Sodium Hypochlorite Dosing Calculation: From Chlorine Dose to Pump Flow

A sodium hypochlorite dosing calculation has one core line: chlorine needed per hour divided by chlorine contained in each liter of solution equals the solution feed rate, which is the flow your metering pump must deliver. The dose itself depends on chlorine demand and the residual you need, and it should come from jar tests or commissioning. Every number below is an assumed case used only to show the arithmetic. This guide is for operators at water, wastewater and cooling water plants, and for engineers sizing chlorine feed pumps.

Why the calculation must be based on available chlorine

Available chlorine expresses the oxidizing power of hypochlorite as an equivalent amount of chlorine gas. A drum labeled "10%" or "10% available chlorine" usually means weight percent, roughly 10 g of chlorine equivalent per 100 g of solution.

The most common field mistake is reading a design dose of 2 mg/L as 2 mg of product per liter of water. Process doses are almost always stated as chlorine, and the volume of product needed is much larger. Mix those units up and the pump size can be off by several times. Before any sodium hypochlorite dosing calculation, ask whether the mg/L figure is as chlorine or as product.

Sodium hypochlorite dosing calculation in three steps

Step 1: chlorine demand per hour

Chlorine demand (g/h) = water flow (m³/h) × dose (mg/L as chlorine)

Since 1 m³ is 1000 L, mg/L multiplied by m³ gives grams directly.

Step 2: available chlorine per liter of solution

Solution strength (g/L) = weight fraction × solution density (kg/L) × 1000

Do not assume a density of 1. Hypochlorite solution is heavier than water, and treating it as water skews the result. Use the supplier's certificate or check it with a hydrometer.

Step 3: solution feed rate

Feed rate (L/h) = chlorine demand (g/h) ÷ solution strength (g/L)

A worked sodium hypochlorite dosing calculation (assumed values)

Assume a flow of 100 m³/h, a tested dose of 2 mg/L as chlorine, delivered product labeled 10% available chlorine, and a measured density of 1.15 kg/L.

1. Chlorine demand: 100 × 2 = 200 g/h
2. Solution strength: 0.10 × 1.15 × 1000 = 115 g/L
3. Feed rate: 200 ÷ 115 ≈ 1.74 L/h

If you dilute the product 1:1 with water in a day tank, strength drops to about half and the feed rate roughly doubles to about 3.5 L/h. A higher flow can be easier for a small solenoid pump to meter, but the tank empties faster.

To set the real dose, see typical sodium hypochlorite dosage for drinking, reuse and wastewater and how to set the hypochlorite dose for reclaimed water, then confirm with a chlorine demand test.

Strength loss: why the sodium hypochlorite dosing calculation needs margin

Hypochlorite decomposes in storage, faster when it is warm, exposed to light or more concentrated. Drums left outside in summer can lose noticeable strength within weeks.

That affects your sodium hypochlorite dosing calculation in two ways:

• Use measured strength, not the label. Test on delivery and periodically in storage, and raise pump output as strength drops.
• Leave capacity margin. A common practice is to size the pump at roughly 1.3 times the maximum calculated feed, while also covering the extra volume needed once strength has decayed, so the pump does not live at full stroke.

Do not oversize either. If the calculation says 1.74 L/h and you buy a pump rated at tens of liters per hour, the stroke length ends up very short and accuracy suffers. For low flows like this, a solenoid diaphragm metering pump is usually the right fit.

Checks after the sodium hypochlorite dosing calculation

Head material and gas binding. Hypochlorite releases gas, and gas trapped in the pump head stops delivery. Choose oxidation-resistant head materials with a degassing valve, as covered in sodium hypochlorite dosing pump selection.

Injection pressure. Into a pressurized main, the pump rating must exceed the maximum line pressure. Into an open basin, fit a backpressure valve to prevent siphoning.

Residual feedback. Flow and water quality change, so a fixed feed rate is often short one hour and high the next. Where possible add a residual chlorine analyzer for flow-paced or residual-trim control.

Safety: hypochlorite releases chlorine gas on contact with acids. Wear goggles and chemical-resistant gloves when handling it, and keep its tank away from acid storage.

Three errors we see in a sodium hypochlorite dosing calculation

The first is mixing up weight percent and trade percent. Some suppliers quote strength as grams per liter or as trade percent, which is already a volume basis, so multiplying by density again overstates the strength.

The second is forgetting the dilution step. Operators dilute the day tank, but the pump setting is still based on the original strength, so the plant is underdosed for weeks.

The third is calculating at average flow only. Size on peak flow and peak dose, then check that minimum flow still sits inside the pump's turndown. A sodium hypochlorite dosing calculation done at both ends saves a pump change later.

Conclusion

Every sodium hypochlorite dosing calculation follows one path: chlorine demand divided by solution strength gives the feed rate. State the dose as chlorine and confirm it by testing, calculate strength from measured weight percent and measured density, and recalculate after any dilution. Then size the pump with margin and add degassing, backpressure control and residual feedback. Send us your flow, tested dose and product strength and contact our engineers to check pump flow and configuration.

FAQ

What if the label says 10% but the measured strength is lower?

Use the measured value. Hypochlorite loses strength in storage, so test it and recalculate the feed rate.

My dose is given as product, not chlorine. Do I still convert?

Yes. Find the available chlorine in that product, convert the dose to a chlorine basis, then follow the steps above so you do not mix units with the design.

Does diluting the product reduce disinfection?

No, as long as the total chlorine delivered is unchanged. Dilution only raises pump flow. Softened water for dilution helps limit scaling.

How do I estimate daily product use from a sodium hypochlorite dosing calculation?

Multiply the product feed rate by daily operating hours, then size tanks for a short storage period so strength does not fall too far.

The calculated flow is too small for my pump. What now?

Dilute the product and rerun the sodium hypochlorite dosing calculation with the new strength, or move to a smaller solenoid pump. Running a large pump at a very short stroke is hard to keep accurate.