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Fixed-Dome (Chinese Model)

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Digester Types
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Fixed-Dome (Chinese Model)

A fixed-dome digester is a buried brick or concrete pit with a gas-tight dome on top: gas collects under the dome and pushes slurry up and out through an overflow, so the gas pressure rises and falls with the amount stored. The dome must be airtight, built by a skilled mason, and strong enough against the gas trying to lift it: a 2.5 m dome with 0.5-1 m of slurry displaced carries 24-48 kN of uplift.

Part of Biogas Digesters

The fixed-dome or Chinese model is an underground masonry digester whose dome holds the gas and displaces slurry as it fills.

The Dome Is The Gas Holder And The Pressure Vessel

The parent describes the design: an underground brick or concrete chamber with a dome-shaped top, no moving parts, a life over 20 years, a skilled-masonry requirement, and a household size of 6-10 m³. As gas collects in the dome, the slurry level in the digester is pushed down and slurry is forced up and out through the overflow pipe into a tank, so the stored gas is at a pressure equal to the height difference of slurry.

That height sets the pressure. Slurry weighs about 1,000 kg/m³, so each metre of slurry head gives 1,000 × 9.81 = 9.81 kPa, about 100 cm of water column, against the 5-15 cm that a burner wants (Pressure Regulation). A fixed dome therefore delivers gas at far above burner pressure when full and almost none when empty, and needs a regulator or a pressure-reducing design.

Uplift On The Dome: Worked Example

Take a dome with a base diameter of 2.5 m (an assumed size for a 6-10 m³ plant). Its horizontal cross-section is π ÷ 4 × 2.5² = 4.91 m². Gas pressure pushes up on the dome as on a piston of that area:

  • 0.5 m of displaced slurry: 4.9 kPa × 4.91 m² = 24 kN, about 2.5 tonnes of force.
  • 1.0 m: 9.81 kPa × 4.91 = 48 kN, about 4.9 tonnes of force.

The dome’s weight, the earth packed over it and the ring wall that holds it must exceed that, or a full plant tries to lift its roof. The corpus’s dome guides treat loads pressing down on a dome (Dome Building, Arch Principles); a gas dome is loaded from below, and the numbers above are why masonry quality and earth cover matter. Limit the overflow height to what the dome can bear.

What You Need

Method

Step 1: Site and dig. Close to the animals, downhill and at least 30 m from drinking water, in ground that stays dry; shore the pit and never leave anyone alone in it.

Step 2: Foundation and wall. Compact the base, lay a foundation course and build the wall to the springing level.

Step 3: Dome. Build it gas-tight with no gaps; it is the one structure that cannot be repaired from inside.

Step 4: Plaster and seal multiple coats (Waterproofing & Sealing).

Step 5: Test with water, then with gas pressure, before loading.

Step 6: Cover the dome with earth and fence the outlet tank.

Common Mistakes

  • A dome that passes the water test but leaks gas. Gas finds hairline cracks water does not.
  • No relief, so over-pressure lifts or cracks the dome.
  • Leaving the inspection plug unsealed, which lets gas out and air in.
  • No cover or rail on the outlet tank.

Pits, domes and gas

The digester pit, the dome space and the outlet tank all hold methane, carbon dioxide and hydrogen sulfide. Do not climb into a dome or tank. If a repair inside is unavoidable, treat it like the well-shaft entry in Safety Protocol: empty it, ventilate it for a long time, never go alone, keep a person outside on a line, and never judge the air by smell or by a flame, since hydrogen sulfide dulls the nose and a flame can ignite methane. A rescue into a gas-filled space kills the rescuer (Anaerobic Digestion Science, Safety Protocol). Never test for gas with a flame.

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