Sodium metabisulfite [SMBS] is the typical chlorine reducing agent of choice for larger RO systems. A dosing rate of 2.0 to 3.0 ppm SMBS per 1.0 ppm chlorine so as to include an industrial safety factor for brackish water RO systems with at least 20 seconds of reaction time.

SMBS is preferred to be used for larger water treatment systems than other chemical reducing agents such as carbon filters and sulphur dioxide due to its advantage of reducing the chance of bacteria growth than produces biological fouling.

When dissolved in water, sodium bisulphite (SBS) is formed from SMBS:

  1. Na2 S2 O5 + H2 O → 2 NaHSO3
  2. SBS is then reducing hypochlorous acid according to:
  3. NaHSO3 + HOCl → HCl + NaHSO4

SMBS should not be cobalt-activated. Although the dechlorination itself is rapid, good mixing is required to ensure completion; static mixers are recommended. The injection point is preferably downstream of the cartridge filters in order to have these protected by chlorine. In this case, the SMBS solution should be filtered through a separate cartridge before being injected into the RO feed. Dechlorinated water must not be stored in tanks.

Online ORP analyser would give the details of the traces of free chlorine if any present in water and would dump the water if any traces found.

SMBS Dosing System is installed with following components:
  1. Single or compartment solution preparation tank.
  2. Chemical dosing pumps single or multiple
  3. Valve’s fittings
  4. Interconnecting piping
  5. Instruments like pressure gauges
  6. Level gauges, level switches
  7. PRV
  8. Sensors
  9. Control panel including interconnecting cabling