Which of the following coagulants is commonly used to settle sludge in a clarifier?

Alum, or aluminum sulfate, is commonly used as a coagulant in water and wastewater treatment processes, particularly in settling sludge in clarifiers. This compound works effectively by destabilizing suspended particles in the water, allowing them to clump together and form larger aggregates, known as flocs. When added to the water, alum reacts with the dissolved particles and facilitates the removal of these impurities by promoting sedimentation. The resulting flocs can then be more easily settled out in the clarifier, improving the overall clarity and quality of the treated water.

Other options, while they have specific purposes in wastewater treatment, do not primarily serve as coagulants for settling sludge. Lime is primarily utilized for pH adjustment and can assist in precipitation processes but is not a traditional coagulant. Polymer may help in the flocculation process to enhance settling but is typically used after the initial coagulation step. Sodium hypochlorite is mainly used as a disinfectant and does not play a role in the coagulation or settling of sludge. Therefore, alum stands out as the most suitable choice for this application.