Anaerobic Digestion of Wastewater Treatment Plant Primary Sludge for Biogas and Energy Recovery

This study evaluated the anaerobic digestibility of primary sludge from two wastewater treatment plants (WWTPs), Leeuwkuil and Rietspruit. Anaerobic biodegradation produces biogas as an energy carrier. Sludge from the primary settling tanks was tested in batch mode as a mono-substrate, without pretreatment or external inoculum. Proximate and ultimate analyses were used to estimate theoretical methane production. Anaerobic digestibility tests were then performed using an Automatic Methane Potential System (AMPTS® II, Bioprocess Control). The volatile-to-total solid (VS/TS) ratios were 71 for Leeuwkuil and 13 for Rietspruit. Theoretical methane yields for Leeuwkuil sludge were 257–293 L/kg VS. For Rietspruit, the Buswell and Dulong methods gave negative theoretical BMP values (−76 and −15 L/kg VS), suggesting these models may be unsuitable for high-oxygen-content substrates. Measured methane production was 11.3 L/kg VS for Leeuwkuil and 4.8 L/kg VS for Rietspruit, indicating low anaerobic digestibility relative to solid content. Leeuwkuil primary sludge nevertheless showed better potential as a co-substrate for methane production than Rietspruit sludge. Rietspruit sludge may pose challenges for anaerobic digestion, though pretreatment or co-digestion could improve performance. Based on measured methane productivities, each WWTP could generate about 0.5 MWh of electricity per day from biogas. The study shows that primary sludge digestibility depends strongly on the physico-chemical characteristics of the influent wastewater. Primary sludge can often be improved for digestion through chemical/physical pretreatment and co-digestion with secondary sludge or suitable agro-industrial organic residues.