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Iowa Wastewater Operator Grade IV Exam ACTUAL
EXAM QUESTIONS AND VERIFIED ANSWERS WITH
RATIONALES JUST RELEASED
Summarized Exam Topics Covered (Actual Iowa Grade IV)
• Activated sludge process control (F/M, SRT, MLSS, RAS, WAS)
• Nutrient removal (biological nitrogen and phosphorus)
• Trickling filters and RBCs (hydraulic/organic loading, ponding, flies)
• Secondary clarification (solids loading, blanket depth, denitrification)
• Disinfection (chlorine, dechlorination, UV, contact time, CT)
• Sludge digestion (anaerobic – VA/alkalinity, gas production, temperature)
• Sludge thickening and dewatering (gravity belt, centrifuge, polymer)
• Lagoon systems (loading, algae, sludge accumulation)
• Advanced treatment (filtration, membranes, tertiary processes)
• Laboratory procedures (DO, pH, alkalinity, solids, microscopy)
• Process troubleshooting (filaments, bulking, foaming, rising sludge)
• Pumps, blowers, and conveyances (VFDs, cavitation, packing)
• Safety (confined space, H₂S, lockout/tagout, PPE)
• Iowa regulations (NPDES, permit limits, reporting)
• Math (detention time, loading rates, SVI, SRT, percent removal)
• Instrumentation and SCADA (probes, calibration, failure modes)
1. A complete-mix activated sludge plant operates at an SRT of 2 days. Effluent is turbid with many
single cells. Which action best raises SRT?
A) Increase RAS rate
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B) Decrease WAS rate
C) Increase aeration DO
D) Add polymer to final clarifier
Answer: B
Rationale: Lower WAS rate increases solids retention time (SRT), allowing biomass to mature.
2. You measure DO of 0.3 mg/L in the aeration basin. Mixed liquor is dark brown with a septic odor.
What is the immediate remedy?
A) Reduce RAS flow
B) Increase aeration blower output
C) Increase WAS to lower MLSS
D) Add nitrate to basin
Answer: B
Rationale: DO below 0.5 mg/L can cause filamentous growth and poor treatment; increase aeration.
3. A gravity belt thickener processes 20 gpm of waste activated sludge at 0.8% solids. Polymer dose is 8
lb/ton dry solids. What is daily polymer usage (lb/day)?
A) 12.8 lb/day
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B) 7.7 lb/day
C) 25.6 lb/day
D) 15.4 lb/day
Answer: B
Rationale: 20 gpm × 0.008 lb solids/gallon = 0.16 lb solids/min × 1440 min/day = 230.4 lb solids/day;
230.4÷2000=0.1152 ton × 8 lb/ton = 0.92 lb/day? Correction: 20 gpm = 28800 gpd × 8.34 lb/gal × 0.008 =
1921 lb solids/day ÷2000 = 0.96 ton ×8 = 7.68 lb/day → closest 7.7.
4. A final clarifier experiences solids washout after a power failure. The operator restarts RAS pumps
after 2 hours. What should be checked first?
A) Effluent weir level
B) Sludge blanket depth
C) DO in return line
D) Scraper torque
Answer: B
Rationale: After power loss, sludge settles and may consolidate; measure blanket before returning to
avoid overloading.
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5. A digester’s biogas CO₂ content rises from 30% to 40% while CH₄ drops from 70% to 60%. Volatile
acids are 1200 mg/L, alkalinity 2400 mg/L. What is the VA/alkalinity ratio and interpretation?
A) 0.5 – normal
B) 0.5 – slightly stressed
C) 0.5 – severe imbalance
D) 0.5 – recovering
Answer: B
Rationale: Ratio 0.5 indicates stress; 0.4 desired; >0.8 indicates souring.
6. An RBC shaft bearing runs hot and vibrates. The unit is partially submerged. What is the likely cause?
A) Low DO in tank
B) Excessive biomass thickness on media
C) High pH in wastewater
D) Low rotational speed
Answer: B
Rationale: Heavy biomass overloads the shaft, causing bearing stress and vibration.
Iowa Wastewater Operator Grade IV Exam ACTUAL
EXAM QUESTIONS AND VERIFIED ANSWERS WITH
RATIONALES JUST RELEASED
Summarized Exam Topics Covered (Actual Iowa Grade IV)
• Activated sludge process control (F/M, SRT, MLSS, RAS, WAS)
• Nutrient removal (biological nitrogen and phosphorus)
• Trickling filters and RBCs (hydraulic/organic loading, ponding, flies)
• Secondary clarification (solids loading, blanket depth, denitrification)
• Disinfection (chlorine, dechlorination, UV, contact time, CT)
• Sludge digestion (anaerobic – VA/alkalinity, gas production, temperature)
• Sludge thickening and dewatering (gravity belt, centrifuge, polymer)
• Lagoon systems (loading, algae, sludge accumulation)
• Advanced treatment (filtration, membranes, tertiary processes)
• Laboratory procedures (DO, pH, alkalinity, solids, microscopy)
• Process troubleshooting (filaments, bulking, foaming, rising sludge)
• Pumps, blowers, and conveyances (VFDs, cavitation, packing)
• Safety (confined space, H₂S, lockout/tagout, PPE)
• Iowa regulations (NPDES, permit limits, reporting)
• Math (detention time, loading rates, SVI, SRT, percent removal)
• Instrumentation and SCADA (probes, calibration, failure modes)
1. A complete-mix activated sludge plant operates at an SRT of 2 days. Effluent is turbid with many
single cells. Which action best raises SRT?
A) Increase RAS rate
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B) Decrease WAS rate
C) Increase aeration DO
D) Add polymer to final clarifier
Answer: B
Rationale: Lower WAS rate increases solids retention time (SRT), allowing biomass to mature.
2. You measure DO of 0.3 mg/L in the aeration basin. Mixed liquor is dark brown with a septic odor.
What is the immediate remedy?
A) Reduce RAS flow
B) Increase aeration blower output
C) Increase WAS to lower MLSS
D) Add nitrate to basin
Answer: B
Rationale: DO below 0.5 mg/L can cause filamentous growth and poor treatment; increase aeration.
3. A gravity belt thickener processes 20 gpm of waste activated sludge at 0.8% solids. Polymer dose is 8
lb/ton dry solids. What is daily polymer usage (lb/day)?
A) 12.8 lb/day
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B) 7.7 lb/day
C) 25.6 lb/day
D) 15.4 lb/day
Answer: B
Rationale: 20 gpm × 0.008 lb solids/gallon = 0.16 lb solids/min × 1440 min/day = 230.4 lb solids/day;
230.4÷2000=0.1152 ton × 8 lb/ton = 0.92 lb/day? Correction: 20 gpm = 28800 gpd × 8.34 lb/gal × 0.008 =
1921 lb solids/day ÷2000 = 0.96 ton ×8 = 7.68 lb/day → closest 7.7.
4. A final clarifier experiences solids washout after a power failure. The operator restarts RAS pumps
after 2 hours. What should be checked first?
A) Effluent weir level
B) Sludge blanket depth
C) DO in return line
D) Scraper torque
Answer: B
Rationale: After power loss, sludge settles and may consolidate; measure blanket before returning to
avoid overloading.
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5. A digester’s biogas CO₂ content rises from 30% to 40% while CH₄ drops from 70% to 60%. Volatile
acids are 1200 mg/L, alkalinity 2400 mg/L. What is the VA/alkalinity ratio and interpretation?
A) 0.5 – normal
B) 0.5 – slightly stressed
C) 0.5 – severe imbalance
D) 0.5 – recovering
Answer: B
Rationale: Ratio 0.5 indicates stress; 0.4 desired; >0.8 indicates souring.
6. An RBC shaft bearing runs hot and vibrates. The unit is partially submerged. What is the likely cause?
A) Low DO in tank
B) Excessive biomass thickness on media
C) High pH in wastewater
D) Low rotational speed
Answer: B
Rationale: Heavy biomass overloads the shaft, causing bearing stress and vibration.
