Effect of co-substrate ratio on pH, redox potential, and cumulative gas from pineapple pulp residue and cow manure

Abstract Dark fermentation efficiency is highly influenced by the ratio of substrate, concentration of total solids (TS), and redox level. Herein, it was investigated to assess the impact of varying co-substrate ratios of cow manure and pineapple pulp residue on gas production under constant TS concentration at 8%. Three substrate ratios (70:30, 50:50, and 30:70, w/w) were fermented in batch at 35°C. The gas yield was determined by a water displacement method, and pH and oxidation–reduction potential (ORP) were determined at intervals. The 70:30 proportion gave the highest total volume of gas (39 mL), while the 50:50 proportion gave much less gas yield (6 mL), and the 30:70 proportion gave only 0.8 mL. ORP values in treatments showed fluctuating instability, with greater positive values corresponding to less gas formation. These findings indicate that substrate ratio and TS control are crucial parameters governing the fermentation environment, and they exert significant influences on agro-industrial residue gas productivity. Additional studies with gas composition measurement are recommended to validate hydrogen selectivity and optimize the process conditions.