Membranes need to be replaced about every three years. If you use them a lot, they will not last as long. They work by only allowing certain sized particles to get through. It's like a screen that has holes in it that certain sizes get through. Different membranes are rated for different sizes. 1 micron is standard, and the .5 micron is a little better (usually only on Spectrapure brand RO filters).
The Deionization filter works by having particles that are charged in the filter (it is a resin), and they attract certain other charged particles. They don't last as long, usually a certain amount of water filtered. Some have a color change indicator to show when it's exhausted. The best is to have both, since RO is better at removing heavy metals like copper, and the DI is better at removing silicates and phosphates. If I had to choose only one, I'd go with RO.
Sonny
