Characteristics of greenhouse gas emission in three full-scale wastewater treatment processes

摘要

Three full-scale wastewater treatment processes, Orbal oxidation ditch, anoxic/anaerobic/aerobic (reversed A~2O) and anaerobic/anoxic/aerobic (A~2O), were selected to investigate the emission characteristics of greenhouse gases (GHG), including carbon dioxide (CO_2), methane (CH_4) and nitrous oxide (N_2O). Results showed that although the processes were different, the units presenting high GHG emission fluxes were remarkably similar, namely the highest CO_2 and N_2O emission fluxes occurred in the aerobic areas, and the highest CH_4 emission fluxes occurred in the grit tanks. The GHG emission amount of each unit can be calculated from its area and GHG emission flux. The calculation results revealed that the maximum emission amounts of CO_2, CH_4 and N_2O in the three wastewater treatment processes appeared in the aerobic areas in all cases. Theoretically, CH_4 should be produced in anaerobic conditions, rather than aerobic conditions. However, results in this study showed that the CH_4 emission fluxes in the forepart of the aerobic area were distinctly higher than in the anaerobic area. The situation for N_2O was similar to that of CH_4: the N_2O emission flux in the aerobic area was also higher than that in the anoxic area. Through analysis of the GHG mass balance, it was found that the flow of dissolved GHG in the wastewater treatment processes and aerators may be the main reason for this phenomenon. Based on the monitoring and calculation results, GHG emission factors for the three wastewater treatment processes were determined. The A~2O process had the highest CO_2 emission factor of 319.3 g CO_2/kg COD_(removed), and the highest CH_4 and N_2O emission factors of 3.3 g CH_4/kg COD_(removed) and 3.6 g N_2O/kg TN_(removed) were observed in the Orbal oxidation ditch process.