How is the ferric chloride dose determined?

The dose is determined by a jar test on your own water. A row of jars receives different doses, then rapid mixing, slow mixing and settling copy the plant. The lowest dose that meets the target for turbidity, colour or phosphate, with margin, becomes the starting dose. It is then confirmed on the plant and re-tested whenever the raw water changes.

What does "mg/L as Fe" mean in ferric dosing?

It means the dose counts only the iron, not the whole chemical. The same dose can be stated as product, as pure FeCl₃ or as Fe, and each gives a different number. For a 40% solution, multiply the dose as product by 0.40 to get FeCl₃, then by 0.344 to get Fe. Always write the basis next to the number.

What is flow-proportional dosing?

Flow-proportional dosing links the metering pump to the plant flow meter. When flow rises, the pump speeds up; when flow falls, it slows down. The dose in mg/L therefore stays constant. Without it, a fixed pump rate overdoses at low flow and underdoses at high flow. Many plants add an online trim based on water quality.

How do you calculate the ferric chloride pump rate?

Multiply the dose in mg/L as product by the water flow in m³/h. That gives grams of product per hour. Divide by the product density in kg/L times 1,000 to get litres per hour. If your dose is stated as FeCl₃ or as Fe, convert it to a dose as product first. Our coagulant dose calculator does this for you.

Is ferric dosing for phosphorus removal different?

The method is the same, but the sizing basis differs. For phosphorus removal, the dose is first estimated as a molar ratio of iron to phosphorus, then confirmed by jar testing. Tighter phosphorus limits need a higher ratio, and the dosing point matters. Our phosphorus removal guide gives the typical ratios, dosing points and a worked example.

Ferric Chloride Dosing in Water Treatment

A ferric chloride dose is not taken from a table. It is found by jar testing your own water, stated in a clear unit (as product, as FeCl₃ or as Fe), then converted into a pump setting that follows plant flow. Flow pacing then holds that dose as plant flow changes; when the water changes, repeat the jar test.

Why there is no single "right" ferric dose

Ferric chloride works as a coagulant because the Fe³⁺ ion hydrolyses on contact with water. It forms ferric hydroxide flocs that destabilise and collect particles, colour and organic matter. How much ferric chloride that takes depends on the water, not on the product:

Turbidity and colour. More particles and more natural organic matter usually need more coagulant.
Alkalinity and pH. Ferric chloride consumes alkalinity as it hydrolyses. Low-alkalinity water can drop out of the useful pH window.
Temperature. Cold water usually slows floc formation.
The goal. Clarifying drinking water, conditioning sludge and removing phosphorus are different targets with different dose logic.

The same product can need very different doses at two plants, or in summer and winter. For a comparison with other coagulants, see ferric chloride vs PAC vs alum.

Step 1: find the dose with a jar test

A jar test is a bench-scale copy of your clarification process. It is the standard way to set a coagulant dose before you change anything on the plant.

A typical jar test sequence

Sample the raw water at the point where you will dose. Test it soon after sampling.
Fill a row of jars with equal volumes, usually on a gang stirrer.
Dose a range. Give each jar a different ferric chloride dose, bracketing the expected answer.
Rapid mix briefly to disperse the coagulant, as your flash mixer does.
Slow mix to grow flocs, matching your flocculator.
Settle, then measure what matters: turbidity, colour, residual iron, pH, and phosphate if that is the target.
Pick the lowest dose that meets the target with margin. A higher dose that gives only a slightly clearer jar usually costs more sludge and alkalinity.

Use a diluted stock of the same product you dose on the plant, and repeat the test when the raw water changes.

Our jar test optimiser finds the lowest dose in your own jar results that meets your target.

Step 2: state the dose in the right units

This is where most dosing mistakes start. A dose written simply as "mg/L of ferric" can mean three different amounts of iron, depending on the basis:

Dose basis	What is counted	How to convert
As product	mg of the delivered solution per litre of water	No conversion; this is the basis for pump settings
As FeCl₃	mg of pure ferric chloride per litre	As product × 0.40 for a 40% solution
As Fe	mg of iron per litre	As FeCl₃ × 0.344 (iron is about 34.4% of FeCl₃ by mass)

For our 40% grade, one litre of product weighs about 1.43 kg and carries about 13.8% iron by mass. Our phosphorus removal guide shows how that comes to about 197 g of iron per litre of product.

Write the basis next to every dose, check the basis of any limit you work to, and use the delivered concentration: our product is specified at 40% ± 1% FeCl₃, and each batch certificate of analysis gives the measured value. Our buyer's checklist for ferric chloride 40% solution explains the concentration basis and what the certificate reports.

In drinking-water treatment, also check the total dose against the product's listing. Our ferric chloride is listed under NSF/ANSI/CAN 60 with a listed maximum use level of 100 mg/L. Under the standard, that level is set for the product as evaluated, not for its iron content, and it covers the total quantity used in the treatment train, whatever the number of dosing points. So compare your dose as product with the listed figure; if your dose is stated as FeCl₃ or as Fe, convert it to a dose as product first.

Step 3: turn the dose into a pump setting

Once the dose is fixed, convert it into a product flow. With the dose as product:

Product flow (L/h) = dose (mg/L as product) × water flow (m³/h) ÷ (product density in kg/L × 1,000)

This works because 1 mg/L equals 1 g/m³. If your dose is stated as FeCl₃, first divide it by 0.40 to get the dose as product. If it is stated as Fe, divide by 0.138 instead. Our coagulant dose calculator does this arithmetic and also converts the dose into monthly product use.

Flow-proportional dosing

Plant flow is rarely constant. If the pump runs at a fixed rate, the real dose rises at low flow and falls at high flow. Flow-proportional (flow-paced) dosing solves this. The plant's flow meter sends a signal to the metering pump, and the pump output follows the flow, so the dose in mg/L stays constant.

Many plants add a trim: an online measurement, such as settled turbidity or phosphate, nudges the dose within set limits.

Dosing for phosphorus removal

Phosphorus removal follows the same three steps, with one change: the dose is sized as a molar ratio of iron to phosphorus (Fe:P), then confirmed by jar testing. The ratio climbs as the phosphorus target gets tighter, and the dosing point matters as much as the dose. Our phosphorus removal guide covers the Fe:P ratios, dosing points and a worked example. Our phosphate removal calculator applies a chosen Fe:P ratio to your flow.

Keep the dose honest

Check the pump with a drawdown test, and compare product used with the dose you think you give.
Track the result: settled turbidity, residual iron, pH and alkalinity.
Re-test when things change: raw water, season, product batch or target.

Product specifications for our 40% grade are on the ferric chloride 40% (CHIMIFLOC FR 4014) page. For hazard and handling information, see the safety data sheet (on request).

References

US Environmental Protection Agency. Nutrient Control Design Manual, EPA/600/R-10/100, August 2010: chapter 3, metal salt dose and molar ratio; chapter 9, chemical addition points and jar testing (checked 6 October 2026).
NSF International / ANSI. NSF/ANSI/CAN 60-2020, Drinking Water Treatment Chemicals: Health Effects, section 2.21 (definition of maximum use level), as incorporated by reference by the Virginia Department of Health (checked 6 October 2026).
NSF International. Drinking water treatment chemicals listings, NSF/ANSI/CAN 60, company search "chimi" (checked 6 October 2026).
PubChem, National Library of Medicine. Ferric chloride (CID 24380): molar mass used for the iron-share conversion (checked 6 October 2026).