Valorization of fruit and vegetable waste in a novel three-stage hybrid anaerobic digester for enhanced biogas production: Performance study and microbial community analysis

Abstract

The study proposes the performance of a three-stage hybrid mesophilic anaerobic digester comprising a suspended-growth hydrolytic reactor, a hybrid (suspended-growth + attached-growth) acidogenic down-flow reactor, and a hybrid methanogenic up-flow reactor, for enhanced biogas production. Commingled fruit and vegetable waste (FVW) – shredded to sizes 1 – 4 mm – plus 2.5 L of tap water served as the feedstock in 8 different batch combinations, where the weight of the waste was increased from 3.5 to 7 kg. Decoupling the three major anaerobic steps ensured desirable working pH ranges for the hydrolytic (4.02 – 6.79), the acidogenic (5.69 – 6.62), and the methanogenic stages (6.94 – 7.7). Triplicate studies of each batch type vis-à-vis all three stages were performed. The highest cumulative soluble chemical oxygen demand percentage removal of 96.3 % was noticed in the case of batch run 5. The maximum biogas yield of 781.8 ml/g CODremoved was corresponding to batch 7, with 6.5 kg FVW, whereas the lowest biogas yield, 735.8 ml/g CODremoved, was corresponding to batch 1. The produced biogas from the methanogenic digester was of high quality (≤ 0.01 % H2S) and constituted 58–62 % CH4 and 6–7 % H2. Microbial community analysis confirmed the presence of pertinent bacterial and archaeal groups, such as Firmicutes, Proteobacteria, Spirochaetes, and Methanothrix, including key homoacetogenic genera, such as Acetobacterium Woodii, Desulfotomaculum, and Desulfitobacterium.