FAQ: Digestate and fertilisers

Yes, the solid fraction of digestate can be composted, which reduces odour nuisance, improves structure and can make its management easier.

Composting requires the addition of structural material (e.g. straw, wood chips), aeration and temperature control. The result is a stable product that, after legal requirements are met, can be an organic fertiliser or soil improver. Nitrogen losses during composting are significant unless the process is well managed. For waste, permits and quality tests apply.

Sources: Rozporządzenie (UE) 2019/1009 — produkty nawozowe UE — EUR-Lex

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Technically yes, but ammonia and methane emissions are then highest, and the requirements for tightness, distances and covering may be stricter. A closed tank with gas recovery is the preferred solution.

Sources: FNR — Fachagentur Nachwachsende Rohstoffe; KTBL — Kuratorium für Technik und Bauwesen in der Landwirtschaft; Dyrektywa Rady 91/676/EWG (azotanowa) — EUR-Lex

Discussed in the article: Digestate storage: tanks, lagoons, covers and required capacity

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The amount of total nitrogen per unit of mass is similar or slightly higher (most of the nitrogen stays in the digestate, while carbon escapes as biogas), but the share of ammonium nitrogen is higher, so the nitrogen is available to plants faster and is more prone to volatilisation.

Sources: FNR — Fachagentur Nachwachsende Rohstoffe; KTBL — Kuratorium für Technik und Bauwesen in der Landwirtschaft; Rozporządzenie (UE) 2019/1009 (produkty nawozowe UE) — EUR-Lex

Discussed in the article: Digestate composition: nitrogen, phosphorus, potassium, sulphur and organic matter

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Digestate may contain some of the microorganisms present in the feed, and their number depends on temperature, retention time and the type of substrates; for waste, hygienisation is sometimes required.

Thermophilic digestion reduces pathogens more effectively than mesophilic. For slurry and agricultural substrates the risk before digestion is similar to that of raw slurry, and after digestion it is sometimes lower. To limit the risk, hygienisation, time intervals before harvesting crops eaten raw and application rules are used. The detailed requirements result from the regulations for waste and animal by-products.

Sources: Quality management of digestate from biogas plants used as fertiliser — IEA Bioenergy Task 37

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No, ordinary mesophilic digestion is not equivalent to the required hygienisation — the rules on animal by-products set separate time and temperature conditions.

Digestion reduces the number of pathogens, but not always to the degree required by law. For category 3 materials, among others, treatment at 70 °C for an hour with size reduction to a specified particle size is used, unless the regulations permit another method. The specific requirements and exceptions result from Regulation 1069/2009 and Implementing Regulation 142/2011.

Sources: Rozporządzenie (WE) nr 1069/2009 — uboczne produkty pochodzenia zwierzęcego — EUR-Lex; Quality management of digestate from biogas plants used as fertiliser — IEA Bioenergy Task 37

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No, digestion does not break down plastic or glass, so contaminants present in the feed remain in the digestate and can end up in the field if they are not removed earlier.

That is why it is so important to clean waste before the digester and to select separation technology after the process. Microplastics and glass in digestate limit its use as a fertiliser and may result in non-compliance with quality requirements. Digestate quality checks include an assessment of physical contaminants. Packaged waste should be unpacked in a depackager, not tipped in together with the packaging.

Sources: Quality management of digestate from biogas plants used as fertiliser — IEA Bioenergy Task 37

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No, the composition of digestate depends on the substrates, dry matter, retention time and technology, so the content of nitrogen, phosphorus and potassium has to be determined by analysis.

Digestate from slurry has a low dry matter and a moderate N content, and from protein waste a much higher one. A significant share of the nitrogen is present as ammonium, which acts quickly on plants but easily volatilises as NH₃. The dose and timing of application are selected according to analysis results, crop needs and the limits in force, not on averages from the literature.

Sources: Utilisation of digestate from biogas plants as biofertiliser — IEA Bioenergy Task 37; Quality management of digestate from biogas plants used as fertiliser — IEA Bioenergy Task 37

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No, ammonium nitrogen is quickly available to plants, but it easily volatilises as ammonia and can be leached, so effectiveness is determined by the method and timing of application.

A high pH of the digestate (usually above 7.5) increases the share of NH₃ and the losses during spreading. They are limited by soil injection or rapid incorporation, low temperature and moist soil, and the absence of frozen ground. Nitrogen use is measured by an effectiveness coefficient relative to mineral fertilisers, which depends on conditions. Well-planned fertilisation reduces losses and the threat to waters.

Sources: Utilisation of digestate from biogas plants as biofertiliser — IEA Bioenergy Task 37

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No, separation splits the digestate into a solid and a liquid fraction, and the nutrients are distributed between them — phosphorus goes mainly to the solid fraction, and ammonium nitrogen and potassium to the liquid one.

The solid fraction is easier to transport over longer distances and can be composted or dried, while the liquid one is suitable for fertilising near the plant. Separation reduces the storage volume of the liquid part and makes phosphorus management easier. Effectiveness depends on the type of separator (press, centrifuge) and the addition of coagulants. The benefits must be assessed together with energy and handling costs.

Sources: Utilisation of digestate from biogas plants as biofertiliser — IEA Bioenergy Task 37

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The Nitrates Directive introduces it for natural fertilisers in nitrate vulnerable zones; in Poland the nitrates action programme (Regulation of the Council of Ministers of 31 January 2023, Journal of Laws (Dz.U.) 2023, item 244) applies throughout the country. Always check the regulations and action programme in force.

Sources: Dyrektywa Rady 91/676/EWG (dyrektywa azotanowa) — EUR-Lex; Ministerstwo Rolnictwa i Rozwoju Wsi — gov.pl; KTBL — Kuratorium für Technik und Bauwesen in der Landwirtschaft

Discussed in the article: Applying digestate to fields: rates, timing and the 170 kg N/ha limit

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Digestate is best spread during the growing season and with near-ground or soil application (e.g. trailing hoses, injection tines), which limits ammonia losses and odour nuisance.

Splash-plate spreading leads to higher NH₃ emissions. Weather conditions affect losses: cool and humid days are more favourable than heat. Dates, distances from waters and buildings, and doses must be observed. A good logistics plan takes into account equipment working time, soil compaction and seasonal restrictions.

Sources: Utilisation of digestate from biogas plants as biofertiliser — IEA Bioenergy Task 37

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Digestate supplies plants with nutrients and contains organic matter, but it has less of it than compost, so its effect on humus is limited.

During digestion the readily degradable matter is used up, and a more stable fraction remains. The effect on soil depends on the dose and frequency of application and on soil structure. The solid fraction after separation or after composting enriches the soil better in organic matter. Excessive doses risk losses to waters: nitrogen mainly through nitrate leaching, phosphorus mainly through surface runoff and erosion.

Sources: Quality management of digestate from biogas plants used as fertiliser — IEA Bioenergy Task 37

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The digestate tank must hold the production over the period in which it may not be applied to fields (usually several months), which in Poland results from the nitrates action programme; for liquid natural fertilisers the programme requires tank capacity for at least 6 months.

The regulations specify minimum capacities of tanks for liquid natural fertilisers on farms, as well as periods when fertilising is prohibited. Digestate from a biogas plant is usually treated similarly to slurry if it contains animal excrement, but the specific requirements depend on classification and permits. The investor should plan capacity with a margin and check the current nitrates programme.

Sources: Pytania i odpowiedzi do Programu azotanowego — gov.pl

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The rules on timing, doses and storage of nitrogen fertilisers affect how farms can use digestate as a fertiliser. The details depend on the classification of the digestate and the substrates used for digestion.

Sources: Rozporządzenie Rady Ministrów z 12 lutego 2020 r. — Dz.U. 2020 poz. 243 (ELI); Dyrektywa Rady 91/676/EWG (azotanowa) — EUR-Lex

Discussed in the article: The new nitrates programme of 2020: fertilisation rules cover the whole of Poland

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Nitrogen losses are limited by using soil application, covering tanks, avoiding spreading in heat and wind, and adjusting the dose to crop needs and the weather.

The greatest NH₃ losses occur in the first hours after spreading, so rapid incorporation into the soil is effective. Tanks with a surface crust limit emissions, and covers (e.g. floating) limit them even more. Reducing losses improves the profitability of fertilisation and reduces environmental pollution.

Sources: Pytania i odpowiedzi do Programu azotanowego — gov.pl

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Since the act entered into force, digestate products have a separate definition in the Act on Fertilisers and Fertilisation and are subject to notification to the plant protection inspectorate and to requirements for documentation, quality and use.

Sources: Ustawa z 13 lipca 2023 r. o ułatwieniach w przygotowaniu i realizacji inwestycji w zakresie biogazowni rolniczych — Dz.U. 2023 poz. 1597; Ustawa o nawozach i nawożeniu — ISAP

Discussed in the article: Digestate in the act: notification to the inspectorate, batch documentation and a ban on application to frozen soil

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It depends on the substrates and the mode of placing on the market. Digestate from slurry and manure is usually treated, in the context of limits, as a natural fertiliser; for other substrates (e.g. biowaste) separate rules apply. Confirm the status in the plant documents and with the competent authority.

Sources: Dyrektywa Rady 91/676/EWG (dyrektywa azotanowa) — EUR-Lex; Ministerstwo Rolnictwa i Rozwoju Wsi — gov.pl; KTBL — Kuratorium für Technik und Bauwesen in der Landwirtschaft

Discussed in the article: Applying digestate to fields: rates, timing and the 170 kg N/ha limit

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No, the status of digestate is determined by the type of substrates: digestate from waste is in principle waste and can be a fertiliser only after legal requirements are met, whereas digestate from an agricultural biogas plant based on approved substrates can in Poland be a digestate product subject to the Act on Fertilisers and Fertilisation.

The Act on Fertilisers and Fertilisation defines "digestate products" as organic substances from the production of agricultural biogas made from animal excrement, straw and other non-hazardous natural substances originating from agricultural or forestry production, or from other substrates that do not endanger the health of people, animals or the environment, specified in the implementing regulations to the act on agricultural biogas plants; their transfer for agricultural use requires a written contract (the parties keep it for at least 3 years). For waste, decisions are needed (e.g. a processing permit) and compliance with the requirements of the act on waste or on fertilisers. Regulation (EU) 2019/1009 allows fertilising products with CE marking to be placed on the market if they meet quality and safety standards.

Sources: Dyrektywa 2008/98/WE w sprawie odpadów — EUR-Lex; Rozporządzenie (UE) 2019/1009 — produkty nawozowe UE — EUR-Lex; Ustawa o nawozach i nawożeniu (tekst jednolity, Dz.U. 2024 poz. 105) — ISAP

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Phosphorus in digestate is partly available to plants, and its effectiveness depends on the chemical form, soil reaction and type of digestate; it is usually comparable with other organic fertilisers.

In separation, phosphorus concentrates in the solid fraction, which can be dosed at a different time. Because of the risk of eutrophication of waters, on farms with a high concentration of animals phosphorus doses are sometimes the main limit on application. Soil and digestate tests help to plan fertilisation without excess.

Sources: Utilisation of digestate from biogas plants as biofertiliser — IEA Bioenergy Task 37

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Most of all where the digestate has a high concentration of ammonium nitrogen (poultry manure, protein wastes), there are problems with managing the nitrogen or with ammonia inhibition, and cheap heat is available. With small nitrogen loads, the investment usually does not pay back.

Sources: Rajagopal R., Massé D.I., Singh G. (2013), A critical review on inhibition of anaerobic digestion process by excess ammonia; FNR — Fachagentur Nachwachsende Rohstoffe; DBFZ — Deutsches Biomasseforschungszentrum

Discussed in the article: Ammonia stripping from digestate: how it works, what it costs and what is produced

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No. It is a policy document that announces legislative proposals. Quantitative targets become mandatory only when they are included in adopted legislation.

Sources: Komunikat COM(2020) 381 — strategia „od pola do stołu” (EUR-Lex); Komisja Europejska — strategia „od pola do stołu”

Discussed in the article: The "Farm to Fork" strategy: fewer fertilisers, fewer nutrient losses, more biogas from agriculture

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No. The CE mark is voluntary; the producer may sell digestate under national rules. CE makes sales across the EU easier, but requires conformity assessment and compliance with quality criteria.

Sources: Rozporządzenie (UE) 2019/1009 — EUR-Lex; Komisja Europejska — nawozy

Discussed in the article: EU Fertilising Products Regulation applied from 16 July 2022: CE marking for digestate

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The dose of digestate is set according to the nitrogen (and phosphorus) content and the needs of the crops, and it may not exceed the limits resulting from the regulations, including 170 kg N/ha per year from natural fertilisers.

The limit of 170 kg N/ha applies to nitrogen from natural fertilisers (slurry, manure, liquid manure). The Polish nitrates action programme (Regulation of the Council of Ministers of 31 January 2023, Journal of Laws (Dz.U.) 2023, item 244) also includes in its table of natural fertilisers the digestate product from agricultural biogas plants, and the tabulated nitrogen content value can be replaced by the result of a test from an authorised laboratory. Application dates, requirements for sloping, frozen and flooded land, and tank capacities also apply. The details are regulated by the nitrates action programme and the Act on Fertilisers and Fertilisation — the current version of the regulations should be checked and the dose calculated for the specific farm.

Sources: Quality management of digestate from biogas plants used as fertiliser — IEA Bioenergy Task 37; Pytania i odpowiedzi do Programu azotanowego — gov.pl; Rozporządzenie Rady Ministrów z 31 stycznia 2023 r. w sprawie Programu działań azotanowych — Dz.U. 2023 poz. 244

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A biogas plant must ensure lawful management of digestate, keep records, have suitable tanks and document the use of fertilisers on fields.

Depending on the substrates, digestate is waste or a product. For waste, decisions, entries in BDO and transfer cards are needed. For use in agriculture — a fertilisation plan and compliance with the requirements of the nitrates programme. Contracts with recipients specify doses, dates and responsibility. Well-designed digestate logistics can be as important as substrate logistics.

Sources: Ustawa z 13 lipca 2023 r. o ułatwieniach w inwestycjach w zakresie biogazowni rolniczych, Dz.U. 2023 poz. 1597; Dyrektywa 2008/98/WE w sprawie odpadów — EUR-Lex

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It is worth testing dry matter, organic matter, pH, total and ammonium nitrogen, phosphorus, potassium, magnesium, and in waste also heavy metals and physical contaminants.

Regular tests make it possible to set fertiliser doses, assess process quality and meet legal requirements. Samples are taken after mixing the tank, and at application — from the spreader. Results from a single date do not replace constant monitoring, because the composition changes with the feed recipe. The laboratory should use recognised methods and state the uncertainty of the result.

Sources: Quality management of digestate from biogas plants used as fertiliser — IEA Bioenergy Task 37

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It is liquid digestate from a biodigester usually fed with manure. It contains plant nutrients, including nitrogen in ammonium form, and is used as an organic fertiliser.

Sources: Africa Biogas Partnership Programme — Hivos; Clemens et al., Africa Biogas Partnership Program: A Review of Clean Cooking Implementation through Market Development in East Africa — Energy for Sustainable Development (2018); The potential of small-scale biogas digesters to improve livelihoods and long-term sustainability of ecosystem services in Sub-Saharan Africa: final report — GOV.UK (DFID)

Discussed in the article: Bioslurry from household biodigesters in East Africa: digestate as a fertiliser for smallholdings

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Digestate (the post-fermentation residue) is the material leaving the digester after biogas production, containing most of the fertiliser components of the substrate, and is most often used as an organic fertiliser.

It contains nitrogen (largely as ammonium nitrogen), phosphorus, potassium and organic matter. Compared with raw slurry it is more homogeneous, has a smaller odour nuisance and usually more readily available nitrogen. It can be applied in liquid form or after separation. The rules of use and sale depend on the type of substrates — in some cases digestate is waste.

Sources: Utilisation of digestate from biogas plants as biofertiliser — IEA Bioenergy Task 37

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Digestate that is waste is classified according to the waste catalogue in group 19 06, e.g. 19 06 05 (liquor from anaerobic treatment of animal and vegetable waste) or 19 06 06 (digestate from anaerobic treatment of animal and vegetable waste).

The choice of code depends on the origin of the waste and its form (liquid or solid mass). Municipal waste has separate codes in this group (19 06 03 and 19 06 04), and a digestate product from an agricultural biogas plant using approved substrates is usually not waste, provided that it meets the conditions resulting from the regulations, so in that case it does not receive a waste code. The code is entered in the records in BDO and in transfer documents. Incorrect classification results in problems with collection or inspection, so in doubtful cases it is worth seeking the opinion of the voivodeship authority or an adviser.

The basis is the regulation of the Minister of Climate of 2 January 2020 on the waste catalogue (Journal of Laws (Dz.U.) 2020, item 10); the status is determined by the type of substrates and the way the material is managed, not by the name of the material.

Sources: Dyrektywa 2008/98/WE w sprawie odpadów — EUR-Lex; Rozporządzenie Ministra Klimatu z 2 stycznia 2020 r. w sprawie katalogu odpadów (Dz.U. 2020 poz. 10); Rozporządzenie Ministra Klimatu w sprawie katalogu odpadów (Dz.U. 2020 poz. 10) — ISAP; Ustawa o odpadach (Dz.U. 2013 poz. 21) — ISAP

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It has a higher pH and a larger share of ammonium nitrogen, and less organic matter, which binds ammonia and limits infiltration into the soil. This usually means a higher risk of losses after surface spreading.

Sources: Dyrektywa (UE) 2016/2284 (NEC) — EUR-Lex; UNECE — Konwencja LRTAP (wytyczne dotyczące redukcji emisji amoniaku); KTBL — Kuratorium für Technik und Bauwesen in der Landwirtschaft

Discussed in the article: Ammonia emissions from digestate: application techniques and rapid incorporation

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Digestate has less organic dry matter than the feed, because part of the matter has been converted into biogas, while the mineral dry matter remains.

During digestion the organic dry mass is lost (in proportion to the degree of degradation), while the mineral composition remains, so the ash content of the dry matter rises. The water content is usually similar to the feed or slightly higher (part of the organic dry matter goes into the biogas), if it was not additionally diluted. When planning logistics, the amount of digestate corresponding to the feed minus the gas must be taken into account.

Sources: Utilisation of digestate from biogas plants as biofertiliser — IEA Bioenergy Task 37

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Digestate usually has a pH of 7.5–8.5, because bicarbonates and ammonia are formed during digestion, and volatile fatty acids are consumed.

An alkaline reaction increases the volatility of ammonia, so application requires care. On the other hand, digestate slightly de-acidifies acid soils, although the effect is small compared with liming. At a pH above 8, NH₃ losses rise quickly. The pH is measured on a fresh sample.

Sources: Utilisation of digestate from biogas plants as biofertiliser — IEA Bioenergy Task 37

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Further digestion takes place in the digestate tank and can release methane, and storage and application release ammonia and nitrous oxide, so these emissions enter the plant's climate balance.

An open tank in the warm season can emit significant amounts of CH₄. The solution is a sealed cover, preferably with gas recovery, a longer time in the digester or post-digestion. During application, soil injection limits NH₃ losses. Emissions from the digestate tank are taken into account, among others, in the methodology for calculating greenhouse gas emission reductions under RED II (separate default values for digestate in an open and a closed tank). The EU Methane Regulation (EU) 2024/1787, on the other hand, concerns the energy sector (oil, natural gas, coal) and does not cover digestate tanks.

Sources: Utilisation of digestate from biogas plants as biofertiliser — IEA Bioenergy Task 37; Rozporządzenie (UE) 2024/1787 w sprawie redukcji emisji metanu — EUR-Lex; Dyrektywa (UE) 2018/2001 (RED II), załączniki V i VI — EUR-Lex

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