Ammonia stripping removes dissolved ammonia from wastewater by turning it into a gas and carrying it away in air or steam. For a given duty there is usually more than one way to do it, and the right choice depends on the chemistry of the water, the energy available and how much fouling the plant can tolerate. This post compares the three options offered for a large petrochemical wastewater duty: conventional steam stripping at elevated pH, thermal air stripping with no pH adjustment, and a vortex (cyclone) stripper.
The challenge: two big streams, one tight discharge target
A petrochemical complex in the Middle East needed to strip ammonia from two parallel wastewater streams. Each stream ran at 77 m³/h normally and up to 90 m³/h at maximum, so the combined duty was 154 to 180 m³/h. The design conditions were:
| Parameter | Inlet | Required outlet |
|---|---|---|
| Ammonia | About 400 ppm as ammonia (330 ppm as nitrogen) | Below 10 ppm |
| pH | 9.0 to 9.5 | Below 9.0 |
| Temperature | Ambient, about 20 to 30°C | Below 55°C |
| COD | About 350 ppm | Not specified |
| Suspended solids | Below 5 ppm | Not specified |
That is a reduction of roughly 97.5% in ammonia, at very high flow. A guarantee was offered that the discharge would be met at maximum flow with the specified inlet concentration.
The approach: three offers
Option 1: steam stripping at elevated pH (the tender specification)
This follows the specification as written. Per stripper, the scope was two plate-type heat recovery exchangers for the incoming and outgoing water, a stainless steel stripping column about 2.2 m in diameter and 15 m tall, overhead air coolers, an accumulator drum built to ASME VIII, and the associated pumps, valves and instruments. The columns are made from 316 stainless steel.
Option 2: thermal air stripping, no pH adjustment
This uses a process developed for landfill leachate in the Far East. It needs no chemical addition other than a small flow of anti-foam, and relies on heat alone to release the ammonia. Four plants of this type had been built, ranging from about 9 m³/h to 75 m³/h, taking influent ammonia of roughly 4,000 to 6,700 mg/l down to below 100 mg/l. For this duty the column would be 304 stainless steel, about 3.0 m in diameter and 18 m tall, with a humidification tower or air cooler.
Option 3: vortex (cyclone) stripping
This uses the same thermal principle as option 2, but replaces the packed column with a cyclone chamber. The stripping chamber would be about 2.4 m in diameter and 10 m tall, with a matching humidification chamber. The main advantage is that residual solids and material build-up are contained within a system designed to be serviced, avoiding the extensive cleaning that packing can need. The column height is also reduced. Of the three offers it had the lowest capital cost by a clear margin.
What the detailed design looked like
The more detailed scope for the vortex option shows how much plant sits around the stripper. Each train included plate heat exchangers to pre-heat the influent and cool the effluent, shell-and-tube heat exchangers to heat the water to process temperature, a humidification tower to pre-heat the stripping air and cool the exiting water, and the cyclone strippers. The ammonia-laden air, at about 40,000 m³/h and 70°C, would be destroyed in a flare, and a separate flare was already on site. Steam demand was in the region of 14 to 17 tonnes per hour. Anti-foam dosing was estimated at about 50 litres per hour, subject to commissioning.
The control philosophy was deliberately simple. Experience on earlier installations suggested a simple control system gives the most stable results, because the variables are few and the key task is initial setting-up and tuning. Flows and valve positions are pre-set, and three PLC panels (a master and one per stripper) coordinate the plant and report faults as summary alarms.
Why the options differ
| Steam stripping | Thermal air stripping | Vortex stripping | |
|---|---|---|---|
| pH adjustment | Elevated pH | None | None |
| Column size | About 2.2 m x 15 m | About 3.0 m x 18 m | About 2.4 m x 10 m |
| Fouling risk | Packing | Packing | Contained, serviceable |
| Relative capital cost | Highest | Slightly lower | Lowest |
The client was asked to say which option was of interest so that further detail could be provided for that technology alone.
What to consider for your own project
- Know your inlet chemistry. pH, ammonia, COD, chlorides and solids all affect which process is viable.
- Count the utilities. Steam and cooling demand can dominate running cost. Steam raising was excluded from this scope.
- Decide what happens to the ammonia. Here it was sent to a flare. Recovery equipment, for example as ammonium salt, was available but excluded.
- Think about cleaning. Packing that fouls needs shutdowns. Equipment designed for cleaning in place can reduce downtime.
- Check discharge limits. For UK projects, start with the Environment Agency’s guidance on discharge permits.
FAQ
How does ammonia stripping work?
Wastewater is heated, or its pH raised, so dissolved ammonia becomes a gas, which is carried away by air or steam.
Do you always need to raise the pH?
No. One of the options here relied on temperature alone and needed only a small amount of anti-foam.
What is a vortex stripper?
It replaces a packed stripping column with a cyclone chamber, which reduces height and contains solids build-up.
What happens to the stripped ammonia?
In this design the ammonia-laden air went to a flare for destruction.
Weighing up an ammonia stripping duty?
Send us your flow and water analysis and we will compare process routes for you. For a smaller-scale example, see our post on bulk ammonia reduction with ammonium sulphate recovery.