Quantification of CO2 and N2O Emissions from a Pilot-Scale Aerobic Digester, Towards the Validation and Calibration of the First Activated Sludge Model for Aerobic Digestion (AeDM1)

In this study, a pilot aerobic digester was developed and operated to monitor N2O and CO2 emissions using the off-gas technique. A 30-days monitoring campaign was carried out to evaluate the impact of aerobic digestion (AeD) in Greenhouse Gas (GHG) estimation. After the achievement of the equilibrium conditions for a conventional AeD, a monitoring campaign was performed assuming 20 days as sludge retention time. The N2O gas flux was found equal to 71.7 mgN2O m−2min−1 against 16914 mgCO2 m−2min−1 calculated for CO2, demonstrating that strong aerobic oxidation processes occur inside the digester. In terms of equivalent CO2, N2O covers the 55% of the total CO2,eq emissions and CO2 the 45%. The experimental campaigns were coupled with the development of a mathematical model for AeD, named Aerobic Digestion Model No. 1 (AeDM1). The Morris Method allowed us to carry out a sensitivity analysis on the main kinetic parameters, resulting that the maximum specific growth rate of heterotrophs is the more sensitive parameter. After the model calibration, the experimental results on the pilot digester were used to validate the model, inserting the data collected during the experimental tests as model inputs.