The C/N ratio (carbon-to-nitrogen ratio) is the parameter that measures the nutrient balance of the digester diet. The optimal range is between 20 and 30: below 15 there is excess nitrogen and a risk of ammonia inhibition; above 35 nitrogen is lacking and digestion slows down. Almost no substrate is in the ideal range on its own (slurry is around 5-15, straw 50-100), so it is balanced by mixing in co-digestion. This article explains what it is, how to calculate it properly, the problems of imbalance and how to adjust it.
The C/N ratio is one of the first numbers a technician looks at when designing the diet of a digester. It summarises, in a single value, whether the substrate mix is balanced in nutrients or not.
Understanding it well avoids two of the most common problems in biogas: ammonia inhibition (when nitrogen is in excess) and slow digestion (when it is lacking).
This is the what is of the C/N ratio: its definition, the correct calculation, why the optimum is 20-30 and how it is adjusted with co-digestion.
Definition of the C/N ratio
The C/N ratio is the relationship between the carbon and the nitrogen available in a substrate or a mix. It is the parameter that summarises, in a single number, the nutrient balance of the diet the digester receives.
Carbon is the energy source of the microorganisms; nitrogen, the material with which they build their proteins and grow. If either of the two is lacking or in excess, the consortium works worse.
Correct calculation: biodegradable carbon vs total carbon
The C/N ratio is calculated by dividing the total carbon by the total nitrogen of the substrate, both on a dry basis. But there is an important nuance: not all carbon is equal.
What really matters is the biodegradable carbon, not the total carbon. A lignocellulosic substrate such as straw has a lot of carbon, but a good part of it is in the lignin, which is barely digested. Calculating the C/N on total carbon gives an overly optimistic picture of a substrate that, in practice, contributes little useful carbon.
Optimal ranges: why 20-30
The optimal range for anaerobic digestion is between 20 and 30. Within that margin, the consortium has energy (carbon) and nitrogen in a balanced proportion to grow and produce methane without accumulating toxic by-products.
Below 20, nitrogen starts to be in excess; above 30, it starts to be lacking. The extremes are what cause problems.
Low C/N ratio (<15): the free ammonia problem
A low C/N ratio, below 15, means excess nitrogen. That nitrogen is released as ammonium during digestion and, in its free form (NH₃), inhibits the methanogenic archaea.
It is the typical problem of concentrated livestock substrates, such as slurry. It is covered in detail in the post on free ammonia.
High C/N ratio (>35): the nitrogen deficit
A high C/N ratio, above 35, means a nitrogen deficit. There is plenty of carbon, but the microorganisms do not have enough nitrogen to grow, so digestion slows down and the carbon is poorly used.
This is what happens with very fibrous substrates such as straw or dry plant residues.
Co-digestion as a tool to balance the C/N
Since almost no substrate is on its own in the 20-30 range, the solution is anaerobic co-digestion: mixing substrates with opposite C/N to bring the whole mix closer to the optimum. These are the typical values:
| Substrate | Typical C/N | Profile |
|---|---|---|
| Pig slurry | 5-15 | Low (excess N) |
| Cattle slurry | 10-20 | Low-medium |
| Fruit and vegetable waste | 25-35 | Close to the optimum |
| Maize silage | 30-50 | High |
| Straw | 50-100 | Very high (N deficit) |
Combining a slurry (low C/N) with a silage or straw (high C/N) balances the diet in theory, although this will need to be confirmed after the first weeks at the plant or with a semi-continuous laboratory experiment. With these values, the proportion of each substrate is calculated so that the mix falls within 20-30, as in agroindustrial co-digestion. Biogas from slurry is a clear case where the low C/N forces the addition of a carbon-rich co-substrate.
Common errors in determining the C/N
- Using total carbon instead of biodegradable carbon, overestimating the contribution of fibrous substrates.
- Calculating the C/N with generic table values instead of real analysis: the C/N varies a lot with origin and season.
- Forgetting that the C/N is only the starting point: the real balance also depends on degradability and on other nutrients (phosphorus, sulphur, trace elements).
- Focusing only on the C/N and not monitoring ammonia or the FOS/TAC during operation.
Frequently asked questions about the C/N ratio
What is the C/N ratio in anaerobic digestion?
The C/N ratio (carbon-to-nitrogen ratio) is the proportion between the carbon and the nitrogen available in a substrate or a mix. It measures the balance of the main nutrients in the digester diet: carbon provides energy and nitrogen, the material for the microorganisms to grow. It is one of the key parameters for designing co-digestion mixes.
What is the optimal C/N ratio?
The optimal range of the C/N ratio for anaerobic digestion is between 20 and 30. Within that margin, the microbial consortium has carbon and nitrogen in a balanced proportion to produce methane stably. Outside that range, problems appear.
What happens if the C/N ratio is very low or very high?
If the C/N is very low (below 15), there is excess nitrogen, which is released as ammonium and can inhibit the methanogenic archaea. If it is very high (above 35), there is not enough nitrogen for the microorganisms to grow and digestion slows down, wasting the carbon. Both extremes reduce biogas production; the goal is to get close to the 20-30 range.
How is the C/N ratio balanced with co-digestion?
Generally, by mixing substrates with opposite C/N. A livestock slurry (low C/N, a lot of nitrogen) is combined with a carbon-rich substrate such as silage, straw or plant residues (high C/N), so that the resulting mix falls within the optimal range of 20-30. That calculation of proportions is the basis of diet design in co-digestion and is adjusted with the analytical characterisation of each substrate.
Smallops and diet design
A poorly calculated C/N unbalances the diet and limits production. At Smallops we characterise your substrates and calculate the optimal mix with an Operational Excellence Diagnosis.
Is the diet of your digester well balanced?
Request a Smallops Operational Excellence Diagnosis: we analyse the real C/N of your substrates and calculate the mix that maximises production without unbalancing the process.
References and standards
Wang, X. et al. (2012). Optimizing feeding composition and carbon/nitrogen ratios in anaerobic co-digestion. Bioresource Technology, 120, 78-83.
Yen, H.W. & Brune, D.E. (2007). Anaerobic co-digestion of algal sludge and waste paper. Bioresource Technology, 98 (1), 130-134.
Hills, D.J. (1979). Effects of carbon:nitrogen ratio on anaerobic digestion of dairy manure. Agricultural Wastes, 1 (4), 267-278.
Wu, X. et al. (2010). Improvement of anaerobic digestion of solid wastes. Energy & Fuels, 24 (8), 4543-4549.