Reverse osmosis permeate is near-pure water: corrosive, unbuffered and unfit for distribution without conditioning. This guide covers remineralisation, Langelier stabilisation, boron polishing and disinfection to deliver compliant, non-aggressive drinking water.
Low TDS, zero alkalinity and a negative saturation index make raw permeate aggressive to pipework
Single-pass SWRO permeate typically carries 150–400 mg/L TDS with negligible calcium and alkalinity. Water this soft has almost no buffering capacity, so small additions of acid or CO₂ swing the pH sharply.
With a strongly negative Langelier Saturation Index (LSI), permeate dissolves cement linings and corrodes ferrous mains, releasing iron and causing “red water” complaints. A positive-to-neutral CCPP is the design target.
Seawater boron (4–5 mg/L) passes the first RO pass as uncharged boric acid. Single-pass permeate may exceed the WHO 2.4 mg/L guideline, and demineralised water tastes flat — both are corrected in post-treatment.
Designing to a calcium-carbonate-precipitation-potential the distribution network can tolerate
Langelier Saturation Index: LSI = pH − pHs, where pHs is the pH at calcium carbonate saturation. Aggressive permeate sits at LSI ≈ −3 to −5; the conditioning target is roughly −0.5 ≤ LSI ≤ +0.5 with a positive Calcium Carbonate Precipitation Potential (CCPP) of 4–10 mg/L as CaCO₃.
Typical potable targets: calcium hardness 40–120 mg/L as CaCO₃; total alkalinity 40–100 mg/L as CaCO₃; pH 7.5–8.5; CCPP slightly positive. Hardness and alkalinity are delivered together by dissolving calcium carbonate or by lime + CO₂ dosing.
Three established routes to add calcium and alkalinity at potable scale
CO₂ is dissolved into the permeate to drop pH, then the acidic water percolates through a bed of calcium carbonate, dissolving it to add Ca²⁺ and bicarbonate alkalinity in a fixed 1:1 molar ratio. Robust and chemical-light, but limited dissolution rate and bed maintenance.
Hydrated lime (Ca(OH)₂) slurry plus carbon dioxide gives precise, fast control of both hardness and alkalinity, independent of contact time. Requires a lime preparation/saturator skid and careful slurry handling, but scales cleanly to large plants.
A controlled fraction of pre-treated seawater or brackish water is blended back to lift mineral content. Cheap where a suitable source exists, but constrained by the salinity, boron and pathogen load it reintroduces — usually a supplement, not a sole solution.
Meeting the boron guideline and delivering a stable disinfectant residual
Raising second-pass feed pH above 9.5 converts boric acid to borate ion, which the membrane rejects efficiently — the standard route to sub-1 mg/L boron for sensitive crops and WHO compliance.
Where only a polishing step is needed, boron-selective ion-exchange resin removes the residual fraction without a full second pass, regenerated with acid and caustic.
Primary disinfection (chlorine or UV) is followed by chloramine formation to carry a stable residual through long distribution networks with minimal disinfection by-products.
Caustic or CO₂ stripping trims the delivered pH into the 7.5–8.5 band after remineralisation, locking the CCPP positive before the water enters the clear well.
Reynolds & Bauhm engineers remineralisation, boron polishing and disinfection packages matched to your membrane configuration and distribution network.
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