What is Digestate? The Difference Between Digestate and Slurry and How UK Farms Store Both
Digestate is one of the most valuable yet least understood byproducts in UK agriculture. As anaerobic digestion becomes an increasingly common part of farm operations and renewable energy strategies across Britain, more land managers, farmers, and AD operators are handling digestate for the first time and discovering that its storage and application requirements are more specific than they might expect.
Understanding what digestate is, what it contains, how it differs from slurry, and what the law requires for its storage is not just a regulatory matter. It directly affects soil health, crop yield, and the commercial returns from an anaerobic digestion system. This guide covers everything UK farms and AD operators need to know, including how to choose the right tank for storing both digestate and slurry safely and compliantly.
What is Digestate?
Digestate is the nutrient-rich material that remains after organic matter has been broken down through anaerobic digestion. Anaerobic digestion is a biological process in which microorganisms break down organic materials in the absence of oxygen, inside a sealed vessel called a digester or anaerobic digester. The two main outputs of the anaerobic digestion process are biogas, a mixture of methane and carbon dioxide used to generate renewable energy and renewable natural gas, and digestate, the residual material left behind once the digestion process is complete.
Digestate is derived from the feedstock fed into the anaerobic digester. Feedstocks commonly used in UK AD systems include animal manure, food waste, crop residues, sewage sludge, and purpose-grown energy crops. The exact composition of the digestate reflects the feedstock used, the hydraulic retention time within the reactor, and the efficiency of the AD process itself.
Digestate is a valuable material and digestate is a valuable organic fertiliser rather than a waste product. It contains valuable nutrients including nitrogen, phosphorus, and potassium, the three primary nutrients required for crop growth. The nutrient content and overall soil health benefits of digestate make it a direct replacement for synthetic fertilisers on many UK farms. When applied correctly, digestate can improve soil structure, increase soil organic matter, and deliver measurable benefits to overall soil health. The use of digestate as an organic fertiliser also supports recycling of nutrients such as nitrogen and phosphorus back into the agricultural system, reducing reliance on energy-intensive manufactured inputs.
What Does Digestate Contain?
The nutrient content and physical composition of digestate vary depending on the feedstock and AD system used, but digestate contains valuable nutrients across the following categories:
Nitrogen: available in both ammoniacal and organic forms. The nutrients such as nitrogen in digestate are directly plant-available and comparable to synthetic fertilisers in immediate crop uptake effect.
Phosphorus and Phosphate: digestate contains significant phosphorus and phosphate levels. The phosphate content must be accounted for in nutrient management planning under UK farming rules.
Potassium: digestate is nutrient-rich in potassium, making it especially valuable for grassland and arable cropping systems.
Organic Matter: the organic materials remaining after digestion contribute to soil organic matter when applied to land, improving soil structure, water retention, and microbial activity over time. The effects on soil from repeated digestate application include improved water holding capacity and a measurable improvement in overall soil health.
Dry Matter and Fibre: the dry matter content determines whether digestate separates into liquid and solid fractions. Digestate fibre, found in the solid portion, contributes to longer-term organic matter additions to the soil. Liquid digestate releases its nutrients more rapidly.
Methane Potential and Biogas Production: while the majority of methane and biogas is captured during the AD process, the portion of the digestate that remains can still have residual biogas production potential. Covered storage is important for capturing any residual emissions and reducing the carbon footprint of the operation.
Digestate can also contain trace heavy metals depending on the feedstock composition. This is why digestate quality and compliance with PAS 110 matters for operators intending to use digestate as a certified biofertiliser.
Liquid Digestate and Solid Digestate: What is the Difference?
After leaving the anaerobic digester, digestate is often separated into two fractions. Understanding liquid and solid digestates and their different characteristics is important for storage planning, nutrient management, and land application decisions.
Liquid digestate is the high-moisture, low-dry-matter fraction. It contains the majority of the readily available ammoniacal nitrogen, making it the most immediately crop-available portion of the digestate. Liquid digestate is typically spread on land using a tanker or trailing shoe applicator and is the most common form used directly as an organic fertiliser replacement on UK arable and grassland farms.
Solid digestate is the fibrous, higher-dry-matter fraction. The digestate fibre it contains releases nutrients more slowly and contributes more significantly to long-term soil organic matter improvement. Solid digestate is often composted or used as a soil conditioner. It is easier to handle and transport than liquid digestate and can be stored in a covered clamp rather than a liquid storage tank.
Whole digestate, which has not been separated, contains both liquid and solid fractions and requires larger storage volumes than separated liquid digestate alone.
What is Slurry? How Does it Differ from Digestate?
Slurry is a mixture of animal excreta, urine, water, and small amounts of bedding material produced directly on farm from housed livestock. It is collected under slatted floors or scraped from yards and stored before being applied to land as an organic fertiliser.
The distinction between slurry and digestate matters for both practical and regulatory reasons.
Slurry has not been through any digestion process. It retains a high proportion of its original organic matter, contributing more bulk organic matter to soil over time. Slurry nitrogen is present in both ammoniacal and organic forms, with a higher proportion remaining in organic form compared to well-processed liquid digestate. Slurry also has a significantly more variable composition, and biogas production from exposed slurry contributes to methane emissions if the tank is uncovered.
Digestate has been through the AD process. The anaerobic digestion process converts a greater proportion of nitrogen into ammoniacal nitrogen, making digestate faster acting and more predictable as a nutrient source. The digestion process also reduces the volume of organic matter, pathogen levels, and odour. Emission reductions from using digestate rather than raw slurry are a recognised environmental benefit of the AD system.
Both materials require adequate on-farm storage under the Silage, Slurry and Agricultural Fuel Oil (SSAFO) regulations, and both must be applied to land in accordance with the Farming Rules for Water and, in nitrate vulnerable zones, under the Nitrates Regulations 2015.
Both digestate and slurry storage are governed by the Silage, Slurry and Agricultural Fuel Oil (SSAFO) Regulations, last updated in 2010. These regulations set minimum standards for the construction, capacity, and maintenance of storage systems for both materials.
Under SSAFO, slurry storage systems must store a minimum of four months of production with no drainage to watercourses permitted. In practice most agronomists recommend five to six months of storage capacity to allow flexibility around weather and land application windows.
For digestate, the Environment Agency classifies whole digestate and separated liquid digestate as liquid organic manures under the SSAFO framework. This means the same structural and capacity requirements apply. The ad system used must be complemented by a compliant storage solution; the benefits of AD are only fully realised when the digestate produced can be stored safely until the correct application window.
For digestate that meets the PAS 110 quality standard, a different regulatory position applies. Certified digestate is classified as a manufactured fertiliser rather than a waste product, removing it from the waste management regulatory framework entirely. This is a significant compliance and commercial advantage for AD operators managing large volumes. The ad plane of compliance, from digester to land application, must be documented and maintained to retain this status.
Failure to comply can result in enforcement action from the Environment Agency, unlimited fines, and criminal prosecution in the most serious cases involving water pollution.
How UK Farms Store Digestate and Slurry: Tank Solutions
Choosing the right storage tank is one of the most consequential infrastructure decisions on any farm or AD site. The tank must be structurally sound, chemically compatible with the stored material, sized for the required storage period, and compliant with SSAFO requirements.
Corrugated Steel Slurry and Digestate Tanks
Our corrugated steel slurry and digestate tanks are manufactured from Magnelis coated steel, offering significantly greater corrosion resistance than standard galvanised steel. Tanks are available from 280m3 to 1,250m3, certified to British and European structural standards including BS 5502, and suitable for whole digestate, liquid digestate, and livestock slurry. Unlike fixed concrete tanks, modular corrugated steel tanks can be installed without extensive groundworks and can be expanded or relocated as storage requirements change.
Choosing the Right Liner
The liner is the critical component that makes any corrugated steel tank suitable for digestate or slurry storage. Our liner kits are manufactured in-house:
Landflex ES: the recommended liner for digestate and slurry storage, with outstanding resistance to ammoniacal nitrogen, acids, and biogas byproducts. This is the standard specification for our slurry and digestate tanks.
Butyl Rubber: the premium liner for clean water storage on the same site, suitable for irrigation tanks used alongside AD systems.
HDPE: used where the stored liquid has a high solids content requiring abrasion resistance and frequent internal cleaning.
Tank Covers and Roof Kits
Covering a digestate or slurry tank delivers important operational and compliance benefits. An uncovered tank emits ammonia, increases nitrogen losses, contributes to greenhouse gas emission, and can breach permit conditions. Rainfall entering an uncovered tank increases storage volume rapidly.
Our roof kits include heavy-duty steel roof covers and the Aqua-Float floating cover, which sits directly on the liquid surface, preventing up to 98% of evaporation and dramatically reducing ammonia emission and odour. All covers include lockable access hatches for safe inspection and agitation access.
Using Digestate as a Fertiliser: Benefits and Best Practice
The land application of digestate is governed by the Farming Rules for Water, the Nitrates Regulations in nitrate vulnerable zones, and the specific conditions of any Environment Agency permit held by the AD operator.
The benefits of AD and the use of digestate as a fertiliser include a reliable nitrogen supply that reduces dependence on synthetic fertilisers, improved soil structure and organic matter over repeated applications, phosphorus and potassium inputs that reduce mineral fertiliser requirements, reduced greenhouse gas emission compared to leaving organic waste untreated, and recycling of nutrients to the soil that would otherwise be lost. The effects on soil from consistent digestate application over several seasons include measurable improvements in soil conditioner properties, improve soil structure, and overall soil health.
Digestate application timing is critical. Liquid digestate applied in spring ahead of crop uptake delivers the highest nitrogen use efficiency and the best crop yield outcomes. Application outside of growing periods, in waterlogged conditions, or on steeply sloping land increases the risk of runoff and must be avoided.
Frequently Asked Questions About Digestate
What is digestate in simple terms?
Digestate is the nutrient-rich material left behind after organic matter has been broken down in an anaerobic digester. It is a byproduct of the anaerobic digestion process and is used as an organic fertiliser on agricultural land because it contains valuable nutrients including nitrogen, phosphorus, and potassium. Digestate is a valuable alternative to synthetic fertilisers and supports nutrient recycling across the agricultural system.
Is digestate the same as slurry?
No. Slurry is raw livestock excreta and washings collected directly on farm. Digestate is the residual material produced by the anaerobic digestion of organic feedstocks. Both require SSAFO-compliant storage, but digestate has a higher proportion of available nitrogen, lower odour, reduced pathogen levels, and lower greenhouse gas emission potential compared to untreated slurry.
What are liquid and solid digestate?
Liquid digestate is the low-dry-matter fraction produced after mechanical separation of whole digestate. It is fast acting and nitrogen-rich, applied to land by tanker or trailing shoe. Solid digestate is the fibrous, higher-dry-matter fraction used as a soil conditioner or composted for longer-term organic matter and digestate fibre addition to the soil.
Do I need a permit to spread digestate?
It depends on whether your digestate meets the PAS 110 quality standard. Certified digestate is classified as a manufactured fertiliser and is not subject to waste management regulations. Non-certified digestate requires either an exemption registration or an Environmental Permit from the Environment Agency for land application.
What size digestate storage tank do I need?
Under SSAFO regulations, you must store a minimum of four months of production. The right tank size depends on your AD system output, the separation ratio of liquid to solid digestates, and your intended spreading schedule. Our team provides free site surveys and can calculate the correct storage capacity for your operation.
Can the same tank store both digestate and slurry?
Yes, provided the tank is correctly specified for both materials. A Landflex ES lined corrugated steel tank is suitable for both liquid digestate and livestock slurry and can be sized to accommodate the combined storage requirement of a mixed farm AD system. Contact our team to discuss combined storage solutions.
Conclusion
Digestate is a valuable organic fertiliser and a natural byproduct of anaerobic digestion, but handling it correctly requires a clear understanding of what it contains, how it differs from slurry, and what UK law requires for its storage and application. Getting this right protects your land, watercourses, compliance position, and the agronomic returns from your AD investment.At Butek Tanks, we have been manufacturing corrugated steel storage tanks since 1965. Our slurry and digestate tanks, liner kits, and roof kits provide complete, SSAFO-compliant storage solutions for farms and AD operators across the UK. Contact our team today to arrange a free site survey.