In standard treatment, how much calcium hydroxide should be added for each mg/L of alum?

To determine how much calcium hydroxide should be added for each mg/L of alum in standard treatment, it's important to recognize the relationship between alum and calcium hydroxide in water treatment processes.

Alum, or aluminum sulfate, is used as a coagulant in water treatment, and when it is added to water, it lowers the pH. To counteract this and maintain the desired pH level for effective coagulation and clarification, calcium hydroxide (lime) is used as a pH adjuster.

The ratio of calcium hydroxide to alum is based on the stoichiometry of the chemical reactions involved in the coagulation process. Studies and operational guidelines suggest that approximately 0.4 mg/L of calcium hydroxide is needed for each mg/L of alum. This ratio helps ensure that the pH is stabilized after alum addition, allowing for optimal coagulation and removal of impurities.

Therefore, the correct answer reflects this established guideline for balancing the effects of alum addition in water treatment processes.