Factors Affecting Nitrous Oxide Emissions from Activated Sludge Wastewater Treatment Plants—A Review

Nitrous oxide (N2O) is a greenhouse gas contributing to ozone layer depletion and climate change. Wastewater treatment plants (WWTPs) contribute significantly to the global anthropogenic N2O emissions. The main factors affecting N2O emissions are the dissolved oxygen concentration (DO), the nitrite accumulation, the rapidly changing process conditions, the substrate composition and COD/N ratio, the pH, and the temperature. Low DO in the nitrification process results in higher N2O emissions, whereas high aeration rate in the nitration/anammox process results in higher N2O production. High DO in the denitrification inhibits the N2O reductase synthesis/activity, leading to N2O accumulation. High nitrite accumulation in both the nitrification and denitrification processes leads to high N2O emissions. Transient DO changes and rapid shifts in pH result in high N2O production. Ammonia shock loads leads to incomplete nitrification, resulting in NO2− accumulation and N2O formation. Limiting the biodegradable substrate hinders complete denitrification, leading to high N2O production. A COD/N ratio above 4 results in 20–30% of the nitrogen load being N2O emissions. Maximum N2O production at low pH (pH = 6) was observed during nitrification/denitrification and at high pH (pH = 8) during partial nitrification. High temperature enhances the denitrification kinetics but produces more Ν2O emissions.