Even the cheap R.O. setups on "The Bay" have at least 4 stages, so you are getting some extra filtration from the pre-filters and the carbon post filter.
Distilled water is the purest, but most small home units would have to run 24/7 to keep up.
Prefilters are for emoving large solid materials (sediments)and chlorine. A typical carbon prefilter removes chlorine well butnot choroamine. Prefilters do ot removed dissolved solids. The b carbon will remove a few pertoluem ditlates. The smam horizintal DI filters t
ypically sold with Chep\ap RO DI systems are very near worthless. For one they settles and then water flows through the open channels bipassing the reisn, plus they have such a small capacity their life span is very, very short.
Distilled water is seldom the purest unless you have a glass distilllation apparatus and do the distillation your self to assure only vapors at specific temperatures are condensed and there fore bottled. Almost no paharmecutical companies, laboratories or electronics manafacturers that need ultra pure water use distillers. They use RODI filters with a high grade resin and usuallu use at least two DO cartridges and if they start with water above about a TDS of 150 to 200 they run thw water first through a water softener.
If your talking about filter like this:
http://cgi.ebay.com/New-Portable-4st-50GPD-Reverse-Osmosis-RO-Water-Filter_W0QQitemZ380175432185QQcmdZViewItemQQptZSmall_Kitchen_Appliances_US?hash=item588438b9f9 Don't bother as it will cost twice as much to operate as a beter filter with all prefilters d and DI filters being in a standard vertical housing. And due to the vertical mountings you will not get good prefiltartion or post filtration. Toyr substandard RO menbrane will not last as long as it will have to deal with poorly prefiltered water. The RO membare will aslo only function witha rejection rate of 80% to 85%.
This is a good filter except for the near worthless carbon post filter mounted horix zontally on top. Yu would stll need to add at least one carbon filter cartridge to use if you have chloroamine in your water. Just replace the DI cartridge with the chloramine filter cartridge. The mebarne (DOW Filmtech 75 gph) is the best smaller RO membrane sold. If you want zero TDS water just add another filter cartridge if you use the existing one for DI.
All the others use crappy RO menmbranes like the chaeap one that will only reject 80% to 85%. DI resins are expensive and have very limited capacity ies. They should never be expected to remove more than a very few TDS.
Unless your willing to spend the effort to remount horizontal DI filters (and other Prefilters) vertically do not bother buying a system that uses them.
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