To produce biogas, you need organic matters like animal manure, animal wastes and plant matters. These organic matters added to the biogas digester are called feedstock. Every organic matter has a carbon-nitrogen (C/N) ratio. For proper biogas production, the C/N ratio of the feedstock mix should be between 20 and 30. Micro-organisms in the biogas digester feed on carbon (as carbohydrates) and nitrogen (as protein, nitrates, ammonia, etc.) to produce biogas. They use carbon as an energy source and nitrogen for building cell structures. These organisms use up carbon about 30 times faster than they use nitrogen so they require a carbon-nitrogen ratio closer to 30. Let’s look at what happens if your feedstock mix does not meet this ratio.

1. C/N Ratio too Low (Too much nitrogen): If there is too much nitrogen, the carbon will be exhausted first and biogas production will stop. The remaining nitrogen will be lost as ammonia gas (NH₃). This loss of nitrogen decreases the fertility of the effluent (i.e. the feedstock after it has pass through the digester). It will also lead to foul ammonia smell.
2. C/N Ratio too High (Too much carbon): If there is too much carbonin the feedstock, nitrogen will be used up and carbon left over. This will make digestion to slow down.

Some feedstock will not meet the C/N requirement of 20-30 and so they have to be combined with other feedstock. If a feedstock has low C/N ratio, combine it with another feedstock with high C/N ratio. The reverse is also the case.

C/N Ratio of Various Feedstocks
Below is a table listing the C/N ratios of various organic materials. Use tables like this with caution because they can be very misleading for at least two reasons:

1. The C/N ratio measured chemically in the laboratory is often not the same as the C/N ratio availableto the bacteria as food (some of the carbon and nitrogen could be indigestible to the bacteria, e.g. straw, lignin, sawdust etc.).
2. The nitrogen or carbon content of even a specific kind of plant or animal waste can vary tremendously according to the age and growing conditions of the plant; the diet, age, degree of confinement, etc., of the animal.

So, tables like this should be used only for approximation.

The best way to get accurate C/N values is by testing your feedstock in a lab and avoid using feedstock that are hard to digest (e.g. sawdust, straw). The available nitrogen content of organic materials is best generalized and presented as total nitrogen (% of dry weight). For carbon, only the non-lignin carbon content should be considered since lignin is not digestible.

Now let’s look at some examples:

Example 1:

Human excreta have a C/N ratio of about 8 and that of rice straw is 73. For 1 kg of human excreta, how much rice straw should be mixed to achieve a C/N ratio of 25?

Solution:

Here is the formula you use:

R = [W₁R₁ + W₂R₂]/ (W₁+W₂)

Where W₁ & W₂ = Weight of human excreta and rice straw respectively; R₁ & R₂ = C/N ratio of human excreta and rice straw respectively; R = the C/N ratio we want to get after mixing the 2 feedstock.

25  = [1kg x 8 + Rkg x 73]/ (1 + R )kg

Solving this you get R= 0.354 kg.

Therefore for each kg of human excreta 0.35 kg (350 gm) of rice straw should be mixed.

Note: If you have more than 2 feedstock the formula becomes:

R = [W₁R₁ + W₂R₂ + W₃R₃ + W₄R₄ + W₅R₅+…] / (W₁+W₂+ W₃+ W₄+ W₅+…)

Example 2
Calculate the C/N ratio of 50kg pig manure (C/N=14) and 50kg vegetable produce (C/N=19).

Solution:

Using formula, R = [W₁R₁ + W₂R₂]/ (W₁+W₂)

R = [50×14 + 50×19]/(50+50) = 1650/100 = 16.5.

Since the C/N ratio does not fall within 20-30, this mix is not suitable for use in a biogas digester. Using this mix will lead to loss of nitrogen in the form of ammonia. This will reduce the fertilizing value of the effluent.

Example 3
Calculate the amount of vegetable produce (C/N=19) that should be added to 50kg pig manure (C/N=14) to give a C/N ratio of 25.

Solution

Using the formula: R = [W₁R₁ + W₂R₂]/ (W₁+W₂), we have,

25 = (50×14 + W₂x19)/(50+W₂)

W₂ = -91.7kg. Ignore the negative sign. So we’ll need 91.7kg of vegetable produce.

And finally, that is how you calculate the C/N ratio of feedstocks. If you have any questions or contributions, please ask by posting a comment. Also remember to register in our forum http://www.farmersjoint.com/member.php?action=register  to gain access to experts that will answer your questions. You also gain access to quality, member-only ebooks. Register here.