Methane Production Using Anaerobic Co-Digestion of Swine and Nejayote Wastewater: Synergic Effects and Kinetic Modeling Studies

Anaerobic co-digestion of substrates offers synergistic benefits, enhancing methane production and improving the operational stability of wastewater treatment. The present study, for the first time, evaluated the biochemical methane potential and kinetics modeling performance of two regional wastewater streams—swine wastewater (SW) and nejayote wastewater (NW)—under mesophilic batch conditions. Five substrate ratios (SW/NW: 100/0 to 0/100) were tested, and interaction effects were measured using the co-digestion performance index (CPI). All mixtures demonstrated synergistic effects, with CPI values ranging from 1.12 to 1.26. NW exhibited the highest methane yield (438 ± 25 NL-CH4/kgCODT-removed), nearly twice that obtained for SW (227 ± 18 NL-CH4/kgCODT-removed). In addition, co-digestion improved the methane yield of SW as mono-digestion, with production increasing from 281.8 ± 12.4 to 304.7 ± 27.8 NL-CH4/kgCODT-removed in all mixtures. The methane production kinetics were analyzed using six mathematical models. The multi-phase Gompertz model provided the best fit (R2 > 0.99), while the two-phase model offered the best balance of accuracy and simplicity according to Akaike’s criterion. The present model effectively described the diauxic patterns of methane production resulting from substrate heterogeneity with an error of <8% for all experimental assays.