FprEN 18256
(Main)Digestate - Determination of the residual biogas potential
- Abstract
This document specifies a method for the determination of the residual biogas potential in digestate in an anaerobic environment.
NOTE This method has been validated in an interlaboratory study with specific products that were present on the market during the study (as included in Annex B, being solid digestate).
- Status
- Not Published
- Publication Date
- 20-Jan-2027
- Technical Committee
- CEN/TC 223 - Soil improvers and growing media
- Drafting Committee
- CEN/TC 223/WG 4 - Analytical methods
- Current Stage
- 5020 - Submission to Vote - Formal Approval
- Start Date
- 01-Oct-2026
- Due Date
- 21-Dec-2024
- Completion Date
- 01-Oct-2026
- Mandate
- M/XXX - M/XXX
FprEN 18256 is a CEN draft European Standard for measuring the residual biogas potential (RBP) of digestate. It specifies a laboratory method for testing digestate in an anaerobic environment, so laboratories and technical teams can assess how much biogas the material can still produce after digestion.
What does FprEN 18256 specify?
FprEN 18256 specifies one method for determining the residual biogas potential in digestate. The method is organized into scope, normative references, terms and definitions, principle, apparatus, reagents, sampling, procedure, calculation, test report, and validation, with two informative annexes.
Sampling is handled through a separate standard, and the procedure then covers sample preparation, inoculum preparation, gas measurement, and calculation of RBP.
| Annex | What it covers |
|---|---|
| Annex A | Specific information on the RBP test, including an example reaction vessel setup |
| Annex B | Performance characteristics from the interlaboratory study |
What are the key requirements of FprEN 18256?
FprEN 18256 sets out a controlled batch test in sealed vessels under anaerobic conditions. In practice, this means the laboratory must keep oxygen out, control temperature, measure the gas produced, and normalize the result to standard conditions.
Test principle and equipment
Clause 4 says the test uses closed reaction vessels filled with sample, inoculum, and nutrient solution, then measures the biogas by manometric or volumetric methods. Clause 5 lists the needed equipment, including a temperature-controlled facility, gastight vessels, pressure or volume measuring devices, a barometer, mixing, a balance, a pH meter, and sieves.
This matters because RBP results depend on tight control of leaks, temperature, and measurement method.
Sample handling and inoculum preparation
Clause 7 points users to EN 12579:2024 for sampling. Clause 8.1 requires the sample to be transported, stored, and prepared according to prEN 13040-1:2025, and to be characterized by total solids (TS) and volatile solids (VS).
The sample should be prepared within 24 hours after sampling, or stored for up to 1 month at 3 °C ± 2 °C under anaerobic or anoxic conditions. The inoculum should come from a biogas digester, be sieved to 5 mm or less, and be pre-digested for at least 5 days at test temperature without added nutrients or carbon sources.
Reaction setup and test run
Clause 8.3 requires an inoculum-to-sample ratio of 2 on a volatile-solids basis, with final total solids in the vessel between 10 g/l and 50 g/l. The vessel also needs nutrient solution and enough water to leave headspace, and a blank vessel must be prepared with inoculum but no sample.
The headspace must be flushed with oxygen-free inert gas, preferably N2, with turnover equal to 5 times the headspace volume. Clause 8.4 says the test may use manometric or volumetric gas measurement, runs for 28 days, and requires pH checks and regular recording of gas data.
Calculation and reporting
Clause 9 gives the formulas for standardizing gas volumes and calculating cumulative biogas production and RBP. The result is expressed per g VS and corrected for the blank vessel, with a mass correction factor when the inoculum masses differ.
Clause 10 sets the minimum test report content. Users need to report the sample identification, number of replicates, mean RBP, test temperature, duration, gas measurement method, end-of-test net daily biogas production, and any optional or non-standard work cycles.
Validation and performance
Clause 11 states that the method was validated according to ISO 5725-2. Annex B reports interlaboratory study data for solid digestate, which is the matrix used to establish the performance characteristics.
What terms does FprEN 18256 define?
- Biogas - A gas mixture mainly made of methane (CH4) and carbon dioxide (CO2) produced by anaerobic degradation of organic material.
- Inoculum - Biological material with active anaerobic microorganisms that starts and supports degradation.
- Working volume - The usable volume in the reaction vessel occupied by inoculum, sample, nutrients, and water.
- Headspace - The remaining volume in the vessel above the working volume where biogas collects.
- Residual biogas potential (RBP) - The amount of biogas the digestate can still produce under the test conditions.
- Total solids (TS) - The dry matter content used to set the loading of the test.
- Volatile solids (VS) - The organic fraction used to express loading and the final RBP result.
Who uses FprEN 18256?
FprEN 18256 is used by laboratories that test digestate from anaerobic digestion. It is also relevant to technical and quality staff working with soil improvers, growing media, and other organic fertilizers, where digestate stability affects how the material is assessed and handled.
The method is useful when a buyer, producer, or laboratory needs a consistent way to compare digestate samples and document the remaining biogas-forming potential.
Which standards are used with FprEN 18256?
- CEN/TS 17732:2022 - Supplies the terminology used for soil improvers and growing media.
- EN 12579:2024 - Covers sampling of soil improvers and growing media.
- prEN 13039:2025 - Provides the method for determining organic matter content and ash, used here for VS.
- prEN 13040-1:2025 - Covers sample preparation, dry matter, moisture, and laboratory bulk density.
- EN 45501 - Supports the weighing instrument requirements for the balance.
- ISO 5725-2 - Gives the method used to validate repeatability and reproducibility.
What does the FprEN 18256 document contain?
FprEN 18256 contains the test principle, apparatus list, reagent recipes, sample and inoculum preparation steps, and the full calculation route for RBP. It also includes the test report items that should be recorded for traceability.
Annex A provides a figure of a reaction vessel arrangement for the RBP test. Annex B presents the interlaboratory study and the performance characteristics derived from solid digestate data, which helps users judge how the method performed in validation.
Relations
- Effective Date
- 28-Jan-2026
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Frequently Asked Questions
FprEN 18256 is a draft published by the European Committee for Standardization (CEN). Its full title is "Digestate - Determination of the residual biogas potential". This standard covers: This document specifies a method for the determination of the residual biogas potential in digestate in an anaerobic environment. NOTE This method has been validated in an interlaboratory study with specific products that were present on the market during the study (as included in Annex B, being solid digestate).
This document specifies a method for the determination of the residual biogas potential in digestate in an anaerobic environment. NOTE This method has been validated in an interlaboratory study with specific products that were present on the market during the study (as included in Annex B, being solid digestate).
FprEN 18256 is classified under the following ICS (International Classification for Standards) categories: 65.080 - Fertilizers. The ICS classification helps identify the subject area and facilitates finding related standards.
FprEN 18256 has the following relationships with other standards: It is inter standard links to EN ISO 24025-2:2020. Understanding these relationships helps ensure you are using the most current and applicable version of the standard.
FprEN 18256 is associated with the following European legislation: EU Directives/Regulations: 2003/2003, 2019/1009; Standardization Mandates: M/564, M/564 AMD 1, M/564 AMD 2, M/588, M/XXX. When a standard is cited in the Official Journal of the European Union, products manufactured in conformity with it benefit from a presumption of conformity with the essential requirements of the corresponding EU directive or regulation.
FprEN 18256 is available in PDF format for immediate download after purchase. The document can be added to your cart and obtained through the secure checkout process. Digital delivery ensures instant access to the complete standard document.
Standards Content (Sample)
SLOVENSKI STANDARD
01-december-2025
Digestat - Določanje potenciala bioplina, ki se še lahko sprosti iz digestata (RBP)
Digestate - Determination of the residual biogas potential
Gärrest - Bestimmung des Restbiogaspotentials
Digestat - Détermination du potentiel de production de biogaz résiduel
Ta slovenski standard je istoveten z: prEN 18256
ICS:
65.080 Gnojila Fertilizers
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
DRAFT
EUROPEAN STANDARD
prEN 18256
NORME EUROPÉENNE
EUROPÄISCHE NORM
October 2025
ICS 65.080
English Version
Digestate - Determination of the residual biogas potential
Digestat - Détermination du potentiel de production de Gärrest - Bestimmung des Restbiogaspotentials
biogaz résiduel
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 223.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 18256:2025 E
worldwide for CEN national Members.
prEN 18256:2025 (E)
Contents Page
European foreword . 3
Introduction . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Principle . 6
5 Apparatus . 6
6 Reagents . 6
7 Sampling . 7
8 Procedure . 7
8.1 Storage and preparation of sample . 7
8.2 Sampling, storage and preparation of inoculum . 8
8.3 Starting the procedure . 8
8.4 Measurement methods . 9
9 Calculation . 9
10 Test report . 11
11 Validation of the method . 11
11.1 Validation in accordance with ISO 5725-2. 11
11.2 Performance characteristics. 11
Annex A (informative) Specific information on RBP - test . 12
Annex B (informative) Performance characteristics of the method . 13
Bibliography . 15
prEN 18256:2025 (E)
European foreword
This document (prEN 18256:2025) has been prepared by Technical Committee CEN/TC 223 “Soil
improvers and growing media”, the secretariat of which is held by NEN.
This document is currently submitted to CEN Enquiry.
This document has been prepared under a standardization request addressed to CEN by the European
Commission. The Standing Committee of the EFTA States subsequently approves these requests for its
Member States.
prEN 18256:2025 (E)
Introduction
Anaerobic digestion is a biological process leading, in absence of oxygen, to the degradation of organic
carbon and to the production of biogas, a gas mixture composed mainly by methane (CH ) and carbon
dioxide (CO ). The degradation process is carried out by a wide range of mesophilic or thermophilic
microorganisms and applied on an industrial scale as an exploitation strategy to produce added-value
products from crops, livestock manure, agro-industrial by-products, biowaste and sludges. The digestate
obtained is a stabilized, nutrient-rich material, and a suitable component for soil improvers, growing
media and other organic fertilizers.
When the organic material is not efficiently converted into biogas, the digestate coming out from the plant
is unstable and continues to produce biogas in the final storage tank.
The assessment of the digestate’s stability can be carried out through the residual biogas potential (RBP)
test which represents the amount of biogas that could still be produced from the organic material present
at the end of the anaerobic digestion process.
prEN 18256:2025 (E)
1 Scope
This document specifies a method for the determination of the residual biogas potential in digestate in
an anaerobic environment.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
CEN/TS 17732:2022, Soil improvers and growing media — Terminology
EN 12579:2024, Soil improvers and growing media — Sampling
prEN 13039:2025, Soil improvers and growing media — Determination of organic matter content and ash
prEN 13040-1:2025, Soil improvers and growing media — Sample preparation — Part 1: Sample
preparation for chemical and physical tests, determination of dry matter content, moisture content and
laboratory bulk density
EN 45501, Metrological aspects of non-automatic weighing instruments
3 Terms and definitions
For the purposes of this document, he terms and definitions given in CEN/TS 17732:2022 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp/
— IEC Electropedia: available at https://www.electropedia.org/
3.1
biogas
mixture of gases mainly composed of CH (methane) and CO (carbon dioxide) and produced by the
4 2
anaerobic degradation of organic material
3.2
inoculum
biological material with active anaerobic microorganisms able to initiate and enhance the degradation
process
3.3
working volume
usable volume within a reaction vessel taken up by inoculum, sample, nutrients solution and water
3.4
headspace
remaining volume within the reaction vessel obtained by subtraction of the working volume, occupied by
the biogas
prEN 18256:2025 (E)
4 Principle
The test is carried out under anaerobic conditions and in a temperature-controlled environment. Closed
reaction vessels are filled with adjusted quantities of inoculum, sample and a nutrient solution. The
biogas is cumulated within the headspace of the reaction vessel and its amount and composition are
measured by manometric or volumetric methods. The residual biogas potential (RBP) is referred to the
standard conditions of pressure and temperature and to the amount of organic matter (volatile solids)
added to the reaction vessel.
5 Apparatus
5.1 Test facility, temperature controlled room, climate cabinet or water bath, mesophilic temperature
adjustable to 38 °C ± 2 °C.
The laboratory may carry out the analysis in thermophilic temperature 52 °C ± 1 °C.
5.2 Reaction vessels, glass or steel gastight vessels (with a volume of 0,5 l to 2,5 l).
5.3 Pressure transducer, operating range of 0 kPa to 50 kPa and a maximum uncertainty set lower
than 1,5 % of the full scale.
5.4 Volume displacement devices, liquid displacement gasometers, lubricated gas syringes and gas
counters.
5.5 Barometer, to measure atmospheric pressure.
5.6 Mixing device, shaking Table 120 rpm ± 20 rpm or magnetic stirring unit.
5.7 Balance, meeting EN 45501 class II tolerances with a scale interval of 0,01 g.
5.8 pH meter, with slope adjustment and temperature control.
5.9 Sieves, 5 mm square aperture size or smaller.
6 Reagents
6.1 Water with a specific conductivity not higher than 0,2 mS/m at 25 °C, free from the elements to be
determined.
6.2 NaOH (0,5 mol/l)
6.3 HCl (0,5 mol/l)
6.4 Trace elements stock solution
Dissolve the following masses of chemical compounds in 1000 ml water (see 6.1):
— 0,05 g of MnCl · 4H O;
2 2
— 0,005 g of H BO ;
3 3
— 0,005 g of ZnCl ;
— 0,003 g of CuCl ;
prEN 18256:2025 (E)
— 0,001 g of Na MoO4 · 2H O;
2 2
— 0,1 g of CoCl · 6H O;
2 2
— 0,01 g of NiCl · 6H O;
2 2
— 0,005 g of Na SeO .
2 3
6.5 Complete nutrient solution
Dissolve the following masses of chemical compounds in 1000 ml water (see 6.1):
— 0,27 g of KH PO ;
2 4
— 1,12 g of Na HPO · 2H O;
2 4 2
— 0,53 g of NH Cl;
— 0,075 g of CaCl · 2H O;
2 2
— 0,10 g of MgCl · 6H O;
2 2
— 0,02 g of FeCl · 4H O;
2 2
— 10 ml of trace elements stock solution (see 6.4).
Adjust the pH of the medium, if necessary, with dilute mineral acid or alkali to 7 ± 0,2 (see 6.2 and 6.3).
6.7 N gas
6.8 Microcrystalline cellulose (CAS 9004-34-6)
7 Sampling
Sampling is not part of the method specified in this document. Follow EN 12579:2024 dealing with soil
improvers and growing media. It is important that the laboratory receives a sample that is representative
of the product under consideration. The sample should not have been damaged or changed during
transport or storage.
8 Procedure
8.1 Storage and preparation of sample
Transportation, reception and storage of samples shall be determined according to prEN 13040-1:2025,
Clauses 6 and 7; samples preparation shall follow Clauses 9.2.1, 9.3 and 9.4. The sample shall be
characterized by the following parameters: total solids (TS) according to prEN 13040-1:2025 for dry
matter and volatile solids (VS) according to prEN 13039:2025 for organic matter. Volatile solids shall be
submitted to the preparation procedure within 24 hours after sampling. Otherwise, the sample shall be
stored (max. 1 month) at 3 °C ± 2 °C under anaerobic/anoxic condition.
+
NOTE For further characterization of the sample, the determination of pH, ammonium content (N-NH ),
volatile fatty acid profile (VFA) and the chemical oxygen demand (COD) is applicable.
prEN 18256:2025 (E)
8.2 Sampling, storage and preparation of inoculum
The inoculum (3.2) should come from a biogas digester working at mesophilic (or thermophilic)
condition; it shall be sieved with a maximum square aperture size of 5 mm to remove the particles
retained and then pre-digested without the addition of any nutrients and carbon sources
...