Integrating Sewage Sludge Co-Gasification in WWTPs for Increasing Overall Energetic Performance

This study evaluates the integration of sewage sludge digestion with biomass co-gasification to enhance energy recovery and operational efficiency in wastewater treatment plants (WWTPs). The activated sludge process, while effective for wastewater treatment, generates large quantities of sludge that require further stabilization. Anaerobic digestion is widely used, producing biogas but generating a biosolid that requires final disposal. Thermochemical processes, particularly gasifica-tion, offer a promising alternative to finding a valorization option for the digestate. Nevertheless, the high mineral content and ash fraction in digested sludge present operational challenges, reducing gasification efficiency and energy output. Integrating biomass as a supplementary feedstock in co-gasification addresses these limitations by increasing the mixture’s energy content and improving process performance. This study developed a comprehensive simulation model to evaluate the technical, energetic, and economic implications of this integrated approach, considering factors such as raw material prices, energy market conditions, and plant configuration. Results show that co-gasification with biomass can substantially increase available energy output (from 392 kW_elec from the digestion process to 660-804 kW_elec when integrated with sludge co-gasification). The ap-proach significantly reduces the need for auxiliary fossil fuels, leading to important savings. How-ever, the economic feasibility is highly sensitive to fluctuations in energy and raw material prices, as well as policy frameworks governing biomass utilization and sludge disposal. Sustainability is maximized when the process uses low-cost, residual biomass resources. However, feed-in tariffs are needed to ensure stability and must be linked to electricity and natural gas prices. In the present study, a value of 3 - 6 c€/kWh was needed to achieve profitability when natural gas prices fall below 50 €/MWh. Overall, integrating sludge digestion with biomass co-gasification offers a viable path-way to advance energy self-sufficiency and circular economy objectives in WWTPs.