Chloramine, which more than 30% of US cities now use as a disinfectant, is more difficult to remove from water than free chlorine. It can be removed with exactly the same treatment strategies that are used for chlorine, but greater contact time is needed.
Activated carbon is the best remover of free chlorine, and with enough residence time it removes chloramine effectively. However, when chloramine is involved in an oxidation-reduction reaction, ammonia is left behind, and ammonia is a severe problem for aquarium owners.
Reverse osmosis units with at least two carbon prefilters should remove chloramine nicely, but ammonia must still be dealt with.
According to Francis S. DeSilva ("Supplying DI Water for the Aquarium Industry") from Water Technology magazine's website:
Ammonia can be present in water in two forms, either ammonium hydroxide (NH3) or as the ammonium ion (NH4).
When the pH of the water is less than 7 the ammonia is present as the ammonium ion. As pH increases above 7, more of the ammonia is present as ammonium hydroxide.
The ammonium ion is readily removed by cation resin. Good removal capacity can be expected in waters low in hardness. Waters that are high in hardness will have decreased capacity due to the simultaneous affinity and removal of calcium, magnesium and the ammonium ion.
What this means is that a postfilter cartridge containing standard water softener resin should remove ammonia nicely.
Reverse osmosis removes the hardness and decreases the pH, assuring that the ammonia exists in the ammonium ion state that is readily removed by standard cation (water softening) resin.
Here is a clip from another article on ion exchange which throws illumination on the same concept:
Ammonia is present in almost all waters to some degree. In many cases, it is deliberately added to chlorinated feedwaters to reduce THM precursors. It is well removed by deionization processes but not by softening.
The general approach to ammonia removal would be primary and polishing softeners. Ammonia is removed preferentially to sodium by cation resin, but is displaced by calcium and magnesium. Therefore, a single softener will remove ammonia during the initial part of its cycle but will then release the ammonia as it becomes exhausted with hardness.
Softening can be used effectively for ammonia removal if the primary softener is allowed to load to a hardness endpoint and a polishing softener is used to remove ammonia. In this case, the primary softener must not be operated to hardness breakthrough, as this would cause an ammonia spike and lead to elevated ammonia levels in the final product water.
(whole house & well units)