Massachusetts Wastewater Practice Exam

The correct response identifies pH adjustment as a common first step in the treatment of chromium, cyanide, and other industrial wastes due to its critical role in influencing the solubility and mobility of these contaminants. Both chromium and cyanide have specific pH ranges where they can exist in various forms, some of which can be more toxic or less amenable to treatment than others.

For example, adjusting the pH can help convert chromium from its more toxic hexavalent form to the less toxic trivalent form, which is much easier to remove through precipitation or other treatment processes. Similarly, cyanide is more easily treated at specific pH levels, where it can be made to react with other chemicals, ultimately leading to its neutralization.

By controlling the pH, treatment facilities can enhance the effectiveness of subsequent removal processes such as precipitation, where metals and other contaminants are solidified and can be removed from wastewater. This initial step sets a foundation for the successful treatment of these hazardous industrial wastes, leading to safer discharge into the environment or further treatment.