ELI5: How do water softener systems work? Are they magic? Adding salt gets rid of other minerals. . . HOW? I’ve never understood this. I’m 54, male.

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They use a principle called ion exchange. They have big filter cartridges filled with plastic beads. However, the beads are made from a special type of plastic, which has negative charges on its surface. Positively charged ions – such as sodium, calcium, magnesium and so on, are attracted to the negative charges on the beads and get stuck. Calcium and magnesium, which cause water “hardness” are more strongly positively charged than sodium, so if there is a choice, the calcium and magnesium will stick to the beads in preference to the sodium, and will push the sodium off into the water. The first step in the process is to load the beads with sodium. This is done by taking a concentrated salt solution, and running it through the cartridge. There is so much sodium in the brine, that it pushes any calcium or magnesium off the beads. After about a 30 minute soaking time, the waste brine then goes down the drain, carrying the calcium/magnesium with it. The cartridge then gets rinsed out with fresh water, and then gets switched in line with the water supply. As water flows past the beads, the calcium/magnesium in the water preferentially stick to the beads, and the sodium is released. At some point, all the sodium on the beads is depleted and the beads are saturated with calcium/magnesium, and the system needs to be regenerated again – so the system mixes up a salt solution and sends it through the beads, and the process repeats. Most water softeners have a water meter or timer, which triggers regeneration – the trigger needs to be adjusted for how much calcium is in the water (and in the case of a timer system, how much water is used on a daily basis).


Here’s how I would explain to an actual 5 year old: The beads in the softener have millions of little hands. They hold onto regular old salt until something they like better goes by (like hardness or certain other things). When they’re full of things they like and you want them to let go (regenerate), you put a lot of salt in there and they let go of the things they had grabbed onto because they want to touch all the salt going by. When the the salt is starting to run out at the end of the rinse, there’s nothing left but salt to grab onto. So they hold onto it until something better comes by. Rinse and repeat (literally).


How is your sex relevant to this?


There is another sort of water softening system. This uses polyphosphate salts and was common in laundry and dishwashing products. The polyphosphates bind to the magnesium and calcium ions in hard water, preventing them from reacting with soap to form scum, or precipitating out to form scale. The polyphosphates are in the product, and dont need extra equipment such as columns or filters.


Salt is actually two types of atoms bound together, and they have a loose bond – once salt gets into water, it dissolves. This means the atoms float around by themselves. However, some other molecules can form from the leftovers, molecules which do not have a loose bond. These include the other minerals, which bind with parts of the dissolved salt, and drop to the bottom as solid materials, which are easily filtered out. Now, a water softener uses some other smart tricks to remove the salt, but this is the basic version – salt dissolves, other materials form from the leftovers. We can use the same principle to clean water of lots of other unwanted things, and I recall a science project we had in high school where we went to a sewage plant and collected some, well, sewage water, and had to clean it using, basically, filtering and this method. My group’s water ended up crystal clear – but quite toxic, as we hadn’t figured out how to get the pH value down to normal levels.