Co-digestion calculator
Estimate the production of methane, biogas, energy and CHP output from a mixture of substrates. The methane potential (BMP) comes from a database of 270 substrates and 1,866 records from publications; dry matter and plant parameters are explicit assumptions that you can change.
The model is additive: it does not account for synergies or inhibition in co-digestion (e.g. by ammonia), retention time or process temperature. The result is a preliminary estimate, not a plant design.
Calculator
Plant parametersdefaults are assumptions — change them for your project
Assumption: 85%. The share of the laboratory-test potential obtained in a continuous digester (retention time, incomplete degradation).
Assumption: 53%. Typically about 50–60% depending on the feed composition; used only to convert methane into biogas.
Assumption: 40%. Check the unit's data sheet.
Assumption: 43%. Useful heat recovered from engine cooling and exhaust gas.
Assumption: 8000 h. Used for the electrical output and the capacity of a biomethane plant.
All digesters, excluding the digestate tank. Needed for OLR and HRT; we assume a feed density of 1 t/m³.
Affects the C/N thresholds and the assessment of ammonia risk.
Affects the assessment of the dry matter of the feed.
Add a substrate and enter the mass to see the result. You can also .
How we calculate
For each component the calculator determines the volatile solids (VS) load: mass × dry matter × VS share of the dry matter. The load multiplied by the BMP gives the theoretical amount of methane: BMP is given in m³ CH₄ per tonne of VS (numerically the same as NmL CH₄/g VS in the publications). The sum over all components is multiplied by the share of the potential realised.
- CH₄ [Nm³/year] = Σ mass × DM × VS × BMP × share of potential realised
- biogas [Nm³/year] = CH₄ ÷ CH₄ share of the biogas; hourly flows are averages over 8760 h
- energy in the methane [MWh/year] = CH₄ × 9.97 kWh/Nm³ (lower heating value of methane at normal conditions) ÷ 1000
- electrical output [kWₑ] = energy × electrical efficiency ÷ operating time; heat = energy × thermal efficiency
- biomethane equivalent [Nm³/h] = CH₄ ÷ 0.97 ÷ operating time — biomethane with 97% CH₄, without methane losses in upgrading
- OLR [kg VS/(m³·d)] = daily VS load ÷ active volume; HRT [d] = active volume ÷ daily feed volume (density 1 t/m³)
- C/N of the mixture = Σ VS ÷ Σ (VS ÷ C/N) — the VS-weighted harmonic mean (the arithmetic mean overstates C/N); calculated when the components with a C/N in the database make up at least 90% of the VS
The uncertainty range next to the typical result is the same calculation for the lowest and highest published BMP of each component (unchanged for a component with its own BMP value). The spread results from differences in varieties, harvest date, sample preparation and test methodology; we describe it on the substrate pages and in the BMP methodology.
Dry matter and volatile solids change the result considerably and differ between farms and batches of raw material. If the database holds a typical composition of the substrate, the calculator inserts it with a hint (range and source); otherwise the fields are empty and must be filled in — ideally with the results of a laboratory analysis of your own material.
What the model does not account for
- synergies and antagonisms in co-digestion — a mixture can perform better or worse than the sum of its components;
- process inhibition, including by ammonia (protein-rich substrates, poultry litter), hydrogen sulphide and fatty acids;
- degradation kinetics at a given retention time, temperature, mixing and losses during silage storage;
- the plant's own consumption of electricity and heat and the availability of the CHP unit.
Process assessment
Separately from the yield, the calculator assesses the process risk: it checks the shares of substrates with particular characteristics (e.g. fat-rich, easily degradable, without buffering), dose limits from the substrate descriptions, OLR, HRT, the dry matter of the feed and C/N according to rules with thresholds from the literature (including the FNR guide, IEA Bioenergy Task 37, studies of co-digestion of fats and glycerol). The result is a status: stable mixture, needs monitoring or high risk of process failure — with an explanation, sources and a proposal to reduce the doses (“Fit doses to limits”). The assessment does not change the calculated yield: we add neither “synergy bonuses” nor penalties.
Investment decisions require your own BMP tests of representative samples, a composition analysis and a process design.
Worked example
A mixture of 10,000 t/year of maize silage and 20,000 t/year of cattle slurry, BMP = weighted means from our database. Dry matter and volatile solids are illustrative values, not data from a specific plant. Default parameters: 85% of the BMP potential realised, 53% CH₄ in the biogas, CHP efficiency 40% electrical / 43% thermal, 8000 operating hours.
| Substrate | Mass t/year | DM % | VS % DM | BMP m³ CH₄/t VS | VS load t/year | CH₄ Nm³/year |
|---|---|---|---|---|---|---|
| Maize silage (whole crop) | 10,000 | 33 | 95 | 341.0 | 3,135 | 908,680 |
| Cattle slurry | 20,000 | 8 | 80 | 236.3 | 1,280 | 257,094 |
In total 1,165,774 Nm³ of CH₄ per year (on average 133.1 Nm³/h), i.e. 251.1 Nm³/h of biogas with 53% CH₄. The chemical energy of the methane is 11,623 MWh/year, which at an efficiency of 40% and 8000 operating hours corresponds to a cogeneration unit of about 581 kWₑ (4,649 MWh of electricity and 4,998 MWh of heat per year) or about 150 Nm³/h of biomethane. The silage gives 78% of the methane although it makes up only one third of the feed mass.
The dry matter of the feed is 16.3% — below the 30% that the FNR guide “Leitfaden Biogas” takes as the maximum for a wet-digestion mixture. The digester contents have less dry matter than the feed because part of the organic matter is converted into biogas.
Questions about the calculator
How do I calculate biogas production from a mixture of substrates?
For each substrate the feed mass is multiplied by its dry matter (DM) content, the share of volatile solids (VS) and the BMP methane potential. The sum over all components is corrected for the share of the potential realised in the plant, and biogas production is obtained by dividing the amount of methane by the CH₄ share of the biogas.
Where do the BMP values in the calculator come from?
From the FeedstockData database: for each substrate we compile BMP test results from peer-reviewed publications and databases. By default the calculator uses the weighted mean and, to assess uncertainty, the lowest and highest published result. Every substrate has a page with a table of records and a bibliography.
Why does the calculator result differ from the output of my biogas plant?
The calculator is an additive model: it assumes that each component yields as much methane as follows from its BMP, independently of the others. It does not account for synergy or inhibition effects (e.g. by ammonia or fatty acids), retention time, temperature, mixing or storage losses. It is a preliminary estimate, not a plant design.
What does the share of the BMP potential realised mean?
BMP is the maximum methane yield from a laboratory test that usually lasts several dozen days. In a continuous plant part of the organic matter leaves the digester undegraded, so we assume that only part of the potential is obtained. The default 85% is an assumption to be changed, not a measured value.
How does the calculator work out the electrical output of the CHP?
It multiplies the chemical energy of the methane (9.97 kWh per 1 Nm³ of CH₄) by the electrical efficiency of the unit and divides the result by the annual operating time. With the default 40% and 8000 h, 1 million Nm³ of CH₄ per year corresponds to an output of about 500 kWₑ.
What are OLR and HRT?
OLR (organic loading rate) is the amount of volatile solids fed per day per 1 m³ of active volume, in kg VS/(m³·d). HRT (hydraulic retention time) is the average time the feed stays in the digester, in days. The calculator works them out once the active volume is given, assuming a feed density of 1 t/m³.