RO Feed Water De-Chlorination using Sodium Meta-bisulfite, Bisulfite, Sulfite

Advanced Reverse Osmosis Technology

RO Feed Water De-Chlorination - Sodium Bisulfite, Sodium Meta-bisulfite or Sodium Sulfite can all be used to de-chlorinate the RO feed water. De-chlorination is essential with polyamide type membranes.

Sodium Metabisulfite (SMBS = Na2S2O5) is a 100% active solid and dissolves in water to form sodium bisulfite. It is 100% active. The fumes from mixing with water can be irritating.

Na2S2O5 + H2O >>>> 2NaHSO3 (2 moles of Sodium Bi-sulfite)

De-chlorination reaction with bleach (HOCl):

HOCl + NaHSO3 >>> NaCl + H2SO4

Sodium Bisulfite (SBS = NaHSO3) is liquid and usually sold as a 40% active.

De-chlorination reaction with bleach (HOCl):

HOCl + NaHSO3 >>> NaCl + H2SO4

Sodium Sulfite (SS = Na2SO3) is a liquid, usually with a maximum active of 20%.

De-chlorination reaction with bleach (HOCl):

HOCl + Na2SO3 >>> HCl + Na2SO4

A 40% sodium bisulfite solution (either as supplied or made up with metabisulfite) will probably be the most stable solution to use.

The SBS solution should be dosed as close to the RO system as possible (to keep as much of the pre-treatment as possible in contact with chlorine – e.g. cartridge filters). However, if the free chlorine level is high, the SBS should be dosed prior to the antiscalant injection point (or antiscalant dosage adjusted to compensate for chlorine attack). Some antiscalants are attacked by free chlorine. The antiscalant and SBS dosing point should be far enough apart to prevent neat product mixing (SBS and antiscalant can often be mixed when diluted correctly, but pH differences of the neat products can cause problems).

For further information or assistance please contact Reverse Osmosis Chemicals International on +44 (0) 161 877 2334 or e-mail info@rochemicals.com.