There are two basic types of water softeners: automatic water softeners (also known as self-regenerating water softeners) and exchange tank systems.

Automatic Water Softeners

Residential automatic water softeners are plumbed into the home’s water supply and work by eliminating dissolved minerals through a process called ion exchange. Inside each water softener is a mineral tank that is filled with small plastic beads (also known as resin) that are negatively charged. To balance the charge, positively charged sodium ions are present on the beads. A separate brine tank holds a sodium chloride (salt) or potassium chloride solution, which is used to regenerate the softener. Under normal usage, hard water is passed through the mineral tank. The calcium and magnesium ions in the hard water have a stronger positive charge than the sodium or potassium ions on the resin. Therefore, the calcium and magnesium ions replace the sodium or potassium ions on the resin. The water flowing through the softener is now considered “soft” because the majority of the calcium and magnesium in the water has been replaced with sodium or potassium.

Eventually, there will not be enough sodium left on the resin to effectively soften the water. Then the softener has to be regenerated. This process is usually done during the middle of the night because soft water is not available during the regeneration. To start the regeneration, salt water from the brine tank is sent to the mineral tank. The high levels of sodium or potassium in the brine force the calcium and magnesium off the resin, replacing it with sodium or potassium. The chloride present in the brine water simply stays in solution. After regenerating the mineral tank, the brine solution is flushed to the sewer. New salt or potassium chloride must be added to the brine tank on a regular basis to replace the salt or potassium chloride that is used to regenerate the mineral tank. Because chloride is not used up during the exchange process, eventually all of the chloride added to the mineral tank will end up being sewered as spent brine.

Exchange Tank Softeners

Exchange tank softeners work in a manner similar to automatic water softeners, but feature a removable mineral tank that is replaced with a fresh mineral tank when the sodium on the resin is depleted. The depleted tanks are regenerated by water conditioning services at off-site facilities that are permitted to treat and discharge salty wastes, and thus, do not put chloride into the Santa Clara River.