CWEA ADVANCED WATER TREATMENT | VERIFIED EXAM
QUESTIONS AND ANSWERS - LATEST VERSION 2026/2027
CWEA Advanced Water Treatment
1. Q: What is the primary function of the activated sludge process?
ANSWER To remove dissolved and suspended organic matter
(BOD/COD) and nutrients from wastewater using biological floc.
2. Q: What is the "Food to Microorganism" (F/M) ratio? ANSWER The
pounds of BOD applied per day per pound of Mixed Liquor Volatile
Suspended Solids (MLVSS) under aeration.
3. Q: What is a typical F/M ratio for a conventional activated sludge
plant? ANSWER 0.2 to 0.5 lbs BOD/lb MLVSS/day.
4. Q: What is Sludge Volume Index (SVI)? ANSWER A measure of the
settling characteristics of the sludge, defined as the volume (in mL)
occupied by 1 gram of sludge after 30 minutes of settling.
5. Q: What is considered a "good" SVI? ANSWER Between 50 and 100
mL/g.
6. Q: What does an SVI > 150 indicate? ANSWER Sludge bulking (poor
settling).
7. Q: What is Solids Retention Time (SRT) also known as? ANSWER
Mean Cell Residence Time (MCRT).
8. Q: What is the difference between SRT and Sludge Age? ANSWER
They are conceptually similar; Sludge Age refers only to the solids
in the reactor, while SRT often includes solids in the secondary
clarifier and recycle lines.
,9. Q: What is the typical Mixed Liquor Suspended Solids (MLSS)
concentration for extended aeration? ANSWER 3,000 to 5,000
mg/L.
10. Q: What is "Return Activated Sludge" (RAS)? ANSWER Sludge
settled in the secondary clarifier that is returned to the aeration
basin to maintain the biomass population.
11. Q: What is "Waste Activated Sludge" (WAS)? ANSWER The
excess sludge removed from the system to control the SRT and
solids inventory.
12. Q: What is filamentous bulking? ANSWER A condition where
filamentous bacteria overgrow, extending floc structure and
preventing settling.
13. Q: What are three common causes of filamentous bulking?
ANSWER Low DO, Low F/M ratio, and septic influent (high sulfides).
14. Q: What type of filament is associated with low DO
conditions? ANSWER Sphaerotilus natans or Type 1701.
15. Q: What is "pin floc"? ANSWER Very small, weak floc particles
that do not settle well, often caused by over-aeration or starvation
(old sludge).
16. Q: What is viscous bulking? ANSWER Bulking caused by
excessive amounts of polysaccharides (biological slime), often
linked to high F/M or septicity.
17. Q: What is the purpose of a selector? ANSWER To favor the
growth of floc-forming bacteria over filamentous bacteria by
creating a high F/M environment before the main basin.
18. Q: What is the difference between an anaerobic selector and
an anoxic selector? ANSWER Anaerobic has no nitrate or oxygen
(used for Phosphorus removal); Anoxic has nitrate but no oxygen
(used for Denitrification).
19. Q: What is "rising sludge"? ANSWER Sludge that floats to the
surface after settling, usually caused by denitrification in the
clarifier (nitrogen gas bubbles lifting the sludge).
, 20. Q: How do you distinguish between rising sludge and bulking?
ANSWER Bulking stays at the surface; rising sludge usually comes
up in clumps after the sludge has settled.
21. Q: What is the Modified Ludzack-Ettinger (MLE) process?
ANSWER A pre-anoxic configuration where internal mixed liquor
recirculation (RAS) brings nitrate to an anoxic zone for
denitrification.
22. Q: What is the Bardenpho process? ANSWER A 4- or 5-stage
process designed for high levels of nitrogen and phosphorus
removal.
23. Q: What is the VIP process? ANSWER Virginia Initiative Plant;
a high-rate activated sludge process using a basin-in-series
configuration and anaerobic/anoxic zones.
24. Q: What is the function of a "step feed" activated sludge
system? ANSWER Distributing influent flow along the length of the
aeration tank to balance organic loading and oxygen demand.
25. Q: What is the "Oxic/Anoxic" process? ANSWER A
modification where aeration is cycled on and off in the same tank to
achieve nitrification and denitrification (simultaneous nitrification-
denitrification).
26. Q: What is contact stabilization? ANSWER A process where
biomass absorbs organics in a small contact tank, then settles, and
goes to a separate re-aeration tank to stabilize.
27. Q: What is the meaning of "Endogenous Respiration"?
ANSWER The phase where microorganisms consume their own cell
tissue because external food sources are depleted.
28. Q: What is the "zoogleal film"? ANSWER The slime layer of
bacteria in trickling filters.
29. Q: What is the primary difference between Rotating Biological
Contactors (RBC) and trickling filters? ANSWER RBC media rotates,
alternately exposing the biomass to wastewater and air.
30. Q: What is Dissolved Oxygen (DO) rebound? ANSWER A
sudden increase in DO concentration usually indicating that the food
source has been exhausted.
QUESTIONS AND ANSWERS - LATEST VERSION 2026/2027
CWEA Advanced Water Treatment
1. Q: What is the primary function of the activated sludge process?
ANSWER To remove dissolved and suspended organic matter
(BOD/COD) and nutrients from wastewater using biological floc.
2. Q: What is the "Food to Microorganism" (F/M) ratio? ANSWER The
pounds of BOD applied per day per pound of Mixed Liquor Volatile
Suspended Solids (MLVSS) under aeration.
3. Q: What is a typical F/M ratio for a conventional activated sludge
plant? ANSWER 0.2 to 0.5 lbs BOD/lb MLVSS/day.
4. Q: What is Sludge Volume Index (SVI)? ANSWER A measure of the
settling characteristics of the sludge, defined as the volume (in mL)
occupied by 1 gram of sludge after 30 minutes of settling.
5. Q: What is considered a "good" SVI? ANSWER Between 50 and 100
mL/g.
6. Q: What does an SVI > 150 indicate? ANSWER Sludge bulking (poor
settling).
7. Q: What is Solids Retention Time (SRT) also known as? ANSWER
Mean Cell Residence Time (MCRT).
8. Q: What is the difference between SRT and Sludge Age? ANSWER
They are conceptually similar; Sludge Age refers only to the solids
in the reactor, while SRT often includes solids in the secondary
clarifier and recycle lines.
,9. Q: What is the typical Mixed Liquor Suspended Solids (MLSS)
concentration for extended aeration? ANSWER 3,000 to 5,000
mg/L.
10. Q: What is "Return Activated Sludge" (RAS)? ANSWER Sludge
settled in the secondary clarifier that is returned to the aeration
basin to maintain the biomass population.
11. Q: What is "Waste Activated Sludge" (WAS)? ANSWER The
excess sludge removed from the system to control the SRT and
solids inventory.
12. Q: What is filamentous bulking? ANSWER A condition where
filamentous bacteria overgrow, extending floc structure and
preventing settling.
13. Q: What are three common causes of filamentous bulking?
ANSWER Low DO, Low F/M ratio, and septic influent (high sulfides).
14. Q: What type of filament is associated with low DO
conditions? ANSWER Sphaerotilus natans or Type 1701.
15. Q: What is "pin floc"? ANSWER Very small, weak floc particles
that do not settle well, often caused by over-aeration or starvation
(old sludge).
16. Q: What is viscous bulking? ANSWER Bulking caused by
excessive amounts of polysaccharides (biological slime), often
linked to high F/M or septicity.
17. Q: What is the purpose of a selector? ANSWER To favor the
growth of floc-forming bacteria over filamentous bacteria by
creating a high F/M environment before the main basin.
18. Q: What is the difference between an anaerobic selector and
an anoxic selector? ANSWER Anaerobic has no nitrate or oxygen
(used for Phosphorus removal); Anoxic has nitrate but no oxygen
(used for Denitrification).
19. Q: What is "rising sludge"? ANSWER Sludge that floats to the
surface after settling, usually caused by denitrification in the
clarifier (nitrogen gas bubbles lifting the sludge).
, 20. Q: How do you distinguish between rising sludge and bulking?
ANSWER Bulking stays at the surface; rising sludge usually comes
up in clumps after the sludge has settled.
21. Q: What is the Modified Ludzack-Ettinger (MLE) process?
ANSWER A pre-anoxic configuration where internal mixed liquor
recirculation (RAS) brings nitrate to an anoxic zone for
denitrification.
22. Q: What is the Bardenpho process? ANSWER A 4- or 5-stage
process designed for high levels of nitrogen and phosphorus
removal.
23. Q: What is the VIP process? ANSWER Virginia Initiative Plant;
a high-rate activated sludge process using a basin-in-series
configuration and anaerobic/anoxic zones.
24. Q: What is the function of a "step feed" activated sludge
system? ANSWER Distributing influent flow along the length of the
aeration tank to balance organic loading and oxygen demand.
25. Q: What is the "Oxic/Anoxic" process? ANSWER A
modification where aeration is cycled on and off in the same tank to
achieve nitrification and denitrification (simultaneous nitrification-
denitrification).
26. Q: What is contact stabilization? ANSWER A process where
biomass absorbs organics in a small contact tank, then settles, and
goes to a separate re-aeration tank to stabilize.
27. Q: What is the meaning of "Endogenous Respiration"?
ANSWER The phase where microorganisms consume their own cell
tissue because external food sources are depleted.
28. Q: What is the "zoogleal film"? ANSWER The slime layer of
bacteria in trickling filters.
29. Q: What is the primary difference between Rotating Biological
Contactors (RBC) and trickling filters? ANSWER RBC media rotates,
alternately exposing the biomass to wastewater and air.
30. Q: What is Dissolved Oxygen (DO) rebound? ANSWER A
sudden increase in DO concentration usually indicating that the food
source has been exhausted.
