Methane Production and Kinetic Modeling for Co-digestion of Manure with Lignocellulosic Residues

Anaerobic digestion (AD) of animal manure and lignocellulosic residues is gaining increased interest as a result of their wide availability, optimum physicochemical characteristics, high methane potential, and absence of conflict with the human food chain compared to energy crops. The aim of this study was to assess the biomethanation process of two lignocellulosic substrates, wheat straw (WS) and meadow grass (MG), with cattle manure (CM) under thermophilic (53 °C) conditions, focusing on nutrient availability in the reaction mixtures, along with C/N ratios. Results showed that, with the use of 50% WS on an organic matter basis in the feedstock and substitution of the rest of the volatile solids (VS) component share between CM and MG (25:75, 50:50, and 75:25), the methane yield can be increased by 20–24% compared to WS mono-digestion, with a methane production rate of 27, 23, and 22 N mL of CH₄ g–¹ of VS day–¹, respectively. Moreover, the positive effects of coupled biological reactions in the reaction mixture of co-digestion were explained using the synergistic effect value (η). The η value was calculated using estimated and experimental methane yields. Furthermore, in MG co-digestion, where 75% VS originated from MG and the rest was distributed as a 25:75 mixture of CM and WS, a 14% enhancement in the methane yield was shown in comparison to MG mono-digestion, with the maximum methane production rate of 25 N mL of CH₄ g–¹ of VS day–¹ in batch experiments. Finally, the best co-digestion results with the highest methane yield (up to 25%) and lowest lag phase (6–7 days) were achieved when 75% organic matter originated from CM. The combination presenting the above-mentioned increase in the methane yield also showed a methane production rate of 22 N mL of CH₄ g–¹ of VS day–¹. It was concluded that increasing the MG share in co-digestion improves the feedstock digestibility and also gives the higher methane production rate. In contrast, a high WS share increases the lag phase and is a detriment to the biodegradability. Finally, through co-digestion of two lignocellulosic substrates of different physicochemical characteristics with CM, the overall biodegradability compared to single-substrate digestion is improved and the methane yield is enhanced.