Which of the following components is/are NOT removed in the secondary treatment of sewage?

The correct options are:
• Suspended settleable organic solids
• Pathogens
• Fat and grease

Explanation:
Sewage treatment is generally divided into three main stages: primary, secondary, and tertiary treatment.

1. Primary (Physical) Treatment:
This stage involves the physical removal of large and small particles from sewage through filtration and sedimentation. Large floating objects, as well as suspended settleable organic solids (which form the primary sludge), and floating materials like fat and grease, are separated and removed during this physical step. Because these are already removed during the primary stage, they are not targeted or removed in the secondary treatment.

2. Secondary (Biological) Treatment:
This stage relies on microorganisms (bacteria and fungi) to consume and break down the remaining biodegradable organic material. The primary focus of secondary treatment is the removal of colloids, including dissolved organic matter, thereby significantly reducing the Biochemical Oxygen Demand (BOD) of the sewage.

3. Tertiary (Chemical/Advanced) Treatment:
Even after secondary treatment, disease-causing microorganisms (pathogens) may still persist in the effluent. These pathogens are typically removed during the tertiary (disinfection) stage using agents like chlorine, ozone, or UV radiation before the water is safely discharged or reused.

Consequently, suspended settleable organic solids, pathogens, and fat/grease are NOT the components removed during the secondary treatment stage.