To keep treated leachate under 40°C in hot weather, install a plate heat exchanger and cool its water circuit with an air blast cooler, an evaporative cooling tower or an electric chiller. The right choice depends on how cold the discharge must be, how much power you can spare and how much space you have. This article compares the three using a Hong Kong leachate treatment plant as the worked example.
The challenge: summer temperatures at discharge
Thermal ammonia stripping plants heat leachate to strip out the ammonia. They recover most of the heat, but treated liquid still leaves warmer than ambient. At one site, additional cooling was needed to keep the exit leachate below 40°C through the summer months.
This is not unusual. Discharge consents and downstream treatment processes often set temperature limits, because warm effluent can harm biological treatment and reduce dissolved oxygen in receiving water.
The approach: a plate heat exchanger and a cooled water circuit
The first element is the same in every option: a plate heat exchanger on the exit leachate. Plates were recommended because they are easy to maintain and clean, an important feature for a liquid that can foul. The heat exchanger then needs a cooled water circuit to carry the heat away.
Three ways of cooling that circuit were offered.
| Air blast cooler | Evaporative cooling tower | Electric chiller | |
|---|---|---|---|
| Achievable temperature | About 38°C | Up to about 36°C | Up to about 32°C |
| Fan or compressor power | 12 kW | 5 kW fan | 152 kW |
| Circuit pump | 5.5 kW | 5.5 kW | 5.5 kW |
| Footprint | Fitted above existing coolers | About 2 m x 2 m | About 2.4 m x 3 m |
| Lead time | 6 to 7 weeks | 2 to 3 weeks | 8 weeks |
Equipment would also need about four weeks of sea freight from the UK.
Why the choice matters
Air blast cooler
It uses no water and is simple, but it can only cool to a few degrees above ambient. At 38°C discharge there is little margin under a 40°C limit. Its advantage here was that it could be mounted above existing units, saving ground space.
Evaporative cooling tower
It is the lowest-power option, because evaporation does most of the work, and it reaches a lower temperature than the air blast cooler. The trade-off is that it consumes make-up water and needs water treatment to control scale and biological growth. It was also the quickest to deliver.
Electric chiller
It gives the lowest temperature and the greatest margin. It also absorbs more than 12 times the fan power of the air blast cooler, which adds to the running cost every hour it operates. It makes sense only if the temperature limit is tight enough that the other two cannot meet it.
What was excluded
Installation and all associated pipework and fittings were outside the scope. Check these early because they can add considerably to the total.
What to consider for your own project
- Start from the limit. Know the maximum permitted discharge temperature and the hottest ambient temperature you design for.
- Look at the approach temperature. The gap between the cooled water and the ambient air determines which technology can meet the target.
- Count the running power. A chiller’s absorbed power can dwarf the rest of the cooling package.
- Check water supply. A cooling tower needs make-up water and treatment.
- Allow for fouling. Choose a heat exchanger you can open and clean.
FAQ
Why cool leachate at all?
Warm discharge can breach consent limits and disrupt downstream biological treatment.
Which cooling option uses least power?
In this comparison, the evaporative cooling tower, at about 5 kW for the fan, against 12 kW for the air blast cooler and 152 kW for the chiller.
Can an air blast cooler get below ambient temperature?
No. It can only approach the ambient air temperature, which is why it gave the highest discharge temperature of the three.
Why use a plate heat exchanger?
It is compact and easy to dismantle for cleaning.
Is your effluent too warm?
Talk to us about your discharge temperature and we will help you choose a cooling option.