Hot- and Cold-Water Treatment

Chemical treatment of potable waters is limited to small concentrations of inexpensive, nontoxic chemicals, such as polyphosphates, orthophosphates, and silicates. Zinc polyphosphate has been used for more than 60 years in controlling corrosion of steel in municipal water systems. Zinc orthophosphate is effective in controlling corrosion of steel, copper, and lead. 

Sodium silicate treatment in the amount of about 4-15 ppm Si02 is sometimes used by individual owners of buildings in soft-water areas. This treatment reduces red water caused by suspended rust resulting from corrosion of ferrous piping, and it also eUminates blue staining by water that has passed through copper or brass piping. 

Domestic or industrial hot-water heaters of galvanized steel through which hot aerated water passes continuously are not protected reliably in all types of water by nontoxic chemical additions such as silicates or polyphosphates. Adjustment of the saturation index to a more positive value, as discussed earlier, is sometimes helpful. Often, cathodic protection or use of nonferrous metals, such as copper or 70% Ni-Cu (Monel), is the best or only practical measure. 

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