Do Reverse Osmosis water filtration systems produce water with negligible amounts of minerals, or is an additional deionization component required?

Question

I want to purchase some type of water filtration system to produce water that contains negligible amounts of minerals. Currently I purchase distilled water from Walmart, and I want my water filtration system to be that good.

I was under the impression that a standard home reverse osmosis system with a carbon filter would do this. However, MoreBeer states:

Reverse osmosis. Reverse osmosis (RO) involves passing water through a series of individual pressurized membrane filters that remove organics, inorganics, microbes, and some minerals. The “bulk water” machines frequently found in grocery stores often use reverse osmosis to treat the local water and usually carbon filter it as well; this water usually sells for $ 0.25–0.35/gallon. It is essentially similar to very soft spring water or distilled water.

I am confused because they say it removes "some" minerals, but then state that the R.O. water you buy at a store is "essentially similar" to distilled water. It goes on to imply that Deionization/Ion Exchange would remove all the minerals.

If I really want to get 0ppm water (or insignificant amounts of minerals), would a reverse osmosis system be sufficient, or would I need an additional ion exchange component?

[Edit: added the term deionization, which apparently is different than ion exchange]

Answer 1

Compared to any kind of ground water, RO has fewer minerals. It's not quite as pure as double distilled water that might be used in a (biology) lab, but very close. Possibly cleaner than rain water (which picks up CO2 from the air, and dust). For brewing, the amount of minerals is negligible.

Ion exchange, on the other hand, doesn't remove minerals. It simply exchanges one that might be bad (like calcium: bad for for your plumbing, lead: bad for health) with one that is acceptable, like sodium. AFAIK these systems don't do anything to the negative ions (Cl-, SO4--, etc); they are mainly intended to remove metals and calcium. And since they are only replacing them with other ions, you won't necessarily end up with pilsener water.

Purified drinking water, like Dasani and maybe the stuff at your local store, might be RO with minerals added to improve the taste. Maybe it would be fine for some styles, if you can find out what's really in it.

Answer 2

I purchased a RO system with a Deionization componenet. Apparently, deionization is different than ionexchange. Pepi's answer mentions that ion exchange adds sodium but removes calcium. Deionization only adds hydroxide and hydrogen, which in turn combine and form water.

I measured the TDS, with a $15 reader from Amazon, of both the water coming out of the RO and the water coming out of the DI (which also went through the RO). Here were my results:

Tap water: 398 ppm TDS
Filtered through RO: 80 ppm TDS
Filtered through RO and DI: 0 ppm TDS

So, to answer the question of whether or not RO can reproduce the distilled water I buy from walmart (0 ppm TDS), the answer is No. However, the combination of RO plus DI does result in 0 ppm.

However, given that my tap water is extremely bad (398 ppm TDS), perhaps this would not hold true for all types of water.