Anaerobic digestion is the breakdown of manure, food scraps and crop residue by microbes in the absence of oxygen, in four stages that end in biogas, a mix of 55-70% methane and 30-45% carbon dioxide with 0.1-3% hydrogen sulfide, plus a nutrient-rich digestate. The methane burns as fuel, but the hydrogen sulfide in the same gas is lethal and dulls the sense of smell, so a digester is both a fuel source and a gas hazard.
Part of Biogas Digesters
Anaerobic digestion is the oxygen-free microbial decomposition of organic matter into methane-rich biogas and a residual slurry.
Four Stages, Each By A Different Group Of Microbes
When organic matter rots without oxygen (Anaerobic Conditions), as in a swamp or a landfill, specialised microbes work in a chain (the parent’s four stages):
- Hydrolysis: proteins, fats and carbohydrates are broken into simpler molecules.
- Acidogenesis: the simple molecules become volatile fatty acids.
- Acetogenesis: the acids become acetic acid, hydrogen and CO₂.
- Methanogenesis: methane-producing archaea make CH₄ from acetic acid and hydrogen.
The whole process takes 20-60 days depending on temperature and feedstock (parent). The detail is on Microbiology of Methane Production, and the controls on N Ratio & Feedstock Selection and Temperature & Retention Time.
What Comes Out
| Output | Amount | Use |
|---|---|---|
| Methane (CH₄) | 55-70% of the gas | fuel |
| Carbon dioxide (CO₂) | 30-45% | inert, displaces oxygen |
| Hydrogen sulfide (H₂S) | 0.1-3% | corrodes, and is lethal |
| Water vapour | saturated | condenses in pipes |
| Digestate | almost as much as went in | fertiliser (Digestate as Fertilizer) |
Energy: pure methane gives about 33-36 MJ per cubic metre (35.8 at 0°C, 32.8 at 25°C), so biogas at 55-70% methane holds about 18-25 MJ/m³, which is 5-7 kWh. The parent calls it “roughly 60% as energy-dense as natural gas”.
Where The Feed Comes From, And What It Was For
A digester eats what a settlement would otherwise use: manure is the main fertiliser (Animal Manure), straw is bedding, feed and building fibre, food scraps are feed for pigs and poultry (Pig Husbandry), and a crop grown just for the digester takes food land. The digestate returns the nutrients (see the fertiliser page) but the carbon that left as gas does not come back as humus. The feedstock discussion is on the C:N page, with its food cost.
The gas is a hazard before it is a fuel
Hydrogen sulfide is 34 ÷ 29 = 1.18 times as heavy as air and pools low. Raw biogas at 0.1% H₂S carries 1,000 ppm and at 3% carries 30,000 ppm; in air, the US NIOSH “immediately dangerous to life” level is 100 ppm and about 1,000 ppm is reported to cause collapse within a breath or two (both recalled, not opened here). At around 100 ppm and above it dulls the sense of smell, so a rotten-egg smell that fades does not mean the air has cleared: it can mean the nose has stopped working. Pure methane is 0.55 times as heavy as air, rises, and explodes at 5-15% in air, but raw biogas, with 30-45% carbon dioxide mixed in, is 0.84-0.99 times as heavy as air (16.04 and 44.01 g/mol weighted by 70% and 55% methane, against 28.96 for air), nearly neutral, so do not assume a biogas leak rises away or sinks away. Carbon dioxide is 1.52 times as heavy and sits in pits. Biogas displaces oxygen. Never go into a digester, its pit or its tanks, and never go in after someone; ventilate and haul out with a line from outside. No flame near any part of a digester. See Gas Hazards (Methane, CO) (written for coal workings) for where each gas collects.
What’s Next
- Learn next: Digester Types, to choose a design
- Combine with: Using Biogas, for what the gas can do