WTO Sedimentation and Filtration 2 — Questions and Answers
Question 1: What is the primary purpose of tube settlers installed in a sedimentation basin?
- Increase flow velocity through the basin
- Reduce effective settling depth to improve particle removal (Correct answer)
- Add coagulant chemicals to the water
- Aerate the water before filtration
Correct answer: Reduce effective settling depth to improve particle removal
Tube settlers reduce the effective settling depth, allowing particles to settle over shorter distances and dramatically improving clarifier capacity.
Question 2: A rapid sand filter is backwashed when head loss reaches approximately what level above the clean bed loss?
- 1–2 feet
- 5–8 feet (Correct answer)
- 15–20 feet
- 25–30 feet
Correct answer: 5–8 feet
Most rapid sand filters are backwashed when head loss reaches roughly 5–8 feet above initial clean-bed head loss, indicating significant clogging.
Question 3: Which phenomenon causes a settled floc layer to resuspend and carry over the effluent weir during high flow periods?
- Density currents
- Short-circuiting
- Sludge blanket upset (Correct answer)
- Weir loading excess
Correct answer: Sludge blanket upset
A sludge blanket upset occurs when rising flow velocities exceed the settling rate of accumulated floc, causing it to resuspend and overflow the effluent weir.
Question 4: The Hazen-Williams equation is used in filtration systems primarily to calculate:
- Chemical dosing rates for coagulation
- Head loss through filter media (Correct answer)
- Settling velocity of discrete particles
- Turbidity removal efficiency
Correct answer: Head loss through filter media
The Hazen-Williams equation is used to estimate head loss (friction loss) through filter media and piping in water treatment systems.
Question 5: In a dual-media filter, anthracite coal is placed on top of sand because:
- Coal is cheaper than sand
- Coal has a lower specific gravity and larger grain size, allowing it to remain on top after backwash (Correct answer)
- Coal removes taste and odor compounds
- Sand is too fine to be placed on top without washing away
Correct answer: Coal has a lower specific gravity and larger grain size, allowing it to remain on top after backwash
Anthracite's lower specific gravity (1.4–1.6 vs. 2.65 for sand) and larger grain size cause it to stratify above the denser, finer sand after backwash.
Question 6: What condition is indicated when a filter's turbidity breakthrough occurs early in the filter run, shortly after returning to service?
- Media is too deep
- Filter ripening is incomplete or improper backwash (Correct answer)
- Backwash rate was too low
- Coagulant dose was too high
Correct answer: Filter ripening is incomplete or improper backwash
Early turbidity breakthrough after returning to service typically indicates incomplete filter ripening, often caused by over-washing or failure to use a filter-to-waste period.
Question 7: Weir overflow rate (WOR) in a sedimentation basin is expressed in units of:
- Gallons per minute per square foot
- Million gallons per day per linear foot of weir (Correct answer)
- Nephelometric turbidity units per day
- Pounds of solids per square foot per day
Correct answer: Million gallons per day per linear foot of weir
Weir overflow rate is expressed as flow per unit length of weir (gpd/ft or MGD/ft), which controls the velocity at which water leaves the basin and entrains floc.
WTO Sedimentation and Filtration 2
What is the primary purpose of tube settlers installed in a sedimentation basin?