A fermentation vessel can be any clean, food-safe container (a 60-200 litre plastic drum, a glass carboy or a wooden barrel) fitted with an airlock made from a tube dipping into a jar of water, so carbon dioxide can leave but air cannot enter. Leave empty space above the wash for foam, never seal the lid shut, and stand the vessel outdoors or in a ventilated room, because a 100-litre ferment gives off about 4.2 cubic metres of CO₂.
Part of Ethanol Fuel Production
Fermentation vessel design covers the container, lid and airlock that let a wash ferment safely without air or bacteria getting in.
The Airlock Is Also The Pressure Relief
An airlock is a tube from the vessel lid with its free end under a few centimetres of water in a jar. Gas pushes through the water and bubbles out; the water seal stops air going the other way. If the vessel cools and its gas contracts, water can be drawn a little way up the tube, so keep the jar clean and the tube’s other end clear of the wash. The seal also limits pressure: the vessel can never rise above the weight of the water over the tube end. Each centimetre of water is about 98 pascals, so 5 cm of water is 5 × 98 = about 490 Pa, roughly 0.5% of atmospheric pressure, far less than a drum or carboy can take.
Flow is not the constraint. If the main fermentation emits 4.2 m³ (100 L of wash) over about 3 days, that is about 58 litres an hour, under 1 litre a minute, which a 10 mm tube carries at about 0.2 metres a second. What blocks airlocks is foam and solids. A wider tube, and a vessel filled no more than about three-quarters full, keep foam out of it.
What You Need
- Vessel: food-grade plastic drum (60-200 L), glass carboy or demijohn, or a wooden barrel (traditional but harder to clean)
- Lid or bung with a hole for the tube; the seal around the tube should be tight but the lid must be able to lift if the tube blocks
- Tube and jar for the airlock, with clean water
- A cleaning method: boiling water, and rinsing with the clean-water source the community trusts
Method
Step 1: Choose the vessel. Avoid anything that has held fuel, pesticide or strong chemicals, and avoid uncoated zinc, galvanised or aluminium surfaces, which wash acids can corrode. Plastic and glass are easiest to clean; a wooden barrel needs scrubbing and keeps contamination in its pores.
Step 2: Clean. Scald with boiling water and let it cool; a fermenter with leftover sour wash will seed the next batch with vinegar bacteria (Vinegar Production describes what they make).
Step 3: Fill. Leave a quarter or more of the vessel empty for foam.
Step 4: Fit the airlock and check the bubbling begins within 6-24 hours (Monitoring & Troubleshooting).
Step 5: Place it at 25-30°C, outdoors in the shade or in a room with permanent air openings at floor and ceiling level. Label it plainly.
Common Mistakes
- Making the lid airtight with no relief. Gas has nowhere to go and the container splits or the lid fires off.
- Letting the airlock dry out. Check the water weekly; a dry tube lets air and fruit flies in.
- Using a pit or cellar. CO₂ is 1.52 times as heavy as air and collects in low closed spaces.
A fermenter is a gas generator
The CO₂ from a ferment cannot be seen or smelled and displaces breathable air in a closed room, pit or cellar. Never ferment where air cannot move, never enter a fermentation pit or cellar without airing it, and never go down after someone who has collapsed: ventilate and use a line. Label every container “fermenting wash, not for drinking”, because a fuel wash is not safe to drink.
What’s Next
- Learn next: Monitoring & Troubleshooting, for reading the airlock and the hydrometer
- Combine with: Anaerobic Conditions, on why the airlock matters