Pilot-scale anaerobic digestion of on-farm agro-residues: Boosting biogas production and digestate quality with thermophilic post-digestion

Environmental challenges associated with the disposal of organic farm waste and the growing demand for renewable energy underscore the importance of anaerobic digestion (AD), a process that converts organic matter into biogas and nutrient-rich digestate, offering a sustainable solution for waste management and energy production. This study evaluated a two-step AD process using cow manure and yeast extract through mono- and co-digestion trials at pilot-scale, serving as a preliminary step to assess feasibility and performance prior to scaling up for full-scale implementation on a dairy farm. Two 72 L continuously stirred tank reactors were operated, with the primary reactor maintained at mesophilic conditions and the secondary reactor acting as a thermophilic post-digester. This configuration was used to assess the influence of thermophilic post-digestion on biogas yield and digestate quality. During the mono-digestion of manure, mesophilic digestion yielded 138 L CH4/kg VS, while thermophilic post-digestion provided an additional methane recovery of 100 L CH4/kg VS. Co-digestion with yeast extract significantly enhanced methane yield, increasing it 1.77-fold to 421 L CH4/kg VS in the two-stage AD system. However, co-digestion resulted in elevated hydrogen sulphide (H2S) levels, posing potential challenges for biogas purification. Additionally, higher and more fluctuating volatile fatty acid concentrations were observed compared to manure mono-digestion. The quality and safety of the digestates remained comparable between mono- and co-digestion treatments, suggesting that co-digestion with yeast extract can offer advantages for manure-based AD systems, although an effective H2S mitigation strategy is recommended to optimise process sustainability.