For a large leachate flow, two independent ammonia stripping lines that share one thermal destructor give you capacity, resilience and heat recovery in one package. This design for a landfill extension in Hong Kong treats up to 2,500 m³ a day of leachate from about 5,300 mg/l ammonia nitrogen to below 150 mg/l, a reduction of more than 97%, using landfill gas as the fuel.
The challenge: high flow, strong leachate
| Parameter | Design value |
|---|---|
| Flow | 2,500 m³/day |
| Ammonia nitrogen in | 5,300 mg/l |
| COD in | 15,000 mg/l |
| Ammonia nitrogen out | below 150 mg/l |
| Effluent temperature | 40°C |
| Biogas available | 4,800 Nm³/h at 50% methane |
Climate also matters: the design covers cold conditions at 10°C and hot, humid conditions at 35°C and 90% relative humidity.
The approach: two lines, shared destructor
Each line is rated for 55 m³/h, and either can run while the other is shut down for cleaning. Both feed their ammonia-laden air into a single shrouded thermal destructor, which burns the ammonia and the fuel gas together.
Following the flow
- Heating. Leachate is heated in a shell-and-tube heater using heat from the destructor exhaust via an economiser.
- Stripping. It enters the top of a column about 24 m tall. Roughly 13 m is stripping section and 6 m is a cooling section, in one structure made of a high-grade corrosion-resistant stainless steel.
- Air. Atmospheric air passes up through the cooling section, then steam is injected to bring it to the right conditions before it enters the stripper.
- Destruction. Ammoniated air leaves the top and goes to the thermal destructor as primary and secondary combustion air.
- Heat return. Exhaust gas heats the economiser. Heat is also recovered from exiting leachate and from the ammoniated air before combustion, and is passed to the incoming leachate.
Why share the destructor
- One destructor is cheaper than two and easier to control.
- Each line still has its own heater, steam generator, column and fans, so one can be isolated.
- The economiser is built as five cassettes, for easier cleaning.
The trade-off is that a destructor outage stops both lines. That single point of failure should be weighed in the maintenance plan.
What else is in the package
- Feed water treatment: a steam-fed de-aerator that removes about 80% of dissolved gases and a duplex demineralisation system.
- Air blast coolers for the second condenser stage.
- A control panel with a large operator display.
- Supply, supervision of installation and commissioning. Civil design, installation of mechanical and electrical works and approvals are outside the scope.
What to consider for your own project
- Know your fuel gas. The plant depends on a steady landfill gas supply.
- Plan cleaning. Two lines allow cleaning without a full shutdown.
- Check materials. Leachate is aggressive, so use high-grade stainless steel.
- Allow for buffer storage. Inlet and outlet tanks are normally by others.
For UK landfills the Environment Agency’s guidance on managing landfill gas is the starting point.
FAQ
Why use landfill gas as the heat source?
It is available on site and is otherwise flared.
What happens to the ammonia?
It is burned to nitrogen and water in the thermal destructor.
Can one line handle the flow alone?
Each is rated for 55 m³/h, so one line treats about half the maximum.