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NC Phase W practice questions

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Question 1Water Treatment

In a conventional surface-water treatment train, which process step is primarily designed to remove dissolved organic compounds and taste-and-odor constituents using granular media?

  1. Granular activated carbon adsorption Correct answer.
  2. Rapid mix coagulation
  3. Secondary clarification
  4. Sodium hypochlorite disinfection

Explanation

Granular activated carbon provides adsorption sites that bind dissolved organics, taste-and-odor compounds, and some synthetic chemicals as water passes through the bed. Coagulation and clarification target particles and turbidity rather than dissolved organics, and disinfection inactivates pathogens without removing dissolved carbon.

Question 2Water Treatment

A water treatment operator measures turbidity in NTU. Which instrument is the standard choice for this measurement in a conventional plant?

  1. Turbidimetric titrator
  2. Conductivity meter
  3. Nephelometer Correct answer.
  4. Colorimeter

Explanation

A nephelometer measures light scattered at 90 degrees by suspended particles and reports results in nephelometric turbidity units. Colorimeters measure absorbance of specific wavelengths, conductivities measure ionic content, and titrators measure concentration by reagent addition.

Question 3Water Treatment

What is the primary purpose of adding a coagulant such as aluminum sulfate during rapid mix?

  1. To raise the pH of the finished water above 9.0
  2. To neutralize particle surface charge and promote floc formation Correct answer.
  3. To provide alkalinity for corrosion control
  4. To oxidize dissolved iron and manganese in the raw water

Explanation

Coagulants hydrolyze to positively charged species that neutralize the negative surface charge on colloids, allowing particles to collide and aggregate. Raising pH above 9, oxidizing metals, and supplying alkalinity are separate treatment objectives handled by other chemicals or processes.

Question 4Water Treatment

During a jar test, the operator observes that floc forms quickly but remains small and pin-floc settles poorly at higher coagulant doses. Which adjustment is most appropriate to evaluate next?

  1. Increase the coagulant dose further to strengthen the floc
  2. Test a coagulant aid or flocculant polymer at a lower primary coagulant dose Correct answer.
  3. Raise the rapid-mix intensity to shear the floc into larger particles
  4. Shorten the slow-mix time to prevent floc breakup

Explanation

Pin floc and poor settling at high coagulant doses often indicate overdosing, so testing a lower primary dose with a polymer flocculant can build larger, denser floc. Increasing coagulant, shortening slow mix, or intensifying rapid mix would generally worsen floc formation or breakup.

Question 5Water Treatment

A filter run ends because of headloss long before turbidity breakthrough. Which operational change is most likely to extend the run?

  1. Increase backwash frequency to reduce accumulated solids in the filter
  2. Reduce the filtration rate to lower solids loading per unit area Correct answer.
  3. Decrease coagulant dose to reduce the amount of floc captured
  4. Switch to shallower media depth to reduce headloss

Explanation

Headloss-limited runs reflect solids accumulation and flow resistance, so reducing the filtration rate lowers solids loading and headloss buildup. More frequent backwashing does not change the loading rate, and reducing coagulant or media depth would degrade particle removal or shorten the run.

Question 6Water Treatment

Which parameter is most important to monitor continuously to confirm that coagulation is properly destabilizing particles in a conventional plant?

  1. Raw water temperature
  2. Filtered water turbidity Correct answer.
  3. Backwash flow rate
  4. Finished water fluoride residual

Explanation

Filtered water turbidity is the primary indicator that coagulation, flocculation, and filtration are working together to remove particles. Temperature affects coagulation but is a raw water characteristic, while fluoride residual and backwash flow rate do not directly confirm particle destabilization.

Question 7Water Treatment

A utility must achieve 3-log removal/inactivation of Giardia cysts. Which combination of treatment processes is typically credited toward this goal?

  1. Backwash plus filter-to-waste
  2. Aeration plus granular activated carbon
  3. Rapid mix plus sedimentation
  4. Filtration plus disinfection (CT) Correct answer.

Explanation

Regulatory log-credit frameworks assign Giardia removal to filtration and inactivation to disinfection measured by CT (disinfectant concentration times contact time). Coagulation, aeration, adsorption, and backwash are not credited for pathogen log removal or inactivation in that framework.

Question 8Water Treatment

Chlorine residual decays faster in water with high levels of dissolved organic matter. What is the most likely explanation?

  1. Organics increase the CT value required for disinfection
  2. Organics consume chlorine and form disinfection byproducts Correct answer.
  3. Organics raise the pH and reduce chlorine solubility
  4. Organics catalyze conversion of free chlorine to chloramine

Explanation

Natural organic matter reacts with free chlorine, consuming the disinfectant and forming regulated disinfection byproducts such as trihalomethanes. Organic matter does not simply raise pH, change the CT requirement, or convert free chlorine to chloramine without ammonia.

Question 9Water Treatment

Which condition is most likely to cause short filter runs with early turbidity breakthrough?

  1. Adequate slow-mix flocculation time
  2. Improper coagulation producing weak, poorly settled floc Correct answer.
  3. Low raw water turbidity
  4. Filter media that has been freshly backwashed

Explanation

Poor coagulation produces small, weak floc that penetrates the filter bed, causing early breakthrough and shortened runs. Freshly backwashed media, low raw turbidity, and adequate flocculation generally support rather than degrade filter performance.

Question 10Water Treatment

What is the primary role of a sedimentation basin following flocculation?

  1. To adsorb dissolved organic compounds
  2. To allow floc to settle and reduce solids loading on filters Correct answer.
  3. To add alkalinity for corrosion control
  4. To disinfect the water before filtration

Explanation

Sedimentation provides quiescent conditions where floc settles by gravity, reducing the particle load reaching the filters. Disinfection, adsorption, and alkalinity addition are performed by other unit processes.

Question 11Water Treatment

A plant uses free chlorine for primary disinfection. At a given residual, which water quality change would most reduce disinfection efficiency?

  1. An increase in contact time in the clearwell
  2. An increase in pH from 7.0 to 8.5 Correct answer.
  3. An increase in chlorine dose
  4. A decrease in water temperature from 20 to 10 degrees C

Explanation

At higher pH, free chlorine shifts toward the less effective hypochlorite ion, reducing disinfection efficiency for a given residual. Lower temperature also slows disinfection, but the pH shift is the classic chemistry effect; longer contact time and higher dose improve, not reduce, efficiency.

Question 12Water Treatment

Which backwash condition is most important to avoid when cleaning a granular media filter?

  1. Excessive bed expansion that washes media to waste Correct answer.
  2. Backwash duration sufficient to remove captured solids
  3. Backwash water turbidity that starts high and clears
  4. Bed expansion of about 20 to 30 percent

Explanation

Excessive expansion can carry media over the wash trough and out of the filter, reducing media depth and degrading future runs. Moderate expansion, initial dirty backwash water that clears, and adequate duration are all normal and desirable.

Question 13Water Treatment

A groundwater source contains dissolved iron and manganese. Which treatment approach is most appropriate before filtration?

  1. Oxidation followed by precipitation and filtration Correct answer.
  2. Coagulation with alum alone without oxidation
  3. Ion exchange softening only
  4. Granular activated carbon adsorption alone

Explanation

Dissolved iron and manganese must be oxidized to insoluble forms that can precipitate and be removed by filtration. Coagulation alone, softening, or carbon adsorption do not reliably convert and remove these dissolved metals.

Question 14Water Treatment

Which chemical is commonly used to adjust finished water pH upward for corrosion control without adding significant hardness?

  1. Sodium hypochlorite
  2. Sodium hydroxide Correct answer.
  3. Ferric chloride
  4. Aluminum sulfate

Explanation

Sodium hydroxide is a strong base used to raise pH and adjust alkalinity for corrosion control. Alum and ferric chloride are coagulants that lower pH, and sodium hypochlorite is a disinfectant.

Question 15Water Treatment

A distribution system sample shows a free chlorine residual of 0.2 mg/L at a location far from the plant. Which action best addresses the likely cause?

  1. Evaluate chlorine demand and consider boosting residual or reducing decay in the system Correct answer.
  2. Lower the clearwell contact time
  3. Increase coagulant dose at the plant
  4. Reduce filter backwash frequency

Explanation

Low residual far from the plant usually reflects chlorine decay from pipe demand, long travel time, or nitrification, so demand assessment or residual boosting is appropriate. Coagulant dose, backwash frequency, and clearwell contact time do not directly correct distribution residual loss.

Question 16Water Treatment

What does the term CT represent in disinfection practice?

  1. Chlorine demand plus chlorine residual
  2. Coagulant dose divided by turbidity
  3. Disinfectant residual concentration multiplied by contact time Correct answer.
  4. Contact tank volume divided by flow rate

Explanation

CT is the product of disinfectant residual concentration (C) and effective contact time (T), used to demonstrate inactivation credit. The other expressions describe different plant calculations and do not represent the CT concept.

Question 17Water Treatment

A utility observes increased trihalomethane formation in finished water. Which operational strategy most directly reduces THM formation?

  1. Enhance removal of natural organic matter before chlorination Correct answer.
  2. Raise clearwell pH to 9.5
  3. Increase coagulant dose without jar testing
  4. Increase chlorine dose to improve disinfection

Explanation

THMs form when chlorine reacts with natural organic matter, so removing precursors before chlorination is the most direct control strategy. Increasing chlorine dose or raising pH can worsen or shift byproduct formation, and untested coagulant changes risk poor performance.

Question 18Water Treatment

Which statement best describes the purpose of filter-to-waste following backwash?

  1. To bypass the filter during high demand periods
  2. To add coagulant to finished water
  3. To send backwash water directly to the clearwell
  4. To divert initial filtered water until turbidity returns to acceptable levels Correct answer.

Explanation

Filter-to-waste diverts the first filtered water after backwash, when turbidity and particle passage are elevated, until effluent quality stabilizes. It does not route water to the clearwell, bypass filters for demand, or dose coagulant.

Question 19Water Treatment

A plant must lower finished water lead corrosion risk in a system with low alkalinity. Which adjustment is generally recommended?

  1. Increase chloride-to-sulfate ratio
  2. Increase alkalinity and adjust pH to promote protective scale formation Correct answer.
  3. Decrease pH to make water less scaling
  4. Add more free chlorine to oxidize lead

Explanation

Low alkalinity water is often corrosive, and increasing alkalinity while adjusting pH helps form protective scales that reduce lead leaching. Lowering pH, raising the chloride-to-sulfate ratio, or oxidizing lead with chlorine would tend to increase corrosion risk.

Question 20Water Treatment

Which factor most strongly governs the settling velocity of a discrete particle in a sedimentation basin?

  1. Particle diameter and density difference from water Correct answer.
  2. Basin wall material
  3. Filter media size
  4. Chlorine residual

Explanation

For discrete settling, velocity depends on particle size and the density difference between the particle and water, as described by Stokes' law. Wall material, chlorine residual, and filter media size do not determine particle settling velocity in the basin.

Question 21Water Treatment

A surface water plant experiences an algal bloom causing high pH and taste-and-odor complaints. Which combination of actions is most appropriate?

  1. Stop backwashing to preserve biomass
  2. Increase filter rate to move water faster
  3. Optimize coagulation for changed pH and apply powdered activated carbon Correct answer.
  4. Reduce disinfection to limit byproduct formation

Explanation

Algal blooms change pH and add organic taste-and-odor compounds, so coagulation must be reoptimized and powdered activated carbon applied for adsorption. Increasing filter rate, cutting disinfection, or stopping backwash would compromise treatment and water quality.

Question 22Water Treatment

Why is it important to maintain a measurable disinfectant residual throughout the distribution system?

  1. To remove dissolved iron from pipes
  2. To increase water hardness for consumer acceptance
  3. To provide a barrier against microbial contamination after treatment Correct answer.
  4. To reduce the need for corrosion control

Explanation

A residual disinfectant protects finished water from microbial contamination that can enter during distribution, such as through main breaks or cross-connections. It does not harden water, remove iron, or replace corrosion control.

Question 23Water Treatment

A filter shows negative headloss (a vacuum) during operation. What is the most likely cause?

  1. Media or underdrain blockage restricting flow through the filter Correct answer.
  2. Low coagulant dose
  3. Excessive backwash expansion
  4. High raw water alkalinity

Explanation

Negative headloss indicates the pressure below the media is lower than atmospheric, usually from a blockage in the media or underdrain that restricts flow. Backwash expansion, coagulant dose, and alkalinity do not create vacuum conditions in a filter.

Question 24Water Treatment

Which monitoring approach best verifies that a UV disinfection reactor is delivering the required dose?

  1. Counting backwash cycles per day
  2. Measuring chlorine residual downstream
  3. Tracking UV transmittance and reactor flow against validated dose curves Correct answer.
  4. Measuring filter effluent turbidity only

Explanation

UV dose depends on water transmittance and flow through a validated reactor, so monitoring these parameters against validation curves confirms dose delivery. Chlorine residual, turbidity alone, and backwash counts do not verify UV dose.

Question 25Water Treatment

What is the main reason for adding fluoride to drinking water in many utilities?

  1. To reduce the incidence of dental caries Correct answer.
  2. To improve coagulation efficiency
  3. To remove manganese from source water
  4. To control corrosion in pipes

Explanation

Fluoridation is practiced to reduce dental caries in the population served. It is not a coagulation aid, corrosion-control chemical, or manganese removal process.

Question 26Water Treatment

Which raw water characteristic most directly increases the required coagulant dose for a surface water plant using alum?

  1. High turbidity and particle surface area Correct answer.
  2. Low temperature with stable pH
  3. High alkalinity relative to turbidity
  4. Low dissolved organic carbon concentration

Explanation

Coagulant demand rises with the amount of particulate and colloidal material that must be destabilized and swept into floc, so higher turbidity and particle surface area consume more coagulant. Alkalinity affects pH buffering rather than being the primary driver of dose for particle removal.

Question 27Water Treatment

In a conventional treatment train, what is the principal purpose of rapid mix?

  1. To remove dissolved gases by aeration
  2. To disperse coagulant rapidly and uniformly through the water Correct answer.
  3. To provide contact time for disinfection
  4. To build large, settleable floc particles

Explanation

Rapid mix supplies intense, brief turbulence so coagulant is distributed throughout the flow before hydrolysis and adsorption reactions are complete. Floc growth is the job of the later, gentler flocculation stage.

Question 28Water Treatment

A jar test shows that the lowest turbidity occurs across a narrow range of coagulant dose, with turbidity rising sharply on either side. What does this pattern most likely indicate?

  1. Charge neutralization is the dominant mechanism and the optimum dose is near the charge-neutral point Correct answer.
  2. The coagulant is being overdosed only at the highest doses
  3. The water contains no alkalinity and coagulation cannot proceed
  4. The floc is too large to be measured reliably

Explanation

A sharp turbidity minimum indicates that particle destabilization depends on neutralizing the surface charge, so both underdosing and overdosing leave residual repulsion or restabilization. Sweep-floc systems tend to show a broader plateau of good performance.

Question 29Water Treatment

Which statement correctly describes the velocity gradient (G) used in flocculation design?

  1. G is expressed in units of inverse seconds and reflects mixing energy dissipated per unit volume Correct answer.
  2. Tapered flocculation means G increases from the first to the last stage
  3. G is independent of basin volume and water viscosity
  4. Higher G always improves floc formation regardless of floc strength

Explanation

The velocity gradient represents the square root of power dissipated per unit volume divided by viscosity, giving units of reciprocal seconds. Designers taper G downward through flocculation stages because floc becomes fragile as it grows.

Question 30Water Treatment

A plant's floc is pin-floc that carries over the weirs and blinds the filters. Which corrective action is most appropriate?

  1. Reduce coagulant dose below the jar-test optimum
  2. Extend gentle flocculation time and verify coagulant dose and pH are at optimum Correct answer.
  3. Increase filter loading rate to push floc through the media
  4. Increase the flocculation velocity gradient substantially

Explanation

Pin-floc results from insufficient floc growth, often from too little gentle mixing time or a non-optimal coagulation chemistry. Extending slow flocculation and confirming dose and pH address the root cause.

Question 31Water Treatment

In a rectangular horizontal-flow sedimentation basin, which design change most directly reduces the risk of density currents and short-circuiting?

  1. Increasing the surface overflow rate
  2. Reducing the basin depth while keeping detention time constant
  3. Removing the effluent weir and using a single point withdrawal
  4. Installing baffles and improving inlet energy dissipation Correct answer.

Explanation

Baffles and well-dissipated inlet conditions spread flow across the basin and suppress density currents that otherwise carry particles to the outlet prematurely. Reducing short-circuiting improves actual detention time and settling performance.

Question 32Water Treatment

Which condition would most likely increase the surface overflow rate required to achieve a given particle removal in a settling basin?

  1. An increase in the number of basins in service
  2. An increase in basin length with the same width and flow
  3. A decrease in the settling velocity of the design particle Correct answer.
  4. A decrease in plant flow rate

Explanation

The surface overflow rate must be at or below the settling velocity of the particle to be removed, so a slower-settling design particle requires a lower overflow rate and thus more surface area. Adding basins or reducing flow lowers the applied overflow rate.

Question 33Water Treatment

A gravity filter is operating with a headloss that has reached the available driving head while effluent turbidity remains acceptable. What is the appropriate operational response?

  1. Increase coagulant dose to reduce headloss
  2. Reduce the filter loading rate without backwashing
  3. Backwash the filter to restore clean-bed headloss Correct answer.
  4. Continue running until turbidity exceeds the limit

Explanation

When terminal headloss is reached, the filter can no longer pass design flow and must be backwashed to remove accumulated solids. Waiting for turbidity breakthrough risks compliance exceedance and media problems.

Question 34Water Treatment

During backwash of a dual-media anthracite-over-sand filter, which condition indicates that the media beds may have intermixed?

  1. Anthracite appears at the surface after backwash Correct answer.
  2. Air scour is applied before the water wash
  3. The filter bed expands by about 20 percent during backwash
  4. Turbidity in the backwash water rises and then clears

Explanation

Anthracite is less dense than sand and should remain on top; if sand appears above anthracite or the layers blend, the media have intermixed. Excessive backwash rate or inadequate stratification during backwash is a common cause.

Question 35Water Treatment

What is the primary function of a filter underdrain in a granular media filter?

  1. To remove dissolved organics by adsorption
  2. To support the media and distribute backwash water uniformly Correct answer.
  3. To provide disinfection contact time
  4. To measure effluent turbidity

Explanation

The underdrain supports the media and gravel and distributes backwash water and air evenly across the bed. Uniform distribution prevents media disturbance and ensures effective cleaning.

Question 36Water Treatment

A utility using free chlorine wants to maintain an effective CT for Giardia inactivation while reducing regulated disinfection byproduct formation. Which operational change best supports both goals?

  1. Adding chlorine before coagulation to maximize contact
  2. Increasing chlorine dose and shortening contact time
  3. Switching to a higher chlorine dose at a warmer temperature
  4. Improving upstream removal of DBP precursors and using a lower chlorine residual with adequate contact time Correct answer.

Explanation

Removing precursor organic matter before chlorination lowers DBP formation while allowing a lower residual to achieve the required CT through adequate contact time. Prechlorination typically increases precursor reactions and DBP formation.

Question 37Water Treatment

Which statement best describes the role of the baffling factor in disinfection CT calculations?

  1. It corrects for the effect of water temperature on disinfectant decay
  2. It adjusts theoretical contact time to account for short-circuiting and actual flow patterns Correct answer.
  3. It sets the minimum required log inactivation for viruses
  4. It converts chlorine residual into a concentration-time product

Explanation

The baffling factor reduces the theoretical detention time to a conservative effective contact time that reflects imperfect flow patterns. Better baffling yields a higher factor and more credited contact time.

Question 38Water Treatment

A clearwell is credited with disinfection contact time. Which change would most reduce the credited CT without changing the chlorine dose?

  1. Increasing the water temperature
  2. Reducing the plant flow rate
  3. Improving inlet and outlet baffling
  4. Lowering the clearwell water level and reducing effective volume Correct answer.

Explanation

Credited contact time depends on effective volume and flow, so lowering the water level shortens detention time and reduces CT. Better baffling and lower flow would increase credited CT.

Question 39Water Treatment

Which operational practice best minimizes the risk of nitrification in a chloraminated distribution system?

  1. Reducing system pH to below 6.5
  2. Increasing free ammonia to stabilize the residual
  3. Maintaining a stable chloramine residual and controlling free ammonia Correct answer.
  4. Switching to free chlorine only during winter months

Explanation

Nitrifying bacteria thrive when free ammonia is present and chloramine residual is weak, so controlling ammonia and maintaining a stable residual limit their growth. Utilities also monitor for nitrite and ammonia trends as early indicators.

Question 40Water Treatment

A distribution system experiences a sudden loss of pressure below 20 psi in a main break area. What is the most appropriate immediate response?

  1. Increase chlorine dose and take no other action
  2. Issue a precautionary boil water notice and begin corrective actions Correct answer.
  3. Flush the area and resume normal operation without notification
  4. Wait for confirmed bacteriological samples before any public notice

Explanation

A pressure loss below the regulatory threshold creates a pathway for contamination, so a precautionary notice and corrective actions such as flushing, repair, and sampling are required. Waiting for sample results delays protection of public health.

Question 41Water Treatment

Which parameter is typically monitored as an early warning of nitrification in a chloraminated system?

  1. Calcium hardness
  2. Turbidity in the clearwell
  3. Nitrite concentration Correct answer.
  4. Fluoride concentration

Explanation

Nitrite is produced by ammonia-oxidizing and nitrite-oxidizing bacteria and rises before chloramine residual collapses, making it a useful early indicator. Ammonia and chloramine residual trends are monitored alongside it.

Question 42Water Treatment

A utility must reduce lead and copper corrosion in a low-alkalinity, low-pH finished water. Which combination is generally recommended?

  1. Increase pH and alkalinity to promote a protective scale Correct answer.
  2. Reduce pH to keep metals soluble
  3. Lower pH and reduce alkalinity
  4. Add chloride to increase conductivity

Explanation

Raising pH and alkalinity favors formation of a protective passivating scale on pipe surfaces, reducing metal release. Very low pH and alkalinity leave water aggressive toward plumbing materials.

Question 43Water Treatment

Which statement best describes the purpose of a corrosion control treatment study under the Lead and Copper Rule?

  1. To size filters for turbidity removal
  2. To calculate disinfection CT requirements
  3. To set the maximum contaminant level for lead
  4. To determine the optimal treatment to minimize lead and copper at customer taps Correct answer.

Explanation

Corrosion control studies evaluate treatment options such as pH adjustment, alkalinity adjustment, and inhibitor addition to find the most effective approach for that water. The results support the utility's optimal corrosion control treatment designation.

Question 44Water Treatment

A utility plans to use UV disinfection for Cryptosporidium inactivation. Which monitoring approach best verifies that the required dose is delivered?

  1. Measuring free chlorine residual downstream of the reactor
  2. Measuring turbidity only at the plant intake
  3. Recording lamp hours without flow measurement
  4. Measuring UV absorbance and flow to compute the validated reduction equivalent dose Correct answer.

Explanation

UV dose is verified by combining measured UV transmittance and flow with the reactor's validated dose algorithm to compute the reduction equivalent dose. Chlorine residual does not verify UV dose.

Question 45Water Treatment

Which condition most reduces the effectiveness of UV disinfection for a given reactor and flow?

  1. Low iron concentration in the water
  2. High UV lamp intensity
  3. Low UV transmittance caused by turbidity and organics Correct answer.
  4. Short lamp sleeve fouling

Explanation

Low UV transmittance means less light penetrates the water, reducing the dose received by pathogens at a given lamp output and flow. Utilities must account for transmittance in the validated dose calculation.

Question 46Water Treatment

A plant treating a groundwater with high dissolved iron and manganese plans to use oxidation followed by filtration. Which sequence is most appropriate?

  1. Oxidize the dissolved metals to particulate forms, then filter Correct answer.
  2. Adjust pH downward before oxidation
  3. Add a sequestering agent and avoid filtration
  4. Filter first, then oxidize the filtered water

Explanation

Dissolved iron and manganese must be oxidized to insoluble particles before they can be captured by a filter. Filtering first allows dissolved metals to pass through and later precipitate in the distribution system.

Question 47Water Treatment

Which process is most effective at removing dissolved organic carbon that serves as a precursor to disinfection byproducts?

  1. Granular media filtration alone
  2. Sedimentation without chemical addition
  3. Enhanced coagulation with an optimal coagulant dose Correct answer.
  4. Fluoridation

Explanation

Enhanced coagulation increases removal of dissolved organic carbon by optimizing coagulant dose and pH for precursor removal. Plain sedimentation and filtration remove particles but little dissolved organic matter.

Question 48Water Treatment

An operator observes rising effluent turbidity from a filter immediately after backwash that clears within a few minutes. Which practice best reduces this post-backwash turbidity spike?

  1. Returning the filter to service immediately at full rate
  2. Increasing the backwash rate further
  3. Using filter-to-waste or a slow controlled start after backwash Correct answer.
  4. Reducing the filter media depth

Explanation

Filter-to-waste or a slow rate start allows the initial ripening period to pass before filtered water enters the clearwell. This controls the turbidity spike that occurs while the bed restratifies and captures particles.

Question 49Water Treatment

A utility wants to reduce the formation of chlorination byproducts while maintaining primary disinfection credit. Which strategy is most consistent with regulatory practice?

  1. Removing precursors before chlorination and optimizing the point of chlorine application Correct answer.
  2. Increasing the free chlorine dose and shortening contact time
  3. Chlorinating raw water to maximize contact time
  4. Switching to a higher chlorine residual throughout the system

Explanation

Removing precursors before chlorine is applied and choosing an application point that minimizes reaction with organics reduces byproduct formation while preserving disinfection credit. Prechlorination of raw water typically increases byproduct formation.

Question 50Water Treatment

Which statement best describes the relationship between filter media effective size and backwash expansion?

  1. A larger effective size generally requires a higher backwash rate to achieve the same percent expansion Correct answer.
  2. Backwash expansion is determined only by water temperature
  3. Effective size has no influence on the backwash rate needed for a given expansion
  4. A smaller effective size generally requires a higher backwash rate

Explanation

Coarser media have higher settling velocities and require greater upflow velocity, and thus a higher backwash rate, to achieve a given bed expansion. Temperature also affects water viscosity and expansion but is not the only factor.