Carbon footprint of a conventional wastewater treatment plant: An analysis of water-energy nexus from life cycle perspective for emission reduction

Reducing greenhouse gas (GHG) emissions is of far-reaching implications for the sustainable development of a conventional domestic wastewater treatment plant (WWTP). GHG emissions of the WWTP in different working loads were monitored to quantify the influence of critical processes on GHG emissions and shed light on the relations between direct and indirect GHG emissions with the water-energy nexus. The life cycle carbon footprint of the WWTP was assessed to find the most critical impact factor. A conceptual model of direct and indirect GHG emissions was established based on the water-energy nexus and life cycle assessment (LCA). Results showed that direct GHG emissions were highest in medium working load in May. While high indirect GHG emissions were in February, with the highest working load. Methane (CH4) emission rates, from 44.39 ± 42.49 to 4.94 ± 2.29 kg CO2e·d−1, decreased along the treatment processes due to oxidation. Dissolved CH4 in wastewater mainly came from the influent that was 0.08 ± 0.04 kg CH4·d−1. The increase of nitrous oxide (N2O) emissions and dissolved N2O occurred in the secondary treatment stage and was affected by the C/N ratio. The operation and maintenance phase had the most significant carbon footprint due to 86.4% of indirect GHG emissions from electricity usage through LCA. Electricity usage and glucose dosing were significantly correlated with N2O emissions but had no contribution to CH4 emissions. The conceptual model qualitatively demonstrated that direct and indirect GHG emissions reduced as electricity usage decreased based on the water-energy nexus. This study provided new insight into the analysis of GHG emissions from WWTPs.