WTO - Water Treatment Operator Sedimentation and Filtration Questions and Answers 1 — Questions and Answers
Question 1: Which of the following principles primarily governs the process of sedimentation in a water treatment clarifier?
- Centrifugal Force
- Brownian Motion
- Gravity (Correct answer)
- Electrostatic Attraction
Correct answer: Gravity
Sedimentation is a physical water treatment process that uses gravity to remove suspended solids from water. In the relatively still water of a clarifier or settling basin, particles heavier than water will settle to the bottom due to the force of gravity.
Question 2: A water treatment plant is treating raw water with low turbidity and high quality. To reduce capital and operational costs, which treatment scheme would be most appropriate?
- Conventional Filtration
- Direct Filtration (Correct answer)
- Slow Sand Filtration
- Diatomaceous Earth Filtration
Correct answer: Direct Filtration
Direct filtration is a suitable treatment method for high-quality source waters with low turbidity. This process omits the sedimentation step that is part of conventional filtration, thereby saving space and reducing costs. Coagulation, flocculation, and filtration are still utilized.
Question 3: An operator observes a rapid increase in head loss across a gravity filter, and the finished water turbidity is approaching the regulatory limit. What is the most immediate and appropriate action to take?
- Increase the coagulant dose.
- Decrease the filtration rate.
- Initiate a backwash cycle. (Correct answer)
- Add a polymer aid to the influent.
Correct answer: Initiate a backwash cycle.
A rapid increase in head loss (differential pressure) and turbidity breakthrough are key indicators that a filter is clogged with accumulated particles and needs to be cleaned. Initiating a backwash cycle is the standard procedure to clean the filter media by reversing the flow of water and flushing out the trapped solids.
Question 4: According to Stokes' Law, which characteristic of a suspended particle will result in the fastest settling velocity in a sedimentation basin?
- Low density and small diameter
- High density and small diameter
- Low density and large diameter
- High density and large diameter (Correct answer)
Correct answer: High density and large diameter
Stokes' Law describes the settling of particles in a fluid. The settling rate is directly proportional to the particle's density and the square of its diameter. Therefore, particles that are larger and denser will settle out of the water much faster than smaller, less dense particles.
Question 5: During a filter backwash, what is the primary purpose of the initial air scour phase?
- To disinfect the filter media with oxygen.
- To break up the compacted filter bed and loosen trapped particles. (Correct answer)
- To stratify the filter media by size.
- To warm the backwash water to improve cleaning efficiency.
Correct answer: To break up the compacted filter bed and loosen trapped particles.
The air scour step in a backwash procedure involves pushing compressed air up through the filter. This action causes the filter bed to expand and agitates the media, effectively breaking up the compacted layer of solids (the filter cake) and loosening the particles that have been trapped within the media, preparing them to be washed away by the water phase of the backwash.
Question 6: A water treatment plant using conventional filtration is experiencing an issue with floc carryover from the sedimentation basin to the filters. Which of the following is a likely cause?
- Excessively long detention time in the basin.
- Flow velocity through the basin is too high. (Correct answer)
- The coagulant dosage is too high.
- The raw water temperature has significantly increased.
Correct answer: Flow velocity through the basin is too high.
In a sedimentation basin, the water velocity must be slow enough to allow floc particles to settle out by gravity. If the flow rate is too high, the velocity will be excessive, and the floc will not have sufficient time to settle and will be carried over to the filters, increasing the load on them.
WTO - Water Treatment Operator Sedimentation and Filtration Questions and Answers 1
Which of the following principles primarily governs the process of sedimentation in a water treatment clarifier?