Post-aerobic digestion of waste sludge: performance analysis and modelling of nitrogen fate under alternating aeration

Combined anaerobic–aerobic sludge digestion has been demonstrated as a valid technological solution to improve the sludge stabilization process, in terms of solid reduction and sludge dewaterability properties. In this study, we investigated the fate of all nitrogen species (ammonia, nitrites, nitrates) in the sequential digestion process when the post-aerobic step is operated with intermittent aeration. Nitrogen course has been followed in the two digesters operated in semi-continuous mode and in batch nitrification–denitrification kinetic tests. Two-stage digestion was applied to real waste activated sludge. High volatile solid removal efficiencies (47 and 26 % in the anaerobic and aerobic phases, respectively) confirmed the potential of this technology. Moreover, the post-aerobic stage allowed to successfully achieving nitrogen removal through the simultaneous nitrification–denitrification process. Nitrification and denitrification efficiencies were 95 and 70 %, respectively. Batch tests for nitrification and denitrification were also carried out to investigate the process kinetics. A process model has been formulated and calibrated with a first set of experimental kinetic data. Evaluated kinetic parameters were employed in the validation phase successfully performed (correlation coefficients R[ 0.98) with different series of experimental data.