Which of the following is a chemical treatment in water treatment process?

The correct option is Coagulation.

Water treatment processes are categorized into physical, chemical, or biological methods based on how contaminants are removed. Let us evaluate each of the options to understand their roles in water treatment:
1. Screening: This is a physical pretreatment process. It involves passing raw water through bar screens or mesh screens to physically intercept and remove large floating debris, such as leaves, branches, plastics, and rags.
2. Sedimentation: This is a physical process that utilizes gravity. Water is kept in a quiet basin, allowing heavier suspended solids to settle to the bottom over time so they can be removed as sludge.
3. Filtration: This is a physical or mechanical process where water passes through a porous medium, such as sand, gravel, or anthracite. It traps and removes remaining suspended particles and impurities that did not settle during sedimentation.
4. Coagulation: This is a chemical treatment process. Very fine suspended impurities (colloids) in water carry negative electrical charges that cause them to repel one another and remain suspended. During coagulation, chemical coagulants (such as aluminum sulfate or ferric chloride) are added to the water. These chemicals undergo chemical reactions to neutralize the electrical charges on the colloidal particles, destabilizing them so they can collide and stick together to form larger masses (flocs). Because this process relies on chemical additives and charge-neutralization reactions, it is classified as a chemical treatment.