Monitoring the aeration efficiency and carbon footprint of a medium-sized WWTP: experimental results on oxidation tank and aerobic digester

Abstract

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 kg_CO2/kg_bCOD and 0.007 kg_CO2,eq/kg_bCOD, respectively. For an AeD, 3 × 10^-10 kg_CO2/kg_bCOD direct CO₂ emissions were measured, while CO_2,eq emissions from N₂O were 4 × 10^-9 kg_CO2,eq/kg_bCOD. The results for the AS tank and the AeD were used to estimate the net carbon and energy footprint of the entire plant.