Strategies for the Revalorization of Sewage Sludge in a Waste Water Treatment Plant Through the Integration of Hydrothermal Processing

Different strategies for sewage sludge management have been implemented to minimize the environmental impacts and benefit from the resources embedded in this waste stream. In that regard, Hydrothermal treatments (HTTs) of biomass can contribute generating high-value products (hydrochar) and enhancing biogas generation. In this work, different strategies were analysed for implementing HTTs at WWTPs considering the effect of 2 different process temperatures (160 and 250 °C) on different sewage sludge samples (primary, secondary and digestate sludge). Determination of carbon, nitrogen and phosphorus fate and mass and energy balances in hydrochar and process waters were carried out. HTT promoted solubilization of nitrogen, phosphorus and organic carbon up to 89%, 13% and 124% respectively. Moreover, biomethane yield increased up to 168% reaching methane concentrations up to 79% in the biogas. High heating values of hydrochars ranged from 12.0 to 18.2 MJ kg¯¹. The integration of HTT as an intermediate step between two-step anaerobic digestion process showed an increase in the net energy balance of up to 124% in comparison with the conventional anaerobic digestion process.