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Colorado Wastewater Treatment Operator Class D Exam
ACTUAL EXAM COMPLETE QUESTIONS AND CORRECT
ANSWERS LATEST UPDATE THIS YEAR
Colorado Class D Wastewater Treatment Operator exam
SUMMARIZED EXAM TOPIC COVERAGE (Point Form)
• Basic math for loading, flow, detention time, weir overflow rate, surface overflow rate, and
sludge age
• Safety: confined space, lockout/tagout, H2S, PPE, oxygen deficiency, chlorine safety
• Collection systems: lift stations, cleaning, inspection, wet wells, odor control
• Preliminary treatment: screens (bar, fine), grit removal, comminutors, flow equalization
• Primary treatment: sedimentation tanks, scum removal, sludge collection, BOD/ TSS removal
efficiency
• Secondary treatment (activated sludge, fixed film): aeration, MLSS, RAS, WAS, F/M ratio,
settling, DO control
• Disinfection: chlorine (gas, hypochlorite), dechlorination, contact time, UV, safety
• Sludge handling: thickening, digestion (aerobic/anaerobic), belt press, drying beds, polymer
addition
• Ponds/lagoons: operation, algae, short-circuiting, effluent quality
• Recordkeeping, reporting, permits (CDPHE), effluent limits, compliance, sampling procedures
• Basic equipment: pumps (centrifugal, positive displacement), valves, meters, maintenance
• Laboratory: DO, pH, chlorine residual, settleability (SV30), microscopic exam basics
1. A Class D operator notices a sudden drop in dissolved oxygen in the aeration basin from 2.5 mg/L to
0.8 mg/L within two hours. What is the most likely immediate cause?
A) Increase in return activated sludge rate
B) Slug load of high-BOD wastewater
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C) Normal diurnal variation
D) Air temperature decrease
Answer: B – High-BOD slug increases microbial oxygen demand, quickly depleting DO.
2. During a routine morning check, the wet well level indicator shows a rising level but the pump running
light is off. What should the operator do first?
A) Call a supervisor
B) Check the pump circuit breaker and control panel
C) Add chlorine to the wet well
D) Increase the weir setting
Answer: B – Verify power and control settings before assuming mechanical failure.
3. A bar screen is manually cleaned only once per shift, but debris is accumulating between cleanings.
What is the best solution?
A) Reduce plant flow
B) Increase cleaning frequency or install mechanical rakes
C) Bypass the screen during peak flow
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D) Add coagulant upstream
Answer: B – More frequent cleaning prevents plugging and protects downstream equipment.
4. A treatment pond has heavy algae growth on the surface, and effluent TSS is rising. What is the most
likely cause?
A) Too much wind mixing
B) Short hydraulic detention time due to high flow
C) Low pH from industrial discharge
D) Overdosing chlorine
Answer: B – Short detention prevents algae settling, carrying solids out in the effluent.
5. An operator measures a chlorine residual of 1.5 mg/L after 20 minutes of contact time. State permit
requires 1.0 mg/L minimum after 15 minutes. What action is required?
A) Reduce chlorine dose immediately
B) Maintain current dose – the requirement is met
C) Bypass the contact tank
D) Dechlorinate before discharge
Answer: B – Residual and contact time exceed permit minimum; no change needed.
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6. A small plant uses a belt filter press for dewatering. The cake is too wet (85% moisture). What is the
first operational adjustment?
A) Increase polymer dose
B) Reduce belt speed
C) Increase hydraulic feed rate
D) Turn off the wash water
Answer: B – Slower belt allows more drainage time, producing drier cake.
7. While entering a chlorine feed room, your H2S monitor reads 10 ppm. What should you do?
A) Put on a dust mask and investigate
B) Immediately exit and ventilate the room
C) Increase chlorine feed to bind H2S
D) Call the lab to verify
Answer: B – 10 ppm H2S exceeds safe limits; immediate evacuation and ventilation required.
8. A flow meter shows 0.8 MGD entering the plant, but the primary clarifier effluent weir appears
overloaded with solids. What is the most probable problem?
A) Flow meter is reading low – actual flow is higher
Colorado Wastewater Treatment Operator Class D Exam
ACTUAL EXAM COMPLETE QUESTIONS AND CORRECT
ANSWERS LATEST UPDATE THIS YEAR
Colorado Class D Wastewater Treatment Operator exam
SUMMARIZED EXAM TOPIC COVERAGE (Point Form)
• Basic math for loading, flow, detention time, weir overflow rate, surface overflow rate, and
sludge age
• Safety: confined space, lockout/tagout, H2S, PPE, oxygen deficiency, chlorine safety
• Collection systems: lift stations, cleaning, inspection, wet wells, odor control
• Preliminary treatment: screens (bar, fine), grit removal, comminutors, flow equalization
• Primary treatment: sedimentation tanks, scum removal, sludge collection, BOD/ TSS removal
efficiency
• Secondary treatment (activated sludge, fixed film): aeration, MLSS, RAS, WAS, F/M ratio,
settling, DO control
• Disinfection: chlorine (gas, hypochlorite), dechlorination, contact time, UV, safety
• Sludge handling: thickening, digestion (aerobic/anaerobic), belt press, drying beds, polymer
addition
• Ponds/lagoons: operation, algae, short-circuiting, effluent quality
• Recordkeeping, reporting, permits (CDPHE), effluent limits, compliance, sampling procedures
• Basic equipment: pumps (centrifugal, positive displacement), valves, meters, maintenance
• Laboratory: DO, pH, chlorine residual, settleability (SV30), microscopic exam basics
1. A Class D operator notices a sudden drop in dissolved oxygen in the aeration basin from 2.5 mg/L to
0.8 mg/L within two hours. What is the most likely immediate cause?
A) Increase in return activated sludge rate
B) Slug load of high-BOD wastewater
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C) Normal diurnal variation
D) Air temperature decrease
Answer: B – High-BOD slug increases microbial oxygen demand, quickly depleting DO.
2. During a routine morning check, the wet well level indicator shows a rising level but the pump running
light is off. What should the operator do first?
A) Call a supervisor
B) Check the pump circuit breaker and control panel
C) Add chlorine to the wet well
D) Increase the weir setting
Answer: B – Verify power and control settings before assuming mechanical failure.
3. A bar screen is manually cleaned only once per shift, but debris is accumulating between cleanings.
What is the best solution?
A) Reduce plant flow
B) Increase cleaning frequency or install mechanical rakes
C) Bypass the screen during peak flow
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D) Add coagulant upstream
Answer: B – More frequent cleaning prevents plugging and protects downstream equipment.
4. A treatment pond has heavy algae growth on the surface, and effluent TSS is rising. What is the most
likely cause?
A) Too much wind mixing
B) Short hydraulic detention time due to high flow
C) Low pH from industrial discharge
D) Overdosing chlorine
Answer: B – Short detention prevents algae settling, carrying solids out in the effluent.
5. An operator measures a chlorine residual of 1.5 mg/L after 20 minutes of contact time. State permit
requires 1.0 mg/L minimum after 15 minutes. What action is required?
A) Reduce chlorine dose immediately
B) Maintain current dose – the requirement is met
C) Bypass the contact tank
D) Dechlorinate before discharge
Answer: B – Residual and contact time exceed permit minimum; no change needed.
, Page 4 of 111
6. A small plant uses a belt filter press for dewatering. The cake is too wet (85% moisture). What is the
first operational adjustment?
A) Increase polymer dose
B) Reduce belt speed
C) Increase hydraulic feed rate
D) Turn off the wash water
Answer: B – Slower belt allows more drainage time, producing drier cake.
7. While entering a chlorine feed room, your H2S monitor reads 10 ppm. What should you do?
A) Put on a dust mask and investigate
B) Immediately exit and ventilate the room
C) Increase chlorine feed to bind H2S
D) Call the lab to verify
Answer: B – 10 ppm H2S exceeds safe limits; immediate evacuation and ventilation required.
8. A flow meter shows 0.8 MGD entering the plant, but the primary clarifier effluent weir appears
overloaded with solids. What is the most probable problem?
A) Flow meter is reading low – actual flow is higher
