Dewatering dose calculations are simple only after the basis is fixed. The same pump setting can represent very different treatment if one person reports active chemical, another reports as-supplied product and a third reports prepared-solution volume. Every calculation should state sample/feed volume, target dose basis, product or stock concentration, density where required and actual pump calibration.
This control belongs to a connected operating system. Use the dewatering engineering pillar, process-flow control and chemistry selection together; changing one boundary can move the constraint elsewhere.
The core batch-dose relationship
For a jar or batch test, the active chemical mass required is m = D × V when D is in mg/L and V is in L, giving m in mg. If a prepared stock has concentration Cstock in mg/mL, the addition volume is Vstock = m / Cstock. The formula is arithmetic; the selected dose is a treatability-test variable, not an industry target.
Worked jar example—illustrative only
Assume a 1.0 L sample, an illustrative target dose of 80 mg/L active chemical and a prepared stock containing 10 mg/mL active. Required active mass = 80 × 1.0 = 80 mg. Stock addition = 80 / 10 = 8.0 mL. The 80 mg/L dose and 10 mg/mL stock concentration are hypothetical values chosen only to demonstrate unit conversion; neither is a recommended treatment or make-down setting. Actual preparation must remain within the selected product’s approved instructions.
Continuous active-mass rate
For flow Q in m³/h and active dose D in mg/L, the active mass rate is ṁactive = D × Q / 1000 kg/h, because 1 mg/L equals 1 g/m³. This assumes the dose is defined against the same feed-flow basis being measured.
Worked continuous example—illustrative only
At an illustrative 120 mg/L active dose and 25 m³/h feed, active mass rate = 120 × 25 / 1000 = 3.00 kg/h. If the as-supplied product is 20 wt% active with density 1.10 kg/L, product volume rate = 3.00 / (0.20 × 1.10) = 13.64 L/h. These values demonstrate the conversion only; actual product concentration, density and treatment dose must come from the product data and field test.
Three bases that must never be mixed
| Dose basis | What it means | Common error |
|---|---|---|
| Active chemical | Mass of active treatment species per feed volume | Using product mass as if it were 100% active |
| As-supplied product | Mass or volume of commercial product per feed volume | Comparing products with different active content on an unequal basis |
| Prepared solution | Volume of diluted/made-down solution | Changing make-down concentration but keeping the same pump setting |
Solution make-down belongs in the calculation record
Record the product mass/volume added, make-down water volume, product density when needed and the final prepared-solution basis. Follow the product instructions for concentration and aging; a generic “1% polymer solution” is not a universal rule. Polymer preparation can be concentration-sensitive and poor make-down invalidates an otherwise correct dose calculation.
Calibrate the pump in delivered units
Pump stroke, percentage or speed is not a chemical dose. Verify actual delivered volume over a known time at the operating suction/discharge condition, then convert that volume to chemical mass from the prepared-solution concentration. Recheck calibration after product viscosity, suction arrangement, pump maintenance or concentration changes materially.
Uncertainty can be larger than the arithmetic
- Feed flowmeter bias or batch-volume error.
- Wrong active content or product density entered into the calculation.
- Prepared solution not at the assumed final volume/concentration.
- Pump slip, gas lock, blocked injection quill or suction starvation.
- Dose recorded against feed flow while the chemical pump and feed pump are not synchronized.
Chemical-handling boundary
A correct dose calculation is not authorization to handle, mix or dispose of a product. Use the current SDS, product data and site procedure to verify compatibility, approved storage and make-down, PPE, eyewash and spill controls, and the permitted residue and container route. Never infer safe handling from concentration or pump rate alone.
Chemical dosing should close mathematically from jar to skid: treatment dose → active mass rate → product/solution rate → measured pump delivery. Keep the basis explicit and the field trial becomes reproducible.
Common questions
How do I convert a jar-test dose in mg/L to chemical volume?
Multiply dose by sample volume to get chemical mass, then divide by the stock-solution concentration. Keep active versus as-supplied basis explicit.
How do I calculate continuous active chemical use?
For dose in mg/L and feed in m³/h, active chemical kg/h = dose × flow / 1000.
Can I set polymer dose from pump percentage alone?
No. Pump percentage must be converted through an actual delivery calibration and the prepared-solution concentration before it represents a treatment dose.
