Oral presentation: Co-digestion of grass silage and cattle slurry for grid decarbonisation and phosphorus sequestration

Decarbonising electricity while addressing agricultural nutrient surpluses is a critical challenge in livestock-intensive regions. This study evaluates an integrated system designed to deliver net-negative emissions electricity alongside phosphorus sequestration and nutrient recovery. The environmental performance of an anaerobic digestion system co-digesting grass silage and the solid fraction of cattle slurry was evaluated using life cycle assessment in accordance with the ISO 14040 and ISO 14044. The system integrates digestate management using a decanter centrifuge separation, belt drying with ammonia capture, and pyrolysis, producing biochar for the substitution of carbon black in cement tiles, as well as bio-oil, syngas, and electricity via a ~500 kWe combined heat and power unit. The functional unit was defined as 1 kWhe of electricity exported to the grid. The system was designed to operate for 8,000 hours annually using locally sourced feedstocks. The results indicate that the system can export 3.1 GWhe of electricity per year and achieves net greenhouse gas emissions of -0.2537 kg CO2e per kWhe, a reduction of 0.4307 kg CO2e per kWhe compared to the UK grid electricity. Substantial environmental benefits arise from biochar substitution for carbon black, the displacement of synthetic fertilisers, and the utilisation of surplus heat to offset natural gas. Total ammonia emissions were estimated at a low level of 2.3 g per kWhe, owing to the advanced digestate management. The system also adds value by enabling long-term phosphorus sequestration of 7.8 g phosphorus per kWhe in biochar. Cattle slurry separation reduced the greenhouse gas and ammonia emissions from storage and land application by 11% and 17%, respectively. The results demonstrate that integrated anaerobic digestion coupled with advanced digestate management can produce net-negative greenhouse gas electricity while enhancing nutrient circularity and reducing agricultural emissions. This system, therefore, represents a viable pathway for sustainable energy production in livestock-intensive regions.