Ensiling for anaerobic digestion (AD): a review of key considerations to maximise methane yields

Growth in anaerobic digestion (AD) has expanded the use of silage to preserve crops intended for renewable energy generation. Preservation of seasonal crops and their residues is critical in a process that needs continuous feeding. Whilst the impact of different crops and harvesting times on methane formation is relatively well understood, to date the specific considerations for maximising methane yields through management practice have been largely ignored. The present paper reviews the current state of knowledge on silage practice for biogas production and specifically on the factors affecting methane yields of ensiled crops, as well as their influence on the silage quality, and provides suggestions for further research in the field. Data shows that ensiling is able to conserves 93% of crops gross energy when good practices are followed. Shorter chop length (7-10 mm), lower DM feedstock (20-35%) and lower compaction values (less than 250 kg/m3) are used to achieve higher biogas yields. Increase biogas production can also be obtained by managing the organic acids production with the preferential formation of acetic acid and ethanol instead of lactic acid via enzymatic or microbial additives. The review outcomes show that more research is required to provide a clear-cut distinction between the requirements for ensiling crops for AD versus crops for animal feed.