Anaerobic digestion of waste activated sludge with incineration bottom ash: Enhanced methane production and CO2 sequestration

Development of anaerobic digestion (AD) technology in China for WWTP sludge treatment is hindered by low biomethane yield and costly biogas upgrading. This study investigated the feasibility of using a byproduct waste, sludge incineration bottom ash, as additive to promote AD performance. The effectiveness of ashes addition was investigated with varied dosages (0.6-1.5 g/g dry matter (DM) of sludge) to enhance thermophilic AD of sludge in batch experiments in terms of methane production and CO2 sequestration. The optimal dosage was 0.9 g/g DM, which increased the methane production by 26.6%, shortened the methanization lag phase by 32.4% and increased the maximum methane production rate by 36.0%. Kinetics modelling results show that ashes additive accelerated hydrolysis, acidogenesis and degradation of volatile fatty acids (VFAs) by up to 139.6%, 87.1% and 100.9%, respectively. High dosage at 1.5 g/g DM output the greatest sludge hydrolysis and VFAs production but inhibited methanogenesis. Moreover, the amount of sequestered CO2 resulting from basic cations (primarily Ca2+) leaching in ashes-amended digesters was linearly correlated (R-2 > 0.99) with the dosage of ashes additive. This indicates that ashes addition facilitated in-situ carbonation in AD system and that Ca2+ leaching remains rate-limiting. The results demonstrated that sludge incineration bottom ash could be used as an effective additive material to improve AD of sludge, thus enhancing the technical and economic feasibility of AD technology for sludge treatment.