Characteristics of direct CO2 emissions in four full-scale wastewater treatment plants

Abstract

Direct CO2 emissions from wastewater treatment plants (WWTPs), which are commonly excluded from the greenhouse gas emission inventory as proposed by the International Panel on Climate Change, should be partly taken into consideration, because some of the fossil organic carbons (i.e. detergents, cosmetics, and pharmaceuticals) are also biodegradable, leading to the emission of CO2 during wastewater treatment. For the purpose of understanding the characteristics of direct CO2 emissions during biological nutrient removal from urban WWTPs, we investigated four different processes. Full-scale tests were carried out continuously during a two-year period, consisting of the anoxic/anaerobic/oxic (A2O) process, the anoxic/oxic (AO) process, the oxidation ditch, and the sequencing batch reactor (SBR) processes. Our experimental results show large differences in CO2 fluxes in the various treatment tanks (or periods), e.g. the aerated units/periods in four different WWTPs all contributed more than 96.0% of the entire direct CO2 emission caused by aerobic respiration and aeration stripping. On the other hand, CO2 emissions from non-aerated areas were quite low giving rise to small amounts of CO2 emissions during anaerobic metabolism. The direct emission of CO2 when treating per m3 wastewater (g CO2/m3 wastewater) corresponded with the following descending order among the four processes: SBR (347.34) > oxidation ditch (343.78) > A2O (175.68) > AO (173.37). However, this order changed into SBR (0.97) > oxidation ditch (0.76) > AO (0.68) > A2O (0.58) when treating per kilogram COD (kg CO2/kg COD).