Optimization of Temperature to Enhance Anaerobic Co-Digestion of Vegetable Waste with Cattle Rumen Content: Maximizing the Biogas Yield

Abstract

The global challenge of managing organic waste sustainably continues to intensify, with vegetable waste (VW) and cattle rumen content (CRC) representing major contributors to environmental degradation and underused energy resources. This study optimised the effects of temperature on biogas production, methane content, and volatile fatty acid (VFA) dynamics during VW–CRC co-digestion under mesophilic (35–48 °C) and thermophilic (50–60 °C) conditions. Batch digesters (5 L) were operated at VW:CRC ratios of 70:30 and 50:50, organic loading rates of 2–4 kg VS/m³/day, a 40-day retention time, and pH 6.8–7.2. The highest biogas yield was 0.62 L/g VS with 62% methane at 42 °C and a 70:30 ratio 38% higher than VW mono-digestion and 18% higher than thermophilic operation at 55–60 °C. Temperature optimisation halved the lag phase, increased volatile solids (VS) reduction to 65%, and limited peak VFA to approximately mg/L versus >2200 mg/L at temperature extremes. First-order kinetic modelling at the temperature optimum gave hydrolysis and methanogenesis rate constants of 0.12 d^-1 and 0.08 d^-1, respectively. ASPEN Plus simulation of a 1 m³ continuous stirred-tank reactor (CSTR) at 30-day hydraulic retention time (HRT), informed by batch kinetics, projected stable biogas production with approximately 60% methane. The findings from the findings support scalable biogas systems in Nigeria and sub-Saharan Africa, where there is a significant supply of VW and CRC in markets/farms and slaughterhouses that have not been utilised, which can be used for rural energy, waste management, and reduction of greenhouse gases in line with Sustainable Development Goals 7, 12, and 13.