While it does not cause a drinking water health problem for people or pets, "Hardness" is still one of the most important water quality considerations for homeowners. Too much hardness (generally >180 mg/l) is what prevents you from using tap water in your clothes iron, it’s what over time clogs your faucet screens and shower heads, it’s what accumulates in your coffee maker and causes a bitter taste, and it’s what creates cloudiness in your ice cubes.

Other effects of hard water include:

• produces soap scum with reduced lathering; most noticeable on tubs and showers;
• produces white mineral deposits on dishes; more noticeable on clear glassware; and
• reduces the efficiency of devices that heat water. As hardness deposits build in thickness, they act like insulation, reducing the efficiency of heat transfer.
• builds up and gradually clogs pipes

The most common components of water hardness are calcium and magnesium, although iron,strontium and manganese may contribute. Since most of the hardness is calcium (Ca) and most of the calcium is in the form of calcium carbonate (CaCO3), hardness in water is reported as if all the hardness were CaCO3 and laboratories report the mass (mg - milligrams) contained in a volume (L - liter) of water. Sometimes it is reported as grains per gallon.

The level of hardness in water is ranked below from soft to very hard:

Soft:0 - 17.1 mg/l or 0 –1 grains per gal (17.1 mg/l equals 1 grain per gallon)

Slightly hard :17.1 – 60 mg/l or 1 - 3.5 grains per gallon

Moderately hard :60 – 120 mg/l or 3.5 - 7.0 grains per gallon

Hard :120 – 180 mg/l or 7.0 - 10.5 grains per gallon

Very hard :180 mg/l & over or 10.5 grains per gallon & over

Removing hardness from water is called "softening" and can be accomplished by various methods such as lime softening, ionic exchange and ultra-filtration.

The Centralia Water Treatment Plant currently doesn’t soften water as part of its treatment process, as the hardness ranges are almost always in the desirable range of 50 to 180 mg/l.

Waters with a hardness of less than 50 mg/l may have a corrosive effect in which common trace metals such as copper, iron, lead and zinc could be released into your water at higher concentrations.

A protective film of CaCO3, for this reason is desirable in the distribution lines and to monitor the finished water’s aggressiveness or scale forming proprieties, the "Langelier Index" test is used.

Water pH and chemical feed rates are adjusted accordingly to maintain the proper amount of coating in the water lines.