Enhancing the biomethane production from lignocellulosic residues through bioaugmentation of anaerobic digestion

Bioaugmentation of anaerobic digestion (AD) systems is considered a cost-effective and environmentally friendly strategy to combat incomplete digestion of recalcitrant lignocellulosic substrates. This study investigated the lowest microbial inoculum size required for once-off bioaugmentation of AD cultures to enhance biomethane yield and process performance. The batch, laboratory-scale anaerobic co-digestion was carried out using pretreated corn stover (PCS) and food waste (FW), with cellulolytic Bacillus subtilis, Serratia marcescens and Bacillus licheniformis. The bioaugmentation screening was accomplished through a stepwise increase in the microbial loading using an initial standardised concentration of 0.4 × 1011 colony-forming units (CFU)/mL within the system. Bioaugmentation decreased the digestion time by up to 11 days. The inoculation of B. subtilis at a microbial concentration of 20 × 1011 CFU/mL (4.85 g DCW/L) improved the biomethane yield by 34% compared to the unaugmented control and produced 525 NmL CH4/gVS. Additionally, S. marcescens at 12 × 1011 CFU/mL doubled the volumetric methane productivity from 0.47 ± 0.02 to 1.04 ± 0.02 mL/(mL.day) when compared to the unaugmented control. The application of Nanopore sequencing after AD, to investigate the microbial community dynamics and structure in this treatment, underlined 43.52, 7.69 and 25.26% increases in the bacterial alpha diversity, namely the Shannon-, Simpson- and Observed indices, respectively. Moreover, a high abundance of between 50 and 80% of the Firmicutes population was identified.