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/tonne
Detailed guidance on digestate in agriculture can be found on the WRAP website, this includes information on the four-year Digestate & Compost in Agriculture Project, which looks at the use of quality anaerobic digestate and compost in agriculture.
Where can digestate be used?
Before investing in AD it is important to consider where and how the digestate will be used. If it is not to be considered waste, digestate must meet the standards set out in the Quality Protocol and PAS110 (England & Wales) and the SEPA position statement (Scotland).
The Quality Protocol sets out criteria for the production of quality outputs from anaerobic digestion of biowaste. Producers and users are not obliged to comply with the Quality Protocol. If they do not, the digestate will be considered to be waste and waste management controls will apply to its handling, transport and application.
The Publicly Available Specification (called PAS110) for digestate, derived from the anaerobic digestion of source-segregated biodegradable materials creates an industry specification against which producers can verify that the digested materials are of consistent quality and fit for purpose. If an AD plant meets the standard, its digestate will be regarded as having been fully recovered and to have ceased to be waste, and it can be sold with the name “Bio-fertiliser”. PAS110 is for the digestate product, the QP is about safeguards and process needed to achieve PAS110.
The Anaerobic Digestate Quality Protocol is not applicable in Scotland, however SEPA have published a Regulatory Position Statement which is to be followed and used in conjunction with PAS110 certification. Also note that in Scotland under the Zero Waste Plan digestate which is not PAS110 certified and produced in accordance with the SEPA position statement above, will not be counted towards recycling targets even if it is currently produced and used under an exemption.
The Northern Ireland Environment Agency (NIEA) published a Northern Ireland Regulatory Position Statement explaining how digestate will be regulated, in July 2010 and at the same time adopted the AD Quality Protocol.
The Biofertiliser Certification Scheme (BSC) provides assurance that biofertiliser (the BSC name for digestate) is safe and of good quality. Renewable Energy Assurance Ltd., a subsidiary of the Renewable Energy Association, administers the Scheme for England, Scotland, Wales and Northern Ireland. You can find more about how to become certified on the Biofertiliser Certification Scheme website.
Digestate enhancement and treatment
Digestate can be used whole, spread on land with tankers or umbilical pipe lines. Alternatively, it can be separated in to liquor and fibres, which have differing distributions of nutrients. The liquor should contain less than 6% dry matter. Separated liquor can be spread more easily to growing crops. Separated fibre can be used fresh as a soil conditioner or, after further aerobic composting to stabilise it, a material suitable for making into a compost product.
WRAP have produced a study which identifies digestate enhancement and treatment techniques.
How to plan digestate use on your land
Digestate's major drawback is that it is a pre-determined mix of nutrients that cannot be altered. For more information on the use of digestate see Defra's nutrient planning handbook, RB209.
There must be enough land in the vicinity of the digester that can accept the digestate within the restrictions of Nitrate Vulnerable Zones (NVZs); 62% of land in England and 4% in Wales falls within NVZs. To work out the amount of land required to accommodate the digestate you produce, see the AD calculator.
Digestate derived from AD plants treating animal by-products and approved by Animal and Plant Health Agency (APHA) is subject to a grazing ban once the digestate is used on land. Livestock must not be allowed access to the land during this time period.