Landfill mining can turn an existing dump into a fuel source and free up tipping space at the same time. In a proposal for a city landfill in South-East Asia, waste already buried was dug out, screened into a solid refuse fuel (SRF) and fed to pyrolysis plants in three phases totalling 12.5 MW. The design aimed to cut the waste needing final landfill by as much as 93%.

The challenge: a full landfill and a growing city

The city collected around 500 to 600 tonnes of municipal waste a day, mostly household, with an organic fraction assumed at about 64%. The existing dump held roughly 140,000 tonnes. Mass-burn incineration was ruled out because wet, high-organic tropical waste burns poorly and costs a great deal to run. Source separation was not realistic in the short term either.

The approach: let the landfill do the pre-treatment

The core idea was to landfill fresh waste loosely, about one metre deep, so air can reach it and it stabilises aerobically. Months later, much of the organic matter has degraded to soil, moisture has fallen and plastic bags are damaged. Mining that waste is then easier than treating it fresh:

• The waste is drier (around 30 to 40% moisture instead of much higher).
• Bags can be opened by a bladed trommel rather than an expensive bag opener.
• Roughly 100,000 tonnes of the 140,000 in situ were expected to be recoverable after stabilisation.
• Fresh waste is tipped into areas just mined, so the available residence time is as long as possible.

From landfill to fuel

1. Mining. Excavators dig 10 to 12 tonnes an hour and work about 10 hours a day.
2. Screening. A rotary trommel removes material over 60 mm, mainly soil and fines, at up to 10 tonnes an hour each.
3. Covered storage. The primary stage works by day and the secondary by night, so a covered store gives 24 hours of buffer, at about 50 m² per tonne an hour of throughput.
4. Shredding and drying. Pyrolysis needs SRF under 25% moisture and under 50 mm. Shredders (10 t/h each) cut size and thermal-screw dryers (4 t/h each) take moisture from about 40% to 20 to 25%.
5. Feed hopper. Sized for about six hours, because stopping a pyrolyser is costly.

Roughly 20% of what is mined is still organic. That goes to tunnel composters for around eight weeks. The proposal treated compost sales as a windfall because waste-derived compost struggles in an open market.

The three phases

Together that is 12.5 MW and roughly 215,000 tonnes mined a year. The design basis for SRF was a dry calorific value of about 15 MJ/kg at 20% moisture and 85% availability. In Phase I, 140 tonnes a day of mined waste gave about 105 tonnes of wet SRF and about 82 tonnes dry, with rejects and moisture making up the rest.

Power generation and clean-up

Each pyrolysis module heats the SRF without oxygen to make a fuel gas, gasifies the char, and burns the gas in a thermal oxidiser. The hot exhaust heats the pyrolysis tubes and then a boiler, which raises steam for a turbine. Flue gas passes through sodium bicarbonate injection for acid gases, a bag filter, a mercury capture filter and catalytic nitrogen oxide reduction before a 15 m stack. The proposal compared expected emissions to limits in the EU waste incineration directive and showed the design well inside them, including particulates, hydrogen chloride and dioxins. These are design values, not measurements.

Why it works, and trade-offs

• Waste returned to the landfill. Screening rejects (about 7% of mined waste) and ash (about 5% of pyrolyser feed) go back into the space created by mining, so no new landfill volume is needed.
• Two sites to cut diesel. The first-stage separation sits in the middle of the landfill, so excavators and rejects travel only short distances, while the covered secondary stage sits beside the shredders.
• Water. Full capacity needs around 1,620 m³/day, from leachate treatment and a nearby river.
• Land and jobs. The plant needs roughly one hectare and 20 to 25 local jobs, with sorting done by people already working informally on the site.
• Leachate. Collecting and managing leachate replaces uncontrolled escape.

The weak points are fuel supply, since the waste is a finite resource, and the need for a power purchase agreement that covers the cost of generation. The proposal said that if waste proved insufficient, a financial evaluation would decide which phases to cut.

What to consider for your own project

1. Characterise the buried waste. Age, depth, moisture and inerts decide how much is usable.
2. Plan the tipping pattern. Loose, shallow placement helps stabilisation but uses more area.
3. Size storage to the shift pattern. If one stage runs 10 hours and the other 24, the buffer matters.
4. Treat rejects as part of the design. They need a permitted place to go.
5. Test for odour and leachate. Mining disturbs both.

For UK guidance on landfill and waste regulation, see the Environment Agency’s published guidance. For a related RDF project, see our RDF feasibility post.

FAQ

What is landfill mining?

Excavating buried waste to recover fuel, materials and space.

Why not burn fresh waste?

Wet tropical waste has low energy value and would use more energy to dry than it yields without separation.

What happens to what cannot be used?

Rejects and ash go back to the landfill in the space freed by mining.

Thinking about landfill mining?

Tell us about your site and tonnages and we will outline a phased route.