To maintain water quality within acceptable limits (Art. 14.3), water supplied to a building usually must undergo some form of treatment. Whether treatment should be at the source or after transmission to the point of consumption is usually a question of economics, involving hydraulic features, pumping energies and costs, and possible effects of raw water on transmission mains.
Treatment, in addition to disinfection, should be provided for all water used for domestic purposes that does not fall within prescribed limits. Treatment methods include screening, plain settling, coagulation and sedimentation, filtration, disinfection, softening, and aeration. When treatment of the water supply for a building is necessary, the method that will take the objectionable elements out of the raw water in the simplest, least expensive manner should be selected.
Softening of water is a process that must be justified by its need, depending on use of the water. With a hardness in excess of about 150 ppm, the cost of softening will be offset partly by the reduction of soap required for cleaning. When synthetic detergents are used instead of soap, this figure may be stretched considerably. But when some industrial use of water requires it, the allowable level for hardness must be diminished appreciably.
Since corrosion can be costly, corrosive water must often be treated in the interest of economics. In some cases, it may be enough to provide threshold treatment that will coat distribution lines with a light but protective film of scale. But in other cases boiler-feed water for high-pressure boilers, for example it is important to have no corrosion or scaling. Then, deaeration and pH control may be necessary.
(The real danger here is the failure of boiler-tube surfaces because of overheating due to scale formation.)