Inoculum-dependent microbial communities and the inoculum-to-substrate ratio regulate biomethane potential during co-digestion of solid wastes

Abstract

Anaerobic co-digestion of organic wastes is a sustainable method for recovering renewable energy, but its performance is highly dependent on factors such as the type of inoculum used and the inoculum-to-substrate ratio (ISR). The impact of inoculum type and ISR on biogas production in the anaerobic co-digestion of paper waste (PW) and chicken manure (CM) was evaluated in this study. Mesophilic (35 ± 2 °C) anaerobic digestion assays were carried out in the presence of three (3) inoculum types: anaerobic digester sludge (ADS), cow dung (CD), and rumen waste (RW) and three (3) ISRs (1:1, 1:2 and 1:3 on a volatile solid basis). Each of the inocula proved to be viable in producing biogas in an anaerobic environment ( p > 0.05), which validated their appropriateness. Of the tested inocula, ADS had the best biogas production with a yield of 375 to 580 mL g −1 VS, which demonstrated good digestion performance. On the contrary, CD and RW had low yields of biogas. The highest ISR to produce biogas using all inoculums was 1:1, which produced 580 mLgVS − 1 ADS, 500 mLg −1 VS CD, and 565 mLg −1 VS RW. Beyond this ratio, the increase in ISR slowed down the biogas production, implying the inhibitory effect through substrate congestion. Illumina MiSeq sequencing of high throughput microbial community analysis of inocula showed unique inoculum specific changes in composition. Methanocelleus , Anaerolina , and Methanosarcina , which are important hydrolytic/fermentative, syntrophic, and methanogenic groups, dominated the ADS inoculum. The most prevalent cellulolytic, fermentative and methanogenic microorganisms in the CD inoculum were Ruminococcus , Ethanoligenens and Methanobrevibacter . The RW inoculum was typified by the majority of Methanoculleus , Methanosarcina , Anaerolina , and Petrimonas sulfuriphila , which operated as cellulolytic, fermentative, syntrophic, and hydrogenotrophic/acetoclastic methanogenic microorganisms. The differences in microbial composition in close relation to the biogas activity indicate the regulatory power of the type of inoculum on the microbial functions in AD. In general, the choice of ADS as inoculum and an optimal ISR of 1:1 is important in improving the performance of anaerobic digestion and biogas production from PW and CM. The research gives an understanding of the interactions between the inoculum, microbe, and biogas and helps to optimize anaerobic system to achieve a better recovery of renewable energy.