Purpose of filtration in conventional water treatment: What is the primary contaminant class targeted by filtration (after sedimentation and before disinfection)?

Introduction / Context:
Filtration (e.g., rapid sand filters, dual-media filters) follows clarification to polish the water. Its core purpose is to remove the remaining fine particles that cause turbidity and can shield microorganisms from disinfectants.

Given Data / Assumptions:

• Conventional treatment sequence: coagulation–flocculation–sedimentation–filtration–disinfection.
• Use of rapid gravity filters with appropriate backwashing.

Concept / Approach:

Filtration physically strains and adsorbs fine suspended matter and microflocs, reducing turbidity and particle counts. While it does reduce microbial counts significantly, complete pathogen inactivation is achieved by subsequent disinfection. Colour and odour may be partly reduced but typically require activated carbon or oxidation for reliable removal.

Step-by-Step Solution:

Verification / Alternative check:

Turbidity performance goals (e.g., NTU limits) are standard compliance metrics for filters, confirming their primary role.

Why Other Options Are Wrong:

• (a) and (b) are not primary filtration objectives; rely on carbon/oxidation.
• (d) claims complete pathogen removal; final inactivation is via disinfection.
• (e) incorrectly suggests equal removal of all classes.

Common Pitfalls:

• Assuming filtration alone ensures microbial safety without disinfection.
• Overlooking headloss development and backwash scheduling that affect turbidity capture.

Final Answer:

Turbidity (fine suspended and colloidal particles).