Combined effects of thermal pretreatment and increasing organic loading by co-substrate addition for enhancing municipal sewage sludge anaerobic digestion and energy production

Abstract In this work, the thermal pretreatment of municipal sewage sludge (MSS) associated to its anaerobic co-digestion with the olive processing wastewater (OPW) was investigated for increasing complex organic matter disintegration and bio-conversion into energy. The hydrolysis of MSS (47%–49%) was performed at 120 °C during 30 min of pretreatment, which was confirmed by Fourier transform infrared (FTIR) spectroscopy. Results showed that increasing total solid (TS) from 0.58% to 3.2% has no effect on the suspended volatile solid (VS) disintegration. Anaerobic co-digestion of pretreated MSS (PMSS) mixed with OPW was investigated and the effect of increasing OPW proportion on the bio-methane potentials (BMP) was examined. The anaerobic digestion of better ratios of PMSS/OPW(80%/20% and 70%/30%) was also performed in sequencing batch reactors (ASBRs). The high biodegradation yields of VS and phenols (81% and 92%, respectively) were obtained at PMSS/OPW of 70%/30% corresponding to a methane yield of 0.441 L/gVSinlet. Therefore, thermal pre-treatment of MSS and OPW addition improved significantly methane yield (50%–160%) and wastes stability. Furthermore, they increased total energy production from 30.9 kW h/ton to 93.5 kW h/ton, which would provide 0.56 M€/year net benefits only from the electric power, which is considered interesting. The excess thermal energy should be used for wastes pretreatment and digester heating.