Primary Treatment of Wastewater Flashcards
T/F: Primary clarifiers are capable of removing 100% of the influent TSS and and BOD
False: The clarifier can only remove the settleable portion of the TSS and BOD
Which of the following components is most likely to be in the sludge at the bottom of a primary clarifier?
a: Ammonia
b: Soluble BOD
c: Grease
d: Settleable solids
D
The main purpose of primary treatment is to:
a: Remove grit and larger debris to protect downstream equipment
b: Reduce the loading to the secondary treatment process
c: Convert particulate BOD into soluble BOD for easier treatment
d: Generate electricity by reducing the organic load to the digester
B
T/F: Clarifiers rely on density differences to separate particles from wastewater.
True
T/F: Two particles have the same density. The particle with more surface area will settle faster than the particle with less surface area.
False. The particle with more surface area will settle slower, not faster.
T/F: Primary sludge will continue to compact and achieve higher total solids concentrations for as long as it remains in the clarifier.
False. Compaction is complete after about 2 hours.
Which of the following particles should reach the bottom of the clarifier first?
a: Grease
b: Feather
c: Sand
d: Lettuce
C
What phenomena in a clarifier push particles up and out of the clarifier?
a: Drag
b: Surface tension
c: Gravity
d: Surface overflow rate
D
Surface overflow rate is comparable to:
a: Detention time
b: Velocity
c: Weir loading rate
d: Density
B
The sludge layer that forms at the bottom of the clarifier is called the:
a: Schmutzdecke
b: Float
c: Blanket
d: Emulsion
C
A potential negative consequence of leaving primary sludge in the clarifier for an extended period of time might be:
a: Hydrogen sulfide gas
b: Higher solids concentrations
c: Lower solids concentrations
d: Breakdown of grit particles
A
T/F: A well-operated primary clarifier will always remove 40% of total BOD.
False. The percent removal depends on the amount of particulate and soluble BOD entering the clarifier.
T/F: Primary clarifier performance is limited by SOR, HDT, and the characteristics of the incoming wastewater.
True
T/F: Biodegradable organic solids are measured by both the BOD and VSS laboratory tests.
True
The influent to a primary clarifier contains 190 mg/L of TSS. The effluent contains 125 mg/L of TSS. Find the percent removal.
a: 26.1%
b: 34.2%
c: 65.7%
d: 87.4%
B
The influent to a primary clarifier contains 250 mg/L of BOD. Fifty mg/L is soluble and 200 mg/L is particulate. If the clarifier removes 50% of the particulate BOD, what will the total BOD concentration be in the primary effluent?
a: 50 mg/L
b: 100 mg/L
c: 125 mg/L
d: 150 mg/L
D
A typical HDT for a primary clarifier would be:
a: 30 minutes
b: 1.5 hours
c: 3 hours
d: 6 hours
B
T/F: Fillets are used to fill in the corners of square and octagonal clarifiers.
True
T/F: Imhoff tanks settle primary sludge in the upper compartment and digest it in the lower compartment.
True
T/F: Generally, 1.5 m (5 ft) of freeboard is provided. This allows excess water to be stored when there is a blockage or downstream equipment malfunction.
False: most clarifiers have 0.5 to 0.7 m (1.5 to 2.0 ft) of freeboard.
This device helps prevent floatable material from escaping over the weir:
a: Energy dissipater
b: Rake
c: Scum baffle
d: Wear strip
C
In a circular clarifier, the sludge hopper is typically located:
a: along the outer edge of the clarifier
b: Near the center of the clarifier
c: Adjacent to the scum baffle
d: Below the scum box
B
Most primary clarifiers are smaller than 100 m (300 ft) in diameter for this reason:
a: Wind can create currents in larger clarifiers
b: Larger sludge collection mechanisms are not available
c: Difficulty in reaching equipment for cleaning
d: Skimmer arm weight cannot be supported
A
T/F: Most circular, primary clarifiers use a center-feed, peripheral-rim, take-off flow pattern.
True
T/F: The feedwell directs flow down toward the bottom of the clarifier while increasing flow velocity.
False: The feedwell reduces inlet velocity.
A WRRF has three primary clarifiers, but only two of them are currently in service. The influent velocity is currently 2 m/s (6.6 m/s). The operator should:
a: Record the influent velocity on the log sheet
b: Place the third clarifier into service
c: Adjust thee feedwell baffles
d: Take a clarifier out of service
B
The purpose of the feed well in a primary clarifier is to:
a: Prevent short-circuiting
b: Channel the flow to the effluent weir
c: Aerate and mix the influent wastewater
d: Collect scum and sludge
A
T/F: Weirs for circular clarifiers generally consist of rectangular openings spaced at 0.3 m (1-ft) intervals.
False: Weirs for circular clarifiers are typically v-notch weirs
T/F: The plows on a circular clarifier scraper mechanism are angled to direct sludge to the center hopper.
True
T/F: The drive for the sludge collection mechanism is typically mounted on the wall of a circular clarifier.
False: The drive is typically mounted on the center platform
This type of launder projects into the clarifier center and is mounted on the outer wall:
a: Inset launder
b: Outboard launder
c: Cantilevered launder
d: Inboard launder
D
One potential disadvantage of an inboard launder is:
a: Weir length is increased compared to an inset launder
b: Lack of a mounting location for the weir plate
c: Floatable material can become trapped under the launder
d: Difficulty in reaching both weir plates for cleaning
C
One advantage of using and an inset or cantilevered launder is:
a: Increases total weir length
b: Difficulty reaching weir plates for cleaning
c: Floatable material can become trapped
d: Increases weir loading rate
A
Scum and other floatable material:
a: Passes under the scum baffle and into the effluent
b: Is pushed by the skimmer arm onto the egress ramp
c: Consists primarily of grease and sand
d: Should be removed upstream in preliminary treatment
B
T/F: Clarifier drive units typically consist of a motor and three sets of reducers.
True
T/F: A sprocket is a toothed wheel that is used in combination with a chain or belt.
True
T/F: Gears transfer power from one machine to another.
True
A clarifier drive mechanism has a broken shear pin. The operator should:
a: Replace the shear pin and restart the drive
b: Determine and correct the source of the overload condition
c: Decrease the drive motor speed
d: Adjust the gear-to-pinion teeth ratio
B
The clarifier mechanism is hung up on the clarifier floor. The resulting torque load reaches 45% of the maximum design drive torque. As a result, the worm gear shifts inside the gearbox causing:
a: Destruction of the spur gear
b: Drive motor to shut down
c: Rake speed to decrease
d: Overtorque alarm to trigger
D
When two gears are placed next to one another, the smaller gear is called the:
a: Gear
b: Idler
c: Pinion
d: Worm
C
This type of gear may be used when it is necessary to connect a horizontal shaft to a perpendicular gear:
a: Sprocket
b: Worm
c: Idler
d: Pinion
B
Two gears are operating together. The smaller gear is spinning at 2000 rpm. The larger gear spins at 500 rpm. How much more torque does the larger gear have?
a: Both gears have the same torque
b: Twice as much
c: Four times less
d: Eight times as much
C
When a reducer is placed between a motor and a piece of equipment:
a: Speed is reduced and torque is increased
b: Speed is increased and torque is reduced
c: Speed is reduced and torque is reduced
d: Speed is increased and torque is increased
A
T/F: The flow in a rectangular clarifier is radial.
False: Flow is longitudinal from one end to the other
T/F: A chain-and-flight system consists of two, continuous parallel loops of chains.
True
T/F: A perforated inlet baffle may be used to break up jetting action and disperse flow in a rectangular clarifier.
True
This type of sludge collection mechanism is commonly used with rectangular clarifiers:
a: Nierfratzer spiral
b: Chain and flight
c: Window shade plow
d: Logarithmic spiral plow
B
When a high-high torque alarm is triggered, this may also occur:
a: Tooth loss on the pinion gear
b: Collector mechanism may speed up
c: Load indicator shows zero
d: Shear pin breaks
D
A minimum of ______ sprockets is needed to support a chain-and-flight system that removes both settleable and floatable materials:
a: 1
b: 2
c: 3
d: 4
D
The drive unit for a rectangular clarifier is located:
a: On the clarifier wall
b: Underneath the inlet baffle
c: Parallel to the scum skimmer
d: In the influent manifold
A
The inlet manifold:
a: Is typically 25% of the tank width
b: Distributes flow evenly across the tank
c: Requires a minimum of 10 openings
d: Incorporates a waterfall cascade
B
One disadvantage of submerged launders over transverse weirs is:
a: Diminished algae growth
b: Easier to remove scum from the clarifier surface
c: Trapped rags and debris can be difficult to remove
d: Higher flow velocities
C
Plastic or metal wear shoes at either end of a clarifier end of a clarifier flight:
a: Support the weight of the flights
b: Do not require replacement
c: Compensate for stretching of the chain
d: Assist in cleaning the scum dipper
A
Historically, cast iron chains and wooden flights were used. Today, most systems use stainless steel or plastic with fiberglass flights for this reason:
a: Plastic chains are less likely to decay in sunlight
b: Cast iron is more expensive
c: U.S. EPA requires the use of food grade materials
d: These materials are lighter and easier to maintain
D
A transverse collection trench may be used in this situation:
a: Longitudinal trench will not fit the space
b: Length to width ratio is less than 3:1
c: Multiple chain-and-flight systems are used in parallel
d: Expected sludge concentrations exceed 10%
C
The trough rotation on a rotating skimmer:
a: Typically operates on a timed cycle
b: Turns in the direction of flow towards the weir
c: May be triggered by an approaching skimmer arm
d: Cannot be used with submerged launders
C
T/F: Hydraulic detention time can be defined as the amount of time, on average, water remains in a vessel.
True
T/F: As SOR increases, the velocity of water through the clarifier decreases.
False. Surface overflow rate is another name for velocity specific to clarifiers.
All of the following variables can affect the settling velocity of a particle except:
a: Size
b: Shape
c: Density
d: Color
D
A long, warm collection system may have this effect on the influent wastewater:
a: Decreases particle size
b: Increases ratio of inorganic to organic particles
c: Decreases ammonia concentration
d: Increases opportunities for flocculation
A
As wastewater temperatures decrease in a primary clarifier:
a: Particles will settle slower
b: Odor potential increases
c: Particles will settle faster
d: Water becomes less viscous
A
A small WRRF decides to accept hauled septage waste from a nearby campground. On days when septage is received:
a: Percent removal of TSS increases
b: Percent removal of soluble BOD increases
c: Percent removal of both TSS and BOD decreases
d: Percent removal is unaffected
C
Cold water affects settling velocity because:
a: cold water contains more dissolved gases
b: There is less drag on particles
c: Microbial activity increases as temperature drops
d: Water is denser at colder temperatures
D
These conditions tend to entrain grease and prevent it from being skimmed off in the primary clarifier:
a: Presence of ducks
b: Low pH, high temperatures
c: Lack of turbulence in the sewer
d: High pH, low temperatures
B
