Post treatment – Remineralization of permeate water
Post treatment of the reverse osmosis permeate water is very important to avoid corrosion of downstream piping and equipment and add taste to the water.
The RO product water is slightly acidic (pH 5–6) and soft with low alkalinity, while distilled water approaches zero hardness with an alkalinity not likely to exceed 2 mg/l. Low alkalinity waters are aggressive to metal and asbestos cement pipes and take up calcium from mortar-lined pipes. They are unsuitable for distribution and for drinking purposes since they are without minerals and tasteless.
The RO product water should therefore be remineralized, in order to acquire: a) a minimum alkalinity of 80 mg/l as CaCO3, b) Calcium Carbonate Precipitation Potential CCPP of 3–10, c) a positive Langelier Index LSI (0–0.5).
Suitable treatment methods include: hydrated lime or calcite and carbon dioxide dosing; carbon dioxide dosing followed by filtration through limestone and subsequent lime caustic dosing to raise the pH above the saturation value ; sodium bicarbonate and calcium chloride dosing; blending with a suitable brackish water from an artesian source.