Agricultural anaerobic co-digestion of grass clippings and manure

Problem statement

This technology improves the digestion of pig manure and uses grass for energy generation.

Executive summary

The technology entails the co-digestion of pig manure with grass clippings, improving the ammonia accumulation in the digester. The co-digestion with pig manure improves the flowing of grass, as monodigestion of grass can be technically challenging due to its fibrous nature.

Technology description

The anaerobic co-digestion of pig manure and grass is advantageous over the monodigestion of either feedstock. Up to 20% grass can be added to the digester without detrimental effects due to the fibrous nature of grass. The digestion is done in a continuous stirred reactor, with a mixture of grass and pig manure being fed daily and digestate being removed also daily. The best combination is grass plus separated pig manure to be able to adjust the composition of solid manure plus pig slurry. This enables the reduction of the ammonia content in the digester while still guaranteeing a proper dry matter content. Biogas is produced, which can either be purified into biomethane or be burned in a CHP for heat and electricty production. The digestate can be used as fertilizer.

Market deployment considerations

Consideration of local renewable energy and bio-fertiliser market conditions and supporting policies.

Environmental considerations

Manure digestion is prefered over direct spreading in the field; grass digestion is prefered over composting