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NE Water/Wastewater Operator practice questions

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Question 1Drinking Water Treatment

Which chemical is the most widely used primary coagulant in drinking water treatment?

  1. Calcium hydroxide slurry
  2. Sodium silicofluoride
  3. Sodium hypochlorite solution
  4. Aluminum sulfate (alum) Correct answer.

Explanation

Alum is the traditional coagulant: it reacts with natural alkalinity to form a gelatinous aluminum hydroxide floc that sweeps and adsorbs fine particles. The other listed chemicals serve disinfection, pH/softening, or fluoridation duties.

Question 2Drinking Water Treatment

What is the purpose of the flocculation step in a conventional surface water plant?

  1. To disinfect the water with chlorine before it reaches the filters
  2. To remove turbidity by passing water through graded media
  3. To strip dissolved gases such as carbon dioxide from the water
  4. To gently stir the water so small particles clump together Correct answer.

Explanation

Flocculation is slow, gentle mixing after rapid mix that encourages microscopic particles and microfloc to collide and grow into larger, settleable flocs. Rapid mixing disperses the coagulant; flocculation builds floc size.

Question 3Drinking Water Treatment

Turbidity in drinking water is measured and reported in which units?

  1. Parts per billion
  2. Nephelometric turbidity units Correct answer.
  3. Milligrams per liter
  4. Microsiemens per centimeter (conductivity units)

Explanation

Turbidity is an optical measurement of light scattered by suspended particles and is expressed in nephelometric turbidity units (NTU). Milligrams per liter and parts per billion describe mass concentration, not light scatter.

Question 4Drinking Water Treatment

What is the main purpose of backwashing a granular media filter?

  1. To increase the surface loading rate on the filter
  2. To add a coagulant coating to the media grains
  3. To sanitize the media with a strong chlorine solution
  4. To flush captured solids out of the media bed Correct answer.

Explanation

Backwashing reverses flow upward through the bed, expanding it so that accumulated solids are scoured loose and carried to waste. This restores head loss and keeps the filter producing acceptable turbidity.

Question 5Drinking Water Treatment

Why is a chlorine residual maintained in the distribution system?

  1. To keep disinfecting the water throughout the piping network Correct answer.
  2. To raise the pH and thereby reduce corrosion of distribution piping
  3. To prevent trihalomethane formation inside storage tanks
  4. To oxidize dissolved iron and manganese in the finished water

Explanation

A residual protects water quality after treatment by inactivating microorganisms that may enter through cross-connections, main breaks, or storage. It is a regulatory requirement in distribution systems for this reason.

Question 6Drinking Water Treatment

Fluoride is added to drinking water primarily to accomplish which result?

  1. Improve the taste of stored water
  2. Inhibit growth of algae in reservoirs
  3. Reduce the incidence of dental caries Correct answer.
  4. Control corrosion of lead service lines

Explanation

Community water fluoridation adjusts the natural fluoride level to the optimum for the climate so that tooth enamel becomes more resistant to decay. The target concentration is typically around 0.7 mg/L.

Question 7Drinking Water Treatment

Which chemical is commonly fed to raise the pH of finished water?

  1. Lime (calcium hydroxide) Correct answer.
  2. Carbon dioxide
  3. Aluminum sulfate
  4. Sodium chloride

Explanation

Lime is an alkaline material that consumes carbon dioxide and raises pH, and it is widely used for stabilization and softening. Carbon dioxide moves pH in the opposite direction.

Question 8Drinking Water Treatment

In a conventional sedimentation basin, particles are removed mainly by which mechanism?

  1. Oxidation by dissolved oxygen
  2. Gravity settling to the basin floor Correct answer.
  3. Ion exchange with resin media
  4. Adsorption onto granular activated carbon

Explanation

Sedimentation relies on the density difference between floc and water, so particles with sufficient settling velocity reach the sludge zone. Design is based on surface overflow rate, not on chemical reaction.

Question 9Drinking Water Treatment

A jar test shows the lowest settled turbidity at a coagulant dose of 15 mg/L. What should the operator do next?

  1. Add additional chlorine to oxidize the remaining particles
  2. Turn off coagulant feed because settled turbidity is already low
  3. Shift the coagulant feed toward the best-performing dose Correct answer.
  4. Increase the filter loading rate to compensate for turbidity changes

Explanation

Jar tests bracket the best chemical dose and pH under simulated plant mixing conditions. The result is then applied cautiously to the plant and confirmed by monitoring settled and filtered turbidity.

Question 10Drinking Water Treatment

A water has a chlorine demand of 1.8 mg/L and the operator must hold a free residual of 0.5 mg/L. What chlorine dose is required?

  1. 0.5 mg/L
  2. 3.6 mg/L
  3. 1.3 mg/L
  4. 2.3 mg/L Correct answer.

Explanation

Dose equals demand plus the desired residual, because the demand portion is consumed by reducing agents and other chlorine-reactive material. Adding 1.8 and 0.5 gives 2.3 mg/L.

Question 11Drinking Water Treatment

The required CT is 60 mg·min/L. If the measured disinfectant residual is 0.8 mg/L, what contact time is required?

  1. 60 minutes
  2. 48 minutes
  3. 75 minutes Correct answer.
  4. 0.8 minutes

Explanation

CT is the product of disinfectant concentration and contact time, so time equals CT divided by concentration. Dividing 60 mg·min/L by 0.8 mg/L gives 75 minutes.

Question 12Drinking Water Treatment

A filter receives 2.0 MGD and has 400 ft² of surface area. What is the loading rate in gpm/ft²?

  1. 0.72 gpm/ft²
  2. 3.5 gpm/ft² Correct answer.
  3. 5.0 gpm/ft²
  4. 2.0 gpm/ft²

Explanation

Convert flow to gallons per minute (2,000,000 ÷ 1,440 = about 1,389 gpm) and divide by the bed area of 400 ft². The result is approximately 3.5 gpm/ft², a typical value for a conventional rapid sand filter.

Question 13Drinking Water Treatment

Why must turbidity be low before disinfection is judged effective?

  1. Turbidity raises the CT value above the required level
  2. Particles can shelter microorganisms from the disinfectant Correct answer.
  3. Turbidity consumes fluoride and lowers its concentration
  4. Turbidity raises pH and converts chlorine to chlorate

Explanation

Microorganisms adsorbed onto or embedded within particles are physically shielded from disinfectant contact and oxidant demand is also higher. That is why filtered turbidity limits are tied directly to pathogen inactivation requirements.

Question 14Drinking Water Treatment

Which chemical is added in lime-soda softening to remove noncarbonate hardness such as calcium sulfate?

  1. Hydrated lime
  2. Soda ash Correct answer.
  3. Sodium hypochlorite
  4. Ferric chloride

Explanation

Lime precipitates carbonate hardness by supplying hydroxide and raising pH, but sulfate and chloride hardness need an external carbonate source. Soda ash supplies that carbonate so calcium can precipitate as calcium carbonate.

Question 15Drinking Water Treatment

Aeration of groundwater is most commonly applied to remove which substance?

  1. Fluoride
  2. Hydrogen sulfide Correct answer.
  3. Sulfate
  4. Nitrate

Explanation

Air stripping volatilizes dissolved gases, and hydrogen sulfide is a common groundwater gas that causes objectionable odor and corrosion. Aeration also raises dissolved oxygen, which helps oxidize iron and manganese.

Question 16Drinking Water Treatment

Manganese greensand filters are used primarily to remove which contaminants?

  1. Trihalomethanes and synthetic organics
  2. Nitrate and chloride
  3. Dissolved iron and manganese Correct answer.
  4. Radium and uranium

Explanation

Greensand is a manganese-coated glauconite media regenerated with permanganate; it oxidizes and filters out soluble iron and manganese. The oxidized precipitates are then captured in the bed and removed by backwashing.

Question 17Drinking Water Treatment

Trihalomethanes form when chlorine reacts with which type of material in the water?

  1. Natural organic matter Correct answer.
  2. Nitrate from agricultural runoff
  3. Suspended sand and clay particles
  4. Dissolved calcium and magnesium

Explanation

Chlorine oxidizes and substitutes onto organic molecules such as humic and fulvic substances, producing trihalomethanes and other disinfection byproducts. Removing precursors before chlorination is the primary control strategy.

Question 18Drinking Water Treatment

Under the Surface Water Treatment Rule, conventional filtration plants must achieve at least what removal and inactivation of Giardia lamblia?

  1. 90 percent (1-log)
  2. 99.9 percent (3-log) Correct answer.
  3. 99 percent (2-log)
  4. 99.99 percent (4-log)

Explanation

The rule sets a 3-log (99.9 percent) Giardia requirement and a 4-log (99.99 percent) virus requirement for systems using conventional treatment. Credits are assigned to filtration and disinfection based on demonstrated performance.

Question 19Drinking Water Treatment

At what pH range does free chlorine exist predominantly as hypochlorous acid, the more effective disinfecting form?

  1. Free chlorine speciation is independent of pH
  2. Above 9.5
  3. Below about 7.5 Correct answer.
  4. Around 8.5 to 9.0

Explanation

Hypochlorous acid dissociates to hypochlorite with a pKa near 7.5, so at lower pH most free chlorine is HOCl, which penetrates cell walls far more readily than the charged hypochlorite ion. Above pH 8 the less effective hypochlorite form dominates.

Question 20Drinking Water Treatment

What is the purpose of placing a layer of anthracite coal above the sand in a dual-media filter?

  1. To increase solids holding capacity and lengthen filter runs Correct answer.
  2. To raise the pH of the filtered water
  3. To supply oxygen to nitrifying bacteria
  4. To disinfect the water as it passes through the bed

Explanation

Anthracite is lighter and coarser than sand, so it stays on top after backwashing and traps larger particles in the upper bed. This stratifies solids through the depth and allows longer runs with slower head loss buildup.

Question 21Drinking Water Treatment

A clearwell holds 100,000 gallons and receives a flow of 1.0 MGD. The basin has a baffling factor of 0.5. What effective contact time (T10) should be used in the CT calculation?

  1. 72 minutes Correct answer.
  2. 36 minutes
  3. 24 minutes
  4. 144 minutes

Explanation

Theoretical detention time is 100,000 gal divided by 1,000,000 gal/day, or 0.1 day, which equals 144 minutes. Multiplying by the baffling factor of 0.5 gives an effective T10 of 72 minutes for CT credit.

Question 22Drinking Water Treatment

Ultraviolet light applied at typical drinking water doses is credited mainly for inactivating which organism?

  1. Total coliforms
  2. Cryptosporidium parvum oocysts Correct answer.
  3. Legionella bacteria
  4. Hepatitis A virus

Explanation

UV damages nucleic acids and is highly effective against Cryptosporidium oocysts, which are notoriously resistant to chlorine-based disinfectants. Viruses generally require much higher UV doses than protozoa, and UV provides no residual.

Question 23Drinking Water Treatment

After excess-lime softening raises the pH to about 11, carbon dioxide is often added to the water. What is the purpose of that step?

  1. To oxidize any remaining dissolved iron and manganese in the water
  2. To raise finished water alkalinity above raw water levels
  3. To increase the finished water's disinfecting power
  4. To reduce the pH and stabilize the water against scaling Correct answer.

Explanation

Recarbonation with carbon dioxide converts excess hydroxide and carbonate so the pH drops back to a stable range and calcium carbonate does not continue to precipitate. Without it, the water would be scale-forming and hard on downstream processes.

Question 24Drinking Water Treatment

A chloraminated distribution system shows declining total chlorine residual, falling pH, and increasing nitrite in dead-end mains. What is the most likely cause?

  1. Excessive fluoride feed at the treatment plant
  2. Scaling of pipe walls by calcium carbonate
  3. A sudden increase in free chlorine generation
  4. Nitrification occurring within the distribution system Correct answer.

Explanation

Nitrifying bacteria oxidize ammonia to nitrite and nitrate, which consumes the chloramine residual, depresses pH and alkalinity, and raises nitrite in slow-moving areas. Utilities respond with flushing, breakpoint chlorination, and chloramine residual control.

Question 25Drinking Water Treatment

A utility needs to reduce trihalomethane formation in its distribution system. Which change addresses the problem at its source?

  1. Adding more fluoride to complex the dissolved organic molecules
  2. Enhancing coagulation to remove more natural organic matter Correct answer.
  3. Changing the filter support gravel to a coarser gradation
  4. Increasing the free chlorine dose so that it oxidizes the precursors

Explanation

Trihalomethanes form from chlorine reacting with natural organic precursors, so removing those precursors before chlorination reduces formation. Enhanced coagulation means adjusting pH and increasing coagulant dose to maximize total organic carbon removal.

Question 26Drinking Water Treatment

A treatment credit of 3-log removal for a pathogen means that what fraction of the organisms is removed?

  1. 3 percent
  2. 99.99 percent
  3. 99 percent
  4. 99.9 percent Correct answer.

Explanation

Each log unit represents a tenfold reduction, so 3 log corresponds to a reduction factor of 10^3, or 99.9 percent removal. Four log would be 99.99 percent and 2 log would be 99 percent.

Question 27Drinking Water Treatment

A nephelometric turbidimeter measures which property of a water sample?

  1. Concentration of free chlorine residual
  2. Acidity or alkalinity of the sample
  3. Dissolved mineral concentration in mg/L
  4. Light scattered by suspended particles Correct answer.

Explanation

A nephelometer passes light through the sample and measures light scattered at roughly 90 degrees by suspended and colloidal material, reporting the result in NTU. It responds to particulates, not to dissolved minerals, alkalinity, or disinfectant residual.

Question 28Drinking Water Treatment

What is the purpose of the flocculation step in a conventional treatment plant?

  1. To gently stir the water so floc particles grow larger Correct answer.
  2. To raise pH so dissolved metals can precipitate
  3. To strip dissolved gases by exposing water to air
  4. To disinfect the water before it reaches the filters

Explanation

Flocculation uses slow, gentle mixing after rapid mix so that destabilized particles collide and agglomerate into larger, denser floc that settles and filters well. Vigorous mixing would shear the floc apart, so the energy input is deliberately low.

Question 29Drinking Water Treatment

A routine distribution sample is positive for total coliform bacteria. What does this result most directly indicate?

  1. A possible breach of treatment or distribution integrity Correct answer.
  2. The sample container must have been contaminated during collection
  3. A chlorine residual above 4.0 mg/L was present
  4. The water certainly contains infectious viruses

Explanation

Total coliforms are used as indicator organisms, so a positive result signals that treatment barriers or distribution integrity may have failed and that pathogens could be present. It does not prove that pathogens are present, and it triggers repeat sampling and follow-up investigation.

Question 30Drinking Water Treatment

Filter headloss is best described as which of the following?

  1. The pressure head lost as water moves through the media Correct answer.
  2. The rate at which water passes through the media
  3. The difference in turbidity between influent and effluent
  4. The volume of water used during a backwash cycle

Explanation

Headloss is the drop in hydraulic head between the water above the media and the underdrain, caused by friction and by solids trapped in the bed. Rising headloss is a standard trigger for backwashing.

Question 31Drinking Water Treatment

Under the Surface Water Treatment Rule, filtered water turbidity must not exceed which value in at least 95 percent of monthly measurements?

  1. 5 NTU
  2. 4 NTU
  3. 0.3 NTU Correct answer.
  4. 1.0 NTU

Explanation

The rule sets a limit of 0.3 NTU in at least 95 percent of monthly turbidity measurements for conventional and direct filtration plants, with no single measurement above 1 NTU. Meeting 0.3 NTU supports the pathogen removal credit the plant claims.

Question 32Drinking Water Treatment

A plant feeds alum at 25 mg/L to a flow of 2.0 MGD. How many pounds of alum per day are required?

  1. 209 lb/day
  2. 50 lb/day
  3. 417 lb/day Correct answer.
  4. 834 lb/day

Explanation

Use the standard formula: pounds per day = dose (mg/L) x flow (MGD) x 8.34. Here 25 x 2.0 x 8.34 equals about 417 lb/day of alum product.

Question 33Drinking Water Treatment

A sample has a total chlorine residual of 1.2 mg/L and a free chlorine residual of 0.7 mg/L. What is the combined chlorine concentration?

  1. 1.9 mg/L
  2. 0.5 mg/L Correct answer.
  3. 1.2 mg/L
  4. 0.7 mg/L

Explanation

Combined chlorine is the difference between total and free chlorine, so 1.2 minus 0.7 equals 0.5 mg/L. Combined chlorine represents chloramines formed when chlorine reacts with ammonia nitrogen.

Question 34Drinking Water Treatment

Which sequence correctly orders a conventional surface water treatment train?

  1. Coagulation, sedimentation, flocculation, disinfection, then filtration
  2. Sedimentation, filtration, coagulation, flocculation, then disinfection
  3. Filtration, coagulation, flocculation, sedimentation, then final disinfection
  4. Coagulation, flocculation, sedimentation, filtration, then disinfection Correct answer.

Explanation

Coagulant is dispersed by rapid mix, floc is grown by flocculation, solids are settled, remaining particles are captured by filtration, and disinfection is applied last so that turbidity does not shield organisms. Placing disinfection before filtration wastes disinfectant on solids and raises byproduct formation.

Question 35Drinking Water Treatment

Alum coagulation is generally most effective in which pH range?

  1. 7.8 to 8.5
  2. 9.0 to 10.5
  3. 5.5 to 7.0 Correct answer.
  4. 3.0 to 4.5

Explanation

Alum produces its most effective positively charged hydrolysis species and best turbidity removal at roughly pH 5.5 to 7.0, which is why many plants adjust pH before coagulant feed. Above about pH 8 the alum species become less effective and solubility of aluminum increases.

Question 36Drinking Water Treatment

Granular activated carbon contactors are used primarily to do which of the following?

  1. Add fluoride ion to help prevent dental caries
  2. Adsorb organics that cause taste and odor Correct answer.
  3. Remove nitrate and arsenic by anion exchange
  4. Neutralize excess lime remaining after softening

Explanation

Activated carbon has a large internal pore structure that adsorbs dissolved organic molecules, including taste and odor compounds and precursors of disinfection byproducts. The carbon becomes exhausted over time and must be replaced or regenerated.

Question 37Drinking Water Treatment

A residential ion exchange softener removes which contaminants from water?

  1. Calcium and magnesium Correct answer.
  2. Iron bacteria and viruses
  3. Nitrate and arsenic
  4. Chloride and sulfate

Explanation

Cation exchange resin swaps sodium for the hardness ions calcium and magnesium, reducing scale formation and soap consumption. The resin is regenerated with a concentrated brine solution.

Question 38Drinking Water Treatment

Under the Lead and Copper Rule, what is the lead action level at the 90th percentile?

  1. 1.3 mg/L
  2. 0.015 mg/L Correct answer.
  3. 0.05 mg/L
  4. 0.005 mg/L

Explanation

The lead action level is 0.015 mg/L at the 90th percentile; exceeding it triggers corrosion control treatment, public education, and possibly source water or lead service line actions. The corresponding copper action level is 1.3 mg/L.

Question 39Drinking Water Treatment

Under the Stage 2 Disinfectants and Disinfection Byproducts Rule, the TTHM maximum contaminant level, expressed as a locational running annual average, is which value?

  1. 0.100 mg/L
  2. 0.250 mg/L
  3. 0.060 mg/L
  4. 0.080 mg/L Correct answer.

Explanation

The TTHM MCL is 0.080 mg/L (80 micrograms per liter) calculated as a locational running annual average at each monitoring site. The companion HAA5 standard is 0.060 mg/L.

Question 40Drinking Water Treatment

Alum is fed to a low-alkalinity surface water. Which effect should the operator expect?

  1. A rise in pH caused by the release of hydroxide ions into solution
  2. An increase in alkalinity from sulfate reduction
  3. A sharp pH drop unless alkalinity or caustic is added Correct answer.
  4. No change in pH under any circumstance

Explanation

Alum consumes alkalinity as it hydrolyzes, releasing hydrogen ions and driving pH down. When raw water alkalinity is low there is little buffering capacity, so pH can fall sharply and supplemental alkalinity or caustic soda is needed to hold the coagulation optimum.

Question 41Drinking Water Treatment

Orthophosphate is dosed into finished water to serve which purpose?

  1. To form a protective film that limits metal leaching Correct answer.
  2. To precipitate manganese within the filter bed
  3. To provide a disinfectant residual throughout the system
  4. To raise alkalinity and reduce the chlorine demand

Explanation

Orthophosphate is a corrosion inhibitor that forms a low-solubility film on pipe surfaces, reducing the release of lead and copper into the water. It is not a disinfectant and does not remove metals in the filter.

Question 42Drinking Water Treatment

Potassium permanganate is commonly fed at a water plant to accomplish which goal?

  1. To supply the coagulant needed to form floc
  2. To raise the pH of the finished water before chlorination
  3. To soften water by precipitating calcium carbonate
  4. To oxidize iron, manganese, and taste and odor compounds Correct answer.

Explanation

Permanganate is a strong oxidant that converts soluble ferrous iron and manganous manganese to insoluble forms removable by filtration and destroys many taste and odor compounds. It is dosed carefully because excess permanganate imparts a pink color to finished water.

Question 43Drinking Water Treatment

A utility switches from free chlorine to chloramines for secondary disinfection. Which outcome is expected?

  1. Greater inactivation of viruses within the clearwell
  2. Complete elimination of all disinfection byproducts
  3. Shorter residual life but stronger oxidizing power
  4. A longer lasting residual with lower THM formation Correct answer.

Explanation

Chloramines are far more stable than free chlorine, so the residual persists longer in large distribution systems, and they form substantially fewer trihalomethanes because they react less readily with organic matter. The tradeoff is weaker disinfection strength, so chloramines are used as a secondary rather than primary disinfectant.

Question 44Drinking Water Treatment

A filter with 300 square feet of surface area operates at 4 gpm per square foot. What volume does it produce during a 24-hour run?

  1. 3.46 million gallons
  2. 1.73 million gallons Correct answer.
  3. 2.88 million gallons
  4. 0.72 million gallons

Explanation

Multiply loading rate by area to get flow: 4 gpm/ft2 x 300 ft2 equals 1,200 gpm. Over 1,440 minutes that is 1,728,000 gallons, or about 1.73 million gallons.

Question 45Drinking Water Treatment

A plant must dose 1.0 MG of water at 2.0 mg/L as Cl2 using calcium hypochlorite that is 65 percent available chlorine. How many pounds of product are required?

  1. 33.4 lb
  2. 25.7 lb Correct answer.
  3. 10.8 lb
  4. 16.7 lb

Explanation

Pure chlorine needed is 2.0 mg/L x 1.0 MG x 8.34, or 16.7 lb of Cl2. Because the product is only 65 percent available chlorine, divide by 0.65 to get about 25.7 lb of product.

Question 46Drinking Water Treatment

Chlorine is used to oxidize 1.2 mg/L of dissolved ferrous iron. Using a stoichiometric requirement of about 0.64 mg/L of Cl2 per mg/L of Fe2+, what chlorine dose is needed?

  1. 0.77 mg/L Correct answer.
  2. 1.9 mg/L
  3. 0.32 mg/L
  4. 1.2 mg/L

Explanation

Multiply the iron concentration by the stoichiometric ratio: 1.2 mg/L x 0.64 equals about 0.77 mg/L of chlorine. In practice operators feed above the theoretical value because chlorine also exerts demand from other reducing substances and a residual must remain.

Question 47Drinking Water Treatment

When ozone is used as a primary disinfectant, which regulated byproduct requires particular attention in waters containing bromide?

  1. Trihalomethanes
  2. Chlorite
  3. Bromate Correct answer.
  4. Haloacetic acids

Explanation

Ozone oxidizes naturally occurring bromide to bromate, a regulated probable human carcinogen, so bromate control (by limiting ozone dose, lowering pH, or removing bromide) is a key design constraint. Trihalomethanes and haloacetic acids are associated with chlorine, and chlorite with chlorine dioxide.

Question 48Drinking Water Treatment

Ammonia is added ahead of chlorine at a chlorine-to-ammonia weight ratio of about 5 to 1. Which chlorine species predominates in the finished water?

  1. Monochloramine Correct answer.
  2. Dichloramine
  3. Nitrogen trichloride
  4. Free hypochlorous acid

Explanation

A weight ratio near 5 to 1 of Cl2 to ammonia-nitrogen corresponds roughly to a 1-to-1 molar ratio, the condition that favors monochloramine formation. Higher chlorine ratios progressively form dichloramine and nitrogen trichloride, which cause taste and odor complaints.

Question 49Drinking Water Treatment

A jar test is run on a surface water sample at a treatment plant. Which result does this test most directly provide to the operator?

  1. The chlorine residual needed in the finished water
  2. The optimum coagulant dose and coagulation pH Correct answer.
  3. The hardness that the finished water will carry
  4. The proper media gradation for the filters

Explanation

A jar test simulates rapid mix, flocculation, and settling on small beakers so the operator can compare settled water clarity across several chemical doses and pH values. The dose and pH that give the best clarify settle and filter performance are then scaled up to the plant. It is a coagulation optimization tool rather than a measure of disinfection or finished water quality.

Question 50Drinking Water Treatment

A clarifier receives a flow of 1.5 MGD and has a water surface area of 1,000 square feet. What is the surface overflow rate, in gallons per minute per square foot?

  1. 10.4 gpm per square foot
  2. 0.10 gpm per square foot
  3. 1.04 gpm per square foot Correct answer.
  4. 4.2 gpm per square foot

Explanation

Convert the flow to gallons per minute by dividing by 1,440 minutes per day: 1,500,000 divided by 1,440 equals about 1,042 gpm. Dividing that flow by the 1,000 square feet of surface area gives a surface overflow rate of about 1.04 gpm per square foot. This value describes the settling velocity a particle must exceed to be removed in the basin.