CO2e/m3. The carbon footprint of WWTPs varies by treatment technology, scale, and operational conditions, ranging from 61 to 161 kg CO2e per population equivalent (PE) annually. Through anaerobic digestion with biogas recovery and anoxic phase optimization, emissions can be reduced by 38% to 1018 tCO2e/year [95% CI: 860–1190]. The findings underscore that achieving carbon neutrality requires extending accounting beyond plant boundaries to include effluent exports, sludge management, and urban infrastructure integration. This paper provides a transparent, practitioner-ready framework for understanding, quantifying, and mitigating carbon emissions from wastewater treatment, with particular emphasis on SBR technology.