Highly efficient anaerobic co-digestion of food waste and horticultural waste using a three-stage thermophilic bioreactor: Performance evaluation, microbial community analysis, and energy balance assessment

The thermophilic anaerobic co-digestion of food waste and horticultural waste with a three-stage bioreactor was investigated to produce renewable energy, focusing on performance evaluation, microbial community analysis, and energy balance assessment. The results of this study indicated that the average methane yield (0.42 +/- 0.06 L/gVS) in the bench-scale three-stage thermophilic bioreactor was approximately 45% and 31% higher than that of the one and two-stage bioreactors, respectively. The average volatile solid (VS) reduction achieved in the bench-scale three-stage thermophilic bioreactor was 63.0 +/- 8.2%, which was 61% and 32% higher than that of the one and two-stage bioreactors, respectively. Pyrosequencing analysis of microbial communities in simulated thermophilic three-stage co-digestion experiments showed that bacterial phyla Firmicutes, Thermotogae and Bacteroidetes dominated in all three types of bioreactors. However, significant differences were observed at the genus level of bacterial communities between the one, two and three-stage bioreactors. Indeed, due to selective enrichment, the dominant methanogenic pathways in the three-stage bioreactor shifted from hydrogenotrophic methanogenesis at a low organic loading rate (OLR) to a balance of hydrogenotrophic and acetoclastic pathways at high OLR. Furthermore, assessments of the energy balance and the economic viability of the bench-scale system demonstrated that the three-stage thermophilic co-digestion could be a promising process for the treatment of food waste and horticultural waste as well as for bioenergy recovery at an industrial scale.