Cascading Biomethane Recovery from Primary and Bioprocessed Food and Corn Stover Wastes: Anaerobic Thermophilic Co-Digestion from Batch to 40 L Scale-Up

In this study, thermophilic anaerobic digestion (TAD, 60 °C) was evaluated as a downstream waste-to-energy step within a cascading thermophilic biorefinery using four interrelated substrates—primary cafeteria wastes (pCFWs) and primary corn stover wastes (pCSWs) were evaluated without physicochemical pretreatment. Their corresponding secondary substrates, secondary cafeteria waste (sCFW) and secondary corn stover wastes (sCSWs), were residual solids generated after thermophilic bioethanol production and exopolysaccharide production, respectively. To our knowledge, this is the first study to demonstrate sequential thermophilic valorization in which primary wastes are untreated and the remaining biotreated secondary residues are subsequently converted into biomethane, adding each step to bioeconomy. Biomethane potential was quantified to determine how substrate composition and upstream bioprocessing influence methane yield and biodegradability. In the batch, pCFW achieved the highest biodegradability (84% VS reduction) but suffered rapid acidification at higher loadings, whereas pCSW was hydrolysis-limited by lignocellulosic recalcitrance. Upstream bioprocessing (biological pretreatment) improved digestibility, with sCSW exhibiting a 1.8-fold increase in methane yield (300 L CH4 kg−1 VS) relative to pCSW. All co-digestion treatments outperformed monodigestion, with the best-performing (among the tested) sCFW:sCSW ratio of 3:1 delivering the highest methane yield (413 L CH4 kg−1 VS) and VS reduction (95.8%). Scale-up in a 40 L fed-batch reactor achieved methane productivities of 49–142 L CH4 kg−1 VS per feeding cycle, reaching stable operation after two cycles, with cumulative methane production of ~800 L CH4 kg−1 VS for secondary wastes compared to ~550 L CH4 kg−1 VS for primary wastes. Microbial analysis revealed dominance of syntrophic acetate-oxidizing bacteria (Acetomicrobium, 22.6%) and hydrogenotrophic methanogens (Methanothermobacter, 72.3%). Therefore, biologically pretreated wastes enabled higher methane recovery and improved solids destruction under thermophilic conditions and demonstrates circular conversion of wastes into renewable biomethane.