What is the main role of alum and ferric chloride in water treatment processes?

The primary role of alum and ferric chloride in water treatment processes is coagulation. Coagulation is a crucial step in water treatment where these chemicals effectively aggregate and bind together small particles and impurities present in the water. When alum (aluminum sulfate) or ferric chloride is added to water, they form positively charged ions that attract negatively charged particles and colloids, leading to the formation of larger clusters called flocs. This aggregation helps in the removal of suspended solids, turbidity, and other contaminants, making the water clearer and safer for consumption.

This process is essential in preparing the water for subsequent treatment stages, such as sedimentation and filtration, ensuring that a more thorough purification can be achieved. Other options, such as disinfection, pH adjustment, and fluoridation, involve different chemicals or processes that target specific aspects of water treatment but do not play the fundamental role of physically removing particles as coagulation does.