Activated Sludge Exam

Free Sample Exam

Questions: 43

Passing score: 24 / 43 (55.8%)

Answer every question, then submit the exam to see your result and any areas that may need additional study.

Question 1 of 43

What is a primary function of a diffused aeration system in an activated sludge process?

To dewater waste activated sludge.

To supply oxygen and help mix the contents of the aeration basin.

To screen large debris from raw wastewater.

To remove settled sludge from the bottom of the secondary clarifier.

Report this question (opens in a new tab)

Question 2 of 43

Compared with coarse-bubble diffusers under similar conditions, what is a common advantage of fine-bubble diffusers?

They remove all suspended solids directly from the aeration basin.

They cannot become fouled or restricted.

They generally provide greater oxygen transfer efficiency.

They eliminate the need for blowers.

Report this question (opens in a new tab)

Question 3 of 43

Shortly after an unusual industrial discharge enters the plant, oxygen uptake by the mixed liquor drops sharply even though dissolved oxygen and aeration equipment appear adequate. What should the operator suspect?

The organic loading must have decreased normally and the timing of the unusual industrial discharge is unrelated.

The biological population is necessarily becoming more active because adequate dissolved oxygen remains available.

The unusual influent may be inhibiting or toxic to the activated sludge microorganisms.

A secondary clarifier solids-separation problem is the most likely explanation for the sudden decrease in mixed-liquor oxygen uptake.

Report this question (opens in a new tab)

Question 4 of 43

Waste activated sludge has been stabilized and dewatered. What should determine how the resulting material is ultimately used or disposed?

Only the percent solids of the dewatered cake, because sufficiently dry material is automatically acceptable for any disposal option.

Only transportation cost and available hauling capacity, provided the sludge has already been dewatered.

Only whether stabilization has reduced odors, because odor control determines the permitted final use.

Applicable regulatory requirements, permit conditions, and the facility solids-management procedures.

Report this question (opens in a new tab)

Question 5 of 43

What is the most direct way to verify and calibrate the actual output of a liquid chemical metering pump?

Use the pump percentage setting alone as the actual feed rate without measuring delivered volume.

Perform a timed volumetric drawdown or collection test and compare the measured output with the indicated feed rate.

Measure only the chemical storage-tank level once per month and assume short-term pump output is correct.

Increase the pump stroke until process results change and use that response as the calibration.

Report this question (opens in a new tab)

Question 6 of 43

Effluent fecal coliform results are trending upward toward the permit limit while the disinfection system is operating. What is the BEST process-control response?

Increase disinfection to the maximum possible setting without checking flow, equipment performance, or the analytical result.

Reduce disinfection because increasing fecal coliform indicates that the existing treatment intensity is too high.

Make no evaluation until a permit limit has already been exceeded.

Verify the result and evaluate the disinfection equipment and operating conditions, then make appropriate adjustments according to the permit and facility procedure.

Report this question (opens in a new tab)

Question 7 of 43

A plant receives 1.5 MGD containing 20 mg/L ammonia nitrogen. Approximately how many pounds of ammonia nitrogen enter the plant each day? Use pounds/day = mg/L × MGD × 8.34.

125.1 lb/day

25.0 lb/day

500.4 lb/day

250.2 lb/day

Report this question (opens in a new tab)

Question 8 of 43

Why is dissolved oxygen routinely monitored in an activated sludge aeration basin?

To calculate chlorine residual in the final effluent.

To determine whether sufficient oxygen is available for aerobic biological treatment.

To determine the exact sludge blanket depth in the secondary clarifier.

To measure the amount of grit entering the plant.

Report this question (opens in a new tab)

Question 9 of 43

A plant treats 2.0 MGD through an aeration basin with a volume of 1.0 MG and a secondary clarifier with 1,000 square feet of surface area. What are the approximate aeration-basin hydraulic retention time and clarifier surface overflow rate?

24 hours and 1,000 gpd/ft²

12 hours and 1,000 gpd/ft²

12 hours and 2,000 gpd/ft²

24 hours and 2,000 gpd/ft²

Report this question (opens in a new tab)

Question 10 of 43

A treatment process requires 60 pounds per day of active chemical. The chemical product is 15 percent active ingredient by weight. Approximately how many pounds of chemical solution must be fed per day?

400 lb/day

900 lb/day

300 lb/day

510 lb/day

Report this question (opens in a new tab)

Question 11 of 43

A plant receives 2.0 MGD with 200 mg/L BOD. The aeration basin contains 1.0 MG at 2,500 mg/L MLVSS. Using 8.34 as the conversion factor, what is the approximate F/M ratio?

0.16 lb BOD/lb MLVSS-day

1.6 lb BOD/lb MLVSS-day

0.016 lb BOD/lb MLVSS-day

6.25 lb BOD/lb MLVSS-day

Report this question (opens in a new tab)

Question 12 of 43

The activated sludge has poor settling, SVI has increased substantially, and the settled sludge appears bulky. What information would be especially useful when investigating possible filamentous bulking?

Influent flow and BOD loading alone, without examining the mixed-liquor biological population.

Secondary clarifier detention time alone, because filamentous bulking can be identified from hydraulic data without examining the sludge.

Microscopic examination of the activated sludge together with relevant process data.

SVI and sludge blanket measurements alone, because they can confirm the presence and type of filamentous organisms without microscopy.

Report this question (opens in a new tab)

Question 13 of 43

Why is scheduled preventive maintenance important for activated sludge aeration blowers?

It eliminates the need for dissolved oxygen measurements.

It is needed only after a blower has completely failed.

It helps maintain reliable air delivery and reduces the risk of unexpected blower failure.

It guarantees that the activated sludge process will never require monitoring.

Report this question (opens in a new tab)

Question 14 of 43

What are two important functions that a mechanical aerator can provide in an activated sludge basin?

Screenings removal and grit classification.

Laboratory calibration and sample preservation.

Sludge dewatering and final effluent filtration.

Oxygen transfer and mixing of the basin contents.

Report this question (opens in a new tab)

Question 15 of 43

An activated sludge process develops a much higher SVI than its normal historical range. What does this MOST directly indicate?

The sludge has become more voluminous and its settling characteristics may have deteriorated.

The influent flow has necessarily stopped.

The aeration basin contains no microorganisms.

A higher SVI always means better settling.

Report this question (opens in a new tab)

Question 16 of 43

Which method provides the most direct field measurement of sludge blanket depth in a secondary clarifier?

Calculate the blanket depth from SVI alone without making a measurement in the clarifier.

Use an aeration-basin dissolved oxygen probe to locate the sludge interface.

Use an appropriate sludge-blanket sampler or interface detector and determine the sludge interface relative to the known clarifier depth.

Use the RAS flow percentage as the blanket depth regardless of actual clarifier conditions.

Report this question (opens in a new tab)

Question 17 of 43

SCADA shows a return-sludge control valve at 100 percent open, but the local position indicator shows about 45 percent open and measured RAS flow is low. What is the BEST response?

Assume the SCADA indication is correct and increase unrelated process settings until RAS flow rises.

Increase the valve command above 100 percent to compensate for the low measured flow.

Change the SCADA display to 45 percent so it agrees with the local indicator without investigating the equipment.

Verify the actual valve position and RAS flow and investigate the actuator, feedback signal, and control system before relying on the SCADA indication.

Report this question (opens in a new tab)

Question 18 of 43

An activated sludge system contains approximately 25,000 pounds of suspended solids. Approximately 2,500 pounds of solids leave the system each day through wasting and the effluent. What is the approximate solids retention time?

2.5 days

5 days

25 days

10 days

Report this question (opens in a new tab)

Question 19 of 43

During a major wet-weather event, secondary clarifier flow increases sharply and effluent suspended solids begin to rise even though mixed liquor settleability has not changed significantly. What is a likely concern?

A loss of nitrification alone is the most likely cause of the immediate increase in effluent suspended solids.

High hydraulic loading may be causing solids carryover from the secondary clarifier.

Filamentous bulking is the most likely cause even though settleability has not changed significantly.

The RAS concentration must have become too high, regardless of the sharp increase in clarifier flow.

Report this question (opens in a new tab)

Question 20 of 43

What does MLVSS generally represent when evaluating an activated sludge process?

The volume of return activated sludge pumped each day.

The amount of dissolved oxygen in the mixed liquor.

The volatile portion of mixed liquor solids, commonly used as an estimate of the biological and organic fraction.

Only the inorganic grit present in the aeration basin.

Report this question (opens in a new tab)

Question 21 of 43

After a motor has been reinstalled and coupled to a pump, the unit develops abnormal vibration even though the motor and pump each appeared satisfactory before coupling. What should be checked before continued operation?

Increase the coupling tightness until the vibration decreases without checking shaft alignment.

Increase motor speed to determine whether the shafts align themselves during operation.

Assume alignment is correct because both pieces of equipment appeared satisfactory before they were coupled.

The alignment between the motor shaft and the pump shaft, along with the coupling and mounting condition.

Report this question (opens in a new tab)

Question 22 of 43

A plant is nitrifying successfully, but alkalinity is steadily decreasing and pH is approaching the lower end of the process operating range. Why is this important?

The decrease in alkalinity indicates denitrification is consuming alkalinity faster than nitrification can produce it.

Continued alkalinity consumption can allow pH to fall and eventually inhibit nitrification.

The falling pH should improve nitrification because nitrifying organisms perform better as buffering capacity decreases.

Alkalinity is important only for chemical phosphorus precipitation and does not affect biological ammonia oxidation.

Report this question (opens in a new tab)

Question 23 of 43

A return activated sludge pump is indicated as running, but the measured RAS flow is much lower than normal. What is the BEST response?

Assume the RAS flow must be normal because the pump RUN indicator is on.

Immediately stop all return sludge pumping without investigating.

Verify actual pump and system conditions and investigate why the expected RAS flow is not being produced.

Increase waste sludge flow to compensate without checking the RAS system.

Report this question (opens in a new tab)

Question 24 of 43

During biological denitrification, what happens to nitrate under appropriate anoxic conditions?

Ammonia is oxidized to nitrate as the principal denitrification reaction.

Nitrate is biologically reduced and ultimately converted to nitrogen gas.

Nitrate is removed only by incorporation into new biomass and is not converted to a gaseous form.

Nitrate is retained unchanged while dissolved oxygen is converted to nitrogen gas.

Report this question (opens in a new tab)

Question 25 of 43

When inspecting an open aeration basin, what is an important general safety practice?

Use designated access areas and follow the facility fall-protection and personal protective equipment requirements.

Remove protective barriers because they make process observation more difficult.

Climb over guardrails whenever a closer view is needed.

Work alone at the basin edge regardless of facility procedures.

Report this question (opens in a new tab)

Question 26 of 43

A 30-minute settled sludge volume is 300 mL/L and the MLSS concentration is 3,000 mg/L. What is the sludge volume index, or SVI?

900 mL/g

100 mL/g

10 mL/g

30 mL/g

Report this question (opens in a new tab)

Question 27 of 43

Which sequence best describes the basic operation of a sequencing batch reactor, or SBR?

Decant continuously while filling and mixing are permanently stopped.

Screen, incinerate, chlorinate, and landfill.

Settle, pump raw sludge, screen, and aerate once per month.

Fill, react, settle, and decant.

Report this question (opens in a new tab)

Question 28 of 43

Which statement BEST distinguishes primary sludge from waste activated sludge?

Primary sludge is mainly settled raw-wastewater solids, while waste activated sludge is primarily biological solids removed from the activated sludge process.

Primary sludge and waste activated sludge are identical materials produced at the same point in the treatment process.

Waste activated sludge consists mainly of inorganic grit removed before biological treatment.

Primary sludge is generated only by aeration, while waste activated sludge is removed only by screening.

Report this question (opens in a new tab)

Question 29 of 43

A plant wastes 20,000 gallons per day of sludge containing 0.8 percent solids. Assuming the sludge weighs approximately 8.34 pounds per gallon, about how many pounds of dry solids are wasted per day?

1,334 lb/day

134 lb/day

667 lb/day

13,344 lb/day

Report this question (opens in a new tab)

Question 30 of 43

What happens to ammonia nitrogen during successful nitrification?

It is biologically oxidized to nitrite and then primarily to nitrate.

It is removed only by primary clarification.

It is converted into suspended sand.

It is converted directly into grit.

Report this question (opens in a new tab)

Question 31 of 43

What is the primary purpose of return activated sludge, or RAS?

To add raw wastewater directly to the final effluent.

To return settled biological solids from the secondary clarifier to the aeration basin.

To permanently remove excess solids from the treatment system.

To remove grit from the influent channel.

Report this question (opens in a new tab)

Question 32 of 43

Which sequence BEST represents the functions of a typical sludge-treatment train when the goals are to reduce volume, stabilize biodegradable solids, and then produce a dewatered cake?

Dewatering, followed by screening, followed by return activated sludge pumping.

Aeration-basin mixing, followed by primary clarification, followed by screening.

Disinfection, followed by grit removal, followed by thickening.

Thickening, followed by stabilization or digestion, followed by dewatering.

Report this question (opens in a new tab)

Question 33 of 43

A pump motor is receiving normal voltage, but its amperage draw has risen significantly above its normal operating value while pump performance deteriorates. What should the operator investigate?

Reduce the recorded amperage value because normal voltage proves that the motor cannot be overloaded.

A developing mechanical or hydraulic overload, binding condition, bearing problem, or pump obstruction.

Increase the overload setting so the higher current becomes the new normal operating condition.

Assume the increased current indicates improved pump efficiency and continue operating without investigation.

Report this question (opens in a new tab)

Question 34 of 43

A packed pump stuffing box has been tightened until visible leakage stops, and the packing area begins to overheat. What is the BEST maintenance response?

Increase motor speed so additional shaft movement cools the overheated packing.

Continue operating until the packing wears enough to begin leaking again.

Inspect the packing and adjust the gland according to the manufacturer's procedure so the required controlled leakage and cooling are maintained.

Tighten the gland further because zero leakage is always the correct operating condition for packed stuffing boxes.

Report this question (opens in a new tab)

Question 35 of 43

Which operating sequence BEST supports enhanced biological phosphorus removal in an activated sludge process?

Maintain high dissolved oxygen in every zone so phosphorus is never released by the biomass.

Eliminate sludge wasting so all phosphorus accumulated in the biomass remains permanently in the treatment system.

Maintain only an anoxic zone with nitrate present and avoid subsequent aerobic treatment.

Provide an anaerobic zone followed by an aerobic zone and remove phosphorus-rich biomass through sludge wasting.

Report this question (opens in a new tab)

Question 36 of 43

An activated sludge plant treats 2.0 MGD. The MLSS concentration is 3,000 mg/L and the RAS concentration is 9,000 mg/L. Assuming steady-state conditions and negligible effluent solids, what RAS flow approximately satisfies a secondary-clarifier solids balance?

2.0 MGD

1.0 MGD

1.5 MGD

0.5 MGD

Report this question (opens in a new tab)

Question 37 of 43

A blower is operating normally, but discharge pressure is higher than usual and airflow to the aeration basin has decreased. What should the operator investigate?

A low secondary clarifier sludge blanket, because clarifier blanket depth directly controls blower discharge pressure.

Insufficient blower speed only, so the blower speed should be increased before checking the downstream air system for restrictions.

A higher biological oxygen demand alone, because increased process oxygen demand directly causes blower discharge pressure to rise while airflow falls.

Possible restrictions such as fouled diffusers, restricted air piping, or incorrect valve positions.

Report this question (opens in a new tab)

Question 38 of 43

If an operator increases the waste activated sludge rate while other conditions remain approximately constant, what will generally happen to solids retention time?

Solids retention time will always double.

Solids retention time will generally decrease.

Solids retention time will become infinite.

Solids retention time is unrelated to sludge wasting.

Report this question (opens in a new tab)

Question 39 of 43

A persistent, stable foam develops on an aeration basin and repeatedly returns after being removed from the surface. What is the BEST approach?

Increase RAS flow to the maximum possible rate without investigating the cause.

Assume the foam can only be caused by excessive aeration and make no other evaluation.

Stop collecting all process data until the foam disappears.

Evaluate biological and operating conditions such as sludge age, loading, dissolved oxygen, and microscopic observations.

Report this question (opens in a new tab)

Question 40 of 43

Which statement best describes a typical characteristic of an extended-aeration activated sludge process?

It generally uses a very short solids retention time and high F/M ratio to minimize aeration time.

It intentionally maintains a short sludge age to prevent endogenous respiration and biological solids stabilization.

It normally operates without return activated sludge because biomass is retained only on fixed media.

It generally uses a relatively long solids retention time and low F/M ratio, with significant aeration requirements.

Report this question (opens in a new tab)

Question 41 of 43

Dissolved oxygen in an aeration basin falls well below its normal operating range while influent loading has increased. What is the BEST operator response?

Increase sludge wasting without checking aeration or loading conditions.

Immediately stop all aeration because influent loading increased.

Verify the condition, evaluate oxygen demand and aeration capacity, and adjust aeration as appropriate.

Ignore the low dissolved oxygen because loading has no effect on oxygen demand.

Report this question (opens in a new tab)

Question 42 of 43

A wastewater plant adds filtration after secondary clarification. What is a typical purpose of this tertiary treatment step?

To increase the solids concentration entering the secondary clarifier.

To provide additional removal of suspended material before final discharge or further treatment.

To replace all biological treatment with screening.

To return all final effluent directly to the aeration basin.

Report this question (opens in a new tab)

Question 43 of 43

An equipment motor starts and runs normally in HAND mode but does not start in AUTO mode when the process condition calls for operation. What should be investigated?

The AUTO interlocks should be bypassed permanently so the motor responds whenever a start command is expected.

The motor windings only, because normal HAND operation proves the automatic control circuit is operating correctly.

The automatic command, process sensor, control logic, interlocks, and associated control circuitry.

The pump or driven equipment must be replaced before the automatic control signal is checked.

Report this question (opens in a new tab)