Why RO Membrane must be dechlorinated?
Dechlorination
When RO or NF membrane is used in the RO/NF process, the feed must be dechlorinated to prevent oxidation of the membrane. RO membranes have some chlorine tolerance before noticeable loss of salt rejection is observed. The first sign of chlorine attack on RO/NF membrane is loss of membrane flux followed by an increase in membrane flux and salt passage. Eventual degradation may occur after approximately 200–1,000 hours of exposure to 1 mg/L of free chlorine (200–1,000 ppm-h tolerance). The rate of chlorine attack depends on various feed water characteristics. Under alkaline pH conditions, chlorine attack is faster than at neutral or acidic pH. An acidic pH is preferred for better biocidal effect during chlorination. Chlorine attack is also faster at higher temperatures and higher concentrations of heavy metals (e.g., iron), that can catalyze membrane degradation. Since oxidation damage is not covered under warranty, Technical person always recommends removing residual free chlorine by pretreatment prior to exposure of the feed water to the membrane. Other oxidizing agents such as chlorine dioxide, hydrogen peroxide, ozone, and permanganate are capable of damaging RO/NF membranes also if not used properly.
Residual free chlorine can be reduced to harmless chlorides by activated carbon or chemical reducing agents. An activated carbon bed is very effective in the dechlorination of RO feed water according to following reaction:
C+2Cl2 +2H2O→4HCl+CO2
Sodium metabisulfite (SMBS) is commonly used for removal of free chlorine and as a biostatic. Other chemical reducing agents exist (e.g., sulfur dioxide), but they are not as cost-effective as SMBS.
When dissolved in water, sodium bisulfite (SBS) is formed from SMBS: Na2S2O5 + H2O → 2 NaHSO3
SBS then reduces hypochlorous acid according to: 2NaHSO3 + 2HOCl → H2SO4 + 2HCl + Na2SO4
In theory, 1.34 mg of sodium metabisulfite will remove 1.0 mg of free chlorine. In practice, however, 3.0 mg of sodium metabisulfite is normally used to remove 1.0 mg of chlorine.
The SMBS should be of food-grade quality and free of impurities. SMBS should not be cobalt-activated. Solid sodium metabisulfite has a typical shelf life of 4–6 months under cool, dry storage conditions. In aqueous solutions, however, sodium bisulfite can oxidize readily when exposed to air. A typical solution life can vary with concentration as follows:
Concentration (wt %) Solution life
10 1 week
20 1 month
30 6 months
Although the dechlorination itself is rapid, good mixing is required to ensure completion. Static mixers are recommended. The recommended injection point is downstream of the cartridge filters in order to protect the filters 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.
When RO/NF membranes are fouled with heavy metals such as Co and Cu, residual SBS (up to 30 ppm) partially converts to oxidants under the presence of excessive oxygen. When there is a heavy potential for metal fouling, SBS dosing amount control must be optimized and oxidation conditions of the concentrate must be monitored by an oxidation-reduction potential (ORP) meter.
The absence of chlorine should be monitored using an oxidation-reduction potential (ORP) electrode downstream of the mixing line. 175 - 200 mV threshold readings of the ORP have been typically applied. The electrode signal shuts down the high pressure pump when chlorine is detected.
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