What characterizes primary treatment of sewage?

• A. Disinfection with chlorine
• B. Biological oxidation
• C. Chemical precipitation
• D. Physical removal of large objects, followed by settling

Answer: (D)

Primary treatment uses physical separation to remove solids from wastewater before any biological or chemical treatment. It focuses on taking out the big stuff and letting the rest settle by gravity. Screens and grit removal grab large objects, and then the wastewater moves to settling tanks where suspended, settleable solids sink to form sludge while the clarified liquid moves on to later treatment stages. This step reduces the load on downstream processes by removing the material that would clog equipment and consume a lot of oxygen.

Disinfection with chlorine is a downstream step that targets pathogens, not solids removal. Biological oxidation is the realm of secondary treatment, where microorganisms break down dissolved and suspended organic matter. Chemical precipitation is used for particular contaminants or nutrient removal in some systems, not the main goal of primary treatment.