What is soft/ hard water and why is soft preferred?
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What is hard water?
Hardness is caused by the dissolved metal salts of calcium and magnesium, usually in carbonate form, along with other metals in lesser amounts. Hard water is found throughout the world. The hardest sources are usually underground where water and rock have lots of time to interact. The rainfall and snowmelt that percolates through the soil into the aquifer starts out soft but contains dissolved CO2. This leaches minerals like calcium or magnesium carbonate out of the rock turning the water hard.
Calcium carbonate, mostly found in limestone, is not very soluble so only a little heat makes it precipitate out. It’s the whitish scale in your kettle and is also found in stalactite and stalagmite formations in caves. Magnesium carbonate is found in a rock called dolomite. It’s very insoluble and ends up in your kettle too. These minerals aren’t harmful to humans or animals but play havoc with hot water heaters, dishwashers, laundry, kettles and even showering.
Hardness is commonly measured in grains, which is a US unit of measure. One grain is equivalent to 64.8 mg of dissolved calcium carbonate in 1 US gallon (3.8 litres). Soft water is 3.5 grains or less. Very hard water is anything over 10.5 grains.
Where I live has some of the hardest water in all of North America at 38.
Water softening
The easiest and most common way of treating hard water is the use of a water softener. The main component is a tank containing tens of thousands of plastic resin beads made of cross linked sodium polystyrene sulfonate. This synthetic chemical (invented in the 1930s) is known as an ion exchange resin and functions to remove calcium and magnesium ions from your water. There’s also a water soluble drug version, without the cross links, used to lower potassium levels in people.
When you turn on a tap or run the shower, hard water flows into the resin tank and comes in contact with the resin beads. Due to concentration gradients, calcium and magnesium ions in the hard water displace atoms of sodium bound to the active sulfonate groups in the resin. The released sodium ions combine with the carbonate ions left behind in the water to form sodium carbonate, Na2CO3. The newly softened water flows from the resin tank’s outlet, through the supply lines and out to the tap.
Sodium carbonate is also known as washing soda, a centuries old household cleaner and laundry additive. It’s thousands of times more soluble in water than calcium or magnesium carbonate. So it won’t precipitate out as scale or interfere with your soap which is great news for your plumbing, dishwasher and washer, water heater, kettle and even washing up. Softened water does have slightly elevated levels of sodium but not enough to cause danger. It affects taste though and I prefer hard water for drinking.
The resin in your softener can only soften so much water. At some point all the sodium ions will be exchanged and your softener will no longer function. Back in the old days (I remember this) you’d call to have the tank swapped out but nowadays all you need to do is run the recharge cycle on your machine.
Recharge timing is automated on modern machines using a flow meter that measures how much water you have used. Older softeners use a clock timer that would be set to recharge every few days, wasting a lot of salt and water. The recharge is usually set to run in the middle of the night when no one is turning on taps or flushing toilets.
The recharge cycle has a number of stages and takes a few hours to finish. The most important part of this for today’s explanation is the brine cycle. Water softeners have a salt tank that stores up to 100 kg of salt. It is partially filled with very salty water. Brine is drawn from this tank into the resin tank and, due to the very high concentration of sodium, reverses the ion exchange on the resin beads I’d mentioned earlier. The calcium and magnesium form salt compounds with the chlorine from the brine and are washed down the drain into the sewer.
After the resin is refreshed with sodium ions (and rinsed) the last step is the refill cycle on the brine tank. Fresh water is pumped into the tank which dissolves some more salt for the next recharge. So every few weeks you’ll need to add some salt to the tank. It’s a special high purity salt made just for softeners so don’t use anything else. 20kg bags are somewhere around ten bucks give or take.
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