Biogas offers alternative fertiliser opportunity

Digestate is still a relatively new source of alternative fertiliser, but with more biogas plants coming on stream in the UK an increasing number of farmers are applying it on their land to help reduce artificial fertiliser costs.

Latest figures from the National Non-Food Crops Centre and WRAP show there are 106 anaerobic digestion plants outside the water industry, processing up to 5.1m tonnes of food and farm waste every year. Around 90-95% of material fed into an AD plant comes out as digestate, so there is a potentially significant source of fertiliser for growers to tap into.

But the amount of digestate available on the “open market” could be considerably less than 5m tonnes as many plants operate relatively closed systems where digestate is spread back on to the land immediately around the facility, often as part of some formal or informal deal for supplying feedstock.

The main opportunities for growers wanting to use digestate, but with no financial stake in an AD plant, therefore come from stand-alone food waste-based plants, says WRAP’s David Tompkins. Many such units tend not to have sufficient land area for spreading all the digestate produced, so need to look to neighbouring farmers and growers to take the material.

“AD capacity is almost certain to increase over the next few years as there are still millions of tonnes of food waste that could potentially be used to generate biogas. Nutrients from digestate could supplement or replace nutrients from artificial fertilisers, with the arable east of England probably providing the best market development opportunity.”

Recognising the value

Mr Tompkins says farmers and AD operators now better appreciate the value of digestate as an alternative fertiliser, which will help drive the market forward.

“Three or four years ago there were still a lot of questions over using digestate and many operators had to pay people to take it. However, a lot of the concerns have been addressed with the introduction of the Quality Protocol [see panel] and we are now seeing occasional examples where farmers are even being charged for the nutrient value of the digestate.”

Most of the fertiliser value of digestate is in the ammonium, which he reckons is worth around £5/t for food waste-based digestate, but nutrient content and fertiliser value will vary depending on feedstock used.

There is a wide range of ways in which AD operators are supplying digestate, with some offering a complete service that includes nutrient planning, haulage and spreading, while others may deliver the digestate for free, leaving the grower to pay for spreading. Costs vary, but an example haulage cost in Wales is £7/t for 25 miles, while spreading might be nearer £3-5/t, he says. “There’s a real diversity of models and costs out there, so it really comes down to what works for the individuals involved.”

Know what you’re getting

Mr Tompkins says any growers considering taking digestate for the first time should ask a few key questions of their supplier:

  • What is the agronomic value of the digestate (nutrient content)?
  • What type of digestate is it and what feedstocks does it come from?
  • Is it accredited under the BCS (see below)
  • How much is available and when?
  • How should it be spread and who will do this?

He encourages farmers to use digestate that is accredited by the independent Biofertiliser Certification Scheme (BCS), which was set up to recognise digestates that meet the PAS110 standards of the Quality Protocol. “This is the best way to make sure the end product is safe, consistent quality and fit for purpose,” he says.

“The AD Quality Protocol applies in England, Wales and Northern Ireland and requires digestates to meet an identified standard (or standards) and the only one that’s recognised at the moment is PAS110. In Scotland, SEPA do not require the AD Quality Protocol to be used, so digestates complying with PAS110 alone can be considered products.”

The standard specifies controls on input materials and the AD management system and associated technologies, as well as setting minimum quality criteria for whole digestate, separated fibre and separated liquor.

The Biofertiliser Certification Scheme applies across the UK and is a voluntary scheme. Currently just 10 AD operators are registered with the scheme, but BCS’s Ciaran Burns says it is working with another 30-35 operators that are going through the validation process.

“The scheme is all about giving confidence to farmers and the market that digestate is a safe and effective product to use. Farmers using certified digestate can have confidence in the quality and consistency of the material and be assured that it is free from contaminants.

“At the moment there’s nothing to say farmers have to use digestate that meets our certification scheme, but it is recommended by farm assurance schemes such as Red Tractor.”

Using BCS-accredited digestate can also remove the need for a costly environmental permit if food or other waste was used in the original feedstock, he notes (see regulations panel for more).

What is digestate?

Digestate is a nutrient-rich substance produced from the anaerobic digestion process that can be used as a fertiliser. It can be used whole (typically 5-6% dry matter), or separated into solid and liquid parts. Solid fibrous material is similar to compost and can be useful as a soil improver, while the nutrient-rich liquor can displace artificial fertiliser.

All the nitrogen, phosphorous and potassium present in the feedstock will remain in the digestate. The NNFCC estimates typical values for nutrients as:

  • Nitrogen: 2.3-4.2kg/t
  • Phosphorous: 0.2-1.5kg/t
  • Potassium: 1.3-5.2kg/t

However, the nutrients are more available than in raw slurry, meaning it is easier for plants to make use of the nutrients. This can be particularly valuable for land within Nitrate Vulnerable Zones where applications of organic nitrogen are restricted.

Regulations explained

Digestate regulation can be quite complex and ultimately depends on what feedstocks are being used in the first place and whether the Environment Agency classes them as “waste”.

Anything classed as waste will require either a costly environmental permit or an exemption.

Generally, where the only waste feedstock to an AD plant is agricultural manure and slurry, or where non-waste feedstocks such as crops grown specifically for AD are used with the manure or slurry, the digestate is not waste if it is used in the same way that undigested manure and slurry would normally be used (for example, spread as a fertiliser on agricultural land) and would not need to be authorised by the EA.

But, if other wastes such as food wastes are digested on their own or with manure, slurry or crops grown for AD, then the storage and spreading of the digestate on land will require EA authorisation (for example, permit or exemption), unless it is certified under the BCS. Facilities using food waste covered by Animal By-Products Regulations will also be subject to those regulations and AD operators are required to pasteurise digestate before use.

There are regulatory controls on allowing livestock access to land treated with digestates from animal by-products or to crops harvested from land treated with ABP-digestates, therefore farmers will need to keep additional AHVLA records.

More on this topic

Supplying anaerobic digestor plants

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