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

The correct option is Coagulation.

To understand why coagulation is classified as a chemical treatment, let us examine the nature of each water treatment process listed:

1. Screening: This is a physical process. It uses physical barriers (screens) to block and remove large floating objects such as leaves, twigs, and plastic waste from the water.
2. Sedimentation: This is a physical process. It relies on gravity to allow heavy suspended particles to settle to the bottom of a basin or tank over time.
3. Coagulation: This is a chemical process. Fine suspended particles and colloidal matter in water carry negative electrical charges, which cause them to repel one another and prevent them from settling. During coagulation, chemical coagulants (such as alum, or aluminum sulfate) are added to the water. These chemicals react with the water and the impurities, neutralizing the electrical charges. This charge neutralization allows the fine particles to come together and adhere to one another, forming larger particles called flocs.
4. Filtration: This is a physical process. Water is passed through layers of sand, gravel, or charcoal to physically trap and remove the remaining fine particles and impurities.

Since coagulation uniquely depends on the addition of chemical substances to alter the chemical behavior and charges of the particles, it is classified as a chemical treatment in the water purification process.