Greenhouse Gas Emissions from Municipal Wastewater Treatment Plants

Southeast Virginia resembles much of America in that urban, suburban, and rural areas can all be found within the region. As Virginia’s second most populous metropolitan area and one of its largest in square mileage, it can be viewed as a sort of microcosm of the rest of Virginia and the country. Not only are its demographics and geography representative of mainstream America, its wastewater treatment processes are also typical of those found in communities across the country.Wastewater treatment processes can be performed by aerobic and anaerobic means, both of which result in the emission of greenhouse gases, namely methane (CH4) and nitrous oxide (N2O). Methane is a byproduct of organic material being treated using anaerobic technologies, and nitrous oxide results from aerobic processes. However, greenhouse gas emissions resulting from wastewater treatment account for only a very small portion of total municipal greenhouse gas emissions.Methane gas emissions are frequently a byproduct of municipal wastewater treatment processes. Anaerobic digestion is a process in which bacteria assimilates organic matter in the absence of oxygen, resulting in the production of methane gas. Methane gas can also be produced when untreated wastewater decomposes anaerobically.Nitrous oxide emanates from municipal wastewater that is rich in organic matter containing nitrogen and is produced during nitrification and denitrification. Nitrification takes place in aerobic conditions, changing ammonia into nitrate, and is necessary because ammonia is toxic, creates a high oxygen demand on receiving waters, and can lead to algal blooms. Denitrification takes place in anaerobic conditions, changing nitrate to nitrous oxide.A large municipality in Southeast Virginia funded its own greenhouse gas inventory for the year 2010 that was carried out through a major university in the metropolitan area. Methane gas emissions resulting from the treatment of wastewater generated within the municipality were determined to be 431 metric tons of methane gas. Direct and indirect nitrous oxide emissions from municipal wastewater treatment together accounted for 12.8 metric tons of nitrous oxide. Converting both methane and nitrous oxide emissions to million metric tons of carbon dioxide equivalent (MMTCO2E) and adding them together resulted in a total of 0.01 MMTCO2E, which accounted for 0.3% of the municipality’s total greenhouse gas emissions.