Hard water softeners (water softeners) are the most famous for water treatment using ion exchange resins.
A water softener is a device that exchanges the hardness of calcium ions　and magnesium ions contained in water with sodium ions (softening) with a cation exchange resin.

Water with high hardness is called “hard water”,　and water with low hardness is called “soft water”.
The World Health Organization (WHO) defines the standards for hard water　and soft water as follows, based on the total hardness * 1.
* 1 Sum of hardness derived from calcium　and hardness derived from magnesium in water.

Classification	Numerical value
Soft water	Less than 60mg/L
Medium hard water	Less than 60～120mg/L
hard water	Less than 120～180mg/L
Very hard water	180mg/L　or more

* Standards for soft water　and hard water (Guidelines for WHO drinking water quality standards)

According to the tap water quality standard, the total hardness is 300 mg / L　or less, but in most cases, tap water is 100 mg / L　or less,　and tap water can be said to be medium hard water　or soft water.
However, even a small amount of hardness may cause scale in piping etc. depending on the conditions. Also, if you use detergent　or soap in water with a high hardness component, the detergent component　and the hardness component will combine to form an insoluble metal soap, resulting in poor foaming. Therefore, use cationic resin to remove almost all the hardness contained in the raw water before use.

By the way, the figure below shows the difference in foaming when tap water　and water from which the hardness component of tap water has been removed by a water softener are mixed with detergent　and stirred up　and down. Tap water is also soft with a total hardness of 50 mg / L, but there was a large difference in foaming.