Across the UK, there are Anaerobic Digestion (AD) plants that process biodegradable feedstocks to produce digestate and biogas. Large sites and those that process waste feedstocks require environmental permitting. However, any AD plant in England that meets either, or both, of the following is applicable for waste (and/or permit) exceptions:

• Sites with small scale biogas plants (<400 kWth input aggregated capacity of biogas engines/boilers).
• Sites processing only energy crops (non-waste) feedstocks.

Larger plants will need their own environmental permit. Any waste exemptions must be registered with the Environment Agency (EA), and written evidence of exemptions (from exemptions/permits) should be requested from the EA in the first instance.

Waste exemptions for agricultural and non- agricultural digestors

The T24 exemption is for on-farm AD plants that are utilising manure, slurries, animal bedding and plant tissue wastes produced on the farm.

Alternatively, the T25 exemption is for commercial/industrial AD plants that are utilising food and garden wastes (no hazardous or animal by-product wastes) produced either on-site or bought in. Please note that the AD process is different from composting, which has its own exemption and limits (T23).

Both T24 and T25 exemptions cover:

• Generation of heat (and power) for either use on-site or exporting to the national grid.
• Processing of the allowed feedstocks.
• Maximum site capacities* of 1250 m³ for agricultural digestors, and 50 m³ for non-agricultural digestors.
• No allowances for venting or excessive flaring of biogas; plant breakdowns must be notified and acknowledged prior to flaring.
• Feedstocks must stay within the digestor for at least 28 days.
• No spreading of digestate on site(s) covered under the exemption.

*This refers to the combined volumetric capacity of the following: a) all liquid storage and treatment tanks; b) all digesters; c) digestate storage tanks.

Changes to any of the following could affect any pre-existing exceptions:

• On-site capacities.
• The type of feedstocks being fed to the digester.
• End use of the digestate.
• Any combustion plants onsite.

Before making changes to your site, make sure you check if any limits within applicable exemptions could be exceeded.

Spreading of digestate is excluded from both T24 and T25 exemptions. Therefore additional permissions are required if the digestate is not being sold onto another party for end use (U10 for agricultural land and U11 for public and communal gardens/land). However, it is the producer’s duty of care to ensure that whoever is receiving the digestate also has the relevant waste exemptions for digestate spreading.

For more information regarding what is covered under these exemptions, please see the linked official EA guidance.

What about non-waste digestate?

There are also considerations regarding non-waste digestate, which is produced from non-waste feedstocks or solely from manure (which is then considered to be the same as raw manure spreading with lower environmental impacts). Digestate not classified as a waste must undergo an end of waste assessment, the anaerobic digestate quality protocol, prior to being marketed and sold as a non-waste product.

If you are unsure whether any recent/planned changes to your AD plant has/will invalidate your current exemptions, our skilled and experience engineers are on hand to support you.

Further information about whether your AD plant, and/or subsequent biogas combustion plant(s) need a permit is in our previous blog: Does your AD Plant need an Environmental Permit?

If you need support with an AD site, AD permit, combustion plant permit, exemption(s), or any further advice on anything discussed in this blog, please call the team on 024 7669 8899.