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PA Water/Wastewater Operator practice questions

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Question 1Drinking Water Treatment

What is the primary purpose of coagulation in a surface water treatment plant?

  1. Remove dissolved gases
  2. Raise pH and reduce corrosion
  3. Destabilize particle charges Correct answer.
  4. Kill pathogenic bacteria

Explanation

Coagulation adds a chemical that neutralizes the negative surface charges on suspended particles so they can collide and stick together during flocculation. It is not a disinfection, gas-stripping, or pH-adjustment step.

Question 2Drinking Water Treatment

A jar test is performed primarily to determine which of the following?

  1. The chlorine demand of the finished water
  2. The design detention time of the clearwell
  3. The optimum coagulant dose and pH Correct answer.
  4. The filter backwash rate

Explanation

A jar test simulates coagulation and flocculation on a bench scale using several coagulant doses and pH values. The dose that produces the best floc and clearest settled water is selected for the plant.

Question 3Drinking Water Treatment

Turbidity in finished drinking water is reported in which units?

  1. mg/L
  2. ppm
  3. grains per gallon
  4. NTU Correct answer.

Explanation

Turbidity is an optical measure of light scattering by suspended particles and is reported in nephelometric turbidity units (NTU). It is not a mass concentration.

Question 4Drinking Water Treatment

What is the main function of a sedimentation basin following coagulation and flocculation?

  1. Adsorb dissolved organics
  2. Add fluoride
  3. Remove settleable floc Correct answer.
  4. Disinfect the water

Explanation

Sedimentation follows flocculation so that floc grown large enough can settle out before the water reaches the filters. This reduces the solids load on the filters and extends filter runs.

Question 5Drinking Water Treatment

Which chemical is most commonly used as a primary coagulant in drinking water treatment?

  1. Alum Correct answer.
  2. Sodium fluoride
  3. Sodium hypochlorite
  4. Calcium hydroxide

Explanation

Aluminum sulfate, or alum, is the most widely used primary coagulant for surface water. It forms a gelatinous hydroxide floc that enmeshes and settles particles.

Question 6Drinking Water Treatment

Water systems must maintain a disinfectant residual in the distribution system primarily to do which of the following?

  1. Guard against contamination after treatment Correct answer.
  2. Control taste and odor
  3. Remove hardness
  4. Prevent corrosion of distribution pipes and fixtures

Explanation

A disinfectant residual protects the distribution system from microbial contamination that could enter through main breaks, cross connections, or storage tanks. It is not intended to control corrosion, soften water, or manage taste and odor.

Question 7Drinking Water Treatment

A rapid sand filter should be backwashed when which condition occurs?

  1. The coagulant feed is shut off
  2. Head loss or turbidity reaches its limit Correct answer.
  3. The chlorine residual drops
  4. The filter has operated for exactly 24 hours straight

Explanation

Filters are backwashed when terminal head loss is reached or when effluent turbidity rises, indicating solids breakthrough. A fixed run time alone is not the controlling criterion.

Question 8Drinking Water Treatment

Fluoride is added to drinking water to do which of the following?

  1. Improve the taste and odor of the water
  2. Reduce dental decay Correct answer.
  3. Soften the water
  4. Kill bacteria

Explanation

Fluoride at controlled concentrations strengthens tooth enamel and reduces dental caries. It is not a disinfectant and does not affect hardness or taste.

Question 9Drinking Water Treatment

A clearwell holds 500,000 gallons. If the plant flow is 2.5 MGD, what is the detention time?

  1. 4.8 hours Correct answer.
  2. 0.2 hours
  3. 2.4 hours
  4. 9.6 hours

Explanation

Detention time equals volume divided by flow. 500,000 gal divided by 2,500,000 gal/day equals 0.2 day, which is 0.2 times 24 = 4.8 hours.

Question 10Drinking Water Treatment

A settling basin with a surface area of 1,500 ft2 receives 3.0 MGD. What is the surface overflow rate?

  1. 4,500 gpd/ft2
  2. 500 gpd/ft2
  3. 2,000 gpd/ft2 Correct answer.
  4. 200 gpd/ft2

Explanation

Surface overflow rate equals flow divided by surface area. 3,000,000 gpd divided by 1,500 ft2 equals 2,000 gpd/ft2.

Question 11Drinking Water Treatment

If a chlorine dose of 4.0 mg/L produces a residual of 0.8 mg/L, what is the chlorine demand?

  1. 0.8 mg/L
  2. 3.2 mg/L Correct answer.
  3. 4.8 mg/L
  4. 5.0 mg/L

Explanation

Chlorine demand is the difference between the dose applied and the residual remaining: 4.0 minus 0.8 equals 3.2 mg/L.

Question 12Drinking Water Treatment

Adding alum to a water with low alkalinity will generally have what effect?

  1. Lower the pH Correct answer.
  2. Remove all dissolved oxygen
  3. Raise the pH significantly
  4. Have no effect on pH

Explanation

Alum reacts with natural alkalinity and consumes it, releasing hydrogen ions and lowering pH. If alkalinity is insufficient, lime or caustic must be added to prevent the pH from dropping too far.

Question 13Drinking Water Treatment

Chloramines are formed when free chlorine reacts with which substance?

  1. Sulfate
  2. Sodium
  3. Ammonia Correct answer.
  4. Fluoride

Explanation

Free chlorine reacts with ammonia to form monochloramine, dichloramine, and trichloramine, depending on the chlorine-to-ammonia ratio and pH. This reaction is the basis of chloramination.

Question 14Drinking Water Treatment

Which is the principal precursor for trihalomethane formation during chlorination?

  1. Humic substances Correct answer.
  2. Nitrate
  3. Calcium carbonate
  4. Dissolved iron

Explanation

Natural organic matter, especially humic and fulvic substances, reacts with chlorine to form trihalomethanes and other disinfection byproducts. Removing organics before chlorination is a key control strategy.

Question 15Drinking Water Treatment

In a dual-media filter with anthracite over sand, the anthracite layer serves mainly to do which of the following?

  1. Support the gravel layer
  2. Disinfect the water
  3. Remove dissolved salts and minerals
  4. Allow deeper solids penetration Correct answer.

Explanation

Anthracite is coarser and lighter than sand, so it sits on top and creates larger openings at the surface. This lets solids penetrate deeper into the bed, using more of the filter's capacity and lengthening filter runs.

Question 16Drinking Water Treatment

Aeration of groundwater is commonly used to do which of the following?

  1. Remove fluoride
  2. Add chlorine residual
  3. Increase calcium hardness substantially
  4. Oxidize iron and strip hydrogen sulfide Correct answer.

Explanation

Aeration exposes groundwater to air, oxidizing dissolved iron and manganese into insoluble forms that can be filtered out. It also strips hydrogen sulfide and carbon dioxide.

Question 17Drinking Water Treatment

A key limitation of UV disinfection is that it does which of the following?

  1. Produces large amounts of trihalomethanes
  2. Requires very long contact tanks
  3. Is ineffective against bacteria
  4. Provides no lasting disinfectant residual Correct answer.

Explanation

UV light inactivates microorganisms at the point of exposure but leaves no residual in the water. A chemical disinfectant is therefore typically added downstream to maintain distribution system protection.

Question 18Drinking Water Treatment

Ozone treatment of water containing bromide can produce which regulated disinfection byproduct?

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

Explanation

Ozone oxidizes bromide in the source water to bromate, a regulated disinfection byproduct. This is a key reason ozone dose and bromide levels are monitored.

Question 19Drinking Water Treatment

Which disinfectant is generally considered most effective for inactivating Cryptosporidium oocysts?

  1. Silver ions
  2. Ultraviolet light Correct answer.
  3. Chloramines
  4. Free chlorine at high dose

Explanation

Cryptosporidium oocysts are highly resistant to free chlorine and chloramines. UV light at appropriate doses is effective at inactivating them, which is why many systems use UV as a barrier.

Question 20Drinking Water Treatment

The DPD colorimetric test is used to measure which of the following?

  1. Dissolved oxygen concentration
  2. pH
  3. Free and total chlorine residual Correct answer.
  4. Turbidity

Explanation

The DPD method reacts with chlorine to produce a color whose intensity is measured and converted to a chlorine concentration. It can distinguish free chlorine from total chlorine.

Question 21Drinking Water Treatment

A clearwell provides 30 minutes of contact time with a baffling factor of 0.5. If the free chlorine residual is 1.0 mg/L, what is the CT value?

  1. 60 mg-min/L
  2. 15 mg-min/L Correct answer.
  3. 30 mg-min/L
  4. 0.5 mg-min/L

Explanation

CT equals disinfectant concentration times effective contact time. Effective time is 30 minutes times the 0.5 baffling factor, or 15 minutes, so CT is 1.0 mg/L times 15 minutes, or 15 mg-min/L.

Question 22Drinking Water Treatment

On a chlorine demand curve, the breakpoint represents the point at which which condition occurs?

  1. The pH of the water is neutral
  2. Chlorine demand first begins to increase
  3. Chloramines are at their highest concentration
  4. Ammonia is oxidized and free chlorine appears Correct answer.

Explanation

Before the breakpoint, chlorine is consumed by reducing agents and ammonia. At the breakpoint the ammonia is essentially oxidized, and further chlorine additions produce a free chlorine residual.

Question 23Drinking Water Treatment

A filter measuring 20 ft by 30 ft is backwashed at 15 gpm/ft2. What is the total backwash flow rate?

  1. 9,000 gpm Correct answer.
  2. 300 gpm
  3. 600 gpm
  4. 90,000 gpm

Explanation

The filter area is 20 ft times 30 ft, or 600 ft2. Multiplying area by the backwash rate gives 600 ft2 times 15 gpm/ft2, or 9,000 gpm.

Question 24Drinking Water Treatment

At a pH of about 7.0, free chlorine in water is present mainly as which species?

  1. Hypochlorite ion, with little hypochlorous acid
  2. Chloride ion
  3. Chloramine
  4. Hypochlorous acid, with some hypochlorite ion Correct answer.

Explanation

Hypochlorous acid is the dominant free chlorine species below the pKa of about 7.5, so at pH 7 roughly three-quarters is HOCl. HOCl is a much stronger disinfectant than the hypochlorite ion.

Question 25Drinking Water Treatment

A rapid sand filter shows a sudden increase in effluent turbidity while head loss remains normal. What is the most likely cause?

  1. Media upset or breakthrough Correct answer.
  2. A partially clogged filter underdrain
  3. A decrease in raw water turbidity
  4. A sudden increase in coagulant dose

Explanation

A sudden turbidity increase with normal head loss usually means solids are passing the filter, as from media upset, breakthrough, or a bypass around the bed. The bed is not clogging, since head loss has not risen.

Question 26Drinking Water Treatment

What is the primary purpose of running a jar test at a water treatment plant?

  1. To measure the finished water chlorine residual
  2. To determine the optimum coagulant dose Correct answer.
  3. To estimate the plant's daily water demand
  4. To calculate the filter loading rate in gpm/ft2

Explanation

A jar test simulates coagulation and flocculation on a small scale using several chemical doses so the operator can select the dose that produces the best floc and the clearest settled water. It is a dosing trial, not a measurement of residual or flow.

Question 27Drinking Water Treatment

What is the function of the graded gravel layer placed beneath the sand in a rapid sand filter?

  1. To disinfect the filtered water as it leaves
  2. To increase the filter's loading rate
  3. To support the sand above the underdrains Correct answer.
  4. To adsorb dissolved organic compounds

Explanation

Graded gravel supports the sand bed and keeps the media in place while allowing filtered water and backwash water to pass freely into the underdrain system. It provides no disinfection or adsorption.

Question 28Drinking Water Treatment

Powdered activated carbon is most commonly added to treated water for which purpose?

  1. To control taste and odor compounds Correct answer.
  2. To raise the pH of the water
  3. To soften the water by precipitating calcium
  4. To inactivate Giardia cysts

Explanation

PAC adsorbs taste- and odor-causing compounds such as geosmin and MIB and is dosed as a slurry, usually ahead of filtration. It is not a disinfectant, a softening chemical, or a pH adjuster.

Question 29Drinking Water Treatment

An operator must change a chlorine gas cylinder. Which protective equipment is required for this task?

  1. A face shield and rubber gloves only
  2. A self-contained breathing apparatus Correct answer.
  3. A hard hat and steel-toed boots
  4. A dust mask and safety glasses

Explanation

Chlorine gas is acutely toxic, and dust masks and cartridge respirators do not reliably protect against it at leak concentrations. Only an SCBA or supplied-air respirator should be used where a chlorine release is possible.

Question 30Drinking Water Treatment

Why must a public water supply well be fitted with a sanitary seal?

  1. To allow accurate measurement of the static water level
  2. To increase the well's yield from the aquifer
  3. To remove iron and manganese from the water
  4. To seal the casing against surface contamination Correct answer.

Explanation

The sanitary seal grouts the annular space around the casing so surface runoff and shallow contaminated water cannot travel down the outside of the casing into the aquifer. It has no effect on yield, water-level measurement, or water quality treatment.

Question 31Drinking Water Treatment

Why is finished water turbidity kept low before disinfection?

  1. Turbidity raises the pH of the finished water
  2. Turbidity causes chlorine to evaporate from storage tanks
  3. Particles can shield microorganisms from the disinfectant Correct answer.
  4. Turbidity makes the water more corrosive to pipe walls

Explanation

Microorganisms attached to or embedded in particles are protected from chemical disinfectants, so the required CT cannot be achieved even at a normal residual. Low filtered water turbidity is therefore a prerequisite for claiming disinfection credit.

Question 32Drinking Water Treatment

A finished water has a pH of 5.8 and is attacking distribution piping. Adding sodium hydroxide solution will generally do which of the following?

  1. Lower the pH and increase the water's corrosiveness
  2. Add hardness and cause scaling throughout the system
  3. Have no measurable effect on pH or on corrosion
  4. Raise the pH and reduce the water's corrosiveness Correct answer.

Explanation

Sodium hydroxide is a strong base; feeding it raises pH and moves the water toward calcium carbonate saturation, which reduces attack on metal pipe and favors a protective scale. It adds sodium, not calcium or magnesium hardness.

Question 33Drinking Water Treatment

What is the main purpose of coarse screens at a surface water intake?

  1. To disinfect the raw water before treatment
  2. To remove large debris that could damage pumps Correct answer.
  3. To settle out fine silt and clay particles before filtration
  4. To remove dissolved gases from the raw water

Explanation

Bar screens and trash racks at the intake protect pumps, valves, and downstream equipment from logs, rags, and other large floating or submerged debris. They do not remove dissolved substances or fine particles.

Question 34Drinking Water Treatment

A plant treats 2.0 MGD and the desired coagulant dose is 15 mg/L. How many pounds per day of coagulant are needed?

  1. 250 lb/day Correct answer.
  2. 500 lb/day
  3. 30 lb/day
  4. 125 lb/day

Explanation

Feed rate in pounds per day equals the dose in mg/L times the flow in MGD times 8.34, the weight in pounds of one gallon of water times one mg/L. Here 15 x 2.0 x 8.34 is about 250 lb/day.

Question 35Drinking Water Treatment

The short period of elevated turbidity that often occurs immediately after a filter is returned to service following backwash is called which of the following?

  1. Media fluidization
  2. Filter breakthrough
  3. Air binding
  4. Filter ripening Correct answer.

Explanation

Ripening, also called filter maturation, is the interval while a freshly washed bed re-establishes its filter cake and surface conditioning; effluent turbidity is elevated at start-up and then declines. Many plants waste this initial filtrate until turbidity is acceptable.

Question 36Drinking Water Treatment

A filter bed is 30 inches deep and expands 30 percent during backwash. What is the expanded bed depth?

  1. 36 inches
  2. 45 inches
  3. 39 inches Correct answer.
  4. 33 inches

Explanation

Expanded depth equals the original depth multiplied by one plus the fractional expansion: 30 inches x 1.30 equals 39 inches. Enough expansion is needed to release collected solids without washing media into the troughs.

Question 37Drinking Water Treatment

A clearwell gives 20 minutes of detention time with a baffling factor of 0.6. If the free chlorine residual leaving the clearwell is 0.8 mg/L, what is the CT value?

  1. 4.8 mg/L-min
  2. 12.0 mg/L-min
  3. 16.0 mg/L-min
  4. 9.6 mg/L-min Correct answer.

Explanation

Effective contact time is detention time multiplied by the baffling factor (20 x 0.6 equals 12 minutes), and CT is that effective time multiplied by the residual (12 x 0.8 equals 9.6 mg/L-min).

Question 38Drinking Water Treatment

Lime-soda ash softening is used to remove which constituents from water?

  1. Dissolved nitrate and nitrite
  2. Calcium and magnesium hardness Correct answer.
  3. Trihalomethanes and other organic compounds
  4. Suspended sand and grit

Explanation

Lime and soda ash raise pH and supply carbonate and hydroxide so that calcium and magnesium precipitate as carbonate and hydroxide solids, which are then removed in a clarifier. It is a hardness removal process, not a nutrient, organics, or grit removal process.

Question 39Drinking Water Treatment

A manganese greensand filter is regenerated by applying which chemical?

  1. Potassium permanganate Correct answer.
  2. Sodium hypochlorite
  3. Sulfuric acid
  4. Sodium chloride brine

Explanation

Greensand carries a manganese dioxide coating that is consumed as it oxidizes dissolved iron and manganese; potassium permanganate restores the oxidized coating during regeneration. Brine is used to regenerate ion exchange resin, not greensand.

Question 40Drinking Water Treatment

A water has a Langelier Saturation Index of negative 1.5. What does this indicate?

  1. The water is exactly at equilibrium with calcium carbonate
  2. The water is undersaturated and likely to be corrosive Correct answer.
  3. The water is heavily supersaturated and will scale
  4. The water has excessive hardness and must be softened

Explanation

A negative index means the water is undersaturated with respect to calcium carbonate and tends to dissolve protective scale, so it is corrosive. A positive index indicates a scaling tendency, and zero indicates balance.

Question 41Drinking Water Treatment

Which process is best suited to removing dissolved salts (TDS) from a brackish groundwater?

  1. Granular activated carbon adsorption
  2. Aeration followed by sand filtration
  3. Reverse osmosis Correct answer.
  4. Conventional coagulation, flocculation, and filtration

Explanation

Dissolved ions are far too small to be removed by filtration or adsorption, so a membrane process such as reverse osmosis is used to reject them. Coagulation, aeration, and carbon adsorption target particles, gases, and organics respectively.

Question 42Drinking Water Treatment

A clarifier's sludge blanket is rising steadily and effluent turbidity is increasing. Which action is most appropriate?

  1. Increase the plant flow rate to flush the clarifier
  2. Increase sludge wasting to lower the blanket Correct answer.
  3. Increase the coagulant feed rate immediately
  4. Reduce sludge withdrawal to thicken the sludge

Explanation

A rising blanket means solids are entering the clarifier faster than they are being removed, so more sludge must be wasted to keep the blanket below its design level. Actions that add solids or raise the overflow rate will worsen the carryover.

Question 43Drinking Water Treatment

Why must free chlorine residual be measured at the sampling point rather than in a sample bottle transported to the lab?

  1. Because glass permanently absorbs chlorine from the sample
  2. Because sample pH changes significantly in the lab
  3. Because chlorine residual dissipates as the sample sits Correct answer.
  4. Because DPD reagent reacts only at low temperatures

Explanation

Free chlorine is unstable and continues to react with demand in the sample and with the container, so the measured residual falls as time passes. It must be measured in the field immediately to represent conditions in the main.

Question 44Drinking Water Treatment

Sodium hypochlorite solution stored in a warm, sunlit tank will generally do which of the following?

  1. Become more stable the longer it is stored
  2. Increase in strength as water evaporates
  3. Lose available chlorine strength over time Correct answer.
  4. Convert entirely to chloride with no loss of disinfecting power

Explanation

Hypochlorite decomposes to chloride and chlorate, and the decay rate rises with temperature, sunlight exposure, and concentration. Tanks should be shaded, kept cool, and turned over frequently.

Question 45Drinking Water Treatment

After backwashing, an operator finds sand accumulating in the wash water troughs. What is the most likely cause?

  1. The underdrain laterals are partially plugged
  2. The filter is being operated at too low a loading rate
  3. The coagulant feed rate has been set too low
  4. Backwash rate is too high, over-expanding the bed Correct answer.

Explanation

Media carryover occurs when the bed expands beyond the design level and the upward velocity lifts grains into the troughs. The backwash rate should be reduced to produce the target expansion for the current water temperature.

Question 46Drinking Water Treatment

Under the Surface Water Treatment Rule, a conventional filtration plant must achieve 3.0-log removal and inactivation of Giardia lamblia. If filtration is credited with 2.5 log, what inactivation must disinfection provide?

  1. 5.5 log
  2. 0.5 log Correct answer.
  3. 3.0 log
  4. 2.5 log

Explanation

Credits from filtration and disinfection are added together to meet the required total, so disinfection must supply the difference: 3.0 minus 2.5 equals 0.5 log. Treating the credits as additive rather than complementary would over- or under-design the disinfection step.

Question 47Drinking Water Treatment

A GAC contactor holds 2,400 cubic feet of media and receives 500 gpm. What is the empty bed contact time?

  1. 12 minutes
  2. 48 minutes
  3. 36 minutes Correct answer.
  4. 72 minutes

Explanation

Convert the flow to cubic feet per minute by dividing by 7.48 gallons per cubic foot, giving about 66.8 ft3/min, then divide media volume by flow: 2,400 divided by 66.8 is about 36 minutes. EBCT is a key design and operating parameter for organic removal on GAC.

Question 48Drinking Water Treatment

To form monochloramine while limiting trihalomethane formation, in what order are the chemicals normally applied?

  1. Chlorine first, then ammonia after brief contact Correct answer.
  2. Ammonia alone, with no free chlorine added at all
  3. Both chemicals added at the same point with rapid mixing
  4. Ammonia first, then chlorine after a 30-minute delay

Explanation

Chlorine is applied first so a short free-chlorine contact period oxidizes some taste and odor compounds and satisfies ammonia demand, and ammonia is then added to convert the remaining free chlorine to monochloramine. Applying them in the reverse order tends to favor dichloramine formation and leaves no free-chlorine contact.

Question 49Drinking Water Treatment

After excess-lime softening, carbon dioxide is added to the water to do which of the following?

  1. Lower the pH and stabilize the water against scaling Correct answer.
  2. Provide disinfection credit by inactivating bacteria
  3. Raise the pH so that more magnesium precipitates
  4. Oxidize dissolved iron and manganese for removal

Explanation

Excess-lime softening leaves the water at high pH and supersaturated with calcium carbonate, which can scale filters and pipes. Recarbonation with carbon dioxide converts hydroxide and carbonate to bicarbonate, lowering pH and bringing the water toward calcium carbonate equilibrium.

Question 50Drinking Water Treatment

Under the Lead and Copper Rule, a system exceeds the lead action level when the 90th percentile tap sample concentration is greater than which value?

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

Explanation

The lead action level is 0.015 mg/L and the copper action level is 1.3 mg/L, both evaluated at the 90th percentile of first-draw tap samples. Exceeding an action level triggers corrosion control, public education, or source water treatment rather than an acute health-based violation.