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Wastewater Treatment Plant Operator practice questions

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Question 1Preliminary and Primary Treatment (screening, grit removal, sedimentation)

What is the primary purpose of a bar screen at the headworks of a wastewater treatment plant?

  1. Remove dissolved organic matter that would otherwise carry over
  2. Disinfect the flow before it reaches the biological process
  3. Remove large solids that could damage or clog downstream equipment Correct answer.
  4. Separate grit and heavy sand from the incoming flow by gravity

Explanation

Screens are physical barriers that capture rags, sticks, plastics, and other large debris before the flow reaches pumps, grit chambers, and clarifiers. They protect equipment from clogging and damage but do not remove dissolved matter, disinfect, or serve as grit removal units.

Question 2Preliminary and Primary Treatment (screening, grit removal, sedimentation)

How are screenings removed from raw wastewater normally handled after collection?

  1. Discharged back to the collection system after being ground
  2. Returned to the head of the plant so the organics can be degraded
  3. Spread directly on land without any further treatment step
  4. Dewatered and then landfilled or incinerated with other solids Correct answer.

Explanation

Screenings are highly putrescible and are dewatered (often in a press or bagger) and then disposed of by landfill or incineration. Recycling them to the plant head or the sewer simply re-introduces the debris that the screen was installed to remove.

Question 3Preliminary and Primary Treatment (screening, grit removal, sedimentation)

Why is a grit removal unit installed ahead of primary sedimentation?

  1. To add oxygen and keep the raw wastewater fresh
  2. To reduce the organic load on the primary clarifier
  3. To protect pumps and digesters from abrasion and accumulation Correct answer.
  4. To raise the pH so that downstream settling improves

Explanation

Grit is dense, abrasive, inorganic material that wears pump impellers and accumulates in channels and digesters, reducing their capacity. Removing it early also keeps clarifier hoppers and sludge lines free of deposits.

Question 4Preliminary and Primary Treatment (screening, grit removal, sedimentation)

Primary sludge is best described as which of the following?

  1. The grit that settles out before the clarifier is reached
  2. The biological film that grows on clarifier walls and baffles
  3. The grease and oil skimmed from the clarifier surface
  4. The settled solids pumped from the bottom of a primary clarifier Correct answer.

Explanation

Primary sludge is the mass of raw settleable solids that accumulates on the clarifier floor and is drawn off from the sludge hopper. Floating grease and oil form scum and are removed separately by the skimmer.

Question 5Preliminary and Primary Treatment (screening, grit removal, sedimentation)

Which material is removed by the skimmer in a primary clarifier?

  1. Return activated sludge floc
  2. Settled grit and sand
  3. Dissolved nitrogen and phosphorus
  4. Grease, oil, and other floating matter Correct answer.

Explanation

Light material that rises to the surface, such as grease, oil, waxes, and plastics, forms a scum layer that the skimmer pushes into a scum trough or hopper. Settled solids are removed from the bottom by sludge collection equipment.

Question 6Preliminary and Primary Treatment (screening, grit removal, sedimentation)

A Parshall flume measures wastewater flow using which principle?

  1. The pressure drop across an orifice plate
  2. The speed of a propeller set in the channel
  3. The velocity of flow downstream of the throat
  4. The depth of water upstream of the throat Correct answer.

Explanation

In a Parshall flume the converging section accelerates the flow and creates a measurable depth at a fixed reference point; the upstream head is related to flow by a published rating equation. The flume has no moving parts and does not rely on orifice plates or propellers.

Question 7Preliminary and Primary Treatment (screening, grit removal, sedimentation)

Primary clarifiers are most commonly built in which configuration?

  1. Deep narrow hoppers with no appreciable surface area
  2. Rectangular or circular tanks with mechanical sludge collectors Correct answer.
  3. Earthen basins that have no scraping mechanism at all
  4. Sealed tanks that exclude all air from the headspace

Explanation

Primary clarifiers need a large surface area and a modest depth so that particles can settle, and they use chain-and-flight or rotating mechanisms to move sludge to a hopper. Sealed tanks and earthen basins without collection equipment cannot remove sludge reliably.

Question 8Preliminary and Primary Treatment (screening, grit removal, sedimentation)

What is the purpose of the bypass or overflow channel provided at a mechanically cleaned bar screen?

  1. Provide a metered route for measuring plant flow
  2. Store accumulated screenings until they are hauled away
  3. Carry flow around a blinded screen to prevent flooding Correct answer.
  4. Route flow to the grit chamber when the screen is dirty

Explanation

If the screen blinds and the water level upstream rises to a set point, flow passes over a bypass weir so the headworks and upstream sewer do not flood. The bypass is an emergency relief route, not a normal operating path.

Question 9Preliminary and Primary Treatment (screening, grit removal, sedimentation)

What horizontal velocity is typically maintained in a grit channel so that grit settles while organic solids stay in suspension?

  1. About 1 ft/s (0.3 m/s) Correct answer.
  2. About 0.1 ft/s (0.03 m/s)
  3. About 7 ft/s (2.1 m/s)
  4. About 15 ft/s (4.6 m/s)

Explanation

A horizontal velocity near 1 ft/s allows dense grit particles to settle while lighter organic material is carried through to the next process. Much lower velocities drop organics with the grit, and much higher velocities scour grit onward.

Question 10Preliminary and Primary Treatment (screening, grit removal, sedimentation)

A primary clarifier holds 120,000 gallons of wastewater. At a flow of 0.5 MGD, what is the detention time?

  1. 24.0 hours
  2. 11.5 hours
  3. 2.9 hours
  4. 5.8 hours Correct answer.

Explanation

Detention time equals tank volume divided by flow: 120,000 gal divided by 500,000 gal/day equals 0.24 day, which is about 5.8 hours. Typical primary clarifier detention times fall in the 1.5 to 2.5 hour range at design flow, so this value would be considered long.

Question 11Preliminary and Primary Treatment (screening, grit removal, sedimentation)

A primary clarifier receives 0.5 MGD and has a surface area of 1,000 square feet. What is the surface overflow rate?

  1. 50 gpd/ft²
  2. 250 gpd/ft²
  3. 1,000 gpd/ft²
  4. 500 gpd/ft² Correct answer.

Explanation

Surface overflow rate is flow divided by surface area: 500,000 gallons per day divided by 1,000 square feet equals 500 gpd/ft². This value is the settling velocity a particle must exceed to be removed in the tank.

Question 12Preliminary and Primary Treatment (screening, grit removal, sedimentation)

A clarifier receives 1.0 MGD and has 200 feet of effluent weir length. What is the weir loading rate?

  1. 5,000 gpd/ft Correct answer.
  2. 500 gpd/ft
  3. 2,500 gpd/ft
  4. 1,000 gpd/ft

Explanation

Weir loading rate equals flow divided by total weir length: 1,000,000 gallons per day divided by 200 feet equals 5,000 gpd/ft. High weir loading can pull solids over the weir and degrade effluent quality.

Question 13Preliminary and Primary Treatment (screening, grit removal, sedimentation)

Which description corresponds to Type I settling in a clarifier?

  1. Discrete particles settle individually and do not flocculate Correct answer.
  2. Particles settle as a concentrated blanket with a distinct interface
  3. Particles agglomerate as they fall and settle faster with depth
  4. Solids are compressed at the bottom under the weight above them

Explanation

Type I settling applies to discrete, nonflocculating particles whose size, shape, and density stay constant, so they settle at a constant terminal velocity. Flocculent, hindered, and compression settling describe the other three regimes and involve changing particle character or concentration.

Question 14Preliminary and Primary Treatment (screening, grit removal, sedimentation)

Why are inlet baffles installed in a primary clarifier?

  1. To increase the surface area available for settling
  2. To keep the sludge blanket in suspension near the outlet
  3. To dissipate energy and reduce short-circuiting of flow Correct answer.
  4. To add oxygen to the wastewater as it enters the tank

Explanation

Baffles spread the incoming flow across the tank and dissipate its velocity so that water does not travel directly to the outlet weir. This improves the effective detention time and prevents short-circuiting that would carry solids over the weir.

Question 15Preliminary and Primary Treatment (screening, grit removal, sedimentation)

A primary clarifier receives suspended solids at 300 mg/L and discharges effluent at 90 mg/L. What is the suspended solids removal efficiency?

  1. 60 percent
  2. 30 percent
  3. 90 percent
  4. 70 percent Correct answer.

Explanation

Removal efficiency is the influent minus effluent concentration divided by the influent concentration: (300 - 90) divided by 300 equals 0.70, or 70 percent. This is within the usual performance range for a well-operated primary clarifier.

Question 16Preliminary and Primary Treatment (screening, grit removal, sedimentation)

What fraction of influent BOD5 is typically removed by well-operated primary sedimentation alone?

  1. 5 to 10 percent
  2. 60 to 75 percent
  3. 25 to 40 percent Correct answer.
  4. 85 to 95 percent

Explanation

Primary clarifiers remove roughly 25 to 40 percent of BOD5, mostly the portion associated with settleable solids, along with 50 to 70 percent of suspended solids. Higher BOD removal requires biological treatment or chemical enhancement.

Question 17Preliminary and Primary Treatment (screening, grit removal, sedimentation)

What is the function of a grit classifier or grit washer?

  1. Grind grit into fine particles that pass on with the flow
  2. Wash organics from grit and dewater it for disposal Correct answer.
  3. Measure the volume of grit so hauling can be scheduled
  4. Add chemicals that increase the specific gravity of grit

Explanation

A classifier uses a slow screw or rake in a settling basin to rinse lighter organic material back to the liquid stream while dewatering the heavier grit for hauling. Cleaner grit reduces odors and disposal costs.

Question 18Preliminary and Primary Treatment (screening, grit removal, sedimentation)

How does a mechanically cleaned bar screen differ from a manually cleaned bar screen?

  1. It allows a much higher velocity through the bar openings
  2. It has no openings and relies on gravity to settle solids
  3. Its rake removes accumulated solids on a timed or headloss cycle Correct answer.
  4. It must be installed downstream of the primary clarifier

Explanation

On a mechanically cleaned unit, a rake automatically lifts screenings on a timer or when headloss reaches a set point, so the screen stays open without operator attention. Manually cleaned screens require an operator to pull the debris by hand.

Question 19Preliminary and Primary Treatment (screening, grit removal, sedimentation)

What is the role of the air supply in an aerated grit chamber?

  1. Raises the dissolved oxygen level so that grit is oxidized and floats away
  2. Cools the wastewater so that grease congeals and can be settled out
  3. Rolls the flow so light organics stay suspended while grit settles Correct answer.
  4. Grinds the grit into finer particles that are carried on to the clarifier

Explanation

Air diffusers along one side create a rolling, spiral flow pattern that keeps lighter organic material in suspension while dense grit drops into the hopper below. The air also helps keep the wastewater fresh and helps strip some volatiles.

Question 20Preliminary and Primary Treatment (screening, grit removal, sedimentation)

How does a vortex-type grit chamber separate grit from wastewater?

  1. By bubbling air through the flow to float grit to the surface
  2. By spinning the flow in a conical hopper so grit migrates to the center and settles Correct answer.
  3. By passing the flow over inclined plates that trap grit
  4. By spinning the flow in a drum and scraping grit off the walls

Explanation

A rotating propeller or tangential inlet creates a vortex in a conical basin; the swirling flow throws denser grit toward the center and downward into the hopper while organics are carried out with the flow. The unit is compact and has a short detention time.

Question 21Preliminary and Primary Treatment (screening, grit removal, sedimentation)

Why should grit removed at the headworks be relatively free of organic matter?

  1. Organics dissolve and return to the flow, raising the plant BOD
  2. Organics make the grit too light to settle in the grit chamber
  3. Organics in grit are putrescible and cause odors and handling problems Correct answer.
  4. Organics increase the abrasiveness of grit and damage the classifier

Explanation

Organic material in the grit stream decomposes, creating odors, attracting vectors, and complicating landfill disposal. Washed grit is largely inert mineral material that can be handled and disposed of with far less nuisance.

Question 22Preliminary and Primary Treatment (screening, grit removal, sedimentation)

What is the purpose of adding a coagulant such as alum ahead of a primary clarifier?

  1. Raise the pH so that downstream nitrification is favored
  2. Strip dissolved gases from the flow and control odors
  3. Kill pathogenic organisms before the disinfection step
  4. Coagulate fine and colloidal solids so more solids and phosphorus are removed Correct answer.

Explanation

Chemically enhanced primary treatment neutralizes the surface charge on colloidal particles so they aggregate into settlable floc, increasing TSS, BOD, and phosphorus removal. The result approaches what a secondary clarifier alone would achieve, at the cost of added sludge and chemical handling.

Question 23Preliminary and Primary Treatment (screening, grit removal, sedimentation)

If primary sludge is held too long in a clarifier hopper, why can the sludge blanket rise to the surface?

  1. The scraper compacts the solids into a floating raft
  2. The sludge becomes denser than the surrounding water
  3. Decomposition produces gas bubbles that buoy the solids Correct answer.
  4. Incoming flow reverses direction and lifts the solids

Explanation

Prolonged storage allows anaerobic decomposition and denitrification to generate gas bubbles that become trapped in the sludge, reducing its effective density. The gasified solids then float, carrying solids over the weir and degrading effluent quality.

Question 24Preliminary and Primary Treatment (screening, grit removal, sedimentation)

What happens to the headloss through a bar screen when rags and debris accumulate on the bars?

  1. It decreases because the openings become smoother
  2. It decreases until the rake is operated automatically
  3. It increases, raising the upstream water level Correct answer.
  4. It stays the same because flow is held constant

Explanation

As the openings blind, the available flow area shrinks and resistance rises, so the water level upstream of the screen climbs. That rising level is often used to trigger the automatic rake and, at a higher set point, the bypass.

Question 25Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

In the activated sludge process, the food-to-microorganism (F/M) ratio expresses which of the following?

  1. BOD applied per unit mass of microorganisms in the aeration tank per day Correct answer.
  2. Air supplied per unit volume of wastewater treated
  3. Return sludge flow divided by influent flow
  4. Solids leaving the plant per unit mass of BOD applied

Explanation

The F/M ratio compares the organic food entering the aeration tank with the mass of organisms available to consume it, usually as pounds of BOD per pound of MLVSS per day. It is a key control parameter because it links loading, biomass inventory, and sludge production.

Question 26Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

What is the primary purpose of returning activated sludge (RAS) from the secondary clarifier to the aeration tank?

  1. To disinfect the secondary effluent
  2. To maintain a concentrated population of microorganisms in the aeration tank Correct answer.
  3. To add alkalinity to the mixed liquor
  4. To waste excess microorganisms from the system

Explanation

RAS returns settled biological solids to the aeration basin so that a high, controlled population of organisms stays in contact with the incoming food. Without RAS, organisms would be flushed out faster than they could reproduce.

Question 27Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

A facultative lagoon is best described as a pond that:

  1. is sealed and used only for dewatering digested solids
  2. is designed strictly for chemical phosphorus precipitation
  3. has an aerobic upper zone and an anaerobic bottom zone Correct answer.
  4. is kept fully aerobic at all depths by mechanical diffusers

Explanation

Facultative ponds stratify biologically: algae and surface mixing keep the upper layer aerobic, while settled solids decompose anaerobically on the bottom. The two zones coexist and exchange intermediates such as organic acids and gases.

Question 28Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

Which group of bacteria oxidizes ammonia to nitrite in the activated sludge process?

  1. Nitrosomonas Correct answer.
  2. Zoogloea
  3. Nitrobacter
  4. Acinetobacter

Explanation

Ammonia-oxidizing bacteria such as Nitrosomonas convert ammonia to nitrite, and nitrite-oxidizing bacteria such as Nitrobacter then convert nitrite to nitrate. Both steps together make up nitrification.

Question 29Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

What dissolved oxygen level is typically targeted in the aerobic zone of an activated sludge aeration tank?

  1. 1 to 2 mg/L Correct answer.
  2. 0.1 to 0.3 mg/L
  3. 6 to 8 mg/L
  4. Zero, with the basin kept fully anaerobic

Explanation

A residual of about 1 to 2 mg/L is normally maintained so that aerobic organisms, including nitrifiers, are not oxygen-limited. Higher residuals waste blower energy without improving treatment, and near-zero levels favor filaments and odors.

Question 30Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

What is the primary purpose of the media in a trickling filter?

  1. Straining suspended solids from the flow like a mechanical screen
  2. Providing surface area for attached microorganisms to grow Correct answer.
  3. Adding alkalinity to the wastewater as it flows down the filter
  4. Cooling the wastewater before it reaches the secondary clarifier

Explanation

Trickling filter media give attached organisms a large surface on which to form a biological film that adsorbs and stabilizes organic matter. The media also provide void space for air movement and for the film to slough when it becomes too thick.

Question 31Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

An oxidation ditch is best described as which type of process?

  1. A chemical precipitation process
  2. A high-rate trickling filtration process
  3. An anaerobic digestion process
  4. An extended aeration activated sludge process Correct answer.

Explanation

Oxidation ditches are looped channels with brush or disc aerators that circulate mixed liquor, giving long detention and low organic loading characteristic of extended aeration. The long solids retention time supports nitrification and reduces excess sludge.

Question 32Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

Sludge age, or mean cell residence time (MCRT), is defined as the:

  1. time the sludge spends in the secondary clarifier
  2. average time the solids remain in the treatment system Correct answer.
  3. duration of the 30-minute settleability test
  4. detention time of the wastewater in the aeration tank

Explanation

MCRT is the mass of solids held in the system divided by the mass of solids leaving the system each day, which equals the average residence time of a solid particle. It is controlled mainly by the waste sludge rate.

Question 33Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

The sludge volume index (SVI) is reported as the:

  1. cubic feet of air delivered per gallon of mixed liquor in the basin
  2. milliliters of settled sludge per gram of solids after 30 minutes Correct answer.
  3. percentage of influent suspended solids captured by the clarifier
  4. kilograms of BOD applied per kilogram of mixed liquor solids per day

Explanation

SVI is computed by dividing the settled sludge volume in milliliters per liter by the mixed liquor suspended solids concentration in grams per milliliter after a 30-minute settle. Typical well-settling sludge falls roughly in the 80 to 150 range.

Question 34Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

Which condition most commonly promotes filamentous bulking in an activated sludge system?

  1. Maintaining dissolved oxygen above 4 mg/L in the basin
  2. Persistently low dissolved oxygen in the aeration basin Correct answer.
  3. Operating at a very high return sludge rate
  4. Feeding polymer continuously to the secondary clarifier

Explanation

Filamentous organisms outcompete floc formers when oxygen is scarce, because their long filaments reach oxygen more efficiently. Low DO, low F/M, nutrient deficiency, and septic or high-sulfide influents are classic bulking causes.

Question 35Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

An activated sludge plant operated at a 2-day solids retention time fails to nitrify. What is the most likely reason?

  1. Nitrifiers grow slowly and are washed out of the system Correct answer.
  2. A short SRT strips too much alkalinity from the mixed liquor
  3. Nitrifiers require a strictly anaerobic environment to grow
  4. A short SRT causes the clarifier sludge blanket to rise

Explanation

Nitrifying bacteria reproduce much more slowly than heterotrophs, so a system that wastes solids aggressively removes them faster than they can grow. Nitrification generally requires a solids retention time well above the minimum generation time, often 8 to 15 days depending on temperature.

Question 36Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

Approximately how much alkalinity is consumed per milligram of ammonia nitrogen oxidized to nitrate?

  1. About 7.1 mg of alkalinity as CaCO3 Correct answer.
  2. No alkalinity is consumed; nitrification actually produces alkalinity
  3. About 1.5 mg of alkalinity as CaCO3
  4. About 0.5 mg of alkalinity as CaCO3

Explanation

Nitrification consumes roughly 7.14 mg of alkalinity as CaCO3 for each milligram of ammonia nitrogen fully oxidized, which is why some plants must add alkalinity to sustain it. Municipal wastewater often has just enough alkalinity to nitrify only part of its ammonia.

Question 37Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

What condition is required in the anoxic zone for biological denitrification to occur?

  1. Free dissolved oxygen above 2 mg/L throughout the zone
  2. A free chlorine residual of at least 0.5 mg/L
  3. Nitrate present with a biodegradable carbon source and no free dissolved oxygen Correct answer.
  4. High dissolved oxygen with no available carbon

Explanation

Denitrifiers use nitrate as the electron acceptor in the absence of free oxygen and need an degradable carbon source as the electron donor. If oxygen is present, they prefer it and nitrate reduction stops.

Question 38Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

In a biological nutrient removal plant, how do phosphate-accumulating organisms behave in the anaerobic and aerobic zones?

  1. They require a zone with neither oxygen nor nitrate to take up phosphate
  2. They take up phosphate anaerobically and release it aerobically
  3. They remove phosphate only when nitrifying bacteria are absent
  4. They release phosphate anaerobically and take up phosphate aerobically Correct answer.

Explanation

Phosphate-accumulating organisms store carbon anaerobically while releasing phosphate, then use that stored carbon aerobically to take up more phosphate than they released. Waste sludge removal is what actually takes phosphorus out of the plant.

Question 39Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

The sludge blanket in a secondary clarifier is rising to the surface with visible gas bubbles while the effluent remains fairly clear. What is the most likely cause?

  1. A sludge blanket that is too shallow
  2. A polymer overdose causing over-flocculation
  3. Denitrification occurring in the sludge blanket Correct answer.
  4. Filamentous bulking from a low F/M ratio

Explanation

When nitrate-rich sludge sits long enough in a clarifier, denitrifiers consume the nitrate and release nitrogen gas bubbles that lift solids to the surface. Shortening the blanket residence time, increasing return rates, or reducing nitrate entering the clarifier controls it.

Question 40Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

What most commonly causes ponding on the surface of a trickling filter?

  1. Biological growth that plugs the void spaces in the media Correct answer.
  2. Dissolved oxygen above 4 mg/L in the filter flow
  3. Hydraulic loading that is too low with cold wastewater
  4. Recirculation at a ratio above 1:1

Explanation

Excessive or uneven biological growth, often with high organic loading or insufficient sloughing, fills the media voids so water cannot drain. Restoring flow, reducing loading, or flushing the media with high hydraulic doses corrects the condition.

Question 41Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

In a facultative lagoon, dissolved oxygen is normally lowest just before sunrise because:

  1. bacteria stop respiring when the sun goes down
  2. photosynthesis intensifies after dark
  3. nitrifying bacteria consume oxygen only in darkness
  4. algae consume oxygen through respiration at night without producing any Correct answer.

Explanation

During daylight, algae produce oxygen by photosynthesis faster than the pond consumes it. At night photosynthesis stops while algae, bacteria, and sediment continue to respire, so dissolved oxygen declines until dawn.

Question 42Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

An aeration tank holds 1.0 million gallons at an MLSS of 2,500 mg/L. Waste sludge is drawn at 0.05 MGD with a concentration of 8,000 mg/L, and solids in the effluent are negligible. What is the approximate mean cell residence time?

  1. About 2.5 days
  2. About 6.3 days Correct answer.
  3. About 0.16 days
  4. About 16 days

Explanation

Solids in the system are 2,500 × 8.34 × 1.0 = 20,850 lb, and solids wasted daily are 8,000 × 8.34 × 0.05 = 3,336 lb/day. Dividing gives about 6.3 days, which is a typical MCRT range for nitrifying activated sludge.

Question 43Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

A secondary clarifier with a 60-foot diameter receives 1.5 MGD. What is the approximate surface overflow rate?

  1. About 265 gpd per square foot
  2. About 530 gpd per square foot Correct answer.
  3. About 2,100 gpd per square foot
  4. About 1,060 gpd per square foot

Explanation

The surface area is π × 30² ≈ 2,827 square feet, and 1,500,000 gpd divided by 2,827 gives about 530 gpd per square foot. Overflow rate is checked against design standards because excessive values carry solids over the weirs.

Question 44Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

If the solids retention time of an activated sludge plant is increased while organic loading stays the same, what generally happens?

  1. Sludge production is unchanged but effluent BOD rises sharply
  2. Less sludge is wasted and the F/M ratio falls Correct answer.
  3. More sludge must be wasted and the F/M ratio rises
  4. The F/M ratio rises and oxygen demand increases

Explanation

A longer SRT builds a larger biomass inventory for the same food supply, which lowers the F/M ratio. Because more of the substrate is used for maintenance rather than growth, net sludge production per pound of BOD drops.

Question 45Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

In a modified Ludzack-Ettinger nutrient removal train, what is the purpose of the internal mixed liquor recycle?

  1. To carry nitrate-rich mixed liquor back to the anoxic tank for denitrification Correct answer.
  2. To waste excess solids from the aeration tank
  3. To return settled sludge from the clarifier to the aeration tank
  4. To feed supplemental carbon into the aerobic zone

Explanation

The internal recycle moves nitrified mixed liquor from the aerobic zone to the anoxic zone, supplying the nitrate that denitrifiers reduce to nitrogen gas. Without it, nitrate would pass to the clarifier and out with the effluent.

Question 46Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

A secondary clarifier produces a turbid effluent made up of fine particles that will not settle, and the SVI is unusually low. Which condition does this describe?

  1. Pin floc from overaeration or low organic loading Correct answer.
  2. Denitrification in the sludge blanket
  3. Filamentous bulking
  4. A sludge blanket that is too high

Explanation

Pin floc consists of small, weakly aggregating particles that settle poorly even though the sludge is dense and has a low SVI. It is associated with low organic loading, long sludge age, and excessive aeration, and it produces a cloudy effluent.

Question 47Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

What effect does recycled nitrate entering the anaerobic zone of a biological phosphorus removal plant have?

  1. It has no effect because phosphate-accumulating organisms do not respond to nitrate
  2. It increases phosphorus removal by supplying oxygen to the organisms
  3. It prevents phosphate release and reduces phosphorus removal Correct answer.
  4. It makes the anaerobic zone septic and produces hydrogen sulfide

Explanation

Nitrate carried into the anaerobic zone is used as an electron acceptor, allowing organisms to take up carbon without releasing phosphate. The anaerobic selector then fails to give phosphate-accumulating organisms the advantage they need, and phosphorus removal deteriorates.

Question 48Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

Why does a trickling filter usually need a lower organic loading rate to nitrify than to remove BOD?

  1. Nitrifiers grow slowly and need longer contact in a low-rate filter Correct answer.
  2. Nitrification occurs only at temperatures above 40 degrees Celsius
  3. Nitrification is inhibited whenever oxygen is present
  4. Nitrifiers require a completely anaerobic biofilm

Explanation

Nitrifying organisms grow more slowly than the heterotrophs that remove BOD, so they are crowded out in the heavily loaded upper portions of a filter. Lower organic loading and greater depth or recirculation give them the time and oxygen they need.

Question 49Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

A stable, dark brown foam persists on the aeration basin surface and the sludge has a high SVI. This is most often associated with which condition?

  1. An influent pH that stays below 5
  2. A long sludge age combined with a high influent load of fats, oils, and grease Correct answer.
  3. Routine chlorination of the return sludge
  4. A very low sludge age and a high F/M ratio

Explanation

Nocardia-type filamentous organisms are hydrophobic and produce stable brown foam, and they proliferate with long sludge ages, low F/M, and abundant fats, oils, and grease. Control relies on reducing the foam-causing substrate, adjusting sludge age, or selective wasting.

Question 50Secondary (Biological) Treatment (activated sludge, trickling filters, lagoons, nutrient removal)

Which group of microorganisms is primarily responsible for converting ammonia to nitrite in the activated sludge process?

  1. Heterotrophic floc formers such as Zoogloea
  2. Ammonia oxidizers such as Nitrosomonas Correct answer.
  3. Filamentous bacteria such as Microthrix
  4. Sulfur bacteria such as Beggiatoa

Explanation

Nitrification is a two-step autotrophic process in which ammonia-oxidizing bacteria such as Nitrosomonas convert ammonia to nitrite, and nitrite-oxidizing bacteria then convert nitrite to nitrate. These organisms use inorganic carbon and obtain energy from the oxidation reaction, and they grow slowly compared with heterotrophs.