The best landfill leachate treatment design treats the works as one system, not as a chain of separate processes with separate targets. A 2,000 m³/day leachate pre-treatment works proposed for a landfill extension in Hong Kong shows how: thermal ammonia stripping takes the ammonia, a membrane bioreactor (MBR) removes the organics, a screw press dewaters the sludge and activated-air units deal with odour.
The challenge: leachate changes over decades
Leachate is not constant. The design considered three phases of a landfill’s life:
- Early acetogenic: high COD/BOD and high ammonia. The stripping plant is not used.
- Early methanogenic: lower COD/BOD, high ammonia. The stripping plant is still not used.
- Developed methanogenic: low COD/BOD and high ammonia. The stripping plant operates.
This matters because biological removal of ammonia needs a carbon source. Early leachate has plenty of carbon, so biology can handle the ammonia. As the landfill matures, carbon falls but ammonia stays high, and that is when a thermal stripper earns its place.
The treatment train
Equalisation tank
Leachate is collected from the landfill and balanced here before treatment.
Thermal ammonia stripping plant
Landfill gas heats the leachate to about 70°C. It then flows down a column against a stream of air, which carries the ammonia away. The air passes to a thermal oxidiser, which converts ammonia to nitrogen and water. Two columns run duty-duty: both operate at high flows, and one can be shut down at lower flows.
The design links two heat loads. One heats the leachate and air; the other is the oxidiser. Economisers capture heat from the oxidiser exhaust and return it to the first load. Stripping before biological treatment avoids buying a carbon source such as methanol for nitrification and denitrification. The stripper does not change BOD or COD.
The main maintenance issue is carbonate scale on the column packing, especially where calcium is high. Two measures address it: polypropylene random packing with additives that discourage scale, and the ability to clean columns by slow recirculation of acid.
Membrane bioreactor
An MBR is an activated sludge process with a membrane to hold back the biomass. It separates the retention time of the liquid from the retention time of the sludge, so tanks can be smaller while biomass concentration is high. The design duty point is 15,000 mg/l of suspended solids in the tank, with membranes able to work up to 20,000 mg/l. With pores below 0.2 micrometres, the effluent is clarified and largely disinfected, and the flow does not depend on how well sludge settles.
Sludge dewatering
The specification asked for 30% dry solids. The sludge is conditioned with coagulant and flocculant, settled in a lamella unit, thickened in a screw drum and pressed in a rotary screw press, chosen because it is continuous, robust and needs little operator attention. Solids go to the landfill; the liquid returns to the equalisation tank.
Odour control
Air vented from under the MBR covers, about 7,500 Nm³/h, is treated by an activated-air unit. It produces oxygen radicals on a UV-lit catalytic mesh and injects them into a side stream, which is about 5 to 10% of the main flow (375 to 750 Nm³/h), so odour molecules are oxidised within centimetres. The unit needs about 10 kW. A Hong Kong transfer station trial was cited as giving odour reduction near 100%.
What to consider for your own project
- Design for all three leachate phases. A plant right for year one may be wrong for year twenty.
- Check carbon against nitrogen. The BOD to ammonia ratio decides how much the stripper must do.
- Plan scale control for columns where calcium is high.
- Use landfill gas as heat when it is available. See the Environment Agency’s LFTGN03.
- Include odour from day one. Covered tanks need vent treatment.
Our earlier article on thermal ammonia stripping at a landfill covers the stripping step in more depth.
FAQ
Why strip ammonia before the MBR?
It saves buying carbon for nitrification and denitrification, and relieves the biology.
What is MLSS?
Mixed liquor suspended solids, a measure of biomass concentration in the tank.
How are odours destroyed?
Activated oxygen radicals oxidise the odorous compounds into harmless products.