17. Secondary sewage treatment involves
(1) Physical removal of solids from polluted water by filtration and sedimentation
(2) removal of chemical remains by precipitation
(3) removal of dissolved organic compounds by activated sludge or trickling filter
(4) Removal of microbial pathogens by chlorination or ozonization

Concept of secondary treatment

Secondary sewage treatment is the biological stage that follows primary settling.

• Its main goal is to remove dissolved and colloidal organic matter (measured as BOD/COD) using microbial processes in systems such as activated sludge or trickling filters.

• Therefore, statement (3) correctly describes secondary treatment.

Option-wise explanation

1. Physical removal of solids by filtration and sedimentation

   • This describes primary treatment, where screens, grit chambers, and primary clarifiers remove large and settleable solids.

   • It is not secondary treatment.

2. Removal of chemical remains by precipitation

   • Chemical precipitation (e.g., for phosphorus or specific industrial pollutants) is usually associated with primary or tertiary/advanced treatment, not the standard definition of secondary biological treatment.

3. Removal of dissolved organic compounds by activated sludge or trickling filter – correct

   • In activated sludge, aerated tanks allow bacteria to oxidize organic matter; flocs then settle in secondary clarifiers.

   • In trickling filters, wastewater passes over biofilm-coated media where microbes degrade organics.

   • Both are canonical secondary biological treatment processes.

4. Removal of microbial pathogens by chlorination or ozonization

   • This is disinfection, typically part of tertiary / final treatment, carried out after secondary treatment to kill remaining pathogens before discharge or reuse.

So, secondary sewage treatment is best described by option (3).