Landfill gas is the best fuel for a thermal ammonia stripper because the ammonia-laden air can be used as combustion air. Diesel cannot do this, so the destructor must be about 50% larger and burns a lot of fuel. In a tender for a plant treating roughly 2,000 m³/day of landfill leachate, that difference decided the whole plant size.
The challenge: destroy the ammonia and supply the heat
A thermal ammonia stripper heats leachate and passes it down a packed column against a stream of air. Ammonia moves from the liquid to the air. The ammonia-laden air then has to be dealt with, and the usual answer is a thermal oxidiser at 850°C, which converts ammonia to nitrogen and water.
The same combustion also generates heat, which can be recovered to warm the incoming leachate and raise steam. The question for the designer is what to burn. The tender asked for a diesel back-up in addition to the landfill gas supply, so the proposal compared the options.
The approach: sizing a plant for 2,000 m³/day
The design basis was:
- Leachate flow of 2,000 m³/day, with ammonium design strength of roughly 4,500 mg/l and a maximum of about 5,100 mg/l.
- A treated effluent target of 200 mg/l ammonium.
- Landfill gas of about 50% methane, supplied at 2,250 to 2,750 Nm³/h at 50 mbar.
- Strict emission limits on the oxidiser exhaust, including NOx, CO, SO₂, benzene, vinyl chloride and non-methane organic compounds.
Two lines, one destructor
The plant has two complete stripping lines in a duty and standby arrangement, each capable of the full 2,000 m³/day, with a single shared thermal destructor. A line can run alone, or both can share the load at 50% each.
The reasoning is that servicing a destructor typically needs only a few hours of downtime. A second full destructor would be an unnecessary expense. The stripping columns, pumps and heat exchangers are the items that need cleaning and maintenance, so those are duplicated.
How the heat is reused
- The incoming leachate is pre-heated in a plate heat exchanger by treated leachate leaving the system.
- Hot exhaust from the oxidiser passes through an economiser that makes steam, which is injected into the stripping air.
- Humid air carries more heat, so the stripping column stays at the right temperature.
- The treated leachate is cooled to a discharge temperature below 40°C.
Why landfill gas wins: the fuel comparison
When the oxidiser burns landfill gas, the ammonia-laden air feeds the burners as primary and secondary combustion air. The gas provides the heat for destruction and the plant needs little extra fresh air.
Diesel burners cannot use the ammonia-laden air as primary air. They need fresh air for their forced-draught fans, and the ammonia air can only serve as cooling air at the tertiary stage. The consequences, from the proposal:
| Fuel | Approximate destructor rating | Approximate fuel use |
|---|---|---|
| Landfill gas | About 15 MW | About 2,750 Nm³/h |
| Diesel | About 23 MW, around 50% larger | About 1,870 kg/h |
| LPG | About 15 MW, as for landfill gas | About 1,160 kg/h |
A gaseous fuel such as LPG can also use the ammonia air as primary air, which brings the rating back to about 15 MW. That makes LPG a far better back-up than diesel. The proposal therefore left ports for future diesel burners, but recommended gaseous fuels.
Running costs and utilities
- Electricity: about 200 kW per running line.
- Water: 8 to 14 m³/h.
- Chemicals: only an anti-foam agent, at around 1 litre per hour.
The low chemical demand is a major advantage over pH-driven stripping, which needs large quantities of acid and alkali.
Programme and acceleration options
The normal programme allowed 6 weeks for preliminary design, 6 for detailed design and 8 for drawings. After design approval, which can take up to six months, came 27 weeks of procurement and fabrication, an estimated 20 weeks of installation and 4 weeks of commissioning. No manufacture started until all approvals were in.
The proposal also described ways to compress the programme when needed:
- Design: doubling the engineering team could shorten a 14-week drawing phase to about 10 weeks.
- Components: paying expedited delivery fees can save 4 to 6 weeks, depending on the supplier.
- Fabrication: more workshops, extra technician teams and overtime.
- Shipping: air freight takes 5 to 10 days, compared with 28 to 40 days by sea from the UK to the site.
What to consider for your own project
- Tell the supplier what fuel you will actually have. The destructor size depends on it.
- Check gas pressure. This design needed landfill gas at 50 mbar, with boosters provided separately.
- Take the emission limits seriously. Sulphur in the gas affects SO₂, and the destruction efficiency governs trace compounds such as benzene.
- Decide how much standby you need. Duplicating the stripping lines, rather than the destructor, gave resilience at lower cost.
- Allow time for approvals. They can add months before manufacture begins.
If the ammonia is better recovered than destroyed, see our post on thermal stripping with an acid scrubber.
FAQ
What is a thermal ammonia stripper?
It heats wastewater and passes it through a packed column against warm air. Ammonia moves into the air, and the air is then treated, for example in a thermal oxidiser.
Why use a thermal oxidiser for the ammonia?
It destroys the ammonia completely at about 850°C and needs no chemicals, unlike an acid scrubber, which produces a salt that must be handled.
Can the plant run without landfill gas?
Yes, with diesel or LPG burners, but the destructor must be larger and the running cost is higher. Diesel is the least attractive option.
Does the process rely on bacteria?
No. It is based on physical chemistry, so there is no biological population to maintain or upset.
Treating leachate with high ammonia?
Contact us with your leachate analysis and available fuel or waste heat, and we will recommend a stripping configuration.