Enhancing methane production from rice crop residues via pretreatment and co-digestion with cattle or swine slurry

Improper management of residues from rice cultivation and livestock farming can exert severe pressure on the environment. Anaerobically digesting these residues for biogas production is a plausible solution to mitigate their environmental impact. However, due to the low degradability of rice straw, pretreatment is imperative to enhance its methane production. Therefore, this study investigated the effects of eight thermal and four thermochemical pretreatment methods on the biodegradability of rice straw. Results indicated that the thermochemical pretreatment, with NaOH, increased methane production by 13%–15%, while thermal pretreatment increased it by 2%–5%, compared to untreated rice straw. Subsequently, the most effective pretreatment was used in co-digestion batch experiments with cattle and swine slurries separately to determine the optimal ratio of pretreated rice straw (p-RS) with each slurry. Co-digestion of p-RS with swine slurry increased methane production up to 59%, while co-digestion with cattle slurry increased it by 14%, compared to the mono-digestion of each slurry. Moreover, a techno-economic analysis was employed to assess the feasibility of establishing a biogas power plant treating the optimum co-digestion mixture. Based on our assessment, the plant can generate an annual revenue and net profit of approximately €2,418,000 and €1,510,000, respectively. The payback period was estimated at 2.98 years, while the return on investment and profit margin were 33.5% and 62.4% per year, respectively.