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When organic waste such as food waste, human excrement and animal manures are decomposed in an anaerobic digester they make three end products: biogas, liquid fertiliser and sludge.

Biogas

The biogas produced from the breakdown of waste is predominantly methane and carbon dioxide and it can then be burnt directly as a fuel for cooking or other uses. The amount of biogas which can be generated is dependant on a number of factors. However, when processing under optimal conditions an AD system with a capacity of 10m3 could be expected to produce over 7m3 of biogas per day. The calorific value of this biogas is approximately equivalent 17kg of green wood per day (8kg dry wood) or over 6 tonnes per year.

The potential of the Anaerobic Digestion (AD) process to make biogas and bio-fertiliser

Resources from waste

Liquid fertiliser

Total nitrogen      5.2 kg/m3

NH4-Nitrogen       3.4 kg/m3

Phosphate (P2O5) 2.5 kg/m3

Potassium (K2O)   4.2 kg/m3

Outputs from AD

Why we do it

The liquid fraction produced is rich in nutrients. This can then be applied to fields to give water and nutrients to the growing crops. The containment of the waste whilst decomposing limits evaporation so precious water is conserved. Similarly, the nutrient chemical in the waste are broken down by the digestion process so that the material is more suited to use as a fertiliser. The liquids contain some of the nutrients and the solids also. Typical nutrients levels found in digestate are: