Which step adds a chemical to the system?

• A. Coagulants added
• B. Filtration
• C. Potable water enters distribution system
• D. Screening

Answer: (A)

Coagulation is the step where chemicals are added to the water. Coagulants like alum or ferric salts are introduced during rapid mixing to neutralize particle charges and help tiny particles clump together into larger flocs. This chemical addition makes downstream removal by sedimentation and filtration much more effective, improving water clarity and contaminant removal. Filtration is a physical process that removes particles but doesn’t add chemicals. Screening removes large debris mechanically, also not chemical. Water entering the distribution system represents water after treatment, not the chemical addition step.