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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 NO and CO 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 CO and NO emissions from the AS tank were 0.14 kg/kg and 0.007 kg/kg, respectively. For an AeD, 3 × 10 kg/kg direct CO emissions were measured, while CO emissions from NO were 4 × 10 kg/kg. The results for the AS tank and the AeD were used to estimate the net carbon and energy footprint of the entire plant.

Data about Greenhouse Gas (GHG) emissions from settling units in\ud wastewater treatment plants (WWTPs) are limited, probably because of the\ud increased difficulties in evaluating direct emissions when there is absence of an\ud induced air stream through the liquid volume (Caivano et al. 2016). Particularly,\ud gas samples collection is not immediate and easy due to the low off-gas flow\ud leaving the liquid surface.\ud In this study, a modified off-gas apparatus is proposed, to avoid these\ud experimental problems. A floating hood was connected to a blower to simulate\ud the wind action and encourage the gas stripping. The incoming air flow rates\ud were fixed to 4, 9, and 16 Nl min−1, simulating a wind velocity of 1.05, 2.36,\ud and 4.19 m/s, respectively, in order to measure GHG emissions from a full-scale\ud plant in several conditions. The same experimental conditions and a reproducible\ud sampling apparatus were employed to measure GHG emissions also\ud from a pilot plant. The monitoring of the full-scale plant shows that the concentrations\ud of N2O and CO2 in the off-gas change rapidly, demonstrating the\ud stripping effect induced by the blower air flow. A peak is reached and then a\ud rapidly decrease is observed, proving a gradual decrease of mass transfer phenomena.\ud As expected, the peak value increases with increasing the wind speed,\ud whereas the time at which the peak is observed decreases. Regarding the\ud pilot-scale plant, the results show the slow diffusion phenomena occurring in a\ud closed system, preventing the mass transfer from the liquid to the gaseous phase