What is Digestate? Find Out More About This Natural Fertilizer

There is a simple answer to the question of what Digestate is. Digestate is what remains after anaerobic digestion (biogas digestion), and it is comprised of the solid fibrous material fraction emanating from the AD bio-reactor, and the liquid portion.

Anaerobic digestion draws carbon, hydrogen, and oxygen from the feedstock. However, the essential plant nutrients (N, P, and K) remain largely in the digestate. This means that digestion is a quite remarkable process as it is accomplished without substantially reducing the potential fertilising properties of the feedstock material.

It’s like being able to take a quart out of a pint pot! In effect, by using AD, we are extracting largely just the energy and leaving the quantity of plant nutrients largely unaffected.

Key Takeaways About “What is Digestate?”

• Digestate is the nutrient-rich material left after anaerobic digestion. It contains both a fibrous solid fraction and a liquid fraction after biogas has been produced.
• Most plant nutrients remain in digestate. Nitrogen, phosphorus, and potassium (N-P-K) are largely retained during digestion, allowing digestate to function as an effective fertiliser.
• Two main types of digestate are produced. The solid content of the slurry as it leaves the digester tank is fibrous and rich in organic matter, while the liquid portion of the digestate contains soluble nutrients such as ammonium and phosphate.
• Nutrients become more available to plants. Anaerobic digestion mineralises nutrients, increasing the proportion of plant-available forms such as ammonium nitrogen.
• Digestate can improve soil structure. The organic material improves soil friability, moisture retention, and overall soil health when applied at appropriate rates.
• Digestate reduces odour compared with raw manure. Anaerobic digestion can reduce odour nuisance by around 80%, making digestate more acceptable for land application.
• Digestate supports sustainable nutrient recycling. Using digestate closes the nutrient loop by returning valuable nutrients and organic matter from waste streams back to agricultural soils.
• Quality depends heavily on the feedstock. The nutrient content and contamination risks vary depending on whether the digestion feedstock is agricultural waste, food waste, sewage sludge, or mixed municipal waste.
• Contaminant control is essential. Digestate quality management must address potential pathogens, heavy metals, plastics, glass, and other physical impurities.
• Standards such as PAS110 help ensure digestate quality. In the UK, compliance with end-of-waste protocols allows digestate to be marketed as a certified product rather than treated as waste.

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Digestate Composition

The composition of the fertilising agents in a digestate depends to a large extent on the feedstock, and can, therefore, vary substantially. However, the availability of nutrients is invariably higher in digestate than in untreated organic waste.

Nutrients (N-P-K) are mineralised during digestion, which allows improved plant uptake compared to untreated organic waste. For instance, digestate has 25% more accessible NH4-N (inorganic nitrogen) and a higher pH value than untreated liquid manure.

Another advantage of the digestion stage is that it reduces the odour nuisance by about 80%. The use of digestate also benefits the humus balance in the soil. Thus, it can be used as a fertiliser or a soil amendment in agriculture, landscaping, parks, and, if suitably free from plastic and glass, from PAS110 certified sources, even in gardens.

The use of digestate manure as a fertiliser permits the creation of a highly sustainable nutrient cycle and maintains or improves soil structure due to the application of organic matter.

Uses

Digestate is usually described as a soil conditioner. When applied to soils at an optimum rate it has been shown by research to improve soil structure. There is also evidence that the microorganisms which also remain after the digestion process which get transferred into the soil can increase the levels of beneficial bacteria in soils. This is only beneficial when the digestate is added to soils in which the healthy microorganisms have become depleted due to what many people consider to be intensive farming and chemical usage.

Acidogenic digestate has been shown to provide:

• improved moisture retention and
• an increased organic content.

It can also perform a function to protect soils against erosion.

The added organic material in digestate will break down further, aerobically in the soil, releasing additional nutrients.

Methanogenic digestate provides nutrients for plant growth.

Fibrous digestate can also be used as:

• an eco-friendly filler to provide improved structural characteristics to composite plastics
• a board with insulating properties for use in the building industry
• a cattle litter for stock held indoors.

Mixed waste digestate was used in growth trials and was shown to promote healthy growth results for crops, with a surprising potential for improving crop disease resistance.

In fact, when applied to plant leaves, it has been shown to inhibit some plant diseases in a mechanism that appears to induce resistance.

Soil-borne diseases can also be reduced, and general stimulation of biological activity in the soil has been reported as an indirect effect of optimum digestate application.

Advantages and Benefits from the Application of Digestate to Land

So, anaerobic digestion is nowadays seen as a method to treat the organic wastes and provide a useful natural fertiliser, which is the digestate, and in the UK moves are underway to ensure that it can be sold as a product after producers follow applicable “end-of-waste” protocols.

(Image Ref: Pahl (1999). “Environmental Factors in Pig Production – Description of potential emissions, causes, abatement, and legislation”.)

However, in order to extract the maximum recovery value from these wastes, the digestate should demonstrably have a useful purpose, and benefit should be derived from its production. No specific research on the benefits of AD digestate has yet been published (written in 2009).

However, there have been WRAP-funded studies carried out by what was then the Composting Association. Those studies have been published and show crop yield data for different application rates and seasons of application, which conclusively show the benefits of compost, which comprise:

1. a more even release of N than manures over time
2. soil structure improvements in improved friability and moisture retention
3. raised carbon content within the soils by a significant but not large amount

The above was shown to be reflected in increased crop yield and reduced watering requirements/improved resilience to drought.

Its main advantage is that it has a high nutrient content. Its quality should be acceptable for purposes such as soil amendment or landscaping.

Digestate quality can be assessed on 3 criteria:

1. chemical,
2. biological and
3. physical aspects.

The chemical aspects of quality management of digestate are related to the presence of:

• nutrients (NPK)
• heavy metals and other inorganic contaminants
• persistent organic contaminants.

Potential Contaminants

Agricultural wastes can contain persistent organic contaminants such as pesticide residues or antibiotics.

Industrial organic waste, sewage sludge, and household waste digestate can contain aromatic, aliphatic and halogenated hydrocarbons, PCBs, PAHs, etc. in trace concentrations, but nevertheless at a concentration still capable of being considered hazardous in some circumstances.

According to their origin, organic waste can contain hazardous materials (vectors), which can result in new routes of transmission of pathogens and diseases between animals, humans, and the environment. The quality control of this type of biomass is therefore essential in relation to the biological treatment.

The main problems are related to these:

• pathogens
• seeds
• transmissible spongiform encephalopathy (TSE).

Soils experts like to lump all but the first listed of these together and call them PTEs (Potentially Troublesome Elements).

The possible presence of the above impurities in the digestate, and more obviously the presence of visible small pieces of waste materials (e.g., glass and plastics) can cause a negative public perception of the AD technology.

Digestate users must take care to preserve the wholesome appearance of the soils where digestate is applied. Aesthetic damage to the environment must be avoided, and unfortunately, these considerations will tend to increase the operational costs for digestate use.

The most frequent physical impurities are:

• plastic and rubber
• metal
• glass, ceramic, sand and stones
• cellulosic materials (wood, paper, etc.).

Once again, the physical contamination of the digestate inevitably depends upon the nature of the feedstock (wastes), the nature of the pre-treatments applied, and the biological treatment (digestion) itself.

This brings us back to the issue of the manner in which the material is collected. For the digestion of MSW, source segregation is more efficient than mixed collection, because mechanical pre-treatments are not as effective in removing contaminants as pre-sorting of waste types by householders, which provides the best elimination of potential contaminants at source.

The Special Case of Pig Farm Manure Feedstock Digestate

In the case of digesters at pig farms, or digesting pig farm manure, copper is at times added to the food used in the piggery to accelerate growth by increasing food conversion rates. Zinc is added for the same purpose and to counteract the toxicity caused by high copper concentration.

These heavy metals will be found in the pig slurry; it is important to monitor their levels in the digestate, because the reduction of heavy metals after digestion is not feasible, and therefore, limits have to be applied and respected in each country. (See http://www.epa.ie/pubs/advice/brefs/intensive%20agriculture.pdf and Table 39 – Heavy Metal Concentrations in Slurry and Dry Manure. ).

The European Commission has suggested limit values for metals composts and digestates in the Working Document on Biological Treatment of Biodegradable Waste (See the Working document: biological treatment of biodegradable waste (second draft), European Commission, 2001. Subsequently, limits for the UK have been set for PAS100 composts, and within PAS110 for AD.

Frequently Asked Questions (FAQs)

What is digestate?

Digestate is the material that remains after organic matter has been processed through anaerobic digestion to produce biogas. It consists of a liquid fraction and a fibrous solid fraction containing nutrients and organic matter.

Is digestate a fertiliser?

Yes. Digestate contains valuable nutrients such as nitrogen, phosphorus, and potassium (N-P-K), making it an effective organic fertiliser and soil conditioner when applied correctly.

What is the difference between acidogenic and methanogenic digestate?

Acidogenic digestate is mainly fibrous and rich in structural plant material such as lignin and cellulose. Methanogenic digestate is typically a liquid or slurry containing soluble nutrients like ammonium and phosphates.

How does digestate improve soil quality?

Digestate improves soil structure, increases organic matter content, enhances moisture retention, and can stimulate beneficial microbial activity in soils.

Does digestate smell like manure?

No. Anaerobic digestion significantly reduces odour. Digestate typically produces around 80% less odour than untreated organic wastes or raw manure.

Can digestate contain contaminants?

Yes. Depending on the feedstock, digestate may contain heavy metals, pathogens, pesticide residues, plastics, glass, or other impurities. Quality control and feedstock management are essential.

Why is source separation important for digestate quality?

Separating organic waste at the source reduces contamination by plastics, metals, and other materials that are difficult to remove during mechanical pre-treatment.

What standards regulate digestate quality in the UK?

In the UK, digestate quality is commonly regulated under the PAS110 standard, which ensures the material meets requirements for safe agricultural use and end-of-waste status.

Can digestate increase crop yields?

Research shows that digestate can improve crop yields due to its nutrient content and its positive effects on soil structure and biological activity.

Are there risks associated with digestate application?

Potential risks include nutrient over-application, heavy metal accumulation, pathogen transmission, and contamination from physical impurities. Proper testing, standards compliance, and responsible application rates help mitigate these risks.

[Published July 2020. Updated March 2026.]