, Question 1
A conventional activated sludge plant experiences a sudden decrease in
effluent ammonia removal despite adequate dissolved oxygen and unchanged
influent flow. The SRT is 4 days and mixed liquor temperature is 12°C. Which
is the most likely cause?
A. Nitrifying bacteria washout due to insufficient SRT
B. Filamentous bulking causing poor settling
C. Denitrification occurring in the aeration tank
D. Phosphorus precipitation inhibiting nitrifiers
Correct Answer: A - Nitrifying bacteria washout due to
insufficient SRT
RATIONALE
At 12°C, the minimum SRT for nitrification is approximately 6-8
days; an SRT of 4 days is below this threshold, causing washout of
slow-growing nitrifiers. Filamentous bulking affects settling, not
ammonia oxidation directly. Denitrification in the aeration tank would
not reduce ammonia removal, and phosphorus precipitation does not
inhibit nitrifiers.
Question 2
In an enhanced biological phosphorus removal (EBPR) system, which
condition is essential for selecting polyphosphate-accumulating organisms
(PAOs) over glycogen-accumulating organisms (GAOs)?
A. Alternating anaerobic and aerobic zones with volatile fatty acids
(VFAs) in the anaerobic zone
B. Continuous aerobic conditions with high dissolved oxygen
C. Addition of ferric chloride in the anaerobic zone
D. Maintaining a low SRT to favor fast growers
Correct Answer: A - Alternating anaerobic and aerobic zones
with volatile fatty acids (VFAs) in the anaerobic zone
Page 2
, RATIONALE
PAOs require alternating anaerobic/aerobic cycling and VFA uptake
under anaerobic conditions to gain a competitive advantage over
GAOs. Continuous aerobic conditions do not select for PAOs, ferric
chloride would chemically precipitate phosphorus, and low SRT
would wash out PAOs.
Question 3
A trickling filter plant achieves 85% BOD removal but experiences significant
odors and ponding. The most likely cause is:
A. Excessive hydraulic loading causing biofilm sloughing
B. Insufficient ventilation leading to anaerobic conditions
C. Low organic loading resulting in endogenous respiration
D. High recirculation ratio diluting the influent
Correct Answer: B - Insufficient ventilation leading to anaerobic
conditions
RATIONALE
Ponding and odors in trickling filters often result from poor
ventilation, which creates anaerobic zones and prevents uniform
biofilm growth. Excessive hydraulic loading causes sloughing but not
typically ponding. Low organic loading and high recirculation do not
directly cause ponding and odors.
Question 4
Which disinfectant produces the highest concentration of disinfection
byproducts (DBPs) such as trihalomethanes when applied to wastewater
effluent containing high organic matter?
A. Chlorine dioxide
B. Ozone
C. Free chlorine
D. Ultraviolet (UV) light
Page 3
, Correct Answer: C - Free chlorine
RATIONALE
Free chlorine reacts with natural organic matter to form
trihalomethanes and haloacetic acids, especially at high organic
concentrations. Chlorine dioxide produces fewer THMs but generates
chlorite. Ozone and UV do not produce THMs, though ozone can
form bromate.
Question 5
A sludge dewatering centrifuge produces cake with 22% solids, but the centrate
has high suspended solids and polymer consumption is excessive. Which
corrective action is most appropriate?
A. Increase polymer dose and reduce bowl speed
B. Decrease polymer dose and increase differential speed
C. Increase feed rate and reduce polymer dose
D. Optimize polymer dosage and adjust differential speed to improve
capture
Correct Answer: D - Optimize polymer dosage and adjust
differential speed to improve capture
RATIONALE
High centrate solids and excessive polymer indicate poor flocculation
and inadequate capture. Optimizing polymer dose and adjusting
differential speed can improve solids capture and reduce polymer
usage. Simply increasing polymer or reducing bowl speed may not
address the root cause.
Question 6
In a membrane bioreactor (MBR), a sudden increase in transmembrane
pressure (TMP) and decrease in permeability is observed. Which is the most
likely cause?
Page 4
A conventional activated sludge plant experiences a sudden decrease in
effluent ammonia removal despite adequate dissolved oxygen and unchanged
influent flow. The SRT is 4 days and mixed liquor temperature is 12°C. Which
is the most likely cause?
A. Nitrifying bacteria washout due to insufficient SRT
B. Filamentous bulking causing poor settling
C. Denitrification occurring in the aeration tank
D. Phosphorus precipitation inhibiting nitrifiers
Correct Answer: A - Nitrifying bacteria washout due to
insufficient SRT
RATIONALE
At 12°C, the minimum SRT for nitrification is approximately 6-8
days; an SRT of 4 days is below this threshold, causing washout of
slow-growing nitrifiers. Filamentous bulking affects settling, not
ammonia oxidation directly. Denitrification in the aeration tank would
not reduce ammonia removal, and phosphorus precipitation does not
inhibit nitrifiers.
Question 2
In an enhanced biological phosphorus removal (EBPR) system, which
condition is essential for selecting polyphosphate-accumulating organisms
(PAOs) over glycogen-accumulating organisms (GAOs)?
A. Alternating anaerobic and aerobic zones with volatile fatty acids
(VFAs) in the anaerobic zone
B. Continuous aerobic conditions with high dissolved oxygen
C. Addition of ferric chloride in the anaerobic zone
D. Maintaining a low SRT to favor fast growers
Correct Answer: A - Alternating anaerobic and aerobic zones
with volatile fatty acids (VFAs) in the anaerobic zone
Page 2
, RATIONALE
PAOs require alternating anaerobic/aerobic cycling and VFA uptake
under anaerobic conditions to gain a competitive advantage over
GAOs. Continuous aerobic conditions do not select for PAOs, ferric
chloride would chemically precipitate phosphorus, and low SRT
would wash out PAOs.
Question 3
A trickling filter plant achieves 85% BOD removal but experiences significant
odors and ponding. The most likely cause is:
A. Excessive hydraulic loading causing biofilm sloughing
B. Insufficient ventilation leading to anaerobic conditions
C. Low organic loading resulting in endogenous respiration
D. High recirculation ratio diluting the influent
Correct Answer: B - Insufficient ventilation leading to anaerobic
conditions
RATIONALE
Ponding and odors in trickling filters often result from poor
ventilation, which creates anaerobic zones and prevents uniform
biofilm growth. Excessive hydraulic loading causes sloughing but not
typically ponding. Low organic loading and high recirculation do not
directly cause ponding and odors.
Question 4
Which disinfectant produces the highest concentration of disinfection
byproducts (DBPs) such as trihalomethanes when applied to wastewater
effluent containing high organic matter?
A. Chlorine dioxide
B. Ozone
C. Free chlorine
D. Ultraviolet (UV) light
Page 3
, Correct Answer: C - Free chlorine
RATIONALE
Free chlorine reacts with natural organic matter to form
trihalomethanes and haloacetic acids, especially at high organic
concentrations. Chlorine dioxide produces fewer THMs but generates
chlorite. Ozone and UV do not produce THMs, though ozone can
form bromate.
Question 5
A sludge dewatering centrifuge produces cake with 22% solids, but the centrate
has high suspended solids and polymer consumption is excessive. Which
corrective action is most appropriate?
A. Increase polymer dose and reduce bowl speed
B. Decrease polymer dose and increase differential speed
C. Increase feed rate and reduce polymer dose
D. Optimize polymer dosage and adjust differential speed to improve
capture
Correct Answer: D - Optimize polymer dosage and adjust
differential speed to improve capture
RATIONALE
High centrate solids and excessive polymer indicate poor flocculation
and inadequate capture. Optimizing polymer dose and adjusting
differential speed can improve solids capture and reduce polymer
usage. Simply increasing polymer or reducing bowl speed may not
address the root cause.
Question 6
In a membrane bioreactor (MBR), a sudden increase in transmembrane
pressure (TMP) and decrease in permeability is observed. Which is the most
likely cause?
Page 4
