Nitrous oxide and methane emissions from different treatment processes in full-scale municipal wastewater treatment plants

Abstract

Nitrous oxide (N 2O) and methane (CH 4) are two important greenhouse gases (GHG) emitted from biological nutrient removal (BNR) processes in municipal wastewater treatment plants (WWTP). In this study, three typical biological wastewater treatment processes were studied in WWTP of Northern China: pre-anaerobic carrousel oxidation ditch (A+OD) process, pre-anoxic anaerobic-anoxic-oxic (A-A/A/O) process and reverse anaerobic-anoxic-oxic (r-A/A/O) process. The N 2O and CH 4 emissions from these three different processes were measured in every processing unit of each WWTP. Results showed that N 2O and CH 4 were mainly discharged during the nitrification/denitrification process and the anaerobic/anoxic treatment process, respectively and the amounts of their formation and release were significantly influenced by different BNR processes implemented in these WWTP. The N 2O conversion ratio of r-A/A/O process was the lowest among the three WWTP, which were 10.9% and 18.6% lower than that of A-A/A/O process and A+OD process, respectively. Similarly, the CH 4 conversion ratio of r-A/A/O process was the lowest among the three WWTP, which were 89.1% and 80.8% lower than that of A-A/A/O process and A+OD process, respectively. The factors influencing N 2O and CH 4 formation and emission in the three WWTP were investigated to explain the difference between these processes. The nitrite concentration and oxidation–reduction potential (ORP) value were found to be the dominant influencing factors affecting N 2O and CH 4 production, respectively. The flow-based emission factors of N 2O and CH 4 of the WWTP were figured out for better quantification of GHG emissions and further technical assessments of mitigation options.