Farm digestate, also known as farm-based digestate, refers to the residual material that is left over after the anaerobic digestion of organic waste generated on a farm. The organic waste can include animal manure, crop residues, and other agricultural byproducts.
The anaerobic digestion process breaks down the organic material, resulting in the production of biogas and a nutrient-rich liquid or solid digestate. The digestate can be used as a fertilizer or soil amendment, providing valuable nutrients such as nitrogen, phosphorus, and potassium to grass and crops.
Farm digestate can be an effective and sustainable alternative to traditional chemical fertilisers. By using digestate as a fertiliser, farmers can reduce their reliance on synthetic fertilisers and improve the health of their soil. Additionally, using digestate as a fertiliser can help reduce greenhouse gas emissions, as the organic matter in the digestate is returned to the soil rather than being left to decompose and release methane, a potent greenhouse gas.
“By modelling alternative farming systems, AD is confirmed as an ideal pollution control, achieving savings of 32–44% CO2eq, with the best improvement for full confinement herds. Exploiting the CO2 component of biogas and the ability to use CH4 to power farm vehicles are seen as routes to achieve an objective of 50% reduction but will require technology advances before becoming an affordable reality “
Dr Richard Blanchard
Centre for Renewable Energy Systems Technology,
Nutrients in digestate are valuable
“At Gask Farm we used to spend £52,000 a year on fertilisers, but we’ve been using digestate for 6 years and now we only spend £10,100. You have to balance these savings against the costs of application, but on wheat and barley crops we only apply a fifth of the fertiliser that we used to. The winter oilseed rape that we grow receives no fertiliser – the nutrients and trace elements in the digestate do the job for us.”
Andrew Rennie, farmer and AD operator
“Digestate is an excellent biofertiliser which can help farmers to reduce their costs. ”
Nicola Dunn, NFU
Digestate is a nutrient-rich substance produced by anaerobic digestion that can be used as a fertiliser. It consists of left over indigestible material and dead micro-organisms – the volume of digestate will be around 90-95% of what was fed into the digester. Digestate is not compost, although it has some similar characteristics. Compost is produced by aerobic micro-organisms, meaning they require oxygen from the air.
By using digestate instead of synthetic fertilisers derived from natural gas, we can save energy, cut consumption of fossil fuels and reduce our carbon footprint.
All the nitrogen, phosphorous and potassium present in the feedstock will remain in the digestate as none is present in the biogas. However, the nutrients are considerably more available than in raw slurry, meaning it is easier for plants to make use of the nutrients.
The exact composition of digestate is determined by the plants diet. However, some typical values for nutrients are:
- Nitrogen: 2.3 – 4.2 kg/tonne
- Phosphorous: 0.2 – 1.5 kg/tonne
- Potassium: 1.3 – 5.2 kg/tonn