Chemical Dosing Calculation for Wastewater: From mg/L to Pump L/h

A chemical dosing calculation for wastewater is a chain of unit conversions. Multiply plant flow (m³/h) by the target dose (mg/L), divide by 1,000, and you have kilograms per hour of active chemical. Divide that by solution strength and density, and you have the liters per hour your metering pump must deliver. The only number that has to come from your own site is the dose itself, and that should be confirmed by jar testing and commissioning, not copied from another plant. This guide is written for wastewater operators, process engineers and contractors sizing feed pumps for coagulants, polymer, acid, caustic or hypochlorite.

The chemical dosing calculation in three steps

Put everything in m³/h, mg/L and kg/h first. Mixed units cause more mistakes than the math.

1. Active chemical (kg/h) = flow Q (m³/h) × dose C (mg/L) ÷ 1,000. One mg/L equals one g/m³, so flow times dose gives g/h, and dividing by 1,000 gives kg/h.
2. Product required (kg/h) = active chemical ÷ active content of the delivered product. Skip this step if the dose was already specified as product.
3. Pump flow (L/h) = active chemical ÷ (solution strength as a decimal × density in kg/L). A 10% solution goes in as 0.1. Entering 10 makes the answer 100 times too small.

You can merge them into one line, pump flow = Q × C ÷ (1,000 × strength × density), but separate steps make errors easier to spot: active chemical for the process engineer, product weight for purchasing, L/h for the pump vendor.

The pounds formula for US plants

Plants working in million gallons per day use the pounds formula: lb/day = flow (MGD) × dose (mg/L) × 8.34. It is the same chemical dosing calculation in US units.

Where the dose comes from: converting jar test results

Most people get stuck on the dose, not the formula. Influent varies by plant and season, so the starting dose should come from a jar test on your own water and be fine-tuned against effluent results after startup.

In a jar test you add a stock solution, so you need to turn milliliters back into mg/L:

Dose (mg/L) = stock added (mL) × stock strength (g/L) ÷ sample volume (L)

A dilute 1% stock is roughly 10 g/L, so 1 mL in a 1 L jar equals about 10 mg/L. Run a series of jars from low to high, compare floc size, settling and supernatant clarity, and take the lowest dose that does the job as the starting value for your chemical dosing calculation. Our guide to finding the right PAC dosage range shows how to set up the jar series.

Active basis or product basis?

This is where a chemical dosing calculation most often goes wrong. Two people can say "the dose is X mg/L" and mean different things. Polyaluminum chloride is often specified by its Al₂O₃ content, sodium hypochlorite by available chlorine, and caustic soda or sulfuric acid by the percentage of pure compound. Size the pump on product when the designer meant active basis, and the pump comes out too small.

Ask which basis the dose uses, then take the active content and density from the supplier's certificate of analysis. Do not assume a density of 1 for strong liquid chemicals; they are noticeably heavier than water, and using 1 overstates the flow.

Which flow rate goes into the chemical dosing calculation?

Use peak flow and maximum dose to size the pump, so it keeps up at the worst hour. Use average flow to estimate daily chemical use and operating cost. If the plant data is in m³/day and the lift station only runs part of the day with dosing interlocked to it, divide by the real operating hours, not by 24, or the pump will be undersized several times over.

Daily usage and tank size from the chemical dosing calculation

Daily active chemical (kg/day) = average flow (m³/day) × dose ÷ 1,000. Divide by active content to get product per day for ordering.

Size the day tank from how long one batch should last: usable volume (L) = average pump flow (L/h) × hours between batches. Mixing once a shift instead of once a day changes the tank size a lot. If you want the tank, mixer and pumps supplied as one unit, see our packaged dosing skids.

Diluting product: what changes in the chemical dosing calculation

Polymer is usually made down to a weak solution, and some sites dilute liquid products so the pump runs mid-stroke. Active chemical does not change:

Neat volume × neat strength = diluted volume × diluted strength

In step 3, use the diluted strength. Pump flow goes up accordingly. A common trap is a design based on neat product, followed by on-site dilution, and then a pump that cannot keep up even at full stroke.

From L/h to a metering pump

The chemical dosing calculation gives a theoretical maximum. Allow roughly 1.3 times that figure when picking the pump, and make sure its rated pressure is above the real pressure at the injection point. For setting stroke length and speed and calibrating after installation, see working out metering pump flow from water flow. Pumps can be filtered by flow and pressure in our metering pump range.

Check the math against real usage

After a few weeks, compare chemical used with water treated over the same period: actual dose (mg/L) = active chemical used (kg) × 1,000 ÷ water treated (m³). It is the chemical dosing calculation run backward. If it is far from the jar test value, calibrate the pump first, then check whether the chemical strength has changed, and only then adjust the dose setpoint.

Safety note: wear goggles and chemical-resistant gloves when handling acid, caustic or hypochlorite, add chemical slowly to water when diluting, and stop and depressurize the pump before opening the head or the discharge line.

Conclusion

A chemical dosing calculation for wastewater comes down to three steps: flow times dose gives active chemical, active content gives product, and solution strength and density give pump L/h. Confirm the dose with jar tests and commissioning, check whether it is stated on an active or product basis, separate peak flow from average flow, and use diluted strength when you dilute. To see where each chemical goes in the process, read about wastewater chemical dosing points. Send us your flow, chemical, strength and injection pressure, and contact our engineers to check the pump size.

FAQ

Can I do a chemical dosing calculation before a jar test?

Yes, as a rough first pass, but pick a pump with plenty of turndown and run the jar test before or during commissioning to fix the real dose.

Is mg/L the same as ppm in a chemical dosing calculation?

For wastewater and dilute solutions, 1 mg/L is close enough to 1 ppm. For dense concentrated chemicals the two drift apart.

How do I handle two chemicals dosed at the same point?

Run a separate chemical dosing calculation for each one. Each has its own dose, strength and density, and each normally gets its own pump.

My numbers are right but the effluent still fails. Why?

The math only makes sure the right amount goes in. Results also depend on the injection point, mixing energy, pH and temperature, so check those before changing chemicals.

How do I work out strength when I mix a dry product?

Use weight: strength = chemical weight ÷ total solution weight. Log product and water for each batch so the chemical dosing calculation stays accurate.