  Upgrading effluent standards of wastewater treatment plants (WWTPs) and
repairing sewerage systems leads to contradictions and synergies between water
pollution control and climate change mitigation. This affects historical
trajectories and characteristics of greenhouse gas (GHG) emissions from China's
WWTPs, which stay inadequately studied. Here we establish emissions inventories
of China's WWTPs using plant-level WWTP operational data. We find that removed
amount of chemical oxygen demand and ammonia nitrogen increased 0.8 and 1.3
times during 2009-2019, while WWTP GHG emissions increased 1.8 times, being 6
times national GHG emissions growth rate. Increasing sludge yield and
electricity intensity became primary driving factors in 2015 because of
stricter effluent standards and lower influent contaminant concentration. We
defined Functional Unit-Gini coefficient to quantify divergence of WWTP GHG
emissions, which grew from 0.20 in 2009 to 0.29 in 2019. Diversified sludge
disposal methods and energy structure increased the inequality, while upgrading
effluent standards decreased it.
