Desalination - Precision From Intake to Distribution

At the intake, sodium hypochlorite (NaOCl) is dosed for two distinct functions: continuous seawater chlorination to suppress marine biological growth, and intermittent shock dosing to prevent bacteria from adapting to a constant chlorine concentration.
This dual-mode dosing strategy requires metering equipment that can switch between steady-state feed and high-rate pulse injection without compromising accuracy in either mode.

Pretreatment is where the feedwater is prepared for the RO membranes, and it is the stage with the highest diversity of chemical injection points.

Typical pretreatment dosing includes:

• Sulfuric acid (H₂SO₄) for pH adjustment — lowering the feed pH keeps calcium carbonate in its dissolved form and reduces the scaling potential (measured as the Langelier Saturation Index, or LSI) before the water reaches the membranes.

• Ferric chloride (FeCl₃) as a coagulant — injected before filtration to destabilize suspended solids, organic matter, and colloidal particles, preventing them from reaching and fouling the membrane surfaces.

• Polyelectrolyte as a flocculant — bridges the coagulated particles into larger, settleable flocs for removal in clarifiers or media filters. Dosing accuracy here directly affects filter run times and the solids loading carried into the RO system.

• Sodium bisulfite (NaHSO₃) for dechlorination — removes residual free chlorine that would otherwise oxidize and irreversibly damage the polyamide RO membranes. The target is to reduce free chlorine to below 0.05 mg/L before the feed contacts the membranes.

• Biocide injection — where required to eliminate algae and control biological growth that has survived or developed after the initial chlorination stage.

• Anti-scalant/dispersant injection — to prevent mineral scale formation (calcium carbonate, calcium sulfate, barium sulfate, silica) on membrane surfaces.

At the first-pass RO stage, anti-scalant (scale inhibitor) is the primary chemical dose. As the RO membranes concentrate the dissolved solids in the feed water, mineral saturation levels increase rapidly — particularly for sparingly soluble salts like calcium sulfate and barium sulfate. Anti-scalant dosing prevents these minerals from precipitating onto membrane surfaces, which would reduce flux, increase salt passage, and shorten membrane element life.

In two-pass SWRO configurations, the second-pass RO system targets the removal of specific contaminants that are not fully rejected in the first pass — most notably boron. To improve boron rejection across the second-pass membranes, caustic soda (NaOH) is injected to increase the feed pH, shifting the boron equilibrium from boric acid (poorly rejected) to the borate ion (well rejected).

After desalination, the permeate is soft, slightly acidic, low in mineral content, and aggressive toward distribution infrastructure if left unconditioned. Post-treatment dosing transforms RO permeate into water that meets drinking water standards and protects downstream piping:

• Calcium hypochlorite for product water chlorination — ensures microbiological safety through the distribution network.
• Lime (calcium hydroxide, Ca (OH)₂) to increase pH and add calcium hardness. Lime also reacts with CO₂ to form calcium bicarbonate, which builds a protective coating layer on pipeline surfaces to prevent water erosion — a critical function for the long-term integrity of distribution infrastructure.
• Carbon dioxide (CO₂) for product water passivation — works in combination with lime to achieve the target calcium carbonate equilibrium and stabilize the water chemistry.
• Sodium fluoride for fluoridation where required by local drinking water regulations to prevent dental decay.
• Remineralization through lime milk and CO₂ injection to restore calcium and magnesium content to levels appropriate for potable use — essential because RO permeate is essentially demineralized water that would be corrosive and flat-tasting without conditioning.
• Boron elimination — in cases where first and second-pass RO do not achieve sufficient boron removal; additional caustic soda injection may be required in the post-treatment stage.

Membrane cleaning restores RO performance when fouling causes flux decline or salt passage increases. 

Cleaning-in-place (CIP) and chemically enhanced backwash (CEB) protocols require accurate chemical preparation and repeatable solution transfer:

• Acid cleaning — citric acid or hydrochloric acid solution at pH 2–3, circulated for 30–60 minutes to dissolve calcium carbonate, metal oxides, and inorganic scale deposits.
• Alkaline cleaning — sodium hydroxide (NaOH) with surfactant solution at pH 11–12, circulated for 30–60 minutes to remove organic fouling, biofilm, and colloidal material.
• Citric acid preparation — mixing citric acid powder with water in a preparation tank, where Milton Roy mixers ensure homogeneous solution concentration before transfer to the CIP system.
• SMBS preparation — mixing sodium metabisulfite powder with water for dechlorination solution, again requiring reliable agitation and consistent batch preparation.

The pretreatment stages — particularly coagulation, flocculation, and clarification — generate sludge that requires treatment before disposal. Polymer (polyelectrolyte) preparation and injection is the primary dosing application in sludge treatment, conditioning the sludge for thickening and dewatering.