Digester bacteria need carbon and nitrogen in a ratio of about 25-30:1, and a nitrogen-rich manure must be mixed with a carbon-rich material to get there: chicken manure (C:N 7) needs roughly 3.7 kg of dry straw (C:N 80) for each kilogram of dry manure to reach 25:1, and pig manure (14) about 1.1 kg. Every feedstock also has another use, as feed, bedding or fertiliser, that the gas takes away.
Part of Biogas Digesters
The carbon-to-nitrogen ratio is the weight of carbon in a feedstock divided by the weight of nitrogen; digesters work best near 25-30:1, which sets how feedstocks are blended.
The Parent’s Feedstock Table
| Feedstock | C:N | Gas (m³ per tonne) | Parent’s note |
|---|---|---|---|
| Cow manure | 25:1 | 30-50 | ideal, most common |
| Pig manure | 14:1 | 40-60 | too much nitrogen alone |
| Chicken manure | 7:1 | 50-80 | must mix with carbon-rich material |
| Food waste | 15-25:1 | 80-150 | excellent gas producer |
| Straw | 80:1 | 20-30 | too carbon-rich alone |
| Human waste | 8:1 | 20-30 | needs pathogen management |
| Grass clippings | 20:1 | 40-60 | good supplement |
Mixing Arithmetic: Weighted By Nitrogen, Not By Weight
If two materials have the same carbon fraction, the mixture’s C:N is the harmonic mean, weighted by dry mass: 1 ÷ R = f ÷ R₁ + (1 − f) ÷ R₂, where f is the dry-mass fraction of material 1. Different materials do not in fact have the same carbon fraction, so this is an estimate to be corrected from your own tests; the parent gives ratios but no carbon contents.
Chicken manure (R₁ = 7) with straw (R₂ = 80) to reach R = 25: 1 ÷ 25 = 0.04, so 0.04 = f ÷ 7 + (1 − f) ÷ 80 = 0.1429 f + 0.0125 − 0.0125 f, giving 0.0275 = 0.1304 f and f = 0.211. That is 21% manure and 79% straw by dry weight, or 3.74 kg of straw per kg of manure (about 3.7 kg). The same working for pig manure (14) gives 1.14 kg of straw per kg, about 1.1 kg. For R = 30, f = 0.160 and straw per kg of manure is 5.26.
| Blend with straw (80:1) | For 25:1 | For 30:1 |
|---|---|---|
| Chicken manure (7) | 3.74 kg straw per kg | 5.26 |
| Human waste (8) | 3.09 | 4.40 |
| Pig manure (14) | 1.14 | 1.83 |
| Food waste at 15 | 0.97 | 1.60 |
| Grass clippings (20) | 0.36 | 0.80 |
| Cow manure (25) | none needed | 0.32 |
All figures are dry mass. Fresh mass is dry mass divided by (1 − moisture). If chicken manure were 70% water and straw 10% (assumptions: dry a sample and weigh it to find yours), 1 kg of dry manure is 3.3 kg fresh and 3.74 kg of dry straw is 4.2 kg as baled: 1.2 kg of baled straw per kg of fresh manure.
Then Dilute To A Slurry
The parent says a digester wants about 8-10% total solids. The blend above has 1 + 3.74 = 4.74 kg of dry matter; at 8-10% solids that is 4.74 ÷ 0.10 = 47 kg to 4.74 ÷ 0.08 = 59 kg of slurry, so about 42-54 kg of water to add.
What Each Feedstock Is No Longer Doing
- Food waste (80-150 m³/tonne, or about 1.4-3.75 GJ at 18-25 MJ/m³) is the traditional feed of pigs and poultry (Pig Husbandry); feed the animals first.
- Straw (20-30 m³/tonne, only about 0.36-0.75 GJ, 100-210 kWh) is bedding, feed and building fibre, and gives the worst gas per tonne of the table.
- Manure is the farm’s main nutrient source (Animal Manure); the digestate returns the nutrients, but not the carbon that left as gas (Digestate as Fertilizer).
- Grass clippings compete with hay and green manure (Hay Making).
- Human waste can be composted into safe fertiliser (The Science of Humanure Composting); digesting it needs the pathogen controls on the digestate page.
- Dedicated crops for the digester take food land, and the corpus’s guides (Caloric Budget for a Community) say food comes first.
Common Mistakes
- Feeding chicken manure alone. Too much nitrogen: ammonia builds and inhibits the culture.
- Feeding straw alone. Too little nitrogen, and floating fibre crusts.
- Mixing by fresh weight with a dry-mass recipe.
Mixing and handling feedstock
Fresh and fermenting manure, especially chicken and pig manure, releases ammonia and hydrogen sulfide in closed sheds and pits. Mix in the open air, never in a pit, and do not breathe the dust of dry chicken manure. Human waste carries disease (Waste Disposal).
What’s Next
- Learn next: Temperature & Retention Time
- Combine with: Pile Construction, whose compost pile uses the same 25-30:1 target for a different process