Digestate
Digestate is the residue from methanization.
Depending on its composition and texture, it can be used as a type II fertilizer (for the liquid phase), soil amendment (for the solid phase), or composted.
The spreading of digestate is described on a specific page:
Methanization digestates and their agronomic benefits
Principles of digestate formation and main characteristics
During methanization, different families of anaerobic bacteria degrade complex organic chains into simple elements. The four steps of this degradation - hydrolysis, acidogenesis, acetogenesis, and methanogenesis (see Figure 1.1) - occur simultaneously in the digester, mostly under mesophilic conditions (35-40°C) and with a substrate retention time of 40 to 60 days.
During digestion, about two-thirds of the biodegradable organic matter is converted into biogas (composed of CH4 and CO2).
At the end of this process, the residual organic matter, dissolved minerals, and water make up the digestate (see Figure 1.2). It is stored in a pit directly connected to the digester.
⚠️The composition of digestate depends on the input materials, digestion processes, and possible post-treatment.
Agronomic properties of digestates
Raw digestates
General characteristics
Compared to the substrate from which it originates, a digestate has the following properties:
- pH is higher (alkaline);
- the rate of nitrogen mineralization (as NH4+) relative to total N is higher;
- the C/N ratio is lower;
- the dry matter content is lower.
Organic matter composition and amendment value
In digestate, the organic matter exists in three forms (see Figure 1.2):
- a biodegradable (or labile) organic matter, highly mineralizable, composed of soluble sugars and part of hemicellulose. This fraction serves as energy and nutrient sources for soil bacteria and earthworms;
- a poorly biodegradable (or stable) organic matter, composed of lignin and cellulose. This fraction is decomposed mainly by fungi. It is a precursor of humic matter and thus improves the clay-humus complex;
- living organic matter made up of microorganisms that transform and store organic elements into mineral elements accessible to plants (mineralization).
Only part of the fresh organic matter in substrates (about ⅔) is converted into biogas. The degradation rate of this fresh organic matter depends on the nature of inputs, retention time, and technology used.
The lignin fraction (stable) is not attacked by digestate bacteria. The humification potential of digestate remains unchanged compared to the substrates it originates from. When digestate is spread, this stable organic matter can bind with soil clay and strengthen the soil clay-humus complex, essential for water retention and many nutrients. Furthermore, the soil biological activity would be improved, as well as the soil porosity.
Fertilizing value
Methanization is a process that preserves elements not incorporated into biogas. Total amounts of fertilizing elements N, P, K, and trace elements in the substrate are thus conserved in the digestate and are very variable depending on inputs.
However, methanization transforms some of these elements. Organic nitrogen in the substrate (mainly proteins and urea) is partly mineralized into ammoniacal form, which is a more readily assimilable form for plants (see Figure 1.3). The ammoniacal nitrogen proportion can represent up to 65% of the total nitrogen in raw digestates or the liquid fraction of digestates.
Digestate also contains nitrogen in organic form that was not degraded during methanization. Part of this nitrogen mineralizes during the year following application, under the action of soil organisms. The other part is stored, associated with carbon, in the soil organic matter (humification). The humified fraction will mineralize later at the same rate as soil organic matter.
Therefore, two types of effects of digestate on nitrogen nutrition of crops can be considered:
- a short-term effect linked to uptake by the receiving crop (and possibly the following crop) of part of the mineral nitrogen and mineralized organic nitrogen;
- a long-term effect linked to the modification of soil organic nitrogen stock and its mineralization rate.
On average, raw digestate has a higher assimilable nitrogen content than manure or slurry, due to nitrogen inputs from possible external substrates (increased total nitrogen) and mineralization of organic nitrogen during digestion (high ammoniacal nitrogen %).
The question of short-term phosphorus bioavailability in digestates compared to substrates is unresolved. During methanization, part of phosphorus is solubilized in the liquid fraction and then forms insoluble mineral substances that precipitate at the bottom and on digester walls. This leads to very variable immediate phosphorus bioavailability in digestate depending on studies. In some cases, it is improved, while in others, it is reduced compared to starting substrates. In all cases, the long-term bioavailability of digestate phosphorus would be equivalent to that of a soluble mineral phosphorus form.
For potassium, mostly present in soluble form in digestate, methanization does not alter its bioavailability for plants.
Sanitary quality
Overall, classic mesophilic digestion (around 37°C) allows elimination of about 99% of pathogenic germs (reduction factor of 100), and thermophilic digestion (around 55°C) 99.99% of them (reduction factor of 10,000).
Thus, thermophilic digestion is considered a hygienizing treatment for products with high pathogen concentrations. Mesophilic digestion is suitable for lightly contaminated products but can be supplemented by hygienizing treatments (pasteurization, post-composting, etc.) if necessary.
Regarding micropollutants, methanization achieves high elimination rates of organic pollutants and pathogenic germs. Polycyclic aromatic hydrocarbons are partially degraded; residual amounts are fixed to the solid matter, with very low release to the liquid phase. Metallic micropollutants are not eliminated but fixed in the solid fraction in a non-bioavailable form.
Odors
Several studies report odor reduction enabled by methanization, due to the depletion of easily degradable organic matter, which is also responsible for bad odors. This degradation occurs in the closed space of the digester, so olfactory nuisances related to storage and spreading of organic matter are greatly reduced. However, these results are often expert opinions and require validation by olfactometric measurements.
Fluidity
Degradation of substrate organic matter by methanization results in a decrease in dryness and thus an improvement in digestate fluidity compared to the substrate. This can have positive agronomic consequences by facilitating infiltration into soil of digested slurries, for example.
Phase separation
Some digestates undergo a treatment called “phase separation”, for example by centrifugation or using a screw press, which allows isolation of:
- a solid fraction concentrated in organic matter and phosphate elements, usable as a base amendment;
- a liquid fraction concentrated in nitrogen, mainly in ammoniacal form, and potassium (Figure 1.4).
Phase separation leads to differentiated management of digestate based on technical criteria (spreading method, field distance, etc.), economic (spreading cost), and agronomic (field and crop amendment or fertilization needs).
Regulatory context
General framework
Methanization digestate, depending on its composition and dry matter content, can be considered both as an «amendment» (which modifies or improves physical, chemical, or biological soil properties) or as a «fertilizer» playing a fertilizing role (providing plants with elements directly useful for nutrition).
In all cases, it is a material intended to be spread on agricultural soils. Thus, management of methanization digestates is governed by two types of regulations (see Table 2.1):
- regulations aiming to control the agronomic quality and sanitary safety of digestate, addressed to the digestate producer;
- regulations targeting the receiving environment of these digestates (farms), introducing additional requirements for digestate spreading in certain zones.
| Digestate production | |
|---|---|
| Regulatory texts | Content |
| Legislation on classified installations: under ICPE sections 2781-1 and 2781-2 and authorization*, registration, or declaration regimes | Provisions on:
|
| Regulation (EU) No 142/2011 (implementing Regulation No 1069/2009 of the European Parliament and Council):
establishing sanitary rules applicable to animal by-products and derived products not intended for human consumption and the decree of April 9, 2018 setting national technical provisions related to the use of animal by-products and derived products in biogas plants, composting plants or “local composting”, and slurry use. |
Provisions on:
|
| Digestate use | |
| Vulnerable zone (NVZ) | Nitrates Directive 91/676/EEC:
|
| Non-vulnerable zone | Code of good agricultural practices |
| Any zone | For an operator receiving digestate, the ICPE regulation specifies that they must sign a written contract with the digestate producer, specifying mutual commitments and responsibilities.
Spreading rules established in ICPE decrees apply. |
* Farms under authorization under ICPE section 2781‑2 must comply with articles 36 to 42 of the decree of February 2, 1998, relating to emissions of any kind from ICPE.
It should also be noted that digestate classified as waste cannot be sold; if sold, it must have Market Authorization (AMM).
Classified installations for environmental protection (ICPE)
Methanization installations are subject to legislation on Classified Installations for Environmental Protection. The ICPE section No. 2781 is specific to methanization. It defines a regime of declaration with periodic control (DC), a regime of registration (E), and a regime of authorization (A). Currently, the regimes are defined according to Table 2.2:
The installation regime (authorization, registration, or declaration) defines in particular the measures to be respected on the methanization site and in terms of spreading, with details on substrates and digestates.
Nitrates Directive
Digestate spreading is also subject to the Nitrates Directive (Directive 91/676/EEC), which aims to reduce water pollution caused by nitrate leaching from agricultural practices.
Action programs defined under this Directive contain a set of measures whose implementation is mandatory in designated vulnerable nitrate zones (NVZ) or across the entire territory.
Within this framework, methanization digestate is classified as “type II nitrogen fertilizers”. These are “low C/N nitrogen fertilizers” (i.e., less than or equal to 8), containing organic nitrogen and a variable proportion of mineral nitrogen.
Rules applicable in vulnerable zones cover:
- nitrogen fertilization balance and establishment of record-keeping documents (fertilization plan and practice record book)
- application periods for organic and mineral fertilizers and their use near watercourses and on sloping land
- prohibition of spreading on snow-covered, frozen, or flooded soil
- vegetative soil cover during intercrop periods and obligation of unfertilized grass or wooded strips 5 meters wide along watercourses and water bodies over 10 ha
- compliance with minimum storage durations for Livestock effluents and the maximum annual nitrogen quantity of livestock effluents spread.
Sources
- Sophie Carton and Quentin Bulcke. Use of digestates in agriculture. [03/11/25]. https://projet-methanisation.grdf.fr/cms-assets/2022/06/Utilisation-des-digestats-en-agriculture-web_compressed.pdf
- ADEME, AILE, Solagro, Trame. September 2011. On-farm Methanization - Practical Guide
- ADEME. January 2019. Setting up an on-farm methanization unit
- APESA (coordinator), BIOMASSE NORMANDIE, RITTMO. January 2015. Regulatory and legal framework for agricultural methanization and composting activities, Study for ADEME
- Arvalis (coordinator), ADEME, CTIFL, Terres Inovia, IFV, ITAVI, IDELE, IFIP. 2020. GES’TIM+ Guide
- Lorraine Regional Chamber of Agriculture. May 2019. Technical Sheet Methanization Digestates: Optimizing soil return to benefit agronomic and economic benefits - Trial results and farm monitoring - 4-year summary
- Lorraine Chamber of Agriculture. October 2019. Practical guide for project leaders and advisors - Methanization digestates: Optimizing soil return to benefit agronomic and economic benefits
- Comifer. May 2013. Nitrogen fertilization calculation, Methodological guide for establishing local prescriptions
- CPE Artois Picardie. Methanization units: Digestate spreading, Methodological guide. 2020 edition
- Doublet S., Leclerc B., Couture C. and Berger S. December 2004. Agronomic and sanitary quality of digestates from methanization - Final report
- EREP SA and EAWAG. 24/09/2009. State of the art of (cost-effective) methods for elimination, concentration, or transformation of nitrogen for small/medium agricultural biogas installations - Final report
- Western CUMA Regional Federation. 2018. Properly spreading digestates from methanization, Technical sheet within the TEpLis project
- Gaillot B. and Lavarde P. (coordinator). July 2015. Spreading on agricultural land of fertilizing materials from residual origin - Prospective mission on regulatory framework and monitoring methods - CGEDD report No. 009801-01, CGAAER No. 14074
- Levasseur P., Soulier A., Lagrange H., Trochard R., Foray S., Charpiot A., Ponchant P. and Blazy V. 2019. Agronomic valorization of pig, cattle, sheep, goat, poultry, and rabbit livestock effluents. RMT Livestock and Environment, Paris, 83 pages
- Nitrawal asbl, Aisne Chamber of Agriculture, Nord-Pas de Calais Regional Chamber of Agriculture. Choosing your organic fertilizer spreading equipment. Sheet within the SUN (Sustainable Use of Nitrogen) project
