A pH-driven ammonia stripper suits a small, space-constrained site that needs to cut ammonia sharply and cannot run a biological plant. In a design for an indoor facility treating about 120 m³/day of wastewater, raising the pH and stripping with air was proposed to take ammonia from 1,500 mg/l down to below 40 mg/l, with a separate reactor to polish out hydrogen peroxide.
The challenge: high ammonia, hydrogen peroxide and no room outdoors
A design-and-build enquiry came from a microelectronics-related technology park in East Asia. The wastewater carried two problems at once:
- Ammonia at up to 1,500 mg/l, to be reduced to less than 40 mg/l. That is a removal of roughly 97%.
- Hydrogen peroxide (H₂O₂) at up to 1,500 mg/l, to be reduced to less than 10 mg/l.
The flow was about 120 m³/day, at a pH of 7 to 9, a temperature of 15 to 30°C and about 100 mg/l of suspended solids. Land was at a premium, and the plant had to sit inside a building with limited height and door width.
The approach: choosing the process
Four routes to ammonia removal were considered. The reasoning is useful for any similar enquiry.
| Option | Main drawback for this site |
|---|---|
| Biological removal, such as a sequencing batch reactor | Needs a large land area and may need an expensive carbon source |
| pH-driven air stripping | Needs pH-adjusting chemicals held on site |
| Steam stripping | A more complex process train, so likely to cost more |
| Thermally driven stripping | Needs a waste heat source, otherwise premium energy makes it too costly |
No waste heat was identified and land was scarce, so pH-driven stripping was proposed. The chemistry is simple. Raising the pH converts ammonium in the water to dissolved ammonia gas, and air passed through the water carries it away. Caustic soda is preferred for this. Lime has worked in some cases.
Three stripping stages instead of one tall column
An indoor site changes the geometry. A single stripper column would have been too tall for the building, so the same duty is split across three columns in series. Every item is also sized to fit inside an ISO shipping container, which means it passes through the building’s doors.
The modular package comprises:
- Two equalisation tanks with a transfer pump set, to smooth out variations in flow and strength.
- A mixing vessel where the pH is raised by dosing pump, with pH measured by a sampling probe.
- A sand filter with backflush, to protect the packing from solids.
- A hot water system, rated at 20 m³/h at 60°C, because stripping works better when the water is warm.
- Three GRP stripper columns with packing, packing cleaning systems and anti-foam dosing.
- An acid scrubber that captures the ammonia from the stripping air, so it is not released to the room or the atmosphere.
- A dedicated hydrogen peroxide polishing reactor.
- Two air blowers, ducting, control valves, a PLC and instrumentation.
Why a separate hydrogen peroxide reactor
Air stripping removes a certain amount of hydrogen peroxide as a side effect, but not reliably enough to guarantee less than 10 mg/l from 1,500 mg/l. A dedicated polishing reactor makes the second target a designed outcome. It is a good example of a rule that applies widely: if the discharge limit covers two pollutants, each needs its own stage of treatment.
Why pre-packaging helps
Building the units as separate skid modules allows pre-delivery testing in the factory. On site the work reduces to final assembly. The proposal also kept a clear boundary. Chemical storage, inlet and outlet buffer storage, the feed pump, civil works, the building and the power supply were left outside the supplier’s scope, so they must be planned by the project team early.
Programme
The proposed timetable ran to about 52 working weeks:
- Detailed specification: 6 weeks
- Design: 8 weeks
- Procurement and fabrication: 24 weeks
- Installation on site: 10 weeks (estimated, by others)
- Commissioning and performance testing: 4 weeks
This excludes time for the client’s engineers to approve the design, which should be added to any realistic programme.
What to consider for your own project
- Be clear about your chemical supply. A pH-driven stripper needs a safe, reliable store of caustic soda or lime, and a plan for pH adjustment of the treated water if your discharge consent requires it.
- Measure the full wastewater range. Peak ammonia strength matters more than the average when sizing the stripper.
- Check for other pollutants. Hydrogen peroxide here needed its own stage. Solids, foam-forming compounds and scaling metals can all affect a stripper.
- Look at the building early. Height, door width and floor loading can decide whether you need one column or several.
- Consider the ammonia you capture. An acid scrubber turns it into a salt solution that needs a disposal or reuse route.
For UK sites, discharge limits and permitting depend on the receiving water or sewer. Start with the Environment Agency’s environmental permits guidance.
FAQ
How does a pH-driven ammonia stripper work?
Raising the pH shifts dissolved ammonium to ammonia gas. The wastewater trickles down a packed column against a flow of air, which carries the gas out of the water.
Why not use biological treatment?
Biological nitrification needs a large tank volume and, for high-strength flows, may need an added carbon source. Where space is tight, stripping is more compact. Stripping also does not depend on a bacterial population, which a sudden change in wastewater can upset.
What happens to the stripped ammonia?
In this design, an acid scrubber captures it from the air. The resulting solution is a salt that can be reused or disposed of.
Does stripping leave the water at the right pH?
No. The treated water leaves at a high pH, so a neutralisation step may be needed before discharge. In this case, discharge pH adjustment was left outside the supplier’s scope.
Planning an ammonia treatment plant?
Send us your flow, ammonia concentration and site constraints and we will tell you which process is likely to suit.