In primary treatment, what is added to assist suspended solids in settling?

• A. Chemically treated polymers
• B. Chlorine gas
• C. Aeration
• D. UV irradiation

Answer: (A)

In primary treatment, suspended solids settle more quickly when they’re helped to clump together. Chemically treated polymers act as coagulants and flocculants: they neutralize particle charges and bridge together small particles into larger aggregates. These larger flocs become heavy enough to settle in the primary clarifier, so more solids are removed early in the process. Chlorine gas, aeration, and UV irradiation are used for disinfection or oxygenation, not to aid settling in primary treatment.