Sounds like what you have is a mixed-bed deionizer and filter. The carbon
handles organics. The beads are a mixture of cation and anion exchange
resins. (If I recall correctly, they are based on a polystyrene resin
modified to have cationic or anionic sites.) When the filters are new, the
cation resin will swap hydrogen ions for metal ions, ammonium, etc., and
the anion resin will swap hydroxyl resins for anions (sulfate, chloride,
fluoride, nitrate, etc.) If you didn't have both in the cartridge, you'd
get either a fairly acidic or basic effluent. With a mixed bed, you get
roughly neutral.
Regenerating an ion exchange resin is a simple matter: Just put it in a
solution with a high concentration of the ion you want to replenish. For a
cation-exchange resin, this would be strong acid, like hydrochloric acid.
For an anion-exchange resin, you might use sodium hydroxide. Once a mixed
bed is depleted, however, you have a problem. You can't combine strong acid
and strong base to regenerate the resins for obvious reasons. If mixed-bed
systems are to be regenerated, you need to use something like a saturated
salt solution. This, of course is just the wrong thing to use if growing
CP. So, unless someone is aware of an inorganic compound that is very
soluble and at least not harmful to CP, the best thing to do with a spent
cartridge is to dispose of it.
This is one reason that people use RO systems; there is just the membrane
that might be considered disposable (they do have a finite lifetime). On
the other hand, RO systems can waste tremendous amounts of water, depending
on how they are operated. What system you use depends on what you have in
short supply, I guess. RO if you have lots of water, and possibly a use for
the waste stream, DI if water is in short supply.
(I grow my few plants under lights in the basement, along with the orchids,
and collect condensate from the dehumidifier all summer for use in the
winter.)
Carl Gustafson,
Ex-chemist (polymers, phosphates, process & product development)