Which greenhouse gases are typically included in a wastewater GHG inventory and why?

• A. Methane (CH4) and nitrous oxide (N2O) due to anaerobic processes and nitrification/denitrification; CH4 has high global warming potential; accounting guides mitigation.
• B. Carbon dioxide only.
• C. Oxygen and nitrogen.
• D. Sulfur dioxide.

Answer: (A)

Wastewater GHG inventories focus on gases produced by microbial processes in sewer networks and treatment facilities. The gases most typically included are methane and nitrous oxide. Methane forms under anaerobic conditions in sewer systems, primary and secondary sludge, and digesters, where organic matter is broken down without oxygen. It’s a potent greenhouse gas, with a global warming potential many times that of CO2, so capturing and reducing methane can significantly cut emissions. Nitrous oxide arises during nitrification and denitrification in nitrogen removal steps of treatment. Although emitted in smaller amounts than methane, N2O is far more potent per unit mass, giving it a substantial climate impact that warrants careful accounting. Carbon dioxide is produced as well, but much of the CO2 from wastewater is considered biogenic and part of natural carbon cycling, so it is not counted in the same way as CH4 and N2O in many wastewater inventories. Oxygen and nitrogen are elements, not greenhouse gases, and sulfur dioxide is not a primary concern in standard municipal wastewater inventories. Therefore, methane and nitrous oxide are included because they come from anaerobic and nitrification/denitrification processes and have high global warming potentials.