A dry cell anaerobic digester treats mixed municipal waste in sealed concrete cells that are filled, left to digest for about 48 days, then emptied in rotation. In a proposal for a municipal landfill operator in East Asia, ten cells of roughly 8,000 m³ each were sized for about 460 tonnes of waste a day, with an estimated gas yield near 1,500 Nm³/h and 3 MW of exported power.

The challenge: mixed waste, no source separation

The operator received several hundred tonnes of raw household waste every day and sent it to landfill. The waste was typically not separated at source, and a large share was not biodegradable. A typical analysis showed roughly half organic food waste, around 5% vegetation, and the rest ash, ceramics, plastics, glass, metals, paper and textiles.

Wet processes such as continuously stirred digesters need clean, liquid-like feedstock and struggle with grit, plastics and bulky items. The question was how to get gas and a stabilised residue from waste of this kind without an expensive, delicate front end.

The approach: treat the digester as a controlled landfill

The proposal describes dry cell digestion as a structured form of landfill gas generation. The advantage over a landfill is that the waste composition is known, the cell is sealed and the gas production can be managed. That gives more predictable performance than a landfill cell that fills over years.

Light pre-treatment

Waste arrives at a covered tipping area, which stops rain getting into the load. It then passes through two trommels, a coarse screen of about 80 mm followed by a finer one of about 25 mm, and a picking line. Large or bulky items are removed by hand. About 90% of the waste was expected to go forward to the digester cells, which keeps the front end simple.

Filling and sealing the cells

Each cell is a concrete box. The design assumed ten cells, each about 8 m high, 25 m wide and 40 m long, which gives a volume of roughly 8,000 m³ per cell. Walls are about 0.75 m thick at the base, tapering towards the top.

• A movable conveyor on top of the cell feeds waste through four trap doors in the roof, one after another, so the waste forms an even layer.
• When a cell is full the trap doors are sealed.
• Leachate is then circulated through the waste to start digestion. Using the cell’s own liquid means very little leachate needs separate treatment.

Finding the equilibrium point

After about 8 days, gentle suction is applied to the cell. The operator adjusts it until the rate of gas extraction matches the rate of gas production. This is the equilibrium point. Too much suction pulls air into the cell, and too little lets gas escape. Once a cell is balanced it needs less attention and gas continues to be drawn for the rest of the retention time of about 48 days.

Gas pipes, valves and sampling outlets allow each cell to be checked daily for gas composition and controlled independently. A manifold joins the cells to one pipeline.

Rotating the cells

The number of cells is chosen so that, as soon as one is full, the next has reached the end of its retention time and has just been emptied. In practice that means ten cells and a filling time of about a week and a half per cell, so there is a steady, continuous gas flow rather than a peak and trough from a single batch. A cell is emptied, the digestate is sorted and the cell is refilled.

Gas treatment and power

• A gas dewatering unit cools the gas below its dew point (about 25°C) so that moisture drops from saturated to a maximum of about 80% relative humidity at no more than 40°C, which protects the engines.
• The gas goes to gas engines. If the engines shut down, it is diverted to a standby flare running at 1,000°C.
• A SCADA system on the dewatering skid logs and tracks data and can be linked to a control room.

The design basis was about 65 Nm³ of biogas per tonne of waste with a methane content of 55% to 57%. At roughly 85% availability the proposal estimated 23,000 MWh a year of exported electricity. About 40% of the waste going into the digester was expected to remain as stabilised material suitable for composting, at around 60% moisture.

Why it works: a robust process for rough feedstock

Biological breakdown of organic matter in the absence of oxygen is well established. Defra’s Anaerobic Digestion Strategy and Action Plan explains how the process works and where it fits in UK waste policy. The dry cell approach suits mixed waste because:

• Grit, plastics and non-biodegradable items do not damage pumps or mixers, because there are no pumps or mixers moving the waste.
• Gas production can be managed cell by cell.
• After digestion the residue is biologically stabilised, which makes sorting easier.

The trade-offs are land and civil works. Ten cells need around 7 rai (roughly 1.1 hectares), and the delivery time depends heavily on how quickly the concrete is built. The proposal quoted 12 to 18 months, assuming the civil works kept to schedule. Stabilised residue from mixed waste also needs a market or a disposal route, and the compost value of such material should not be assumed.

What to consider for your own project

1. Know your waste. Composition and tonnage drive gas yield. Treat a first estimate as indicative and verify it with a waste analysis.
2. Start with a feasibility study. This project proposed a front-end feasibility study and front-end engineering design before committing to build, so that cell dimensions and gas figures could be confirmed.
3. Plan the rotation. Match the number of cells to retention time and filling time so there is always one cell filling and one finishing.
4. Decide where the residue goes. Compost, landfill cover or disposal each need their own approvals and testing.
5. Provide for standby. Engines stop for maintenance, so a flare is needed to deal with the gas.

FAQ

What is a dry cell anaerobic digester?

It is a set of sealed concrete cells in which solid waste is digested without added water or stirring. Leachate is recirculated to keep the process active, and gas is drawn off through pipework.

How long does waste stay in a cell?

In this design, about 48 days after the roughly 8-day start-up period, then the cell is emptied and refilled.

How much gas does municipal waste produce?

This proposal assumed about 65 Nm³ per tonne with 55% to 57% methane. Actual yield depends on the organic content and moisture of your waste.

Can it treat mixed household waste without source separation?

Yes, with trommel screening and a picking line to remove bulky items. Better separation raises gas yield and improves the residue.

Looking at waste-to-gas for your site?

Tell us your waste tonnage and composition and we will outline a digester layout for you.