Slow sand filter design detail: The finished floor or underdrain bed of a slow sand filter is generally provided with which longitudinal slope for drainage and cleaning?

Introduction / Context:
Slow sand filters rely on uniform flow through fine media and periodic scraping of the schmutzdecke. Proper floor slope ensures effective drainage during cleaning and prevents stagnant pockets that could foster biological nuisance.

Given Data / Assumptions:

• Conventional slow sand filter with underdrain system.
• Gravity drainage to a sump during maintenance.

Concept / Approach:
A gentle longitudinal slope of about 1 in 100 on the floor (or underdrain bed) is commonly adopted to facilitate complete drainage without causing excessive headloss or construction complexity. Steeper slopes are rarely necessary; flatter slopes risk puddling.

Step-by-Step Solution:

Verification / Alternative check:
Facility O&M manuals frequently specify 1 in 100; some designs use 1 in 200 where site constraints dictate, but 1 in 100 remains a widely taught value.

Why Other Options Are Wrong:
1 in 50 is unnecessarily steep; 1 in 75 is steeper than typical; 1 in 200 may be acceptable but is flatter than the common recommendation; 1 in 500 is too flat for dependable drainage.

Common Pitfalls:
Confusing slow sand with rapid gravity filter backwash floors; misplacing slope direction relative to drain channel.

Final Answer:
1 in 100