How do alum and ferric salts function in water treatment?

Alum and ferric salts play a crucial role in water treatment primarily through the process of coagulation and flocculation. When added to water, these substances react with the natural alkalinity present, resulting in the formation of floc, which consists of larger aggregates of particles. This floc, made up of both the added coagulants and suspended impurities, can then be easily removed from the water through sedimentation or filtration.

The formation of floc is essential as it helps in bringing together fine particles that would otherwise remain suspended in water, not allowing for effective treatment. The resultant floc is larger and heavier than the individual particles, thus facilitating their removal from the water.

Other options listed are not applicable to the function of alum and ferric salts. They do not work by increasing water's pH, nor do they decrease water temperature or operate through reverse osmosis. Instead, their primary function revolves around improving the removal of suspended solids, thereby enhancing water clarity and quality.