What is the primary purpose of primary treatment?

• A. To disinfect the water.
• B. To settle solids and capture floatable materials.
• C. To control excess NH3 and Total P.
• D. To remove big objects from the influent to prevent damage to pumps.

Answer: (B)

Primary treatment uses physical processes to remove solids and floatables from wastewater before the biological treatment stages. In a primary clarifier, gravity allows settleable solids to drop to the bottom as sludge, while lighter materials rise to the surface and are skimmed off as scum. This step reduces the solids load and some organic material entering the next stage, making subsequent treatment more effective. Disinfection and nutrient removal occur later in the process, not during primary treatment. Large objects are typically removed earlier through screening, not in the primary treatment step. So the main purpose is to settle solids and capture floatable materials.