Methane is made by two partner groups: fast bacteria that turn manure into acids, hydrogen and CO₂, and slow archaea (methanogens) that turn acetic acid and hydrogen into methane. The archaea need a pH of 6.8-7.5, no oxygen and no poisons, so feeding faster than they can work builds acid, drops the pH and stops the gas; the cure is to stop feeding for 5-7 days and raise the pH.
Part of Biogas Digesters
The microbiology of a digester is the partnership between acid-forming bacteria and methane-forming archaea, and the conditions that keep it balanced.
Two Teams With Different Speeds
The first three stages in the parent’s chain (hydrolysis, acidogenesis, acetogenesis) are done by bacteria that grow quickly and tolerate a wide range of conditions. The last, methanogenesis, is done by archaea, a separate group of single-celled organisms, which grow slowly, are strict anaerobes (oxygen harms them) and are sensitive to acidity, temperature swings and poisons. The parent describes the final step as archaea converting acetic acid and hydrogen to CH₄. Two reactions are involved: CH₃COOH → CH₄ + CO₂ (from acetic acid) and CO₂ + 4 H₂ → CH₄ + 2 H₂O (from hydrogen and carbon dioxide). The textbook split is that most of the methane comes by the acetic acid route (recalled, not from the corpus).
Mass check for the acetic acid reaction: 60.05 g of acetic acid gives 16.04 g of methane, 0.27 kg of methane per kg of acid.
The Digester Is A Balance, And Feed Rate Sets It
Acid-formers can produce acid faster than the archaea consume it. If you add too much feedstock at once, acid builds, pH falls, the archaea slow, still more acid accumulates, and gas production stops. The parent’s cure: stop feeding for 5-7 days and add wood ash or lime to bring the pH back to 6.8-7.5 (the parent gives 1-2 kg of ash or lime, an amount that should be scaled to the digester size and added in small steps, checking pH as you go, because lime in excess can kill the culture too).
What Poisons The Culture
The parent lists cleaning chemicals, antibiotics in animal manure and heavy metals, which can kill the bacteria. A second source is oxygen: air let in through a leak or an open inlet harms the archaea. A third is a sudden temperature change (Temperature & Retention Time). A fourth is an excess of nitrogen-rich feed such as chicken manure by itself, which releases ammonia (N Ratio & Feedstock Selection; ammonia inhibition is recalled, not in the corpus).
What You Need
- Active culture: to start or restart a digester, fresh slurry from a working digester or fresh cow manure (parent)
- pH strips or red cabbage extract to read the slurry (pH Testing, written for soap)
- A log: feed amount, pH, gas production, temperature
- A regular feed, not a surge
Method: Keep The Balance
Step 1: Seed a new digester with a working digester’s slurry or fresh cow manure.
Step 2: Feed small amounts daily and raise slowly.
Step 3: Read the pH weekly; if it drops, stop feeding and treat.
Step 4: Keep chemicals, medicines and disinfectants out of the feed stream.
Common Mistakes
- Dumping a week’s feed at once.
- Adding disinfected wash water or medicated manure.
- Adding lime by the handful and overshooting.
A sour digester vents sour gas
Troubleshooting a digester means opening ports and handling slurry that releases hydrogen sulfide, carbon dioxide and methane. Work in open air, never lean into an opening, never enter, and keep flames away. Hydrogen sulfide dulls the sense of smell (Anaerobic Digestion Science).
What’s Next
- Learn next: N Ratio & Feedstock Selection
- Combine with: Temperature & Retention Time