Which of the following is a step in preliminary treatment?

• A. Biological Digestion
• B. Ultraviolet Disinfection
• C. Screening
• D. Chlorination

Answer: (C)

Preliminary treatment focuses on protecting the plant by removing large solids and grit before the water moves on to the rest of the processes. The step that accomplishes this best is screening, which uses bars or screens to catch large debris such as rags, plastics, and other bulky materials. This early removal prevents clogs and damage to pumps and pipes, making downstream treatment more effective. Ultraviolet disinfection and chlorination are disinfection steps applied after solids are removed and usually occur later in the treatment sequence, not in preliminary treatment. Biological digestion is part of the biological treatment stage that breaks down organic matter, occurring after preliminary and primary treatment. Therefore, screening is the correct preliminary treatment step.