Quick answer: Biological nitrification-denitrification is the default and the cheapest at normal concentrations. Above roughly 500 mg/L, air stripping or struvite precipitation usually comes first to knock the load down. Ammonia is the parameter most often missed in winter, because nitrifying bacteria grow slowly and are temperature-sensitive.
Why ammonia fails when COD passes
This is the most common complaint we hear, and the cause is biological rather than mechanical. The organisms that remove COD grow quickly and tolerate a wide range of conditions. Nitrifying bacteria, which convert ammonia to nitrate, grow far more slowly and are much more sensitive to temperature, pH, alkalinity and toxicity.
A plant sized comfortably on COD load can therefore be genuinely short of the reactor volume and sludge age that nitrification needs. Three specific traps:
- Sludge age too short. Nitrifiers are washed out faster than they reproduce. Increasing sludge age is the first thing to check and costs nothing.
- Alkalinity exhausted. Nitrification consumes alkalinity. When it runs out, pH falls and the process stalls — this looks like a toxicity problem but is a chemistry problem.
- Cold weather. Nitrification rates fall sharply as temperature drops. A plant that passes in summer can fail every winter with nothing else changed.
Before considering new equipment, work through the six reasons wastewater plants fail their discharge permit — several apply directly here.
The four routes compared
| Route | Works best at | Strengths | Watch out for |
|---|---|---|---|
| Nitrification–denitrification (biological) | Low to moderate ammonia | Cheapest to run; removes nitrogen completely to nitrogen gas | Needs long sludge age, alkalinity, warmth and a carbon source for denitrification |
| Air stripping | High ammonia | Handles concentrations biology cannot; fast and robust | Needs pH raised well into the alkaline range; transfers ammonia to air, so off-gas must be scrubbed |
| Breakpoint chlorination | Low ammonia, polishing only | Rapid, simple, good final trim | High chemical cost; produces chlorinated by-products; impractical at high load |
| Struvite precipitation | High ammonia with phosphate present | Recovers a usable fertiliser product | Needs magnesium dosing and the right phosphate ratio; chemical cost |
At high concentration the economical pattern is usually two stages: strip or precipitate the bulk, then let biology polish the remainder. Sending very high ammonia straight to a biological stage inhibits the biomass, which is self-defeating.
Getting biological nitrogen removal right
Complete nitrogen removal needs both halves of the process, and the second half is the one people forget:
- Nitrification (aerobic) converts ammonia to nitrate. This removes ammonia but leaves nitrogen in the water as nitrate.
- Denitrification (anoxic) converts nitrate to nitrogen gas, which leaves the system. It needs a carbon source, and it returns roughly half the alkalinity that nitrification consumed.
If your permit limits only ammonia, nitrification alone satisfies it. If it limits total nitrogen, you need both. Putting the anoxic zone first and recycling nitrate back to it lets the incoming effluent’s own COD serve as the carbon source, which avoids buying methanol — worth specifying if your effluent has COD to spare.
One practical warning: denitrification happening where you do not want it — in the clarifier — lifts sludge to the surface in clumps with visible gas bubbles, and that sludge leaves with your effluent. It looks like bulking but needs the opposite remedy.
What to have measured before asking for a proposal
- Ammonia nitrogen concentration, and total nitrogen if your permit limits it
- Flow rate, with the peak
- Alkalinity — routinely omitted and frequently the actual constraint
- Temperature range across the year, especially the winter minimum
- COD, to judge whether there is carbon available for denitrification
- Phosphate, if struvite recovery is being considered
- The discharge limit itself, and whether it is ammonia or total nitrogen
Sizing the biological stage around these figures is covered in how to size an industrial wastewater treatment plant, and the equipment set in industrial wastewater treatment equipment and complete systems.
Frequently asked questions
Why does my plant pass COD but fail ammonia?
Nitrifying bacteria grow much more slowly than COD-removing organisms and need a longer sludge age. A plant sized on COD can be short of the volume and sludge age nitrification requires. Check sludge age and alkalinity first — both are free to check.
Why does ammonia removal get worse in winter?
Nitrification rates fall sharply with temperature, and nitrifiers are the slowest organisms in the plant. A reactor that performs in summer can be undersized for the same load in winter. Design for the winter minimum, not the annual average.
What alkalinity do I need for nitrification?
Nitrification consumes alkalinity as it runs. When alkalinity is exhausted, pH falls and the process stalls. Measure alkalinity at the reactor, not the inlet. If it is being depleted, dosing alkali restores performance without any new equipment.
When should I use air stripping instead of biological treatment?
At high ammonia concentrations, where biology would be inhibited. Stripping handles loads biology cannot, but it needs pH raised into the alkaline range and the off-gas must be scrubbed. The usual pattern is strip the bulk, then polish biologically.
Does removing ammonia remove total nitrogen?
No. Nitrification converts ammonia to nitrate, which is still nitrogen in the water. To remove nitrogen from the system you also need an anoxic denitrification stage that converts nitrate to nitrogen gas. Check which your permit actually limits.
My sludge floats to the top of the clarifier in clumps. Why?
Usually denitrification occurring in the clarifier: nitrogen gas bubbles form and lift the sludge blanket. It resembles bulking but needs the opposite fix. Look for gas bubbles in the floating clumps to tell them apart before changing anything.
Send your ammonia concentration, flow, temperature range and discharge limit and we will propose the route: request an ammonia removal proposal.