Ammonia contamination is one of the most persistent challenges in wastewater management, particularly in leachate from landfill sites, digestate liquors, and industrial effluents. Left untreated, elevated ammonia levels can severely damage aquatic ecosystems and breach discharge consent limits. Air stripping remains one of the most reliable and scalable technologies for tackling this problem, and it can be implemented in two distinct ways depending on site conditions and available resources.
What Is Ammonia Stripping?
Ammonia stripping is a physical treatment process that transfers dissolved ammonia from the liquid phase of wastewater into a gaseous phase using air. Once volatilised, the ammonia gas is either released, captured in an acid absorber, or destroyed via thermal oxidation. The process is highly effective and can be tailored to suit widely varying flow rates and site infrastructures.
There are two established approaches to achieving this transfer:
- pH-driven stripping — raising wastewater pH to approximately 11–12
- Thermally-driven stripping — raising wastewater temperature to approximately 65–70°C
Both methods exploit the same underlying chemistry — shifting the ammonium/ammonia equilibrium so that free ammonia gas is released — but they differ significantly in operating costs, capital investment, and suitability depending on what energy resources are already available on site.
Choosing Between pH-Driven and Thermally-Driven Systems
Selecting the right stripping technology is very much a site-specific decision, driven by the availability of waste heat, energy costs, and chemical pricing in the local market.
Where waste heat is readily available — for example from an engine, flare, or steam system — thermally-driven stripping typically delivers substantially lower operating costs, since it eliminates the need for chemical dosing entirely.
Where no waste heat source exists, pH-driven stripping is often the more economical route, although this depends heavily on local energy and chemical costs. Because of this trade-off, a proper technical and economic assessment is essential before committing to either pathway.
pH-Driven Air Stripping
In this method, lime or caustic soda is dosed into the wastewater to raise the pH into the 11–12 range. This chemical addition serves a dual purpose: it shifts the ammonium-ammonia balance towards free ammonia gas, and it also precipitates certain salts and larger organic molecules, helping to reduce residual COD.
The typical process flow is:
- pH adjustment via lime or caustic soda dosing
- Filtration to remove suspended solids ahead of the stripping column (essential, since high solids loads can rapidly foul the packing media)
- Stripping in a packed-bed column, where the partial pressure of ammonia drives its transfer from liquid to gas phase
- Optional multi-staging for very low residual ammonia targets
- Optional acid absorption of the exhaust air to capture stripped ammonia
- Final neutralisation of the treated effluent with acid before discharge
Because large volumes of air are required per unit volume of wastewater to achieve low residual concentrations, operating costs can rise quickly at scale. This means pH-driven stripping tends to be more economically viable at lower flow rates, though the ultimate decision depends on full process economics.
Thermally-Driven Air Stripping
Thermally-driven systems achieve ammonia volatilisation by heating the wastewater rather than adjusting its chemistry. This generally involves a more complex plant setup, including:
- The stripping column itself
- An inlet heater
- A cooling tower for inlet air humidification
- A steam-raising facility to fully saturate the stripping air
- An optional exit air condenser to maximise overall efficiency
- An optional thermal oxidiser or ammonia gas scrubbing column to destroy or recover stripped ammonia
While capital costs are typically higher than a pH-driven system, thermally-driven stripping avoids ongoing chemical costs and can achieve superior removal rates — making it highly attractive wherever waste heat (from biogas engines, flares, or steam) is already part of the site infrastructure.
Performance and Removal Rates
Both technologies can achieve significant reductions in ammonia concentration, though thermally-driven systems generally edge ahead in peak performance.
Thermally-driven units can achieve ammonia removal of up to 98.5%, while pH-driven units typically achieve up to 90% removal — with residual concentrations as low as 50 mg/l achievable in standard configurations.
Technical Specifications
| Parameter | Detail |
|---|---|
| Flow rates available | 24 to 5,000 m³ per day |
| Materials of construction | Polyethylene and GRP (low temperature) up to stainless steels and high-grade alloys (elevated temperature, high-corrosion duties) |
| Heat sources | Engine exhaust gases, flare exhaust gases, high-temperature hot-water streams, steam |
| Chemical requirements | Caustic soda/lime (pH adjustment); nitric, hydrochloric or phosphoric acid (pH re-adjustment); anti-foaming agent as needed |
| Energy requirement | ~450 MJ per m³ of wastewater treated (equivalent to 25 m³ of 50%-methane biogas) |
| Waste heat capacity example | A 1 MW electrical engine provides sufficient exhaust heat (~50% of electrical power) to treat 80–100 m³/day of leachate |
| Pre-treatment | Coarse filtration for solids over 2 mm diameter |
| Land requirement (example) | A 200 m³/day stripper occupies approximately 120 m² |
| Removal efficiency (pH-driven) | Up to 90% |
| Removal efficiency (thermally-driven) | Up to 98.5% |
| Residual ammonia (standard config.) | As low as 50 mg/l |
Applications
Ammonia stripping systems of this type are widely used for:
- Landfill leachate treatment
- Anaerobic digestate liquor polishing
- Industrial and agricultural wastewater treatment
- Sites seeking to meet strict nitrogen discharge consent limits ahead of final effluent discharge
Because flow rates from 24 up to 5,000 m³ per day can be accommodated, these systems scale from small on-site installations to large centralised treatment facilities.
Turnkey Delivery and Piloting
For organisations evaluating whether ammonia stripping is right for their site, turnkey design, build and operate services are available, along with pilot plant trials for new installations. This allows operators to validate performance and economics on their actual wastewater stream before committing to a full-scale system, backed by a range of instrumentation and control options for ongoing process management.
Key Takeaways
- Ammonia stripping can be achieved via pH adjustment (to pH 11–12) or thermal heating (to 65–70°C)
- Thermally-driven systems achieve up to 98.5% removal but carry higher capital costs, offset by lower operating costs where waste heat is available
- pH-driven systems achieve up to 90% removal, with lower capital costs but ongoing chemical expenses
- Systems scale from 24 to 5,000 m³ per day, with a compact footprint (~120 m² for a 200 m³/day unit)
- Site-specific factors — energy availability, chemical costs, and flow rate — determine the most economical choice
- Pilot trials and turnkey delivery options are available to de-risk new installations
Get in Touch
Choosing between pH-driven and thermally-driven ammonia stripping requires careful evaluation of your site’s energy resources, wastewater characteristics, and discharge targets. Our technical team can help assess your specific requirements, model the operating economics, and design a turnkey solution — including pilot plant trials to validate performance before full-scale investment. Contact our Technical Sales Department today to discuss the right ammonia stripping solution for your site.