For growers close to an anaerobic digestion plant, digestate can be a valuable way of delivering at least some of a crop’s nutrient requirements
While it is difficult to pin down whether growers are paying for digestate from AD plants, being paid to take it or taking it for free, there is no question that it is valuable as a fertiliser, according to ADAS principal soil scientist John Williams.
Nitrogen is the most significant crop nutrient supplied by digestate, but its levels and availability to the crop vary depending on the type of digestate used, timing and application method, he says.
There are three types of digestate – whole digestate, which has a dry matter content of about 5%, digestate liquor, and separated fibre, typically around 20-25% dry matter.
Nitrogen tends to be present in higher levels in digestate derived from food waste than farm-sourced crop and manure-based material, says Mr Williams.
“Food waste digestate will typically have around 5kg/cu.m of total N with 80-90% of this in a readily available form.”
Typical nutrient values for the different types of digestate are included in AHDB’s Nutrient Management Guide (RB209).
While there is potential for the nitrogen to be taken up by vigorously growing crops such as oilseed rape in autumn, spring application, particularly for cereals, is likely to supply more crop available nitrogen than autumn timings, as a result of reduced nitrate leaching losses, says Mr Williams.
Data from the Waste and Resources Action Programme’s (WRAP) Digestate and Compost in Agriculture (DC-Agri) project (www.wrap.org.uk/content/digestate-and-compost-agriculture-dc-agri) indicate that crop available nitrogen supply of food-based digestate to cereal crops was 55% of total nitrogen for spring applications compared with 15% for autumn applications.
Using dribble bars to apply digestate helps reduce ammonia emissions and it is important to apply it evenly, says Mr Williams.
“Dribble bars allow applications to growing crops in spring and umbilical systems help prevent soil damage by reducing the weight of application machinery. Flow meters are very valuable to provide an accurate measure of how much you are applying.”
Uneven applications can lead to high nutrient loading in some parts of a field and low nutrient loading in others, while over-application can lead to lodging and environmental losses, he warns.
“A sensible application rate for food-based digestate is 20-25cu.m/hectare – maybe a bit higher for crop-based digestate. It is important to match application rate and nutrient supply to crop need.”
Mr Williams calculates that two, 25cu.m/ha applications of food-based digestate in spring would supply about half of the typical nitrogen requirement of a winter wheat crop.
As well as nitrogen, small amounts of P, K and S are also available from digestate, typically 1kg/cu.m, 2kg/cu.m and 1kg/cu.m, respectively, for food-based material and slightly higher levels from farm-based digestate, says Mr Williams.
However, for growers looking to boost soil organic matter levels, there are better choices, he says.
“If using whole digestate or the liquid fraction the amount of organic matter is low. If you are looking to build organic matter up it is better to use farmyard manure, compost or biosolids.”
Ensuring digestate is of good quality is important to avoid contamination issues.
According to RB209, digestate that is certified under the Biofertiliser Certification Scheme and farm-sourced digestate does not normally need an environmental permit or exemption to be in place for application to land.
Applications of non-certified digestate from non-agricultural sources must be applied to land under the Environmental Permitting Regulations. The Biofertiliser Certification Scheme sets baseline quality specifications, set by the British Standards Institution Publicly Available Specification 110 (PAS110).
Mr Williams says: “If it is PAS110-certified you can have confidence the material is of good quality. It has to be pasteurised, there has to be segregation of plastics and there are controls on the types of feedstock used.
“If you are going to take it onto your land, why not visit the site to understand how the process works and the measures that are put in place to ensure good quality digestate is produced.”
PAS110-certified material will also include a nutrient analysis of the digestate and for non-certified digestate, it is important to get one, says Mr Williams.
Where digestate is being kept on-farm, safe storage is crucial to avoid it entering watercourses, he says.
“It also has a high ammonium content, so covered stores are best to reduce the risk of ammonia emissions.”
According to RB209, with 80-90% of nitrogen in whole and liquor digestate in the readily available form, applications are subject to closed-spreading periods in NVZs. In separated fibre (typically 25% dry matter), usually less than 30% of the total nitrogen is in the readily available form and applications would not be subject to closed spreading periods in NVZs, but it is important to have the fibre analysed to check the readily available N levels.
Source: RB209 and John Williams, ADAS
A typical 2MW crop-fed AD plant will typically take 35-40,000 tonnes of crop/annum and produce 22,000t of digestate, about 18,000t of liquid and 4,000t of solid, according to Innes McEwen, head of farming at Future Biogas.
The company’s 11 plants are predominantly crop-fed, with a small percentage of manures but no food waste. Digestate is separated into the liquid and solid forms for more efficient storage, he says.
Taking into account the value of first year available nutrients, calculated at £5.20/cu.m, spreading liquid digestate within four miles of the plant is cost neutral or beneficial at a lower mileage, assuming a haulage rate of £3.10/cu.m and spreading cost of £2.10/cu.m, says Mr McEwen.
The recent increase in fertiliser prices mean its nutrient value is closer to £5.80/cu.m, he adds.
Good communication between farmer and AD plant operator is important, he says. “If, for example, rainfall has diluted the digestate, the farmer should be informed so they can adjust application rates accordingly.”
Mr McEwen says he knows of a farm in Norfolk where maize has been grown solely using digestate with two applications of 35cu.m/ha each.
“We tried strip tillage and made a channel of digestate then drilled maize seed above it. We used a dribble bar over the growing crop which worked ok.”
He also reports interest from the organic sector.
“They may be willing to pay for it to travel further, but we need to be sensible.”
James Astor, chairman of Regen Holdings, has one food waste AD plant near Kettering and provides feedstock and services to about 20 more.
While the nutrient content of digestate varies between plants, N content is typically 5kg/cu.m and the value of digestate in terms of overall nutrients is about £5/cu.m, he says.
Although arrangements between AD operators and farmers vary, he says an example of an agreement is an AD plant supplying and applying digestate free in the first year and in year two typically reaching a deal with the farmer who may pay up to £3/cu.m at certain times of year, with greater demand for digestate in March/April for growing crops than in autumn for application to stubble, says Mr Astor.
“It is a dynamic market which takes a while to establish. It is ideally spread within a few miles of the plant. I am aware of some travelling over 30 miles but this is not cost-effective.”
There are currently 600 AD plants in UK.
Restoration of higher tariff levels for the Renewable Heat Incentive was announced by the Government in May, with the green gas industry forecasting that as many as 40 AD plants may be built over the next two years as a result of this, according to the Anaerobic Digestion and Bioresources Association.
However, James Astor, chairman, Regen Holdings, says growth is more likely to be in the crop-based rather than food waste sector because, at a local level, there is competition between plants for feedstock in the latter case.