Which stage adds coagulants in public water treatment?

• A. Screening
• B. Filtration
• C. Coagulation/flocculation
• D. Distribution

Answer: (C)

Coagulation and flocculation is the stage where coagulants are added to water to make tiny, suspended particles gather into larger clumps. In natural or treated waters, many particles are colloidal and carry a negative surface charge, which keeps them from sticking together. Adding coagulants—such as alum or ferric salts—introduces positively charged species that neutralize those negative charges. With the charges neutralized, particles lose their repulsion and begin to collide and stick together, forming larger clumps called flocs.

This mixing is done in two steps: a rapid, thorough mix to disperse the coagulant quickly, followed by a gentler, longer mixing (flocculation) to encourage collisions and growth of the flocs. Once flocs are formed, they can be more easily removed in the next treatment step, typically filtration (and sometimes sedimentation, depending on the plant design).

Screening happens earlier and mainly removes large debris; filtration then physically removes the coagulated material; distribution is the stage where treated water is delivered to the system while maintaining residual disinfectant. Disinfection usually follows filtration, ensuring microbiological safety as water moves through the distribution system.