Sedimentation and Filtration Flashcards
7 cards from real WTO practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Sedimentation and Filtration flashcards as text
What is the main disadvantage of a high backwash rate in a rapid sand filter?
Answer: It can cause media loss by washing fine sand out of the filter
Excessively high backwash rates expand the bed too much and can wash fine media grains out through the underdrain, resulting in media loss and increased replacement costs.
In an upflow solids-contact clarifier, what serves as a primary flocculation and settling zone?
Answer: The recirculating sludge blanket
Upflow solids-contact clarifiers rely on a suspended sludge blanket through which raw water passes upward, providing contact between incoming particles and recirculating floc.
The term 'negative head' in filtration refers to:
Answer: Pressure below atmospheric occurring within the filter bed, which can cause air binding
Negative head occurs when head loss through a clogged filter section exceeds the available pressure head, creating sub-atmospheric pressure that can release dissolved gases and cause air binding.
Which of the following conditions promotes formation of mud balls in a rapid sand filter?
Answer: Excessive coagulant leading to strong floc with insufficient backwash energy
Mud balls form when strongly coagulated solids bind filter grains together and backwash energy is insufficient to break them apart, leading to clumps that grow over successive filter runs.
A sedimentation basin is experiencing density currents. What is the most likely cause?
Answer: Temperature or turbidity differences between inflow and basin water
Density currents occur when inflowing water has a different density (due to temperature or suspended solids) than basin water, causing it to flow along the floor or surface rather than settling uniformly.
What is the purpose of the underdrain system in a rapid sand filter?
Answer: Uniformly collect filtered water and distribute backwash water across the filter floor
The underdrain uniformly collects filtered effluent during a filter run and evenly distributes backwash water upward through the media to ensure thorough cleaning.
When evaluating filter performance under the Surface Water Treatment Rule (SWTR), the maximum allowable combined filter effluent (CFE) turbidity is:
Answer: 0.3 NTU in 95% of samples per month
The SWTR requires that combined filter effluent turbidity not exceed 0.3 NTU in 95% of measurements taken each month, with no single measurement exceeding 1 NTU.