Greenhouse gas emission by centralized wastewater treatment plants in Chinese industrial parks: Inventory and mitigation measures

Wastewater treatment plants (WWTPs) are an important end-of-pipe measure for pollution control and a main energy consumer in industrial parks. WWTPs remove pollutants from wastewater to protect water environments, but produce greenhouse gas (GHG) emissions at the same time. Currently, China has more than 500 national industrial parks (NIPs), and centralized WWTPs are requisite components of environmental infrastructure in these parks. A set of computational methods are established to measure the three main GHGs emitted by the WWTPs in the NIPs, and these GHGs are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). A total of 344 centralized WWTPs in 152 typical NIPs were considered, and their emissions were carefully evaluated. The key findings are as follows, 5.64 million tons (Mt) of CO2-equivalent (CO2-eq) was emitted, and the emissions include 1.63 Mt CO2-eq from on-site emissions, 1.45 Mt CO2-eq from off-site emissions due to energy consumption, and 2.56 Mt CO2-eq from off-site emissions associated with sludge disposal. Aerobic biological treatment is the most frequently used process, and the GHGs emissions associated are 940.6–1,006.9 t CO2-eq/Mt wastewater, which is much lower than the average value (1,441.8) for other kinds of treatment processes. For sludge disposal, incineration is widely applied (131 WWTPs, 86%) to dispose of more than half of residual sludge (1.1 Mt dry sludge per year, 55.7%). However, the 131 WWTPs that used incineration only produced 43% of the total GHG emissions. Scenario analysis showed that building materials applications are the best method of sludge disposal due to low off-site (sludge-related) GHG emissions (0.39 Mt CO2-eq/a), while landfilling (4.23 Mt CO2-eq/a) has the highest emission. Finally, five key measures are proposed to mitigate GHG emissions while considering multiple stakeholders.