N2O and CO2 Emissions from Secondary Settlers in WWTPs: Experimental Results on Full and Pilot Scale Plants

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