Engineering calculators · Chemical dosing
Chemical dosing calculator
Turn a treatment dose in mg/L into a dosing pump rate, daily and annual chemical use, storage days, bund volume and cost. Preset chemicals carry typical product strengths and densities for ferric and aluminium coagulants, sodium hypochlorite, caustic, acids and made-down polymer; a custom entry takes any supplier data. Coagulant doses can be entered as metal, as active compound or as product, and an optional influent phosphorus figure gives the metal to phosphorus molar ratio.
Inputs
Choosing a preset fills in the typical strength, basis, density and price; edit them to match your supplier's certificate of analysis. Hypochlorite is quoted in % w/v as available chlorine (14% w/v = 140 g Cl2/L); coagulants in % w/w of the salt or of Al2O3. For the polymer preset the "product" is the 0.2% made-down solution, with the price per tonne of neat polymer.
Results
- Pump runs at 53% of maximum, inside the 10 to 90% band where metering pumps are accurate.
- Dose of 15.0 mg Fe/L is within the usual 5.00 to 60.0 mg Fe/L range.
- 26.1 days of storage at the design dose, within the 7 to 90 day range usual for a bulk tank.
- Bund the store to at least 110% of the largest tank (11.0 m³ for one tank of 10.0 m³) or 25% of the total stored, whichever is greater (CIRIA C736).
- Fe:P molar ratio of 1.4 is within the 1 to 4 range typical for chemical phosphorus removal; 1.5 to 2.5 is common for effluent P of about 1 mg/L.
- Handling: Thin but corrosive and staining; PVC-U, PE or PVDF wetted parts, no stainless steel.
Sensitivity to dose
Use this to size the pump and store for the range a jar test or seasonal change may need. All other inputs as above.
| Dose, mg Fe/L | Pump, L/h | Pump, % of max | Product, kg/d | Storage, days | Cost a year |
|---|---|---|---|---|---|
| 7.50 | 7.99 | 27 | 272 | 52.1 | £27,828 |
| 11.3 | 12.0 | 40 | 408 | 34.8 | £41,743 |
| 15.0 | 16.0 | 53 | 545 | 26.1 | £55,657 |
| 18.8 | 20.0 | 67 | 681 | 20.9 | £69,571 |
| 22.5 | 24.0 | 80 | 817 | 17.4 | £83,485 |
| 30.0 | 32.0 | 107 | 1,089 | 13.0 | £111,314 |
How the calculation works
- Active content of the product. For a strength quoted % w/w, grams of active per litre = strength × 10 × SG; for % w/v it is strength × 10. For a metal basis, multiply by the metal mass fraction of the compound: Fe in FeCl3 = 55.85/162.2 = 0.344, Fe in Fe2(SO4)3 = 0.279, Al in Al2O3 = 0.529.
- Mass dosed per day = dose (mg/L) × flow (m³/d), which is g/d because 1 mg/L × 1 m³ = 1 g. Product volume = mass ÷ grams per litre on the chosen basis; product mass = volume × SG.
- Pump rate = product volume × (1 + dilution) ÷ 24, in L/h. Turndown = rate ÷ pump maximum. The dose equivalents convert between product, active compound and metal so a figure on any basis can be checked.
- Storage days = tank volume ÷ daily product volume. Bund = 110% of the tank [CIRIA C736] [EA].
- Cost = product mass × price per tonne; per m³ treated = cost per day ÷ flow.
- Metal to phosphorus ratio = (mg metal/L ÷ molar mass of metal) ÷ (mg P/L ÷ 30.97) [M&E ch. 6].
Worked example with the default inputs
Ferric chloride 40% w/w, SG 1.42, dosed at 15 mg Fe/L to 5,000 m³/d. Active content = 40 × 10 × 1.42 = 568 g FeCl3/L, of which 34.4% is iron: 196 g Fe/L. Iron needed = 15 × 5,000 = 75,000 g/d, so product = 75,000 ÷ 196 = 384 L/d = 16.0 L/h, or 545 kg/d. A 30 L/h pump runs at 53%; a 10 m³ tank lasts 26.1 days and needs an 11.0 m³ bund. At £280/t the cost is £152.48/d, £55,657 a year. With 6 mg P/L in the influent the Fe:P molar ratio is (15/55.85)/(6/30.97) = 1.4.
What this cannot tell you. It does not choose the dose: that comes from jar tests, titration (for pH correction) or breakpoint curves (for chlorination), and varies with season and influent. It does not model the chemistry of alkalinity consumption (ferric chloride consumes about 2.7 mg CaCO3 per mg Fe), sludge production, chlorine demand and decay, or the mixing needed at the dosing point. Pump selection also needs the back-pressure, suction conditions, and the stroke and speed range of the specific pump; the viscosity notes are a reminder, not a specification.
Sources
- CIRIA C736 (2014). Containment systems for the prevention of pollution (secondary containment: 110% of the largest tank or 25% of total storage)
- Environment Agency. Storing oil and chemicals: guidance on secondary containment and bund design
- Metcalf & Eddy / AECOM (2014). Wastewater Engineering, 5th ed., ch. 6 (chemical unit processes: coagulant chemistry, phosphorus precipitation stoichiometry) and ch. 12 (disinfection)
- WEF (2017). Design of Water Resource Recovery Facilities, MOP 8, 6th ed., ch. 9 chemical feed and storage
Frequently asked questions
Should I dose ferric as Fe or as product?
Jar tests and phosphorus removal stoichiometry are expressed as mg Fe/L (or mg Al/L), so design on the metal basis and let the calculator convert to litres of product. Operators and suppliers usually talk in litres or kilograms of product, and pump settings are in L/h of whatever is in the day tank, so all three are reported.
How much should I dilute a dosing chemical?
Dilute only when the neat pump rate falls below about 10% of the smallest sensible pump, or when the chemical must be mixed quickly at the dosing point. Dilution water must be suitable: hard water precipitates with caustic and coagulants, and chlorinated water degrades polymer. Hypochlorite and concentrated sulphuric acid are normally dosed neat; sulphuric acid dilution is dangerously exothermic.
What bund volume do I need for the chemical store?
UK practice (CIRIA C736 and Environment Agency guidance) is 110% of the largest tank or 25% of the total stored volume, whichever is greater, with incompatible chemicals such as acid and hypochlorite in separate bunds. The bund must resist the chemical, have no uncontrolled drain, and keep tank fittings and the delivery point inside the contained area.
Why is the metal to phosphorus ratio above 1?
The stoichiometry FePO4 or AlPO4 needs one mole of metal per mole of P, but competing hydroxide precipitation, organic and polyphosphate forms, and the need for a low residual all push the practical ratio up. Typical values are 1.5 to 2.5 mol/mol for an effluent of about 1 mg P/L and 3 or more below 0.5 mg P/L. The extra metal becomes hydroxide sludge, which is why sludge production rises with chemical P removal.
Related reading: chemical dosing system design, chemical phosphorus removal, coagulation and flocculation guide.