North Carolina Wastewater Operator Exam Practice Test

You already know the feeling: you can recite what MLSS stands for, but the exam doesn’t ask you to define it. It hands you a rising sludge blanket, a DO reading that doesn’t match the blower log, and four answers that all sound reasonable. This practice test is built for that gap — the space between knowing the terms and knowing what to do with them.

It’s independent study material, written to mirror the style and difficulty of North Carolina’s wastewater operator certification exams. It isn’t produced or endorsed by NC DEQ or the Water Pollution Control System Operators Certification Commission (WPCSOCC), and none of the questions are pulled from an actual exam.

What you’re actually practicing

Most of the questions read like a shift log, not a flashcard. A few examples of the format:

• Ammonia is climbing even though DO looks fine — what do you check next, and why?
• Given flow, BOD, and detention time, calculate the loading and decide if the basin is undersized.
• A clarifier is losing solids over the weir during a wet-weather event — hydraulic problem or blanket problem?
• WAS rate hasn’t changed but SRT has drifted — what’s the likely cause?

Every question comes with an explanation that walks through the reasoning, not just the letter. When you miss one, you’ll know which process concept to go back and study, not just which answer was correct.

Topics covered

The question bank tracks the technical areas in NC’s Needs to Know (NTK) framework for Biological Wastewater grades WW1 through WW4:

Math gets its own emphasis throughout — flow conversions, pounds-per-day loading, detention time, surface overflow rate, F/M, SRT, chemical feed rates, percent removal. NC DEQ hands you the official formula sheet at the exam and won’t let you bring your own, so the goal here is fluency: you should recognize which formula a scenario calls for almost before you finish reading it.

About the actual NC certification exam

A quick refresher, since a lot of this changes from state to state and even year to year:

• Who runs it: WPCSOCC, under NC DEQ, based on the grade-specific NTK material.
• Grades: Biological Wastewater WW1 through WW4, tied to the classification of the facility you’re certifying for.
• Format: Multiple choice. Passing score is 70%.
• Schedule: NC DEQ currently offers wastewater exams four times a year at various sites — locations and times can shift, so go by your official exam notification, not last year’s schedule.
• Formula sheet: Provided at the exam, and it’s the only one you’re allowed to use.
• Eligibility: WW1 needs a high school diploma or GED, minimum age 18, and the certification school completed. WW2 through WW4 stack on top of the prior grade plus experience (or a qualifying degree). If you don’t have the experience yet, you may be able to sit for the exam as an Operator-in-Training — just know an OIT can’t serve as ORC or backup ORC.
• Applying: You’ll file NC DEQ’s current exam application, attach your certification-school completion paperwork, and pay the processing fee ($101 on the 2026 application). Postmark it at least 30 days before your exam date — DEQ is specific about that deadline.

Requirements and fees do get updated, so cross-check anything above against NC DEQ’s current wastewater operator exam page before you apply.

How to use this to actually pass

A few things that make a measurable difference:

1. Read the NTK document for your grade first. The practice questions are reinforcement, not a replacement for it.
2. Don’t skip the math. It’s the section people cram the night before and it’s the section that costs the most points from rushing.
3. When you miss a question, don’t just note the right letter. Figure out why the other three are wrong — that’s usually where the real understanding is.
4. Watch your units. MGD vs. GPM, mg/L vs. lbs/day — a lot of wrong answers on these exams come from an otherwise-correct calculation with the wrong unit attached.
5. Untimed first, timed later. Build accuracy before you build speed.
6. Go back to your weak spots. It’s tempting to keep answering questions you’re already good at because it feels productive. It isn’t.

Sample Questions and Answers

Question 1. A secondary clarifier at a biological wastewater treatment plant has shown a gradual increase in effluent suspended solids over three days. The mixed liquor in the aeration basin appears normal, but the clarifier blanket is becoming deeper. A settleability test shows that the sludge settles slowly and occupies a large volume after 30 minutes. Which operating condition should the operator investigate FIRST?

A. Excessive chlorine contact time
B. Poor sludge settleability associated with filamentous growth
C. Excessive grit removal
D. Low influent screening efficiency

Correct Answer: B

Answer Explanation: Option B is correct because a rising clarifier blanket combined with slow, bulky settling sludge strongly suggests a sludge settleability problem, commonly associated with filamentous organisms. Filamentous growth can produce an open, poorly compacting sludge structure that remains suspended and can eventually carry over the clarifier weir. The operator should evaluate microscopic observations, dissolved oxygen conditions, nutrient availability, sludge age, food-to-microorganism conditions, and return/waste activated sludge rates before making a process adjustment. The problem is occurring in the biological solids separation process, not in preliminary screening or chlorine contact. Early identification is important because continued blanket accumulation can result in significant effluent TSS violations and loss of solids inventory from the biological process.

Why the other options are incorrect:

Option A concerns disinfection and would not explain a deep secondary sludge blanket.

Option C affects removal of heavy inorganic material and does not normally cause poor activated-sludge settling.

Option D concerns preliminary treatment and does not explain the observed biological sludge characteristics.

Study Guide:
Remember that bulking sludge is primarily a settling and compaction problem, often associated with filamentous organisms.
When clarifier blankets rise, evaluate settleability, RAS/WAS rates, DO, sludge age, and microscopic characteristics before making major process changes.

Question 2. An activated-sludge plant normally operates with an aeration basin dissolved oxygen concentration of approximately 2.0 mg/L. During a high-flow event, the operator observes that DO has fallen to 0.3 mg/L while ammonia in the effluent begins increasing. Blowers are operating, but the air delivery appears reduced. What is the MOST appropriate immediate operational response?

A. Reduce all return activated sludge pumping
B. Increase wasting substantially
C. Restore adequate oxygen transfer to the aeration process
D. Increase final effluent chlorination

Correct Answer: C

Answer Explanation: Option C is correct because the combination of very low aeration-basin DO and increasing effluent ammonia indicates that nitrifying organisms may not be receiving sufficient oxygen to oxidize ammonia effectively. The operator should first investigate the reduced air delivery, including blower operation, valves, diffusers, air headers, and basin mixing. Nitrification is particularly sensitive to inadequate dissolved oxygen because nitrifying organisms require oxygen for ammonia oxidation. Simply increasing chlorine dosage does not correct the biological treatment deficiency and may create unnecessary chemical demand. Likewise, aggressive wasting could further reduce the nitrifying biomass available for treatment. The priority is to restore reliable aeration and confirm that DO and biological performance recover.

Why the other options are incorrect:

Option A may alter solids inventory but does not directly correct inadequate oxygen transfer.

Option B could remove valuable nitrifying organisms and make ammonia treatment worse.

Option D disinfects the final effluent but does not correct biological ammonia oxidation.

Study Guide:
Low DO can cause rapid deterioration in carbon removal and especially nitrification.
When ammonia rises with low basin DO, investigate the air system and oxygen transfer capacity before changing sludge wasting or disinfection.

Question 3. A plant operator receives the following laboratory results from a secondary treatment system:

Influent BOD₅ = 220 mg/L
Effluent BOD₅ = 22 mg/L

What is the approximate BOD₅ removal efficiency?

A. 80%
B. 85%
C. 90%
D. 95%

Correct Answer: C

Answer Explanation: Option C is correct because BOD₅ removal efficiency is calculated by subtracting the effluent concentration from the influent concentration, dividing that difference by the influent concentration, and multiplying by 100. The calculation is: (220 − 22) ÷ 220 × 100 = approximately 90%. This indicates that the treatment process removed approximately nine-tenths of the influent biochemical oxygen demand. Operators should understand removal efficiency because it provides a useful process-performance indicator and can help identify deterioration before a discharge limit is exceeded. However, removal efficiency should always be interpreted together with actual effluent concentrations, permit requirements, flow, loading, and other process measurements rather than used as the only indicator of compliance.

Why the other options are incorrect:

Option A understates the removal achieved by the treatment system.

Option B results from an inaccurate calculation of the influent-to-effluent reduction.

Option D overstates the actual percentage removed.

Study Guide:
Use: Removal % = (Influent − Effluent) ÷ Influent × 100.
Always distinguish between percent removal and the actual concentration that must meet the facility’s discharge requirement.

Question 4. During a morning inspection, an operator notices that one final clarifier has a dark, rising sludge blanket and small gas bubbles are visible at the surface. The effluent from that clarifier contains elevated suspended solids. Which condition is MOST likely occurring?

A. Denitrification or anaerobic activity within the settled sludge
B. Excessive screening of influent solids
C. Inadequate chlorine residual in the contact tank
D. Excessive dissolved oxygen in the aeration basin

Correct Answer: A

Answer Explanation: Option A is correct because sludge that remains in the clarifier too long can undergo anaerobic conditions and generate gases. Gas bubbles can attach to sludge particles and cause them to rise to the surface, producing floating sludge and potentially increasing effluent suspended solids. Denitrification can also generate nitrogen gas within settled sludge under appropriate conditions, contributing to rising sludge. The operator should check sludge blanket depth, sludge withdrawal rates, return rates, settling characteristics, and process conditions. The key clue is the combination of a deep or dark blanket and gas-associated rising solids. This is different from conventional poor settling, where sludge generally remains dispersed rather than becoming visibly buoyant because of gas formation.

Why the other options are incorrect:

Option B concerns preliminary solids removal and does not explain gas-associated sludge rising in the clarifier.

Option C affects pathogen control, not the physical mechanism causing sludge to rise.

Option D does not explain gas bubbles forming within settled clarifier sludge.

Study Guide:
A clarifier is not intended to store activated sludge indefinitely.
Long detention of settled sludge can lead to anaerobic conditions, gas formation, and rising sludge, so sludge withdrawal and return should be evaluated.

Question 5. A wastewater plant uses sodium hypochlorite for final effluent disinfection. The chlorine residual suddenly drops while the chemical feed pump continues operating at its normal setting. Flow has increased substantially following a storm. What should the operator investigate FIRST?

A. Whether chlorine demand and hydraulic loading have increased
B. Whether the aeration basin MLSS should immediately be doubled
C. Whether the influent screen openings should be reduced
D. Whether the secondary clarifier should be taken permanently offline

Correct Answer: A

Answer Explanation: Option A is correct because an increase in wastewater flow can increase chlorine demand and reduce effective contact conditions if the chemical feed rate is not adjusted appropriately. Higher organic loading, ammonia, suspended solids, and other chlorine-demanding substances can consume disinfectant before an adequate residual is maintained. The operator should verify actual flow, chemical strength, feed rate, pump calibration, chemical supply, mixing, contact time, and measured residual. The operator should also determine whether the increased flow has altered hydraulic conditions in the contact tank. Simply assuming the pump is functioning because it is running is unsafe; the actual feed rate and chemical concentration should be verified. Disinfection must be evaluated against the facility’s permit requirements and operating conditions.

Why the other options are incorrect:

Option B concerns biological solids inventory and does not directly address chlorine residual.

Option C is unrelated to final disinfectant demand.

Option D is an extreme response that is not justified by the information provided.

Study Guide:
Chlorine residual depends on dose, demand, flow, contact conditions, and chemical strength.
During high flow, verify both chemical feed performance and actual disinfectant residual rather than relying on pump settings alone.

Question 6. An operator calculates that the plant’s return activated sludge flow has increased significantly while the mixed liquor concentration remains stable. The secondary clarifier sludge blanket is low, but the return pump is operating near its maximum capacity. What is the PRIMARY purpose of the return activated sludge system?

A. To disinfect secondary effluent
B. To return settled biological solids to the aeration process
C. To remove grit before primary treatment
D. To increase chlorine contact time

Correct Answer: B

Answer Explanation: Option B is correct because return activated sludge, or RAS, returns settled biological solids from the secondary clarifier to the aeration basin. This maintains an appropriate concentration of microorganisms in the biological treatment process and helps control the solids inventory. RAS flow is an important operating variable because excessive or inadequate return rates can affect clarifier blanket depth, solids residence time, hydraulic loading, and biological process performance. Operators should understand that RAS does not represent the same function as waste activated sludge. RAS generally preserves and recycles biomass within the treatment process, whereas WAS removes excess biological solids from the system. Proper control requires observing clarifier performance rather than adjusting pumps solely by habit.

Why the other options are incorrect:

Option A is the function of disinfection rather than sludge return.

Option C describes preliminary treatment and grit removal.

Option D concerns hydraulic and disinfection design, not activated-sludge recycling.

Study Guide:
RAS returns useful biomass to the aeration basin. WAS removes excess biomass from the process.
Clarifier blanket depth, settling characteristics, MLSS, and process objectives should guide RAS adjustments.

Question 7. A wastewater operator is troubleshooting poor nitrification. The aeration basin has adequate dissolved oxygen, but the plant has recently increased wasting and the sludge age has fallen significantly. Effluent ammonia is increasing. Which explanation is MOST likely?

A. The plant has removed too much chlorine from the process
B. Excessive wasting has reduced the nitrifying biomass inventory
C. Increased wasting always improves nitrification
D. The final clarifier is producing excessive chlorine demand

Correct Answer: B

Answer Explanation: Option B is correct because nitrifying organisms generally grow more slowly than many organisms responsible for carbonaceous BOD removal. Maintaining sufficient solids retention time is therefore critical for nitrification. If an operator increases wasting substantially and lowers sludge age, nitrifying organisms may be removed faster than they can reproduce, causing nitrification performance to decline even though the aeration basin DO remains adequate. The operator should evaluate sludge age, MLSS, wasting rate, ammonia trends, temperature, alkalinity, DO, and clarifier performance before making further changes. A stable DO measurement by itself does not prove that the biological process has enough nitrifying biomass to achieve ammonia oxidation.

Why the other options are incorrect:

Option A has no direct relationship to maintaining nitrifying biomass.

Option C is incorrect because excessive wasting can remove organisms needed for treatment.

Option D does not explain the loss of nitrification caused by reduced solids retention.

Study Guide:
Nitrifiers are relatively slow-growing organisms, so adequate sludge age/SRT is essential.
When ammonia rises after increased wasting, check whether the plant has reduced its nitrifying biomass inventory.

Question 8. A plant’s influent flow meter indicates 2.4 MGD. The operator needs to determine the daily volume in gallons for a chemical dosage calculation. Which value should be used?

A. 240,000 gallons/day
B. 1,440,000 gallons/day
C. 2,400,000 gallons/day
D. 24,000,000 gallons/day

Correct Answer: C

Answer Explanation: Option C is correct because MGD means million gallons per day. Therefore, 2.4 MGD equals 2.4 million gallons per day, or 2,400,000 gallons per day. Converting flow correctly is fundamental to wastewater chemical calculations, loading calculations, hydraulic retention time calculations, and process control. An operator should pay particular attention to units because an error involving a factor of 10 or 100 can produce a significant chemical-feed or process-control error. When using formulas, write the units beside each value before calculating. The NC wastewater examination program provides a specific math formula sheet for use during the examination, making familiarity with units and formula application particularly important.

Why the other options are incorrect:

Option A represents only 0.24 million gallons per day.

Option B represents 1.44 million gallons per day and is not the stated flow.

Option D is ten times greater than the actual daily volume.

Study Guide:
1 MGD = 1,000,000 gallons/day.
For exam calculations, carefully track MGD, gpd, mg/L, lb/day, and minutes so unit conversions do not introduce major errors.

Question 9. During routine operation, an operator notices that the bar screen differential level has increased rapidly and upstream water is approaching the alarm elevation. The screen is visibly covered with rags and other debris. What is the BEST immediate action?

A. Increase chlorine dosage
B. Reduce aeration basin MLSS
C. Safely remove accumulated screenings using the plant’s established procedure
D. Increase RAS flow

Correct Answer: C

Answer Explanation: Option C is correct because the observed headloss and accumulation of screenings indicate that the bar screen is becoming obstructed. Removing the accumulated material restores hydraulic capacity and protects downstream equipment from excessive debris. The operator should follow the facility’s established screening-cleaning procedure and required personal protective equipment because screenings may contain pathogens, sharps, and other hazardous materials. Operators should also investigate why the screen is accumulating unusually large quantities of material and verify that automatic cleaning equipment, if installed, is functioning properly. Ignoring a rapidly increasing upstream level can result in bypass conditions, flooding, pump problems, or hydraulic upset. Chemical or biological process adjustments would not correct a physically obstructed preliminary-treatment unit.

Why the other options are incorrect:

Option A does not remove the physical obstruction.

Option B concerns biological solids and has no relationship to screen headloss.

Option D affects activated-sludge recycling and cannot clear a blocked bar screen.

Study Guide:
Preliminary treatment protects downstream equipment from rags, plastics, grit, and other debris.
Increasing headloss across a screen is a practical indication that cleaning or mechanical inspection is needed.

Question 10. A wastewater plant experiences an unexpected industrial discharge. Within hours, the aeration basin develops unusual foam, the pH begins falling, and oxygen uptake changes sharply. Effluent treatment performance starts deteriorating. What should the operator do FIRST?

A. Assume the condition is normal seasonal variation
B. Document the event and investigate the influent for possible toxic or inhibitory loading
C. Immediately double the sludge wasting rate
D. Stop all laboratory testing

Correct Answer: B

Answer Explanation: Option B is correct because a sudden process response following an unusual industrial discharge can indicate toxic or inhibitory influent loading. Operators should investigate the source, document timing and observations, review process data, collect appropriate samples, and notify responsible plant personnel according to facility procedures and permit requirements. Toxic loading can disrupt microbial activity, change oxygen uptake, alter pH, and impair treatment performance. Automatically doubling wasting could remove organisms that are needed to recover the biological process and would be inappropriate without understanding the cause. Operators should compare influent and process trends, review industrial discharge information, and determine whether operational changes are justified by the evidence.

Why the other options are incorrect:

Option A dismisses a significant process change without investigating its cause.

Option C could worsen the loss of biological inventory.

Option D removes important information needed to diagnose and document the upset.

Study Guide:
A sudden biological upset following an unusual discharge should trigger source investigation, documentation, sampling, and process review.
Do not make aggressive biological-process changes until the likely cause is understood.

Question 11. A secondary clarifier receives a stable mixed liquor concentration, but the sludge blanket repeatedly rises during periods of low plant flow. The operator discovers that the RAS pump is being operated at a fixed percentage of maximum capacity regardless of actual plant flow. Which adjustment principle is MOST appropriate?

A. Evaluate RAS operation in relation to clarifier blanket and process conditions
B. Always operate RAS at maximum capacity
C. Turn off all RAS whenever plant flow decreases
D. Use chlorine residual to determine RAS flow

Correct Answer: A

Answer Explanation: Option A is correct because RAS operation should be evaluated based on actual process conditions rather than a fixed pump setting used under every hydraulic condition. Clarifier performance depends on solids loading, hydraulic loading, settling characteristics, blanket depth, RAS concentration, and the relationship between return flow and plant flow. Operating a pump at an arbitrary percentage of capacity can result in inappropriate solids return and poor clarifier control when conditions change. The operator should trend blanket depth, RAS flow and concentration, MLSS, settling characteristics, and effluent suspended solids. Adjustments should be made deliberately and verified through subsequent observations rather than based solely on a predetermined pump percentage.

Why the other options are incorrect:

Option B ignores the actual solids and hydraulic conditions of the plant.

Option C would stop the normal recycling of biological solids and could quickly disrupt treatment.

Option D has no operational relationship to determining RAS flow.

Study Guide:
RAS is a process-control variable, not simply a pump that should run at one fixed percentage.
Use blanket depth, settling, MLSS, RAS concentration, flow, and effluent quality to evaluate performance.

Question 12. During a routine laboratory check, an operator obtains an unexpectedly high effluent TSS result. The effluent appears visually clear, the clarifier blanket is normal, and other samples from the same period are within the normal range. What should the operator do BEFORE making a major process adjustment?

A. Immediately double the WAS rate
B. Verify the sample, analytical procedure, and laboratory result
C. Shut down the aeration basin
D. Increase chlorine feed by 50%

Correct Answer: B

Answer Explanation: Option B is correct because an isolated laboratory result that conflicts with visual observations and other process data should be verified before making a major operational change. The operator should review sampling location, sample handling, preservation where applicable, analytical procedure, calculations, equipment condition, and quality-control information. A single unexpected value can result from sampling or analytical error, and changing the biological process based on an invalid result can create a genuine treatment problem. Verification does not mean ignoring the result; it means determining whether the result accurately represents plant performance. If confirmed, the operator can then use the data together with clarifier observations and other process measurements to identify the cause.

Why the other options are incorrect:

Option A could unnecessarily alter the biological solids inventory.

Option C is an excessive response unsupported by the available evidence.

Option D does not correct elevated TSS caused by secondary solids carryover.

Study Guide:
Good operators verify unexpected data before making large process changes.
Compare laboratory results with sampling conditions, trends, clarifier observations, and other operating measurements.

Question 13. A plant operator must enter a confined space to inspect equipment in a wastewater wet well. The space has limited access and may contain hazardous atmospheric conditions. Which approach is MOST appropriate?

A. Enter quickly because wastewater spaces are normally safe
B. Enter only after the facility’s confined-space procedures, atmospheric testing, and required controls have been satisfied
C. Enter alone if a coworker knows where the operator is
D. Test the atmosphere only after entering the space

Correct Answer: B

Answer Explanation: Option B is correct because wastewater wet wells and similar confined spaces can contain oxygen-deficient, toxic, or otherwise hazardous atmospheres, as well as engulfment and access hazards. Entry must follow the facility’s confined-space program and applicable requirements, including determining whether the space is permit-required, conducting atmospheric testing as required, establishing appropriate ventilation and communication, using required personal protective equipment, and providing the designated rescue arrangements. Entering simply because the operator has performed the task before is unsafe. Atmospheric testing must be performed according to the entry procedure before exposure and may require continuous or periodic monitoring depending on the conditions. A rescue plan must never depend on improvised entry by another unprotected employee.

Why the other options are incorrect:

Option A ignores potentially fatal atmospheric and physical hazards.

Option C does not provide the required entry controls or rescue protection.

Option D exposes the worker before determining whether the atmosphere is safe.

Study Guide:
Confined-space safety is an entry-control process, not merely an equipment inspection.
Remember the importance of atmospheric testing, ventilation, communication, entry procedures, and rescue planning.

Question 14. A wastewater plant uses a secondary clarifier followed by UV disinfection. Effluent TSS increases significantly while the UV system remains operational at its normal setting. The operator observes more suspended particles in the UV channel. Why can this condition reduce disinfection performance?

A. Suspended solids can shield microorganisms from UV radiation
B. TSS automatically increases UV intensity
C. Suspended solids chemically neutralize all ammonia
D. Higher TSS eliminates the need for UV exposure time

Correct Answer: A

Answer Explanation: Option A is correct because suspended particles can shield microorganisms from UV radiation, reducing the effectiveness of disinfection. UV systems depend on sufficient radiation reaching microorganisms in the wastewater. Elevated TSS or turbidity can interfere with that exposure by absorbing or scattering UV energy and by physically protecting organisms within particles. Therefore, maintaining good secondary clarification is important not only for effluent solids quality but also for downstream disinfection performance. When UV performance changes, the operator should evaluate UV intensity, lamp condition, sleeve cleanliness, flow, transmittance, and effluent quality. Simply increasing UV operation without correcting upstream solids problems may not resolve the underlying cause of poor disinfection.

Why the other options are incorrect:

Option B is opposite of the expected effect; suspended solids can interfere with UV transmission.

Option C is unrelated to UV disinfection and does not describe the treatment mechanism.

Option D incorrectly suggests that increased solids eliminate the need for adequate UV exposure.

Study Guide:
UV disinfection depends on effective radiation reaching microorganisms.
Poor clarification can increase TSS and reduce UV effectiveness by shielding organisms and interfering with UV transmission.

Question 15. An activated-sludge plant has the following operating conditions:

• Influent flow is stable.
• Aeration basin DO is adequate.
• Effluent ammonia has increased steadily.
• Effluent BOD remains acceptable.
• Waste activated sludge has recently been increased substantially.
• Water temperature has not changed significantly.

Which operational factor should receive the MOST attention?

A. Sludge age and nitrifying biomass retention
B. Bar-screen opening width
C. Final chlorine contact tank baffle spacing
D. Grit chamber washing frequency

Correct Answer: A

Answer Explanation: Option A is correct because the pattern strongly points toward a nitrification-specific problem rather than a general carbon-removal failure. Effluent BOD remains acceptable, indicating that the heterotrophic population is still providing effective carbonaceous treatment. At the same time, ammonia has increased after substantial increases in wasting. Nitrifying organisms grow more slowly and require sufficient solids retention time to remain established in the activated-sludge system. Excessive wasting can reduce sludge age and remove nitrifiers from the process faster than they can reproduce. The operator should evaluate SRT, MLSS, WAS rate, ammonia trends, DO, alkalinity, pH, temperature, and clarifier performance. Corrective action should be based on the complete process picture rather than simply increasing aeration.

Why the other options are incorrect:

Option B affects preliminary solids screening and would not explain selective loss of nitrification.

Option C affects downstream disinfection hydraulics and does not control biological ammonia oxidation.

Option D concerns grit handling and is unrelated to the observed ammonia trend.

Study Guide:
When BOD remains acceptable but ammonia rises, think specifically about nitrification.
Check SRT/sludge age, nitrifier retention, DO, pH, alkalinity, temperature, and wasting practices before choosing a corrective action.