What combination forms a floc when alum is added to water?

When alum (aluminum sulfate) is added to water, it interacts with the alkalinity present in the water to form floc during the coagulation process. Alum acts as a coagulant, which means it helps to clump together small particles, suspended solids, and impurities present in the water.

The process begins when alum dissolves in water and releases aluminum ions. These ions neutralize the negative charges on the particles and allow them to agglomerate, or stick together. Alkalinity, primarily due to bicarbonates and carbonates from compounds like calcium carbonate, plays a crucial role because it provides the buffering capacity needed to maintain an appropriate pH during the coagulation process. This combination of aluminum ions from alum and the negatively charged particles in the water leads to the formation of larger aggregates or flocs, which can then be removed from the water through sedimentation or filtration.

Understanding this interaction is essential for water treatment operators, as managing floc formation effectively will enhance the efficiency of the treatment process and ensure better water quality.