A bag or tube digester is a long polyethylene tube, 1-1.5 mm thick and UV-stabilised, laid in a smooth trench: slurry enters one end and leaves the other, and gas collects in the upper part and in a separate storage bag. It is the cheapest and fastest design to build, lasting 3-5 years, and a 1.5 m lay-flat tube 10 m long holds about 7 m³, but it has no strength against puncture, animals or sunlight beyond what you give it.
Part of Biogas Digesters
A bag or tube digester is a flexible plastic tube used as the digester body, with gas held in the tube and a separate gas bag.
The Plastic Is The Pit
The parent describes the design: a long plastic tube (polyethylene, 1-1.5 mm thick, UV-stabilised) in a trench, slurry in at one end and out at the other, gas collecting in the upper portion and in a separate storage bag, a life of 3-5 years, suited to rapid deployment or temporary installations. Because the tube is flexible, its internal pressure is low, set by the tube’s slight stiffness and by any weights placed on the gas bag.
Size From The Geometry
A tube sold flat has a lay-flat width w, so when inflated its circumference is 2w and its diameter is D = 2w ÷ π. For w = 1.5 m (an assumed size): D = 3.0 ÷ 3.1416 = 0.955 m, cross-section π ÷ 4 × 0.955² = 0.716 m². Over a length of 10 m that is 7.2 m³ in total, about 7 m³. If the liquid fills three-quarters of the cross-section (an assumed fill), the slurry volume is 0.75 × 7.2 = 5.4 m³ and the gas space is 1.8 m³. Choose the length from the slurry volume the feed and retention time need (Sizing the Digester).
What You Need
- Tube film, polyethylene, 1-1.5 mm, UV-stabilised, and a spare length for repairs
- A trench with smooth sides, free of stones, glass and roots
- Inlet and outlet pipes, clamped and sealed to the film
- A gas bag and a pipe between them
- A fence or cover against animals and a shade structure against sun
- Plastic repair material: patches and a means of heat-welding or an adhesive that suits polyethylene
Method
Step 1: Dig the trench a little wider than the tube, with a gentle slope from inlet to outlet and smooth walls and floor. Clear every sharp thing out.
Step 2: Lay the tube and tie each end around its pipe with several wraps of clamp, made so that no gas can pass.
Step 3: Fill with water to check for leaks and settle the shape, then drain to the working level.
Step 4: Load the first slurry and seed it with active culture (Microbiology of Methane Production).
Step 5: Fence it and shade it, and inspect for punctures every week. Mark where the pipe runs.
Common Mistakes
- Putting it where animals or children can step on it. One hoof through the film loses the whole gas store.
- No sun protection, which cuts the film’s life.
- A gas bag without weights or a pressure limit, which bursts or never fills.
- Using film that is not UV-stabilised.
A burst bag releases a gas cloud
A punctured tube or bag can empty its gas at once; near people that is methane, carbon dioxide and hydrogen sulfide in one place, and a flame nearby can ignite it. Keep the digester away from cooking fires and houses, fence it, and never walk on the tube. Never enter a trench with a leaking tube (Anaerobic Digestion Science).
What’s Next
- Learn next: Gas Holder Design
- Combine with: Waterproofing & Sealing