Effects of microbial inoculum optimization on biomethane production from paper mill solid residues

The conversion of residual biomass can significantly enhance renewable energy production and reduce greenhouse gas emissions. According to the Brazilian Tree Industry (IBÁ), Brazil produced approximately 23.5 million tons of cellulose and 11 million tons of paper, generating about 0.4 tons of residues for every ton of cellulose produced. This study utilizes a laboratory-scale batch anaerobic reactor to evaluate the effects of microbial inoculum acclimatization and nutritional supplementation on the conversion of cellulose residues into biomethane. Bioreactors that were supplemented with chicken manure and acclimatized inoculum (Batch 04) produced up to four times more biomethane compared to those without supplementation and those using non-acclimatized inoculum. Molybdenum supplementation had no effect on gas production. In Batch 04, the archaeal genera Methanobrevibacter and Methanococcoides, which were dominant in Batch 03, were either undetected or present in very low abundances. Conversely, Methanosarcina and Methanobacterium became the dominant genera in Batch 04, together making up nearly 98% of the archaeal community. Methanosarcina can produce methane by utilizing small organic substrates, while Methanobacterium is hydrogenotrophic archaea. Both batches predominantly contained Bacteroides in their bacterial communities, but Batch 04 showed increased abundances of Parabacteroides, Sphaerochaeta, Ethanoligenes, Aerosphaera, and Candidatus_Falkowbacteria. Similarly, fungal genera such as Mortierella, Fusarium, Pisolithus, Cryptococcus, and Penicillium were more abundant in Batch 04 compared to Batch 03. This study emphasizes the importance of acclimatizing the inoculum and supplementing nutrients in anaerobic digestion reactors for enhancing biomethane production from paper mill residues, which are rich in carbon but deficient in nitrogen.