How a Water Softener Works
Hard water is softened by removing the calcium and magnesium it contains. When hard water passes through the softening system's resin bed, the calcium and magnesium (hardness) ions are removed through an ion exchange process, so only softened water passes through to your home. Once the resin bed fills up with hardness ions, it must be regenerated (cleaned). Salt is typically used in the regeneration process and restores the resin to a "clean" state so the ion exchange process can begin again.
Reverse osmosis (RO) is a separation and water treatment process that uses pressure to force water through a membrane that retains the solute on one side and allows the purified water to pass to the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.
The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). As the membranes get clogged with the matter filtered from the water, the system must go through a rinse cycle to clean the filter. Water used in the rinse cycles can equal 25% to 75% of the amount of filtered water the system produces. It is important to use the RO water judiciously to minimize water waste, as every gallon of water RO water produced can require input of 1.75 gallons potable water.