When water boils and steams, it leaves behind everything that's dissolved in it. A distiller unit just heats up the water until it boils and steams, then condenses the gas (the steam) into a liquid again in cooling coils, then it collects and drips into a resevoir as a liquid. It's 100% pure and has nothing in it. No minerals in it to make it healthy or to stabilize the pH.
Not quite the case. For highest purity from distillation the water must be distilled multiple times (three is usually considered enough) and even then there are traces of ions from the distilling vessel. Consequently, special materials must be used for that. In addition as soon as it is out of the still distilled water will start to dissolve gasses from the air in particular CO2 and oxygen which are quite soluble (especially CO2).
R/O & D/I water is water that is just highly filtered through different types of filtration media (micron carbon filters, membranes and cation/anion resins) to remove 95% to 99.9% of the dissolved solids in your water. The purity of the water depends on the efficiency of the filter media. Usually, it will still contain some trace elements within it, but it's so miniscule, it may be completely indetectible on a TDS meter.
Filtration (microfiltration, RO) and ion exchange are entirely different processes. In RO, for example, the water is forced through a membrane with pores large enough to pass water molecules but not ions. Rejection is from 90 to more than 99% depending on the ion. With exchange, however, cations are adsorbed and for each equivalent so adsorbed one equivalent of hydrogen ions is released. Anions are similarly adsorbed. For each equivalent of anions, one equivalent of hydroxyl ions are released. As anions and cations must balance in the feed the hydrogen and hydroxyl ions are also balanced and the result is that the minerals are replaced by pure water. In modern laboratories the purest water (18 megohm) is made by ion exchange but a few other things are done to to exclude organics (GAC filters) and to prevent CO2 and oxygen from reaching the reservoir.
For homebrewing, each type of water is roughly equivalent but for one thing: Bottled R/O water and distilled water for drinking sold in stores is usually reconstituted with some calcium and magnesium salts to make them safe to drink.
They are perfectly safe without addition of anything. They do taste rather flat however and that is why some minerals are added i.e. for taste - not safety. I have, believe it or not, seen an MSDS for water and while an LD50 of 19 L was listed (no route though - the mind reels) there was no information on toxicity. I guess I should also point out that I drink at least a liter of RO water every day and have been doing so without mineral addition for years.
If you have a home unit, you will need to reconstitute the water yourself. Otherwise, the pH will shift rapidly towards acidity or alkalinity depending on what the pure water interacts with.
Well as pure water has tiny buffering capacity (alkalinity of 2.5 ppm as CaCO3 to pH 4.3) that's hardly surprising. If I eat something I want it's pH to go to 1 -2 as soon as it hits my stomach as that is the pH at which my digestive enzymes do their magic.
That, and to prevent your body from robbing your own bones for the minerals it needs.
This sounds like the quackery that is quoted by the people trying to (and apparently suceeding in) selling "ionized alkaline water" machines. There is no need for you to provide any buffering in any water you drink. Your body provides plenty of bicarbonate for that purpose. And the foods you eat provide plenty of the other minerals. Drinking water is not, even in pretty heavily mineralized stuff, a significant source of minerals. Think about it. If you drink 4 L of water with a sodium content of 100 mg/L that's 400 mg and you are supposed to have a gram a day.
If you live in an area with very hard water (liquid chalk), it's often adviseable to thin out your brewing water by using some distilled or R/O water. Hard water tends to bring out the bitterness in hops to overpowering levels.
I'm starting to feel a little bad jumping on everything you say but it is not hardness that renders hops bitter but rather sulfate. Hardness is generally considered beneficial up to the point where the water starts to taste too 'minerally'. Often this isn't desirable but in some styles it is. The reason a brewer dilutes with RO (or DI) water is, for the most part, to combat alkalinity (bicarbonate ion) which leads to high mash pH. Often times he is after sulfate though in order to avoid that sulfate harshness that can ruin noble hops.
Though calcium is held in high regard in general it seems the best lagers at least are made with softer water so recently there has been a trend towards higher dilutions with the goal of brewing with low overall mineral content (as they have done in Pilsen and Ceske Budejovice for years).
