my sister and BIL have a well for water - their water has a very high sulfur content (not only undrinkable but the plants in his garden dislike it also). They are now in the process of a rainwater catchment system for the plants.

They not only have a RO system for the entire house, they have a second filter system for drinking/cooking water at the sink. I do know the sink system would not filter out the sulfur sufficiently.

Would it help to ask your neighbors about the water and what treatment systems they use?

All water treatment is meaningless unless you have a water report to know what issues your water actually has. RO without the proper pretreatment of the water is a waste of money. It wastes a lot of water as well. If you are considering treating your water, then you need a water treatment professional to design a system for your particular water instead of throwing parts at it without knowing if it's actually doing anything other than flushing dollar bills down the drain. Most people don't actually need RO unless their water is so hard that conventional treatment leaves the water with an off taste. Conventional filtration and softening will take care of 90% of most people's water issues.

Not going to attempt to answer your Q about the "why" - too complex. Frankly I'm not sure if it really matters, but we liked the taste better and it was already here.

However, I may be able to help with the space. In our kitchen reno, we moved the RO filter to the basement and snaked the tubes up to the sink. It's been working great for us and not taking up all that space under the sink.

I did some research a few years ago and at the time RO was the only way to remove nitrates from water. We're on a well and our water tested on the high end of acceptable for nitrates.

Household reverse osmosis units use a lot of water because they have low back pressure. As a result, they recover only 5 to 15 percent of the water entering the system. The remainder is discharged as waste water. You can increase efficiency by paying more for a system with a pump and the energy to run it. But that increases the contaminants in the waste water, so you might not be able to use a grey water system.

Exactly what is or can be removed is controlled by the incoming water quality, the system pressure and the membranes used ... as well as how well the system has been maintained.

Your whole-house carbon filter - if well-maintained - removes particles and most organics. It does little to nothing about dissolved inorganic elements.

But first you need a water report to decide what your problem is and what the best solution is. Don't get a report from anyone selling purification systems. Get an independent one.

Our RO has no waste water. There is no exit from the system. So I don't think anything is being wasted. It doesn't plug in and doesn't use a battery so I'm not sure what energy is required to run it.

You can never go wrong with a reverse osmosis system. They will remove everything harmful in the water, including fluoride, chlorine, nitrates and nitrites, radiation, and everything else. And most systems do waste a lot of water, I have a reverse osmosis system that I can set as low as a 1:1 ratio, but have it set at a 2:1 ratio. But when it comes to your health, reverse osmosis is by far the best option for filtering your water. As a water report will only test for 67 contaminants, most are common, like lead, mercury, chlorine, etc. Meanwhile, there are on average over 400 contaminants in every water sample taken. If done by a private lab, and the fee is paid to test for everything (about $300), you will find that there is a lot more lurking in your water than you had thought, and it will scare you.

Here is a link that might be useful: Reverse Osmosis Systems

RO water is able to remove minerals, salts and chlorine from water, but the question is do you really need it? If your supply water is soft (low in minerals and salts), how much benefit are you really gonna get from it?

Also, all RO systems have waste water - best case scenario you are getting 1/3 good water for every 2/3 of waste water that goes down the drain (generally, more pressure = less waste). You may not see it depending on how it's plumbed but that's how RO and RO/DI systems work. American water is actually very clean and safe, especially if your plumbing is in pretty good shape and not shedding rust etc. So think about if the money spent and water wasted is something you REALLY need.

I've used RO systems for fish tanks with delicate species of fish, and we use sophisticated RO/DI systems here at work (pharma company), but I only use a Britta-type filter at home and that's only because some of the pipes are a bit old.

olivertwist ... it's running on city water pressure, and the waste water is plumbed to the drain of the sink the RO faucet is mounted on. That trickling noise you hear after d\getting a pitcher of water is the waste water running down the drain.

We put an RO system in our basement that runs to a Waterstone filtration faucet and our icemaker. It has a pump.
We have well water which doesn't taste good & we have never been able to drink it or make ice. Our water & ice now taste great....better than the Culligan water we had.