Dry fermentation digests the organic fraction of municipal waste in sealed concrete cells without adding water. A proposal for a city landfill site in South-East Asia combined it with biogas upgrading to compressed biomethane for vehicles and with gasification of the digestate to make electricity. This article explains how the pieces fit together at 800 tonnes of waste a day.
The challenge: wet, organic waste and a city fleet that runs on gas
The waste is 77% wet organic matter, 10% other and 13% plastic, with moisture around 50%. The city already had compressed natural gas distribution and many gas vehicles, so a gas of equivalent quality could be used without vehicle modification. The existing landfill was shallow and uncapped, so little landfill gas was available.
The source also stresses why mass-burn incineration is a poor fit: without sorting, it consumes energy in humid, wet-waste climates.
The design
| Item | Value |
|---|---|
| Waste | 800 tonnes a day |
| Biogas | About 1,540 Nm³/h at 60% methane (60 to 80 Nm³ per tonne) |
| Biomethane | About 14 tonnes a day |
| CO₂ removed | About 30 tonnes a day |
| Delivery pressure | 250 bar |
| Digestate to gasifier | About 368 tonnes a day (dry organic matter) |
| Power from gasification | About 15.5 MW installed, 340 MWh a day |
| Land | About 6 hectares |
How dry fermentation works
Waste goes by loader into large concrete cells of 5,000 to 10,000 m³ with an automated airtight door. Percolate (leachate) is sprayed over the waste to keep it moist and re-inoculate it with bacteria; no stirring is needed. Cells run in batches at around 50% dry matter, normally at mesophilic temperatures (35 to 40°C), though ambient may suffice. Because the non-biodegradable fraction creates water pathways, the waste need not be perfectly sorted, though rubble, large wood and metal are removed at reception.
Cleaning the gas for vehicles
- Water knock-out.
- Compression to about 7 bar g.
- Refrigeration drying to lower the dew point.
- Activated carbon to remove VOCs and H₂S.
- Pressure swing adsorption to remove CO₂.
- Final compression to 200 bar for transport and dispensing.
Quality was to be guaranteed against the Swedish biogas standard SS 155438, with continuous monitoring.
Digestate and power
Digestate does not need liquid-solid separation. It can be composted, but there was little market for mixed-waste compost, so the plan was to dry it as refuse-derived fuel and gasify it. Pyrolysis and gasification make syngas, which is burned in an oxidiser to raise steam for a turbine. Gasification occurs in an oxygen-deficient atmosphere, so there is limited opportunity for dioxins and furans to form. Surplus liquid goes to a sequencing batch reactor, and rejects go to landfill.
What to consider for your own project
- Be honest about revenue. The source treats carbon credits, recyclables and compost as windfall, not base case.
- Secure the contracts. Waste supply, land, gas off-take and power purchase were named as the four pillars.
- Plan for rejects. The landfill must stay open for them.
- Check emissions. Plan for fugitive methane, engine exhaust and flare.
For UK context see the Anaerobic Digestion Strategy and Action Plan.
FAQ
What is dry fermentation?
Batch digestion of stackable waste at about 50% dry matter in sealed cells, with sprayed percolate.
Why gasify the digestate?
It has fuel value, and mixed-waste compost has poor markets.
Why 250 bar?
That is the vehicle fuel delivery pressure used in the design.