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Water Treatment Operator Class I practice questions

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Question 1Water Sources, Quality, and Characteristics

Hardness in a water supply is primarily caused by the presence of which dissolved ions?

  1. Chloride and sulfate
  2. Calcium and magnesium Correct answer.
  3. Nitrate and phosphate
  4. Sodium and potassium

Explanation

Hardness is caused mainly by dissolved divalent metal ions, chiefly calcium and magnesium, and is usually reported as calcium carbonate equivalents. It is this ability to react with soap and form scale that defines hardness in treatment practice.

Question 2Water Sources, Quality, and Characteristics

Turbidity in finished water is most commonly reported in which units?

  1. Standard pH units
  2. Milligrams per liter
  3. Nephelometric turbidity units Correct answer.
  4. Microsiemens per centimeter

Explanation

Turbidity is an optical measurement of light scattering by suspended particles and is reported in nephelometric turbidity units (NTU). Milligrams per liter is used for mass concentration, microsiemens per centimeter for conductivity, and pH is a unitless scale.

Question 3Water Sources, Quality, and Characteristics

A water sample with a pH of 7.0 at 25 degrees Celsius is best described as:

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

Explanation

At 25 degrees Celsius, pH 7.0 represents equal concentrations of hydrogen and hydroxide ions, which is the neutral point. Values below 7 are acidic and values above 7 are basic.

Question 4Water Sources, Quality, and Characteristics

Which organism group is used as the primary indicator of bacteriological contamination in drinking water?

  1. Iron bacteria
  2. Total coliforms Correct answer.
  3. Sulfur-reducing bacteria
  4. Nitrate-oxidizing bacteria

Explanation

Total coliforms are widely used as indicator organisms because their presence suggests contamination or inadequate treatment, and they are easier to detect than many individual pathogens. They do not directly measure viruses, minerals, or chlorine residual.

Question 5Water Sources, Quality, and Characteristics

Compared with surface water, groundwater typically has:

  1. More consistent temperature and lower turbidity Correct answer.
  2. Greater suspended solids and lower mineral content
  3. Higher turbidity and greater temperature swings
  4. Higher dissolved oxygen and more algae

Explanation

Groundwater moves through soil and rock, which filters out many suspended particles and buffers temperature changes, so it generally has lower turbidity and more stable temperature than surface water. It often has higher mineral content because of contact with aquifer materials.

Question 6Water Sources, Quality, and Characteristics

The alkalinity of most natural waters is primarily caused by which constituent?

  1. Silica
  2. Free chlorine residual
  3. Dissolved oxygen
  4. Bicarbonate ion Correct answer.

Explanation

Alkalinity is the capacity of water to neutralize acid, and in most natural waters it is dominated by bicarbonate, with smaller contributions from carbonate and hydroxide. Dissolved oxygen, silica, and chlorine residual do not provide significant acid-neutralizing capacity.

Question 7Water Sources, Quality, and Characteristics

Specific conductance is useful in water treatment because it provides an estimate of:

  1. Free chlorine residual
  2. Turbidity from clay particles
  3. Coliform bacteria density
  4. Total dissolved solids concentration Correct answer.

Explanation

Specific conductance measures the ability of water to carry an electrical current, which depends mainly on the concentration of dissolved ions, so it is used as a surrogate for total dissolved solids. It does not directly measure particles, bacteria, or chlorine.

Question 8Water Sources, Quality, and Characteristics

An aquifer that is bounded above and below by relatively impermeable layers and is under pressure is called:

  1. A perched aquifer
  2. A confined aquifer Correct answer.
  3. A water table aquifer
  4. An unconfined aquifer

Explanation

A confined aquifer is overlain by a low-permeability layer that traps the water under pressure, so the water level in a well may rise above the top of the aquifer. Unconfined and water-table aquifers have a free water surface at atmospheric pressure.

Question 9Water Sources, Quality, and Characteristics

The most common source of elevated nitrate in rural well water is:

  1. Naturally occurring radon gas
  2. Agricultural fertilizer and septic systems Correct answer.
  3. Corrosion of lead plumbing
  4. Chlorination of the well

Explanation

Nitrate commonly enters groundwater from fertilizer application, animal wastes, and septic system effluent. Lead corrosion, chlorination, and radon do not cause elevated nitrate.

Question 10Water Sources, Quality, and Characteristics

Iron and manganese in a water supply most commonly cause:

  1. Staining and metallic taste Correct answer.
  2. Increased pH and alkalinity
  3. Improved disinfection efficiency
  4. Reduced hardness

Explanation

Iron and manganese are aesthetic contaminants that can stain fixtures and laundry and impart a metallic taste or odor. They do not raise pH or alkalinity, improve disinfection, or reduce hardness.

Question 11Water Sources, Quality, and Characteristics

The concentration of dissolved oxygen that water can hold at saturation:

  1. Increases as temperature rises
  2. Is highest at midday in summer
  3. Decreases as temperature rises Correct answer.
  4. Is unaffected by temperature

Explanation

Gas solubility decreases as water temperature increases, so warm water holds less dissolved oxygen at saturation than cold water. This is why thermal pollution and summer heating can lower dissolved oxygen in a water body.

Question 12Water Sources, Quality, and Characteristics

Giardia and Cryptosporidium in surface water supplies most often originate from:

  1. Human and animal fecal contamination Correct answer.
  2. Corrosion of distribution pipes
  3. Naturally occurring minerals in soil
  4. Excessive chlorination

Explanation

Giardia and Cryptosporidium form resistant cysts or oocysts that are shed in the feces of infected humans and animals and can enter surface water. Pipe corrosion, chlorination, and soil minerals are not sources of these organisms.

Question 13Water Sources, Quality, and Characteristics

The most effective first barrier against contamination of a surface water supply is:

  1. Increasing coagulation dose
  2. Flushing storage tanks more often
  3. Adding more chlorine at the tap
  4. Protecting the watershed and source water Correct answer.

Explanation

Source water protection is the first barrier in the multiple-barrier approach because it prevents contaminants from entering the supply in the first place. Treatment adjustments and distribution maintenance are later barriers that address contaminants already present.

Question 14Water Sources, Quality, and Characteristics

A negative Langelier Saturation Index indicates that water is:

  1. At exact calcium carbonate equilibrium
  2. Corrosive and undersaturated with calcium carbonate Correct answer.
  3. Unsafe for human consumption due to bacteria
  4. Scale-forming and oversaturated with calcium carbonate

Explanation

The Langelier Saturation Index compares actual pH with the pH at calcium carbonate equilibrium. A negative value means the water is undersaturated and tends to dissolve calcium carbonate, which can corrode pipes; positive values indicate a scaling tendency, and zero indicates equilibrium.

Question 15Water Sources, Quality, and Characteristics

Saltwater intrusion into a coastal well is most often caused by:

  1. Overpumping that lowers the freshwater table Correct answer.
  2. Low tide in the adjacent ocean
  3. High alkalinity in the aquifer
  4. Excessive rainfall over the recharge area

Explanation

In coastal aquifers, a freshwater lens floats above denser saltwater. Pumping that lowers the freshwater head allows seawater to move inland and contaminate the well.

Question 16Water Sources, Quality, and Characteristics

The color of a water sample after filtration to remove suspended particles is called:

  1. True color Correct answer.
  2. Turbidity color
  3. Apparent color
  4. Surface color

Explanation

True color is measured after suspended solids are removed by filtration or centrifugation, and it is caused by dissolved substances such as organic matter. Apparent color includes both dissolved and suspended material.

Question 17Coagulation, Flocculation, and Sedimentation

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

  1. To inactivate pathogenic bacteria and viruses in the water
  2. To neutralize particle charges so they can agglomerate Correct answer.
  3. To strip dissolved gases such as carbon dioxide
  4. To raise the pH of the water to the saturation point

Explanation

Coagulation destabilizes colloidal particles, which normally carry negative surface charges that keep them apart. Neutralizing those charges allows particles to collide and form larger floc that can be removed by settling and filtration.

Question 18Coagulation, Flocculation, and Sedimentation

Which chemical is the most widely used primary coagulant in drinking water treatment?

  1. Calcium carbonate
  2. Sodium hypochlorite
  3. Sodium chloride
  4. Aluminum sulfate (alum) Correct answer.

Explanation

Alum is the most common primary coagulant; it reacts with natural bicarbonate alkalinity to form a gelatinous aluminum hydroxide floc that enmeshes and sweeps particles from the water.

Question 19Coagulation, Flocculation, and Sedimentation

What is the purpose of the rapid mix (flash mix) step in a coagulation plant?

  1. To give floc particles time to grow large enough to settle
  2. To strip dissolved oxygen from the raw water
  3. To settle out the floc formed in the previous step
  4. To disperse the coagulant quickly and uniformly Correct answer.

Explanation

Rapid mix provides intense turbulence for a short time, usually seconds to about a minute, so the coagulant is dispersed before it reacts locally. Uniform dispersion throughout the flow is necessary for effective charge neutralization.

Question 20Coagulation, Flocculation, and Sedimentation

Flocculation is best described by which of the following?

  1. Gentle mixing that promotes particle-to-particle collisions Correct answer.
  2. Complete quiescence so that particles can settle out
  3. Aeration intended to oxidize iron and manganese
  4. Very intense mixing applied for just a few seconds at high speed

Explanation

Flocculation uses slow, gentle mixing over roughly 15 to 45 minutes. The low velocity gradient encourages collisions and agglomeration without breaking up the floc that forms.

Question 21Coagulation, Flocculation, and Sedimentation

What is the main purpose of a jar test?

  1. To measure the chlorine residual in the finished water
  2. To calculate the required filter backwash rate
  3. To check the accuracy of the plant turbidimeter
  4. To determine the optimum coagulant dose and pH Correct answer.

Explanation

A jar test simulates coagulation, flocculation, and settling on a small scale at several chemical doses. The operator compares floc formation and settled turbidity to choose the best dose and pH before changing plant feed rates.

Question 22Coagulation, Flocculation, and Sedimentation

Which of the following is commonly added as a coagulant aid to strengthen floc and improve settling?

  1. Calcium hypochlorite
  2. Anionic or nonionic polymer Correct answer.
  3. Sodium hydroxide
  4. Powdered activated carbon

Explanation

Polymers act as coagulant aids by bridging between small floc particles, producing larger, denser, faster-settling floc. They are fed in small doses to supplement the primary coagulant.

Question 23Coagulation, Flocculation, and Sedimentation

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

  1. To remove dissolved minerals through ion exchange
  2. To disinfect the water before it reaches the filters
  3. To remove settleable floc particles by gravity Correct answer.
  4. To add alkalinity consumed during coagulation

Explanation

Sedimentation provides quiet conditions and enough detention time for floc formed in flocculation to settle to the bottom as sludge. The clarified water then flows on to the filters, reducing their solids loading.

Question 24Coagulation, Flocculation, and Sedimentation

How is sludge normally removed from the bottom of a sedimentation basin?

  1. By allowing it to pass onto the filters
  2. By backwashing the basin with finished water
  3. By blowdown through a sludge drain Correct answer.
  4. By adding additional coagulant to the basin

Explanation

Sludge accumulates on the basin floor and is drawn off through a sludge drain or hopper, often with the help of a mechanical collector. Removing sludge on a regular schedule keeps the basin volume and detention time within design values.

Question 25Coagulation, Flocculation, and Sedimentation

A treatment plant feeds 20 mg/L of alum. Approximately how much alkalinity, expressed as CaCO3, will the alum consume?

  1. 44 mg/L
  2. 9 mg/L Correct answer.
  3. 20 mg/L
  4. 4.5 mg/L

Explanation

Alum reacts with bicarbonate alkalinity, consuming roughly 0.45 mg/L of alkalinity as CaCO3 for each mg/L of alum fed. For a 20 mg/L dose, 20 x 0.45 is about 9 mg/L.

Question 26Coagulation, Flocculation, and Sedimentation

Alum coagulation is generally most effective in which pH range?

  1. 10.5 to 11.5
  2. 8.5 to 9.5
  3. 5.5 to 6.5 Correct answer.
  4. 2.0 to 3.5

Explanation

Alum forms its best floc in a slightly acidic range, about 5.5 to 6.5, where aluminum hydroxide has its lowest solubility. Above roughly pH 7.5 the floc becomes lighter and less effective, and at very high pH soluble aluminate species form.

Question 27Coagulation, Flocculation, and Sedimentation

A rectangular sedimentation basin is 40 ft wide by 100 ft long and receives a flow of 2.0 MGD. What is the surface overflow rate?

  1. 2,000 gpd/ft2
  2. 500 gpd/ft2 Correct answer.
  3. 1,000 gpd/ft2
  4. 250 gpd/ft2

Explanation

Surface overflow rate is flow divided by the basin surface area. The area is 40 ft x 100 ft = 4,000 ft2, so 2,000,000 gpd divided by 4,000 ft2 equals 500 gpd/ft2.

Question 28Coagulation, Flocculation, and Sedimentation

A rectangular settling basin holds 150,000 gallons and receives a flow of 1.5 MGD. What is the detention time?

  1. 0.24 hours
  2. 24 hours
  3. 2.4 hours Correct answer.
  4. 1.0 hour

Explanation

Detention time equals basin volume divided by flow rate. Dividing 150,000 gal by 1,500,000 gal/day gives 0.10 day, and 0.10 day times 24 hours per day equals 2.4 hours.

Question 29Coagulation, Flocculation, and Sedimentation

Clarified water leaves a settling basin over a weir that is 600 ft long at a flow of 3.0 MGD. What is the weir loading rate?

  1. 500 gpd/ft
  2. 200 gpd/ft
  3. 5,000 gpd/ft Correct answer.
  4. 1,800 gpd/ft

Explanation

Weir loading rate is total flow divided by total weir length. Dividing 3,000,000 gpd by 600 ft gives 5,000 gpd/ft.

Question 30Coagulation, Flocculation, and Sedimentation

A zeta potential measurement close to zero indicates which of the following?

  1. The coagulant dose is far too low to have any effect
  2. The water contains essentially no suspended particles
  3. The particles carry a very strong negative surface charge
  4. Particles have little net charge and tend to agglomerate Correct answer.

Explanation

Zeta potential measures the electrical charge at a particle's shear plane. As coagulant neutralizes the natural negative charge, zeta potential approaches zero, the repulsive barrier disappears, and particles can stick together.

Question 31Coagulation, Flocculation, and Sedimentation

Why are baffles used in flocculation and sedimentation basins?

  1. To reduce short-circuiting and even out the flow Correct answer.
  2. To add turbulence that shears the floc apart
  3. To provide surface area for bacteria to grow on
  4. To raise the water velocity above the channel scour limit

Explanation

Baffles force the water to follow a longer, more uniform path, which reduces short-circuiting and dead zones. Actual detention time then comes closer to the design value and settling is more consistent.

Question 32Coagulation, Flocculation, and Sedimentation

If the flocculation mixer speed is increased well beyond the design value, what is the most likely result?

  1. A lower coagulant demand
  2. No change, since flocculation is unaffected by mixing speed
  3. Floc broken into smaller particles that settle poorly Correct answer.
  4. Larger, denser floc that settles faster

Explanation

Floc is fragile, and excessive velocity gradients tear it apart into smaller, weaker particles. Those particles settle slowly and can carry over the weirs and onto the filters.

Question 33Coagulation, Flocculation, and Sedimentation

How does low raw water temperature typically affect coagulation and flocculation?

  1. The need for a coagulant aid is eliminated
  2. Floc becomes larger and denser, so less coagulant is needed
  3. Reactions slow and floc is lighter and settles slower Correct answer.
  4. There is no measurable effect on coagulation

Explanation

Cold water is more viscous and chemical reactions proceed more slowly, so floc forms slowly and is smaller and less dense. Operators often respond with a higher coagulant dose, a polymer aid, or longer detention time.

Question 34Coagulation, Flocculation, and Sedimentation

What is the likely effect of a large overdose of a polymer coagulant aid?

  1. Complete removal of all alkalinity
  2. Improved filter runs with no downside
  3. An increase in the basin detention time
  4. Restabilization and higher settled turbidity Correct answer.

Explanation

Excess polymer can re-coat particles and restore a net repulsive charge, a condition called restabilization. The result is poorer floc formation and higher turbidity in the settled water.

Question 35Coagulation, Flocculation, and Sedimentation

Why is hydrated lime sometimes fed to coagulated water after the coagulant?

  1. To restore pH and alkalinity consumed by the coagulant Correct answer.
  2. To reduce the volume of sludge produced
  3. To disinfect the water before filtration
  4. To raise the negative zeta potential of the particles

Explanation

Coagulants such as alum consume bicarbonate alkalinity and depress pH. Lime or caustic is added to bring pH back into the optimum range and to produce a non-corrosive finished water.

Question 36Coagulation, Flocculation, and Sedimentation

A jar test shows the lowest settled turbidity at the highest dose tested, and turbidity was still decreasing at that dose. What should the operator do next?

  1. Reduce the rapid mix speed and repeat the same doses
  2. Run another jar test at higher doses Correct answer.
  3. Stop feeding coagulant until turbidity rises
  4. Accept that dose as final without further testing

Explanation

When performance is still improving at the top of the tested range, the true optimum has not yet been bracketed. A new series extending to higher doses is needed to locate the best dose.

Question 37Coagulation, Flocculation, and Sedimentation

A rectangular clarifier has a surface overflow rate of 700 gpd/ft2 and a detention time of 2.5 hours. What basin depth does this correspond to?

  1. About 2.5 ft
  2. About 0.4 ft
  3. About 24 ft
  4. About 9.7 ft Correct answer.

Explanation

Depth equals overflow rate multiplied by detention time when units are consistent. 700 gpd/ft2 times (2.5/24) day gives 72.9 gal/ft2, and dividing by 7.48 gal/ft3 gives about 9.7 ft.

Question 38Coagulation, Flocculation, and Sedimentation

A plant treating high-alkalinity water at pH 8.6 with alum obtains small, light floc and poor settling. Which action is most appropriate?

  1. Reduce flocculation time to prevent floc aging
  2. Increase the filter rate to capture more particles
  3. Lower the pH or try a coagulant suited to high pH Correct answer.
  4. Increase the alum dose substantially without other changes

Explanation

Alum performs best in a slightly acidic range of about 5.5 to 6.5; at pH 8.6 the floc is light and some aluminum remains soluble. Lowering the pH with acid or selecting a coagulant that works well at higher pH is the usual remedy.

Question 39Coagulation, Flocculation, and Sedimentation

Velocity gradient is given by G = square root of P divided by (viscosity x volume). If the power input to a flocculator is increased by a factor of four while volume and viscosity stay constant, G will

  1. increase by a factor of two Correct answer.
  2. increase by a factor of four
  3. decrease by a factor of two
  4. stay the same

Explanation

G varies with the square root of power dissipated per unit volume. Quadrupling the power input multiplies G by the square root of four, which is two.

Question 40Coagulation, Flocculation, and Sedimentation

What is the main reason inclined tube or plate settlers improve clarifier performance?

  1. They shorten the settling distance and add area Correct answer.
  2. They create turbulence that keeps floc suspended
  3. They add coagulant chemicals to the water
  4. They increase basin detention time by several hours

Explanation

In a tube settler a particle only has to fall a short distance before reaching a surface, so it is captured at a much higher overflow rate than in a conventional basin. The modules also provide a large effective settling area within the same footprint.

Question 41Coagulation, Flocculation, and Sedimentation

In a solids-contact (sludge blanket) clarifier, what is the primary removal mechanism?

  1. Particles are removed solely by passing through a bed of sand
  2. Chemicals are added only after the water has been filtered
  3. Dissolved gases are stripped from the water by aeration
  4. Incoming water contacts a sludge blanket that traps the floc Correct answer.

Explanation

Solids-contact units recirculate previously formed sludge so incoming coagulated particles collide with existing floc and are captured in a suspended blanket. This produces clearer supernatant at a higher rise rate than plain settling.

Question 42Coagulation, Flocculation, and Sedimentation

What is the primary purpose of the rapid mix (flash mix) stage in a conventional coagulation-flocculation-sedimentation process?

  1. Build large, dense floc particles that will settle out quickly
  2. Provide disinfection contact time for pathogen inactivation
  3. Rapidly disperse coagulant so it reacts before floc forms Correct answer.
  4. Collect and remove the settled sludge that accumulates on the basin bottom

Explanation

Rapid mix applies intense mixing (typically a velocity gradient of several hundred per second for well under a minute) so the coagulant is dispersed throughout the water and can hydrolyze and neutralize particle charges before visible floc begins to form. If the chemical is not dispersed quickly, local overdoses and uneven coagulation result, and the later flocculation stage must work with poorly conditioned particles.

Question 43Coagulation, Flocculation, and Sedimentation

In the flocculation stage, gentle and slow stirring of the water is used mainly to:

  1. Promote collisions so small particles agglomerate into larger floc Correct answer.
  2. Shear apart existing floc particles so the filters are not blinded
  3. Raise the pH of the water to the optimum range for the coagulant
  4. Keep heavy grit in suspension so it can be carried to the filters

Explanation

Flocculation relies on gentle agitation (typically a velocity gradient of roughly 10 to 75 per second) to create many low-energy particle collisions, causing destabilized particles and microfloc to stick together and grow into larger, denser floc. The mixing must be gentle enough to encourage contact without producing shear forces that tear the growing floc apart.

Question 44Coagulation, Flocculation, and Sedimentation

A clarifier is designed with a surface overflow rate of 600 gpd/ft2. A floc particle whose settling velocity is equal to 400 gpd/ft2 enters the basin. How should this particle be expected to behave?

  1. Completely removed, as long as the basin depth equals the design depth
  2. Completely removed, because settling velocity does not affect removal
  3. Not completely removed; some of these particles will leave with the effluent Correct answer.
  4. Removed only if the detention time in the basin exceeds four hours

Explanation

A particle is completely removed only when its settling velocity equals or exceeds the surface overflow rate, which is flow divided by basin surface area. A particle settling at 400 gpd/ft2 in a basin loaded at 600 gpd/ft2 settles too slowly to reach the sludge zone before the water carries it to the outlet, so it is only partially removed. Basin depth affects detention time but does not change the surface overflow rate criterion for full removal.

Question 45Coagulation, Flocculation, and Sedimentation

A clarifier receives the same flow as before, but the incoming water now carries a much higher suspended solids concentration. What is the most likely consequence?

  1. Only the surface overflow rate rises, so basin performance is unaffected
  2. Only the weir loading rate rises, so the effluent weirs become overloaded
  3. Neither loading rate changes, because both depend only on the flow rate
  4. The solids loading rate rises, which can raise the sludge blanket Correct answer.

Explanation

The solids loading rate is the mass of solids applied per unit of basin surface area per day, calculated from flow multiplied by solids concentration divided by area. Because the flow is unchanged, the hydraulic surface overflow rate and the weir loading rate stay the same, but the extra solids greatly increase the solids loading. When solids accumulate faster than they can be thickened and withdrawn, the sludge blanket rises and can carry over the weirs as increased effluent solids.

Question 46Coagulation, Flocculation, and Sedimentation

A tracer test on a rectangular settling basin shows dye reaching the outlet within a few minutes and an actual detention time about 40 percent of the theoretical detention time. What does this most likely indicate?

  1. The theoretical detention time is wrong, because depth is not part of that calculation
  2. The sludge collector is running too fast and resuspending settled solids
  3. The flow distribution is unusually uniform, so the dye traveled faster than expected
  4. The basin is short-circuiting, so the effective settling volume is reduced Correct answer.

Explanation

Theoretical detention time is basin volume divided by flow rate; a tracer arriving far earlier than that, with a very short measured time, shows that a large share of the flow is moving straight through to the outlet rather than occupying the full basin. This short-circuiting, often caused by inlet jets, density currents, or poor baffling, means only part of the basin volume is doing useful settling work, so effluent quality suffers even though the theoretical detention time looks adequate.

Question 47Filtration

What is the primary purpose of the granular media filtration step in a conventional surface water treatment plant?

  1. Removing suspended particles and turbidity Correct answer.
  2. Stripping dissolved gases that cause taste and odor
  3. Removing dissolved minerals that cause hardness
  4. Inactivating pathogens through chemical disinfection

Explanation

Granular media filters polish water that has already been coagulated and settled, capturing the remaining suspended particles and turbidity. Dissolved constituents such as hardness pass through the bed largely unchanged, and disinfection is a separate downstream process.

Question 48Filtration

In a dual-media filter containing anthracite over sand, why is the anthracite placed on top?

  1. It has a higher specific gravity than sand, so it settles below the sand layer
  2. It is coarser and less dense, allowing deeper solids penetration Correct answer.
  3. It dissolves slowly and buffers the pH of the filtered water
  4. It is finer than sand, so it strains particles at the surface

Explanation

Anthracite has a lower specific gravity than sand, so backwashing leaves it on top of the bed. Because it is also coarser, particles penetrate deeper into the bed, which uses more of the media and lengthens filter runs.

Question 49Filtration

What is the function of the graded gravel layer beneath the sand in a rapid sand filter?

  1. To provide the media that performs most of the particle removal
  2. To disinfect the filtered water as it enters the underdrain
  3. To support the sand bed and keep it from entering the underdrain Correct answer.
  4. To store backwash water for the next cleaning cycle

Explanation

The graded gravel layers hold the sand in place and stop it from washing into the underdrain nozzles during backwash, while still allowing water to pass freely in both directions.

Question 50Filtration

The main objective of backwashing a rapid sand filter is to:

  1. Compress the media bed so that it filters more efficiently
  2. Remove accumulated solids and restore the media's capacity Correct answer.
  3. Increase the effective size of the sand grains
  4. Add coagulant chemical to the surface of the media

Explanation

Backwashing fluidizes the bed and flushes captured solids to waste, restoring both the solids-storage capacity and the hydraulic capacity that the filter lost during the run.