Chlorine will irreversibly damage a polyamide RO membrane, and as such, a chlorine residual cannot be allowed to contact the membrane. This means that once you dechlorinate, surviving bacteria will once again flourish, and dead bacteria will be available as assimilable carbon sources for the survivors.   Many components of existing TOC will also be broken down to AOC (Assimilable organic carbon). These will also become a source of carbon for increases in biological populations and a resulting biomass. This is especially a problem with seawater which has very high organics content.   Furthermore, dead bacteria are suspended solids and are just as likely to foul seawater membranes as any other insoluble organics.

The use of dechlorination chemicals such as bisulfite is also tricky, because overdosing them will scavenge dissolved oxygen, resulting in an anaerobic environment that can encourage growth of certain slime formers such as sulfate reducing bacteria. Carbon filters are not much better, as they become a breeding ground by adsorbing organics and providing an AOC supply to surviving bacteria in the dechlorinated water.

Many wastewater reuse RO systems inject chlorine into their feed water which already contains high levels of ammonia. This produces chloramines which are safe for direct contact with the membrane up to 5 ppm as combined chlorine. This gives the advantage of maintaining a biostatic environment throughout the membrane system. However, if you do not already have ammonia in the water, this would be risky – any failure leading to a loss of ammonium sulfate dosage would result in irreversible damage to your membranes.  The use of chloramines in seawater systems is not viable because bromide salts in seawater will form bromamines which can damage polyamide membranes.

In most cases, it is best to avoid chlorination as it is more likely to increase biofouling issues. Slug dosing non-oxidizing biocides such as DBNPA is generally a better solution for disinfecting the membranes on a periodic basis to extend times between cleanings. Permeate flushing before every shut down, and/or every 12 hours can also help reduce frequency of cleaning.