What is the function of alum in water treatment?

Alum, or aluminum sulfate, is primarily used in water treatment for the process of coagulation. During this process, alum acts as a coagulant that helps in the aggregation of small particles and impurities suspended in water. When added to water, alum dissolves and releases aluminum ions, which neutralize the charges of suspended particles, allowing them to clump together into larger aggregates or 'flocs'. These larger particles can then be easily removed from the water during subsequent filtration processes.

Coagulation is a crucial step in the treatment process as it improves the quality of the water by reducing turbidity and removing contaminants that could otherwise lead to aesthetic or health issues. The floc formed can then settle out or be filtered, thereby enhancing the efficiency of downstream treatment processes such as sedimentation and filtration. The effectiveness of alum in this role makes it an essential component in many water treatment facilities.

In contrast, while alum may have some impact on pH by virtue of its acidic nature, its main role is not as a pH adjuster. It doesn't primarily function as a filtration aid or a disinfectant, as these roles are served by other specific chemicals or processes in water treatment.