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Digestate

Digestate is the generic term for the material remaining after the anaerobic digestion process comprising of undigested solids and the liquid fraction of the input material. Creating markets for the digestate from an AD plant is very important to the plant’s viability. Digestate can take the form of the whole digestate resulting from an anaerobic digestion process, and any subsequently separated fibre (at least 15% dry matter) or liquor (certainly less than 15% dry matter, the more soluble the better for plant up-take).

The AD process consumes relatively little nutrients fed into the reactor and generally enhances the bioavailability of nutrients such as nitrogen, phosphorous and potassium. As such, digestate represents a valuable nutrient source and the stable organic material present can be used as a soil conditioner. In some cases further maturation of the digestate may be required. Considerations over digestate storage design, method of land application and application rates, as well as considerations over Nitrate Vulnerable Zones are necessary in order to minimise methane and ammonia losses and protect land and water resources.

The ADQP was launched in September 2009. The protocol sets out end of waste criteria for the production and use of quality outputs from anaerobic digestion of source-segregated biodegradable waste, lists of waste types and how to use the digestate. Digestate will need to be certified against the BSI PAS 110 certification scheme. The purpose of this PAS is two-fold: to ensure that digested materials are made using suitable inputs and effectively processed by AD for sufficient time; and to ensure that the process has been well-managed and monitored so as to produce digested material that meets market needs and protects the environment.

The regulatory status of digestate produced from an AD plant will therefore depend on the feedstocks to the process and the ability to meet the digestate standard. Digestate from source segregated waste that will be able to meet the PAS 110:2009 and the QP will no longer be considered a waste and is able to exit the waste regulatory framework. Digestate that is not able to meet the PAS 110 and QP will be considered as a waste and will be regulated as such. It still may be applied to land either under an Environmental Permit or with an appropriate exemption under the Environmental Permitting Regulations, both of which are regulated by the Environment Agency / National Resources Wales. Digestate resultant from the digestion of non-wastes for e.g. crops grown specifically for energy production may not require a permit.

In case of a poor quality digestate, it may still be required to be disposed in a landfill or in case of the liquid fraction to be further treated at a wastewater treatment facility either on or off-site. For example, discharge of the effluent to the sewerage system may be required, which needs to be approved.

For more information on PAS110, follow the link: http://www.wrap.org.uk/content/bsi-pas-110-producing-quality-anaerobic-digestate

For more information on the ADQP, follow the link: http://www.wrap.org.uk/content/quality-protocol-anaerobic-digestate

To download a copy of the regulatory statement from the Environment Agency on the production and use of anaerobic digestate from source-segregated biodegradable waste please click here