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MS Water Operator practice questions

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Question 1Water Treatment

In a conventional surface water treatment plant, which process step is designed primarily to remove dissolved natural organic matter and improve taste and odor before disinfection?

  1. Coagulation with alum
  2. Powdered activated carbon addition Correct answer.
  3. Granular media filtration
  4. Fluoridation

Explanation

Powdered activated carbon adsorbs dissolved organic compounds, including taste- and odor-causing substances, and is dosed as a slurry ahead of filtration. Coagulation destabilizes particles but removes only a fraction of dissolved organics, and filtration is a particle barrier. Fluoridation is a public health additive and does not remove organics.

Question 2Water Treatment

A jar test is performed to determine the optimum alum dose for a raw water with high turbidity. Which result indicates the most effective coagulation condition?

  1. No visible floc with slight residual color
  2. Floc that forms only after 30 minutes of slow mixing
  3. Formation of fine pin floc that stays suspended
  4. Rapid formation of large, dense floc that settles clear Correct answer.

Explanation

Effective coagulation produces visible, well-formed floc that settles rapidly and leaves clear supernatant, indicating good particle destabilization and sweep flocculation. Fine pin floc that will not settle usually signals an underdose or poor pH. Absence of floc or very late floc formation indicates inadequate dose, mixing, or pH adjustment.

Question 3Water Treatment

Which statement correctly describes the purpose of the flocculation basin in a conventional treatment plant?

  1. It provides the detention time needed for disinfection.
  2. It provides high-shear mixing to disperse the coagulant.
  3. It provides gentle, tapered mixing to encourage particle collisions and floc growth. Correct answer.
  4. It provides the head loss needed to drive flow through filters.

Explanation

Flocculation follows rapid mix and uses slow, tapered velocity gradients so that destabilized particles collide and agglomerate into settleable floc without being sheared apart. Rapid mix, not flocculation, disperses the coagulant. Disinfection contact time and filter driving head are provided by other units.

Question 4Water Treatment

A sedimentation basin is designed with a surface overflow rate of 600 gallons per day per square foot. What does this value primarily govern?

  1. The rate of coagulant feed
  2. The settling velocity of the smallest particle that can be removed Correct answer.
  3. The detention time available for disinfection
  4. The velocity of water through the filter media

Explanation

Surface overflow rate equals flow divided by basin surface area and, in ideal settling, corresponds to the settling velocity of the slowest particle that is completely removed. It does not set filter velocity, disinfection contact time, or chemical feed rate. Lower overflow rates remove smaller, slower-settling particles.

Question 5Water Treatment

During a filter run, effluent turbidity begins to rise sharply while head loss remains low. Which condition is most likely?

  1. Excessive filter aid polymer feed
  2. Normal end-of-run head loss
  3. Filter breakthrough from media disturbance or channeling Correct answer.
  4. Media clogging from excessive floc loading

Explanation

Breakthrough is the passage of particles through the filter before terminal head loss is reached, producing rising effluent turbidity at relatively low head loss. It can result from media disturbance, channeling, or inadequate coagulation. Clogging produces high head loss with good effluent quality, and excessive polymer would not typically cause low head loss with breakthrough.

Question 6Water Treatment

A plant uses free chlorine for primary disinfection. Which water quality condition reduces disinfection efficiency the most for the same chlorine dose and contact time?

  1. Lower turbidity
  2. Presence of chlorine demand from reduced iron and manganese Correct answer.
  3. Higher water temperature
  4. Higher pH within the allowable range

Explanation

Reduced iron and manganese exert a chlorine demand that consumes free chlorine before it can inactivate pathogens, reducing the residual and CT achieved. Higher temperature increases disinfection kinetics, and lower turbidity improves disinfection by shielding fewer organisms. Higher pH shifts chlorine to the less effective hypochlorite ion, but demand is usually the larger immediate consumer of residual.

Question 7Water Treatment

The CT concept is used to verify inactivation of Giardia and viruses. What two parameters are multiplied to calculate CT?

  1. Residual disinfectant concentration and contact time Correct answer.
  2. Flow rate and basin surface area
  3. Chlorine dose and water temperature
  4. Turbidity and filter run time

Explanation

CT is the product of the residual disinfectant concentration (C, in mg/L) and the effective contact time (T, in minutes) at a defined peak flow and basin baffling condition. The resulting value is compared to required CT tables for the target organism and log inactivation. Dose and temperature affect residual but are not the CT terms themselves.

Question 8Water Treatment

A water system chlorinates with free chlorine and must control trihalomethanes (THMs). Which operational change is most likely to reduce THM formation while maintaining disinfection?

  1. Removing precursor organic matter before chlorination Correct answer.
  2. Raising the finished water pH before chlorination
  3. Increasing the chlorine dose and contact time
  4. Switching from groundwater to a higher-organic surface source

Explanation

THMs form when free chlorine reacts with natural organic matter, so removing precursors by coagulation, softening, or carbon adsorption before chlorination reduces formation. Increasing dose and contact time increases THM formation. Raising pH before chlorination favors THM formation and shifts chlorine speciation. A higher-organic source increases precursors.

Question 9Water Treatment

Which process is most commonly used to remove nitrate from drinking water at a municipal plant?

  1. Fluoridation
  2. Ion exchange or biological denitrification Correct answer.
  3. Conventional coagulation and filtration
  4. Aeration followed by sedimentation

Explanation

Nitrate is dissolved and not removed by coagulation, filtration, or aeration, so treatment relies on ion exchange, reverse osmosis, or biological denitrification. Aeration removes volatile gases such as carbon dioxide and hydrogen sulfide but not nitrate. Fluoridation adds fluoride and removes nothing.

Question 10Water Treatment

A groundwater contains 2.5 mg/L dissolved iron and 0.4 mg/L manganese at pH 7.0. Which treatment approach is appropriate?

  1. Oxidation with chlorine or permanganate followed by filtration Correct answer.
  2. Activated carbon adsorption without oxidation
  3. Lime softening only
  4. Coagulation with alum alone

Explanation

Dissolved iron and manganese must be oxidized to insoluble forms and then filtered out; chlorine, permanganate, or aeration can serve as the oxidant. Coagulation alone does not oxidize dissolved metals, and activated carbon does not remove them. Lime softening can remove some iron but is not the standard approach for manganese at this concentration.

Question 11Water Treatment

What is the primary purpose of adding a filter aid polymer ahead of a rapid sand filter?

  1. To raise the pH of the filtered water
  2. To remove dissolved gases from the water
  3. To disinfect the filter media
  4. To strengthen floc and improve particle capture during filtration Correct answer.

Explanation

Low-dose filter aid polymers bridge and strengthen floc so it is captured in the upper media rather than penetrating and causing breakthrough. They also can reduce initial turbidity spikes after backwash. They are not disinfectants, pH adjusters, or gas-removal agents.

Question 12Water Treatment

A rapid sand filter is backwashed when effluent turbidity is acceptable but head loss reaches the terminal limit. Which backwash condition indicates adequate cleaning?

  1. Backwash water turbidity that continues to rise throughout the wash
  2. Media expansion of about 20 to 30 percent with wash water turbidity peaking then clearing Correct answer.
  3. No media expansion with a short high-rate rinse
  4. Media expansion of 50 percent for one minute

Explanation

Adequate backwash expands the bed enough to fluidize and scrub the media, and the wash water turbidity should rise as dirt is released and then decline as the media clears. Insufficient expansion leaves deposits, while excessive expansion can wash media to waste. Rising turbidity throughout suggests continuing media loss or inadequate cleaning.

Question 13Water Treatment

A treatment plant adds sodium hydroxide to finished water. What is the primary purpose of this addition?

  1. To raise pH and alkalinity for corrosion control and process optimization Correct answer.
  2. To lower the pH for corrosion control
  3. To remove turbidity
  4. To provide disinfection residual

Explanation

Sodium hydroxide is a base used to raise pH and alkalinity, which helps control corrosion in distribution systems and can optimize coagulation and disinfection. It does not lower pH, disinfect, or remove turbidity. Corrosion control often also involves adjusting the carbonate chemistry to form protective scale.

Question 14Water Treatment

A water system using chloramines for secondary disinfection must control nitrification in the distribution system. Which indicator is most useful for detecting nitrification?

  1. Decreasing total chlorine residual with increasing nitrite Correct answer.
  2. Increasing fluoride residual
  3. Rising free chlorine residual with stable total chlorine
  4. Increasing calcium hardness

Explanation

Nitrifying bacteria oxidize ammonia to nitrite and nitrate, consuming ammonia and causing chloramine residual to fall while nitrite appears. A decreasing total chlorine residual with detectable nitrite is a classic indicator. Rising free chlorine with stable total chlorine does not indicate nitrification, and fluoride or hardness changes are unrelated.

Question 15Water Treatment

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

  1. To flocculate particles before sedimentation
  2. To add coagulant to raw water
  3. To backwash filters
  4. To provide contact time for disinfection and store finished water Correct answer.

Explanation

The clearwell stores filtered water and provides the contact time needed for disinfection before distribution. Flocculation occurs in dedicated basins, backwash is driven by pumps or elevated storage, and coagulant is added at rapid mix. Contact time in the clearwell is a key part of CT compliance.

Question 16Water Treatment

A lime softening plant produces sludge that must be handled. Which statement best describes the solids generated?

  1. The sludge is filter media washed to waste during backwash.
  2. The sludge is primarily calcium carbonate and magnesium hydroxide precipitates. Correct answer.
  3. The sludge consists mainly of dissolved sodium chloride.
  4. The sludge is composed of volatile organic compounds removed by aeration.

Explanation

Lime softening raises pH and precipitates calcium as calcium carbonate and magnesium as magnesium hydroxide, which are removed as sludge. Sodium chloride remains dissolved and is not the sludge. Volatile organics are removed by air stripping, and filter media waste is a separate residual stream.

Question 17Water Treatment

A plant treats water with a high concentration of hydrogen sulfide, causing odor complaints. Which treatment step is most directly effective?

  1. Fluoridation
  2. Sedimentation without prior oxidation
  3. Activated carbon alone without oxidation
  4. Aeration or oxidation followed by removal of the oxidized products Correct answer.

Explanation

Hydrogen sulfide is a dissolved gas that can be stripped by aeration or oxidized to sulfate or elemental sulfur, which is then removed by filtration or settling. Sedimentation alone does not remove dissolved sulfide. Carbon alone is quickly exhausted by high sulfide loads, and fluoridation is unrelated.

Question 18Water Treatment

A membrane filtration plant reports a rising transmembrane pressure at constant flux. What does this most likely indicate?

  1. An increase in permeate quality
  2. Membrane fouling or scaling that increases resistance Correct answer.
  3. A decrease in feedwater turbidity
  4. A reduction in membrane surface area

Explanation

At constant flux, transmembrane pressure rises as resistance across the membrane increases from fouling, scaling, or cake buildup. Lower feed turbidity would reduce fouling tendency, and improved permeate quality is not the cause of rising pressure. Membrane surface area is fixed by the installed elements.

Question 19Water Treatment

Which chemical is commonly used as a reducing agent to remove residual chlorine before water enters a reverse osmosis membrane?

  1. Sodium hypochlorite
  2. Alum
  3. Sodium bisulfite Correct answer.
  4. Sodium hydroxide

Explanation

Sodium bisulfite reduces free and combined chlorine to chloride, protecting polyamide membranes from oxidative degradation. Sodium hypochlorite is an oxidant and would damage the membrane. Alum is a coagulant and sodium hydroxide is a base; neither removes chlorine.

Question 20Water Treatment

A utility plans to add ammonia to form chloramines. Which control strategy best minimizes chloramine taste and odor complaints?

  1. Applying ammonia before any free chlorine contact
  2. Lowering pH to favor dichloramine formation
  3. Maintaining a chlorine-to-ammonia ratio and pH that favor monochloramine Correct answer.
  4. Adding excess ammonia to ensure complete reaction

Explanation

Monochloramine forms at chlorine-to-ammonia weight ratios near 4 to 5 to 1 and at pH around 8 to 9, minimizing dichloramine and trichloramine, which cause taste and odor. Excess ammonia encourages nitrification, and low pH favors dichloramine. Ammonia should be added after free chlorine contact, not before.

Question 21Water Treatment

What does a positive coliform result in finished water most directly indicate?

  1. A potential breach in treatment or distribution integrity requiring investigation. Correct answer.
  2. The filter media need replacement.
  3. The water definitely contains pathogenic E. coli.
  4. The chlorine residual is above the target.

Explanation

Total coliforms are indicator organisms; a positive result signals a possible treatment barrier failure, distribution contamination, or sampling problem and triggers investigation and repeat sampling. It does not confirm pathogens such as E. coli unless that organism is specifically identified. Chlorine residual and media condition are checked as part of the investigation but are not what the result indicates.

Question 22Water Treatment

A conventional plant must achieve 3-log inactivation of Giardia. Which combination of treatment steps can be credited toward this requirement?

  1. Filtration credit plus CT-based disinfection credit Correct answer.
  2. Backwash credit plus clearwell storage credit
  3. Rapid mix credit plus flocculation credit
  4. Sedimentation credit plus fluoridation credit

Explanation

Log inactivation and removal credits are assigned to specific barriers, typically filtration for physical removal and disinfection CT for inactivation, and are summed against the required log reduction. Sedimentation, rapid mix, and flocculation receive limited or no pathogen credit under standard tables, and fluoridation and backwash receive none. Clearwell storage contributes only through the contact time used in CT.

Question 23Water Treatment

A water treatment operator observes that filter effluent turbidity spikes immediately after backwash and then declines. Which action best addresses this pattern?

  1. Increasing the filtration rate during the first hour
  2. Reducing backwash duration to save water
  3. Filter-to-waste or slow-start after backwash Correct answer.
  4. Increasing coagulant dose only during backwash

Explanation

Post-backwash turbidity spikes come from media disturbance and residual solids; filter-to-waste or a slow start allows the media to ripen before water is sent to the clearwell. Increasing the filtration rate worsens the spike, and shortening backwash leaves solids in the bed. Coagulant dose is set by jar testing and is not adjusted only during backwash.

Question 24Water Treatment

Which unit process is designed to remove volatile organic compounds such as trichloroethylene from groundwater?

  1. Conventional sedimentation
  2. Lime softening
  3. Chlorination
  4. Granular activated carbon or air stripping Correct answer.

Explanation

Volatile organics are removed by air stripping, which transfers them to the gas phase, or by adsorption on granular activated carbon. Sedimentation and lime softening target particles and hardness, and chlorination can oxidize some compounds but is not a removal process for VOCs and may form byproducts.

Question 25Water Treatment

A plant uses ozone for primary disinfection. Which operational concern is most specific to ozone compared with free chlorine?

  1. It leaves no persistent residual, so a secondary disinfectant is needed. Correct answer.
  2. It forms trihalomethanes as a primary byproduct.
  3. It raises the pH of the water significantly.
  4. It cannot inactivate viruses.

Explanation

Ozone is a powerful disinfectant and oxidant but decomposes rapidly, leaving no measurable residual in the distribution system, so a secondary disinfectant such as chlorine or chloramine is required. Ozone is effective against viruses, has little effect on pH, and does not form THMs; its main byproducts are bromate and assorted aldehydes.

Question 26Water Treatment

In a conventional coagulation-flocculation-sedimentation process, what is the primary purpose of rapid mix?

  1. To provide gentle stirring so floc particles grow large enough to settle
  2. To remove dissolved gases and oxidize iron and manganese
  3. To thicken settled sludge before dewatering
  4. To disperse the coagulant rapidly and promote particle destabilization Correct answer.

Explanation

Rapid mix delivers high-intensity, brief mixing so the coagulant is dispersed throughout the water and contacts particles before hydrolysis products form and become ineffective. Destabilization (charge neutralization or sweep floc) is the goal of this stage. Gentle, prolonged stirring belongs to flocculation, not rapid mix.

Question 27Water Treatment

A jar test series shows that increasing alum dose beyond 30 mg/L steadily raises settled turbidity and lowers pH. What is the most likely explanation?

  1. The higher dose is improving sweep floc and should be adopted
  2. The higher dose is increasing alkalinity and buffering the water
  3. The higher dose is causing restabilization or charge reversal of particles Correct answer.
  4. The higher dose is reducing the coagulant demand of the raw water

Explanation

Excess coagulant can reverse the particle surface charge, so repulsion returns and particles no longer agglomerate, which shows up as rising settled turbidity. Alum also consumes alkalinity, so pH falls as dose increases. Both observations point to overdosing rather than better treatment.

Question 28Water Treatment

Which coagulant is most commonly selected when the raw water has low alkalinity and a low pH that must not be depressed further?

  1. Alum (aluminum sulfate)
  2. Lime (calcium hydroxide)
  3. Polyaluminum chloride Correct answer.
  4. Ferric chloride

Explanation

Polyaluminum chloride is pre-hydrolyzed, so it consumes less alkalinity and depresses pH less than alum or ferric salts for the same degree of destabilization. That makes it attractive for low-alkalinity, low-pH supplies. Lime is a pH-raising chemical and not a primary coagulant.

Question 29Water Treatment

In a rectangular sedimentation basin, what does the surface overflow rate represent?

  1. The depth of sludge blanket maintained on the basin floor
  2. The horizontal velocity of water at the basin midpoint
  3. The mass of solids removed per unit of basin volume
  4. The flow per unit of basin surface area, expressed as a velocity Correct answer.

Explanation

Surface overflow rate is the design flow divided by the basin's plan area, giving units of velocity (for example m/d or gpm/ft2). It is compared with the settling velocity of floc particles to judge whether a basin can capture them. It is not a horizontal velocity or a solids loading measure.

Question 30Water Treatment

A plant's settled water turbidity rises sharply during a two-hour period each afternoon while raw turbidity stays stable. Which investigation is most appropriate first?

  1. Replace the filter media because the grains are rounded
  2. Reduce the clearwell chlorine dose to lower THM formation
  3. Check whether coagulant feed pumps are losing prime or dosing erratically Correct answer.
  4. Increase the filter backwash frequency to twice per shift

Explanation

A repeatable, short-term rise in settled turbidity with stable raw water points to a treatment-side disturbance such as chemical feed instability, not to filter condition. Verifying coagulant dose and pump performance is the logical first step. Filter and disinfection changes address different symptoms and would not explain the timing.

Question 31Water Treatment

What is the primary role of a filter's support gravel layer?

  1. To provide the surface area on which biological nitrification occurs
  2. To store treated water for backwashing without a separate tank
  3. To support the media, distribute backwash flow, and prevent media loss Correct answer.
  4. To chemically adsorb dissolved organic carbon from the water

Explanation

The graded gravel supports the filter media, distributes backwash water through the underdrains, and keeps media from passing into the underdrain system. It does not provide treatment chemistry. Biological nitrification and storage are functions of other processes.

Question 32Water Treatment

A rapid sand filter is operating at a loading rate well above its design rate. Which effect is most likely?

  1. Reduced need for coagulation because the filter does more work
  2. Improved particle capture because the flow pushes floc deeper into the bed
  3. Increased head loss and higher effluent turbidity from solids breakthrough Correct answer.
  4. Lower backwash water use because the bed loads more evenly

Explanation

Exceeding the design loading rate shortens the time available for particle attachment and drives floc deeper or through the bed, producing breakthrough and faster head loss development. Filters are not a substitute for coagulation. Higher rates generally increase, not decrease, backwash demand.

Question 33Water Treatment

Which statement correctly describes the difference between filtration and disinfection as barriers to pathogens?

  1. Filtration and disinfection both work only on dissolved contaminants
  2. Filtration is credited only for virus removal, while disinfection is credited only for protozoa
  3. Filtration physically removes particles, while disinfection inactivates organisms that remain in the water Correct answer.
  4. Filtration inactivates pathogens chemically, while disinfection only removes them physically

Explanation

Filtration removes particulate matter, including many protozoan cysts and bacteria, by physical and attachment mechanisms. Disinfection then inactivates organisms that pass through the filters. The two barriers address pathogens by different mechanisms, which is why multiple-barrier treatment is required for surface water.

Question 34Water Treatment

Which disinfectant produces the most stable residual in a distribution system with long detention times?

  1. Chlorine dioxide
  2. Ultraviolet light
  3. Free chlorine Correct answer.
  4. Ozone

Explanation

Free chlorine leaves a persistent residual that can be measured throughout the distribution system, making it the standard choice for maintaining a disinfectant residual. Ozone and UV leave no lasting residual at all. Chlorine dioxide decays and its residuals are harder to maintain over long detention.

Question 35Water Treatment

A system switching from free chlorine to chloramines for secondary disinfection should expect which change in disinfection capability?

  1. Improved inactivation of Cryptosporidium oocysts compared with free chlorine
  2. No change in disinfection capability because both are chlorine species
  3. Weaker disinfection strength overall, so primary disinfection must be achieved before ammonia is added Correct answer.
  4. Stronger inactivation of viruses and protozoa than free chlorine

Explanation

Chloramines are weaker primary disinfectants than free chlorine, especially against viruses and protozoa, but they are more stable and form fewer regulated byproducts. For that reason the plant must achieve primary inactivation with free chlorine or another disinfectant before ammonia is added to form the chloramine residual.

Question 36Water Treatment

Which condition most favors the formation of nitrogenous disinfection byproducts when chloramines are used?

  1. A high organic nitrogen content in the source water and elevated bromide Correct answer.
  2. A short detention time in the clearwell before ammonia addition
  3. A low pH with excess free chlorine and low ammonia
  4. A high chlorine-to-nitrogen ratio maintained continuously

Explanation

Organic nitrogen compounds in the source water, together with bromide, are precursors for nitrogenous byproducts such as haloacetonitriles and nitrosamines when chloramines are used. A high chlorine-to-nitrogen ratio tends to break down chloramine and produce free chlorine instead. Precursor control and ammonia dosing discipline are the key strategies.

Question 37Water Treatment

What is the principal function of a solids contact clarifier used in lime softening?

  1. To combine mixing, flocculation, and settling with a recycled sludge blanket in one basin Correct answer.
  2. To aerate the water and strip carbon dioxide before lime addition
  3. To recarbonate the water after softening
  4. To filter softened water through granular media

Explanation

A solids contact clarifier brings together rapid mix, flocculation, and clarification in a single unit and uses a recycled slurry of previously formed precipitate to seed additional precipitation. This improves reaction kinetics and produces a dense, easily dewatered sludge. Aeration, filtration, and recarbonation are separate steps.

Question 38Water Treatment

Water is softened by the lime-soda ash process. Which parameter is the best indicator that the process is achieving the intended result?

  1. Filter backwash rate measured during cleaning
  2. Finished water turbidity measured before filtration
  3. Raw water alkalinity measured at the plant intake
  4. Total and calcium hardness measured after recarbonation and filtration Correct answer.

Explanation

The objective of softening is hardness reduction, so total and calcium hardness in the finished water are the direct measures of success. Alkalinity and pH are monitored to control chemical doses but do not by themselves confirm hardness removal. Turbidity reflects clarification performance rather than softening effectiveness.

Question 39Water Treatment

Which process is most effective for removing dissolved arsenic (arsenate) from groundwater at a municipal plant?

  1. Granular activated carbon adsorption alone
  2. Coagulation with ferric chloride followed by filtration Correct answer.
  3. Aeration followed by sand filtration
  4. Simple sedimentation without chemical addition

Explanation

Arsenate is anionic and adsorbs strongly onto iron hydroxide floc, so coagulation with a ferric salt followed by filtration removes it effectively. Plain sedimentation or aeration with sand filtration will not remove dissolved arsenic. Activated carbon has limited capacity for arsenate compared with iron-based treatment.

Question 40Water Treatment

What is the purpose of a corrosion control inhibitor such as orthophosphate in finished water?

  1. To raise the pH above 10 to stop all corrosion
  2. To disinfect the distribution system and control biofilm
  3. To form a protective film on pipe surfaces that limits metal release Correct answer.
  4. To remove hardness and scale from the distribution piping

Explanation

Orthophosphate forms a low-solubility film on the interior of metal pipes, which limits the release of lead and copper into the water. It does not disinfect or soften water. pH adjustment is a separate corrosion control technique, not a function of the inhibitor itself.

Question 41Water Treatment

A utility must reduce lead at customer taps in an older distribution system. Which combination is most consistent with corrosion control practice?

  1. Increase free chlorine to 4 mg/L to oxidize the lead into solution
  2. Blend in more high-alkalinity water to increase scale formation
  3. Raise the finished water pH and add orthophosphate while monitoring lead and copper at taps Correct answer.
  4. Lower the pH to keep scale dissolved and reduce pipe fouling

Explanation

Adjusting pH into the passivating range and adding an orthophosphate inhibitor are the standard approaches to limiting lead and copper release, verified by tap monitoring under the lead and copper rule. Lowering pH increases corrosion and metal solubility. Strong oxidation can mobilize lead and is not a corrosion control strategy.

Question 42Water Treatment

Which treatment process is designed to remove dissolved radium from a groundwater supply?

  1. Rapid sand filtration without chemical addition
  2. Lime softening or ion exchange Correct answer.
  3. Aeration followed by granular activated carbon
  4. UV disinfection at 254 nm

Explanation

Radium is a dissolved cation, so cation exchange or lime softening (which precipitates and removes it with hardness) are effective treatment methods. Aeration and plain filtration remove particles and gases, not dissolved radium. UV inactivates microorganisms and does not remove radionuclides.

Question 43Water Treatment

Aeration is used to remove dissolved gases from groundwater. Which property of the contaminant determines whether air stripping is effective?

  1. Its Henry's law constant and volatility Correct answer.
  2. Its oxidation state in the aquifer
  3. Its tendency to adsorb onto filter media
  4. Its molecular weight relative to water

Explanation

Air stripping transfers a dissolved gas into the air phase, and the driving force is the contaminant's Henry's law constant, which describes its tendency to partition from water into air. High volatility and a high Henry's constant make stripping effective. Molecular weight and adsorption tendency are not the controlling properties.

Question 44Water Treatment

A packed-tower air stripper is treating water contaminated with trichloroethylene. Which operating change would most likely reduce removal efficiency?

  1. Increasing the air-to-water ratio within design limits
  2. Lowering the water temperature before the tower
  3. Reducing the water flow rate through the tower
  4. Reducing the packing height while keeping the same flow Correct answer.

Explanation

Removal efficiency depends on the number of transfer units, which is proportional to packing height. Reducing packing height shortens contact and lowers removal. Increasing the air-to-water ratio and reducing water flow both improve stripping, and colder water increases the Henry's constant for TCE, which also improves removal.

Question 45Water Treatment

What is the purpose of adding fluoride to drinking water in a community water system?

  1. To precipitate excess calcium and prevent scaling
  2. To control corrosion in distribution piping
  3. To reduce the incidence of dental caries in the population Correct answer.
  4. To inactivate viruses that survive filtration

Explanation

Fluoridation is practiced to reduce dental caries, and the target concentration is set to balance benefit against the risk of dental fluorosis. It is not a corrosion inhibitor, a softening chemical, or a disinfectant. Fluoride feed is monitored closely to keep the concentration within the optimal range.

Question 46Water Treatment

A plant adds sodium hypochlorite for disinfection. Which practice best preserves the strength of the stored chemical?

  1. Store it in a warm, sunlit area to keep it from freezing
  2. Store it in a cool, dark, ventilated area and use it within its shelf life Correct answer.
  3. Store it in an open tank to allow off-gassing of chlorine
  4. Dilute it with raw water before storage to reduce its strength

Explanation

Sodium hypochlorite decays faster with heat, sunlight, and high concentration, so cool, dark, ventilated storage and rotation of stock preserve its strength. Dilution with raw water can introduce contaminants and accelerate decay. Open storage allows chlorine loss and creates a safety hazard.

Question 47Water Treatment

Which safety practice is required when an operator changes a chlorine gas cylinder on a treatment plant chlorinator?

  1. Use a dust mask and open the cylinder quickly to purge the line
  2. Wear a supplied-air respirator and follow the plant's chlorine emergency procedure Correct answer.
  3. Work alone to avoid exposing others to the gas
  4. Neutralize any leak with ammonia solution before approaching

Explanation

Chlorine gas is acutely toxic, so cylinder changes require appropriate respiratory protection such as a supplied-air respirator, a second person standing by, and adherence to the facility's emergency and leak-response procedures. Dust masks provide no protection. Ammonia can react with chlorine and is not a safe neutralizing approach.

Question 48Water Treatment

Which statement best describes the role of a residual disinfectant in the distribution system?

  1. It raises the pH to prevent corrosion throughout the system
  2. It removes particulate matter that settles in the mains
  3. It eliminates the need for main flushing and cross-connection control
  4. It maintains a disinfecting presence that limits microbial regrowth after treatment Correct answer.

Explanation

A residual disinfectant continues to inactivate or inhibit microorganisms in the distribution system after treatment, which is why a measurable residual must be maintained. It does not filter particles, adjust pH, or replace operational programs such as flushing and backflow prevention.

Question 49Water Treatment

A plant uses powdered activated carbon to control taste and odor. Which operating condition most reduces its effectiveness?

  1. Adding PAC after filtration with only a few minutes of contact Correct answer.
  2. Choosing a PAC dose determined by a jar test
  3. Adding PAC before coagulation and allowing contact through settling
  4. Adding PAC at a point with good mixing and enough contact time

Explanation

PAC needs adequate contact time and good dispersion to adsorb taste and odor compounds before it is removed with the solids. Adding it after filtration leaves only minutes of contact and no downstream clarification to capture the carbon. Jar testing and well-mixed addition points improve performance.

Question 50Water Treatment

A utility is required to reduce disinfection byproduct precursors before chlorination. Which treatment change most directly targets dissolved organic carbon?

  1. Enhanced coagulation with increased coagulant dose and lowered pH Correct answer.
  2. Raising the finished water pH before chlorine addition
  3. Adding more ammonia to convert free chlorine to chloramine
  4. Increasing the filtration rate to shorten detention

Explanation

Enhanced coagulation removes dissolved organic carbon by increasing coagulant dose and adjusting pH to improve removal, which reduces the precursors that form trihalomethanes and haloacetic acids. Shortening detention or raising pH does not remove precursors. Ammonia changes the byproduct species formed but does not remove the organic carbon.