Breakpoint Chlorination for Ammonia Removal: Estimating the Hypochlorite Dose

Breakpoint chlorination means adding chlorine past the low point on the residual curve, the "breakpoint", where most of the ammonia has been oxidised to nitrogen gas and free chlorine starts to appear. Can a wastewater plant remove ammonia with sodium hypochlorite? Yes, but it is best suited to polishing a small exceedance or handling an emergency, not to carrying the main nitrogen load. The dose is estimated from the ammonia you need to remove multiplied by a chlorine to ammonia ratio. The theoretical mass ratio is about 7.6:1 chlorine to ammonia-nitrogen, real water needs more, and a jar test is the only reliable way to find out how much.

This article is for process engineers and operators at municipal and industrial wastewater plants, and for anyone sizing a metering pump for a breakpoint chlorination system.

How breakpoint chlorination works

The residual does not simply climb as you add chlorine to water containing ammonia.

1. At first, chlorine combines with ammonia to form monochloramine. The measured residual rises, but it is combined chlorine.
2. As dosing continues, the chloramines are oxidised further and break down to nitrogen gas. The residual falls.
3. The lowest point is the breakpoint. Beyond it, any extra chlorine shows up as free chlorine and the reading climbs again.

So the test for having reached breakpoint is a stable free chlorine residual, not a high total chlorine reading. Plenty of sites measure only total chlorine, see a big number, and assume they are done when they are still on the first part of the curve.

When breakpoint chlorination makes sense for ammonia removal

If the biological process is healthy and effluent ammonia only occasionally drifts above the limit, breakpoint chlorination is a practical safety net. It acts within hours.

If influent ammonia is high and nitrification has failed for weeks, forcing the problem with hypochlorite gets expensive in chemical, raises chloride in the effluent and creates chlorinated by-products. Fix the biology first, and treat breakpoint chlorination as a bridge.

Estimating the breakpoint chlorination dose

Step 1: chlorine required

Chlorine required (kg/d) = flow (m³/d) × ammonia-nitrogen to remove (mg/L) × chlorine to ammonia ratio ÷ 1000.

The 7.6:1 figure is the stoichiometric value for ammonia alone. Wastewater also contains nitrite, reduced compounds and organics that compete for chlorine, so the practical ratio is higher. How much higher depends on the water, and only testing will tell you.

Step 2: convert to hypochlorite solution

Hypochlorite needed = chlorine required ÷ available chlorine in the product, then convert to volume with the solution density. Use the measured strength of each delivery, not the label. Hypochlorite decomposes in storage, so the dose has to rise as the product weakens.

Step 3: find the real breakpoint in a jar test

Split a sample of actual effluent into several jars, dose each at a different chlorine level, allow contact time, then measure free chlorine and ammonia and plot the curve. The dose at the lowest point is the breakpoint for that water. Do not buy a large pump on the theoretical ratio alone without a jar test.

Breakpoint chlorination jar test mistakes to avoid

Use fresh effluent taken at the planned injection point, not a sample that has sat in a warm lab for a day, because ammonia and chlorine demand both change. Dose from the same hypochlorite batch you will use on site and check its strength first. Give each jar the same contact time you expect in the plant, and read free chlorine with a method that separates it from combined chlorine. Finally, repeat the test when the effluent changes noticeably, for example after heavy rain or a process upset, since the breakpoint moves with the water.

What affects breakpoint chlorination results

pH is the factor people forget. The reaction runs best near neutral; far from it, the reaction slows and nitrogen trichloride, with its sharp smell, can form. The reaction also consumes alkalinity, so in poorly buffered water the pH drifts down and may need correction.

Mixing and contact time matter too. Inject where the flow is turbulent and allow enough residence time downstream. If the water leaves before the reaction finishes, your readings will mislead you.

Then there is the residual itself. Past breakpoint the effluent carries free chlorine, which can harm receiving waters, so many plants add dechlorination or residual control to meet discharge rules.

Pumps and control for breakpoint chlorination

Breakpoint chlorination uses far more hypochlorite than disinfection, so small solenoid pumps often run out of capacity and a mechanical diaphragm metering pump is usually the better choice. The head must handle hypochlorite and vent gas; see our sodium hypochlorite dosing pump selection guide.

For control, pace the dose from flow and trim it from effluent ammonia or free chlorine, using a residual chlorine analyser. For where dosing points sit across a plant, see municipal wastewater dosing, and for control strategy, read wastewater dosing automation.

Safety: large hypochlorite tanks must be kept well away from acids to prevent chlorine gas release. Stop the pump and relieve pressure before opening pumps or lines, and wear goggles and chemical gloves.

Conclusion

Breakpoint chlorination oxidises ammonia to nitrogen gas and is a useful tool for small exceedances and emergencies, but not a substitute for biological nitrogen removal. Estimate the dose as ammonia to remove times the chlorine to ammonia ratio divided by the product strength, remember that real ratios exceed the theoretical 7.6:1, and confirm with a jar test and free chlorine readings. Send us your flow, ammonia level and site conditions, and contact our engineers for pump and analyser recommendations.

FAQ

How do I know I have passed the breakpoint?

Watch free chlorine. When a free residual appears and rises with dose, you are past breakpoint. Total chlorine alone can mislead.

Will breakpoint chlorination change effluent COD?

It can. Chlorine oxidises some organics and may form chlorinated by-products. Measure before and after to see the effect on your water.

Can hypochlorite handle very high ammonia on its own?

It is not advisable. Chemical use, chloride and by-products all climb. Restore nitrification and use chlorine as a supplement.

Why did pH drop after dosing?

The breakpoint reaction consumes alkalinity. In low-alkalinity water the pH falls, and you may need to add alkali.

What happens if I overdose hypochlorite?

High free chlorine in the effluent can harm receiving waters and downstream biology, and it wastes chemical. Trim the dose from residual readings.