Which substance is commonly used as a coagulant in water treatment?

Aluminum sulfate is commonly used as a coagulant in water treatment processes. Its primary function is to remove suspended solids and impurities from water by promoting the aggregation of these particles into larger clusters, known as flocs. When added to water, aluminum sulfate reacts with natural alkalinity and creates microscopic precipitates that extract contaminants and clarify the water. This process is crucial for producing safe drinking water and ensuring the effectiveness of subsequent filtration and disinfection stages.

The other substances listed serve different purposes. Calcium sulfate is typically utilized in the water treatment industry for hardening water or managing scale, but not as a coagulant. Potassium chloride is often used for water softening and fertilizer purposes rather than coagulation. Sodium phosphate may act as a corrosion inhibitor or nutrient but does not function efficiently as a coagulant like aluminum sulfate does. Thus, aluminum sulfate stands out as the correct choice for its well-established role in the coagulation process in water treatment.