Water conditioning systems soften “hard” water through ion exchange in a brine (very salty) medium. A softener exchanges dissolved minerals for sodium atoms, typically near the water-service entry in a large tank holding rock salt.

 

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Conditioners only soften water; they don’t remove contaminants like microbial agents, heavy metals, pesticides or radiation-producing isotopes from drinking water. See the entry “Water purifier” for a discussion of methods used to remove these hazards from drinking water.

Public-drinking water supplies in the U.S. are free of many contaminants that threaten health. However, minerals such as calcium, iron and manganese – which are commonplace in many parts of the U.S. – aren’t considered health hazards, so water-works operators don’t remove them from community supplies.

Water conditioners come in two major categories: Agents, which are typically added to water to reduce hardness at the point of use, and devices, most of which are permanently installed components of a home plumbing system.

Agents are “active” water softening substances most commonly added to laundry water. Two commonly used hardness reducers are sodium borate (borax) and sodium bicarbonate (baking soda). These chemicals work by capturing and isolating dissolved minerals that can then be easily rinsed away. When in suspension, these minerals can't form soap curd that reduces the effectiveness of detergent.

Most devices, apart from maintenance, operate passively. The user doesn’t need to do anything to benefit from their operation. They’re a permanent part of the plumbing system – softened water flows out of every tap and outlet full time.

Softened water improves the effectiveness of soaps in bathing and cleaning, and greatly reduces the formation of mineral scale and soap scum in sinks, bathtubs and showers.

However, because they take out dissolved minerals and replace them with sodium, softeners may be problematic for people on sodium-restricted diets. In such cases, two-stage treatment is recommended – a water softener, followed by a reverse-osmosis filter designed to trap sodium.

A softening system also requires periodic maintenance to remain effective. You must add rock salt to the brine tank periodically, and the softening system needs to be back-flushed occasionally to remove trapped mineral particles. Automatic back flushing releases salt-laden brine, which has prompted some sanitary districts to ban their use. Brine may also impair home septic systems.