A leachate evaporator can cut landfill leachate to roughly 4% of its original volume using only waste heat from the site’s gas engines, with the vapour destroyed in an enclosed flare. In a proposal for a landfill operator in the Asia-Pacific region, the design took 100 m³ of leachate a day at 0.6% total solids and returned about 4 m³ a day of concentrate at 10 to 15% solids. The question that decided whether it would work was not the evaporator itself. It was whether the engines could supply enough heat.
The challenge: 100 m³ a day, no visible plume
The operator already ran an evaporator that was only just keeping pace with 100 m³ a day. It wanted a second system that was completely separate, so that commissioning could not disturb the existing plant, which might then be kept as a back-up.
Three constraints shaped the brief:
- Concentrate quality. The operator would prefer a concentrate at 5 to 10% solids, around a 90% volume reduction. The design basis agreed was 10 to 15% solids, with the note that a thinner concentrate would simply allow more throughput.
- No visible plume. The site had recently renegotiated its operating consents, and no visible plume was one of the conditions.
- Ammonia. Evaporating leachate drives off ammonia with the water vapour, so the vapour cannot simply be released.
The approach: heat, evaporate, burn
How the process works
Air is heated in a heat exchanger driven by waste heat from the gas engines. The warm air passes through an evaporator unit, where leachate is sprayed over evaporation panels (a packed bed). A fan draws the saturated, ammonia-bearing vapour out and sends it to an enclosed flare, where it is burned at 1,000°C to destroy the ammonia. The concentrate drains to a discharge vessel.
The only moving parts are the pumps and fans, and the system runs automatically, so maintenance and operating effort are low.
Design data
| Parameter | Value | What it means |
|---|---|---|
| Leachate flow | 100 m³/day | Roughly four cubic metres every hour |
| Inlet total solids | 0.6% | A relatively weak leachate |
| Outlet total solids | 10 to 15% | About 4 m³/day of concentrate, a 96% volume reduction |
| Average temperature range | 14.7 to 23.7°C | Mild, but cold enough that pipework is insulated |
| Average humidity range | 77 to 89% | Humid ambient air limits how much water it can pick up, so the air has to be heated well above ambient |
Heat from four engines
The proposal fitted simple economiser heat exchangers to four of the site’s existing gas engines. The operator’s main concern was whether that heat would be enough, so the next step was a heat and mass balance. Organics normally issues this as part of a front-end engineering design (FEED) package, together with a process flow diagram, P&ID, general arrangement and equipment datasheets. At the client’s request an indicative balance was offered earlier, with the caveat that it was subject to detailed design.
Two practical points came out of the discussions:
- The engines’ exhaust back pressure tolerance was a maximum of about 50 mbar, which limits how much exhaust-side heat exchange is acceptable.
- The site’s engine cooling water already supplied heat to a neighbouring grower, at roughly 89°C out and 65°C return. Any heat taken for the evaporator has to be reconciled with that existing use.
Reusing an existing flare
The site had spare enclosed flares, and the operator wanted to use one to burn the evaporator vapour. The modification was small: three equally spaced 200 mm nozzles in the base, with internal and external flanges, to take the ammoniated air. The existing air louvres did not need changing.
The less obvious issue was turndown. The flare’s 5:1 turndown would sit at the bottom of its range for this duty. The vapour is mostly saturated air, so supplementary landfill gas is needed to hold the combustion temperature. The proposal estimated this at around 700 Nm³/h, subject to detailed design (an earlier estimate was higher at 1,000 Nm³/h). That fuel demand matters because it feeds directly into the payback calculation.
Why it works, and the trade-offs
Evaporation suits sites that have free heat and a need to avoid discharging leachate. Its cost is that the flare must burn a large volume of humid air. The trade-offs the operator raised were these:
- Heat versus gas. The more engine heat available, the less supplementary gas the flare consumes.
- Corrosion. The operator had seen corrosion and erosion from turbulence around the injectors of its existing evaporator. The new design uses spray over a packed bed and 304 stainless steel (Schedule 10) for the evaporator and pipework, with thickness confirmed at detailed design.
- Availability. The operator expected 5 to 8% downtime in the first year and 8 to 10% afterwards. That is easy to meet when it allows for breakdowns and cleaning.
- Inspection. Hatches were accessible. Weekly checks were suggested until performance stabilised, then monthly.
The proposal also had to allow for local seismic loadings, a handrail at the top of the evaporator for safety, and integration with the site’s SCADA system. In the UK, leachate management and gas flaring on landfill both fall under the site’s environmental permit; see the government guidance for landfill operators.
What to consider for your own site
- Quantify the heat first. Get engine heat outputs and the temperatures you can use. Ask for a heat and mass balance before committing capital.
- Decide where the vapour goes. If ammonia is present, it must be burned or otherwise treated.
- Check the flare’s turndown and fuel use. Supplementary gas is a running cost.
- Keep it separate. A stand-alone circuit with its own buffer tank lets you commission without risking the existing plant.
- Allow time. This proposal’s programme ran to about 50 weeks from order, covering a design phase, manufacture, shipping, installation and commissioning.
FAQ
How much can an evaporator reduce leachate volume?
In this design, from 100 m³ a day to about 4 m³ a day, a reduction of around 96%, with the concentrate at 10 to 15% solids. A lower-solids concentrate gives less reduction but higher throughput.
What happens to the ammonia?
It leaves with the vapour and is destroyed by burning at 1,000°C in an enclosed flare.
Can an existing flare be reused?
Often, with modification. Here it needed three 200 mm nozzles in the base. The flare’s turndown range and its supplementary gas demand must be checked.
Is engine waste heat really enough?
It depends on engine size and the temperatures available. That is why a heat and mass balance comes before the final price.
Talk to us about leachate treatment
If you have leachate to manage and engines producing waste heat, we can help you test whether an evaporator makes sense. Get in touch with your flow, leachate strength and engine details.