Typical RO Desalination Chemical Dosages: mg/L & ppm

There is no universal chemical dosage for an RO desalination system. Within the reviewed commercial guidance, 2–5 mg/L of antiscalant on RO feed—often 3 mg/L for an initial SWRO estimate—is a recurring budgeting range, not an operating setpoint. Sulfite dechlorination must be calculated from measured free chlorine, while coagulant, acid, caustic, post-treatment, and cleaning chemicals require their own process targets and reference streams.

These figures are suitable for preliminary industrial RO estimates only. The commissioned dose must follow feedwater analysis, process configuration, the selected chemical’s documentation, and the limits for the installed membrane.

Quick-reference RO chemical dosage table

The values below come mainly from commercial engineering guidance rather than an independent standard. Each row keeps the chemical basis, reference stream, operating mode, and required validation together so the number is not separated from its conditions.

Chemical or application	Purpose	Preliminary value	Basis, stream, mode, and validation
Antiscalant	Limit mineral-scale formation	2–5 mg/L; about 3 mg/L as an initial SWRO estimate1	Usually as-supplied product on RO feed; continuous. Confirm with feed analysis, recovery, concentrate saturation, product projection, and compatibility.
Sodium bisulfite or metabisulfite dechlorination	Remove free chlorine before chlorine-sensitive polyamide membranes	One commercial guide gives 1.47 mg SBS per mg free chlorine theoretically and 1.8–3.0 mg SBS per mg as a practical range.2 Separate commercial guidance uses approximately 3 mg SMBS per mg free chlorine as a preliminary rule.1	Calculate from measured free chlorine as Cl₂-equivalent on RO feed. Verify after adequate mixing with residual-chlorine testing, ORP, or another qualified signal. Do not describe the 1.47 ratio as theoretical SMBS demand.
Ferric chloride	Improve colloid removal before filtration	One supplier guide gives 0.5–5 mg/L as Fe³⁺.1	Raw intake; continuous or demand-dependent. State whether the dose means Fe³⁺, dry ferric chloride, or as-supplied liquid. Establish the actual dose by jar testing and confirm turbidity, SDI, filtration performance, and carryover.
Sulfuric acid	Reduce alkalinity, pH, or scaling tendency	One supplier guide gives 10–50 mg/L as a conditional estimate, mainly for applicable BWRO designs.1	Active acid or as-supplied product on the treated feed; continuous when required. Determine from alkalinity, target pH, and a validated water-chemistry model rather than adopting the range directly.
Sodium hydroxide, second pass	Raise pH for applications such as improved boron rejection	Control to approximately pH 9.5–10.5, rather than using a universal mg/L dose.1	First-pass permeate entering the second pass; continuous feedback or calculated control. Confirm pH and second-pass performance.
Remineralization	Stabilize finished permeate	Example finished-water target: 40–80 mg/L as CaCO₃.1	Permeate; target basis rather than direct reagent dose. Calculate the reagent requirement for the selected lime, calcite, carbon-dioxide, or other process.
Post-treatment chlorine	Maintain a disinfectant residual after RO	Example residual target: 0.5–1.0 mg/L.1	Permeate after RO; residual-controlled. Account for demand and contact conditions. This is not chlorine permitted to reach a sensitive RO membrane.

A low-fouling subsurface intake may require no ferric treatment. An open intake may need coagulation depending on particulate and colloidal loading, seasonal conditions, and the performance of downstream media filtration or ultrafiltration.

Clean-in-place chemicals do not belong in the continuous-dose rows. One commercial guide describes approximately 2% w/w citric acid at pH 2–3 for a low-pH batch clean and a sodium-hydroxide-plus-EDTA treatment controlled to approximately pH 11–12 for a high-pH batch clean.¹ These are example cleaning conditions, not RO-feed dosages. Use them only after checking the cleaning instructions and compatibility limits for the installed membrane.

What mg/L and ppm mean in an RO dosing specification

For dilute aqueous solutions, 1 mg/L is approximately 1 ppm by mass because a liter of water has a mass close to one kilogram.³ This approximation is useful for expressing a small target concentration in the process stream.

It does not allow mg/L and ppm to be substituted blindly when sizing a pump from concentrated stock. A product labeled with a percentage concentration also requires the concentration convention—such as percent by weight—and its density. The pump delivers solution volume, while percent by weight describes chemical mass relative to solution mass.

Every dosage specification should identify whether it means:

• Active ingredient, such as active chlorine
• As-supplied commercial product
• Neat proprietary product, a common basis for antiscalant estimates
• A constituent, such as Fe³⁺ rather than total ferric chloride solution
• A water-quality target, such as pH, disinfectant residual, or mg/L as CaCO₃

The reference stream matters just as much. Chemicals injected before the membrane normally use RO feed flow. Chemicals added after the membrane use permeate flow. A second-pass chemical may use first-pass permeate flow because that stream becomes the feed to the second pass.

Continuous injection must also be kept separate from intermittent treatment. Preservation solutions and batch CIP recipes have different concentration and compatibility requirements. A cleaning concentration must never be copied into a continuous feed-pump setting.

Antiscalant: why 2–5 mg/L is only a starting range

Approximately 2–5 mg/L as supplied on RO feed recurs in the reviewed commercial guidance, with 3 mg/L used as an initial SWRO budgeting value.¹ This does not establish an independent industry standard or mean that every plant should operate within that range.

A peer-reviewed review reports a 360 m³/day brackish-water RO plant that used 6 mg/L antiscalant for more than nine years. That is a secondary report of one plant-specific case, not evidence that 6 mg/L is typical for BWRO or SWRO generally.⁴

TDS alone cannot determine the dose. Waters with the same TDS can have different scaling potential because their ionic composition differs. Relevant constituents may include calcium, alkalinity, sulfate, barium, strontium, silica, phosphate, fluoride, and metals. Temperature, recovery, and concentrate-side conditions also affect scale risk; supplier technical guidance therefore calls for all potential scales to be considered rather than using TDS as the dosing input.⁵

A defensible final dose requires:

1. A complete feedwater analysis
2. The design temperature and intended recovery
3. Concentrate-side saturation calculations
4. A projection for the exact antiscalant product
5. A compatibility review covering the membrane and pretreatment chemicals

More antiscalant is not automatically safer. Product guidance warns that excessive antiscalant can foul RO membranes and that some antiscalants may be incompatible with cationic polymers or coagulants.⁶ Check the selected product rather than assuming that chemicals from different treatment stages can be combined safely.

Chlorination and sulfite dechlorination before the membrane

Intake chlorination and membrane dechlorination are separate operations. Chlorine may be used upstream for an intake or pretreatment objective, but a residual cannot simply be carried into a chlorine-sensitive polyamide membrane. Technical guidance warns that chlorine can irreversibly damage this membrane type.⁵

Chemical terminology matters here. The reviewed commercial RO/UF guide states:

• Theoretical sodium bisulfite demand: approximately 1.47 mg SBS per 1 mg of free chlorine as Cl₂
• Practical sodium bisulfite range: approximately 1.8–3.0 mg SBS per mg of free chlorine, including an operating allowance²

That source states SBS, not SMBS. Sodium metabisulfite forms bisulfite when dissolved, but the chemical names and theoretical mass ratios should not be interchanged without a documented stoichiometric conversion and product-purity basis. For preliminary SMBS estimates, another commercial guide uses approximately 3 mg SMBS per mg of measured free chlorine.¹

The calculation must begin with measured free chlorine at the relevant location, not with a fixed plant-wide sulfite dose. The commercial SBS guide recommends 20–30 seconds of residence time after injection, followed by verification that residual chlorine is non-detect; it also gives below 0.05 mg/L as a pre-membrane target.²

A peer-reviewed review separately cites below 0.1 mg/L residual chlorine as an example RO-feed requirement.⁴ Neither value should be treated as universally acceptable. Verify the chlorine tolerance and warranty requirements for the exact installed membrane.

Overfeeding sulfite is not harmless. The review identifies overdosed sulfite as a possible biofouling trigger and discusses membrane-oxidation risks under some combinations of dissolved oxygen and heavy metals. It also states that metabisulfite dilution tanks must be vented because sulfur dioxide can be generated during mixing.⁴

Coagulant, acid, caustic, post-treatment, and CIP need different controls

Ferric coagulant is used to improve colloid removal ahead of media filtration or ultrafiltration. The commercial estimate of 0.5–5 mg/L as Fe³⁺ is only a starting point.¹ The commissioned dose should come from jar testing followed by checks of turbidity, SDI, filtration performance, and chemical carryover. Before comparing ferric values, establish whether each one is expressed as Fe³⁺, dry ferric chloride, or an as-supplied liquid.

Sulfuric acid is an alkalinity, pH, or saturation-control chemical in applicable RO designs. The commercial 10–50 mg/L range is mainly associated with BWRO estimating and does not establish that acid is required in every plant.¹ Determine the actual quantity from the water chemistry, target pH, and applicable scaling model.

For second-pass boron rejection, sodium hydroxide is better specified by the resulting pH than by a universal mass dose. The cited commercial control target is approximately pH 9.5–10.5 on the second-pass feed.¹ The required product flow must be calculated from the process flow, stock concentration, density, and the amount needed to reach the target pH.

Remineralization also begins with a finished-water target. An example target of 40–80 mg/L as CaCO₃ does not mean adding the same mass of lime directly.¹ Reagent consumption depends on the selected treatment arrangement.

Post-treatment chlorine should be specified as a required residual after demand and contact conditions are considered. An example 0.5–1.0 mg/L residual therefore cannot be converted directly into an identical injection dose.¹

CIP remains a batch operation. The cited 2% w/w citric acid at pH 2–3 and high-pH sodium hydroxide plus EDTA treatment at approximately pH 11–12 are commercial examples only.¹ The installed membrane’s cleaning documentation takes precedence.

Convert mg/L into kg/day and a metering-pump rate

For a continuously dosed chemical:

kg/day = flow in m³/day × dose in mg/L ÷ 1,000³

Example: an RO feed of 10,000 m³/day at an antiscalant dose of 3 mg/L requires:

10,000 × 3 ÷ 1,000 = 30 kg/day

That is 30 kg/day on the stated dosage basis. If 3 mg/L means as-supplied antiscalant, the answer is as-supplied product mass. If it means active ingredient, product strength must still be applied.

For hourly mass:

g/h = flow in m³/h × dose in mg/L³

Do not add another factor of 1,000. One mg/L is numerically equal to one g/m³, so multiplying by m³/h already gives g/h.

For a stock-solution pump:

L/h = process flow in m³/h × target dose in mg/L ÷ stock active concentration in g/L³

For a product specified as percent by weight:

Stock active concentration in g/L = percent number × specific gravity × 10³

A unit-conversion example uses 100 m³/h, a 5 mg/L target, and a 12% w/w hypothetical product with a specific gravity of 1.20:

• Stock concentration = 12 × 1.20 × 10 = 144 g/L
• Required active mass = 100 × 5 = 500 g/h
• Pump rate = 500 ÷ 144 = 3.47 L/h

This demonstrates the arithmetic only. It is not an operating recommendation for chlorine, antiscalant, or any other RO chemical.

Before selecting a pump, check four common errors:

• Using permeate flow for a pre-membrane chemical
• Confusing active dose with as-supplied product dose
• Omitting density when converting weight percent to g/L
• Selecting a pump that cannot meter accurately across the required operating range

Commission the dose instead of copying the table

Turn a preliminary estimate into an operating setpoint through a defined commissioning sequence:

1. Obtain a complete feedwater analysis. Include the constituents needed for scale, coagulation, and compatibility assessment.
2. Identify the service. Distinguish SWRO, BWRO, second-pass RO, and post-treatment.
3. Set recovery and flow basis. Record whether each calculation uses raw intake, RO feed, first-pass permeate, second-pass feed, or final permeate.
4. Run scaling and saturation projections. Evaluate concentrate-side conditions rather than relying on incoming TDS.
5. Check membrane and chemical compatibility. Review antiscalant, coagulant, oxidant, dechlorinant, acid, caustic, and cleaning chemicals.
6. Establish coagulant demand by jar testing. Confirm performance through turbidity, SDI, filtration behavior, and carryover.
7. Confirm the injection point and mixing time. For sulfite dechlorination, use measured free chlorine and verify the result downstream.
8. Calibrate the metering pump. Confirm that its delivered flow matches the calculated requirement.
9. Trend plant response. Use measurements appropriate to the treatment stage, including pH, residual chlorine, ORP or another dechlorination signal, SDI, turbidity, conductivity, differential pressure, normalized permeate flow, and evidence of scale or carryover.

The installed membrane manufacturer’s limits, chemical product documentation, applicable potable-water approvals, and warranty terms take precedence over generic online ranges.

Chemical safety: Never mix acid with hypochlorite. The combination can release toxic chlorine gas, so storage and dosing equipment must keep the chemicals segregated.²

Underdosing can leave scale or oxidant-control objectives unmet. Overdosing can increase chemical cost and, depending on the chemical, contribute to membrane fouling, carryover, or biological growth.⁶

The practical rule is to use 2–5 mg/L antiscalant and chlorine-based sulfite calculations only for estimating and initial investigation. Calculate every chemical on the correct feed or permeate stream, then commission the actual pump setting from site water chemistry, product basis, membrane documentation, and measured plant performance.