Activated Sludge Exam

Free Sample Exam

Questions: 43

Passing standard: 24 / 43 (55.8%)

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Question 1 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

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Question 2 of 43

A plant plans to send stabilized, dewatered WAS to a different disposal facility. The current receiving facility accepts it, but acceptance by the proposed facility is unconfirmed. What must be established before changing destinations?

The material meets the previous destination's acceptance conditions for the new shipment.

The new destination and material meet the applicable acceptance and permit conditions.

The current cake solids percentage matches the plant's last accepted shipment.

The hauler can carry the existing cake mass using its normal transport equipment.

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Question 3 of 43

A RAS pump's rotation is independently confirmed. Its suction pressure has fallen and flow is low, while sludge level at the suction source remains normal. Which check best fits these findings?

Check increased delivered flow as the cause of the measured low flow.

Check whether the motor has stopped despite the confirmed rotation.

Check the suction valve and line for restricted pump supply.

Check low source sludge level despite the verified normal level.

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Question 4 of 43

An operator compares MLSS and MLVSS results while evaluating biomass condition. What does the MLVSS value most reasonably represent?

The fixed mineral residue of suspended mixed-liquor solids.

The viable cell fraction measured by counting individual organisms.

The dissolved organic fraction passing through the solids filter.

The volatile organic fraction of suspended mixed-liquor solids.

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Question 5 of 43

An activated sludge basin is receiving a higher organic load than normal. Which routine measurement most directly helps the operator determine whether aeration is supplying enough oxygen for aerobic treatment?

Solids concentration in the return-sludge line.

Sludge blanket depth in the secondary clarifier.

Oxygen demand in a laboratory BOD incubation.

Dissolved oxygen in the aeration basin.

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Question 6 of 43

An EBPR train has an anaerobic zone followed by an aerobic zone. Phosphorus-rich biomass is settling well, but its removal from the system has been suspended. What process step is missing for sustained net phosphorus removal?

Increase aerobic mixing while retaining the same total biomass.

Waste phosphorus-rich biomass at the required rate.

Return all phosphorus-rich biomass through RAS without wasting.

Increase internal recycle while retaining the same total biomass.

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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

250.2 lb/day

500.4 lb/day

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Question 8 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?

Inspect and adjust the gland to restore the manufacturer's specified leakage.

Retain the zero-leakage setting and use rising temperature as the acceptance criterion.

Tighten the gland further to remove the remaining friction at the packing.

Increase shaft speed to restore cooling without changing the gland adjustment.

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Question 9 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?

300 lb/day

400 lb/day

900 lb/day

510 lb/day

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Question 10 of 43

Net plant wastewater flow is 2.0 MGD. An aeration basin contains 1.0 MG and a secondary clarifier has 1,000 ft² of surface area. Using net plant flow for both nominal design calculations and excluding RAS from the HRT calculation, what are the aeration-basin HRT and clarifier surface overflow rate?

24 hours and 2,000 gpd/ft²

12 hours and 2,000 gpd/ft²

12 hours and 1,000 gpd/ft²

24 hours and 1,000 gpd/ft²

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Question 11 of 43

An activated sludge plant has a net wastewater flow of 2.0 MGD. MLSS entering the secondary clarifier is 3,000 mg/L and RAS is 9,000 mg/L. Assume steady state, negligible effluent solids, and negligible sludge wasting in this simplified balance. What RAS flow approximately satisfies the clarifier solids balance?

1.0 MGD

1.5 MGD

0.5 MGD

2.0 MGD

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Question 12 of 43

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

Fill, settle, decant, and then react on the discharged water.

Fill, decant mixed liquor, settle, and then react.

React, decant mixed liquor, fill, and then settle.

Fill, react, settle, and decant.

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Question 13 of 43

A basin inspection can be completed from a designated guarded platform. A closer unguarded edge would improve the view but is outside the approved access route. Which approach best controls the fall hazard?

Use the unguarded edge if another employee watches from the platform.

Use the unguarded edge if the inspection takes less than a minute.

Use the unguarded edge if the water surface is calm.

Use the guarded platform and the approved inspection method.

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Question 14 of 43

After an unusual industrial discharge, mixed-liquor oxygen uptake falls sharply. Measured DO, temperature, pH, biomass concentration, and biodegradable loading remain near their previous values. Which explanation deserves priority investigation?

An inhibitory constituent may have reduced microbial activity.

Lower biodegradable loading may have reduced demand despite the stable loading.

Lower biomass concentration may have reduced uptake despite the stable solids.

Reduced oxygen availability may have limited activity despite the stable DO.

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Question 15 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?

Grit separation, followed by thickening, followed by dewatering.

Thickening, followed by digestion, followed by dewatering.

Screening, followed by thickening, followed by dewatering.

Thickening, followed by polymer conditioning, followed by dewatering.

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Question 16 of 43

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

Microorganisms store nitrate unchanged in settled floc.

Microorganisms oxidize ammonia toward nitrate.

Microorganisms convert nitrogen gas to ammonia.

Microorganisms reduce nitrate toward nitrogen gas.

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Question 17 of 43

A motor runs in HAND. In AUTO, the verified sensor has reached the start threshold, but the controller's required permissive input remains false. Which check is most directly supported?

Replace the motor before checking the permissive that blocks the automatic command.

Treat successful HAND operation as proof that every AUTO permissive is satisfied.

Determine why the permissive is false before authorizing automatic operation.

Increase the start threshold to eliminate the false permissive input.

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Question 18 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?

10 mL/g

30 mL/g

100 mL/g

900 mL/g

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Question 19 of 43

Ammonia oxidation continues while alkalinity falls and pH approaches the lower operating limit. Which relationship explains why nitrification may become unstable?

Nitrification consumes alkalinity, reducing the buffer against further pH decline.

Nitrification produces alkalinity, making the declining buffer evidence of faster recovery.

Nitrification consumes nitrate, making the falling pH evidence of denitrification.

Nitrification increases buffering as ammonia loading rises, regardless of influent alkalinity.

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Question 20 of 43

A pump motor draws excess current on normal balanced voltage. After safe isolation, qualified personnel find the coupled shaft difficult to rotate; it turns freely when uncoupled from the pump. Which investigation has priority?

Check the pump and coupling for binding or abnormal mechanical load.

Check motor voltage imbalance as the primary cause despite balanced readings.

Check the motor alone as the cause despite its free uncoupled rotation.

Check a control-signal fault as the cause of the observed mechanical resistance.

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Question 21 of 43

Two suspended-growth systems treat comparable loads. System E operates at longer SRT and lower F/M than System C. Which description best fits System E as an extended-aeration process?

Less biomass per unit daily food load, with a shorter solids-age operating strategy.

Less biomass per unit daily food load, with biological solids removed before aeration.

More biomass per unit daily food load, with biomass retained exclusively on fixed media.

More biomass per unit daily food load, with greater opportunity for endogenous respiration.

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Question 22 of 43

Which statement BEST distinguishes primary sludge from waste activated sludge?

Primary sludge comes from bar screening; WAS comes from inorganic grit separation.

Primary sludge comes from tertiary filtration; WAS comes from final chlorine neutralization.

Primary sludge comes from biological growth; WAS comes from raw-water settling.

Primary sludge comes from raw-water settling; WAS comes from biological solids wasting.

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Question 23 of 43

A secondary clarifier is settling biological solids normally, but the aeration basin solids concentration begins to fall because too little settled sludge is being returned. Which flow should the operator evaluate first?

Internal mixed-liquor recycle flow to an anoxic zone.

Primary sludge flow withdrawn from the primary clarifier.

Waste activated sludge flow removed for solids processing.

Return activated sludge flow from the secondary clarifier.

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Question 24 of 43

A plant is evaluating diffuser replacement and wants to reduce blower energy while maintaining comparable process oxygen transfer. Under similar conditions, what advantage can fine-bubble diffusers offer over coarse-bubble diffusers?

More uniform basin DO establishes that blower discharge pressure will be unchanged.

Higher oxygen-transfer efficiency can reduce the air needed for the same transfer.

Lower liquid mixing demand removes the need to assess diffuser fouling.

Larger bubbles can increase the contact surface per unit volume of supplied air.

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Question 25 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?

An open air leak that reduces resistance in the blower discharge path.

A falling basin water level that reduces diffuser submergence pressure.

A restriction in the blower discharge piping, valves, or diffusers.

Reduced resistance from newly cleaned diffusers at the same blower speed.

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Question 26 of 43

A SCADA output command to a RAS valve is 100 percent. Independent inspection finds the stem at about 45 percent travel and confirms low RAS flow. Which conclusion is supported?

The command does not establish that the valve achieved the requested travel.

The command establishes that the local position indication is incorrect.

The stem position establishes that the RAS flow measurement is incorrect.

The low flow establishes that the valve achieved its fully open position.

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Question 27 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?

Shaft alignment, coupling installation, and assembly mounting.

A supply-voltage fault expected to affect the independently operated motor.

A pump impeller fault expected to appear during independent pump operation.

A motor winding fault expected to appear during independent motor operation.

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Question 28 of 43

A surface mechanical aerator keeps solids suspended, but measured DO remains below target after loading increases. What does this combination demonstrate?

Low DO establishes that every part of the basin has lost solids suspension.

Adequate mixing establishes that oxygen transfer meets the current demand.

Mixing can be adequate while oxygen transfer remains insufficient for demand.

Increased loading establishes that the aerator has stopped moving liquid.

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Question 29 of 43

Influent loading rises and verified basin DO falls below target. The blower has available capacity, and the operating plan permits a controlled airflow increase. Which response most directly addresses the oxygen deficit?

Increase airflow under the plan and monitor basin DO and treatment response.

Increase clarifier withdrawal as the initial adjustment while holding airflow unchanged.

Increase sludge wasting as the initial adjustment while holding airflow unchanged.

Increase RAS flow as the initial adjustment while holding airflow unchanged.

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Question 30 of 43

Effluent fecal-coliform results rise. Chlorine mass delivery per day is unchanged, but treated flow has increased 50 percent. Which check most directly evaluates a resulting disinfection change?

Calculate the actual dose per gallon and evaluate contact conditions.

Compare the commanded feed setting with its previous value alone.

Compare the motor run time with its previous value alone.

Compare the total delivered chemical mass with its previous value alone.

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Question 31 of 43

Ammonia in the aeration basin is decreasing while nitrate is increasing and adequate aerobic conditions are present. Which biological process best explains this trend?

Nitrification of ammonia to oxidized nitrogen forms.

Assimilation of nitrogen into new biomass without nitrate production.

Denitrification of nitrate to gaseous nitrogen forms.

Ammonia stripping from water into the gas phase.

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Question 32 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?

25 days

10 days

5 days

2.5 days

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Question 33 of 43

A liquid chemical feed pump indicates 12 gallons per hour, but the operator suspects the actual feed is different. What field check most directly verifies the pump's true output?

Infer volume delivered from one untimed tank-level observation.

Infer volume delivered from motor current during the same interval.

Infer volume delivered from the stroke setting during the same interval.

Measure volume delivered over a timed interval using the approved test.

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Question 34 of 43

Stable foam recurs after surface removal. Microscopy shows abundant branching filamentous organisms in the foam, and SRT has risen above the plant's target. Which investigation best uses these findings?

Evaluate solids-age control and the organisms associated with the foam.

Evaluate blower output as the cause before considering the observed organisms.

Evaluate screen cleaning as the cause before considering the solids-age trend.

Evaluate final disinfection feed as the cause before considering basin biology.

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Question 35 of 43

Aeration-basin mixing is weakening at the same time dissolved oxygen is dropping. Which diffused-aeration function is most directly involved in both symptoms?

Airflow replaces biomass wasting and controls system solids retention time.

Airflow returns settled sludge and controls clarifier underflow concentration.

Airflow provides both oxygen transfer and liquid circulation.

Airflow removes dissolved nutrients by direct physical gas filtration.

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Question 36 of 43

System solids inventory, basin volume, and net wastewater flow remain approximately constant while daily solids loss through wasting increases. Which comparison is correct?

SRT decreases, while nominal basin HRT also increases.

SRT decreases, while nominal basin HRT remains unchanged.

SRT increases, while nominal basin HRT remains unchanged.

SRT remains unchanged, while nominal basin HRT decreases.

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Question 37 of 43

Settled sludge volume rises from 240 to 450 mL/L while MLSS remains at 3,000 mg/L. What does the resulting SVI change most directly show?

The influent flow has increased in proportion to the sludge volume.

The settled sludge occupies less volume per gram of solids.

The mixed liquor contains more solids per liter at unchanged concentration.

The settled sludge occupies more volume per gram of solids.

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Question 38 of 43

A treatment plant relies on two aeration blowers, and loss of either unit would reduce process oxygen capacity. Which maintenance approach best reduces the risk of an unexpected blower outage?

Schedule repairs when loss of airflow first makes a blower unavailable.

Schedule replacement when protective trips permanently stop a blower.

Schedule bearing work after a bearing failure interrupts air delivery.

Schedule inspection and servicing at the specified intervals before failure.

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Question 39 of 43

During wet weather, flow through a secondary clarifier doubles. MLSS and settleability stay near their previous values, while effluent solids rise. What change most directly increases solids carryover risk?

The same surface area now provides twice the hydraulic detention time.

The unchanged settleability establishes a new filamentous-bulking episode.

The same surface area must handle twice the hydraulic flow.

The unchanged MLSS establishes twice the aeration-basin solids inventory.

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Question 40 of 43

SVI rises and settling becomes poor. Microscopy shows abundant filaments bridging between flocs rather than remaining within them. Which explanation is most directly supported?

Hydraulic overflow alone is producing the observed filaments between flocs.

Gas bubbles are producing the observed filament bridges between flocs.

Filament growth is producing an open floc structure that impairs compaction.

Increased mineral grit is producing the observed filament bridges between flocs.

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Question 41 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?

667 lb/day

13,344 lb/day

134 lb/day

1,334 lb/day

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Question 42 of 43

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

Infer the sludge interface from an oxygen profile through the clarifier.

Infer the sludge interface from MLSS and the 30-minute settled volume.

Infer the sludge interface from RAS flow and the design influent flow.

Locate the sludge interface with a blanket sampler or interface detector.

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Question 43 of 43

Secondary effluent still contains suspended material that must be reduced before discharge or reuse. What is the typical purpose of adding filtration after secondary clarification?

Stabilize waste biomass using the same mechanism as sludge digestion.

Oxidize dissolved ammonia using the same mechanism as particle filtration.

Remove dissolved salts using the same mechanism as conventional sand filtration.

Capture suspended particles remaining in clarified secondary effluent.

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