Impacts of digestate recycling as inoculum in percolate-recirculated high-solids anaerobic digestion of source separated organics: Multi-cycle assessment

During high-solids anaerobic digestion (HSAD) operation, fresh feedstock is typically supplemented with digestate from previous batches to serve as an inoculum. This study investigated the long-term effects of digestate recycling as inoculum in bench-scale HSAD systems with percolate recirculation and fed with source-separated organics (SSO). During the first 4 cycles (phase-1), six reactors (R1-R6) were operated under identical conditions, where the feedstock-to-inoculum (F/I) ratio was maintained at 2. Digestate from the previous cycle served as inoculum for the subsequent ones. In the following two cycles (phase-2), 4 reactors (R1-R4) from phase-1 were operated at different digestate recycling ratios, i.e. 40% (R1/R2) and 50% (R3/R4). Reactors operated with 50% digestate recycling consistently achieved higher methane yield and energy recovery efficiencies, faster reductions in H 2 S levels in biogas, and better process stability. Microbial community analysis indicated that 40% digestate recycling promoted microbial diversity in Cycle-5; however, it ultimately led to a less diverse community in Cycle-6. Conversely, 50% digestate recycling resulted in a stable methanogenic consortium dominated by Methanosarcina and Methanoculleus in the digester tank, along with strong enrichment of ammonia-tolerant vadinCA11 in the percolate. Overall, these findings provide practical insights into ensuring process stability and energy recovery efficiency in HSAD.