1. A plant receives flow of 2.5 MGD with influent BOD of 220 mg/L. What is the pounds of
BOD per day?
a) 220 lbs
b) 4,586 lbs
c) 2,500 lbs
d) 1,200 lbs
BOD lbs = flow (MGD) × conc (mg/L) × 8.34 = 2.5 × 220 × 8.34 = 4,586 lbs.
2. During heavy rain, the clarifier weir overflow rate suddenly rises. What immediate action
protects effluent quality?
a) Increase RAS rate
b) Increase wasting to lower MLSS
c) Add polymer at the headworks
d) Bypass primary clarifiers
High weir loading causes solids carryover; wasting reduces sludge blanket and hydraulic
stress.
3. An operator notices rising white foam on the aeration basin. This most likely indicates:
a) Young sludge
b) Old sludge with Nocardia
c) Denitrification in the clarifier
d) Low DO
Nocardia and related filaments cause stable brownish-white foam at older sludge ages.
4. On a secondary clarifier, the sludge blanket depth is 5 ft and rising, effluent turbidity
increasing. What is the first step?
a) Increase RAS flow
b) Increase wasting rate
, c) Reduce aeration DO
d) Add coagulant
Rising blanket with turbidity suggests solids accumulation; wasting removes excess
biomass.
5. A lift station wet well cycles every 2 minutes. This is undesirable because it:
a) Increases energy cost
b) Can overheat the motor and shorten pump life
c) Causes more grit to enter
d) Increases BOD
Rapid cycling overheats motors, increases wear, and risks pump failure.
6. What is the typical detention time for a primary clarifier at design flow?
a) 30-60 minutes
b) 1.5-2.5 hours
c) 6-8 hours
d) 15 minutes
Primary clarifiers are designed for 1.5-2.5 hours to allow settleable solids removal.
7. An operator measures DO in an aeration basin at 0.5 mg/L. The mixed liquor color is
dark, and there is a septic odor. This condition is:
a) Normal for low F/M
b) Severe under-aeration
c) Denitrification only
d) High RAS return
*DO below 1.0 mg/L with dark color and odor indicates anaerobic conditions damaging
biomass.*
8. Which factor most directly affects chlorination efficiency for disinfection?
a) BOD concentration
b) Suspended solids concentration and pH
c) Flow velocity
d) RAS temperature
Solids shield pathogens and chlorine demand increases; pH affects HOCl/OCl- balance.
,9. A secondary clarifier has pin flocs in the effluent. Microscopic exam shows many small,
loose flocs. Likely cause:
a) Old sludge
b) Young sludge (low MCRT)
c) Toxic shock
d) High DO
Young sludge produces small, weak flocs that don’t settle well; increase MCRT.
10. The maximum allowable chlorine residual for a receiving stream in most NPDES permits
is typically:
a) 0 mg/L
b) *0.02 mg/L (total residual chlorine)*
c) 2.0 mg/L
d) 0.5 mg/L
*Aquatic toxicity limits total residual chlorine to <0.02 mg/L in most permits.*
11. A 48-inch force main had a pressure drop of 30 psi over 2 miles. The likely cause is:
a) Air release valve stuck open
b) Slime growth or tuberculation
c) Pump running too fast
d) Low static head
Biological growth and iron deposits increase friction loss, reducing pressure over
distance.
12. An operator adds sodium bisulfite before the effluent enters the receiving stream. This is
for:
a) pH adjustment
b) Dechlorination
c) BOD reduction
d) Odor control
Sodium bisulfite chemically reduces chlorine residual to protect aquatic life.
13. On an RBC (rotating biological contactor), the shafts fail to rotate during cold weather.
Most likely issue:
, a) Motor failure
b) Excess biomass freezing and overloading
c) Low influent BOD
d) High DO
Cold thickens biomass and ice buildup adds weight, stalling shafts.
14. A facultative lagoon has an olive-green color with light algal growth. This indicates:
a) Overloaded condition
b) Healthy, balanced lagoon
c) Anaerobic bottom releasing gas
d) Too much nitrogen
Olive-green indicates active algae producing oxygen, good stabilization.
15. What is the typical MLSS target range for conventional activated sludge?
a) 500-1000 mg/L
b) *2000-4000 mg/L*
c) 5000-7000 mg/L
d) 100-300 mg/L
*Conventional systems maintain MLSS 2000-4000 mg/L for stable BOD removal and
settling.*
16. A weir is overflowing at 20,000 gpd/ft. Plant design weir loading is 10,000 gpd/ft.
Observed effect:
a) Better clarification
b) Solids carryover and higher effluent TSS
c) Lower RAS concentration
d) Reduced sludge volume index
Excessive weir rate pulls solids over the weir before settling is complete.
17. A centrifugal pump operates with the discharge valve nearly closed. This can cause:
a) Cavitation
b) Overheating and damaging the pump
c) Higher efficiency
d) Increased NPSH
BOD per day?
a) 220 lbs
b) 4,586 lbs
c) 2,500 lbs
d) 1,200 lbs
BOD lbs = flow (MGD) × conc (mg/L) × 8.34 = 2.5 × 220 × 8.34 = 4,586 lbs.
2. During heavy rain, the clarifier weir overflow rate suddenly rises. What immediate action
protects effluent quality?
a) Increase RAS rate
b) Increase wasting to lower MLSS
c) Add polymer at the headworks
d) Bypass primary clarifiers
High weir loading causes solids carryover; wasting reduces sludge blanket and hydraulic
stress.
3. An operator notices rising white foam on the aeration basin. This most likely indicates:
a) Young sludge
b) Old sludge with Nocardia
c) Denitrification in the clarifier
d) Low DO
Nocardia and related filaments cause stable brownish-white foam at older sludge ages.
4. On a secondary clarifier, the sludge blanket depth is 5 ft and rising, effluent turbidity
increasing. What is the first step?
a) Increase RAS flow
b) Increase wasting rate
, c) Reduce aeration DO
d) Add coagulant
Rising blanket with turbidity suggests solids accumulation; wasting removes excess
biomass.
5. A lift station wet well cycles every 2 minutes. This is undesirable because it:
a) Increases energy cost
b) Can overheat the motor and shorten pump life
c) Causes more grit to enter
d) Increases BOD
Rapid cycling overheats motors, increases wear, and risks pump failure.
6. What is the typical detention time for a primary clarifier at design flow?
a) 30-60 minutes
b) 1.5-2.5 hours
c) 6-8 hours
d) 15 minutes
Primary clarifiers are designed for 1.5-2.5 hours to allow settleable solids removal.
7. An operator measures DO in an aeration basin at 0.5 mg/L. The mixed liquor color is
dark, and there is a septic odor. This condition is:
a) Normal for low F/M
b) Severe under-aeration
c) Denitrification only
d) High RAS return
*DO below 1.0 mg/L with dark color and odor indicates anaerobic conditions damaging
biomass.*
8. Which factor most directly affects chlorination efficiency for disinfection?
a) BOD concentration
b) Suspended solids concentration and pH
c) Flow velocity
d) RAS temperature
Solids shield pathogens and chlorine demand increases; pH affects HOCl/OCl- balance.
,9. A secondary clarifier has pin flocs in the effluent. Microscopic exam shows many small,
loose flocs. Likely cause:
a) Old sludge
b) Young sludge (low MCRT)
c) Toxic shock
d) High DO
Young sludge produces small, weak flocs that don’t settle well; increase MCRT.
10. The maximum allowable chlorine residual for a receiving stream in most NPDES permits
is typically:
a) 0 mg/L
b) *0.02 mg/L (total residual chlorine)*
c) 2.0 mg/L
d) 0.5 mg/L
*Aquatic toxicity limits total residual chlorine to <0.02 mg/L in most permits.*
11. A 48-inch force main had a pressure drop of 30 psi over 2 miles. The likely cause is:
a) Air release valve stuck open
b) Slime growth or tuberculation
c) Pump running too fast
d) Low static head
Biological growth and iron deposits increase friction loss, reducing pressure over
distance.
12. An operator adds sodium bisulfite before the effluent enters the receiving stream. This is
for:
a) pH adjustment
b) Dechlorination
c) BOD reduction
d) Odor control
Sodium bisulfite chemically reduces chlorine residual to protect aquatic life.
13. On an RBC (rotating biological contactor), the shafts fail to rotate during cold weather.
Most likely issue:
, a) Motor failure
b) Excess biomass freezing and overloading
c) Low influent BOD
d) High DO
Cold thickens biomass and ice buildup adds weight, stalling shafts.
14. A facultative lagoon has an olive-green color with light algal growth. This indicates:
a) Overloaded condition
b) Healthy, balanced lagoon
c) Anaerobic bottom releasing gas
d) Too much nitrogen
Olive-green indicates active algae producing oxygen, good stabilization.
15. What is the typical MLSS target range for conventional activated sludge?
a) 500-1000 mg/L
b) *2000-4000 mg/L*
c) 5000-7000 mg/L
d) 100-300 mg/L
*Conventional systems maintain MLSS 2000-4000 mg/L for stable BOD removal and
settling.*
16. A weir is overflowing at 20,000 gpd/ft. Plant design weir loading is 10,000 gpd/ft.
Observed effect:
a) Better clarification
b) Solids carryover and higher effluent TSS
c) Lower RAS concentration
d) Reduced sludge volume index
Excessive weir rate pulls solids over the weir before settling is complete.
17. A centrifugal pump operates with the discharge valve nearly closed. This can cause:
a) Cavitation
b) Overheating and damaging the pump
c) Higher efficiency
d) Increased NPSH
