I live in an apartment and have had an Aquasafe (same as greenmagi's link) RO
for about a year. I'm quite happy with it.
I don't know if all RO
filters are the same but I can tell you how mine works. There is 1 inlet and 2 exits. One exit goes through all 6 stages of the filter and the other exit (the waste water) only goes through the first three. This water is used to rinse off the garbage filtered out by the RO
To attach the inlet: you will need to drill a small hole in the plastic pipes below your sink. (I don't know what to do if yours are metal and not plastic) You don't even need a drill just a drill bit. There are usually knobs to turn off the water in the plastic tubes, so that you don't need to turn off the water to the whole apartment. Then there is a little attachment included in the kit to attach the inlet hose of the filter to the pipe under your sink. I assumed that replacing this plastic pipe when I leave the apartment will not be hard.
Fully filtered exit: This hose is left open and you simply fill a container and turn off the valve when you are done. There are some RO
systems that come with a storage tank so that there is always fully filtered water available, but these cost $20 to $30 more.
Partially filtered exit (waste): The instructions in my kit said to drill a hole in the elbow pipe below the drain of the sink. There was another attachment to connect the filter's hose to this elbow pipe. But I chose to leave this hose free also. When I use the filter, I jam the waste hose exit into the sink's drain.
When doing water changes, half of the water goes through all 6 stages and is very close to PURE water. The other half goes through only the first three stages of the filter. This filters out chlorine, nitrate, and other crap, but leave the minerals in the water. Doing this way I don't have to add any additives to the water whatsoever. If you wanted to use pure RO
water, I would suggest using "RO
Right" or some other additive specifically designed for RO
Let me know if you have any more questions.
PS I bought mine on ebay for $100 (with shipping) using "buy it now" and a few months later bought another identical one for my brother for only $65 (with shipping) by waiting till I got lucky bidding on auctions.