DELAWARE WASTEWATER OPERATOR CLASS II PRACTICE EXAM – QUESTIONS AND ANSWERS | VERIFIED AND WELL
DETAILED ANSWERS | PLUS RATIONALES | DOWNLOAD AND PASS | LATEST EXAM UPDATE 2026/2027
1. A Class II operator is performing a daily inspection of a 0.5 MGD activated sludge plant and notices a thin,
dark, gelatinous film on the rotating biological contactor (RBC) media. The dissolved oxygen in the RBC tank is
measured at 1.2 mg/L. What is the most probable cause of this condition?
A. Low organic loading
B. High sulfide concentration
C. High pH in the tank
D. Low rotational speed of the media
Correct Answer: B. High sulfide concentration
*Rationale: *The dark, gelatinous film described is characteristic of Beggiatoa or other sulfur-oxidizing bacteria,
which thrive in environments with elevated sulfide levels. This condition is often a result of septicity and low
dissolved oxygen (below 1.5 mg/L). While low D.O. contributes to the environment that allows sulfide to form, the
direct cause of this specific film is the sulfide itself. Low organic loading would result in a thin, pale film (option A),
and pH issues (option C) usually cause sloughing. Low rotational speed (option D) can cause it to become anaerobic
but the specific dark, gelatinous film is the key indicator of sulfide oxidation.
,2. To properly determine the sludge volume index (SVI) for a conventional activated sludge process, which two
laboratory measurements are essential?
A. 30-minute settleability and effluent total suspended solids
B. 30-minute settleability and mixed liquor suspended solids (MLSS)
C. 60-minute settleability and return activated sludge (RAS) concentration
D. 60-minute settleability and mixed liquor volatile suspended solids (MLVSS)
Correct Answer: B. 30-minute settleability and mixed liquor suspended solids (MLSS)
*Rationale: *The Sludge Volume Index (SVI) is a critical indicator of sludge settleability and is calculated using the
formula: SVI = (Settled Sludge Volume (mL/L) after 30 minutes) / (MLSS (mg/L)) * 1000. A 30-minute settleability test
provides the volume, and the MLSS test provides the concentration of solids in the aeration tank. The 60-minute test
is not used for SVI (options C and D). Effluent TSS and RAS concentration are not used in the SVI formula (options A
and C).
3. A centrifugal pump is showing signs of cavitation. Which of the following operational adjustments would be
the most effective and immediate step to correct the problem?
,A. Close the discharge valve slightly.
B. Open the discharge valve completely.
C. Increase the pump speed.
D. Throttle the suction valve.
Correct Answer: A. Close the discharge valve slightly.
*Rationale: *Cavitation occurs when the pressure at the pump suction falls below the vapor pressure of the liquid,
causing bubbles to form and collapse, damaging the impeller. Throttling (closing) the discharge valve increases the
system resistance and reduces the pump's flow rate, which in turn reduces the required Net Positive Suction Head
(NPSHr). This can raise the pressure at the eye of the impeller and help stop cavitation. Opening the discharge valve
(option B) would worsen cavitation by increasing flow and lowering suction pressure. Increasing pump speed (option
C) also worsens cavitation. Throttling the suction valve (option D) further decreases suction pressure and is a
dangerous practice that will significantly worsen cavitation.
4. In a biological nutrient removal (BNR) plant, what is the primary purpose of the anoxic zone in the activated
sludge process?
A. To promote the growth of filamentous organisms for improved floc formation.
B. To provide a zone for denitrification to occur, converting nitrate to nitrogen gas.
, C. To allow for the release of phosphorus from the biomass.
D. To oxidize ammonia to nitrate in the absence of carbon.
Correct Answer: B. To provide a zone for denitrification to occur, converting nitrate to nitrogen gas.
*Rationale: *The anoxic zone is a key component of BNR systems. Its primary function is to facilitate denitrification.
In this zone, dissolved oxygen is absent, but nitrate (NO3-N) is present as an electron acceptor. Facultative
heterotrophic bacteria use the nitrate to oxidize organic matter (BOD) and release nitrogen gas (N2) to the
atmosphere. Option A describes the selector zone, not the anoxic zone. Option C describes the anaerobic zone for
phosphorus release. Option D describes the aerobic zone for nitrification.
5. When using a hypochlorite solution for chlorination, which factor is most critical in calculating the required
feed rate to achieve a specific dosage?
A. The specific gravity of the hypochlorite solution.
B. The temperature of the wastewater being treated.
C. The concentration of available chlorine in the solution.
D. The pH of the hypochlorite solution.
Correct Answer: C. The concentration of available chlorine in the solution.
DETAILED ANSWERS | PLUS RATIONALES | DOWNLOAD AND PASS | LATEST EXAM UPDATE 2026/2027
1. A Class II operator is performing a daily inspection of a 0.5 MGD activated sludge plant and notices a thin,
dark, gelatinous film on the rotating biological contactor (RBC) media. The dissolved oxygen in the RBC tank is
measured at 1.2 mg/L. What is the most probable cause of this condition?
A. Low organic loading
B. High sulfide concentration
C. High pH in the tank
D. Low rotational speed of the media
Correct Answer: B. High sulfide concentration
*Rationale: *The dark, gelatinous film described is characteristic of Beggiatoa or other sulfur-oxidizing bacteria,
which thrive in environments with elevated sulfide levels. This condition is often a result of septicity and low
dissolved oxygen (below 1.5 mg/L). While low D.O. contributes to the environment that allows sulfide to form, the
direct cause of this specific film is the sulfide itself. Low organic loading would result in a thin, pale film (option A),
and pH issues (option C) usually cause sloughing. Low rotational speed (option D) can cause it to become anaerobic
but the specific dark, gelatinous film is the key indicator of sulfide oxidation.
,2. To properly determine the sludge volume index (SVI) for a conventional activated sludge process, which two
laboratory measurements are essential?
A. 30-minute settleability and effluent total suspended solids
B. 30-minute settleability and mixed liquor suspended solids (MLSS)
C. 60-minute settleability and return activated sludge (RAS) concentration
D. 60-minute settleability and mixed liquor volatile suspended solids (MLVSS)
Correct Answer: B. 30-minute settleability and mixed liquor suspended solids (MLSS)
*Rationale: *The Sludge Volume Index (SVI) is a critical indicator of sludge settleability and is calculated using the
formula: SVI = (Settled Sludge Volume (mL/L) after 30 minutes) / (MLSS (mg/L)) * 1000. A 30-minute settleability test
provides the volume, and the MLSS test provides the concentration of solids in the aeration tank. The 60-minute test
is not used for SVI (options C and D). Effluent TSS and RAS concentration are not used in the SVI formula (options A
and C).
3. A centrifugal pump is showing signs of cavitation. Which of the following operational adjustments would be
the most effective and immediate step to correct the problem?
,A. Close the discharge valve slightly.
B. Open the discharge valve completely.
C. Increase the pump speed.
D. Throttle the suction valve.
Correct Answer: A. Close the discharge valve slightly.
*Rationale: *Cavitation occurs when the pressure at the pump suction falls below the vapor pressure of the liquid,
causing bubbles to form and collapse, damaging the impeller. Throttling (closing) the discharge valve increases the
system resistance and reduces the pump's flow rate, which in turn reduces the required Net Positive Suction Head
(NPSHr). This can raise the pressure at the eye of the impeller and help stop cavitation. Opening the discharge valve
(option B) would worsen cavitation by increasing flow and lowering suction pressure. Increasing pump speed (option
C) also worsens cavitation. Throttling the suction valve (option D) further decreases suction pressure and is a
dangerous practice that will significantly worsen cavitation.
4. In a biological nutrient removal (BNR) plant, what is the primary purpose of the anoxic zone in the activated
sludge process?
A. To promote the growth of filamentous organisms for improved floc formation.
B. To provide a zone for denitrification to occur, converting nitrate to nitrogen gas.
, C. To allow for the release of phosphorus from the biomass.
D. To oxidize ammonia to nitrate in the absence of carbon.
Correct Answer: B. To provide a zone for denitrification to occur, converting nitrate to nitrogen gas.
*Rationale: *The anoxic zone is a key component of BNR systems. Its primary function is to facilitate denitrification.
In this zone, dissolved oxygen is absent, but nitrate (NO3-N) is present as an electron acceptor. Facultative
heterotrophic bacteria use the nitrate to oxidize organic matter (BOD) and release nitrogen gas (N2) to the
atmosphere. Option A describes the selector zone, not the anoxic zone. Option C describes the anaerobic zone for
phosphorus release. Option D describes the aerobic zone for nitrification.
5. When using a hypochlorite solution for chlorination, which factor is most critical in calculating the required
feed rate to achieve a specific dosage?
A. The specific gravity of the hypochlorite solution.
B. The temperature of the wastewater being treated.
C. The concentration of available chlorine in the solution.
D. The pH of the hypochlorite solution.
Correct Answer: C. The concentration of available chlorine in the solution.
