Mechanisms of water purification during filtration: Which process is primarily responsible for contaminant removal within filter beds?

Introduction / Context:
Filtration in water treatment removes turbidity and pathogens by several simultaneous mechanisms. Different filter types emphasize different mechanisms (rapid vs. slow sand, dual-media, membrane pretreatment), but in granular media filters, multiple processes act together to achieve high effluent quality.

Given Data / Assumptions:

• Granular media filtration following effective coagulation–flocculation (for rapid filters) or ripened biological layer (for slow sand).
• Appropriate loading rates and backwashing (for rapid filters) or scraping/cleaning (for slow sand).

Concept / Approach:

Key mechanisms include: (a) mechanical straining (size exclusion in pore throats), (b) flocculation/interception and in-bed sedimentation as particles collide and attach, (c) biological metabolism in slow sand filters where a schmutzdecke layer deactivates pathogens, and (d) electrostatic/adsorptive interactions that bind colloids to grain surfaces. No single mechanism alone explains performance across filter types and operating conditions.

Step-by-Step Solution:

Verification / Alternative check:

Pilot studies and filter effluent particle counting demonstrate cumulative removal mechanisms; if coagulation fails, rapid filter removal efficiency drops sharply, underlining the multi-mechanism nature.

Why Other Options Are Wrong:

• Each individual mechanism occurs; limiting to one ignores the integrated nature of granular filtration.

Common Pitfalls:

• Assuming filtration is only straining; in practice, attachment and biology can dominate.
• Overlooking filter ripening and the need for correct backwash regime.

Final Answer:

All the above.