Rapid gravity filtration — key characteristics: Which of the following statements about rapid gravity filters (RGFs) is/are correct?

Introduction / Context:
Rapid gravity filters are the workhorses of modern municipal water treatment, following coagulation–flocculation and sedimentation. Understanding their history, media characteristics, throughput, and pretreatment requirements is essential for design and operation.

Given Data / Assumptions:

• Conventional treatment train: coagulation → flocculation → sedimentation → rapid gravity filtration → disinfection.
• Effective size (d10) for RGF sand ≈ 0.45–0.7 mm.
• Typical unit filtration rates far exceed those of slow sand filters.

Concept / Approach:
G. W. Fuller pioneered rapid sand filtration in the early 20th century. RGFs use coarser media to permit higher rates and are periodically backwashed. Because the media is relatively coarse and the process relies on mechanical straining plus depth filtration, coagulation/flocculation is required upstream to create settleable/flocculable particles that the filter can capture effectively.

Step-by-Step Solution:

Verification / Alternative check:
Design manuals specify backwash rates, air scour, and filter-to-waste steps; all presume prior coagulation and clarification to limit fouling.

Why Other Options Are Wrong:
Each individual statement (a–d) is correct; therefore “All of the above” is the right choice.

Common Pitfalls:
Attempting to run RGFs on raw, high-turbidity water; neglecting headloss development and backwash expansion; confusing slow and rapid filter media sizes.

Final Answer:
All of the above.