What is alum primarily used to treat in water?

Alum, or aluminum sulfate, is primarily used in water treatment for its effectiveness in coagulation to remove phosphorus and particulate matter from water. When added to water, alum forms solid particles that can bind with suspended impurities, which aids in the sedimentation and removal of both organic and inorganic particles. This coagulation process is particularly beneficial in wastewater treatment as it helps control eutrophication by precipitating phosphorus, thus reducing nutrient levels that can lead to algal blooms.

In this context, while chlorine is used for disinfection to manage pathogens, alum does not serve that purpose. Oxygen levels are addressed through aeration processes, and while controlling pathogens is critical in water treatment, it is not the primary function of alum. Therefore, the use of alum in relation to phosphorus demonstrates its crucial role in improving water quality by addressing this particular concern in wastewater treatment.