Quantity of inoculum modulates organic methanation and humification in solid-state anaerobic digestion and aerobic composting hybrid processes

Abstract

In this study, we conducted a comprehensive analysis of the regulatory effects of varying feedstock-to-inoculum (F/I) ratios on the organic matter conversion process within a hybrid solid-state anaerobic digestion (SSAD) - aerobic composting (AC) system. Notably, at an F/I ratio of 1.5, we observed a significant enhancement in the anaerobic fermentation phase, particularly in the microbial community distribution. This adjustment led to a marked enrichment of the acetic acid-utilizing Methanosaeta family, which in turn drove a remarkable increase of over 29.9 % in the volumetric methane production based on reactor volume. Concurrently, the optimized F/I ratio facilitated a more substantial degradation of lignin and cellulose, fostering a conducive environment for the growth of humic acid (HA)-producing microorganisms such as Bacteroidota and Actinobacteria during the aerobic composting phase. The heightened activity of these functional microbes, in conjunction with the upregulation of humification-related genes, culminated in a substantial increase in HA content, ranging from 33.6 % (0.028 g·kg⁻¹ DM) to 60.5 % (1.036 g·kg⁻¹ DM). Thus, our findings demonstrate that by strategically modulating the F/I ratio to 1.5, it is possible to effectively optimize the microbial community structure in the SSAD-AC system, leading to a significant improvement in the efficiency of both organic matter methanation and humification processes.