Assessing sewage treatment plant capacity and process with a focus on carbon emissions

The imperative to reduce carbon emissions and mitigate climate change is prompting a rethink of sewage treatment plant capacity. While larger plants typically offer better energy efficiency and lower management intensity, they require extensive sewer networks, prolonging wastewater retention and potentially increasing greenhouse gas emissions. This study employs the emission factor method to assess carbon emissions (expressed as kg CO₂-eq/m³) of decentralized (500 m³/d) and centralized sewage treatment systems across varying treatment scales (1 × 10⁴, 5 × 10⁴, 10 × 10⁴, 20 × 10⁴, and 50 × 10⁴ m³/d). Results indicate that among the four processes-anaerobic-anoxic-oxic process (AAO), oxidation ditch (OD), cyclic activated sludge system (CASS), and anaerobic-oxic process (AO), the AO process exhibits the highest specific carbon emissions, with an average of 1.266 kg CO₂-eq/m³. Followed by the AAO process, with an average specific carbon emission of 1.259 kg CO₂-eq/m³. CASS and OD processes have relatively lower emissions with average specific carbon emissions of 1.164 and 1.165 kg CO₂-eq/m³_, respectively. Moreover, the calculations reveal that the sewage operation and transportation stages are the primary factors influencing carbon emissions. For OD process, carbon emission in centralized mode is lower than that in decentralized mode when the centralized scale is below 78,850 m³/d, whereas the decentralized mode becomes more advantageous when the centralized scale continues to increase. Similarly, for CASS process, the centralized mode demonstrates a low-carbon benefit when the scale is below 52,100 m³/d, but the decentralized mode outperforms when the scale exceeds this. These findings are significant for practical applications of both centralized and decentralized wastewater treatment systems.