Dechlorination
When RO or NF membrane is used in the RO/NF process, the feed must be dechlorinated to
prevent oxidation of the membrane. FILMTEC™ 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, FilmTec 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