Page 1 of 89
Ohio Wastewater Operator Class A Exam Actual Test
Questions and Correct Answers With Rationales LATEST
THIS YEAR
Summarized Exam Coverage (Class A, Ohio EPA)
Advanced process control (activated sludge, trickling filters, RBCs, lagoons), complex troubleshooting,
biological nutrient removal (BNR), chemical addition (P, N, alkalinity), solids handling (thickening,
digestion, dewatering, incineration), advanced calculations (F/M, MCRT, SVI, O&M troubleshooting),
Ohio EPA rules (OAC 3745), safety (confined space, H2S, lockout/tagout), permit limits, effluent toxicity,
process optimization, lab procedures (DO, pH, TSS, BOD, NH3), pump hydraulics, blower efficiency,
emergency response, and plant management.
1. A Class A operator notices a sudden drop in secondary effluent pH from 7.2 to 5.8. The most likely
immediate cause is:
A) High nitrification in the final clarifier
B) Low alkalinity and nitrification in the aeration basin
C) Excess sodium hydroxide feed
D) Anaerobic digester supernatant return with low VFAs
Answer: B – Nitrification consumes alkalinity (7.14 lb alkalinity/lb NH3-N), crashing pH if alkalinity < 50
mg/L.
, Page 2 of 89
2. A plant using ferric chloride for phosphorus removal sees orange-red foam in the aeration tank. This
indicates:
A) Overdose of polymer
B) Excess iron reducing settleability
C) Ferric chloride reacting with hydrogen sulfide
D) Iron precipitation due to high DO
Answer: C – Iron reacts with H2S to form iron sulfide (black) but excess ferric can cause red/brown foam
from ferric hydroxide or oxidation byproducts.
3. An operator measures a 7-day SVI of 250 mL/g. Mixed liquor TSS is 3,500 mg/L. What should be done
first?
A) Increase wasting to lower MLSS
B) Add coagulant to improve settling
C) Increase RAS rate significantly
D) Check for filamentous bacteria under microscope
, Page 3 of 89
Answer: D – High SVI (>150) suggests bulking; microscopic exam identifies filaments (e.g., Type 1701)
before wasting changes.
4. A Class A plant with BNR experiences rising effluent ammonia (from 1 to 8 mg/L) while nitrate is
dropping. Most probable cause:
A) Toxic shock to nitrifiers
B) Too much alkalinity addition
C) Excessive RAS flow to anoxic zone
D) High DO in anaerobic selector
Answer: A – Loss of nitrification (NH3 up, NO3 down) indicates nitrifier inhibition (e.g., temperature
drop, pH shift, or toxin).
5. A belt filter press produces wet cake (22% solids) instead of normal 28% solids. The first thing to
check:
A) Polymer dose and mix energy
, Page 4 of 89
B) Belt speed (too slow)
C) Sludge feed solids too high
D) Wash water pressure
Answer: A – Polymer is critical; underdosing or poor mixing reduces capture and cake solids; belt speed
adjustment comes after polymer optimization.
6. An operator hears a high-pitched whine from a centrifugal blower. This suggests:
A) Oil level too high
B) Surge condition developing
C) Bearing failure imminent
D) VFD running too slow
Answer: C – High-frequency whine indicates bearing wear; immediate vibration analysis and oil sampling
needed before catastrophic failure.
Ohio Wastewater Operator Class A Exam Actual Test
Questions and Correct Answers With Rationales LATEST
THIS YEAR
Summarized Exam Coverage (Class A, Ohio EPA)
Advanced process control (activated sludge, trickling filters, RBCs, lagoons), complex troubleshooting,
biological nutrient removal (BNR), chemical addition (P, N, alkalinity), solids handling (thickening,
digestion, dewatering, incineration), advanced calculations (F/M, MCRT, SVI, O&M troubleshooting),
Ohio EPA rules (OAC 3745), safety (confined space, H2S, lockout/tagout), permit limits, effluent toxicity,
process optimization, lab procedures (DO, pH, TSS, BOD, NH3), pump hydraulics, blower efficiency,
emergency response, and plant management.
1. A Class A operator notices a sudden drop in secondary effluent pH from 7.2 to 5.8. The most likely
immediate cause is:
A) High nitrification in the final clarifier
B) Low alkalinity and nitrification in the aeration basin
C) Excess sodium hydroxide feed
D) Anaerobic digester supernatant return with low VFAs
Answer: B – Nitrification consumes alkalinity (7.14 lb alkalinity/lb NH3-N), crashing pH if alkalinity < 50
mg/L.
, Page 2 of 89
2. A plant using ferric chloride for phosphorus removal sees orange-red foam in the aeration tank. This
indicates:
A) Overdose of polymer
B) Excess iron reducing settleability
C) Ferric chloride reacting with hydrogen sulfide
D) Iron precipitation due to high DO
Answer: C – Iron reacts with H2S to form iron sulfide (black) but excess ferric can cause red/brown foam
from ferric hydroxide or oxidation byproducts.
3. An operator measures a 7-day SVI of 250 mL/g. Mixed liquor TSS is 3,500 mg/L. What should be done
first?
A) Increase wasting to lower MLSS
B) Add coagulant to improve settling
C) Increase RAS rate significantly
D) Check for filamentous bacteria under microscope
, Page 3 of 89
Answer: D – High SVI (>150) suggests bulking; microscopic exam identifies filaments (e.g., Type 1701)
before wasting changes.
4. A Class A plant with BNR experiences rising effluent ammonia (from 1 to 8 mg/L) while nitrate is
dropping. Most probable cause:
A) Toxic shock to nitrifiers
B) Too much alkalinity addition
C) Excessive RAS flow to anoxic zone
D) High DO in anaerobic selector
Answer: A – Loss of nitrification (NH3 up, NO3 down) indicates nitrifier inhibition (e.g., temperature
drop, pH shift, or toxin).
5. A belt filter press produces wet cake (22% solids) instead of normal 28% solids. The first thing to
check:
A) Polymer dose and mix energy
, Page 4 of 89
B) Belt speed (too slow)
C) Sludge feed solids too high
D) Wash water pressure
Answer: A – Polymer is critical; underdosing or poor mixing reduces capture and cake solids; belt speed
adjustment comes after polymer optimization.
6. An operator hears a high-pitched whine from a centrifugal blower. This suggests:
A) Oil level too high
B) Surge condition developing
C) Bearing failure imminent
D) VFD running too slow
Answer: C – High-frequency whine indicates bearing wear; immediate vibration analysis and oil sampling
needed before catastrophic failure.
