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

• Two-stage AD of cow manure and yeast extract was tested on pilot-scale. • Co-digestion increased biogas production by 1.77 times compared to mono-digestion. • Elevated H 2 S levels were detected in the biogas during co-digestion trials. • The inclusion of a thermophilic post-digestion further improved biogas production. • All digestate types met the requirements for safe and effective organic fertilisers. 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 CH 4 /kg VS, while thermophilic post-digestion provided an additional methane recovery of 100 L CH 4 /kg VS. Co-digestion with yeast extract significantly enhanced methane yield, increasing it 1.77-fold to 421 L CH 4 /kg VS in the two-stage AD system. However, co-digestion resulted in elevated hydrogen sulphide (H 2 S) 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 H 2 S mitigation strategy is recommended to optimise process sustainability.