Digestate: how to turn biogas plant waste into profit

A biogas plant can generate revenue not only from the sale of electricity or biomethane. Anaerobic fermentation leaves behind digestate — a substance that can be used as organic fertiliser or processed into a product with higher added value. For agribusiness, this means lower costs for mineral fertilisers, and for the biogas plant owner — a further economic benefit from the same raw material. 

In this article, the Pro-Energy team will look at what digestate is, how much of it is produced, and how to use, store and sell it. 

What is digestate?

Digestate is the fermented residue remaining in the reactor after biogas production. It is the result of the anaerobic fermentation of organic feedstock — manure, silage, food waste or other biomass. Digestate retains a significant proportion of the nutrients contained in the original feedstock, in particular nitrogen, phosphorus and potassium. During anaerobic digestion, part of the organic matter is converted into biogas and used to generate electricity or biomethane, whilst the rest remains in the digestate.

Digestate fractions

The digestate is separated into two fractions:

• Liquid fraction — 90–95 per cent water containing dissolved nutrients, nitrogen and potassium.
• Solid fraction — 20–30 per cent of dry matter, consisting of organic matter and fibres, which slowly release nutrients into the soil.

The separation is carried out using a separator, and each fraction is then used or sold separately.

How much digestate does BGS produce?

This amounts to approximately 90–95 per cent of the mass of the raw materials fed into the system. For example, a biogas plant on a livestock farm with a capacity of 500 kW processes 30–50 tonnes of raw material per day and produces 28–47 tonnes of digestate daily. Over the course of a year, this amounts to 10,000–17,000 tonnes — a volume that requires a clear disposal plan to be in place as early as the plant’s design stage.

Composition and nutritional value of digestate

The exact composition of the digestate depends on the raw materials, the substrate formulation and the fermentation parameters. It is therefore advisable to analyse it regularly before applying it or selling it.

Indicative figures might look like this:

The exact composition always depends on the raw materials fed into the reactor — this should be taken into account when planning production or sales.

Advantages over manure

Digestate has several advantages: it produces less odour, as the organic matter has already decomposed; it contains no weed seeds, which are destroyed during fermentation; and it contains no pathogens, which are killed at temperatures of 38–55°C. The nitrogen in digestate is in a form that is more readily available to plants, and its consistency is uniform — this makes it easier to apply using machinery.

Advantages over mineral fertilisers

Unlike mineral fertilisers, digestate contains organic matter, which improves soil structure, a complex of trace elements, and is released slowly, meaning it is less likely to be washed away by rain. At the same time, it is cheaper — and sometimes even free for your own farm — and is a more environmentally friendly option.

Ways to use digestate

There are four basic models: spreading on one’s own fields, selling without further processing, composting the solid fraction, and producing pellets.

Method 1: Application to fields (liquid fraction)

The liquid fraction is applied using tankers or hose systems at a rate of 20–40 tonnes per hectare. The best time to apply it is before ploughing or after harvest. It is important to incorporate the digestate into the soil within 24 hours to prevent nitrogen loss. This method is suitable for cereals, maize, sunflowers, rapeseed and perennial grasses. Digestate must not be applied to frozen soil or before rain due to the risk of leaching, and a safe distance from water bodies must be maintained.

Method 2: Composting (solid fraction)

After separation, the solid fraction contains 25–30 per cent dry matter. Composting takes 2–3 months, resulting in a free-flowing, odourless compost. This product is easier to store and transport, commands a higher price when sold, and is even suitable for retail packaging. Compost made from digestate is in demand in vegetable growing, horticulture, greenhouse farming and landscaping.

Method 3: Drying (pelletising)

The solid fraction can be dried using heat from a combined heat and power plant and converted into pellets measuring 4–8 mm with a dry matter content of up to 90 per cent. This is the most expensive product — €80–150 per tonne — and is convenient for storage, transport and precise dosing, with a long shelf life. The downside is the high capital expenditure on the dryer and pelletiser, and the energy intensity of the process.

Method 4: Selling digestate without further processing

The simplest way is to sell the product exactly as it comes out of the station.

Potential buyers could include neighbouring farms, greenhouse complexes, horticultural businesses and producers of organic fertilisers.

Approximate price ranges for this model:

These figures should be used as guidelines for a preliminary economic model, rather than as a guaranteed market price. Before investing, it is necessary to check local demand, logistics and product requirements.

The economics of digestate

Option 1: Own use

The economic benefit of using digestate on one’s own farm stems from the partial replacement of mineral fertilisers. The actual nutritional value depends on the composition of the raw materials and the fermentation process parameters; therefore, the digestate must be analysed before application.

For a preliminary economic assessment, one can estimate savings of €5–15 per tonne of digestate by replacing a portion of mineral fertilisers. If a 500 kW biogas plant produces and uses approximately 15,000 tonnes of digestate per year, the potential savings would amount to €75,000–225,000 per year.

This is an indicative calculation. Actual savings will depend on the composition of the digestate, the nutrient requirements of specific crops, application rates, the cost of mineral fertilisers, as well as the costs of storage, transport and application.

Option 2: Selling the liquid fraction

At an estimated price of €2–5 per tonne and sales of 12,000 tonnes per year, gross revenue will amount to €24,000–60,000.

But there is an important caveat here: the liquid fraction contains a lot of water. The further away the buyer is, the larger a proportion of the price may be ‘eaten up’ by transport costs.

That is why location is crucial for this type of product. A neighbouring farmer may be a better customer than a buyer offering a higher price but located hundreds of kilometres away.

Option 3: Selling compost

If, following separation and composting, approximately 3,000 tonnes of finished product are produced per year, and the selling price is €30–60/t, gross revenue could reach €90,000–180,000. However, this figure must be calculated after deducting the costs of separation, composting, site hire, loading, storage and delivery.

Option 4: Selling pellets

Granulated fertiliser has a higher potential price, but also requires significantly greater investment. In the given scenario, approximately 1,500 tonnes of granules can be produced from 15,000 tonnes of feedstock. At €80–150/t, this amounts to €120,000–225,000 in gross revenue per year. If the equipment costs €200,000–500,000, it is necessary to take into account not only the revenue but also the payback period of the production line: energy consumption, maintenance, staff, packaging and sales.

So, the ‘most expensive’ product is not always the best value for money. For a particular business, the optimal model may be completely different.

Certification and requirements for digestate

The requirements for digestate depend on how it will be used further. If it is used on the producer’s own fields, the key issues are its composition, safety, and compliance with the requirements for the application of fertilisers and soil improvers. If the digestate is intended for systematic sale or export, additional consideration is given to its quality, documentary confirmation of its characteristics and, depending on the target market, conformity assessment procedures.

Requirements in Ukraine

Ukrainian legislation contains a separate definition of “digestate produced in biogas plants”. At the same time, the requirements for the state registration of pesticides and agrochemicals do not apply to such digestate if it is used as an organic fertiliser or soil improver. Therefore, the statement that the state registration of digestate as a fertiliser is mandatory is incorrect.

For quality control of digestate, it is advisable to conduct a laboratory analysis of its composition and properties. Depending on the feedstock and intended use, the content of key nutrients, including nitrogen, phosphorus and potassium, as well as other safety parameters, may be determined.

Requirements for entering the EU market

For the sale of digestate in the EU, the requirements depend on the product’s composition, the feedstock used, the processing technology and the chosen market-entry model. Regulation (EU) 2019/1009 provides for separate categories of component materials for digestate — CMC 4 (Fresh Crop Digestate) and CMC 5 (Digestate Other Than Fresh Crop Digestate).

If the product is intended to be placed on the market as an EU fertilising product, it must comply with the requirements established by the Regulation, and the manufacturer must undergo the prescribed conformity assessment procedure. If compliance is confirmed, the product may bear the CE marking in accordance with the requirements of the Regulation.

Therefore, CE marking is not a universal requirement for any digestate. It applies when the product is placed on the market as an EU fertilising product in accordance with Regulation (EU) 2019/1009.

If digestate is used in organic production

The use of digestate in organic production needs to be considered separately. The mere fact that it is produced in a biogas plant does not automatically mean that it acquires the status of an organic fertiliser. For use in organic production, the requirements of organic production legislation and the rules on permitted fertilisers and soil-improving substances must be met.

Digestate storage

The problem of storage arises due to the seasonality of agricultural use. A biogas plant operates year-round, while fertilisers can only be applied to fields during the appropriate periods.

Lagoons for the liquid fraction

The volume of the lagoon is calculated for 3–6 months of production. Mandatory covering with a film reduces evaporation and odour, while waterproofing protects groundwater from contamination.

Storage areas for the solid fraction

The solid fraction is stored on a concrete site under a shelter, with drainage for collecting the liquid that runs off.

Seasonality of application

The main application takes place in spring and autumn; in summer, it is limited due to crops, while in winter, it is prohibited because of frozen soil. Therefore, storage capacity is required for at least 4–6 months.

Equipment for digestate processing

A separator can serve as a basic element. It separates digestate into liquid and solid fractions. Depending on the project, different types of equipment are used, including screw or drum separators.

The indicative cost of a separator in the specified model is €30–80 thousand.

Composting requires a properly equipped site and equipment for mixing/turning. The indicative investment is €50–150 thousand.

A pelletising line requires a significantly larger amount of equipment: a dryer, granulator, cooler, conveying and packaging systems. The indicative range is €200–500 thousand.

At the same time, the integration of drying with cogeneration is particularly interesting: the heat already generated during electricity generation can be used as a technological resource.

Typical Mistakes When Working with Digestate

Most often, the problem arises not because of the technology itself, but because of a lack of planning.

1. No utilisation plan — digestate accumulates, resulting in environmental fines.
2. Insufficient lagoon capacity — overflow and emergency discharges.
3. Application to frozen soil — fines and water pollution.
4. Exceeding application rates — soil contamination with nitrates.
5. Lack of analyses — unknown composition and ineffective application.
6. Sale without documentation — legal problems with regulatory authorities.

Digestate as an Additional Source of Profit

Digestate is not something that simply needs to be removed from a biogas plant. When properly managed, it can replace part of mineral fertilisers, be sold as an organic product, or be processed into compost or pellets with higher added value.

However, the result depends on the composition of the feedstock, land bank, logistics, technology and target market. Therefore, the digestate management model should be designed simultaneously with the biogas plant, rather than after its launch.

Planning a biogas or biomethane project? Order a consultation with Pro-Energy specialists — we will assess the potential of your feedstock, digestate utilisation options and the economics of the future project.