Compare production rates: the yield (filtration rate) of a rapid gravity (rapid sand) filter is typically how many times that of a slow sand filter?

Introduction / Context:
Water-treatment plants use either slow sand filters (very low rates, high biological action) or rapid gravity filters (much higher rates with prior coagulation–flocculation). Understanding their typical filtration-rate ratio guides footprint and process selection.

Given Data / Assumptions:

• Slow sand filters: ~2–5 m^3/m^2/day (typical design band).
• Rapid gravity filters: ~80–120 m^3/m^2/day (post-coagulation typical band).
• We are asked for a representative multiple (order-of-magnitude comparison).

Concept / Approach:
Taking representative mid-range values (e.g., SSF ≈ 3–4 m^3/m^2/day and RGF ≈ 90–100 m^3/m^2/day) gives a ratio on the order of 20–30. A conservative standard teaching value often cited is about 20 times, which safely represents the magnitude difference for many exam contexts.

Step-by-Step Solution:

Verification / Alternative check:
Design manuals often present RGF throughput an order of magnitude greater than SSF; the 20× figure sits well within the usual range.

Why Other Options Are Wrong:

• 10× or 15×: Understate the typical difference.
• 30× or 35×: Possible for some plants, but higher than a standard benchmark used in teaching questions.

Common Pitfalls:

• Comparing instantaneous backwash/production cycles rather than net filtration rate.
• Ignoring pretreatment needs that enable RGF's higher rates.

Final Answer:
20 times