The Role of  Reverse Osmosis in Removing Chloramines from Water for Aquariums

The water disinfectant chloramine that is being used increasingly by  municipal water treatment plants is bad news for aquarium owners.  Chloramines kill fish,  so there is much interest in removing it from water for aquariums.

Reverse osmosis (RO) has been a favorite of aquarium owners over the years for providing superb-quality water for fish.  There is some concern, however, about RO’s ability to provide chloramine-free water.  The following is an attempt at a non-technical explanation of how RO deals with  chloramine.

First, there is much misinformation,  some of it provided by anti-chloramine groups, that indicates that chloramines are virtually impossible to remove from water.

The fact is, chloramine is removed from water with the same methods that remove chlorine–especially filtration through carbon.   Chloramine reduction just takes longer, which in many cases means that it requires a larger carbon treatment bed and/or greatly reduced flow of water through the bed.  Some carbons, called catalytic carbons, are manufactured especially to treat chloramine and they work much faster than standard carbons.

Chloramine is made by combining chlorine with ammonia.  The removal process involves breaking the bond between chlorine and ammonia then converting the chlorine to harmless chloride. The carbon prefilter of an RO unit (which handles the water very slowly) does a good job of getting rid of the chlorine.  The part that often disturbs people is what happens to the ammonia, since, theoretically, RO membranes aren’t very good at ammonia reduction.

The remaining ammonia can be removed easily by cation exchange,  provided by common water softener resin. There are pH and hardness requirements, however, so that not just any water can be run through a water softener with the assurance that ammonia will be removed.  The reverse osmosis membrane, however,  prepares the water so that  leftover ammonia can be easily removed by cation resin placed after the RO membrane.  Post-RO water is low in hardness and pH,  so a simple and inexpensive cation resin postfilter added to a good RO unit should produce water that is essentially chloramine free.  All filters must be kept fresh to assure success.

Another option is an RO unit with a deionizing (DI) post filter.  The process is the same.  The RO unit’s carbon prefilter breaks down the chloramine and converts the resulting chlorine to chloride.  The RO membrane reduces the total dissolve solids greatly, leaving the DI postfilter free to polish off the ammonia.

Either RO followed by a cation cartridge or a deionizing cartridge should assure excellent, chloramine-free water for fish (or for people, since the same strategies work with undersink drinking water RO units).

References —

Removing Chloramine and Ammonia from Aquarium Water.  (This article provides references to excellent Resin-Tech sources.)

Chloramine and the Reef Aquarium.