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ABC Wastewater Treatment Class IV study materials
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ABC Wastewater Treatment Class IV practice questions

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Question 1Process Control and Operational Troubleshooting

An activated sludge aeration basin shows a steadily decreasing food-to-microorganism (F/M) ratio while MLSS remains constant. This change is most directly caused by which condition?

  1. a shorter mean cell residence time
  2. a higher influent BOD concentration
  3. a lower influent organic load Correct answer.
  4. an increased waste sludge flow

Explanation

F/M compares the daily organic food supply with the biomass inventory in the aeration basin. If MLSS is held constant, a lower influent organic load reduces the food term and lowers the F/M ratio. Wasting changes affect the biomass inventory, and a higher BOD load would raise F/M.

Question 2Process Control and Operational Troubleshooting

An activated sludge sample has an SVI of 250 mL/g. This value most nearly indicates which condition?

  1. complete loss of biological activity
  2. poor settling and bulking Correct answer.
  3. excellent settling and compaction
  4. a very dense sludge with rapid settling

Explanation

SVI is the volume occupied by one gram of settled sludge after a standard settling test. Values around 100 to 150 mL/g are typical for good settling, while values above about 200 mL/g indicate poor settling and likely filamentous bulking. SVI does not measure biological activity directly.

Question 3Process Control and Operational Troubleshooting

In a conventional aerobic activated sludge basin treating municipal wastewater, the usual dissolved oxygen target for reliable nitrification is about which value?

  1. 0.8 mg/L
  2. 6.0 mg/L
  3. 2.0 mg/L Correct answer.
  4. 0.2 mg/L

Explanation

Nitrifying bacteria require adequate dissolved oxygen, and conventional practice targets roughly 1.5 to 2.5 mg/L in the aerobic zone. A target near 2.0 mg/L supports nitrification without wasting excessive blower energy. Values below about 1 mg/L can select filamentous organisms and inhibit nitrification.

Question 4Process Control and Operational Troubleshooting

Sludge blanket solids in a secondary clarifier are rising to the surface in small gas-lifted clumps. Which cause is most likely?

  1. excessive surface skimming
  2. hydraulic short-circuiting
  3. a low sludge blanket
  4. clarifier denitrification Correct answer.

Explanation

When nitrate reaches the anoxic environment of a deep sludge blanket, denitrification produces nitrogen gas bubbles that become trapped in the sludge. The gas lifts clumps of solids to the surface, a condition commonly called rising sludge. Avoiding a deep blanket and reducing nitrate loading to the clarifier help control it.

Question 5Process Control and Operational Troubleshooting

A nitrifying activated sludge basin shows a gradual pH decrease from 7.2 to 6.4 while ammonia removal remains stable. Which cause is most likely?

  1. sulfate reduction in the clarifier
  2. alkalinity consumption by nitrification Correct answer.
  3. alkalinity production by denitrification
  4. carbon dioxide stripping by aeration

Explanation

Nitrification converts ammonia to nitrate and consumes alkalinity, releasing hydrogen ions and tending to lower pH. Denitrification produces alkalinity and tends to raise pH, while aeration strips carbon dioxide and also tends to raise pH. A gradual pH decline during stable nitrification usually points to alkalinity demand.

Question 6Process Control and Operational Troubleshooting

In an anaerobic digester, a volatile fatty acid (VFA) to alkalinity ratio of 0.45 is reported. This value most nearly indicates which condition?

  1. complete stabilization of sludge
  2. a trend toward digester acidification Correct answer.
  3. stable digestion with excess buffering
  4. excessive methane production

Explanation

The VFA-to-alkalinity ratio is a stability indicator for anaerobic digestion. Stable digesters generally remain below about 0.1 to 0.2, while ratios above about 0.3 suggest accumulating acids and increasing risk of souring. A value of 0.45 calls for reduced feed and corrective action.

Question 7Process Control and Operational Troubleshooting

A secondary clarifier has a rising sludge blanket and increasing effluent suspended solids. The RAS flow is well below design and the blanket is high. Which operational adjustment should be made first?

  1. shut down the aeration basin
  2. decrease the RAS flow
  3. increase the RAS flow Correct answer.
  4. increase the waste sludge flow

Explanation

When the sludge blanket is high and RAS flow is below design, solids are accumulating in the clarifier and are at risk of carrying over the weir. Increasing RAS returns settled solids to the aeration basin and lowers the blanket. Wasting can then be adjusted to maintain the desired MCRT.

Question 8Process Control and Operational Troubleshooting

In disinfection, the CT value is calculated by multiplying which two factors?

  1. contact time by flow rate
  2. concentration times contact time Correct answer.
  3. chlorine dose by alkalinity
  4. residual chlorine by pH

Explanation

CT is the product of disinfectant residual concentration and the effective contact time. It is used to judge whether a given disinfectant exposure is sufficient for the target organism and water quality. Flow rate, alkalinity, and pH influence disinfection but are not the two terms multiplied in the CT definition.

Question 9Process Control and Operational Troubleshooting

Aeration basin DO is consistently 0.5 mg/L, and the sludge has an SVI of 220 mL/g with abundant filaments. Which corrective action is most appropriate?

  1. decrease the MCRT by wasting more sludge
  2. add nitrogen to correct a nutrient deficiency
  3. increase aeration to maintain about 2 mg/L DO Correct answer.
  4. reduce the RAS rate to thicken the sludge

Explanation

Low dissolved oxygen in the aeration basin favors filamentous bacteria that can outcompete floc-forming organisms, leading to a high SVI. Restoring the DO to about 2 mg/L addresses the likely selection pressure. Nutrient addition and MCRT changes are considered when other causes are indicated.

Question 10Process Control and Operational Troubleshooting

Persistent stable foam with dark brown color and thick scum on the aeration basin and clarifier is most commonly associated with which condition?

  1. excessive wasting and low MLSS
  2. a very short sludge age and low influent grease
  3. low DO and high F/M
  4. long sludge age and high influent grease Correct answer.

Explanation

Stable dark brown foam is often associated with Nocardia and related filamentous foam-forming organisms. These organisms are favored by long sludge age, high influent fats, oils, and grease, and sometimes low F/M conditions. Short sludge age and low grease tend to reduce this type of foaming.

Question 11Process Control and Operational Troubleshooting

An anoxic zone is intended for denitrification, but effluent nitrate remains high and influent readily biodegradable COD is low. Which adjustment is most appropriate?

  1. decrease the internal nitrate recycle
  2. add a carbon source to the anoxic zone Correct answer.
  3. increase wasting to reduce the MCRT
  4. increase aeration in the anoxic zone

Explanation

Denitrification requires an electron donor, usually readily biodegradable carbon, to convert nitrate to nitrogen gas. If the influent carbon is insufficient, supplemental carbon can restore denitrification and lower effluent nitrate. Aeration would destroy the anoxic condition, and reducing recycle would reduce the nitrate fed to the zone.

Question 12Process Control and Operational Troubleshooting

During a storm, secondary clarifier overflow rate exceeds design, the sludge blanket rises, and effluent TSS increases. Which immediate control action is most appropriate?

  1. stop wasting sludge
  2. increase the aeration basin MLSS
  3. decrease the RAS rate
  4. reduce or equalize clarifier flow Correct answer.

Explanation

The clarifier is being hydraulically overloaded, which shortens detention time and pushes the sludge blanket upward. Reducing or equalizing the flow lowers the overflow rate and helps protect solids capture. Maintaining RAS and avoiding unnecessary solids accumulation are also important during the event.

Question 13Process Control and Operational Troubleshooting

Effluent ammonia rises sharply while influent BOD, DO, pH, and temperature remain near normal. Which explanation is most likely?

  1. nitrifier inhibition or toxicity Correct answer.
  2. a decrease in influent ammonia load
  3. a drop in sludge age from over-wasting
  4. an increase in denitrification recycle

Explanation

A sudden ammonia breakthrough under otherwise normal conditions suggests that the nitrifying population has been inhibited or poisoned. Toxic or inhibitory discharges can reduce nitrification while BOD removal continues. Changes in loading, recycle, or sludge age usually produce a different pattern or are not supported by the stated normal conditions.

Question 14Process Control and Operational Troubleshooting

Oxidation of 1 mg of ammonia nitrogen to nitrate consumes approximately how many milligrams of alkalinity, expressed as CaCO3?

  1. 1.0 mg
  2. 14.3 mg
  3. 7.14 mg Correct answer.
  4. 3.57 mg

Explanation

Nitrification consumes about 7.14 mg of alkalinity as CaCO3 for each mg of ammonia nitrogen oxidized. This stoichiometric demand is why nitrifying plants can lose pH and alkalinity unless sufficient buffering is present. The value is a standard process-control reference.

Question 15Process Control and Operational Troubleshooting

An ORP reading of about -50 mV in an anoxic basin is most consistent with which condition?

  1. strict anaerobic digestion
  2. active chlorine disinfection
  3. denitrification conditions Correct answer.
  4. complete aerobic nitrification

Explanation

ORP is used as an indicator of the oxidation-reduction environment. An anoxic zone with ORP near -50 mV is generally suitable for denitrification, where nitrate is used as the electron acceptor. Strongly negative ORP suggests anaerobic conditions, while positive ORP suggests aerobic conditions.

Question 16Process Control and Operational Troubleshooting

Nitrification in an activated sludge plant deteriorates as wastewater temperature falls to 12°C. Which adjustment would help restore nitrification?

  1. increase the WAS flow
  2. increase the MCRT Correct answer.
  3. decrease the MCRT
  4. lower the DO setpoint

Explanation

Nitrifying bacteria grow more slowly at cold temperatures, so a longer solids retention time is needed to keep them in the system. Increasing MCRT allows the nitrifying population to accumulate despite slower growth. Decreasing MCRT or increasing wasting would tend to wash nitrifiers out.

Question 17Process Control and Operational Troubleshooting

Excessive RAS flow to the aeration basin most directly causes which effect?

  1. a shorter hydraulic detention time Correct answer.
  2. a reduced oxygen demand in the basin
  3. a higher sludge blanket in the clarifier
  4. a lower solids inventory in the aeration basin

Explanation

RAS flow is returned to the aeration basin and adds to the forward flow through the basin. Excessive RAS reduces the hydraulic detention time and can increase hydraulic loading on the clarifier. It does not normally raise the blanket or lower the aeration-basin solids inventory.

Question 18Process Control and Operational Troubleshooting

An aeration basin holds 2.0 million gallons at an MLSS of 3,000 mg/L. WAS is wasted at 0.05 MGD with a solids concentration of 20,000 mg/L; effluent solids are negligible. The MCRT is approximately which value?

  1. 10 days
  2. 3 days
  3. 15 days
  4. 6 days Correct answer.

Explanation

Aeration-basin solids mass is 3,000 mg/L × 2.0 MG × 8.34, or about 50,040 lb. WAS solids mass is 20,000 mg/L × 0.05 MGD × 8.34, or about 8,340 lb/day. Dividing the basin mass by the daily waste mass gives an MCRT of about 6 days.

Question 19Process Control and Operational Troubleshooting

A centrifuge produces cloudy centrate and requires a high polymer dose. Which cause is most likely?

  1. too low feed solids
  2. excessive cake solids
  3. excess polymer causing over-flocculation
  4. low polymer dose or poor mixing Correct answer.

Explanation

Cloudy centrate means fine solids are not being captured, which often indicates insufficient polymer or poor polymer mixing and aging. The high polymer demand may reflect a changing feed or poor conditioning rather than excess polymer. Cake solids and feed solids affect performance but do not by themselves explain cloudy centrate.

Question 20Process Control and Operational Troubleshooting

An industrial wastewater activated sludge plant has an SVI of 250 mL/g, low influent nitrogen and phosphorus, and a high COD:N ratio. Which corrective action is most appropriate?

  1. add nitrogen and phosphorus nutrients Correct answer.
  2. lower the DO setpoint
  3. increase the wasting rate
  4. add a chlorine solution to the aeration basin

Explanation

A high COD:N ratio with low nitrogen and phosphorus indicates nutrient deficiency, which can favor filamentous growth and poor settling. Adding the limiting nutrients improves floc formation and biological treatment. Increasing wasting or lowering DO does not correct the nutrient imbalance.

Question 21Process Control and Operational Troubleshooting

In a membrane bioreactor, transmembrane pressure rises rapidly over a few days while flux and influent loading are stable. Which first response is most appropriate?

  1. stop wasting sludge permanently
  2. install membranes with larger pores
  3. increase the membrane flux setpoint
  4. start the prescribed cleaning sequence Correct answer.

Explanation

A rapid rise in transmembrane pressure at stable flux is a sign of membrane fouling. The operator should follow the manufacturer’s fouling-response procedure, which may include backwashing, relaxation, or chemical cleaning. Increasing flux would accelerate fouling, and changing the membrane pore size is not an operational adjustment.

Question 22Process Control and Operational Troubleshooting

If UV transmittance of secondary effluent falls from 70% to 50% while flow and lamp output stay constant, the delivered UV dose will most likely do which of the following?

  1. increase proportionally
  2. decrease Correct answer.
  3. increase because of particle scattering
  4. remain unchanged

Explanation

Lower UV transmittance means less UV light penetrates the water, so the effective dose at a given lamp output and flow decreases. Reduced dose can allow pathogens to pass if the system is not adjusted. Utilities may improve upstream treatment, reduce flow, or increase lamp power to compensate.

Question 23Process Control and Operational Troubleshooting

Effluent total phosphorus rises above permit while alum feed, pH, and flow are normal. Which check is most appropriate first?

  1. whether the anaerobic digester is souring
  2. whether the coagulant dose is adequate Correct answer.
  3. whether the RAS flow rate is too high
  4. whether the UV disinfection dose is adequate

Explanation

Chemical phosphorus removal depends on an adequate coagulant dose relative to the phosphorus load and the desired residual. If effluent phosphorus rises while pH and flow are normal, the coagulant dose or feed system should be checked first. UV dose, RAS flow, and digester stability are not direct explanations for a phosphorus breakthrough.

Question 24Process Control and Operational Troubleshooting

A centrifugal aeration blower begins to surge, with rapid flow and pressure oscillations. Which action is most appropriate?

  1. increase the discharge pressure setpoint
  2. close the discharge valve completely
  3. open the anti-surge recycle valve Correct answer.
  4. reduce the inlet filter differential pressure

Explanation

Surge occurs when a centrifugal blower operates beyond its stable flow range, causing flow reversal and pressure oscillations. Opening the anti-surge recycle or otherwise increasing flow through the blower restores stable operation. Closing the discharge valve or raising pressure would push the blower further into surge.

Question 25Process Control and Operational Troubleshooting

A low-F/M activated sludge plant has a low SVI, clear supernatant, but fine pinpoint floc escaping in the effluent. Which control adjustment is most appropriate?

  1. decrease wasting to increase the sludge age
  2. add nitrogen to raise the F/M
  3. increase wasting to reduce the sludge age Correct answer.
  4. increase the RAS rate to raise the blanket

Explanation

Pinpoint floc can occur in an old, low-F/M sludge when floc particles remain small and are not captured well in the clarifier. Increasing wasting lowers the sludge age and raises F/M, which can encourage stronger floc formation. Decreasing wasting would make the sludge older and may worsen the condition.

Question 26Process Control and Operational Troubleshooting

At constant pressure and aeration, the saturation concentration of dissolved oxygen in wastewater most likely does which of the following as the water temperature rises from 15°C to 25°C?

  1. Decreases Correct answer.
  2. Remains unchanged
  3. Increases
  4. Increases, then decreases

Explanation

Gas solubility decreases as temperature rises, so warmer water holds less dissolved oxygen at saturation. This lowers the maximum DO available and can reduce basin DO even when airflow is unchanged.

Question 27Process Control and Operational Troubleshooting

A mixed liquor sample has MLSS of 3,500 mg/L and MLVSS of 1,600 mg/L. What does this result most likely indicate about the mixed liquor?

  1. Excessive wasting of volatile solids
  2. A healthy, young sludge with a high organic fraction
  3. A high influent BOD load
  4. An accumulation of inorganic solids in the mixed liquor Correct answer.

Explanation

The volatile fraction is about 0.46, well below the typical 0.65 to 0.80 range for activated sludge. A low volatile fraction usually means inert or inorganic solids, such as grit or chemical precipitate, are accumulating.

Question 28Process Control and Operational Troubleshooting

In an activated sludge aeration basin, the influent BOD mass increases while basin volume and MLSS remain constant. The food-to-microorganism (F/M) ratio will most likely:

  1. Increase Correct answer.
  2. Remain unchanged
  3. Decrease
  4. Become zero

Explanation

F/M is the food mass entering per day divided by the microorganism mass in the basin. With MLSS and volume constant, more influent BOD raises the numerator and therefore the ratio.

Question 29Process Control and Operational Troubleshooting

An operator wants to reduce the sludge age (SRT) of an activated sludge system. Which change is most direct?

  1. Increase the return activated sludge flow
  2. Decrease the waste activated sludge flow
  3. Increase the waste activated sludge flow Correct answer.
  4. Decrease the aeration basin volume

Explanation

SRT is controlled mainly by the rate of solids wasting; increasing WAS flow removes more solids per day and lowers SRT. RAS flow affects return and blanket depth but does not by itself set SRT.

Question 30Process Control and Operational Troubleshooting

A primary clarifier removes less suspended solids during high-flow periods, although sludge collection equipment is working normally. Which condition most likely explains the poorer removal?

  1. A high dissolved oxygen concentration
  2. A low influent BOD concentration
  3. Reduced detention time from short-circuiting Correct answer.
  4. Excessive primary sludge wasting

Explanation

High flow shortens detention time and can cause short-circuiting, leaving less time for particles to settle. Primary clarifier performance falls unless flow is equalized or diverted.

Question 31Process Control and Operational Troubleshooting

Excessive grit is accumulating in the primary clarifier and digesters. Which grit chamber condition most likely allows this to happen?

  1. Grit chamber velocity is too low
  2. Grit chamber air supply is too low
  3. Grit chamber velocity is too high Correct answer.
  4. Grit chamber pH is too low

Explanation

A grit chamber must slow flow enough for dense grit to settle while lighter organics pass. If velocity is too high, grit stays suspended and is carried downstream.

Question 32Process Control and Operational Troubleshooting

A centrifugal sludge pump is noisy, vibrates strongly, and delivers less flow than normal. Which cause is most likely?

  1. A fully closed discharge valve
  2. Cavitation from restricted suction Correct answer.
  3. Normal impeller wear
  4. Excessively high discharge pressure

Explanation

Cavitation occurs when suction restrictions lower pressure enough for liquid to vaporize; collapsing bubbles cause noise, vibration, and reduced flow. Correcting suction restrictions or suction lift is the first response.

Question 33Process Control and Operational Troubleshooting

After high-ammonia filtrate enters the plant, chlorine residual falls sharply at the same chlorine dose. What is the most appropriate immediate response?

  1. Raise the pH without changing dose
  2. Increase the chlorine dose to satisfy the additional demand Correct answer.
  3. Stop disinfection until ammonia clears
  4. Reduce the chlorine dose to avoid overchlorination

Explanation

Ammonia exerts a chlorine demand and shifts the breakpoint, so the same dose may leave little or no free residual. The dose must be adjusted to maintain the required residual and contact time.

Question 34Process Control and Operational Troubleshooting

An aeration basin holds 1.5 million gallons at an MLSS of 2,500 mg/L. Waste activated sludge is wasted at 0.08 MGD with a solids concentration of 8,000 mg/L. Effluent suspended solids are negligible. What is the approximate mean cell residence time (MCRT)?

  1. 3.9 days
  2. 8.9 days
  3. 5.9 days Correct answer.
  4. 12.0 days

Explanation

Aeration basin solids = 1.5 MG × 2,500 mg/L × 8.34 = 31,275 lb. WAS solids = 0.08 MGD × 8,000 mg/L × 8.34 = 5,338 lb/day. MCRT = 31,275 ÷ 5,338 ≈ 5.9 days.

Question 35Process Control and Operational Troubleshooting

In a 30-minute settleometer test, sludge settles to 300 mL/L. The mixed liquor suspended solids concentration is 2,500 mg/L. What is the sludge volume index (SVI)?

  1. 300 mL/g
  2. 83 mL/g
  3. 750 mL/g
  4. 120 mL/g Correct answer.

Explanation

SVI equals the settled sludge volume in mL/L divided by MLSS in g/L. Here 300 ÷ 2.5 = 120 mL/g, which is a typical settling value.

Question 36Process Control and Operational Troubleshooting

A secondary clarifier receives 1.0 MGD influent and 0.5 MGD return activated sludge. The MLSS is 3,000 mg/L and the clarifier surface area is 1,000 ft². What is the approximate solids loading rate?

  1. 30 lb/ft²/day
  2. 50 lb/ft²/day
  3. 37.5 lb/ft²/day Correct answer.
  4. 25 lb/ft²/day

Explanation

Solids loading rate = (influent + RAS flow) × MLSS × 8.34 ÷ clarifier area. Using 1.5 MGD × 3,000 × 8.34 ÷ 1,000 gives about 37.5 lb/ft²/day.

Question 37Process Control and Operational Troubleshooting

A secondary clarifier has an MLSS of 3,000 mg/L and a return sludge concentration of 9,000 mg/L. If the influent flow is 1.0 MGD, what return activated sludge flow is needed to maintain the solids balance?

  1. 0.25 MGD
  2. 0.50 MGD Correct answer.
  3. 1.0 MGD
  4. 0.33 MGD

Explanation

A solids balance around the clarifier gives R = MLSS ÷ (RAS concentration − MLSS). Here R = 3,000 ÷ (9,000 − 3,000) = 0.50, so RAS flow is 0.50 MGD.

Question 38Process Control and Operational Troubleshooting

Aeration basin dissolved oxygen slowly declines over several weeks while influent loading and blower airflow remain constant. Which cause is most likely?

  1. Fouling or plugging of the diffusers Correct answer.
  2. An increase in waste activated sludge flow
  3. A decrease in wastewater temperature
  4. A sudden decrease in RAS flow

Explanation

With loading and airflow unchanged, a gradual DO decline points to reduced oxygen transfer efficiency. Diffuser fouling, scaling, or blockage reduces bubble surface area and oxygen dissolution.

Question 39Process Control and Operational Troubleshooting

A nitrifying activated sludge plant has pH 6.2, rising effluent ammonia, and adequate dissolved oxygen. Which corrective action is most appropriate?

  1. Increase the waste activated sludge flow
  2. Add an anionic polymer to the aeration basin
  3. Reduce the return activated sludge flow
  4. Add alkalinity, such as lime or soda ash Correct answer.

Explanation

Nitrification consumes alkalinity and can drive pH below the range where nitrifiers function well. Adding alkalinity restores pH and supports nitrification when DO is already adequate.

Question 40Process Control and Operational Troubleshooting

An activated sludge plant has an SVI of 250 mL/g and abundant filamentous bacteria. Which operational measure directly targets filamentous organisms in the return sludge?

  1. Add polymer to the aeration basin
  2. Lower the dissolved oxygen setpoint
  3. Chlorinate the return activated sludge Correct answer.
  4. Increase the RAS flow

Explanation

RAS chlorination exposes filamentous organisms to a selective oxidant before they return to the aeration basin. It is used cautiously with settling tests and chlorine residual checks to avoid damaging floc-forming biomass.

Question 41Process Control and Operational Troubleshooting

A gravity belt thickener produces a thin cake and cloudy filtrate while polymer dose is at the normal setpoint. Which adjustment is most appropriate to try first?

  1. Reduce the feed solids concentration
  2. Increase the polymer dose and check mixing Correct answer.
  3. Decrease the polymer dose
  4. Increase the belt speed

Explanation

Cloudy filtrate and poor cake formation usually indicate inadequate flocculation, often from polymer underdosing or poor polymer mixing. Increasing dose slightly and verifying dilution and mixing is the logical first step.

Question 42Process Control and Operational Troubleshooting

A mesophilic anaerobic digester has stable feed, temperature, and pH, but gas production falls and digested solids quality worsens. Which check is most appropriate first?

  1. Lowering the digester temperature
  2. Mixing and feed distribution inside the digester Correct answer.
  3. Gas meter calibration only
  4. Increasing the feed rate

Explanation

Stable feed and temperature but declining gas suggest the biomass is not contacting fresh feed, or that scum and solids have accumulated. Checking mixing, recirculation, and feed distribution is the first diagnostic step.

Question 43Process Control and Operational Troubleshooting

UV disinfection performance declines while flow and influent UV transmittance remain stable. Which check is most appropriate?

  1. Return activated sludge rate
  2. Influent alkalinity
  3. Effluent BOD
  4. Lamp intensity and sleeve fouling Correct answer.

Explanation

If flow and transmittance are stable, the delivered UV dose may be falling because lamps have aged or quartz sleeves are fouled. Intensity monitoring and sleeve cleaning are the appropriate checks.

Question 44Process Control and Operational Troubleshooting

A tertiary sand filter has rapidly increasing head loss and short filter runs while flow is normal. Which cause is most likely?

  1. Low influent suspended solids
  2. High effluent pH
  3. Inadequate backwash or media fouling Correct answer.
  4. Excessive backwash duration

Explanation

Short runs with rapid head loss usually mean the bed is not being cleaned fully or the media is fouled. Backwash rate, duration, air scour, and media condition should be checked.

Question 45Process Control and Operational Troubleshooting

A biological phosphorus removal plant has increasing effluent phosphorus, and the anaerobic zone contains elevated nitrate. Which condition most likely explains the poor phosphorus release?

  1. Excessive wasting of phosphorus-rich sludge
  2. High influent readily biodegradable COD
  3. Nitrate entering the anaerobic zone Correct answer.
  4. Low dissolved oxygen in the aerobic zone

Explanation

Nitrate in the anaerobic zone allows denitrifiers to compete for readily biodegradable COD, and it raises the oxidation-reduction potential. Both effects suppress the anaerobic phosphorus release needed for EBPR.

Question 46Process Control and Operational Troubleshooting

Anaerobic digester supernatant with high ammonia is returned to a nitrifying aeration basin. Within a day, ammonia removal drops sharply; pH is 8.5, DO is 2.5 mg/L, and SRT is adequate. Which inhibition mechanism is most likely?

  1. Free ammonia toxicity to nitrifiers Correct answer.
  2. Chlorine residual in the supernatant
  3. Low alkalinity limitation
  4. Low dissolved oxygen in the basin

Explanation

At high pH, a larger fraction of ammonia is present as un-ionized free ammonia, which is inhibitory to nitrifiers. The combination of high ammonia load and pH 8.5 points to free ammonia toxicity rather than DO or SRT limits.

Question 47Process Control and Operational Troubleshooting

A publicly owned treatment works accepts landfill leachate with very high ammonia and low readily biodegradable COD. Effluent ammonia rises even though aeration capacity and SRT are adequate. Which action is most appropriate?

  1. Decrease the RAS flow
  2. Reduce the leachate feed rate and evaluate nitrification capacity Correct answer.
  3. Add ferric chloride to the aeration basin
  4. Increase the leachate feed rate to dilute ammonia

Explanation

High ammonia leachate adds a large nitrogen load that can exceed the nitrification capacity of the plant, especially when readily biodegradable COD is low. Reducing the leachate feed rate protects nitrification while capacity and alkalinity are evaluated.

Question 48Process Control and Operational Troubleshooting

A phenol spill enters an activated sludge aeration basin. Dissolved oxygen rises sharply despite unchanged airflow, and effluent BOD begins to increase. What is the most appropriate immediate action?

  1. Raise the MLSS by stopping wasting
  2. Increase the RAS flow to dilute the spill
  3. Add polymer to improve settling
  4. Divert or stop the influent and protect the biomass Correct answer.

Explanation

A toxic spill can inhibit oxygen uptake, so DO rises even though aeration is unchanged. The priority is to stop or divert the toxic flow, protect the biomass, and allow recovery before normal loading resumes.

Question 49Process Control and Operational Troubleshooting

A nitrifying activated sludge plant operates at 12°C with an SRT of 4 days. The nitrifier growth rate at 12°C is estimated at 0.25 per day. Using a safety factor of 2.0, what minimum SRT is required, and is the current SRT adequate?

  1. 2 days; adequate
  2. 4 days; adequate
  3. 8 days; inadequate Correct answer.
  4. 8 days; adequate

Explanation

The minimum SRT is the safety factor divided by the growth rate: 2.0 ÷ 0.25 = 8 days. The current 4-day SRT is below that minimum, so nitrification is not adequately supported.

Question 50Process Control and Operational Troubleshooting

A secondary clarifier has a sludge blanket that rises steadily despite maximum RAS flow. The MLSS is 4,000 mg/L and the clarifier solids loading rate is above its design value. Which interpretation is most appropriate?

  1. The blanket is rising because RAS flow is too high
  2. The clarifier is solids-overloaded, so reducing MLSS or influent solids is needed Correct answer.
  3. The clarifier is hydraulically overloaded, so increasing RAS will solve it
  4. The blanket is rising because the sludge is too dense to settle

Explanation

If the blanket rises while RAS is already at maximum and solids loading exceeds design, the clarifier is receiving more solids than it can thicken and return. Reducing MLSS through wasting or reducing solids load is the appropriate response.