OK, time for a brief primer on home water treatments.
Softening: Softening is done to remove hardness ions, which are the divalent cations, mainly Ca+2 and Mg+2. These are the ions that cause buildup in coffee makers and shower heads and in your water heater. The normal procedure is to run the water through an ion exchange resin. The ion exchange resin is loaded with sodium, weakly attached. When the hardness ions come through the resin bed they bump the sodium ions off the resin and attach, because they have more attraction to the resin than the sodium does.
The salt comes into play when the resin is filled up with hardness ions. A strong solution of salt is passed through the resin and the overwhelming concentration of sodium forces the hardness off. This solution is sent down the drain and, once the resin is regenerated, the remaining salt solution is rinsed to the drain as well. In most home softeners this is normally done on a timer in the middle of the night.
The net result of softening is that the ionic content of the water is unchanged, but the bothersome hardness ions are replaced with sodium.
Reverse Osmosis: Reverse osmosis (RO) uses a semipermiable membrane to purify water. You may remember osmosis from highschool biology, where water passes through a semipermiable membrane from an area of low salt (actually ionic) concentration to an area of high salt concentration to dilute the high salt concentration. RO uses pressure to reverse the process. Think of it as a pressure filter that works at the molecular level. It removes all ions, not just hardness, and each pass is about 75% efficient.
In commercial RO systems, part of the untreated water with higher solids is discarded in order to prevent clogging the membrane. In home systems the filter cartridges are periodically replaced when they clog up.
RO membranes are very sensitive to chlorine and particulates, so the water is usually filtered and passed through activated charcoal to remove particulates and chlorine. Membranes are also sensitive to bacterial buildup. That is why commercial systems, and some home systems, treat the water with UV light before passing it into the RO membrane.
The net result of RO treatment of home systems is a reduction of the ionic content of 50 - 80%
Distilled or deionized (DI) water: "Distilled water" is a kind of generic term for water that has had substantially all ions removed. Originally, distillation was the only way to do this. Today, DI water is most often prepared by using ion exchange technology similar to water softening. There are two resins used, one to remove all cations (positively charged ions) and the other to remove all anions (negatively charged ions). In the cation exchangecolumn cations are exchanged for hydrogen (H+), and in the anion exchange column the anions are exchanged for hydroxyl ions (OH-). H+ + OH- --> H2O. The resins are regenerated with hydrochloric acid (HCl) and caustic soda (NaOH) respectively.
All that being said, my phylosophy about water for use in winemaking is that if your water is good enough to drink, it's good enough to make wine with. Even high chlorine is not a problem, because the high organic content of the wine concentrate will overwhelm the chlorine and destroy it.