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Wastewater > Primary Treatment of Wastewater > Flashcards

Primary Treatment of Wastewater Flashcards

1
Q

T/F: Primary clarifiers are capable of removing 100% of the influent TSS and and BOD

A

False: The clarifier can only remove the settleable portion of the TSS and BOD

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2
Q

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

A

D

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3
Q

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

A

B

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4
Q

T/F: Clarifiers rely on density differences to separate particles from wastewater.

A

True

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5
Q

T/F: Two particles have the same density. The particle with more surface area will settle faster than the particle with less surface area.

A

False. The particle with more surface area will settle slower, not faster.

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6
Q

T/F: Primary sludge will continue to compact and achieve higher total solids concentrations for as long as it remains in the clarifier.

A

False. Compaction is complete after about 2 hours.

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7
Q

Which of the following particles should reach the bottom of the clarifier first?
a: Grease
b: Feather
c: Sand
d: Lettuce

A

C

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8
Q

What phenomena in a clarifier push particles up and out of the clarifier?
a: Drag
b: Surface tension
c: Gravity
d: Surface overflow rate

A

D

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9
Q

Surface overflow rate is comparable to:
a: Detention time
b: Velocity
c: Weir loading rate
d: Density

A

B

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10
Q

The sludge layer that forms at the bottom of the clarifier is called the:
a: Schmutzdecke
b: Float
c: Blanket
d: Emulsion

A

C

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11
Q

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

A

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12
Q

T/F: A well-operated primary clarifier will always remove 40% of total BOD.

A

False. The percent removal depends on the amount of particulate and soluble BOD entering the clarifier.

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13
Q

T/F: Primary clarifier performance is limited by SOR, HDT, and the characteristics of the incoming wastewater.

A

True

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14
Q

T/F: Biodegradable organic solids are measured by both the BOD and VSS laboratory tests.

A

True

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15
Q

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%

A

B

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16
Q

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

A

D

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17
Q

A typical HDT for a primary clarifier would be:
a: 30 minutes
b: 1.5 hours
c: 3 hours
d: 6 hours

A

B

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18
Q

T/F: Fillets are used to fill in the corners of square and octagonal clarifiers.

A

True

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19
Q

T/F: Imhoff tanks settle primary sludge in the upper compartment and digest it in the lower compartment.

A

True

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20
Q

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.

A

False: most clarifiers have 0.5 to 0.7 m (1.5 to 2.0 ft) of freeboard.

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21
Q

This device helps prevent floatable material from escaping over the weir:
a: Energy dissipater
b: Rake
c: Scum baffle
d: Wear strip

A

C

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22
Q

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

A

B

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23
Q

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

A

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24
Q

T/F: Most circular, primary clarifiers use a center-feed, peripheral-rim, take-off flow pattern.

A

True

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25
Q

T/F: The feedwell directs flow down toward the bottom of the clarifier while increasing flow velocity.

A

False: The feedwell reduces inlet velocity.

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26
Q

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

A

B

27
Q

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

A

28
Q

T/F: Weirs for circular clarifiers generally consist of rectangular openings spaced at 0.3 m (1-ft) intervals.

A

False: Weirs for circular clarifiers are typically v-notch weirs

29
Q

T/F: The plows on a circular clarifier scraper mechanism are angled to direct sludge to the center hopper.

A

True

30
Q

T/F: The drive for the sludge collection mechanism is typically mounted on the wall of a circular clarifier.

A

False: The drive is typically mounted on the center platform

31
Q

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

A

D

32
Q

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

A

C

33
Q

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

A

34
Q

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

A

B

35
Q

T/F: Clarifier drive units typically consist of a motor and three sets of reducers.

A

True

36
Q

T/F: A sprocket is a toothed wheel that is used in combination with a chain or belt.

A

True

37
Q

T/F: Gears transfer power from one machine to another.

A

True

38
Q

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

A

B

39
Q

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

A

D

40
Q

When two gears are placed next to one another, the smaller gear is called the:
a: Gear
b: Idler
c: Pinion
d: Worm

A

C

41
Q

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

A

B

42
Q

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

A

C

43
Q

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

A

44
Q

T/F: The flow in a rectangular clarifier is radial.

A

False: Flow is longitudinal from one end to the other

45
Q

T/F: A chain-and-flight system consists of two, continuous parallel loops of chains.

A

True

46
Q

T/F: A perforated inlet baffle may be used to break up jetting action and disperse flow in a rectangular clarifier.

A

True

47
Q

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

A

B

48
Q

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

A

D

49
Q

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

A

D

50
Q

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

A

51
Q

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

A

B

52
Q

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

A

C

53
Q

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

A

54
Q

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

A

D

55
Q

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%

A

C

56
Q

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

A

C

57
Q

T/F: Hydraulic detention time can be defined as the amount of time, on average, water remains in a vessel.

A

True

58
Q

T/F: As SOR increases, the velocity of water through the clarifier decreases.

A

False. Surface overflow rate is another name for velocity specific to clarifiers.

59
Q

All of the following variables can affect the settling velocity of a particle except:
a: Size
b: Shape
c: Density
d: Color

A

D

60
Q

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

A

61
Q

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

62
Q

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

A

C

63
Q

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

A

D

64
Q

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

A

B

Wastewater (20 decks)

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