This thread discusses some technical attributes of Zeolite. There are quite a few posts regarding zeolite, but a couple that distill some of the pros/cons are one from waterbear and one from JasonLion.
I've done a search of posts on multiple forums on zeolite and it appears that it does usually filter very well, similar to DE, for a clear water look even at night with lights and has a polished look as well. The fine pores could clog and cause the filter to behave more like regular sand, but I haven't seen reports of that -- perhaps regular backwashing prevents that from occurring. There are reports of the filter material getting into the water, presumably from it getting crushed into finer dust that escapes the filter. As noted, using a sand filter with some added DE has the benefits of finer filtration (though not as good as DE itself) with the ease-of-use of backwashed sand.
One claim that the zeolite manufacturers make (see Zeobest, ZeoSand, Zeobrite, Zelbrite, Zel-Eau, Zeoclere) is that the zeolite prevents or reduces chloramine formation, most saying that this is due to the ammonia adsorption. This simply isn't true! Ammonia combines with chlorine very quickly where at normal pool chlorine concentrations (FC around 10% of CYA) the formation of monochloramine happens with 90% completion in around 40 seconds. So there is no way that the ammonia will get to the zeolite filter before it combines with chlorine (unless someone urinates directly into the skimmer!).
A detailed analysis follows. First, let's look at the rate of formation of monochloramine.
See this link for one of many sources for the rate constant of 3.07x106 for hypochlorous acid combining with ammonia. The Wei & Morris model used a rate constant of 6.13x106 as did Selleck & Saunier. The latest full breakpoint chlorination model is from Jafvert & Valentine (1992) that gives 4.17x106 and one can purchase the paper describing this model here.
0.1 ppm FC equivalent with no CYA (using a larger FC would just make the reaction even faster) is the following (all chlorine "ppm" is relative to chlorine gas)
(0.1 mg/L) / (1000 mg/g) / (70.9064 g/mole) = 1.4x10-6 moles/liter
at a pH near 7.5, half of this is hypochlorous acid, so 7.0x10-7 moles/liter
So the rate of the reaction of chlorine combining with ammonia to form monochloramine is
Rate = Rate Constant * [HOCl] * [NH3] = 3.07x106 * 7.0x10-7 * [Ammonia] = 2.1 * [Ammonia]
and this result is in moles/liter/second
From the above it is seen that ammonia itself combines with hypochlorous acid in under a second (since the net rate factor of 2.1 is >1), but we now need to see how much ammonia there is at a pH of 7.5 since most of it will be ammonium ion and as the ammonia is depleted, ammonium ion will rapidly convert to ammonia so the reaction will continue (with exponential decay).
0.1 ppm total ammonia is the following (all ammonia "ppm" is relative to atomic nitrogen)
(0.1 mg/L) / (1000 mg/g) / (14.0067 g/mole) = 7.1x10-6 moles/liter
There are many sources for the Ka or Kb of ammonium/ammonia, but this link is a pretty clear one. I'll use the Ka constant of 5.6x10-10.
[H+] * [NH3] / [NH4+] = 5.6x10-10
at a pH of 7.5, [NH3] / [NH4+] = 5.6x10-10 / 10-7.5 = 1.8x10-2
so it is clear that most of the total ammonia is in the form of ammonium ion, NH4+.
Total Ammonia = [NH3] + [NH4+] = [NH3] + ([NH3] / 1.8x10-2) so
[NH3] = Total Ammonia / (1 + 1/1.8x10-2) = 7.1x10-6 / 56.55 = 1.3x10-7
so the rate is 2.1 * 1.3x10-7 = 2.6x10-7 moles/liter/second
If I ignore the slowdown as the ammonium ion level drops, I get 7.1x10-6 / 2.6x10-7 = 27 seconds. The 40 seconds for 90% completion I referred to earlier in this post used the accurate exponential decay formulas and used the Jafvert & Valentine rate constant.
Now, let's look at how quickly monochloramine might be removed via ammonia adsorption through multiple turnovers through the zeolite filter. There is an equilibrium between monochloramine and ammonia, though the reaction is very much favored towards monochloramine.
The equilibrium constant for the reaction of chlorine with ammonia to make monochloramine is 2.0x1011 (ratio of 4.17x106 / 2.11x10-5 rate constants from Jafvert & Valentine cited above). In case one thinks that there will be a lot of ammonia left for the filter to remove at equilibrium, the amount would be:
[NH3] = [NH2Cl] / [HOCl] / K = 7.1x10-6 / 7.0x10-7 / 2.0x1011 = 5.1x10-11
[NH4+] = [H+] * [NH3] / 5.6x10-10 = 10-7.5 * 5.1x10-11 / 5.6x10-10 = 2.9x10-9
this is 2.9x10-9 / 7.1x10-6 = 0.04% of the total initial ammonia or resulting monochloramine amount. Even if it were completely removed during each pass of water through the filter, it would take thousands of turnovers to remove the monochloramine via ammonium ion adsorption. However, the removal will have more ammonia replenished from monochloramine while going through the filter so we need to look at that rate.
The reaction rate of replenishing ammonia that is removed has a rate constant of 2.11x10-5 so the rate is:
k * [NH2Cl] = 2.11x10-5 * 7.1x10-6 = 1.5x10-10 moles/liter/second
[EDIT] I've modified the analysis that follows correcting some errors I originally had -- the errors I had made zeolite look better at removing monochloramine via ammonia absorption than would actually occur. [END-EDIT]
Even if we were to assume that the entire pool's amount of monochloramine were exposed to zeolite and that the ammonia were instantly absorbed by the zeolite, then this implies a half-life for monochloramine of -ln(.5)/2.11x10-5 = 32851 seconds or 9.1 hours. Of course, only a fraction of the pool's water is in the filter at any time so even if we assume that the water in the filter is there for 2 minutes, compared to even a quick 3 hour turnover this is a factor of 90 so monochloramine reduction would take over a month assuming 24/7 running of the filter.
Of course, there may be some direct absorption of monochloramine into zeolite, though this has yet to be shown to take place.
The bottom line is that the process of zeolite removing ammonium ion from the water would likely result in a negligible reduction in monochloramine and that in any event it would certainly not prevent it's rapid formation so long as there was chlorine in the water.
The only practical way that zeolite could reduce the amount of chloramine production in the bulk pool water would be by filtering out precursors of such production (i.e. small particles), but it couldn't filter out the ammonia quickly enough if there is chlorine in the water (as demonstrated above) and it has not been shown to filter out urea which is the largest component of sweat and urine that upon oxidation most likely produces dichloramine and trichloramine.
I believe what has happened is that someone used two true facts to draw an incorrect conclusion as follows:
TRUE: Zeolite removes ammonia (via ammonium ion) from water that is filtered
TRUE: Chlorine (hypochlorous acid) combines with ammonia to form monochloramine
FALSE: Therefore, Zeolite prevents or reduces chloramine formation in pools
The reason that the conclusion is false is that it requires the ammonia to persist in the presence of chlorine long enough to be able to be filtered out by the Zeolite and that simply doesn't happen. I'm guessing that one manufacturer posted the claim and then everyone else followed suit, just assuming it was correct.
There is a task force on Zeolites as part of NSF International so I've contacted them to see if I can get more info on where this claim came from. I've also contacted the manufacturers noted above. Hopefully, that will clear things up. [EDIT] I am now a member of the NSF Zeolite task force and am giving input to ensure that the experiments are being done properly so we can finally get some real results with these filters to see if they do reduce monochloramine or other chloramines via any means. [END-EDIT]