Ammonia Nitrogen Removal from Industrial Wastewater: Stripping, Breakpoint Chlorination and Nitrification
Ammonia nitrogen is one of the most widely regulated parameters in industrial effluent and one of the most difficult to remove economically. Limits of 5 to 15 mg/L are now common, and sensitive receiving waters are regulated to 1 mg/L or lower. The three principal removal routes occupy distinct cost and performance envelopes, and selecting between them is governed by influent concentration, salinity, temperature and the availability of a biological treatment stage.
Ammonia is present in effluent from fertilizer production, coke making, landfill leachate, anaerobic digestion liquor, food processing and, critically, from many chemical synthesis routes. In several of these streams, ammonia nitrogen dominates the total nitrogen load, so the choice of removal technology effectively determines the whole nitrogen compliance strategy. Where the flowsheet already includes a biological stage, the ammonia problem is often addressed there, as in membrane bioreactor treatment of leachate with biological nitrogen removal.
Ammonia Chemistry and Why It Matters
Ammonia exists in two forms in water: the ammonium ion, NH4+, and free or un-ionised ammonia, NH3. The equilibrium between them is governed by pH and temperature.
At pH 7 and 20 degrees Celsius, more than 99 percent is present as ammonium ion
At pH 9.5 and 20 degrees Celsius, roughly 50 percent is free ammonia
At pH 11 and 20 degrees Celsius, more than 98 percent is free ammonia
Increasing temperature shifts the equilibrium further toward free ammonia
This equilibrium explains why stripping is a pH-dependent technology and why chlorination must be dosed on a molar basis rather than a mass basis. It also explains why biological nitrification, which consumes the ammonium ion rather than the free form, is unaffected by pH in the same way but has its own narrow optimum near pH 7.5 to 8.0.
Air Stripping
Air stripping transfers free ammonia from the liquid phase to the gas phase by bringing wastewater into intimate contact with air at high pH. It is the technology of choice for concentrated streams, typically above 500 mg/L ammonia nitrogen.
Design Parameters
pH: raised to 10.8 to 11.5 with caustic soda or lime, then readjusted downward after stripping
Temperature: performance improves markedly above 20 degrees Celsius; heating to 30 to 40 degrees Celsius can halve the air requirement
Air-to-liquid ratio: typically 2,000 to 4,000 cubic metres of air per cubic metre of water for a packed tower
Packing: structured or random plastic packing with a specific surface area of 100 to 200 square metres per cubic metre
Height: transfer units are governed by the Henry's law constant, which rises steeply with temperature
Limitations and Disposal
The stripped ammonia must go somewhere. Options include acid scrubbing to recover ammonium sulphate, which is a saleable fertilizer, thermal destruction in a boiler or incinerator, or biofiltration through a nitrifying biofilm. Discharging ammonia-laden air to atmosphere without treatment is increasingly unacceptable under odour and air quality regulation, and in many jurisdictions is explicitly prohibited.
Scaling is the other practical constraint. Raising pH to 11 precipitates calcium carbonate and magnesium hydroxide, so the stripper must be preceded by softening. This is the same scaling chemistry that governs cooling water scale inhibition and corrosion control, and the pretreatment equipment can often be shared.
Breakpoint Chlorination
Breakpoint chlorination oxidises ammonia to nitrogen gas using chlorine. The stoichiometry requires about 7.6 kilograms of chlorine per kilogram of ammonia nitrogen, plus additional chlorine to satisfy the immediate chlorine demand of other reducing substances present.
The Breakpoint Curve
As chlorine is dosed, combined chlorine (chloramines) forms and the residual rises. Further dosing destroys the chloramines, and the residual falls to a minimum at the breakpoint. Beyond the breakpoint, free chlorine residual rises linearly. Operating below the breakpoint leaves chloramines in the effluent, which are both a discharge concern and a source of taste and odour if the receiving water is later chlorinated.
The practical operating window is 0.5 to 1.0 mg/L free chlorine residual after the reaction, which corresponds to a chlorine-to-ammonia mass ratio of 8 to 10. Reaction time is 30 to 60 minutes in a baffled contact tank, with pH controlled between 6.5 and 7.5 to avoid nitrogen trichloride formation at low pH and to accelerate the reaction at neutral pH.
Applications and Drawbacks
Breakpoint chlorination is well suited to low ammonia concentrations, below about 50 mg/L, and to plants that already have chlorination equipment for disinfection. It is fast, robust against shock loads, and requires little additional space. The drawbacks are chemical cost, the formation of disinfection by-products such as trihalomethanes where organics are present, and the need to dechlorinate before discharge. Where similar disinfection chemistry is already required, the incremental cost is modest; the same considerations inform hospital wastewater disinfection requirements.
Biological Nitrification
Nitrification is a two-step biological oxidation. Ammonia-oxidising bacteria convert ammonia to nitrite, and nitrite-oxidising bacteria convert nitrite to nitrate. The overall reaction consumes about 4.57 kilograms of oxygen per kilogram of ammonia nitrogen, of which 3.43 kilograms is used in the first step, and it consumes 7.14 kilograms of alkalinity as CaCO3.
Design Parameters
Sludge age: 12 to 25 days minimum at 15 degrees Celsius to retain slow-growing nitrifiers; longer in cold climates
Dissolved oxygen: 2.0 mg/L minimum, with 2.5 to 3.0 mg/L preferred in the aerobic zone
Temperature: optimum 25 to 32 degrees Celsius; below 12 degrees Celsius the rate falls sharply
pH: 7.5 to 8.0 optimum, inhibited below 6.5
Inhibitors: free ammonia above 50 mg/L, nitrous acid, heavy metals, cyanide and many solvents are inhibitory
Configurations
Conventional activated sludge with a dedicated aerobic zone is the common approach. Moving bed biofilm reactors and integrated fixed-film activated sludge systems provide more nitrifier biomass per unit volume and are therefore more robust against low temperatures and toxic shock. Where alkalinity is limiting, dosing with soda ash or lime is required, because nitrification will acidify the mixed liquor and stop if the bicarbonate buffer is exhausted.
Where both nitrogen and carbon removal are required, the aerated zone is followed by an anoxic zone in which denitrifying bacteria reduce the nitrate produced, using an external carbon source if the wastewater COD is insufficient. For high-strength streams such as leachate, the combination of membrane separation with staged biological treatment, as in membrane bioreactors applied to nitrogen-rich leachate, is generally the most compact solution.
Comparison and Selection
The three technologies are best distinguished by their economic sweet spot.
Air stripping: influent over 500 mg/L, low chloride, softening available, acceptable air emission route. Lowest operating cost per kilogram of ammonia removed, but highest capital cost and largest footprint.
Breakpoint chlorination: influent under 50 mg/L, existing chlorination equipment, space-constrained site. Lowest capital cost, highest chemical cost, disinfection by-product risk.
Biological nitrification: influent 50 to 800 mg/L, existing biological treatment, adequate temperature and alkalinity. Lowest total cost at moderate concentrations, but slow to respond to shock loads and vulnerable to inhibitors.
In practice, many plants combine approaches. A stripper handles the bulk load from a concentrated stream while a nitrifying biological stage polishes to the final discharge limit. Where the plant also treats saline streams, the choice narrows considerably, because salinity depresses the driving force for stripping while it also disrupts nitrifying biomass. In those cases, membrane-based concentration ahead of the ammonia removal step is often the most dependable route, an approach that mirrors the concentration logic used in brine concentrator and crystallizer system design.
Conclusion
Ammonia nitrogen removal should be designed around the concentration band of the actual wastewater, not around a generic technology preference. Stripping wins at high concentration, chlorination wins at low concentration with existing infrastructure, and biological nitrification wins in the middle where temperature and alkalinity permit. In every case, the fate of the removed nitrogen, as gas, as acid scrubber liquor, or as nitrate requiring further reduction, must be resolved at the design stage rather than at commissioning.
Frequently Asked Questions
What is the optimum pH for ammonia stripping?
Between 10.8 and 11.5. Below 10.5 the fraction of free ammonia is too low for economical air rates, and above 11.5 the additional caustic doses mostly precipitate magnesium hydroxide, increasing sludge production without improving stripping. The stripped effluent must then be neutralised to pH 6 to 9 before discharge or biological polishing, as described in ammonia nitrogen removal process design.
How much chlorine is needed to oxidise ammonia?
The theoretical requirement is 7.6 kilograms of chlorine per kilogram of ammonia nitrogen, but in practice 8 to 10 kilograms are needed because chlorine is consumed by other reducing substances and by chloramine formation. Dosing should be controlled by residual free chlorine measurement, targeting 0.5 to 1.0 mg/L after a 30 minute contact period.
Why does nitrification stop working in winter?
Nitrifier growth rate halves for roughly every 5 to 7 degrees Celsius of temperature reduction. A system designed with a 12-day sludge age at 20 degrees Celsius may need 25 days at 10 degrees Celsius to maintain the same biomass inventory. Increasing sludge age, adding biofilm carriers, or insulating and covering tanks are the standard remedies. A fuller examination of biological nitrogen removal design appears in the discussion of membrane biological nitrogen removal for high-strength streams.
