In short — chemical dosing rate formula
Chemical dosing rate is the volume of stock solution that must be injected per unit time to achieve a target concentration in the main fluid flow. The exact formula:
Dosing rate (L/hr) = (Q × C_target) ÷ C_stock
- Q — main flow rate (m³/hr)
- C_target — target dose concentration (ppm = mg/L = g/m³)
- C_stock — stock solution strength expressed as g/L of active chemical = (% w/w × specific gravity × 10). Example: 12% NaOCl with SG ≈ 1.20 → C_stock = 12 × 1.20 × 10 = 144 g/L.
Worked example: Q = 100 m³/hr, target = 5 ppm chlorine, stock = 12% NaOCl (SG 1.20, C_stock = 144 g/L) → Dosing rate = (100 × 5) ÷ 144 = 3.47 L/hr ≈ 58 mL/min. (Mass needed = 100 m³/hr × 5 g/m³ = 500 g/hr ÷ 144 g/L = 3.47 L/hr.)
Used for: chlorination, alum/coagulant dosing, lime dosing, RO antiscalant, caustic / acid dosing, polymer flocculant, and any water-treatment or process metering application.
Worked example — chemical dosing calculation for a 4000 m³/day desalination plant
This is the full, engineer-checkable dose-rate calculation for a typical seawater-RO (SWRO) desalination plant treating 4000 m³/day of feed (= 166.67 m³/hr ≈ 1.93 m³/min). All five core dosing streams are sized from the same general formula:
Dose rate (L/hr) = (Q × C_target) ÷ C_stock
where C_stock (g/L) = % w/w × SG × 10
| Stage / Chemical |
Stock form |
Target dose |
Dose rate |
L/day |
Intake chlorination
(NaOCl) |
12 % NaOCl, SG 1.20
→ 144 g/L active Cl₂ |
2 mg/L Cl₂ |
2.31 L/hr |
≈ 55.5 |
Coagulant
(liquid alum) |
50 % Al₂(SO₄)₃, SG 1.33
→ 665 g/L |
10 mg/L |
2.51 L/hr |
≈ 60.2 |
De-chlorination
(SMBS, pre-RO) |
38 % Na₂S₂O₅, SG 1.23
→ 467 g/L (ratio 1.47:1) |
1.47 mg/L
(1 mg/L res. Cl₂) |
0.52 L/hr |
≈ 12.6 |
| RO antiscalant |
30 % phosphonate polymer, SG 1.10
→ 330 g/L |
3 mg/L |
1.52 L/hr |
≈ 36.4 |
Permeate pH lift
(NaOH on 45 % recovery → 1800 m³/day permeate) |
50 % NaOH, SG 1.52
→ 760 g/L |
15 mg/L |
1.48 L/hr |
≈ 35.5 |
How to use these numbers: the values above are typical mid-range design figures for an SWRO plant of this size. Always confirm doses against (a) the membrane manufacturer's antiscalant product datasheet, (b) jar-test results for the actual seawater turbidity and TOC, and (c) the residual chlorine after the cartridge filters before the RO trains. Sized doses convert linearly with flow — at 8000 m³/day, double every L/hr above.
Pump selection: select diaphragm metering pumps sized to ≈ 2× the calculated rate so the operating point sits at ~50 % stroke for trim accuracy. For the alum and antiscalant dosing rates above (2.51 and 1.52 L/hr), a 5 L/hr stroke-adjustable pump is a typical pick.
↓ Try these numbers in the calculator below ↓
Typical chemical dosages for RO / seawater desalination (mg/L)
Reference ranges for the main dosing chemicals in reverse-osmosis (RO) and seawater-RO (SWRO) desalination pre-treatment. Doses are expressed in mg/L (= ppm = g/m³) of the feed stream unless noted. Use these as a starting point, then confirm against the membrane manufacturer's projection software and jar-test / SDI results.
| Chemical |
Purpose / stage |
Typical dose (mg/L) |
Notes |
| Chlorine / NaOCl | Intake disinfection | 1–3 (shock 3–5) | As free Cl₂; must be removed before RO |
| Ferric chloride (FeCl₃) | Coagulant | 0.5–8 | ≈ 0.2–3 mg/L as Fe; jar-test to set |
| Alum Al₂(SO₄)₃ | Coagulant (alt.) | 5–20 | Higher for turbid surface intakes |
| Antiscalant | RO scale control | 1.5–4 (typ. 2–3) | Phosphonate/polymer; set by projection software |
| SMBS (Na₂S₂O₅) | De-chlorination | 1.5–3 | ≈ 1.47 mg per mg/L residual Cl₂ + 10–30 % excess |
| Sulphuric acid (H₂SO₄) | pH / LSI control | 10–30 | Dose to target feed pH ≈ 6.5–7 |
| Caustic (NaOH) | Permeate pH / boron | 5–20 | On permeate flow, not feed, for 2nd-pass boron removal |
| DBNPA biocide | Non-oxidising biocide | 10–30 | Intermittent, 30–60 min slug 1–3×/week |
Ranges reflect typical membrane-manufacturer design guidance (e.g. DuPont FilmTec, Hydranautics, Toray) for SWRO/BWRO pre-treatment. Convert any mg/L dose to a metering-pump rate with the calculator below: dose rate (L/hr) = (Q × mg/L) ÷ C_stock, where C_stock (g/L) = %w/w × SG × 10.
↓ Convert a dose to L/hr and kg/day in the calculator ↓
Standards & method
✓ Formula independently verified 12 July 2026- Governing standard
- Water-treatment mass balance
- Clauses applied
- Feed (kg/day) = Q(m³/day) × dose(mg/L) ÷ 1000
- Pump rate L/h = (Q × ppm) ÷ (%w/w × SG × 10)
- Core formula
kg/h = Q(m³/h) × dose(mg/L) / 1000 · L/h = kg/h / (strength × SG)- Why this matters
- Dose must be corrected for the active strength of the product AND its specific gravity. Dosing a 12% solution as though it were 100% under-doses by more than eight times.
- Independently verified
- 12 July 2026 — Formula re-derived from the standard and checked numerically against worked reference cases from the code book, not merely tested for “returns a number”.
Why this matters: Divides by product purity and applies specific gravity — omitting either under-doses.
Results are for guidance. Verify against the current edition of the governing standard and have a licensed engineer review before construction or installation.
Chemical Dosing Equation
Calculating the correct dosing pump rate ensures that the proper concentration of a chemical is injected into a moving stream of water or another fluid.
Where:
- Flow = Main water flow rate in m³/hr
- Dose_PPM = Target concentration in mg/L or ppm
- Strength_% = Percentage active ingredient of the stock chemical
- SG = Specific Gravity (density relative to water) of the chemical
Frequently Asked Questions
How to calculate chemical dosing pump rate?
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Multiply the main flow rate by the target PPM dose. Divide that result by the density of the chemical and the percentage of active ingredient.
How do I calculate chemical dosing for a 4000 m³/day desalination plant?
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Convert flow first: 4000 m³/day ÷ 24 = 166.67 m³/hr. Then for each stage, apply Dose rate (L/hr) = (Q × C_target) ÷ C_stock, where C_stock (g/L) = % w/w × SG × 10. Typical SWRO doses: chlorination 2 mg/L → 2.31 L/hr of 12 % NaOCl; coagulant (alum) 10 mg/L → 2.51 L/hr of 50 % liquid alum; SMBS de-chlorination 1.47 mg/L → 0.52 L/hr of 38 % SMBS; antiscalant 3 mg/L → 1.52 L/hr of 30 % phosphonate; permeate NaOH 15 mg/L → 1.48 L/hr of 50 % caustic (on 1800 m³/day permeate at 45 % recovery). See the worked-example table on this page for the full breakdown.
What is chemical dosing in water treatment?
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Chemical dosing is the controlled addition of treatment chemicals to a water or process stream at a precise rate to achieve a target concentration or effect. Common applications include: chlorination (disinfection), pH correction (acid/alkali dosing), coagulation (turbidity removal), fluoridation, antiscalant dosing (preventing scale in membranes and heat exchangers), and corrosion inhibitor dosing.
How do I calculate the chemical dosing rate?
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Chemical dose (mg/L) × Flow rate (m³/h) × 1000 = chemical required (g/h). For liquid chemicals: Pump flow rate (L/h) = [Dose (mg/L) × Process flow (m³/h)] / [Chemical concentration (g/L)]. Example: 2 mg/L chlorine dose, 100 m³/h flow, using 12% sodium hypochlorite (120 g/L): Pump rate = (2 × 100 × 1000) / 120,000 = 1.67 L/h.
What is the difference between ppm and mg/L for dosing?
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For dilute aqueous solutions (water-based), 1 ppm (part per million by mass) ≈ 1 mg/L. This equivalence holds because water density ≈ 1 kg/L, so 1 mg per kg ≈ 1 mg per litre. This approximation breaks down for concentrated solutions or non-aqueous fluids. Always use mg/L for precise dosing calculations in water treatment; ppm is acceptable for approximate field work.
What types of dosing pumps are used in chemical treatment?
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Common dosing pump types: Diaphragm metering pumps — most common; accurate ±1%; handle corrosive and viscous chemicals; suitable for 0.1–1000 L/h at up to 10 bar. Peristaltic (tubing) pumps — fluid only contacts the tubing; no seals; ideal for aggressive chemicals; easy maintenance; lower accuracy (±5%). Piston pumps — high pressure (up to 700 bar) and high flow rates; used in reverse osmosis and high-pressure injection. Solenoid-driven pumps — compact and inexpensive for low flow rates.
What are the most common chemicals used in water treatment dosing?
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Key water treatment chemicals and typical doses: Sodium hypochlorite (NaOCl, 12–15%) — disinfection, 1–5 mg/L as Cl₂; Aluminium sulphate (alum) — coagulation, 10–100 mg/L; Ferric chloride — coagulation, 10–80 mg/L; Sodium hydroxide (caustic soda, 50%) — pH raise, dose to target; Sulphuric acid (98%) — pH lower; Fluorosilicic acid (H₂SiF₆, 20%) — fluoridation, target 0.7 mg/L F⁻; Antiscalant (polymer) — RO membrane protection, 2–10 mg/L.