Estimation of greenhouse gas emission from biological wastewater treatment plants

Wastewater treatment plants, although they may have a small footprint, are known sources of substantial greenhouse gas (GHG) emissions. Two separate municipal wastewater treatment plants (Plant A and Plant B), both providing secondary level of treatment, were evaluated to quantify their GHG emissions and make a comparison. Plant A is a biological aerated filter (BAF) treatment system, while Plant B is an activated sludge (AS) system with two separate bioreactors; one is a plug-flow type and the other is completely mixed. BAF treatment system and the AS system, based on energy consumption, can release, on average, 0.02 and 0.03 kg of equivalent CO₂ per m³ of treated wastewater, respectively. Plant B had significant higher off-site NO₂ emission (0.005 kg of equivalent CO₂ per m³) compared to onsite emission (0.0005 kg of equivalent CO₂ per m³). In comparison, Plant A has overall higher NO₂ emission (0.0075 kg of equivalent CO₂ per m³) than plant B (0.0065 kg of equivalent CO₂ per m³). However, Plant B has higher (overall) methane emissions than Plant A (0.024 and 0.011 kg of equivalent CO₂ per m³, respectively). A regression analysis showed that temperature, SRT, and BOD₅ loading rate strongly influence GHG emission.