Ammonia contamination is one of the most persistent challenges facing operators of landfills, anaerobic digestion plants, and industrial wastewater facilities. Conventional treatment routes – biological nitrification/denitrification or chemical pH-adjusted stripping – can be expensive, chemically intensive, or simply impractical when ammonia loads are very high. Thermally-driven ammonia stripping offers a proven alternative: a process that uses waste heat, rather than chemicals, to strip ammonia from liquid effluent at high rates and with a small physical footprint.
This article explains how the technology works, why it was developed, and where it delivers the greatest value.
The Problem with Ammonia in Wastewater
Ammonia occurs naturally in domestic sewage at concentrations of around 30 mg/l, but in landfill leachate and other organic wastewater streams it can climb to several thousand milligrams per litre. Left untreated, ammonia causes serious environmental harm:
- Toxicity to aquatic life – lethal concentrations for fish range from just 2.5 to 25 mg/l.
- Oxygen depletion – as ammonia is biologically oxidised to nitrate, it exerts a heavy oxygen demand on receiving waters, potentially suffocating aquatic ecosystems.
- Eutrophication – ammonia acts as a fertiliser, triggering excessive growth of stringy bacteria and fungi that disrupt natural watercourses.
Because of these impacts, ammonia discharge limits are tightly regulated, and operators need a dependable, cost-effective way to bring concentrations down to compliant levels.
Two Conventional Routes – and Their Limitations
Historically, operators have relied on one of two approaches:
Biological Treatment
Bacteria in aerated or anoxic reactors convert ammonia to nitrogen gas. This is a robust and well-proven method for wastewater with relatively low ammonia and high carbon content. However, when ammonia levels are very high (roughly above 3,000 mg/l) and there isn’t enough carbon in the waste stream to drive the biological conversion, operators must dose in an external carbon source – an ongoing cost that can become excessive over the life of a project.
pH-Driven Chemical Stripping
This method raises the pH of the wastewater above 11 using lime or caustic soda, converting ammonium ions into ammonia gas, which is then stripped out using large volumes of air (air-to-liquid ratios of around 3,000:1) in a packed column or plate tower. The effluent is often re-dosed with acid afterwards to bring the pH back down for discharge. The major drawback is the ongoing cost of chemical dosing – which can be prohibitive unless a cheap, in-house supply of lime is available.
How Thermally-Driven Ammonia Stripping Works
Thermally-driven ammonia stripping breaks the ammonium-ion bond using heat alone, with only a minor addition of anti-foam agent required. No pH adjustment chemicals and no carbon-source dosing are needed.
The underlying chemistry is straightforward. Ammonia nitrogen exists in water in two forms:
- Dissociated ammonia (NH₄⁺) – the ammonium ion
- Undissociated ammonia (NH₃) – ammonia gas
The equilibrium between the two is governed by:
NH4+ + OH- = NH3 + H2O
The proportion of ammonia present as gas relative to total ammoniacal nitrogen (referred to as “f”) is expressed as:
f = [NH3] / ([NH3] + [NH4+])
Traditionally, this dissociation is pushed towards the gas phase by adding a base such as sodium hydroxide. The key innovation behind thermally-driven stripping is achieving the same dissociation shift using heat energy alone, avoiding pH adjustment entirely. Once ammonia is in gaseous form, it is driven out of the liquid phase with a controlled flow of air, achieved in a purpose-built stripping column.
In a single pass, thermally-driven ammonia stripping can achieve greater than 98.5% removal of ammonia – without a single drop of caustic or acid.
Where Does the Heat Come From?
The single essential input to the process is waste heat. This can be sourced from:
- Combustible waste gases (e.g. landfill gas or biogas)
- Engine exhaust gas
- Excess process steam
- High-temperature hot-water streams
Because the heat is typically otherwise unused (“waste” energy being disposed of), the long-term operating cost of the system can be confined largely to operation, maintenance, and electricity – a substantial saving compared to ongoing chemical or carbon-source purchases.
Disposing of the Stripped Ammonia
Once ammonia gas has been stripped from the liquid phase, it must be dealt with. Several options exist:
- Commercial recovery – reacting the ammonia gas with a sulphate or phosphate ion to produce ammonium sulphate or ammonium phosphate, both of which are used commercially as soil additives. This requires careful market and specification assessment to be commercially viable.
- Catalytic destruction – passing the gas through a catalytic bed, which operates at relatively low temperature and converts ammonia into harmless nitrogen gas.
- Combustion as process air – the preferred approach, where ammonia-laden air is fed into the heat-raising process as combustion air, destroying the ammonia as part of generating the very heat that drives the stripper.
This third option is especially elegant when paired with a low-NOx thermal oxidiser using Exhaust Gas Recycle technology. Detailed exhaust gas analysis has confirmed that NOx formation does not occur within this type of thermal oxidiser – ammonia is instead converted cleanly into nitrogen gas and water. Heat recovered from the thermal oxidiser via conventional economisers is then fed back to power the ammonia stripper, creating a closed, self-sustaining energy loop.
Key Performance Features
| Feature | Performance |
|---|---|
| Single-pass ammonia removal | > 98.5% |
| Footprint efficiency | Up to 840 kg NH₄-N/m²/day |
| Flow rate range | 50 – 3,000 m³/day |
| Maximum influent ammonia treated | Up to 6,700 mg/l reduced to < 100 mg/l |
| Energy requirement | ~450 MJ per m³ of leachate (≈ 25 m³ of 50% methane landfill gas) |
| Land requirement (200 m³/day plant) | ≈ 120 m² |
| Pre-treatment required | Coarse filtration of solids > 2 mm |
| Chemicals required | Anti-foam agent only (process-dependent) |
A 1 MW electrical engine, using roughly 50% of the waste heat in its exhaust, can provide sufficient energy to treat 80–100 cubic metres per day of leachate.
Operations & Maintenance Considerations
As with any continuously operating process plant, thermally-driven ammonia strippers require ongoing attention to keep performance optimal.
Packing Fouling
Carbonate build-up on the stripping column packing is a known issue, driven by specific wastewater chemistry. Systems are designed to tolerate a certain degree of fouling before cleaning is required, and cleaning systems are engineered to restore performance quickly. A wastewater composition analysis up front helps predict expected fouling rates and the level of cleaning sophistication needed.
Process Drift
Plant performance can drift over time as waste stream composition or operating conditions change. Regular, periodic performance reviews – assessing equipment condition, operating regimes, and output against target parameters – help catch and correct this drift before it affects compliance.
Real-World Case Histories
The technology has a long operational track record, with installations running for well over a combined 500,000 hours of compliant operation. A few illustrative examples:
| Site | Flow Rate | Influent Ammonia | Effluent Target | Configuration | Installed | Status |
|---|---|---|---|---|---|---|
| Site #1 | 1,800 m³/day | 7,000 mg/l | < 100 mg/l | Twin columns (50% capacity each) + twin landfill-gas-fired thermal oxidisers | 1997 | Still operating |
| Site #2 | 250 m³/day | 4,000 mg/l | < 100 mg/l | Single column, full capacity | 1998 | Still operating |
| Site #3 | 3,000 m³/day | 1,250 mg/l | < 50 mg/l | Single column, full capacity | 2006 | Still operating |
Site #1 is particularly notable: it was designed with duplicated, half-capacity columns specifically to guarantee spare treatment capacity is always available – a useful design principle for high-criticality applications.
Typical Applications
Thermally-driven ammonia stripping is particularly well suited to:
- Landfill leachate treatment, where waste heat from landfill gas engines or flares is readily available on-site
- High-strength industrial effluents where ammonia concentrations make biological treatment costly or impractical
- Sites seeking to avoid ongoing chemical costs associated with pH-driven stripping or carbon-source dosing for biological nitrogen removal
For background on the wider regulatory context around nitrogen discharge limits, see the US EPA’s overview of nutrient pollution and general information on ammonia toxicity in aquatic environments.
Key Takeaways
- Thermally-driven ammonia stripping removes ammonia using heat alone – no lime, caustic, or carbon-source dosing required.
- Single-pass systems can achieve >98.5% ammonia removal, with a small physical footprint (up to 840 kg NH₄-N/m²/day).
- The process is ideally suited to sites with an existing waste-heat source, such as landfill gas engines, flares, or process steam.
- Stripped ammonia gas can be destroyed cleanly by using it as combustion air in a low-NOx thermal oxidiser, closing the energy loop.
- Proven over more than 15 years and 500,000+ hours of operation across multiple full-scale installations treating flows from 50 up to 3,000 m³/day.
Get in Touch
If your site generates waste heat and struggles with high ammonia loads in leachate or process wastewater, thermally-driven ammonia stripping could deliver substantial cost savings compared to biological or chemical alternatives. Contact our Technical Sales team to discuss your flow rates, ammonia concentrations, and available heat sources, and find out whether a thermally-driven stripper is the right fit for your site.