EFFECT OF BUFFALO-TO-COW MANURE MIXING RATIO ON METHANE YIELD AND PRODUCTION KINETICS DURING MESOPHILIC ANAEROBIC CO-DIGESTION

Abstract

Karachi, Pakistan, faces a chronic natural-gas shortage while its cattle colonies generate substantial quantities of buffalo manure (BM) and cow manure (CM). This study evaluated the effect of BM:CM mixing ratio on methane yield and production kinetics under mesophilic batch digestion. BM and CM were digested alone and in five mixtures (BM:CM = 80:20 to 20:80, VS basis) at 37 °C for 62 days. Kinetics were described using the modified Gompertz model and mixture performance was compared with VS-weighted theoretical yields. BM showed a shorter fitted lag phase than CM (5.2 vs 14.4 d), whereas CM had the higher mean final yield (133 ± 43 vs 109 ± 26 mL CH₄/g VS added). Mixtures containing 40-80% BM increased the fitted maximum methane-production rate from 3.3-4.0 to 4.7-6.5 mL/g VS/d, with the highest rate at 50:50, and reached 80% of their measured final yield 9-17 days sooner than either manure alone. Four mixtures showed apparent positive deviations of 14-23% from weighted theoretical yields; however, final-yield differences were not statistically significant (ANOVA, p = 0.77), and uncertainty in the interaction index did not establish biological synergy. The 20:80 result was sensitive to two anomalously high late measurements. Overall, co-digestion primarily accelerated methane-production kinetics rather than demonstrating a significant increase in ultimate methane yield. These results support further evaluation of flexible BM-CM blending in better-replicated and semi-continuous digestion systems.