Nitrate Removal for Sewage Treatment Plants
Ammonia passes. Nitrate fails. Here is what that means.
If your ammonia result is clean but nitrate is over the limit, your plant is actually working — halfway. It is converting ammonia into nitrate and then stopping. Finishing the job needs one more step that most plants were never built with.
Does This Sound Familiar
Signs your plant nitrifies but does not denitrify
- Ammonia comes back low, nitrate comes back high — the two move in opposite directions.
- Sludge floats to the surface of your clarifier in clumps, sometimes hours after settling normally.
- Fine bubbles rise through the clarifier even though nothing is aerating it.
- Your plant was designed with aeration and clarification, but no anoxic tank.
- Your discharge is to a river, lake or creek where nitrate is a regulated parameter.
The Engineering
Nitrate is what ammonia becomes
Nitrification does not destroy nitrogen. It converts ammonia into nitrite and then into nitrate — a less toxic form, but still nitrogen, and still measured. To actually remove it from the water, nitrate has to be converted into nitrogen gas and released to the atmosphere. That reaction is called denitrification, and it needs the opposite conditions to nitrification.
Nitrifiers need oxygen. Denitrifiers need the absence of it, plus a carbon source to feed on. You cannot do both in the same aerated tank at the same time, which is why a plant built with only an aeration stage will nitrify beautifully and never remove nitrate.
The Fix
How we remove nitrate
1. Add an anoxic stage
A dedicated tank ahead of aeration, mixed but not aerated, where denitrifying bacteria can work. In many retrofits an existing chamber can be converted rather than built new.
2. Recycle the nitrified water
Nitrate is produced downstream in the aeration tank, so nitrified water is returned to the anoxic stage at a controlled rate. Getting that rate right is the difference between removing nitrate and short-circuiting the whole plant.
3. Use the incoming carbon
Raw sewage entering the anoxic stage supplies the carbon the denitrifiers need, so no external chemical is required in most domestic applications.
4. Fix the sludge return
Sludge sitting too long in the clarifier is where unwanted denitrification happens. Timer-controlled wasting and return, tuned to the actual sludge blanket, stops sludge from rising.
5. Give the biology a surface
FBBR-style fixed-film media holds both populations in place and makes the whole nitrogen train more stable through load swings.
6. Verify with a real test
Accredited third-party effluent sampling after the correction, covering the full nitrogen series — ammonia, nitrite and nitrate — not just the one parameter that failed.
Worth Knowing
Nitrogen is measured as a series
Where domestic sewage is part of your stream, ammonia and nitrate are both parameters you are held to, along with phosphate, surfactants and fecal coliform. A design that only targets BOD and solids will pass its first test and fail the ones that matter. We size the nitrogen train as a whole rather than chasing whichever parameter failed most recently. See which parameters apply to your industry.
Send us your nitrogen results
Ammonia, nitrite and nitrate together tell us exactly where your plant stops working.