The efficiency of aeration systems should be monitored to guarantee suitable biological processes. Among the available tools for evaluating the aeration efficiency, the off-gas method is one of the most useful. Increasing interest towards reducing greenhouse gas (GHG) emissions from biological processes has resulted in researchers using this method to quantify N2O and CO2 concentrations in the off-gas. Experimental measurements of direct GHG emissions from aerobic digesters (AeDs) are not available in literature yet. In this study, the floating hood technique was used for the first time to monitor AeDs. The floating hood technique was used to evaluate oxygen transfer rates in an activated sludge (AS) tank of a medium-sized municipal wastewater treatment plant located in Italy. Very low values of oxygen transfer efficiency were found, confirming that small-to-medium-sized plants are often scarcely monitored and wrongly managed. Average CO2 and N2O emissions from the AS tank were 0.14 kgCO2/kgbCOD and 0.007 kgCO2,eq/kgbCOD, respectively. For an AeD, 3 × 10-10 kgCO2/kgbCOD direct CO2 emissions were measured, while CO2,eq emissions from N2O were 4 × 10-9 kgCO2,eq/kgbCOD. The results for the AS tank and the AeD were used to estimate the net carbon and energy footprint of the entire plant.
Keywords: Aeration efficiency; aerobic digestion; carbon and energy footprint; greenhouse gases; off-gas method.