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MD Water Operator practice questions

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Question 1Water Treatment

Which chemical is most commonly added to finished drinking water to provide a residual disinfectant that protects the distribution system?

  1. Lime
  2. Chlorine Correct answer.
  3. Alum
  4. Fluoride

Explanation

Chlorine is applied as a primary and secondary disinfectant because it leaves a measurable residual that continues to inactivate pathogens as water travels through mains and storage. Fluoride, lime, and alum serve other purposes and do not provide a disinfectant residual.

Question 2Water Treatment

A treatment plant operator measures turbidity in filtered water. What does this measurement primarily indicate?

  1. The concentration of dissolved minerals
  2. The alkalinity of the water
  3. The level of disinfectant residual
  4. The amount of suspended and colloidal particles scattering light Correct answer.

Explanation

Turbidity is an optical measurement of light scattered by suspended and colloidal material in the water. It serves as a surrogate for particulate removal efficiency and for the potential presence of pathogens shielded by particles.

Question 3Water Treatment

During coagulation with alum, what is the primary purpose of rapid mixing?

  1. To allow floc particles to grow large and settle
  2. To raise the pH of the raw water
  3. To disperse the coagulant quickly so it contacts particles before hydrolysis completes Correct answer.
  4. To strip dissolved gases from the water

Explanation

Rapid mix provides intense, brief turbulence so the coagulant is dispersed uniformly and can destabilize particles during hydrolysis. Slow mixing and flocculation come afterward and are responsible for building settleable floc.

Question 4Water Treatment

A conventional plant reports filtered water turbidity of 0.12 NTU. Under typical regulatory frameworks, how does this compare to the common combined filter effluent limit of 0.3 NTU?

  1. It exceeds the limit and requires immediate reporting
  2. It is within the limit Correct answer.
  3. It cannot be evaluated without raw water turbidity
  4. It is exactly at the limit

Explanation

The common combined filter effluent standard requires that turbidity not exceed 0.3 NTU in at least 95 percent of monthly measurements. A reading of 0.12 NTU is comfortably below that threshold and indicates effective filtration.

Question 5Water Treatment

Which process is designed to remove dissolved gases such as hydrogen sulfide and volatile organic compounds from water?

  1. Aeration Correct answer.
  2. Coagulation
  3. Disinfection
  4. Sedimentation

Explanation

Aeration brings water into contact with air so volatile dissolved gases transfer from the liquid to the gas phase. It is commonly used to strip hydrogen sulfide, carbon dioxide, and volatile organics, and to oxidize iron and manganese.

Question 6Water Treatment

An operator wants to soften hard water containing calcium and magnesium. Which process directly removes these hardness ions?

  1. Chlorination
  2. Lime softening Correct answer.
  3. Backwashing
  4. Recarbonation alone

Explanation

Lime softening raises pH so calcium precipitates as calcium carbonate and magnesium as magnesium hydroxide, which are then removed as solids. Chlorination, recarbonation alone, and backwashing do not remove hardness.

Question 7Water Treatment

What is the purpose of recarbonation following excess-lime softening?

  1. To add disinfectant residual
  2. To lower the pH and stabilize the water to prevent scale in the distribution system Correct answer.
  3. To increase turbidity for better settling
  4. To raise the pH further and precipitate more calcium

Explanation

After excess-lime softening the pH is very high and the water is supersaturated with calcium carbonate. Recarbonation with carbon dioxide lowers pH and converts the water to a stable condition that will not deposit scale in pipes.

Question 8Water Treatment

In a pressure filter, what does the term 'head loss' describe?

  1. The amount of backwash water used
  2. The depth of filter media in the vessel
  3. The difference between raw and finished pH
  4. The loss of hydraulic head through the filter bed as it collects particles Correct answer.

Explanation

Head loss is the pressure or hydraulic head consumed as water passes through the filter bed. As particles accumulate, head loss rises until the filter reaches its terminal head, triggering backwash.

Question 9Water Treatment

A filter is backwashed when effluent turbidity breaks through or head loss reaches its limit. What is the main objective of backwashing?

  1. To disinfect the filter media with chlorine
  2. To expand the bed and flush out accumulated particles Correct answer.
  3. To add filter aid to the water
  4. To increase the filtration rate

Explanation

Backwashing reverses flow to expand and agitate the media, releasing captured particles and discharging them to waste. This restores both hydraulic capacity and the media's ability to remove particles.

Question 10Water Treatment

An operator observes short filter runs with rapid head loss increase after backwash. Which condition is the most likely cause?

  1. Excessive free chlorine residual
  2. Incomplete media cleaning or media loss leaving a disturbed bed Correct answer.
  3. Excessive backwash duration
  4. Too low a filtration rate

Explanation

Short runs with rapid head buildup usually point to inadequate backwash, media loss, or a disturbed bed that lets solids load the upper media quickly. Proper backwash expansion and media inspection are the usual corrective steps.

Question 11Water Treatment

Which parameter is most commonly monitored to confirm that coagulation is performing well before filtration?

  1. Dissolved oxygen
  2. Zeta potential or streaming current Correct answer.
  3. Specific conductance
  4. Total dissolved solids

Explanation

Zeta potential and streaming current measurements track the surface charge of particles, showing whether the coagulant dose has adequately neutralized charge. Operators use these to fine-tune coagulant dose for optimal particle removal.

Question 12Water Treatment

What does the term 'CT' represent in disinfection practice?

  1. The coagulation time before filtration
  2. The product of disinfectant concentration and contact time Correct answer.
  3. The calcium-to-turbidity ratio
  4. The total chlorine demand of the water

Explanation

CT is the product of disinfectant residual concentration (C) and effective contact time (T). Regulators use CT values to verify that a required level of pathogen inactivation has been achieved.

Question 13Water Treatment

A plant using free chlorine must achieve a target CT. If the measured chlorine residual drops because of high chlorine demand, what happens to the required contact time to maintain the same CT?

  1. It increases Correct answer.
  2. It stays the same
  3. It decreases
  4. It becomes irrelevant

Explanation

CT is the product of concentration and time, so if concentration falls, contact time must rise to keep the same CT value. This is why operators maintain adequate residual and baffling in contact basins.

Question 14Water Treatment

Which disinfectant typically produces the longest-lasting residual in a distribution system?

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

Explanation

Chloramines are more stable than free chlorine and persist longer in distribution systems, which is why many utilities use them to maintain residual in large or warm networks. Ozone and UV provide no lasting residual at all.

Question 15Water Treatment

What is the primary health concern that drives the regulation of disinfection byproducts such as trihalomethanes?

  1. Long-term cancer risk and other chronic health effects Correct answer.
  2. Acute gastrointestinal illness
  3. Taste and odor complaints
  4. Corrosion of distribution piping

Explanation

Trihalomethanes and other disinfection byproducts are regulated primarily because of long-term cancer risk and other chronic health concerns. Utilities balance pathogen control against byproduct formation.

Question 16Water Treatment

Which operational change is most effective at reducing trihalomethane formation when source water contains high natural organic matter?

  1. Adding more alum after disinfection
  2. Removing precursors through enhanced coagulation before chlorination Correct answer.
  3. Switching to a higher filtration rate
  4. Increasing chlorine dose before filtration

Explanation

Trihalomethanes form when chlorine reacts with natural organic matter, so removing those precursors before chlorination reduces formation. Enhanced coagulation and improved clarification are standard precursor-control strategies.

Question 17Water Treatment

What is the purpose of adding a filter aid polymer to filter influent?

  1. To strengthen floc and improve particle capture in the filter Correct answer.
  2. To increase filter backwash rate
  3. To raise the finished water pH
  4. To disinfect the filter media

Explanation

Filter aid polymers bind small floc and particles into stronger, more filterable aggregates, improving capture and lengthening filter runs. They do not disinfect or chemically adjust pH.

Question 18Water Treatment

A water sample has a pH of 6.5. How is this water classified?

  1. Neutral
  2. Strongly basic
  3. Slightly acidic Correct answer.
  4. Strongly alkaline

Explanation

A pH of 7 is neutral; values below 7 are acidic and above 7 are alkaline. A pH of 6.5 is below neutral, so the water is mildly acidic.

Question 19Water Treatment

Which test is used to determine the alkalinity of a water sample?

  1. Measurement with a turbidimeter
  2. Colorimetric measurement with DPD reagent
  3. Titration with a standard acid to a specified endpoint Correct answer.
  4. Gravimetric analysis after evaporation

Explanation

Alkalinity is measured by titrating the sample with standard acid to a defined pH endpoint, such as pH 4.5. The acid volume consumed reflects the water's acid-neutralizing capacity.

Question 20Water Treatment

An operator needs to calculate the lime dose for a flow of 2 million gallons per day. Which information is essential in addition to the target dose in mg/L?

  1. The filter media depth
  2. The daily flow in million gallons per day and the conversion factor 8.34 Correct answer.
  3. The chlorine residual
  4. The plant's backwash schedule

Explanation

Chemical feed in pounds per day equals dose (mg/L) times flow (MGD) times 8.34. The 8.34 factor converts mg/L times MGD to pounds per day, so flow and dose are the required inputs.

Question 21Water Treatment

A plant feeds 25 mg/L of alum at a flow of 1.0 MGD. Using the factor 8.34, what is the approximate feed rate in pounds per day?

  1. 25 lb/day
  2. 2,085 lb/day
  3. 209 lb/day Correct answer.
  4. 8.3 lb/day

Explanation

Feed rate equals dose times flow times 8.34, so 25 mg/L times 1.0 MGD times 8.34 equals about 209 lb/day. This formula is standard for converting a concentration dose and plant flow into a mass feed rate.

Question 22Water Treatment

What is the primary function of a sedimentation basin in a conventional treatment train?

  1. To adjust the pH of the water
  2. To inactivate bacteria with chlorine
  3. To allow floc and settleable solids to settle out by gravity Correct answer.
  4. To remove dissolved gases

Explanation

Sedimentation provides quiescent conditions and adequate detention so floc and other settleable solids can fall out by gravity before filtration. It reduces the solids load on the filters.

Question 23Water Treatment

A plant's raw water has high color and low turbidity. Why can this be difficult to treat with conventional coagulation?

  1. High color prevents pH measurement
  2. Low turbidity causes filters to clog immediately
  3. Color always raises alkalinity
  4. Color-causing organics provide few particles to serve as floc nuclei, so floc formation is poor Correct answer.

Explanation

Dissolved color is not particulate, so there is little suspended material to collide and form floc. Operators often add a weighting agent or adjust coagulant dose and pH to improve floc formation in low-turbidity, high-color water.

Question 24Water Treatment

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

  1. It stores settled sludge for disposal
  2. It provides disinfectant contact time and finished water storage Correct answer.
  3. It houses the coagulation rapid mixers
  4. It removes dissolved iron by oxidation

Explanation

A clearwell stores filtered water and provides the contact time needed for disinfection before the water enters the distribution system. It also buffers supply against demand fluctuations.

Question 25Water Treatment

An operator notices that finished water pH has dropped after increasing the alum dose. What is the most likely explanation?

  1. Alum consumes alkalinity and releases acidity as it hydrolyzes Correct answer.
  2. Alum has no effect on pH
  3. Alum adds alkalinity to the water
  4. Alum raises the pH of the water

Explanation

Alum hydrolysis consumes bicarbonate alkalinity and releases hydrogen ions, which lowers pH. Operators often add lime or caustic to counteract this effect and maintain a stable finished water pH.

Question 26Water Treatment

In a conventional surface water treatment plant, what is the primary purpose of adding a coagulant such as aluminum sulfate to raw water?

  1. To destabilize colloidal particles so they can agglomerate into larger floc Correct answer.
  2. To add hardness minerals that protect distribution piping
  3. To oxidize dissolved iron and manganese in the raw water
  4. To raise the pH of the water before disinfection

Explanation

Coagulants neutralize the negative surface charge on colloids, allowing particles to come together rather than repel one another. This destabilization is the first step that makes floc formation possible before sedimentation and filtration. Raising pH, adding hardness, and oxidizing metals are separate treatment objectives.

Question 27Water Treatment

An operator is calculating the chlorine dose required to achieve a target residual in a clearwell. Which pair of measurements is essential to determine the demand exerted by the water?

  1. Applied dose and residual remaining after contact Correct answer.
  2. Turbidity and color
  3. Alkalinity and hardness
  4. Temperature and conductivity

Explanation

Chlorine demand is the difference between the dose applied and the residual left after a contact period. Without both numbers the demand cannot be computed. Turbidity, alkalinity, hardness, and conductivity influence demand indirectly but do not define it.

Question 28Water Treatment

What is the main purpose of the flocculation stage that follows rapid mix in a conventional treatment train?

  1. To sterilize bacteria with a strong oxidant
  2. To strip dissolved gases from the water
  3. To remove dissolved organic carbon by adsorption onto media
  4. To provide gentle, slow mixing that encourages small particles to grow into settleable floc Correct answer.

Explanation

Flocculation uses slow, gentle agitation so that destabilized particles collide and build larger floc that can settle. Rapid mix disperses the coagulant, while flocculation grows the floc. Sterilization, gas stripping, and carbon adsorption are unrelated processes.

Question 29Water Treatment

Which of the following best describes the purpose of a jar test in a water treatment plant?

  1. To measure the hydraulic loading rate of a filter
  2. To verify that the clearwell provides adequate contact time
  3. To simulate coagulation and flocculation at several chemical doses and select the most effective one Correct answer.
  4. To determine the bacteriological quality of finished water

Explanation

A jar test applies the same mixing energy to several beakers while varying coagulant dose or pH, letting the operator compare floc formation and settling. It is a bench-scale simulation used to optimize chemical treatment. Bacteriological testing, filter loading, and contact time are evaluated by other means.

Question 30Water Treatment

A plant operator records a raw water turbidity of 15 NTU and a filtered water turbidity of 0.10 NTU. What does this indicate about the filter's performance?

  1. The filter is failing and must be taken out of service immediately
  2. The filter is being bypassed
  3. The filter is performing well and is well within typical regulatory limits Correct answer.
  4. The turbidity meter is out of calibration

Explanation

Filtered water turbidity of 0.10 NTU is very low and well below typical regulatory limits, which are generally around 0.3 NTU for conventional filtration. Good removal is reflected by low finished turbidity. Nothing in the data suggests failure, bypass, or meter error.

Question 31Water Treatment

Which chemical is commonly used to raise the pH of finished water and provide corrosion control in the distribution system?

  1. Sodium hypochlorite
  2. Lime or sodium hydroxide Correct answer.
  3. Aluminum sulfate
  4. Sodium chloride

Explanation

Lime and sodium hydroxide are alkaline chemicals used to increase pH and adjust the stability of finished water. Stable water reduces corrosion of pipes and leaching of metals. Hypochlorite is a disinfectant, alum is a coagulant that lowers pH, and sodium chloride is not used for pH adjustment.

Question 32Water Treatment

What does the term 'detention time' describe in a treatment basin?

  1. The length of time a filter runs between backwashes
  2. The time required for a chemical to dissolve in the feed tank
  3. The theoretical time water spends in a basin based on its volume and the flow rate Correct answer.
  4. The interval between calibration checks on a turbidimeter

Explanation

Detention time is calculated by dividing basin volume by flow rate, giving the theoretical residence time. It is used to size basins and evaluate whether processes such as disinfection and sedimentation have adequate contact. Filter run length, chemical dissolution, and calibration intervals are separate concepts.

Question 33Water Treatment

Which of the following is a common symptom that a filter bed may have developed mudballs or media agglomeration?

  1. A decrease in raw water turbidity
  2. A sudden increase in filter effluent pH
  3. Complete loss of media during backwash
  4. Uneven backwash patterns and localized mounding of media Correct answer.

Explanation

Mudballs and agglomerated media cause uneven flow and backwash, often showing as mounds or boils in the bed. These conditions reduce filter effectiveness and typically require air scour or media cleaning. Effluent pH changes, total media loss, and raw turbidity changes are not direct indicators.

Question 34Water Treatment

A plant is treating surface water with low alkalinity using alum. The operator notices the coagulation pH has fallen below the optimum range and floc is poor. Which corrective action is most appropriate?

  1. Add an alkaline chemical such as lime or caustic soda to maintain pH Correct answer.
  2. Increase the alum dose further
  3. Backwash the filters more frequently
  4. Reduce the flocculation mixing intensity

Explanation

Alum consumes alkalinity and depresses pH, which can impair coagulation when natural alkalinity is low. Adding an alkaline chemical restores pH into the optimum coagulation range. Increasing alum would worsen the pH drop, and mixing or backwash changes do not address the chemistry.

Question 35Water Treatment

What is the primary purpose of the sludge collection and removal system in a sedimentation basin?

  1. To remove accumulated solids so the basin retains its designed settling capacity Correct answer.
  2. To aerate the settled solids
  3. To add coagulant to the incoming water
  4. To disinfect the sludge before disposal

Explanation

Solids that settle in a clarifier must be removed to prevent accumulation that reduces basin volume and disrupts flow. Sludge removal maintains effective settling and keeps the process stable. Aeration, disinfection, and coagulant addition are not functions of sludge collection.

Question 36Water Treatment

An operator measures a chlorine residual of 0.2 mg/L at the far end of a distribution system where the target is 0.5 mg/L. Which factor is the most likely cause of the low residual?

  1. Excessive chlorine demand or decay along the pipeline Correct answer.
  2. An alkalinity of 80 mg/L as CaCO3
  3. A turbidity reading of 0.05 NTU
  4. A pH that is too low

Explanation

Chlorine residual declines as it reacts with pipe deposits, organics, and other demand in the distribution system. A low residual at the far end usually reflects demand or decay over travel distance. Low pH and low turbidity would not be expected to cause a residual shortfall.

Question 37Water Treatment

Why is it important to keep the coagulant feed rate proportional to raw water flow in a treatment plant?

  1. To keep the chemical dose constant regardless of flow changes
  2. To reduce the cost of the chemical feed pump
  3. To prevent the coagulant from freezing in the feed line
  4. To maintain a consistent dose in milligrams per liter as plant flow varies Correct answer.

Explanation

Dose is expressed as mass of chemical per volume of water, so the feed rate must track flow to hold a consistent mg/L dose. If flow rises and feed does not, the dose falls and treatment suffers. Pump cost and freezing are unrelated to dose control.

Question 38Water Treatment

A filter run ends because of excessive head loss rather than turbidity breakthrough. Which condition most likely contributes to this pattern?

  1. Filters are being backwashed too aggressively
  2. The clearwell is nearly full
  3. The chlorine residual is too high
  4. Coagulation is producing strong, well-formed floc that is blinding the filter surface Correct answer.

Explanation

Strong floc that collects on the surface of the media can seal the filter and cause rapid head loss buildup, ending the run early. This is sometimes called surface blinding. Aggressive backwash, clearwell level, and chlorine residual do not produce this pattern.

Question 39Water Treatment

What is the main reason a water utility monitors turbidity continuously on individual filter effluents?

  1. To measure the chlorine demand of the filter
  2. To detect filter performance changes quickly so corrective action can be taken Correct answer.
  3. To track chemical costs
  4. To determine the filter backwash rate

Explanation

Continuous turbidity monitoring on each filter provides early warning of breakthrough or media problems, allowing operators to act before finished water quality degrades. It is a performance indicator, not a cost or backwash-rate measurement.

Question 40Water Treatment

A plant using free chlorine for primary disinfection finds that CT requirements are difficult to meet during cold weather. Which explanation is most accurate?

  1. Cold water eliminates the need for disinfection
  2. Cold water decreases the contact time in the clearwell
  3. Cold water increases the decay of chlorine, reducing residual and available CT Correct answer.
  4. Cold water raises the pH of the water, which increases CT

Explanation

Chlorine is less stable and reacts more slowly as a disinfectant in cold water, and decay can reduce the residual that contributes to CT. Operators often must adjust dose or contact time in winter. Temperature does not change clearwell volume or eliminate disinfection requirements.

Question 41Water Treatment

Which of the following best explains why a lime-softening plant might need to add carbon dioxide after softening?

  1. To add hardness back into the water
  2. To increase the chlorine demand
  3. To increase the pH above 11 for disinfection
  4. To stabilize the water and prevent scale deposition in the distribution system Correct answer.

Explanation

Excess-lime softening produces water that is supersaturated with calcium carbonate and prone to scaling. Recarbonation with carbon dioxide lowers pH and stabilizes the water so it does not deposit scale in pipes. It does not add hardness or intentionally raise chlorine demand.

Question 42Water Treatment

An operator needs to determine the filter loading rate in gallons per minute per square foot. Which two measurements are required?

  1. Turbidity and head loss
  2. Filter flow rate and filter surface area Correct answer.
  3. Filter flow rate and media depth
  4. Media depth and backwash rate

Explanation

Loading rate is flow divided by surface area, expressed in gpm per square foot. Media depth and backwash rate are separate design parameters, and turbidity and head loss are performance indicators rather than inputs to the loading calculation.

Question 43Water Treatment

A treatment plant is experiencing taste and odor complaints from customers. Which treatment approach is most directly aimed at removing taste- and odor-causing compounds?

  1. Increasing the filter backwash frequency
  2. Increasing the flocculation time
  3. Raising the clearwell chlorine residual
  4. Adding powdered activated carbon to the treatment process Correct answer.

Explanation

Powdered activated carbon adsorbs many taste- and odor-causing organic compounds and is dosed where contact can occur. It is a common response to episodic taste and odor events. Backwash frequency, chlorine residual, and flocculation time do not directly remove these compounds.

Question 44Water Treatment

An operator calculates that a clearwell holds 500,000 gallons and the plant flow is 5 MGD. What is the approximate theoretical detention time in hours?

  1. 12 hours
  2. 0.4 hours
  3. 24 hours
  4. 2.4 hours Correct answer.

Explanation

Detention time equals volume divided by flow. At 5 MGD, the hourly flow is about 0.208 million gallons per hour, and 0.5 million gallons divided by 0.208 million gallons per hour gives about 2.4 hours. The other values result from arithmetic errors.

Question 45Water Treatment

A utility switches from free chlorine to chloramines for secondary disinfection. What trade-off should the operator expect?

  1. Elimination of all disinfection byproduct formation
  2. Stronger disinfection power with a shorter residual
  3. No change in residual or disinfection performance
  4. Longer-lasting residual with weaker disinfection power against some organisms Correct answer.

Explanation

Chloramines are more stable and persist longer in distribution systems, but they are generally weaker disinfectants than free chlorine for many pathogens. Utilities often use them to reduce byproduct formation while accepting this trade-off. They do not eliminate all byproducts or change nothing.

Question 46Water Treatment

A conventional plant shows good settled water turbidity but rising filtered water turbidity shortly after each backwash. Which cause is most likely?

  1. A clearwell that is too large
  2. Excessive coagulant dose in the raw water
  3. A chlorine residual that is too high
  4. Filter ripening and initial turbidity passage after backwash Correct answer.

Explanation

After backwash, filters go through a ripening period during which capture efficiency is temporarily reduced and some turbidity passes. Operators manage this with filter-to-waste, slow starts, or filter aids. Coagulant dose, chlorine residual, and clearwell size do not explain a post-backwash turbidity spike.

Question 47Water Treatment

An operator is troubleshooting poor floc formation and suspects the rapid mix energy is too low. What is the expected consequence of inadequate rapid mix?

  1. The floc becomes too large and settles out in the rapid mix basin
  2. The coagulant is not dispersed uniformly, so local overdosing and underdosing impair floc formation Correct answer.
  3. The water pH rises sharply
  4. The chlorine demand decreases

Explanation

Rapid mix must disperse coagulant quickly and uniformly so it contacts particles before hydrolysis products form. Poor dispersion creates zones of over- and underdosing that hurt floc quality. It does not produce oversized floc, raise pH sharply, or reduce chlorine demand.

Question 48Water Treatment

A plant's source water alkalinity is 40 mg/L as CaCO3 and the operator wants to use alum at a dose of 40 mg/L. What is the most likely operational concern?

  1. Insufficient alkalinity to buffer the pH change from alum, requiring supplemental alkalinity Correct answer.
  2. The filter loading rate will exceed design limits
  3. The alum will not dissolve at all in the water
  4. Excessive alkalinity causing scale formation in the flocculation basin

Explanation

Alum consumes alkalinity as it hydrolyzes, and each milligram per liter of alum consumes roughly half a milligram per liter of alkalinity as CaCO3. With only 40 mg/L of alkalinity, a 40 mg/L alum dose can depress pH below the optimum range. Supplemental alkalinity or a different coagulant may be needed.

Question 49Water Treatment

An operator observes that chlorine residual in the clearwell is lower than expected even though the dose has not changed. Which condition would most plausibly explain this?

  1. Increased organic matter or ammonia in the raw water raising chlorine demand Correct answer.
  2. A drop in plant flow rate
  3. A decrease in raw water turbidity
  4. A rise in clearwell water level

Explanation

Natural organic matter and ammonia exert chlorine demand, and increases in either consume more chlorine before a residual can be maintained. This can lower clearwell residuals even at a constant dose. Lower turbidity, lower flow, and higher clearwell level would not be expected to reduce residual in this way.

Question 50Water Treatment

Which statement correctly describes the relationship between filter media effective size and uniformity coefficient?

  1. Effective size is the sieve size passing 10 percent of the media, and the uniformity coefficient is the ratio of the 60 percent size to the 10 percent size Correct answer.
  2. Effective size is the media depth, and the uniformity coefficient is the flow rate divided by the surface area
  3. Effective size is the largest particle size, and the uniformity coefficient is the backwash rate divided by the loading rate
  4. Effective size is the average particle diameter, and the uniformity coefficient is the total media depth divided by the media weight

Explanation

Effective size is defined by the sieve opening that passes 10 percent of the media by weight, and the uniformity coefficient is d60 divided by d10. These parameters describe the media gradation and influence filtration and backwash behavior. The other definitions confuse unrelated treatment measurements.