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OH Water/Wastewater Operator practice questions

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Question 1Drinking Water Treatment

Under Ohio's public water system rules, which parameter is required to be monitored at the entry point to the distribution system as a primary disinfectant residual?

  1. Fluoride concentration
  2. Iron and manganese
  3. Free chlorine residual Correct answer.
  4. Total coliform bacteria

Explanation

Free chlorine (or another approved primary disinfectant) must be measured to confirm a residual is maintained entering distribution, because the disinfectant residual protects against microbial regrowth and contamination. Total coliform is a separate distribution monitoring requirement, not a disinfectant residual parameter.

Question 2Drinking Water Treatment

A conventional surface water treatment plant uses coagulation, flocculation, sedimentation, and filtration. What is the primary purpose of the coagulation step?

  1. To remove dissolved gases from the water
  2. To raise the pH to the optimum level
  3. To destabilize suspended particles so they can agglomerate Correct answer.
  4. To kill pathogenic microorganisms

Explanation

Coagulants such as alum neutralize the negative surface charge on fine particles, allowing them to collide and form larger flocs during flocculation. Disinfection and pH adjustment are separate processes with different objectives.

Question 3Drinking Water Treatment

What does the term 'turbidity' measure in a drinking water sample?

  1. The amount of dissolved oxygen present
  2. The cloudiness caused by suspended particles scattering light Correct answer.
  3. The concentration of dissolved minerals
  4. The alkalinity buffering the water

Explanation

Turbidity is an optical measurement of how suspended and colloidal particles scatter light, serving as an indicator of filtration effectiveness and potential pathogen shielding. It does not directly measure dissolved substances.

Question 4Drinking Water Treatment

Which chemical is most commonly added to drinking water to provide a protective residual in the distribution system?

  1. Chlorine Correct answer.
  2. Alum
  3. Sodium hydroxide
  4. Calcium carbonate

Explanation

Chlorine is applied as a primary and secondary disinfectant because it persists in distribution and continues to inactivate microorganisms. Sodium hydroxide and calcium carbonate are used for corrosion control or pH adjustment, and alum is a coagulant.

Question 5Drinking Water Treatment

In a rapid sand filter, what is the purpose of the backwash cycle?

  1. To remove accumulated solids from the filter media Correct answer.
  2. To disinfect the filter media with chlorine
  3. To increase the filtration rate
  4. To add coagulant to the water

Explanation

Backwashing reverses flow and expands the filter bed, flushing out captured particles and restoring the filter's head loss and turbidity removal performance. It is a cleaning step, not a disinfection or coagulation step.

Question 6Drinking Water Treatment

What is the primary health concern associated with elevated nitrate in drinking water?

  1. Skin irritation in children
  2. Dental fluorosis in adults
  3. Methemoglobinemia in infants Correct answer.
  4. Kidney stones in the elderly

Explanation

Nitrate is converted to nitrite in the body, which binds hemoglobin and impairs oxygen transport, causing methemoglobinemia (blue baby syndrome) in infants. This is the basis for the nitrate MCL.

Question 7Drinking Water Treatment

Which of the following best describes the purpose of a clearwell in a surface water treatment plant?

  1. To aerate water and strip volatile organics
  2. To remove color by adsorption
  3. To flocculate particles before sedimentation
  4. To provide contact time for disinfection and store finished water Correct answer.

Explanation

A clearwell provides the detention time needed for disinfectant contact (CT) and stores treated water before pumping to distribution. It is not a flocculation or aeration unit.

Question 8Drinking Water Treatment

What does the term 'CT value' represent in drinking water disinfection?

  1. The concentration of coagulant times temperature
  2. The total coliform count times the sample volume
  3. The disinfectant residual concentration multiplied by the effective contact time Correct answer.
  4. The chlorine demand times the turbidity

Explanation

CT is the product of disinfectant concentration (C) and contact time (T) used to demonstrate that required microorganism inactivation has been achieved. It is compared against regulatory CT tables based on pH, temperature, and target pathogen.

Question 9Drinking Water Treatment

A water sample has a pH of 9.2 and a temperature of 10 degrees C. How does the required CT for free chlorine inactivation of Giardia compare with a sample at pH 7.5 and the same temperature?

  1. The required CT is lower at pH 9.2
  2. The required CT depends only on turbidity
  3. The required CT is higher at pH 9.2 Correct answer.
  4. The required CT is the same at both pH values

Explanation

Free chlorine disinfection efficiency decreases as pH rises because the less effective hypochlorite ion predominates over hypochlorous acid. Higher pH therefore requires a larger CT to achieve the same inactivation.

Question 10Drinking Water Treatment

A plant operator observes that filtered water turbidity has risen from 0.08 NTU to 0.45 NTU after a coagulant feed pump failure. What is the most appropriate immediate response?

  1. Increase the filter backwash frequency only
  2. Increase the chlorine dose to compensate
  3. Restore the coagulant feed and verify jar-test dosing Correct answer.
  4. Reduce the plant flow rate and continue filtering

Explanation

Loss of coagulant destabilizes particle removal, so the operator should restore chemical feed and confirm the correct dose, typically with jar testing, before returning to normal operation. Compensating with disinfection or backwashing does not correct the underlying treatment failure.

Question 11Drinking Water Treatment

A distribution system sample shows a total coliform-positive result. Under the revised Total Coliform Rule, what is the next required action for the system?

  1. Increase the fluoride feed rate
  2. Switch to an alternate disinfectant permanently
  3. Immediately issue a boil water notice
  4. Conduct additional routine samples and assess the cause Correct answer.

Explanation

A single total coliform-positive routine sample triggers repeat sampling and, where required, a level 1 assessment to find and correct the cause. A boil water notice is generally reserved for confirmed E. coli or significant treatment failures.

Question 12Drinking Water Treatment

What is the primary mechanism by which granular activated carbon (GAC) removes organic contaminants from drinking water?

  1. Adsorption onto the carbon surface Correct answer.
  2. Ion exchange with sodium
  3. Precipitation as a solid
  4. Chemical oxidation

Explanation

GAC removes dissolved organics primarily by adsorption, where molecules adhere to the porous carbon surface. It also can support biological activity, but adsorption is the principal removal mechanism for taste, odor, and synthetic organic compounds.

Question 13Drinking Water Treatment

Which operational condition most commonly causes a rapid sand filter to develop negative head or air binding?

  1. Clogged media from accumulated solids and inadequate backwash Correct answer.
  2. High finished water storage level
  3. Low coagulant dose
  4. Excessive backwash duration

Explanation

When media become clogged and head loss becomes excessive, the pressure can drop below atmospheric, releasing dissolved gases and creating air binding. Restoring effective backwash and media cleaning resolves the condition.

Question 14Drinking Water Treatment

Why is it important to maintain a chlorine residual throughout the distribution system rather than only at the treatment plant?

  1. To protect against microbial contamination that may enter the system Correct answer.
  2. To improve the taste of the water
  3. To reduce the water's hardness
  4. To increase water pressure in the mains

Explanation

A residual disinfectant provides ongoing protection against contamination introduced through breaks, cross-connections, or regrowth in the distribution network. It does not affect pressure or hardness and is not intended to improve taste.

Question 15Drinking Water Treatment

A utility is required to report a maximum contaminant level (MCL) violation for total trihalomethanes. Which sampling location is used to determine compliance?

  1. The clearwell effluent
  2. The entry point to the distribution system
  3. The raw water intake
  4. Designated distribution system monitoring points Correct answer.

Explanation

TTHM and HAA5 compliance is based on samples collected at designated distribution system locations, typically representing the highest formation potential. Raw water and treatment plant entry points are not the compliance locations for these disinfection byproducts.

Question 16Drinking Water Treatment

What is the purpose of adding a corrosion inhibitor such as orthophosphate to finished drinking water?

  1. To disinfect the distribution system
  2. To remove turbidity from the water
  3. To adjust the taste of the water
  4. To form a protective film on pipe surfaces and reduce metal leaching Correct answer.

Explanation

Orthophosphate forms a protective coating on pipe walls that limits the release of lead and copper into the water. This is a key corrosion control strategy under the Lead and Copper Rule.

Question 17Drinking Water Treatment

A jar test is performed to determine the optimum alum dose. Which result indicates the best dose?

  1. The sample that requires the longest settling time
  2. The sample with the highest turbidity after settling
  3. The sample with the largest floc and clearest supernatant Correct answer.
  4. The sample with the lowest pH after mixing

Explanation

The optimum coagulant dose typically produces well-formed floc that settles rapidly and leaves the clearest supernatant. High turbidity or excessively low pH may indicate overdosing or poor coagulation conditions.

Question 18Drinking Water Treatment

Which treatment process is most effective at removing dissolved iron and manganese from groundwater?

  1. Sedimentation alone
  2. Granular activated carbon adsorption
  3. Oxidation followed by filtration Correct answer.
  4. Ultraviolet disinfection

Explanation

Dissolved iron and manganese are oxidized (for example with chlorine or potassium permanganate) into insoluble particles that can then be filtered out. Sedimentation alone, GAC, and UV do not reliably remove dissolved metals.

Question 19Drinking Water Treatment

A water treatment plant uses ozone as a primary disinfectant. What is a key operational concern with ozone that differs from chlorine?

  1. Ozone raises the pH of the water significantly
  2. Ozone provides a long-lasting residual in distribution
  3. Ozone cannot inactivate viruses
  4. Ozone must be generated on site and decomposes rapidly Correct answer.

Explanation

Ozone is unstable and must be generated on site, and it decomposes quickly, so it does not provide a lasting distribution residual. A secondary disinfectant such as chlorine is typically needed downstream.

Question 20Drinking Water Treatment

What is the purpose of a solids contact clarifier (upflow clarifier) in a surface water plant?

  1. To combine coagulation, flocculation, and sedimentation in a single unit Correct answer.
  2. To disinfect the water with chlorine
  3. To store finished water for distribution
  4. To remove dissolved gases through aeration

Explanation

A solids contact clarifier mixes chemicals, promotes floc formation, and settles solids in one basin, using a sludge blanket to enhance particle contact and removal. It is not a disinfection or storage unit.

Question 21Drinking Water Treatment

A utility serves 3,300 people and uses conventional filtration. Under the Surface Water Treatment Rule, what minimum combined filter effluent turbidity performance is generally required for systems using conventional filtration?

  1. 95% of monthly samples less than or equal to 0.3 NTU Correct answer.
  2. 50% of monthly samples less than or equal to 0.1 NTU
  3. 100% of samples less than or equal to 5.0 NTU
  4. 95% of monthly samples less than or equal to 1.0 NTU

Explanation

Conventional filtration systems must meet a combined filter effluent limit of 0.3 NTU in at least 95% of monthly measurements, with no individual sample exceeding 1 NTU. These limits reflect the rule's emphasis on reliable particle removal.

Question 22Drinking Water Treatment

An operator calculates that a clearwell holds 250,000 gallons and the plant flow is 1,000 gallons per minute. What is the approximate detention time in minutes?

  1. 25 minutes
  2. 250 minutes Correct answer.
  3. 4 minutes
  4. 2,500 minutes

Explanation

Detention time equals volume divided by flow: 250,000 gallons divided by 1,000 gallons per minute equals 250 minutes. This value is then used with the disinfectant concentration to evaluate CT.

Question 23Drinking Water Treatment

A distribution system has a chlorine residual of 0.2 mg/L at a distant monitoring point while the entry point residual is 1.0 mg/L. What is the most likely explanation?

  1. The water temperature has dropped to freezing
  2. The pH has dropped below 4
  3. The chlorine is reacting with pipe deposits and organic matter as it travels Correct answer.
  4. The chlorine is being created by the pipe material

Explanation

Chlorine residual declines along the distribution system as it reacts with pipe wall deposits, biofilm, and dissolved organics, a phenomenon known as chlorine decay. The operator should investigate the cause and may need to adjust dosing or flush the area.

Question 24Drinking Water Treatment

A plant is required to achieve 3-log inactivation of Giardia cysts. If the measured CT is 60 mg-min/L and the required CT from the tables is 75 mg-min/L at the given conditions, what is the correct interpretation?

  1. The plant has exceeded the requirement and may reduce disinfection
  2. The plant must immediately issue a boil water notice
  3. The CT value is irrelevant because turbidity is low
  4. The plant has not met the required inactivation and must adjust operations Correct answer.

Explanation

The measured CT of 60 is less than the required 75, so the required log inactivation has not been demonstrated and the operator must adjust disinfectant dose, contact time, or pH/temperature conditions. A boil water notice is not automatically required unless a treatment technique violation is confirmed.

Question 25Drinking Water Treatment

A groundwater system under the Ground Water Rule detects E. coli in a source sample. What is the required response?

  1. Reduce the storage tank level
  2. No action is needed if the water is chlorinated
  3. Increase the fluoride dose
  4. Conduct a sanitary survey and correct significant deficiencies, with possible treatment requirements Correct answer.

Explanation

Under the Ground Water Rule, detection of E. coli triggers a requirement to identify and correct significant deficiencies and, where necessary, provide treatment such as disinfection. Chlorination alone does not exempt the system from the assessment and corrective action.

Question 26Drinking Water Treatment

Under the Surface Water Treatment Rule, which pathogen is the primary target for inactivation credit through filtration and disinfection rather than being controlled mainly by source protection alone?

  1. Radium-226
  2. Giardia lamblia cysts Correct answer.
  3. Total dissolved solids
  4. Nitrate-nitrogen

Explanation

The SWTR requires systems to achieve at least 99.9 percent (3-log) removal and inactivation of Giardia lamblia cysts and 99.99 percent (4-log) of viruses. These pathogen reduction requirements are demonstrated through a combination of filtration and disinfection CT. Chemical contaminants such as nitrate, TDS, and radium are addressed under separate primary or secondary standards, not the SWTR pathogen log-credit framework.

Question 27Drinking Water Treatment

In conventional coagulation with alum, what is the primary purpose of rapid mixing (flash mixing)?

  1. To increase the alkalinity consumed by the coagulant reaction
  2. To disperse the coagulant uniformly so it destabilizes particles before flocculation begins Correct answer.
  3. To provide gentle agitation that builds large, settleable floc particles
  4. To strip dissolved gases and oxidize iron in the raw water

Explanation

Rapid mix provides intense, brief agitation (typically seconds to a couple of minutes) so coagulant is dispersed throughout the water and contacts particles while hydrolysis products are forming. This chemical destabilization must occur before slow flocculation, which is what actually grows floc. Slow mixing, aeration, and alkalinity addition are separate unit functions.

Question 28Drinking Water Treatment

What does the term 'filter loading rate' describe in a granular media filter?

  1. The flow rate applied per unit of filter surface area, usually expressed in gpm/ft2 Correct answer.
  2. The head loss across the media measured in feet
  3. The depth of media expanded during backwash
  4. The mass of solids captured per unit of media surface area

Explanation

Loading rate is the hydraulic flow applied to a filter divided by its surface area, commonly reported in gallons per minute per square foot or in m/h. It is a design and operational parameter that affects particle capture and head loss development. Solids capture, bed expansion, and head loss are distinct measurements.

Question 29Drinking Water Treatment

Which unit process is specifically designed to remove volatile organic compounds and dissolved gases such as carbon dioxide from drinking water?

  1. Rapid sand filtration
  2. Lime softening
  3. Chloramination
  4. Aeration Correct answer.

Explanation

Aeration promotes transfer of volatile compounds from water to air by increasing contact between the two phases, and it also raises pH by stripping carbon dioxide. It is commonly used for VOC removal, taste and odor control, and oxidation of iron and manganese. Softening, filtration, and chloramination do not remove dissolved gases.

Question 30Drinking Water Treatment

What is the purpose of the sludge blanket in a solids contact clarifier?

  1. To provide a zone of previously formed floc that intercepts and enmeshes incoming particles Correct answer.
  2. To filter fine particles through a submerged screen
  3. To store coagulant chemical between dosing cycles
  4. To cool the water before disinfection

Explanation

In a solids contact clarifier, mixing, flocculation, and settling occur in one basin, and a suspended blanket of previously formed floc contacts the incoming coagulated water. This contact improves particle capture and produces a clearer overflow. The blanket is not a screen, a chemical storage zone, or a temperature control device.

Question 31Drinking Water Treatment

Which chemical is most commonly added to finished drinking water to raise pH and provide a protective carbonate film on pipe interiors?

  1. Calcium hydroxide (hydrated lime) Correct answer.
  2. Sodium chloride
  3. Aluminum sulfate
  4. Sodium hypochlorite

Explanation

Hydrated lime is a common alkaline chemical used to raise pH and, when the water has adequate calcium and alkalinity, to promote deposition of a calcium carbonate film that reduces corrosion of metal pipe. Hypochlorite is a disinfectant, alum is a coagulant, and sodium chloride is a salt used in ion exchange regeneration.

Question 32Drinking Water Treatment

What is the primary function of a pre-sedimentation basin in a surface water plant drawing from a river with high seasonal turbidity?

  1. To store finished water for peak demand
  2. To remove a portion of heavy solids and reduce the load on downstream coagulation and filtration Correct answer.
  3. To add fluoride to the raw water
  4. To disinfect the raw water before coagulation

Explanation

Pre-sedimentation provides quiescent or lightly aided settling ahead of the main treatment train so that heavy silt, sand, and large floc settle out early. This reduces coagulant demand, sludge production, and the solids load reaching the clarifier and filters. It is not a disinfection, fluoridation, or finished-water storage step.

Question 33Drinking Water Treatment

Which test is used to measure the concentration of a disinfectant residual in finished drinking water?

  1. Gravimetric method
  2. DPD colorimetric method Correct answer.
  3. Turbidimetric method
  4. Membrane filtration method

Explanation

The DPD colorimetric method is a standard field and bench procedure for measuring free and total chlorine residual in drinking water. Turbidimetric testing measures turbidity, membrane filtration is used for microbiological analysis, and gravimetric methods measure mass. Matching the analytical method to the parameter measured is a core operator competency.

Question 34Drinking Water Treatment

A clarifier treating 4.0 MGD has a surface area of 2,500 square feet. What is the surface overflow rate in gallons per day per square foot?

  1. 640 gpd/ft2
  2. 2,500 gpd/ft2
  3. 1,600 gpd/ft2 Correct answer.
  4. 1,000 gpd/ft2

Explanation

Surface overflow rate equals flow divided by surface area, so 4,000,000 gpd divided by 2,500 ft2 equals 1,600 gpd/ft2. This value is compared with design criteria and with the settling velocity of the floc to judge clarifier performance. The other values result from arithmetic errors or from dividing area by flow.

Question 35Drinking Water Treatment

A plant doses 25 mg/L of alum and treats 2.0 MGD. Assuming a solution strength of 1.0 lb alum per gallon of solution and the standard 8.34 lb/gal for water, what is the approximate alum feed rate in pounds per day?

  1. 209 lb/day
  2. 1,250 lb/day
  3. 417 lb/day Correct answer.
  4. 834 lb/day

Explanation

The pounds-per-day formula is dose (mg/L) times flow (MGD) times 8.34, so 25 times 2.0 times 8.34 equals about 417 lb/day. This calculation is used to set chemical feed pumps and to estimate chemical costs and storage needs. Using the wrong conversion factor or halving the flow produces the distractor values.

Question 36Drinking Water Treatment

A rapid sand filter has been running for 40 hours and the operator must decide whether to backwash. Which combination of conditions best justifies initiating a backwash?

  1. Filter run time has reached exactly 24 hours regardless of head loss or turbidity
  2. Turbidity of 0.05 NTU and stable head loss
  3. Terminal head loss has been reached or effluent turbidity has exceeded the trigger, whichever occurs first Correct answer.
  4. Chlorine residual at the filter effluent has dropped below 0.5 mg/L

Explanation

Filters are backwashed when either terminal head loss is reached or effluent turbidity exceeds the established trigger, and the run is ended at whichever limit is hit first. Fixed time alone is not a sufficient criterion because solids loading varies. Turbidity and head loss are the direct indicators of filter condition.

Question 37Drinking Water Treatment

An operator is troubleshooting poor floc formation after alum addition. Which condition is most likely to impair coagulation?

  1. Raw water alkalinity is sufficient to buffer the coagulant
  2. Raw water turbidity is moderate
  3. Raw water pH is far outside the optimum range for the coagulant Correct answer.
  4. Rapid mix detention time is 30 seconds

Explanation

Alum coagulation is pH dependent, and performance drops sharply when pH falls outside the optimum range, often because alkalinity has been consumed. Adequate alkalinity, a short flash mix, and moderate turbidity all support rather than impair floc formation. Operators check pH and alkalinity first when floc is poor.

Question 38Drinking Water Treatment

A utility using free chlorine wants to reduce formation of total trihalomethanes while keeping adequate disinfection. Which operational change is most directly effective?

  1. Increasing the chlorine dose to maintain a higher residual
  2. Shortening the clearwell detention time
  3. Removing more natural organic matter before chlorination and reducing the applied chlorine dose Correct answer.
  4. Raising finished water temperature before disinfection

Explanation

THMs form when chlorine reacts with natural organic matter, so reducing precursor concentrations through enhanced coagulation or GAC and lowering the applied dose reduces formation. Higher doses and longer contact with precursors increase THM production. Temperature is not an operational control that reduces formation.

Question 39Drinking Water Treatment

What is the purpose of the turbidity limit at each individual filter effluent under the Surface Water Treatment Rule?

  1. To set the backwash water flow rate
  2. To determine the required coagulant dose for the next run
  3. To ensure the filter is producing water clear enough to support effective disinfection and to detect breakthrough early Correct answer.
  4. To measure the amount of dissolved iron in the finished water

Explanation

Individual filter effluent turbidity limits exist because particles can shield pathogens from disinfection and because rising turbidity signals filter breakthrough or upset. Monitoring each filter allows operators to catch problems before combined finished water quality degrades. Turbidity is not a measure of dissolved iron, coagulant dose, or backwash flow.

Question 40Drinking Water Treatment

A groundwater source contains 0.6 mg/L of dissolved iron and 0.2 mg/L of dissolved manganese under anoxic conditions. Which treatment sequence is most appropriate?

  1. Chlorination followed immediately by direct filtration without oxidation
  2. Lime softening alone without oxidation
  3. Oxidation with aeration or a chemical oxidant followed by filtration Correct answer.
  4. Activated carbon adsorption without oxidation

Explanation

Dissolved ferrous iron and manganous manganese must be oxidized to insoluble forms before they can be removed by filtration. Aeration or a chemical oxidant such as permanganate or chlorine converts them to particles that filters can capture. Filtration alone or adsorption without oxidation leaves the metals dissolved.

Question 41Drinking Water Treatment

What is the primary operational purpose of the flocculation basin following rapid mix?

  1. To disinfect the water before settling
  2. To add coagulant at a second point
  3. To provide intense shear that breaks apart newly formed floc
  4. To provide gentle, tapered mixing that encourages particles to collide and grow into settleable floc Correct answer.

Explanation

Flocculation uses slow, often tapered mixing so destabilized particles can collide and agglomerate into larger floc without being sheared apart. Intense mixing belongs in rapid mix, and disinfection is a later step. The goal is floc growth, not chemical addition at this stage.

Question 42Drinking Water Treatment

A plant uses chlorine gas as a disinfectant. Which safety provision is most critical for the chlorine room?

  1. A positive-pressure ventilation system that pushes air into the room
  2. A continuously operating exhaust fan at the floor level with a low-level gas detector and alarm Correct answer.
  3. Sealing the room to prevent any air exchange
  4. Storing ammonia in the same room to neutralize leaks

Explanation

Chlorine gas is heavier than air, so a chlorine room needs mechanical exhaust that draws from near the floor, plus a gas detector that alarms and triggers ventilation. The room should be kept under negative pressure relative to adjacent spaces so leaks do not escape into occupied areas. Sealing the room or storing incompatible chemicals nearby increases risk.

Question 43Drinking Water Treatment

What is the primary reason for monitoring the filter-to-waste or filter ripening period after backwash?

  1. To reduce the disinfectant dose needed downstream
  2. To measure the amount of backwash water used
  3. To capture the period of elevated turbidity when a freshly backwashed filter is not yet fully effective Correct answer.
  4. To check the media depth after backwash

Explanation

Immediately after backwash, a filter often passes a turbidity spike as media resettles and captured particles are released. Routing this initial flow to waste or monitoring until turbidity drops prevents degraded water from entering the finished supply. The practice is about turbidity control, not disinfectant dose, backwash volume, or media measurement.

Question 44Drinking Water Treatment

A utility chlorinates water with a free chlorine residual of 1.0 mg/L at pH 7.0. If the pH rises to 8.5 while the dose stays the same, what happens to disinfection effectiveness?

  1. It increases because more hypochlorous acid forms
  2. It decreases because a larger fraction of the chlorine converts to the less effective hypochlorite ion Correct answer.
  3. It increases because the water becomes more alkaline
  4. It stays the same because the residual concentration is unchanged

Explanation

Free chlorine effectiveness depends on the balance between hypochlorous acid and hypochlorite ion, which shifts toward the less biocidal hypochlorite ion as pH rises. At pH 8.5 a smaller fraction remains as hypochlorous acid, so the same residual provides less inactivation. Residual concentration alone does not capture this pH effect.

Question 45Drinking Water Treatment

What is the function of a degasifier or forced-draft aerator following an air stripping tower used for VOC removal?

  1. To remove volatile compounds and carbon dioxide from the water before subsequent treatment Correct answer.
  2. To increase the hardness of the water
  3. To cool the water to near freezing
  4. To add chlorine for oxidation of residual organics

Explanation

Air stripping transfers volatile organics from water to air, and a degasifier or forced-draft aerator continues that transfer while also removing carbon dioxide, which raises pH. This protects downstream processes such as corrosion control and filtration. The unit does not chlorinate, harden, or chill the water.

Question 46Drinking Water Treatment

A plant performs a jar test and observes that the lowest settled turbidity occurs at 15 mg/L alum, while 20 mg/L produces slightly higher turbidity and 10 mg/L produces much higher turbidity. What should the operator do?

  1. Select 10 mg/L to save chemical cost
  2. Abandon coagulation and rely on filtration alone
  3. Select 15 mg/L as the optimum dose and verify with a confirmatory test Correct answer.
  4. Select 20 mg/L because more coagulant is always better

Explanation

The jar test identifies the dose producing the clearest settled water, and 15 mg/L is the minimum dose near that optimum. Operators confirm the result with a repeat or a plant trial before locking in the dose. Overdosing wastes chemical and can restabilize particles, while underdosing leaves turbidity high.

Question 47Drinking Water Treatment

A plant must achieve 4-log virus inactivation using free chlorine. If the required CT for the measured conditions is 12 mg-min/L and the measured CT is 3 mg-min/L, what is the inactivation ratio and the appropriate interpretation?

  1. Ratio of 0.25, so the requirement is not met and more CT is needed Correct answer.
  2. Ratio of 3.0, so the requirement is met
  3. Ratio of 12.0, so the requirement is exceeded
  4. Ratio of 4.0, so the requirement is met

Explanation

The ratio of measured CT to required CT is 3 divided by 12, or 0.25, which is below 1.0 and therefore does not satisfy the 4-log requirement. The operator must increase disinfectant concentration, contact time, or both, or improve pH and temperature conditions that affect CT. A ratio of 1.0 or greater indicates compliance.

Question 48Drinking Water Treatment

A utility using conventional filtration wants to claim Cryptosporidium treatment credit. Which combination of strategies is most consistent with the Long Term 2 Enhanced Surface Water Treatment Rule approach?

  1. A combination of physical removal by filtration and an approved inactivation process such as UV or ozone, with monitoring of source water bin classification Correct answer.
  2. Lowering coagulant dose to reduce sludge production
  3. Reducing filter run times to increase backwash frequency
  4. Increased free chlorine dose alone, because chlorine is highly effective against Cryptosporidium

Explanation

Cryptosporidium oocysts are resistant to free chlorine, so LT2ESWTR credit relies on physical removal by well-operated filtration and on approved inactivation such as UV or ozone, guided by source water monitoring and bin classification. Reducing filter performance or coagulant dose would decrease removal credit. Chlorine alone is not an effective barrier for this organism.

Question 49Drinking Water Treatment

A lime softening plant raises pH to 11.0 for excess lime treatment to remove magnesium hardness. Which operational consequence must the operator plan for?

  1. Chlorine residual will automatically increase at high pH
  2. Turbidity will drop to zero without filtration
  3. The water must be recarbonated or pH adjusted before distribution to avoid scale and corrosion problems Correct answer.
  4. Alkalinity will increase without limit

Explanation

Excess lime softening leaves the water at high pH and often with excess lime, so it must be recarbonated with carbon dioxide or otherwise pH adjusted before distribution. High pH water can cause scale and corrosion issues and reduces free chlorine effectiveness. Filtration is still required, and alkalinity does not increase without limit.

Question 50Drinking Water Treatment

A treatment plant's clearwell baffling factor improves from 0.3 to 0.7 after internal baffles are installed. Assuming the same flow, volume, and disinfectant concentration, how does the available CT change?

  1. CT decreases because baffling reduces the volume available for contact
  2. CT increases because the effective contact time used in the calculation increases Correct answer.
  3. CT increases only if the disinfectant dose is also doubled
  4. CT stays the same because the physical volume has not changed

Explanation

The baffling factor adjusts the theoretical detention time to reflect actual flow patterns, so a higher factor yields a longer effective contact time and therefore a larger CT at the same concentration. Improved baffling reduces short-circuiting and makes the contact basin more efficient. Physical volume and dose are unchanged in this scenario.