That membrane is preceded in the system by sediment and charcoal filters to removed suspended (undissolved) solids and organic molecules.  Often, an additional charcoal "polishing" filter is added after the membrane to be certain of eliminating unusual tastes or odors.

As the membrane would quickly clog up with the dissolved minerals it traps - especially with "hard" water sources - part of the water fed into the system is used to flush them away, greatly extending the life and effectiveness of the membrane.  A standard RO system will use three- or four gallons of "flush" water for every gallon of pure water produced.  Rather than simply dumping it, many folks redirect that flush water for other uses - humidification, watering less sensitive plants in the home or garden, or as is my case, after getting a shot at humidifying the greenhouse, it goes into an artificial pond with goldfish and native frogs, which also serves as a bird-bath and wintertime deer watering hole.  Additionally, it is possible to get a "zero-waste" kit that pumps the flush water into your home hot water supply.

Some things to keep in mind about RO Systems:

1. The typical output is lower than the rated output published.  The rated levels are typically done at 77°F and 65 psi water pressure.  Most households' water supply is cooler than that and at a lower pressure, so if you count on 75% of the rated output, it's closer to reality, although water quality affects that, too.  (For example, "softened" water, in which the Ca & Mg are replaced with Na via salt pellets, is easier to filter than the original hard water, and you'll extend the life of the RO membrane.)  Here is a chart to show you how the temperature and pressure affect the rated output.
    
2. They are not truly "on demand" water systems.  If we have a system with a true output of 72 gallons per day, that's only three gallons per hour, or a bit over three-quarters of a cup per minute.  If you have a large collection of plants to water, you're going to need some sort of storage tank to collect it in between waterings (plastic drums can usually be gotten from your local car wash at no charge).  Systems typically come with a pressurized tank of about 3-gallon capacity, which is often sufficient (larger ones are available).
    
3. RO water is "chemically aggressive", so it pays to use plastic or stainless steel as much as possible for all "wetted" parts.  Having essentially no dissolved solids, pure water is "unbuffered", meaning that the dissolution of the smallest amount of almost anything can significantly affect the pH.  Even carbon dioxide dissolved from the air will lead to the formation of carbonic acid.  I have seen a brass fitting, when exposed to RO water for a long time, have almost all of the zinc removed, leaving a brittle, porous, copper structure that is easily crumbled.

   

Surprisingly enough, it's not all that expensive to use RO water either.  Let's assume you buy a 100 gpd system, replace the sediment filter every six months, the carbon filter every year, and the RO membrane and polishing filter every 2 years, and use 200 gallons a week (20,800 gallons in two years).

For the first two years, you have the purchase price plus the cost of two sediment filters (I include and extra sediment filter and charcoal filter in the original shipment). So your 20,800 gallons cost $204, or just under a penny per gallon. During each successive two-year period, you purchase a membrane, a polishing filter, two carbon filters and 4 sediment filters, for a total of $81, making the pure water cost just over a third of a cent per gallon.