Although pH control is a form of chemical treatment, it is presented here in the general section because, like flow equalization, it is used with biological and physical treatment systems as well as with chemical treatment systems.

Water, the basic substance of all industrial wastewaters, is composed of hydrogen and oxygen. Water dissociates into hydrogen ions and hydroxide ions, as shown in equation 7-2.

In accordance with the law of mass action, discussed in the section entitled Reaction to Produce an Insoluble Solid, the quantities of hydrogen and hydroxide ions are such that the mathematical product of the hydrogen ion concentration, expressed as moles per liter ([H ⁺]), and the hydroxide ion concentration ([OH ]) is always 10 ¹⁴, as shown in equation 7-3.

The hydrogen ion concentration is thus a fundamental property of any aqueous solution. Any liquid for which equation 7-3 does not hold is not an aqueous solution. A strong solution of sulfuric acid is an example. It is a strong acid solution, not an aqueous solution, and its pH value really has no meaning.

The hydrogen ion concentration of an aqueous solution, such as an industrial wastewater, has a major influence on its characteristics. Which substances will dissolve in a given wastewater, as well as how much of a given substance can be dissolved, are two important characteristics. Another is that pH strongly influences the corrosivity of wastewater. The value of the pH of an aqueous solution must be within a...