3.3 Feedstock C:N ratio
Carbon (C) and Nitrogen (N) are both essential nutrients for anaerobic digestion. Carbon is a key component of organic matter and acts as an energy source for the microorganisms involved in anaerobic digestion (AD), while Nitrogen is an essential nutrient for microbial growth and reproduction.
The ratio of carbon to nitrogen, known as the C:N ratio, is a critical parameter in AD. It determines the balance between carbon availability and nitrogen content in the feedstock. An optimal C:N ratio between 16 and 25, ensures that there is enough carbon for energy production while providing an adequate nitrogen supply for microbial growth. If the C:N ratio is unbalanced, either too high or too low, it can negatively impact the AD process, leading to reduced gas production, inhibition of microbial activity, or process instability.
The following table presents C:N ratios for different organic materials.
| Organic material | C:N | Source |
|---|---|---|
| Human excreta | 6 – 14:1 | (Oxfam , 2011) |
| Faecal sludge | 5:1 | (Warda J. B., 2021) |
| Vegetable residue | 12 – 30:1 | (Oxfam , 2011) |
| Cow manure | 6 – 14:1 | (Oxfam , 2011) |
| Chicken droppings | 5 – 9:1 | (Oxfam , 2011) |
| Fresh grass | 12 – 30:1 | (Oxfam , 2011) |
While a C:N ratio for excreta and faecal sludge is generally considered low for optimal AD, the process works because of the ability of methanogenic bacteria to adapt to high concentrations of ammonia. However, biogas production may be less efficient than the optimal C:N ratio. If higher biogas yields are desired, it is generally recommended to aim for a higher C:N ratio by introducing additional organic waste with a higher C:N ratio, such as food waste or agricultural residues, to increase the overall C:N.
