You're investigating reverse osmosis water filters, right? Here's a reverse osmosis water treatment comparison that will help you decide between a reverse osmosis system or a carbon filtration system.
Reverse osmosis, or "RO", is the process of forcing water through a membrane, or filter, with tiny holes. The holes are so tiny that a molecule of water can just barely get through, and the contaminants in the water, such as chlorine, dirt and sediment, nitrates, and many other chemicals, which are larger than the water molecules, can't get through.
Forcing the water through the membrane is a slow process, and most of the water doesn't make it through, which is good news and bad news.
The good news is that the water that doesn't get through goes on down the drain, taking the filtered-out contaminants with it, washing the membrane clean so it can continue to do its job.
The bad news is that a lot of water is wasted. Depending on the system, about 3 to 10 gallons of water flows down the drain for every 1 gallon of water that is cleaned.
The other main kind of water filtration unit for home use is the "SBAC", or "solid block activated carbon" system. These systems use a block of highly-compressed activated carbon to filter water. They don't waste any water.
The contaminants in the water get filtered out by the carbon in two ways. Incredible as it seems, the carbon blocks have up to 150 acres or more (That's over six and a half million square feet!) of surface area in just a pound of carbon, so the contaminants get trapped in the tiny passages that create all that surface area.
Also, through a process known as adsorption, the contaminants are attracted to the carbon particles and held there.
SBAC systems filter out chlorine, dirt and sediment, just as the reverse osmosis units do. They also filter out what are called VOCs ("Volatile Organic Compounds".) These are things like various breakdown products of chlorine, petroleum, pesticides, herbicides, and pharmaceuticals.
These substances have molecules that are smaller than water molecules, so RO units by themselves don't remove them from the water, although most RO units do add a carbon filter to catch the VOC's. (So why not just use carbon filtration in the first place?)
RO units do filter out all the minerals, like calcium, magnesium and potassium. Unfortunately, these are things our bodies need, and that generally make the water taste good. Many authorities think that demineralized water isn't good for us. SBAC units don't filter out the minerals.
RO systems do filter out nitrates, which SBAC systems don't, so if you're in an agricultural area where nitrate levels can get high as fertilizers break down and nitrates get into the water supply, an RO unit is the better choice. Otherwise, for my money the SBAC unit is the better choice.
So there's your reverse osmosis water treatment comparison. Now go ahead and protect your family by purifying your water with a home filtration unit, not by drinking bottled water, which uses huge amounts of petroleum to manufacture the bottles, and huge amounts of landfill space to accommodate the 50,000,000+ bottles discarded each day.