Mass and energy balances of sludge processing in reference and upgraded wastewater treatment plants

This paper describes the preliminary assessment of a platform of innovative upgrading solutions aimed at improving sludge management and resource recovery in wastewater treatment plants. The effectiveness of the upgrading solutions and the impacts of their integration in model reference plants have been evaluated by means of mass and energy balances on the whole treatment plant. Attention has been also paid to the fate of nitrogen and phosphorus in sludge processing and to their recycle back to the water line. Most of the upgrading options resulted in reduced production of dewatered sludge, which decreased from 45 to 56 g SS/(PE × day) in reference plants to 14-49 g SS/(PE × day) in the upgraded ones, with reduction up to 79% when wet oxidation was applied to the whole sludge production. The innovative upgrades generally entail an increased demand of electric energy from the grid, but energy recovery from biogas allowed to minimize the net energy consumption below 10 kWh/(PE × year) in the two most efficient solutions. In all other cases the net energy consumption was in the range of -11% and +28% of the reference scenarios.