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Iowa Wastewater Operator Grade III Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf
- Which of the following is the primary purpose of preliminary treatment
in a wastewater plant?
a) Remove nutrients
b) Remove large solids and debris
c) Reduce BOD
d) Disinfect effluent
Answer: b) Remove large solids and debris
Rationale: Preliminary treatment removes large solids, rags, and debris
that could damage downstream equipment. It is not intended for chemical treatment or BOD removal. 1 / 4
- What is the function of an aeration tank in activated sludge processes?
a) Remove grit
b) Provide oxygen for microbial growth
c) Settle solids
d) Chlorinate effluent
Answer: b) Provide oxygen for microbial growth
Rationale: Aeration tanks supply oxygen to bacteria that degrade organic
matter in wastewater.
- What does BOD5 measure?
a) Total solids
b) Nitrogen content
c) Oxygen demand of biodegradable organics over five days
- pH level
Answer: c) Oxygen demand of biodegradable organics over five days
Rationale: BOD5 measures the oxygen required by microorganisms to
decompose organic matter in five days at 20°C.
- Which microorganism type is primarily responsible for nitrification?
a) Protozoa
b) Bacteria
c) Algae
d) Fungi 2 / 4
Answer: b) Bacteria
Rationale: Nitrification is carried out by chemoautotrophic bacteria, mainly
Nitrosomonas and Nitrobacter, which oxidize ammonia to nitrate.
5. The purpose of secondary clarifiers is to:
a) Remove grit
b) Separate biological solids from treated water
c) Add chlorine
d) Remove fats and oils
Answer: b) Separate biological solids from treated water
Rationale: Secondary clarifiers allow the biomass from the activated
sludge process to settle and separate from the treated effluent.
- Which chemical is commonly used for phosphorus removal in
wastewater?
a) Sodium hypochlorite
b) Aluminum sulfate (alum)
c) Calcium hydroxide
d) Sodium bicarbonate
Answer: b) Aluminum sulfate (alum)
Rationale: Alum reacts with phosphate to form insoluble aluminum
phosphate, which can be removed by settling or filtration.
- What is the typical retention time in a primary clarifier?
- 1–2 hours 3 / 4
- 2–4 hours
- 6–8 hours
- 12–24 hours
Answer: b) 2–4 hours
Rationale: Primary clarifiers are designed for short retention times, usually
2–4 hours, to settle heavy solids.
8. Chlorine is most effective for disinfection when:
- pH is high
- pH is low
c) Temperature is low
d) Ammonia is present
Answer: b) pH is low
Rationale: Chlorine is more effective in acidic conditions because
hypochlorous acid (HOCl), the active disinfectant, predominates at lower pH.
- Which of the following is a sign of filamentous bacteria overgrowth in
activated sludge?
a) High dissolved oxygen
b) Clear effluent
c) Sludge bulking
d) Low pH
- / 4
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