Optimising sewage sludge anaerobic digestion for resource recovery in wastewater treatment plants

Recovering resources from sludge generated during wastewater treatment is both an opportunity and a challenge. Thermophilic anaerobic digestion may optimise biogas production and digested sludge properties for further valorisation, in the framework of a circular economy. Thus, this study aimed at evaluating the effect of operational conditions, i.e. temperature, solids concentration in the sludge, and solids retention time (SRT) on the methane production, volatile fatty acids (VFA) concentration, digested sludge hygienisation, and dewaterability, during long-term anaerobic digestion. This is the first time that sludge anaerobic digestion has been evaluated for over 500 days varying control parameters for assessing concomitantly biogas, VFA, and pathogen removal outcomes with focus on resource recovery. Results showed how by shifting from mesophilic (38 °C) to thermophilic (55 °C) conditions, with a short SRT (10 days) in the reactor, the process performance was optimised. Indeed, the methane production reached a maximum of 0.4 m3CH4/m3reactor·d, with a VFA concentration of 4.0 g COD/L and complete pathogen removal in the digestate, for a safe agricultural reuse. Therefore, the transition from mesophilic to thermophilic anaerobic digesters seems beneficial for the valorisation of by-products and promoting the circular economy in wastewater treatment plants.