High-solid anaerobic digestion of sewage sludge: challenges and opportunities

High-solid anaerobic digestion (HSAD) of sewage sludge is a promising alternative to conventional anaerobic digestion (AD) as it reduces treatment volumes, transportation costs and energy consumption for heating and increases the fertilizing potential of sewage sludge. By centralizing sludge treatment, HSAD represents an opportunity to improve the energy balance of sewage sludge valorization compared to conventional AD. Nevertheless, HSAD of sewage sludge poses several challenges due to structural and rheological characteristics of dewatered sludge and to toxic compounds originating during (H2S, NH3, siloxanes, volatile organic compounds) and before (polyelectrolytes added prior to dewatering) the anaerobic treatment. This work critically reviews HSAD of sewage sludge with the objective to promote process enhancement for energy self-sufficiency and good practices for agricultural utilization of digestate. Advantages and drawbacks of HSAD are discussed in depth and practical solutions to solve critical issues at full scale are proposed. Guidelines for conducting cost-benefit analysis of the process are also provided.