Chlorinated Water and RO Membranes

[swall]

I have been told that chlorinated city water will destroy RO membranes over time. Is this correct? If so, is the damage reversible/repairable? We have a set out we are considering rejuvenating, but they had chlorinated city water run through them for about a year or roughly 300,000 gallons worth.

 

[cub3bead]

If they are cellulose acetate membranes, they are likely ok if you adjusted the pH down to 5.5 first.

If they are thin film composites the chlorine resistance is usually rated in terms of ppm-hours. This number is the product of free chlorine in ppm and the contact time in hours. TFC membranes are typically rated in the range of 1,000 ppm-h before irreversible degradation occurs. If the free chlorine in the city water was greater than 0.1 ppm for the year; the membranes are likely damaged.

You didn't specify which disinfectant your city is using, however, if they are using chloramines then the ppm-h rating for TFC elements is about 300,000 ppm-h.

 

[kenvlach]

Chlorine damage is irreversible, as cub3bead said. With an RO membrane of a type damageable by chlorine, pre-filter with activated carbon.

 

[cub3bead]

An AC filter will take out the chlorine but I have usually scavenged it with sodium bisulfite because in warm climates the AC filter can turn into a giant bug factory and do more harm than good with the resulting biofilm on the membranes.

 

[swall]

Thanks for the replies. We currently are running through a softener and an AC filter. But, we had a set of membranes we took out of service that had been used before the AC filter was in place and I was just wondering about rejuvenating them. Looks like the answer would be NO.

 

[kenvlach]

swall, I don't think we can give a definite answer. How were you monotoring the RO water; was there any deterioration over time?

cub3bead, maybe exchanging one bug for another? Sulfite feeds some Thiobacillus species.

 

[longbow75]

I agree with kenvlach. No exact answer. However a phenomenon known as chemical attack occurs in presence of Cl and a transitional metal such as Fe. It's best to dechlorinate your flow right befoe RO.

P.S. Even if you have a feed Cl concentration less than manifacturer's limit, depending on your concentration factor, it becomes more concentrate at the end elements and stages.

 

[dfmorvan]

Chlorine destroys the TFC membranes. It is irreversible. You will get more flow for the same pressure and conductivity will go up.
To avoid this, I usually use bisulfite and monitor chlorine removal using ORP. Ac is another solution but as cub3bead said sometimes it can cause biofouling problems.