A step-by-step strategic adaptation of anaerobic sludge for high-solids digestion of fruit and vegetable waste
ABSTRACT Dry anaerobic digestion has emerged as a promising alternative for treating and recovering energy from organic solid waste, including fruit and vegetable waste. This study investigated the progressive adaptation of a mesophilic anaerobic sludge, previously used to treat brewery effluents (2.4% Total Solids), to operate under high-solids conditions (13.8%), using the dry fraction of fruit and vegetable waste as the sole substrate. Acclimation was conducted in a mixing reactor at 35°C, with gradual replacement of sucrose by the dry fraction of fruit and vegetable waste and sequential increase in organic concentration from 0.1 to 6.0 kg COD m-3. The adaptation was evaluated through physicochemical parameters, a Specific Methanogenic Activity test, and metagenomic analysis of the microbial community. The results demonstrated a significant biological restructuring, with a reduction in alpha diversity but an increase in functional speciation, reflecting greater resilience to the operational stresses of dry anaerobic digestion. The taxonomic succession showed higher relative abundances of the phyla Proteobacteria and Bacteroidota, and, with adaptation, Nitrospirae, Chloroflexi, and Synergistota stood out. Furthermore, enrichment of the methanogenic archaea genera Methanothrix and Methanobacterium was observed, increasing from 0.13 to 28.9% and from 0.0 to 2.2%, respectively, indicating methane production via complementary acetoclastic and hydrogenotrophic pathways. In Specific Methanogenic Activity assays, the adapted sludge showed an increase of up to 254% in actual methane production compared to the non-adapted inoculum. This increase in methanogenic capacity, coupled with the robust conversion of volatile fatty acids, confirms the effectiveness of the gradual adaptation strategy for dry anaerobic digestion systems. These results contribute to the consolidation of reproducible protocols for inoculum conditioning in the treatment of complex organic waste under high organic concentrations.