The C/N ratio in a substrate mix: why 20–30 and how to balance it

In methane fermentation the most favourable carbon-to-nitrogen ratio (C/N) in the mix is about 20–30: below this limit excess nitrogen is converted into ammonia and inhibits methanogenesis, above it — the bacteria do not have enough nitrogen to build biomass, and the process slows. C/N is a tool for planning the mix, not a rigid requirement; in practice plants outside this range also operate stably if the buffer and adaptation are good.

Typical values for substrates

Values depend on the species, feeding, harvest date and method of storage — the numbers below are indicative and do not replace analysis.

Balance example

Assumptions: a daily mixture — 20 t of maize silage, 30 t of cattle slurry, 3 t of chicken manure. For simplicity we assume that C/N is calculated on the basis of dry matter; example data below:

C/N ≈ 4,500 / 238 ≈ 19. The result is slightly below the lower limit of the 20–30 range; adding a further 5 t of silage would raise C/N to approx. 20, while increasing the share of chicken manure would reduce it still further. It is worth checking free ammonia as well, because the real problem is created by ammonium nitrogen and pH, not by the ratio alone.

Warnings

• C/N calculated from total carbon does not take into account that part of the carbon (lignin) is unavailable. For lignocellulosic substrates the "effective" value is lower.
• Organic nitrogen is released gradually; after a change of mix the ammonium concentration rises with a delay.
• Recirculated digestate returns nitrogen to the system and increases the ammonium concentration.

Correction strategies

1. C/N too low: add a carbon substrate (silage, straw, grasses), limit the share of poultry and pig manures, increase dilution with water or cattle slurry, consider ammonia stripping (described in a separate article).
2. C/N too high: increase the share of slurry or other nitrogen substrates, check trace elements; maize alone without slurry often loses stability.
3. Always: change the mix gradually and verify TAN, pH and FOS/TAC.

C/N planning is worth supplementing with other indicators: the C:N:P:S ratio, fat and protein content, the share of easily degradable VS. In the design of a new plant, good practice is a laboratory-scale test (BMP or continuous) for the planned mix.

Beyond C/N: other components

Besides C/N, phosphorus and sulphur matter. In the literature C:N:P:S proportions of the order of 600:15:5:3 appear as a guide for anaerobic processes — an orientation, not a recipe. Sulphur is needed by the enzymes of methanogens, but its excess is converted into hydrogen sulphide. A deficiency of trace elements (Co, Ni, Se, Mo) can limit the process even with an ideal C/N, especially with a maize mono-substrate.

How to do it in practice

1. Collect analyses of TS, VS, total nitrogen and organic carbon for all substrates.
2. Build a balance spreadsheet: masses × components → C/N of the mix.
3. Check scenarios: winter/summer, failure of delivery of one substrate.
4. Set emergency rules: what to replace a missing substrate with so that C/N stays within a safe range.

Summary for the mix planner

Treat C/N as a preliminary filter: it rejects extremely unbalanced mixes. Then verify the mix through an ammonium nitrogen balance, buffering capacity (TAC), trace element content and a BMP test or continuous test for the adopted recipe. Only the set of this information justifies a decision on a long-term substrate contract.

It is good to record the C/N result together with the date and the recipe, because after a few months you build your own history: which combination gave stable gas, and with which problems with ammonia or a shortage of nutrients appeared. It also allows errors in substrate analyses to be detected faster.