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.