Typically, a reverse osmosis unit is used to treat water used only for drinking and cooking. An activated carbon filter to remove odors and taste. A semi-permeable membrane through which water flows under pressure. A tank to hold the treated water. A drain connection for discharging concentrated contaminants.
Different sizes are available.
They can be under the sink or in a remote location, depending on the size of the water-holding tank. The capacity of the RO system, in gallons per day, would need to be matched to household water needs. Most households find 5 gallons per day adequate. There are maintenance costs, because the RO membrane needs replacing according to the manufacturer's recommended schedule. The cost of a unit needs to be weighed against the type and quantity of contaminants in the water and the concern for safety.