Arizona Water Treatment Operator
Certification Examination Questions
And Correct Answers (Verified Answers)
Plus Rationales 2026 Q&A | Instant
Download Pdf
1. Which of the following is the primary cause of carbonate hardness in
surface water supplies in the arid Southwest?
A. Dissolved magnesium sulfate from evaporite deposits
B. Calcium bicarbonate leached from limestone and caliche soils
C. Sodium chloride from agricultural return flows
D. Silica from volcanic ash layers
Answer: B
Rationale: Carbonate hardness is primarily caused by calcium and
magnesium bicarbonates. In Arizona, limestone (calcium carbonate) and
caliche soils dissolve in water containing carbon dioxide, forming soluble
calcium bicarbonate, which is the dominant source of temporary
hardness.
2. A treatment plant uses a 3-log inactivation credit for chlorine disinfection.
What percentage of Giardia lamblia cysts must be inactivated to achieve
this credit?
A. 90%
B. 99%
C. 99.9%
D. 99.99%
Answer: C
Rationale: A 3-log inactivation means a reduction from 10,000 organisms
, to 10, or a 99.9% reduction (as 1,000 - ,000 = 0.999). For Giardia, the
Surface Water Treatment Rule often requires 3-log inactivation.
3. A flocculator with a velocity gradient of 45 s⁻¹ operates at a water
temperature of 15°C. If the viscosity decreases by 20%, how must the
power input change to maintain the same G value, assuming volume and
water density are constant?
A. Increase by 20%
B. Decrease by 20%
C. Increase by 44%
D. Decrease by 44%
Answer: B
Rationale: The velocity gradient G = sqrt(P / (μ V)). If μ decreases by 20%,
to maintain the same G, P must also decrease by 20% since P is directly
proportional to μ when V and G are constant.
4. Which type of impeller is most appropriate for a mechanical flocculator
designed to promote gentle mixing for large floc formation without
shearing?
A. Turbine impeller with flat blades
B. Paddle impeller with large surface area
C. Propeller impeller with high rotational speed
D. Radial-flow impeller
Answer: B
Rationale: Paddle impellers with a large surface area operate at slow
rotational speeds, creating gentle fluid motion ideal for flocculation. High-
speed impellers like turbines or propellers generate high shear, which
would break up delicate flocs.
5. What is the settling velocity of a particle with a diameter of 0.08 mm,
specific gravity of 2.65, in water at 20°C (kinematic viscosity = 1.0 x 10⁻⁶
m²/s) using Stokes' Law?
A. 0.0047 m/s
B. 0.0057 m/s
, C. 0.047 m/s
D. 0.057 m/s
Answer: B
Rationale: Stokes' Law: v = (g (Sg - 1) d²) / (18ν). v = (9.81 * (2.65-1) *
(0.00008)²) / (18 * 1e-6). This calculates to (9.811.656.4e-9) / 1.8e-5 =
(1.035e-7) / 1.8e-5 = 0.00575 m/s.
6. In a conventional water treatment plant, where is the point of application
for powdered activated carbon (PAC) typically located for the most effective
removal of taste-and-odor compounds?
A. After filtration, just before clearwell
B. In the rapid mix basin
C. In the sedimentation basin effluent channel
D. In the sludge thickener
Answer: B
Rationale: PAC is most effectively applied in the rapid mix basin to provide
high turbulence and contact time with raw water. This allows the carbon
to adsorb compounds before they are removed with sludge in
sedimentation and filtration.
7. The Arizona Department of Environmental Quality (ADEQ) requires
monitoring for disinfection byproducts (DBPs). Which compound is a
regulated DBP and a surrogate for the total organic halide (TOX) formation
potential?
A. Chlorite
B. Bromate
C. Trihalomethanes (total)
D. Chloramines
Answer: C
Rationale: Total Trihalomethanes (TTHMs) are a primary regulated DBP.
They are measured as a class and are a common surrogate for the overall
formation of halogenated organic DBPs, representing a significant
fraction of TOX.
, 8. What is the primary function of a clearwell in a water treatment plant?
A. To provide storage for filter backwash water
B. To allow for contact time for disinfection and storage for treated water
C. To equalize the flow entering the filters
D. To thicken the sludge from sedimentation basins
Answer: B
Rationale: The clearwell provides two critical functions: it provides the
required detention time for disinfectants like chlorine to achieve pathogen
inactivation (CT), and it stores finished water to meet demand
fluctuations.
9. A water sample has a pH of 7.8, alkalinity of 120 mg/L as CaCO₃, and
temperature of 22°C. If the Langelier Saturation Index (LSI) is calculated to
be +0.3, what is the likely condition of the water?
A. Aggressive and corrosive
B. Slightly supersaturated with calcium carbonate and scale-forming
C. Exactly saturated with calcium carbonate
D. Highly corrosive due to low alkalinity
Answer: B
Rationale: An LSI between 0 and +0.5 indicates water is slightly
supersaturated with calcium carbonate. This means it has a tendency to
deposit a protective scale but is not heavily scale-forming. A positive LSI
suggests non-corrosive conditions.
10.Which laboratory test is specifically used to determine the concentration of
suspended solids that will settle out of a water sample over a specified
period, commonly 60 minutes?
A. Total Suspended Solids (TSS)
B. Volatile Suspended Solids (VSS)
C. Settleable Solids
D. Turbidity
Answer: C
Rationale: The settleable solids test, performed using an Imhoff cone,
measures the volume of solids that settle from a one-liter sample in 60
Certification Examination Questions
And Correct Answers (Verified Answers)
Plus Rationales 2026 Q&A | Instant
Download Pdf
1. Which of the following is the primary cause of carbonate hardness in
surface water supplies in the arid Southwest?
A. Dissolved magnesium sulfate from evaporite deposits
B. Calcium bicarbonate leached from limestone and caliche soils
C. Sodium chloride from agricultural return flows
D. Silica from volcanic ash layers
Answer: B
Rationale: Carbonate hardness is primarily caused by calcium and
magnesium bicarbonates. In Arizona, limestone (calcium carbonate) and
caliche soils dissolve in water containing carbon dioxide, forming soluble
calcium bicarbonate, which is the dominant source of temporary
hardness.
2. A treatment plant uses a 3-log inactivation credit for chlorine disinfection.
What percentage of Giardia lamblia cysts must be inactivated to achieve
this credit?
A. 90%
B. 99%
C. 99.9%
D. 99.99%
Answer: C
Rationale: A 3-log inactivation means a reduction from 10,000 organisms
, to 10, or a 99.9% reduction (as 1,000 - ,000 = 0.999). For Giardia, the
Surface Water Treatment Rule often requires 3-log inactivation.
3. A flocculator with a velocity gradient of 45 s⁻¹ operates at a water
temperature of 15°C. If the viscosity decreases by 20%, how must the
power input change to maintain the same G value, assuming volume and
water density are constant?
A. Increase by 20%
B. Decrease by 20%
C. Increase by 44%
D. Decrease by 44%
Answer: B
Rationale: The velocity gradient G = sqrt(P / (μ V)). If μ decreases by 20%,
to maintain the same G, P must also decrease by 20% since P is directly
proportional to μ when V and G are constant.
4. Which type of impeller is most appropriate for a mechanical flocculator
designed to promote gentle mixing for large floc formation without
shearing?
A. Turbine impeller with flat blades
B. Paddle impeller with large surface area
C. Propeller impeller with high rotational speed
D. Radial-flow impeller
Answer: B
Rationale: Paddle impellers with a large surface area operate at slow
rotational speeds, creating gentle fluid motion ideal for flocculation. High-
speed impellers like turbines or propellers generate high shear, which
would break up delicate flocs.
5. What is the settling velocity of a particle with a diameter of 0.08 mm,
specific gravity of 2.65, in water at 20°C (kinematic viscosity = 1.0 x 10⁻⁶
m²/s) using Stokes' Law?
A. 0.0047 m/s
B. 0.0057 m/s
, C. 0.047 m/s
D. 0.057 m/s
Answer: B
Rationale: Stokes' Law: v = (g (Sg - 1) d²) / (18ν). v = (9.81 * (2.65-1) *
(0.00008)²) / (18 * 1e-6). This calculates to (9.811.656.4e-9) / 1.8e-5 =
(1.035e-7) / 1.8e-5 = 0.00575 m/s.
6. In a conventional water treatment plant, where is the point of application
for powdered activated carbon (PAC) typically located for the most effective
removal of taste-and-odor compounds?
A. After filtration, just before clearwell
B. In the rapid mix basin
C. In the sedimentation basin effluent channel
D. In the sludge thickener
Answer: B
Rationale: PAC is most effectively applied in the rapid mix basin to provide
high turbulence and contact time with raw water. This allows the carbon
to adsorb compounds before they are removed with sludge in
sedimentation and filtration.
7. The Arizona Department of Environmental Quality (ADEQ) requires
monitoring for disinfection byproducts (DBPs). Which compound is a
regulated DBP and a surrogate for the total organic halide (TOX) formation
potential?
A. Chlorite
B. Bromate
C. Trihalomethanes (total)
D. Chloramines
Answer: C
Rationale: Total Trihalomethanes (TTHMs) are a primary regulated DBP.
They are measured as a class and are a common surrogate for the overall
formation of halogenated organic DBPs, representing a significant
fraction of TOX.
, 8. What is the primary function of a clearwell in a water treatment plant?
A. To provide storage for filter backwash water
B. To allow for contact time for disinfection and storage for treated water
C. To equalize the flow entering the filters
D. To thicken the sludge from sedimentation basins
Answer: B
Rationale: The clearwell provides two critical functions: it provides the
required detention time for disinfectants like chlorine to achieve pathogen
inactivation (CT), and it stores finished water to meet demand
fluctuations.
9. A water sample has a pH of 7.8, alkalinity of 120 mg/L as CaCO₃, and
temperature of 22°C. If the Langelier Saturation Index (LSI) is calculated to
be +0.3, what is the likely condition of the water?
A. Aggressive and corrosive
B. Slightly supersaturated with calcium carbonate and scale-forming
C. Exactly saturated with calcium carbonate
D. Highly corrosive due to low alkalinity
Answer: B
Rationale: An LSI between 0 and +0.5 indicates water is slightly
supersaturated with calcium carbonate. This means it has a tendency to
deposit a protective scale but is not heavily scale-forming. A positive LSI
suggests non-corrosive conditions.
10.Which laboratory test is specifically used to determine the concentration of
suspended solids that will settle out of a water sample over a specified
period, commonly 60 minutes?
A. Total Suspended Solids (TSS)
B. Volatile Suspended Solids (VSS)
C. Settleable Solids
D. Turbidity
Answer: C
Rationale: The settleable solids test, performed using an Imhoff cone,
measures the volume of solids that settle from a one-liter sample in 60
