WTO Sedimentation and Filtration 3 — Questions and Answers
Question 1: What is the typical surface overflow rate range for a conventional sedimentation basin treating surface water?
- 100–200 gpd/ft²
- 500–1,000 gpd/ft² (Correct answer)
- 2,000–4,000 gpd/ft²
- 10,000–15,000 gpd/ft²
Correct answer: 500–1,000 gpd/ft²
Conventional sedimentation basins treating surface water are typically designed for surface overflow rates of 500–1,000 gpd/ft² to achieve adequate particle removal.
Question 2: During filter backwash, auxiliary surface wash or air scour is used to:
- Cool the filter media to prevent thermal expansion
- Break up mud balls and improve media cleaning (Correct answer)
- Disinfect the filter media between runs
- Increase backwash water temperature
Correct answer: Break up mud balls and improve media cleaning
Auxiliary surface wash or air scour agitates the media to break apart compacted mud balls and organic deposits that backwash water alone cannot dislodge.
Question 3: Stokes' Law predicts that settling velocity of a spherical particle is proportional to:
- The square of the particle diameter (Correct answer)
- The cube root of particle mass
- The particle's turbidity contribution
- The log of particle surface area
Correct answer: The square of the particle diameter
Stokes' Law states that settling velocity is proportional to the square of the particle diameter (d²), so doubling diameter increases settling velocity fourfold.
Question 4: A filter run that produces an initial turbidity spike immediately after startup is best addressed by:
- Increasing coagulant dose to the filter influent
- Using a filter-to-waste period before placing the filter online (Correct answer)
- Reducing backwash flow rate
- Adding sand to the top of the media
Correct answer: Using a filter-to-waste period before placing the filter online
A filter-to-waste period diverts initial poor-quality filtrate to waste until the filter ripens and produces acceptable turbidity.
Question 5: The effective size (ES) of filter sand is defined as the grain diameter at which:
- 50% of the sand by weight is finer
- 10% of the sand by weight is finer (Correct answer)
- 90% of the sand by weight is finer
- 100% of the sand passes the sieve
Correct answer: 10% of the sand by weight is finer
Effective size (ES) is the d10 value — the sieve size that passes 10% of the sand by weight — and controls the initial head loss in a clean filter.
Question 6: What type of sedimentation basin uses a rotating rake mechanism to collect settled sludge at a central drain?
- Rectangular basin
- Circular clarifier (Correct answer)
- Upflow solids contact clarifier
- Contact basin
Correct answer: Circular clarifier
Circular clarifiers use rotating rake or scraper mechanisms that continuously move settled sludge from the basin floor to a central hopper for withdrawal.
Question 7: Which filter media characteristic is described by the uniformity coefficient (UC)?
- The density of the media compared to water
- The ratio of d60 to d10, describing the range of grain sizes (Correct answer)
- The minimum grain size present in the media
- The hydraulic conductivity of the filter bed
Correct answer: The ratio of d60 to d10, describing the range of grain sizes
The uniformity coefficient (UC = d60/d10) quantifies the spread of grain sizes; a lower UC (closer to 1.0) indicates more uniformly sized media.
WTO Sedimentation and Filtration 3
What is the typical surface overflow rate range for a conventional sedimentation basin treating surface water?