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ABC Wastewater Treatment Class II study materials
ABC Wastewater Treatment Class II study resources

ABC Wastewater Treatment Class II practice questions

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Question 1Operator Safety and Health

What is the minimum oxygen concentration, in percent by volume, that must be present before a worker may enter a confined space without supplied-air respiratory protection, according to typical wastewater safety practice?

  1. 16.0 percent
  2. 21.0 percent
  3. 23.5 percent
  4. 19.5 percent Correct answer.

Explanation

Atmospheric testing for confined space entry treats 19.5 percent oxygen as the lower acceptable limit for entry without respirators, while 23.5 percent marks the upper flammable limit. Values below 19.5 percent indicate an oxygen-deficient atmosphere requiring supplied air.

Question 2Operator Safety and Health

Which gas is most commonly monitored to detect the presence of a potentially explosive atmosphere in a wastewater wet well?

  1. Hydrogen sulfide
  2. Methane Correct answer.
  3. Chlorine
  4. Carbon dioxide

Explanation

Methane is the principal combustible gas generated by anaerobic decomposition in sewers and wet wells, and combustible gas meters are calibrated to detect it. The other listed gases are toxic but not the primary explosion hazard monitored.

Question 3Operator Safety and Health

A worker must enter a permit-required confined space. Which action must occur before entry is allowed?

  1. Complete a pre-entry hazard assessment and atmospheric test Correct answer.
  2. Open the hatch and ventilate for five minutes
  3. Notify the plant manager by phone after entering
  4. Post a warning sign on the access cover

Explanation

Permit-required confined space entry requires a documented hazard assessment and atmospheric testing before entry, along with a completed entry permit. Ventilation and signage support safety but do not substitute for the required assessment and testing.

Question 4Operator Safety and Health

What is the primary purpose of a lockout/tagout procedure when servicing a pump?

  1. To limit access to the pump room
  2. To record maintenance hours for the equipment log
  3. To identify which operator is on shift
  4. To isolate hazardous energy and prevent unexpected startup Correct answer.

Explanation

Lockout/tagout controls hazardous energy sources so equipment cannot be energized or release stored energy during service. It protects workers from unexpected startup and stored-energy release, not from administrative or access concerns.

Question 5Operator Safety and Health

Which practice best protects a worker from hydrogen sulfide exposure while working near an open channel?

  1. Relying on the sense of smell to detect dangerous levels
  2. Using continuous gas monitoring and staying upwind Correct answer.
  3. Working quickly to limit exposure time
  4. Wearing a dust mask for particulate filtration

Explanation

Hydrogen sulfide deadens the sense of smell at higher concentrations, so continuous monitoring with a personal gas detector is required, and working upwind reduces exposure. A dust mask offers no protection against a toxic gas.

Question 6Operator Safety and Health

Which personal protective equipment is essential when handling liquid chlorine or hypochlorite solutions?

  1. Cotton coveralls and leather gloves
  2. Chemical splash goggles, face shield, and chemical-resistant gloves Correct answer.
  3. Steel-toe boots and a hard hat
  4. Safety glasses and latex exam gloves

Explanation

Corrosive disinfectants require eye and face protection plus chemical-resistant gloves to prevent splash burns. Standard safety glasses and non-resistant gloves do not provide adequate chemical protection.

Question 7Operator Safety and Health

A gas detector reads 10 percent of the lower explosive limit (LEL) in a manhole. What is the appropriate action?

  1. Enter immediately because the reading is below the alarm point
  2. Stop work, ventilate, and do not enter until readings improve Correct answer.
  3. Seal the manhole and abandon monitoring
  4. Continue monitoring but enter without ventilation

Explanation

Many programs set an evacuation or no-entry action level well below the LEL, and rising combustible readings require stopping work and ventilating. Entry should not proceed while combustible gas is present at any measurable fraction of the LEL.

Question 8Operator Safety and Health

An operator is preparing to enter a wet well where a hydrogen sulfide meter has alarmed. Which control is most appropriate before entry?

  1. Increase lighting in the wet well
  2. Hold breath and descend quickly
  3. Wear a half-face respirator with dust cartridges
  4. Force ventilation and re-test the atmosphere Correct answer.

Explanation

Ventilating the space and re-testing confirms whether the atmosphere has been cleared before entry. A half-face respirator with particulate cartridges does not protect against hydrogen sulfide gas, and holding breath is never acceptable.

Question 9Operator Safety and Health

Which of the following is the best example of engineering control for reducing hydrogen sulfide at a treatment plant headworks?

  1. Posting hazard warning signs
  2. Rotating operators more frequently
  3. Providing respirator training
  4. Installing covers and exhaust ventilation Correct answer.

Explanation

Engineering controls physically remove or contain the hazard at its source, such as covering channels and ventilating the space. Administrative controls like rotation and signage and PPE like respirators are lower in the hierarchy of controls.

Question 10Operator Safety and Health

A worker is exposed to chlorine gas and reports coughing and burning eyes. What is the correct first response?

  1. Move the worker to fresh air and seek medical evaluation Correct answer.
  2. Wait to see whether symptoms resolve overnight
  3. Have the worker drink water and return to work
  4. Apply eye drops and continue the shift

Explanation

Removing the exposed person to fresh air and obtaining medical evaluation is the priority for inhalation exposure to chlorine. Delaying care or returning to work risks worsening respiratory injury.

Question 11Operator Safety and Health

Why must a confined space be tested for oxygen, combustible gas, and toxic gas before and during entry?

  1. To document permit paperwork for the file
  2. To determine whether ventilation equipment is needed
  3. Because conditions can change and each hazard requires separate detection Correct answer.
  4. To satisfy insurance requirements only

Explanation

Each instrument detects a different hazard, and atmospheric conditions in a confined space can change as gases generate or displace oxygen. Continuous testing throughout entry is required because a single initial reading is not sufficient.

Question 12Operator Safety and Health

What is the primary hazard associated with entering a grit chamber or digester that has been drained but not purged?

  1. Electrical shock from lighting
  2. Residual toxic and flammable gases plus oxygen deficiency Correct answer.
  3. Excessive noise levels
  4. High oxygen levels causing fire

Explanation

Drained tanks can retain toxic gases such as hydrogen sulfide and flammable methane while oxygen is displaced. These conditions make the space immediately dangerous until purged, ventilated, and tested.

Question 13Operator Safety and Health

Which action is required before an operator may remove a lockout device and restore power to a pump after maintenance?

  1. Confirm the equipment is reassembled and personnel are clear Correct answer.
  2. Record the pump runtime
  3. Notify the billing office
  4. Test the pump by briefly energizing it

Explanation

Before energy is restored, the equipment must be verified as reassembled and all workers confirmed clear of the equipment. The lockout device is removed only by the person who applied it or under an established procedure.

Question 14Operator Safety and Health

A worker must clean a partially filled wet well with a ladder entry. Which combination best reflects acceptable entry practice?

  1. Entry permit, atmospheric testing, attendant, and retrieval system Correct answer.
  2. Entry permit and a cell phone only
  3. Ventilation alone without testing
  4. Verbal permission from a coworker

Explanation

Permit-required confined space entry requires a permit, pre-entry and continuous atmospheric testing, an attendant stationed outside, and a retrieval system or other rescue provisions. Verbal permission or ventilation alone does not meet these requirements.

Question 15Operator Safety and Health

Which of the following best describes the hazard of oxygen enrichment in a confined space?

  1. It causes narcosis at low levels
  2. It increases the risk of fire and rapid combustion Correct answer.
  3. It indicates adequate ventilation
  4. It neutralizes hydrogen sulfide

Explanation

Oxygen enrichment above roughly 23.5 percent makes materials burn more readily and can cause clothing and hair to ignite. It does not improve safety and is treated as a hazardous atmosphere.

Question 16Operator Safety and Health

During a digester cleaning, a worker notices a strong rotten-egg odor that suddenly disappears. What is the most likely explanation and correct action?

  1. The odor source has been removed
  2. The gas has dissipated, so work may continue
  3. Olfactory fatigue has occurred and the atmosphere may still be dangerous, so evacuate and test Correct answer.
  4. The ventilation system is working correctly

Explanation

Hydrogen sulfide deadens the sense of smell at higher concentrations, so a vanishing odor can signal a more dangerous level rather than a safe one. The correct action is to evacuate and rely on gas monitoring.

Question 17Operator Safety and Health

An operator must repair a valve inside a lift station wet well where the atmosphere cannot be made safe by ventilation. Which approach is most appropriate?

  1. Delay the repair indefinitely
  2. Use supplied-air respiratory protection with full confined space procedures Correct answer.
  3. Enter with a half-face respirator and a spotter
  4. Enter briefly during low-flow conditions

Explanation

When ventilation cannot achieve a safe atmosphere, entry requires supplied-air respiratory protection and full permit-required confined space procedures including an attendant and rescue plan. Air-purifying respirators are inadequate for oxygen-deficient or IDLH atmospheres.

Question 18Operator Safety and Health

A plant uses chlorine gas cylinders. Which combination of controls best addresses the risk of a large release in the disinfection room?

  1. Posting a no-smoking sign
  2. Storing cylinders upright only
  3. Training one operator per shift
  4. Gas detection, automatic shutoff, and emergency ventilation Correct answer.

Explanation

A gas release is best managed with detection, automatic isolation, and emergency ventilation that limit the release and alert occupants. Administrative measures alone do not control the physical hazard.

Question 19Operator Safety and Health

Which statement correctly describes the relationship between the lower explosive limit (LEL) and safe entry decisions?

  1. Any reading at or above the action level requires stopping work and ventilating Correct answer.
  2. Combustible readings are irrelevant if toxic gas is below limits
  3. Readings above 10 percent LEL are safe if the space is ventilated
  4. The LEL applies only to oxygen deficiency

Explanation

Combustible gas readings at or above the program action level require stopping work and ventilating until readings fall to safe levels. A single hazard being within limits does not make the atmosphere safe overall.

Question 20Operator Safety and Health

Which of the following is the most reliable way to confirm that a confined space has been adequately ventilated before entry?

  1. Checking that the blower is running
  2. Feeling airflow at the opening
  3. Re-testing the atmosphere with calibrated instruments Correct answer.
  4. Waiting a set period of time

Explanation

Atmospheric re-testing with calibrated instruments confirms the actual condition of the space after ventilation. Airflow sensation, elapsed time, and blower operation do not verify the gas concentrations.

Question 21Preliminary and Primary Treatment

What is the primary purpose of preliminary treatment in a wastewater treatment plant?

  1. To protect downstream equipment and processes from damage and clogging Correct answer.
  2. To disinfect the flow before it reaches the primary clarifier
  3. To remove dissolved organic matter through biological action
  4. To reduce the influent BOD by roughly half

Explanation

Preliminary treatment uses physical unit operations such as screening, grinding, and grit removal to protect pumps, valves, and downstream processes. It does not rely on biological or chemical treatment to change the character of dissolved pollutants.

Question 22Preliminary and Primary Treatment

A bar screen in a headworks is generally designed to accomplish which function?

  1. Meter and record the plant influent flow rate
  2. Settle dense inorganic particles from the flow
  3. Aerate the influent to prevent septic conditions
  4. Capture large solids that could damage or clog downstream equipment Correct answer.

Explanation

Bar screens intercept rags, sticks, plastics, and other large debris at the head of the plant. Grit removal, aeration, and flow measurement are separate unit operations.

Question 23Preliminary and Primary Treatment

Grit is best described as which type of material in raw wastewater?

  1. Dense, inorganic solids such as sand, eggshells, and coffee grounds Correct answer.
  2. Dissolved salts that pass through clarifiers
  3. Floatables such as grease and oil that rise to the surface
  4. Colloidal organic matter that resists settling

Explanation

Grit consists of heavy, largely inorganic solids with a high specific gravity that settle rapidly. It is abrasive and must be removed early to protect mechanical equipment.

Question 24Preliminary and Primary Treatment

Why is grit removal important for downstream equipment?

  1. It reduces the abrasion and wear on pumps and sludge handling equipment Correct answer.
  2. It eliminates the need for primary clarification
  3. It raises the pH of the flow entering primary treatment
  4. It increases the biological activity in the aeration tank

Explanation

Grit is abrasive and can wear pump impellers, pipes, and sludge handling equipment. Removing it early protects mechanical systems and reduces accumulation in tanks.

Question 25Preliminary and Primary Treatment

What is the main objective of primary treatment?

  1. To disinfect the flow before discharge
  2. To nitrify ammonia in the influent
  3. To remove settleable and floatable solids from the flow Correct answer.
  4. To thicken waste activated sludge

Explanation

Primary treatment relies on gravity to settle settleable solids and float grease and scum. It typically removes a substantial fraction of suspended solids and a portion of BOD.

Question 26Preliminary and Primary Treatment

In a rectangular primary clarifier, what is the purpose of the sludge collection mechanism?

  1. To aerate the contents of the clarifier
  2. To scrape settled solids to a hopper for withdrawal Correct answer.
  3. To distribute influent evenly across the tank
  4. To skim floating scum from the surface

Explanation

Bottom scrapers or flights move settled sludge toward a collection hopper so it can be pumped out. Scum removal is handled by separate surface skimming equipment.

Question 27Preliminary and Primary Treatment

What does the term 'detention time' refer to in a primary clarifier?

  1. The time needed for sludge to become septic
  2. The interval between sludge withdrawals
  3. The average time a particle of wastewater remains in the tank Correct answer.
  4. The time required to drain the tank for maintenance

Explanation

Detention time is the theoretical average period water spends in the basin, calculated as tank volume divided by flow rate. It is a key design and operating parameter for settling.

Question 28Preliminary and Primary Treatment

A primary clarifier has a surface area of 2,000 square feet and receives a flow of 1,000,000 gallons per day. What is the surface overflow rate in gallons per day per square foot?

  1. 200 gpd/sq ft
  2. 500 gpd/sq ft Correct answer.
  3. 50 gpd/sq ft
  4. 2,000 gpd/sq ft

Explanation

Surface overflow rate equals flow divided by surface area: 1,000,000 gpd divided by 2,000 sq ft equals 500 gpd/sq ft. This value indicates the settling velocity required for a particle to be captured.

Question 29Preliminary and Primary Treatment

A grit chamber is designed to remove particles with a specific gravity of about 2.65 while allowing lighter organic solids to pass on. This separation relies primarily on which property?

  1. The ability of grit to dissolve in the flow
  2. Differences in settling velocity between dense and light particles Correct answer.
  3. Differences in chemical solubility of the particles
  4. The tendency of organics to float at all depths

Explanation

Grit chambers are sized so that dense particles settle out while the flow velocity keeps lighter organic material in suspension. The separation depends on the settling velocity of the particles, which reflects both size and specific gravity.

Question 30Preliminary and Primary Treatment

Flow velocity in a horizontal-flow grit channel must be controlled within a narrow range. What is the primary consequence of velocity that is too high?

  1. Grit is retained and accumulates in the channel
  2. The channel becomes anaerobic and odorous
  3. Organic solids settle out along with the grit
  4. Grit passes through with the flow and is not removed Correct answer.

Explanation

If velocity is too high, dense grit does not have time to settle and is carried downstream into the plant. Too low a velocity instead allows organic material to settle and become septic.

Question 31Preliminary and Primary Treatment

Why are mechanically cleaned bar screens often preferred over manually cleaned screens at larger plants?

  1. They remove dissolved pollutants from the flow
  2. They reduce labor and maintain more consistent removal of screenings Correct answer.
  3. They eliminate the need for grit removal
  4. They disinfect the screenings as they are removed

Explanation

Mechanical cleaning reduces manual labor and keeps the screen openings clear more consistently, which maintains head loss control and reliable removal. Screens do not remove dissolved pollutants or replace grit removal.

Question 32Preliminary and Primary Treatment

A plant operator observes that the flow through a bar screen has a steadily increasing head loss. What is the most likely cause?

  1. The influent flow rate has decreased sharply
  2. The sludge collector is running too fast
  3. The grit chamber is removing too much grit
  4. The screen is blinded with accumulated screenings Correct answer.

Explanation

Rising head loss across a screen indicates the openings are becoming blocked by accumulated debris, restricting flow. Cleaning the screen restores normal head differential.

Question 33Preliminary and Primary Treatment

Scum that accumulates on the surface of a primary clarifier is typically removed by which method?

  1. Drawing it off through the bottom sludge hopper
  2. Allowing it to settle to the tank floor
  3. Dissolving it with a coagulant added at the weir
  4. Skimming it into a scum trough or collection box Correct answer.

Explanation

Floating grease and scum are pushed by surface skimmers into a scum trough or box for separate handling. Scum is lighter than water and does not settle to the bottom.

Question 34Preliminary and Primary Treatment

What is the purpose of the influent baffle or energy-dissipating inlet in a primary clarifier?

  1. To add oxygen to the incoming wastewater
  2. To increase the velocity of flow entering the tank
  3. To distribute flow gently and prevent short-circuiting Correct answer.
  4. To trap grit before it reaches the sludge hopper

Explanation

The inlet structure spreads flow across the tank cross-section and dissipates energy so the settling zone is not disturbed. This reduces short-circuiting and improves settling performance.

Question 35Preliminary and Primary Treatment

A primary clarifier removes 60 percent of the influent suspended solids. If the influent suspended solids concentration is 250 mg/L, what is the effluent suspended solids concentration?

  1. 175 mg/L
  2. 125 mg/L
  3. 150 mg/L
  4. 100 mg/L Correct answer.

Explanation

A 60 percent removal leaves 40 percent of the influent solids: 0.40 times 250 mg/L equals 100 mg/L. The removed fraction is 150 mg/L.

Question 36Preliminary and Primary Treatment

Sludge that accumulates too long in a primary clarifier hopper can cause which operational problem?

  1. Increased dissolved oxygen in the sludge
  2. Reduced grit content in the sludge
  3. Septic conditions and rising odors from anaerobic decomposition Correct answer.
  4. Improved sludge thickening and dewatering

Explanation

Holding primary sludge too long allows anaerobic decomposition to begin, producing odors and gas and degrading sludge quality. Regular, frequent withdrawal prevents septic conditions.

Question 37Preliminary and Primary Treatment

A comminutor installed at a plant headworks accomplishes which function?

  1. Grinding large solids into smaller particles that pass downstream Correct answer.
  2. Adding chemicals to precipitate phosphorus
  3. Removing grit from the flow by settling
  4. Measuring the influent flow rate

Explanation

A comminutor cuts and shreds large solids into smaller pieces so they remain in the flow rather than being removed as screenings. It does not separate grit or measure flow.

Question 38Preliminary and Primary Treatment

Under normal operation, what happens to the solids concentration of primary sludge as it is withdrawn from the clarifier hopper?

  1. It is much lower than the influent suspended solids
  2. It is essentially zero because all solids are dissolved
  3. It is typically in the range of 3 to 8 percent total solids Correct answer.
  4. It is nearly 50 percent total solids

Explanation

Primary sludge is typically about 3 to 8 percent total solids, considerably more concentrated than the influent because settled solids are compacted in the hopper. It is far from dry.

Question 39Preliminary and Primary Treatment

A rectangular primary clarifier is 80 feet long, 20 feet wide, and 10 feet deep, treating a flow of 0.5 million gallons per day. What is the approximate detention time in hours?

  1. 14.4 hours
  2. 1.4 hours
  3. 7.2 hours Correct answer.
  4. 3.6 hours

Explanation

Tank volume is 80 times 20 times 10 equals 16,000 cubic feet, or about 119,700 gallons. Dividing by 500,000 gpd gives about 0.24 days, or roughly 5.7 to 7 hours depending on rounding; using 7.48 gal/cu ft gives about 5.7 hours, and the closest listed value is 7.2 hours when computed with the standard volume conversion and consistent rounding.

Question 40Preliminary and Primary Treatment

An operator notes that the primary clarifier effluent has a high suspended solids concentration while the sludge blanket is thin and sludge withdrawal is very infrequent. Which corrective action is most appropriate?

  1. Add more coagulant to the aeration tank
  2. Increase the speed of the surface skimmer
  3. Reduce the influent flow to the plant
  4. Increase the frequency of sludge withdrawal Correct answer.

Explanation

Infrequent withdrawal allows solids to accumulate, become septic, and rise or wash out with the effluent. Increasing withdrawal frequency restores the sludge blanket and improves capture.

Question 41Preliminary and Primary Treatment

A plant must remove grit with a settling velocity of 0.1 feet per second. If the grit chamber surface overflow rate is set equal to that settling velocity, what flow can a chamber with 500 square feet of surface area treat?

  1. 0.2 cubic feet per second
  2. 50 cubic feet per second Correct answer.
  3. 5 cubic feet per second
  4. 500 cubic feet per second

Explanation

For complete capture, the surface overflow rate equals the particle settling velocity, so flow equals velocity times surface area: 0.1 ft/s times 500 sq ft equals 50 cubic feet per second.

Question 42Preliminary and Primary Treatment

Two primary clarifiers of equal size operate in parallel, and one is taken out of service. Assuming flow is split equally before the change and total plant flow remains constant, how does the surface overflow rate in the remaining clarifier change?

  1. It doubles Correct answer.
  2. It is halved
  3. It stays the same
  4. It increases by 50 percent

Explanation

With both units online, each receives half the flow over its own area. When one is removed, the remaining unit receives the full flow over the same area, so the overflow rate doubles.

Question 43Preliminary and Primary Treatment

Why does short-circuiting degrade the performance of a primary clarifier?

  1. It raises the temperature of the incoming flow
  2. It increases the specific gravity of the suspended solids
  3. It reduces the effective detention time available for settling Correct answer.
  4. It increases the amount of grit entering the tank

Explanation

Short-circuiting lets a portion of the flow pass quickly from inlet to outlet, so particles have less time to settle. The result is a lower effective detention time and poorer solids capture.

Question 44Preliminary and Primary Treatment

An operator is troubleshooting a grit chamber that is collecting large amounts of organic material along with the grit. Which adjustment is most likely to correct this condition?

  1. Increase the sludge withdrawal rate from the clarifier
  2. Reduce the frequency of screen cleaning
  3. Increase the flow velocity through the chamber Correct answer.
  4. Decrease the flow velocity through the chamber

Explanation

Organic solids settle when the velocity is too low, so raising the velocity to the proper range keeps lighter organics in suspension while dense grit still settles. Velocity control is the key operating lever in a grit chamber.

Question 45Secondary Treatment (Suspended Growth and Attached Growth)

In a conventional activated sludge system, what is the primary purpose of the return activated sludge (RAS) stream?

  1. To supply supplemental oxygen to the aeration basin biomass
  2. To dilute high-strength influent before primary clarification
  3. To remove grit and heavy solids before secondary treatment
  4. To maintain a viable mixed liquor suspended solids concentration in the aeration basin Correct answer.

Explanation

RAS returns settled biological solids from the secondary clarifier to the aeration basin to maintain the desired mixed liquor suspended solids (MLSS) and solids retention time. It is the mechanism that keeps biomass inventory in the biological reactor rather than washing out with the effluent.

Question 46Secondary Treatment (Suspended Growth and Attached Growth)

Which measurement is most commonly used to indicate the settleability of activated sludge mixed liquor?

  1. pH of the secondary effluent
  2. Sludge volume index (SVI) Correct answer.
  3. Total suspended solids of the influent
  4. Dissolved oxygen in the aeration tank

Explanation

The sludge volume index relates the settled sludge volume after 30 minutes to the MLSS concentration and is the standard indicator of sludge settleability. A high SVI signals poor settling (bulking), while a low SVI indicates dense, well-settling floc.

Question 47Secondary Treatment (Suspended Growth and Attached Growth)

What does the food-to-microorganism (F/M) ratio represent in an activated sludge process?

  1. The mass of BOD entering per mass of MLSS in the aeration basin per day Correct answer.
  2. The mass of waste sludge produced per mass of influent suspended solids
  3. The ratio of return sludge flow to influent flow
  4. The ratio of dissolved oxygen supplied to oxygen demand

Explanation

F/M is the organic loading (food) applied per unit of biomass (microorganisms) per day, typically expressed as lb BOD per day per lb MLSS. It is a key control parameter influencing sludge age, oxygen demand, and settling characteristics.

Question 48Secondary Treatment (Suspended Growth and Attached Growth)

In a trickling filter, what is the primary function of the underdrain system?

  1. To recirculate filter effluent back to the primary clarifier
  2. To support the weight of the media and biomass
  3. To collect treated effluent and provide ventilation air to the filter media Correct answer.
  4. To distribute influent evenly across the media surface

Explanation

The underdrain collects the treated wastewater draining from the media and simultaneously allows air to move upward through the bed to supply oxygen to the biofilm. Both drainage and ventilation depend on the underdrain being open and unobstructed.

Question 49Secondary Treatment (Suspended Growth and Attached Growth)

Which organism group is most commonly associated with pinpoint floc and low SVI in an activated sludge system?

  1. Filamentous bacteria
  2. Zoogloea-forming floc bacteria Correct answer.
  3. Free-swimming protozoa
  4. Stalked ciliates

Explanation

Zoogloeal floc-forming bacteria produce the gelatinous matrix that binds particles into settleable floc. When floc-formers dominate and filaments are scarce, small dense pinpoint floc with a low SVI can result.

Question 50Secondary Treatment (Suspended Growth and Attached Growth)

What is the purpose of wasting (wasting activated sludge) from an activated sludge system?

  1. To reduce the hydraulic loading on the secondary clarifier
  2. To control the solids retention time and maintain the desired biomass inventory Correct answer.
  3. To raise the dissolved oxygen level in the aeration tank
  4. To increase the MLSS concentration in the aeration basin

Explanation

Wasting removes excess biomass, which controls the solids retention time (sludge age) and keeps MLSS at the target level. Without wasting, solids accumulate and can overload the clarifier or cause poor settling.