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IL Water/WW Operator practice questions

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Question 1Source Water and Preliminary Treatment

A community draws its drinking water from a deep sandstone aquifer through a well. How is this source classified?

  1. Reclaimed wastewater
  2. Surface water
  3. Groundwater Correct answer.
  4. A blend of surface water and groundwater

Explanation

Water withdrawn from a saturated geologic formation below the land surface is a groundwater source. Deep aquifers are reached by wells rather than by direct withdrawal from a river, lake, or reservoir.

Question 2Source Water and Preliminary Treatment

What is the main purpose of a bar screen at a plant headworks?

  1. Disinfecting the raw water ahead of the filters
  2. Trapping large debris that could damage downstream equipment Correct answer.
  3. Removing settleable grit before the water reaches the clarifiers
  4. Removing dissolved gases such as carbon dioxide from the raw water

Explanation

Bar screens are the first physical barrier in the plant and catch rags, sticks, plastics, and other large objects before they can wrap pumps, plug pipes, or damage equipment. Grit removal and disinfection are separate processes performed later.

Question 3Source Water and Preliminary Treatment

Grit collected at a treatment plant headworks is best described as which of the following?

  1. Heavy inorganic solids such as sand, gravel, and eggshells Correct answer.
  2. Dissolved minerals that must be removed by ion exchange
  3. Biological growth that develops on wet screen surfaces
  4. Light organic material such as food waste, paper fibers, and vegetable matter

Explanation

Grit is the dense, largely inorganic fraction of the solids, including sand, gravel, cinders, and eggshells, that settles quickly and can abrade pumps and accumulate in tanks. Lighter organic material is kept in suspension and carried on to later processes.

Question 4Source Water and Preliminary Treatment

Turbidity measured on a raw water sample indicates which of the following?

  1. The concentration of dissolved calcium and magnesium ions
  2. The amount of chlorine needed to reach a free residual
  3. Light scattered by suspended and colloidal particles Correct answer.
  4. The total mass of solids retained on a glass fiber filter

Explanation

Turbidity is an optical measurement of light scattered by particles suspended in the water, including clays, algae, and colloids. It responds quickly to changes in source quality and is used as an indirect indicator of particle concentration.

Question 5Source Water and Preliminary Treatment

Compared with water from a deep aquifer, water taken from a river typically has which characteristic?

  1. Lower concentrations of suspended solids
  2. Higher and more seasonally variable turbidity Correct answer.
  3. A temperature that stays constant all year
  4. No need for disinfection before distribution

Explanation

River water is exposed to runoff, storms, and seasonal biology, so its turbidity and overall quality change frequently and generally run higher than in protected groundwater. Deep groundwater has been filtered through soil and is more stable.

Question 6Source Water and Preliminary Treatment

What is the primary objective of a wellhead protection program?

  1. Disinfecting the aquifer with chlorine during recharge
  2. Preventing contaminants from reaching the aquifer near a well Correct answer.
  3. Reducing pumping costs by lowering the water table
  4. Increasing well yield by redeveloping the gravel pack

Explanation

Wellhead protection identifies the land area that contributes water to a well and manages activities there so that contaminants do not reach the aquifer. It is a preventive, land-use based approach rather than a treatment or capacity-improvement measure.

Question 7Source Water and Preliminary Treatment

Which pair of unit processes makes up typical preliminary treatment at a water treatment plant?

  1. Screening and grit removal Correct answer.
  2. Aeration and ion exchange
  3. Filtration and disinfection
  4. Coagulation and flocculation

Explanation

Preliminary treatment is the physical removal of large debris and heavy grit before the main treatment train. Coagulation, filtration, and disinfection are later, principal treatment processes.

Question 8Source Water and Preliminary Treatment

What does a comminutor installed in a headworks channel do?

  1. Adds coagulant chemical to the raw water as it enters the plant
  2. Removes grit from the flow by means of centrifugal action
  3. Measures the rate of flow entering the plant with a weir or flume
  4. Shreds screenings so they can pass on with the flow Correct answer.

Explanation

A comminutor uses rotating cutters to grind screenings in the channel so the material stays in the flow rather than being removed. This protects pumps and downstream equipment while avoiding the handling of wet, raw screenings.

Question 9Source Water and Preliminary Treatment

A grit chamber is designed to hold a velocity of about 1 ft/s so that which condition is achieved?

  1. Grit settles out while lighter organic solids stay in suspension Correct answer.
  2. Pathogenic organisms are inactivated before clarification
  3. Dissolved oxygen stays above 4 mg/L within the chamber
  4. All suspended solids settle to the bottom for removal

Explanation

The design velocity is high enough to keep light organic material moving toward the clarifiers but low enough for dense grit to settle. If the velocity is too low, organics settle with the grit; if too high, grit is scoured out.

Question 10Source Water and Preliminary Treatment

How does a Parshall flume measure flow?

  1. Relating the upstream water depth to flow through a known geometry Correct answer.
  2. Measuring the pressure drop across an orifice plate in the pipe
  3. Weighing the water as it passes over a submerged weir plate
  4. Recording the time required for a float to travel a known distance

Explanation

A Parshall flume is a calibrated open-channel section with a known geometry, so a single depth measurement at a defined point gives flow through a standard rating table. It has no moving parts in the flow and works with only a small head loss.

Question 11Source Water and Preliminary Treatment

In a eutrophic reservoir during summer stratification, the bottom layer typically develops which condition?

  1. Water colder than the surface but fully oxygenated
  2. Anoxic conditions with accumulated iron, manganese, and sulfide Correct answer.
  3. Continuous mixing with the surface layer by wind action
  4. The highest dissolved oxygen and lowest dissolved iron levels

Explanation

The hypolimnion is isolated from the atmosphere and from algal photosynthesis, so oxygen is consumed by decay and reduced iron, manganese, and sulfide accumulate. Withdrawing this water creates treatment problems, particularly after fall turnover mixes it upward.

Question 12Source Water and Preliminary Treatment

Why do many river intakes include several intake ports set at different depths?

  1. To keep dissolved oxygen above saturation at all times
  2. To increase the velocity of water entering the wet well
  3. To allow screens to be cleaned without stopping the flow
  4. To let operators withdraw water of the best available quality Correct answer.

Explanation

Water quality varies with depth, so multiple ports let operators avoid floating debris and algae at the surface and anoxic, iron-laden water near the bottom. The port that gives the best raw quality is selected based on current conditions.

Question 13Source Water and Preliminary Treatment

An earthy, musty taste and odor in a surface water supply is most often associated with which cause?

  1. Corrosion products released from cast iron mains
  2. Compounds such as geosmin and MIB produced by algae Correct answer.
  3. Excess sodium from road salt entering the reservoir
  4. High alkalinity caused by limestone in the watershed

Explanation

Geosmin and 2-methylisoborneol are metabolic products of algae and actinomycetes that cause earthy, musty odors at extremely low concentrations. They are dissolved compounds, so settling alone does not remove them.

Question 14Source Water and Preliminary Treatment

Which organism is highly resistant to chlorine disinfection and must be physically removed by filtration?

  1. Cryptosporidium oocysts Correct answer.
  2. Total coliform bacteria
  3. Iron bacteria
  4. Escherichia coli

Explanation

Cryptosporidium oocysts survive normal chlorine doses, so filtration, and in some cases UV or ozone, provide the barrier against them. Indicator bacteria such as coliforms are far more susceptible to chlorine.

Question 15Source Water and Preliminary Treatment

Well water containing about 1.0 mg/L of dissolved iron is most likely to cause which consumer complaint?

  1. Blue-green staining caused by copper corrosion
  2. Salty taste and elevated sodium in the water
  3. Reddish-brown staining and a metallic taste Correct answer.
  4. White soap scum and scale on plumbing fixtures

Explanation

Dissolved ferrous iron oxidizes when exposed to air, forming rust-colored deposits that stain fixtures, laundry, and dishes and give a metallic taste. Manganese behaves similarly and often accompanies iron.

Question 16Source Water and Preliminary Treatment

Hardness in raw groundwater is caused primarily by which constituents?

  1. Calcium and magnesium dissolved from rock Correct answer.
  2. Nitrate and phosphate from fertilizer
  3. Dissolved carbon dioxide and hydrogen sulfide
  4. Sodium and potassium chloride

Explanation

Hardness is predominantly calcium and magnesium picked up as water moves through limestone, dolomite, and similar formations. It is measured as calcium carbonate equivalents and causes scale and soap consumption.

Question 17Source Water and Preliminary Treatment

The head loss across a bar screen rises steadily during a shift. What does this most likely indicate?

  1. The flow rate entering the plant has dropped
  2. The raw water pump has lost its prime
  3. The grit chamber downstream is removing too much material
  4. The screen is blinding and needs to be cleaned Correct answer.

Explanation

As screenings accumulate, the open area of the screen shrinks and the differential head across it climbs, eventually causing upstream flooding or bypassing. Cleaning the screen with a rake or backwash restores the low head loss.

Question 18Source Water and Preliminary Treatment

Presedimentation ahead of a water treatment plant is used mainly for which purpose?

  1. Adding fluoride to the water before coagulation begins
  2. Removing heavy settleable solids and reducing the load on the plant Correct answer.
  3. Disinfecting the raw water so that filter runs are longer
  4. Cooling the raw water to a temperature suited to coagulation

Explanation

A presedimentation basin provides long, low-velocity detention so sand and other heavy solids drop out before the water reaches the main plant. This reduces solids loading, chemical demand, and sludge production downstream.

Question 19Source Water and Preliminary Treatment

Zebra mussels cause problems for water treatment plants primarily by which mechanism?

  1. Raising the pH of the raw water above 9.0
  2. Using up chlorine residual in the plant
  3. Clogging intake screens, pipes, and small-diameter lines Correct answer.
  4. Producing toxins that pass through conventional treatment

Explanation

Mussel veligers settle and grow on hard surfaces inside intakes, screens, and piping, restricting flow and requiring mechanical, thermal, or chemical control. They are a physical fouling problem rather than a chemical or toxicological one.

Question 20Source Water and Preliminary Treatment

Under the federal Safe Drinking Water Act, what is the purpose of a source water assessment for a public water system?

  1. Determining the required chlorine contact time for disinfection
  2. Setting the maximum contaminant levels allowed in finished water
  3. Certifying the treatment plant's filter effluent turbidity
  4. Evaluating how susceptible the water source is to contamination Correct answer.

Explanation

A source water assessment inventories potential contaminant sources in the area that drains to the intake or well and rates the system's susceptibility to those sources. It informs protection efforts rather than setting treatment standards.

Question 21Source Water and Preliminary Treatment

A rectangular grit channel 3.0 ft wide carries 6.0 cfs. What water depth keeps the average velocity at 1.0 ft/s?

  1. 0.5 ft
  2. 2.0 ft Correct answer.
  3. 3.0 ft
  4. 6.0 ft

Explanation

Velocity equals flow divided by cross-sectional area, so the required area is 6.0 cfs divided by 1.0 ft/s, or 6.0 ft². Dividing that area by the 3.0 ft channel width gives a depth of 2.0 ft.

Question 22Source Water and Preliminary Treatment

A new high-capacity well is placed in service. A shallow dug well 200 ft away soon produces less water. What best explains this?

  1. Total dissolved solids in the aquifer have suddenly increased
  2. Pumping has lowered the water table around the new well Correct answer.
  3. Barometric pressure at the well field has fallen sharply
  4. The shallow well's screen has become plugged by iron bacteria

Explanation

Pumping creates a cone of depression in the water table that spreads outward from the well; a nearby shallow well drawing from the same zone sees a lower water level and reduced yield. This interference is a hydraulic effect, not a change in water chemistry.

Question 23Source Water and Preliminary Treatment

Raw reservoir water has a strong hydrogen sulfide odor and carries 0.3 mg/L of dissolved iron. Which preliminary step is most appropriate?

  1. Raising the pH to 10.5 with lime before settling
  2. Adding fluoride and a corrosion inhibitor
  3. Passing the water through a fine screen
  4. Pre-aerating or chemically oxidizing the raw water Correct answer.

Explanation

Both sulfide and ferrous iron are dissolved and must be oxidized to be removed; aeration or chemical oxidation converts sulfide to sulfate and iron to an insoluble form that settles or filters out, and it also strips odors. Screening and pH adjustment alone do not address dissolved constituents.

Question 24Source Water and Preliminary Treatment

Air flow to an aerated grit chamber is reduced well below the design rate. What is the most likely result?

  1. The chamber velocity rises and scours previously settled grit
  2. Organic solids settle with the grit, producing odorous material Correct answer.
  3. Grit stays in suspension and is carried into the clarifiers
  4. Dissolved oxygen in the chamber rises above saturation

Explanation

The air supply creates the spiral roll that keeps lighter organic particles in suspension while dense grit settles. With too little air, organics settle along with the grit, making the collected material putrescible and odorous.

Question 25Source Water and Preliminary Treatment

Frazil ice threatens a river intake during severe cold weather because it does what?

  1. Raises the water temperature enough to disrupt treatment
  2. Forms crystals in supercooled, turbulent water that clog screens and pipes Correct answer.
  3. Dissolves additional oxygen and accelerates intake corrosion
  4. Builds a solid sheet that floats on the surface only

Explanation

Frazil ice consists of fine, needle-like crystals that form in turbulent supercooled water and adhere to screens, bars, and pipe walls, rapidly blocking flow. Control measures include keeping screens submerged and heated, or providing alternate intake capacity.

Question 26Source Water and Preliminary Treatment

At a surface water intake, what is the primary function of a coarse bar screen?

  1. To settle fine silt and clay before pumping
  2. To remove dissolved minerals that cause hardness
  3. To remove large debris and trash Correct answer.
  4. To disinfect the water and kill bacteria

Explanation

A coarse bar screen is a physical barrier that stops branches, rags, and other large objects before they reach pumps or downstream equipment. It does not provide disinfection, softening, or settling of fine particles.

Question 27Source Water and Preliminary Treatment

A V-notch weir measures flow by relating discharge to which variable?

  1. The velocity of water through a submerged pipe
  2. The pressure drop across an orifice plate
  3. The weight of water collected in a tank
  4. The depth of water above the notch Correct answer.

Explanation

Flow over a V-notch weir is calculated from the head, the water depth measured upstream of the notch. The weir equation uses head raised to the three-halves power along with the notch angle and a discharge coefficient.

Question 28Source Water and Preliminary Treatment

How does a vortex grit chamber separate grit from raw water?

  1. By passing water through a fine mesh screen
  2. By heating the water so grit settles faster
  3. By a rotating flow that throws denser grit outward Correct answer.
  4. By adding chemicals that make grit float

Explanation

A vortex grit chamber uses a rotating flow pattern that drives denser grit particles toward the wall and bottom while the lighter water exits. The grit is then collected and removed.

Question 29Source Water and Preliminary Treatment

Why is a shallow dug well more vulnerable to surface contamination than a deep drilled well?

  1. It is shallow and lacks a deep confining seal Correct answer.
  2. It has a higher pumping rate
  3. It produces harder water
  4. It contains more dissolved oxygen

Explanation

A shallow dug well is completed near the ground surface and typically has little or no deep confining layer above the water. That makes it easier for surface water, septic effluent, or spilled chemicals to reach the well.

Question 30Source Water and Preliminary Treatment

What is the main purpose of destratifying a raw water reservoir with aeration?

  1. To cool the entire reservoir so that algal growth stops completely
  2. To increase the calcium hardness of the stored water before treatment
  3. To keep the water aerobic and limit iron and manganese release Correct answer.
  4. To raise the pH enough to precipitate all of the dissolved calcium

Explanation

Destratification mixes the reservoir and maintains aerobic conditions throughout, which prevents the bottom layer from becoming anoxic. An anoxic hypolimnion can release dissolved iron, manganese, and sulfide from sediments and cause taste and odor problems.

Question 31Source Water and Preliminary Treatment

A river intake is sited a short distance downstream of a municipal wastewater outfall. What is the main water quality concern?

  1. The water will be too cold for treatment
  2. The water will be too hard to treat
  3. The water will contain excess fluoride
  4. The water may carry pathogens and higher organic loads Correct answer.

Explanation

Wastewater outfalls can contribute pathogens, nutrients, and organic matter to the receiving stream. A nearby downstream intake therefore faces higher disinfection demand and greater risk of microbial contamination.

Question 32Source Water and Preliminary Treatment

What does the term groundwater under the direct influence of surface water (GWUDI) mean for a public water system?

  1. The well is located more than 1,000 feet from any river
  2. The source must meet surface water treatment rules Correct answer.
  3. The water is exempt from all treatment requirements
  4. The water can be disinfected only with ultraviolet light

Explanation

A GWUDI source is groundwater that is hydraulically connected to surface water and is vulnerable to surface contamination. Because of that connection, the system must provide treatment comparable to what is required for surface water.

Question 33Source Water and Preliminary Treatment

A new well is completed without grouting the annular space between the casing and the borehole. What problem is most likely?

  1. The pump will operate at a lower pressure
  2. The well will produce water with excessive hardness
  3. The water will contain too much dissolved oxygen
  4. Surface water and shallow contamination can enter the well Correct answer.

Explanation

The annular seal prevents water and contaminants from moving down along the outside of the casing. Without proper grout, shallow groundwater or surface water can short-circuit into the well and degrade the supply.

Question 34Source Water and Preliminary Treatment

Compared with a manually cleaned bar screen, what advantage does a traveling water screen provide at an intake?

  1. It removes debris continuously without manual raking Correct answer.
  2. It disinfects the water as it passes through
  3. It removes dissolved gases from the water
  4. It softens the water by removing calcium

Explanation

A traveling water screen uses moving screen panels and a wash system to remove debris continuously. This reduces the labor needed for manual raking and helps keep head loss stable at the intake.

Question 35Source Water and Preliminary Treatment

A heavy algal bloom develops in a reservoir near the intake. Which operational effect is most likely?

  1. The water will become more corrosive because of low pH
  2. Filters may clog faster and coagulant demand may rise Correct answer.
  3. The raw water hardness will drop sharply
  4. The dissolved oxygen will increase at the bottom

Explanation

Algae add particulate and organic material that can blind filters and consume coagulant. Some algae also produce taste and odor compounds, increasing treatment difficulty.

Question 36Source Water and Preliminary Treatment

What is the main purpose of a sanitary survey of a public water system?

  1. To test the chlorine residual in the distribution system
  2. To identify contamination risks and operational deficiencies Correct answer.
  3. To determine the monthly water bill for customers
  4. To measure the exact flow rate at the plant

Explanation

A sanitary survey is an on-site review of the source, treatment, storage, and distribution facilities. Its purpose is to find conditions that could allow contamination or cause the system to fail to meet requirements.

Question 37Source Water and Preliminary Treatment

A well pump begins delivering water containing sand and sediment. What is the most likely cause?

  1. The pumping rate is too high or the well screen is damaged Correct answer.
  2. The water table has risen above the well casing
  3. The chlorine residual in the well is too high
  4. The well has become too cold

Explanation

Pumping sand usually means the well is being overpumped or the screen or gravel pack has failed. Either condition allows formation material to enter the well along with the water.

Question 38Source Water and Preliminary Treatment

A rectangular grit channel is 2.0 ft wide and carries 4.5 cfs. What water depth keeps the average velocity at 1.0 ft/s?

  1. 0.45 ft
  2. 1.5 ft
  3. 2.25 ft Correct answer.
  4. 4.5 ft

Explanation

Velocity equals flow divided by cross-sectional area, so the required area is 4.5 cfs divided by 1.0 ft/s, or 4.5 square feet. Dividing that area by the 2.0-ft width gives a depth of 2.25 ft.

Question 39Source Water and Preliminary Treatment

A grit chamber removes most 0.2-mm sand but much less 0.1-mm sand. Which statement best explains this?

  1. The chamber velocity is too low to carry any grit
  2. The smaller particles dissolve in the water
  3. The 0.1-mm particles are denser and settle faster
  4. The smaller particles settle more slowly and are more easily carried out Correct answer.

Explanation

Settling velocity decreases as particle diameter decreases, so finer grit needs a longer detention time or a lower overflow rate to be removed. If the chamber is designed for 0.2-mm sand, much of the 0.1-mm material will remain suspended and pass through.

Question 40Source Water and Preliminary Treatment

A well pump short-cycles and occasionally draws air. Which condition best explains this?

  1. The pump is set too high or the well yield is too low Correct answer.
  2. The pressure tank is oversized for the system
  3. The static water level is above the pump intake
  4. The water contains too much dissolved oxygen

Explanation

Short cycling with air entrainment suggests the pump is drawing the water level down to the intake or that the well cannot sustain the pumping rate. The pump may be set too close to the dynamic water level or the well yield may be inadequate.

Question 41Treatment Processes and Facility Operation

Which chemical is most commonly used as the primary coagulant in surface water treatment plants?

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

Explanation

Alum (aluminum sulfate) hydrolyzes in water to form positively charged species that neutralize the negative surface charge on colloidal particles, allowing them to gather into floc. It is typically fed ahead of flocculation, often with a polymer aid.

Question 42Treatment Processes and Facility Operation

What is the primary purpose of the sedimentation basin that follows coagulation and flocculation?

  1. To inactivate viruses and bacteria in the water
  2. To allow settleable floc to be removed by gravity Correct answer.
  3. To strip dissolved gases from the water
  4. To add dissolved minerals back into the water

Explanation

Once floc has formed, the water moves slowly through the basin so gravity can pull the floc to the bottom as sludge. Clarified water then flows on to the filters, where disinfection and other steps are handled separately.

Question 43Treatment Processes and Facility Operation

What is the purpose of backwashing a granular media filter?

  1. To permanently increase the adsorption capacity of the media
  2. To apply a fresh coagulant coating to the media
  3. To flush captured solids out and restore media capacity Correct answer.
  4. To sterilize the media using chlorine gas

Explanation

Backwash water expands the bed and carries trapped particles up and out to waste, restoring both storage capacity and hydraulic capacity. Filters are backwashed when head loss reaches its terminal value or effluent turbidity rises.

Question 44Treatment Processes and Facility Operation

Which chlorine species provides the strongest disinfection in water at a typical drinking water pH?

  1. Chloride ion
  2. Nitrate ion
  3. Hypochlorous acid (HOCl) Correct answer.
  4. Hypochlorite ion (OCl-)

Explanation

Hypochlorous acid is a small, unchanged molecule that penetrates cell walls readily, making it a far stronger disinfectant than the hypochlorite ion. The split between the two forms depends on pH, with HOCl dominant at lower pH values.

Question 45Treatment Processes and Facility Operation

What is the typical target fluoride concentration in finished drinking water for optimal dental protection?

  1. 0.05 mg/L
  2. 0.7 mg/L Correct answer.
  3. 2.5 mg/L
  4. 10 mg/L

Explanation

About 0.7 mg/L is the accepted optimal concentration for community water fluoridation in the United States. Higher levels raise the risk of dental fluorosis, while much lower levels give little decay-prevention benefit.

Question 46Treatment Processes and Facility Operation

In the activated sludge process, what is the main function of the aeration basin?

  1. To settle sludge out of the mixed liquor
  2. To contact wastewater with biomass and dissolved oxygen Correct answer.
  3. To disinfect the wastewater with chlorine
  4. To grind and shred the incoming debris

Explanation

The aeration basin keeps the mixed liquor suspended and supplies dissolved oxygen so the microorganisms can consume organic matter. Settling occurs later in the secondary clarifier, and the biomass is returned to the basin.

Question 47Treatment Processes and Facility Operation

What is the primary purpose of anaerobic digestion of wastewater sludge?

  1. To dewater the sludge into a handleable cake
  2. To thicken the sludge by removing free water
  3. To disinfect the sludge by adding lime
  4. To stabilize organic solids and reduce volatile solids Correct answer.

Explanation

Anaerobic digestion biologically converts volatile solids to gas, mainly methane and carbon dioxide, producing a stabilized, lower-volume product with less odor potential. Thickening and dewatering are separate physical processes.

Question 48Treatment Processes and Facility Operation

What is the purpose of a grit chamber at the head of a wastewater treatment plant?

  1. To remove dissolved BOD from the raw wastewater
  2. To aerate the wastewater before primary settling
  3. To remove heavy, abrasive solids such as sand and grit Correct answer.
  4. To provide chlorine contact time before discharge

Explanation

Grit chambers use a controlled low velocity so dense, inorganic material settles while lighter organic solids stay in suspension. Removing grit protects pumps, pipes, and digesters from abrasion and accumulation.

Question 49Treatment Processes and Facility Operation

A jar test is performed at a water treatment plant to determine which of the following?

  1. The backwash expansion rate needed for the filters
  2. The optimum coagulant dose and pH for good floc formation Correct answer.
  3. The solids loading rate on the secondary clarifier
  4. The chlorine demand of the settled water sample

Explanation

A jar test simulates coagulation, flocculation, and settling at several chemical doses so the operator can select the dose and pH that give the clearest supernatant and best floc. It is the standard bench tool for adjusting coagulant feed.

Question 50Treatment Processes and Facility Operation

How is the CT value used in disinfection calculated?

  1. Filter effluent turbidity multiplied by the filter run time
  2. Disinfectant residual concentration multiplied by contact time Correct answer.
  3. Detention time divided by the plant flow rate
  4. Chlorine dose divided by the contact time in minutes

Explanation

CT is the product of the disinfectant residual remaining at the end of contact and the effective contact time, usually expressed in mg/L-minutes. A higher CT generally means more inactivation, and regulations specify minimum CT values for given conditions.