The coagulant alum reacts with natural alkalinity in water to form which substance?

The correct answer is aluminum hydroxide. When alum (aluminum sulfate) is added to water, it reacts with the natural alkalinity present in the water, which typically includes bicarbonates and carbonates. This reaction leads to the formation of aluminum hydroxide, a compound that plays a crucial role in the coagulation and flocculation processes used in water treatment.

Aluminum hydroxide acts as a coagulant by neutralizing the negative charges on suspended particles in the water, allowing them to aggregate into larger flocs. This aggregation facilitates the removal of these particles during subsequent sedimentation and filtration stages in treatment processes. The presence of aluminum hydroxide is essential for achieving effective clarification and improving water quality.

This reaction is an integral part of the water treatment process, ensuring that impurities are effectively removed and that the treated water meets the required standards for consumption or discharge. Understanding the specific chemical interactions involved in this process is critical for those working in the field of civil engineering and environmental management.