RO Antiscalant Dosing Calculation: From mg/L to Dosing Pump Flow

An RO antiscalant dosing calculation converts a selected feed-water dose into the required chemical mass flow and dosing-pump volume flow. For a dose expressed as supplied commercial product:

• Product consumption (kg/h) = Feed flow (m³/h) × Dose (mg/L) ÷ 1,000

• Neat-product flow (L/h) = Product consumption (kg/h) ÷ Product density (kg/L)

If the antiscalant is diluted before injection, the calculation must also account for the supplied-product mass fraction and the density of the prepared stock solution.

The calculation is only as reliable as its inputs. The recommended dose, the active-ingredient concentration and the prepared-solution concentration are different quantities. A correct unit conversion determines the required feed rate; it does not establish whether the selected dose is suitable for a particular reverse osmosis (RO) system.

1. Which Flow Rate and Dose Basis Should You Use?

Select the dose for the specific RO application

Antiscalant selection should consider feed-water analysis, the expected concentrate chemistry, pH, temperature, recovery, membrane configuration and pretreatment conditions. The selected dose should follow an application assessment and the recommendation for the specific product and membrane system.

A dose used for one formulation should not automatically be transferred to another product, even when both are described as RO antiscalants. Changes in recovery can alter concentrate chemistry, but they do not provide a simple multiplier for calculating the required chemical dose. Adding more chemical is not a substitute for evaluating scaling risk and product compatibility.

Before calculating consumption, confirm how the recommended concentration is defined:

• Supplied-product dose: the mass of the commercial product added per unit volume of treated feed water.

• Active-ingredient dose: the mass of a specifically identified active constituent per unit volume of treated feed water.

• Stock-solution concentration: the proportion of supplied product in the prepared liquid delivered by the dosing pump.

These values are not interchangeable. If a recommendation is already expressed as mg/L of supplied product, do not apply an additional active-content correction.

If a supplier specifies ppm rather than mg/L, confirm the stated basis. A mass-based ppm value represents mg/kg; converting it exactly to mg/L requires the relevant fluid density. The calculations in this article use an explicitly defined mg/L basis.

Use the flow at the dosing point

For a dose specified per litre of RO feed water, use the feed flow entering the defined treatment boundary, rather than the permeate production flow.

For a simple steady-state RO system:

Recovery = Permeate flow ÷ Feed flow

For example, if permeate production is 75 m³/h at 75% recovery, the corresponding feed flow is:

75 ÷ 0.75 = 100 m³/h

Using 75 m³/h instead of 100 m³/h in a calculation based on feed-water dose would underestimate the required product mass by 25%.

Systems with recycle streams or multiple injection points require additional care. Define the stream being treated, identify where fresh chemical enters the system, and account for any chemical returning through recycle. Internal circulation flow is not automatically equivalent to the fresh feed flow requiring a new dose.

2. How to Convert Antiscalant Dose from mg/L to kg/h and L/h

Calculate the required product mass flow

For a dose expressed as supplied commercial product:

Product mass flow (kg/h) = Feed flow (m³/h) × Dose (mg/L) ÷ 1,000

The division by 1,000 converts the units correctly. One cubic metre contains 1,000 litres, while one kilogram contains 1,000,000 milligrams.

The variables are:

Variable	Meaning	Unit
Qf	Feed flow at the defined dosing point	m³/h
Dp	Dose expressed as supplied product	mg/L
mp	Required supplied-product mass flow	kg/h
ρp	Density of the supplied product	kg/L
qp	Neat-product volume flow	L/h

Convert product mass flow into pump volume flow

Once the required mass flow is known, calculate the neat-product volume:

Neat-product flow (L/h) = Product mass flow (kg/h) ÷ Product density (kg/L)

Density matters because kilograms and litres are not interchangeable unless the liquid density is known. Use the density applicable to the supplied product grade and relevant measurement conditions.

Worked example: calculating neat antiscalant flow

The following values are hypothetical calculation inputs. They are not a QINGQUAN product specification or a recommended operating dose.

Assume:

• RO feed flow: 100 m³/h

• Selected supplied-product dose: 3 mg/L

• Assumed neat-product density: 1.20 kg/L

Step 1: Calculate product consumption

100 × 3 ÷ 1,000 = 0.300 kg/h

Step 2: Convert mass flow into volume flow

0.300 ÷ 1.20 = 0.250 L/h

Under these assumptions, the required feed rate is 0.300 kg of supplied product per hour, equivalent to 0.250 L/h at the assumed density.

This calculation demonstrates unit conversion only. The dose must be selected separately according to the water chemistry, membrane requirements and product-specific application recommendation.

When is an active-content correction necessary?

If the target concentration is explicitly defined as active ingredient, the supplied-product mass must be adjusted for the confirmed active mass fraction.

Product mass flow (kg/h) = Feed flow (m³/h) × Active dose (mg/L) ÷ [1,000 × Active mass fraction]

The active mass fraction is expressed as a decimal. For example, a confirmed mass fraction of 0.10 represents 10% by mass.

Apply this correction only when the dose recommendation refers to that identified active ingredient and the product documentation defines the relevant concentration basis. Density, pH or a description such as “concentrated” cannot substitute for missing active-content information.

3. How to Calculate Dosing-Pump Flow for a Diluted Stock Solution

Define the stock concentration correctly

A stock solution containing 10% w/w supplied product contains 10 kg of commercial product in every 100 kg of finished solution. This does not mean that 10% of the solution volume is commercial product, nor does it necessarily mean that the stock contains 10% active ingredient.

For a mass-based stock solution, define:

• wp: supplied-product mass fraction in the prepared solution.

• ρs: density of the prepared stock solution, in kg/L.

• qs: required stock-solution flow, in L/h.

• mp: required supplied-product mass flow, in kg/h.

The stock-flow equation is:

Stock-solution flow (L/h) = Product mass flow (kg/h) ÷ [Product mass fraction × Stock-solution density (kg/L)]

The product mass fraction and stock density must refer to the same prepared solution. Do not automatically substitute the density of the neat product for the density of the diluted stock.

A volume-based dilution recipe requires its own clearly defined calculation basis. A statement such as “10% w/w” cannot be treated as 10 litres of product mixed with 90 litres of water.

Worked example: calculating diluted stock flow

Continue with the hypothetical requirement of 0.300 kg/h of supplied product.

Assume:

• Required supplied-product mass flow: 0.300 kg/h

• Supplied-product mass fraction in stock: 0.10

• Prepared stock-solution density: 1.02 kg/L

The required stock-solution flow is:

0.300 ÷ (0.10 × 1.02) = approximately 2.94 L/h

The stock mass flow is approximately:

2.94 × 1.02 = 3.00 kg/h

At a supplied-product mass fraction of 0.10, this solution delivers approximately 0.300 kg/h of supplied product. Dilution increases the volume delivered by the pump, but it does not change the required supplied-product mass.

Simply multiplying the neat-product volume by ten would ignore the difference between neat-product density and stock-solution density.

For an actual installation, follow the preparation and dilution conditions permitted for the specific formulation. Dilution-water quality, storage conditions and product stability should be checked against the applicable product documentation. The hypothetical 10% stock used here is a calculation example, not a universal preparation instruction.

4. Which RO Antiscalant Product Data Must Be Confirmed?

Match density to the supplied product grade

QINGQUAN lists both acidic and alkaline RO antiscalants. Its published information distinguishes standard and concentrated versions of the acidic product, illustrating why the supplied grade must be identified before a density value is used.

Product and supply version	Published density at 20°C	Published pH of a 1% solution
RO Acidic Antiscalant, standard solution	1.03–1.06 g/cm³	1.5–2.5
RO Acidic Antiscalant, concentrated solution	1.40–1.50 g/cm³	1.5–2.5
RO Alkaline Antiscalant	1.16–1.36 g/cm³	10.0–11.0

Density in g/cm³ has the same numerical value as density in kg/L. However, a published range is not an exact measurement of an individual batch. Using the midpoint of a range produces an estimate; it does not establish the actual density of the delivered product.

A change from a standard to a concentrated product also requires confirmation that the recommended supplied-product dose remains applicable. Updating density alone does not prove that the original dose recommendation is valid for the new grade.

Keep product pH separate from RO operating conditions

The pH values in the table refer to a 1% product solution. They do not establish the operating pH of the RO feed water or determine the correct dose for a particular membrane system.

Product selection requires application conditions and compatibility information, not just a product label. If the membrane, pretreatment or formulation changes, previous compatibility assumptions may need to be reviewed.

5. How to Verify Actual Dosing-Pump Delivery

Compare measured delivery with the calculated target

A calculated flow rate is a target for verification, not proof that the installed pump delivers that volume under operating conditions.

Actual delivery depends on the pump model, liquid properties, control mode and operating conditions. A stroke-length percentage or frequency setting should not be assumed to correspond directly to a specific L/h value unless the pump documentation supports that relationship.

Where an appropriate delivery measurement is available:

Measured flow (L/h) = Delivered volume (L) ÷ Elapsed time (h)

Use a measurement method suitable for the installed equipment and chemical. Compare the measured flow with the calculated target under relevant operating conditions.

Recalculate the delivered dose

For a stock solution with a confirmed and unchanged supplied-product mass fraction and density, the actual average supplied-product dose can be calculated as:

Actual supplied-product dose (mg/L) = 1,000 × Measured stock flow (L/h) × Product mass fraction × Stock density (kg/L) ÷ Feed flow (m³/h)

For the hypothetical example, using a stock flow of 2.94 L/h, a supplied-product mass fraction of 0.10, a stock density of 1.02 kg/L and a feed flow of 100 m³/h gives:

1,000 × 2.94 × 0.10 × 1.02 ÷ 100 = approximately 3.00 mg/L

The small rounding difference results from expressing the calculated stock flow to two decimal places. This calculation verifies the average supplied-product quantity delivered; it does not prove that the selected dose will control scaling under actual water conditions.

When feed flow varies, compare the total product delivered with the total feed-water volume over the same period. An average dose does not establish that the instantaneous dose remained constant throughout that period.

Recheck the calculation when feed flow, supplied product grade, stock concentration or density changes.

Before confirming a dosing arrangement, keep these inputs together:

• Feed flow at the dosing point

• Dose value and its basis

• Supplied product grade

• Stock concentration and density, if dilution is used

• Verified pump delivery under relevant operating conditions

For product-specific inputs that need confirmation, contact QINGQUAN with the intended RO application, supplied product grade and dosing basis.

References

1. DuPont. FilmTec Reverse Osmosis Membranes Technical Manual. Form 45-D01504-en, Revision 20, August 2026.

2. Hydranautics / Nitto. Chemical Pretreatment for RO and NF. Technical Application Bulletin TAB111, February 2022.

3. IUPAC. Compendium of Chemical Terminology, 5th edition, online version 5.0.0, 2025. “Mass fraction.”

4. American Water Chemicals. AWC A-101 Product Information Sheet. Printed document identifier 446-R4 10.01.20.

5. Toray. Reverse Osmosis Membrane Element Operation, Maintenance, and Handling Manual. 06-G-MB1-260430(EN), April 2026.

6. ProMinent. Solenoid Metering Pump, gamma/X, GMXa: Operating Instructions. Part 984586, BA G 067 03/22 EN.