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CA Water Operator practice questions

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Question 1Source Water and Water Quality

Which of the following source waters typically has the most stable temperature and the lowest year-round turbidity?

  1. A shallow lake in a windy region
  2. An agricultural irrigation canal
  3. A wide river downstream of a dam
  4. A deep confined aquifer well Correct answer.

Explanation

Confined aquifers are isolated from surface runoff and weather, so their temperature, turbidity, and microbial quality change very little seasonally. Surface waters respond directly to rainfall, wind, and runoff.

Question 2Source Water and Water Quality

What is the primary drinking water standard (MCL) for nitrate, expressed as nitrogen?

  1. 10 mg/L Correct answer.
  2. 100 mg/L
  3. 1 mg/L
  4. 50 mg/L

Explanation

The nitrate MCL is 10 mg/L measured as nitrogen, which corresponds to about 45 mg/L as nitrate ion. Above this level, infants are at risk of methemoglobinemia.

Question 3Source Water and Water Quality

Turbidity measured in finished (filtered) water is used primarily to indicate:

  1. The degree of hardness stabilization
  2. The adequacy of chlorine residual
  3. The effectiveness of corrosion control
  4. The efficiency of particle removal by filtration Correct answer.

Explanation

Turbidity measures light scattered by suspended particles, so in filtered water it serves as a surrogate for how well the filters are removing particles, including pathogens. It is not a measure of disinfectant residual or corrosion control.

Question 4Source Water and Water Quality

Which pathogen is the most resistant to inactivation by free chlorine?

  1. Cryptosporidium parvum oocysts Correct answer.
  2. Giardia lamblia cysts
  3. Escherichia coli
  4. Enteric viruses

Explanation

Cryptosporidium oocysts are extremely resistant to chlorine and chloramines, so utilities rely on UV, ozone, or physical removal. Giardia is more resistant than bacteria and viruses but far less resistant than Cryptosporidium.

Question 5Source Water and Water Quality

A water sample with a pH of 6.5 is best described as:

  1. Acidic Correct answer.
  2. Neutral
  3. Alkaline
  4. Basic

Explanation

pH is a logarithmic scale in which 7 is neutral at 25 degrees C; values below 7 indicate a higher hydrogen ion activity than pure water, which is acidic. Values above 7 are basic or alkaline.

Question 6Source Water and Water Quality

A routine sample shows total coliform bacteria in the distribution system. This result most directly indicates:

  1. Hardness above the secondary standard
  2. A possible breach in treatment or system integrity Correct answer.
  3. Definite fecal contamination of the water
  4. Excessive chlorine residual in the main

Explanation

Total coliforms are indicator organisms; their presence signals a possible treatment failure, loss of residual, or intrusion pathway. It does not by itself prove fecal contamination, which requires fecal coliform or E. coli confirmation.

Question 7Source Water and Water Quality

Hardness in drinking water is caused primarily by dissolved:

  1. Calcium and magnesium Correct answer.
  2. Chloride and sulfate
  3. Iron and manganese
  4. Sodium and potassium

Explanation

Hardness is defined by the concentration of dissolved calcium and magnesium, usually expressed as mg/L as calcium carbonate. It causes scale and soap consumption but is not a primary health concern.

Question 8Source Water and Water Quality

What is the purpose of a source water assessment under the Safe Drinking Water Act?

  1. To characterize how susceptible a source water is to contamination Correct answer.
  2. To fund construction of new treatment plants
  3. To set maximum contaminant levels for regulated chemicals
  4. To certify distribution system operators

Explanation

Source water assessments delineate the contributing area, inventory potential contaminant sources, and rate the susceptibility of the source. They are informational and do not establish standards or fund construction.

Question 9Source Water and Water Quality

What is the main role of alkalinity when alum is used as a coagulant?

  1. It oxidizes dissolved iron and manganese
  2. It raises the Langelier index toward scaling
  3. It buffers pH so coagulation stays near the optimum range Correct answer.
  4. It provides a lasting disinfectant residual

Explanation

Alum consumes alkalinity and releases hydrogen ions, lowering pH; adequate alkalinity buffers the water so coagulation remains near its optimum pH. If alkalinity is insufficient, lime or caustic is added.

Question 10Source Water and Water Quality

A shallow community well shows 8 mg/L nitrate as nitrogen. Which source is most likely?

  1. Deep bedrock mineral dissolution alone
  2. Agricultural fertilizer and septic system leaching Correct answer.
  3. Corrosion of cast iron pipe
  4. Disinfection byproduct formation

Explanation

Nitrate in shallow groundwater is most commonly associated with fertilizer application, animal wastes, and septic effluent. These diffuse sources readily reach shallow aquifers.

Question 11Source Water and Water Quality

What is the California and federal MCL for arsenic in drinking water?

  1. 50 µg/L
  2. 5 µg/L
  3. 2 µg/L
  4. 10 µg/L Correct answer.

Explanation

The arsenic MCL was lowered to 10 µg/L in 2001 based on cancer risk. The older standard of 50 µg/L was considered insufficiently protective.

Question 12Source Water and Water Quality

California's primary MCL for total chromium in drinking water is:

  1. 5 µg/L
  2. 10 µg/L
  3. 50 µg/L Correct answer.
  4. 25 µg/L

Explanation

The California MCL for total chromium is 50 µg/L, which accounts for both trivalent and hexavalent chromium together. Total chromium is regulated as a single primary standard.

Question 13Source Water and Water Quality

Perchlorate in groundwater is of concern primarily because it:

  1. Causes methemoglobinemia in infants
  2. Interferes with iodine uptake by the thyroid gland Correct answer.
  3. Accelerates corrosion of lead service lines
  4. Forms bromate during ozonation

Explanation

Perchlorate competes with iodide for uptake by the thyroid, potentially disrupting hormone production, especially in pregnant women and infants. It does not act through the mechanisms that cause methemoglobinemia or corrosion.

Question 14Source Water and Water Quality

Sustained overpumping of a coastal aquifer most directly causes:

  1. Rapid recharge from rainfall
  2. Land subsidence confined to inland basins
  3. Increased alkalinity in the pumped water
  4. Seawater intrusion into the freshwater aquifer Correct answer.

Explanation

Pumping lowers freshwater head near the coast, allowing the freshwater-seawater interface to move inland and brackish water to enter wells. Overpumping does not increase recharge.

Question 15Source Water and Water Quality

Permanent land subsidence from groundwater extraction occurs mainly when:

  1. The water table rises above the land surface
  2. The aquifer is confined and artesian
  3. Clay layers compact permanently as head declines Correct answer.
  4. Recharge exceeds pumping

Explanation

Dewatering fine-grained clay and silt layers removes pore pressure that supports the grains, and these layers compact irreversibly. The land surface drops and does not fully recover even if pumping stops.

Question 16Source Water and Water Quality

A utility treats cold water at 5 degrees C with free chlorine. Compared with warmer water, the CT required to inactivate Giardia cysts will be:

  1. Higher, because disinfection is slower at low temperature Correct answer.
  2. Lower, because chlorine is more soluble in cold water
  3. Lower, because cold water slows the organisms
  4. Unchanged, because CT depends only on pH

Explanation

CT tables require greater CT as temperature decreases, because disinfection reactions proceed more slowly in cold water. Higher disinfectant concentration or longer contact time increases the CT achieved, but not the CT required.

Question 17Source Water and Water Quality

Elevated total organic carbon in raw water is a concern mainly because it:

  1. Increases the Langelier index
  2. Consumes alkalinity and raises pH
  3. Reacts with chlorine to form trihalomethanes and other byproducts Correct answer.
  4. Causes scaling in distribution mains

Explanation

Natural organic matter reacts with free chlorine to produce trihalomethanes, haloacetic acids, and other disinfection byproducts. Utilities control this by removing TOC before chlorination or by changing the primary disinfectant.

Question 18Source Water and Water Quality

A raw water has a SUVA of about 5 L/mg-m. This indicates that the natural organic matter is:

  1. Mostly non-aromatic and poorly removed by coagulation
  2. Aromatic and relatively well removed by coagulation Correct answer.
  3. Too low in concentration to form any byproducts
  4. Primarily inorganic carbonate

Explanation

SUVA above roughly 4 L/mg-m indicates aromatic, high-molecular-weight organic matter that coagulates well. Low SUVA indicates hydrophilic material that is removed poorly by coagulation and may require other treatment.

Question 19Source Water and Water Quality

A negative Langelier Saturation Index in finished water suggests that the water is:

  1. Scale-forming
  2. Above the turbidity limit
  3. Corrosive to metal pipe Correct answer.
  4. Over-chlorinated

Explanation

A negative index means the water is undersaturated with calcium carbonate and can dissolve protective scale, exposing metal surfaces to corrosion. Utilities adjust pH, alkalinity, or calcium to reach a slightly positive index.

Question 20Source Water and Water Quality

A wellhead protection area is best described as:

  1. The area contributing recharge to a well within a set travel time Correct answer.
  2. The fixed 100-foot sanitary seal radius required by code
  3. The area where finished water is stored before pumping
  4. The fenced enclosure and paved apron immediately around the wellhead

Explanation

A wellhead protection area is delineated by groundwater flow and time of travel so that potential contaminant sources in the contributing area can be managed. The fenced enclosure alone is far smaller than the contributing area.

Question 21Source Water and Water Quality

Geosmin and 2-MIB in a reservoir most commonly cause:

  1. Earthy and musty taste and odor complaints Correct answer.
  2. Nitrate concentrations above the MCL
  3. Corrosion of copper plumbing
  4. Turbidity high enough to clog filters

Explanation

Geosmin and 2-MIB are metabolites of algae and actinomycetes that are detectable at nanogram-per-liter levels and produce earthy, musty odors. They do not create significant turbidity or affect nitrate.

Question 22Source Water and Water Quality

Iron and manganese in groundwater are primarily a concern because they:

  1. Are precursors to bromate formation
  2. Are acute health hazards at low concentrations
  3. Cause staining and taste complaints as aesthetic problems Correct answer.
  4. Are reliable indicators of fecal contamination

Explanation

Iron and manganese are secondary contaminants with aesthetic effects such as staining of fixtures and laundry and metallic taste. They are not acute health hazards at typical levels and are not microbial indicators.

Question 23Source Water and Water Quality

A clearwell has a baffling factor of 0.5, a theoretical detention time of 40 minutes, and an exit free chlorine residual of 1.0 mg/L. What CT is credited?

  1. 20 mg-min/L Correct answer.
  2. 15 mg-min/L
  3. 40 mg-min/L
  4. 10 mg-min/L

Explanation

Credited contact time is the baffling factor times the theoretical detention time (0.5 times 40 equals 20 minutes), and CT is that time multiplied by the residual (20 times 1.0 equals 20 mg-min/L).

Question 24Source Water and Water Quality

Ozonation of water containing bromide can form bromate. Which strategy best limits bromate formation?

  1. Raise the pH to 10 before ozonation
  2. Chlorinate ahead of the ozone contactor
  3. Reduce the ozone dose and control pH Correct answer.
  4. Increase the ozone dose

Explanation

Bromide is oxidized to bromate during ozonation, and formation increases with ozone exposure and higher pH. Lowering the ozone dose and controlling pH, or removing bromide beforehand, limits bromate.

Question 25Source Water and Water Quality

Air stripping is an effective treatment for removing a groundwater contaminant when the contaminant:

  1. Is a dissolved mineral salt
  2. Has a high Henry's law constant Correct answer.
  3. Has a high molecular weight and low volatility
  4. Is strongly sorbed to soil particles

Explanation

Air stripping transfers volatile compounds from water to air, and efficiency is governed by the Henry's law constant. Volatile solvents such as trichloroethene are removed well, while salts and strongly sorbed compounds are not.

Question 26Source Water and Water Quality

Turbidity in filtered drinking water is monitored primarily because particles:

  1. raise the Langelier saturation index and promote corrosion of pipe walls
  2. react with chlorine to form trihalomethanes in the distribution system
  3. give the water an unpleasant appearance and are regulated only as an aesthetic issue
  4. can shield pathogens from disinfectants and signal filter breakthrough Correct answer.

Explanation

Turbidity is a treatment performance indicator, not merely an aesthetic measurement. Particles can harbor and shield microorganisms from inactivation and can indicate that coagulation or filtration has failed. Because of this, turbidity limits are a condition of pathogen removal credit under surface water treatment rules.

Question 27Source Water and Water Quality

The Stage 2 Disinfectants and Disinfection Byproducts Rule sets the maximum contaminant level for total trihalomethanes (TTHM) at:

  1. 10 micrograms per liter
  2. 80 micrograms per liter Correct answer.
  3. 60 micrograms per liter
  4. 150 micrograms per liter

Explanation

Total trihalomethanes are regulated as a group with a maximum contaminant level of 80 micrograms per liter, calculated as a locational running annual average. The companion limit for the five haloacetic acids is lower, so the two groups are not interchangeable. Exceeding either limit requires operational changes to precursor removal or disinfection strategy.

Question 28Source Water and Water Quality

Under the Surface Water Treatment Rule, a system using conventional filtration must achieve a Giardia lamblia removal and inactivation of at least:

  1. 99.9 percent (3-log) Correct answer.
  2. 99 percent (2-log)
  3. 99.99 percent (4-log)
  4. 99.999 percent (5-log)

Explanation

Conventional filtration systems must achieve a combined 3-log (99.9 percent) removal and inactivation of Giardia lamblia. Removal credit is earned primarily through filtration, and the remainder is met with disinfection contact time. The 4-log requirement in the rule applies to viruses, not to Giardia.

Question 29Source Water and Water Quality

A well screened in gravel 40 feet from a river shows turbidity above 5 NTU and detectable E. coli following heavy rain. For regulatory purposes the source should be:

  1. classified as surface water subject to no microbial treatment
  2. classified as groundwater and exempt from filtration requirements
  3. classified as groundwater under the direct influence of surface water Correct answer.
  4. reclassified as a purchased water source with monitoring waived

Explanation

A groundwater under the direct influence of surface water determination considers proximity to surface water, aquifer geology and the presence of surface water indicators such as turbidity, fecal contamination and temperature shifts. Such sources must be treated under the surface water treatment requirements rather than the groundwater rule alone. Stable, low-turbidity water from a protected aquifer would not trigger the determination.

Question 30Source Water and Water Quality

In a nutrient-rich reservoir, pH rises from 7.5 to 9.0 by late afternoon while total alkalinity drops slightly. The most likely cause is:

  1. anaerobic decay of bottom sediments releasing carbon dioxide
  2. oxidation of ammonia by nitrifying bacteria
  3. dissolution of calcium carbonate from the reservoir bottom
  4. photosynthetic uptake of dissolved carbon dioxide by algae Correct answer.

Explanation

Algal photosynthesis consumes dissolved carbon dioxide during daylight, shifting the carbonate equilibrium and raising pH. Because carbon is being removed from the carbonate system, alkalinity can decline slightly at the same time. pH typically falls again overnight when respiration releases carbon dioxide back into the water.

Question 31Source Water and Water Quality

A utility switches sources and the finished water chloride-to-sulfate mass ratio rises from 0.4 to 1.2. The greatest concern is:

  1. accelerated galvanic corrosion of lead-bearing plumbing Correct answer.
  2. increased calcium carbonate scaling of pipe walls
  3. loss of disinfectant residual from sulfate demand
  4. bromate formation in the clearwell

Explanation

A high chloride-to-sulfate mass ratio increases the water's corrosivity toward lead and galvanic connections, especially where lead solder or brass is present. Chloride tends to destabilize protective scales while sulfate helps maintain them. Utilities switching sources therefore evaluate corrosion control before the change is placed in service.

Question 32Source Water and Water Quality

A river intake shows turbidity spikes above 200 NTU for days after every major storm. The most reliable long-term operational measure to protect the treatment process is to:

  1. lower the coagulant dose to reduce sludge production
  2. withdraw water from an off-stream storage reservoir that settles storm flows Correct answer.
  3. increase the prechlorination dose during storms
  4. reduce filter loading rates during the spike

Explanation

Off-stream storage detains high-turbidity storm flows long enough for sediment to settle, giving the plant a more consistent raw water quality. This reduces coagulant swings and helps keep filtered turbidity within limits. Adjusting chemicals or filter rates does not address the underlying variability of the raw water.

Question 33Source Water and Water Quality

A reservoir is thermally stratified in late summer and the utility draws from deep water near the dam. Compared with surface water, this withdrawal will typically contain:

  1. higher dissolved oxygen and lower manganese concentrations
  2. lower turbidity and lower color than surface water
  3. lower dissolved oxygen and higher iron, manganese and ammonia Correct answer.
  4. higher pH and lower dissolved carbon dioxide

Explanation

The hypolimnion is isolated from the atmosphere and from photosynthesis during stratification, so oxygen is consumed by sediment and biological demand. Reducing conditions release soluble iron and manganese from sediments and favor ammonia accumulation. Water drawn from depth therefore tends to be anoxic and high in dissolved metals and taste-and-odor compounds.

Question 34Source Water and Water Quality

A raw water sample reads 60 color units before filtration and 12 units after passing through a 0.45-micrometer filter. The difference is best explained by:

  1. chlorine residual bleaching the color-causing organics
  2. dissolved humic substances absorbing light at longer wavelengths
  3. analytical error, since filtration should not change a color reading
  4. suspended and colloidal particles counted as apparent color Correct answer.

Explanation

Color measured on an unfiltered sample is apparent color, which includes light scattering by suspended and colloidal material. Filtering through a 0.45-micrometer membrane removes those particles and leaves true color from dissolved organics. A large gap between the two readings points to particulate matter rather than dissolved humic content.

Question 35Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

Which coagulant chemical is most widely used in conventional surface water treatment and lowers pH by consuming natural alkalinity?

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

Explanation

Alum reacts with bicarbonate alkalinity in the raw water to form an aluminum hydroxide floc, and the reaction releases hydrogen ions that depress pH. Lime and caustic soda are bases that raise pH rather than lower it. This is why alkalinity must be monitored whenever alum dose is increased.

Question 36Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

The primary purpose of rapid mixing (flash mixing) in the coagulation process is to:

  1. Allow already-formed floc particles to grow large enough to settle
  2. Strip dissolved gases and add oxygen to the water
  3. Disperse the coagulant rapidly and uniformly through the water Correct answer.
  4. Provide quiescent conditions so particles can settle out

Explanation

Flash mixing creates intense, brief turbulence so the coagulant is spread throughout the flow and contacts particles before floc begins to form. Detention time is typically under a minute and velocity gradients are high. Gentle mixing is reserved for the later flocculation stage.

Question 37Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

Flocculation basins are designed to provide:

  1. Quiescent conditions that allow particles to settle
  2. Aeration to oxidize dissolved iron and manganese
  3. Intense, high-speed mixing for rapid chemical dispersion
  4. Gentle, slow mixing that promotes particle collisions Correct answer.

Explanation

Flocculation uses slow, gentle stirring so destabilized particles collide and agglomerate into larger, denser floc without being sheared apart. Detention times are usually 15 to 45 minutes. Mixing energy is often tapered from inlet to outlet.

Question 38Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

In a conventional treatment plant, the sedimentation basin that follows flocculation is intended mainly to:

  1. Remove dissolved organic compounds by adsorption
  2. Soften the water by precipitating calcium carbonate
  3. Remove settleable floc so the filters are not overloaded Correct answer.
  4. Disinfect the water and provide chlorine contact time

Explanation

Sedimentation removes the bulk of the floc formed in flocculation, leaving only low-turbidity water for the filters. Filters are designed for polishing, not for carrying the full solids load. Overloading filters with settleable solids shortens filter runs and degrades effluent quality.

Question 39Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

The principal job of rapid sand filtration in a conventional surface water plant is to:

  1. Capture residual turbidity and pathogens Correct answer.
  2. Remove settleable floc ahead of sedimentation
  3. Add dissolved oxygen to stabilize the finished water
  4. Remove dissolved minerals that cause hardness

Explanation

Filtration is a polishing step that removes the fine particles and microorganisms that did not settle out. Coagulation and settling ahead of it do the bulk solids removal. This is why filter performance depends heavily on good upstream coagulation.

Question 40Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

A jar test is performed on a raw water sample in order to determine the:

  1. Backwash rate needed to expand the filter bed properly
  2. Optimum coagulant dose and coagulation pH Correct answer.
  3. Chlorine dose required to maintain a distribution residual
  4. Solids loading rate for the sludge thickening process

Explanation

Jar tests simulate rapid mix, flocculation, and settling on a small scale so the operator can compare coagulant doses and pH setpoints. The dose that produces the clearest settled water at the lowest cost is selected. Results are then confirmed by full-scale plant performance.

Question 41Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

Turbidity of finished drinking water is measured and reported in:

  1. Microsiemens per centimeter (µS/cm)
  2. Nephelometric turbidity units (NTU) Correct answer.
  3. Grains per gallon (gpg)
  4. Milligrams per liter (mg/L)

Explanation

Turbidity is an optical measurement of light scattered by suspended particles and is expressed in nephelometric turbidity units. Mass-based units such as mg/L describe weight of solids, not light scatter. Electrical conductivity units describe dissolved ions.

Question 42Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

In a dual-media filter, anthracite is placed above the sand layer because anthracite:

  1. Dissolves slowly and releases coagulant into the filter bed
  2. Is finer than sand and strains particles at the media surface
  3. Is coarser and less dense, so solids penetrate deeper into the bed Correct answer.
  4. Has a higher specific gravity, so it settles faster during backwash

Explanation

Anthracite has a lower specific gravity than sand, so it stratifies above the sand after backwash and can be made coarser. The coarse top layer lets solids penetrate deeper, using more of the bed for storage and lengthening filter runs. The finer sand below provides final polishing.

Question 43Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

A plant treats a low-alkalinity surface water with alum and observes the pH dropping sharply after the dose is increased. The operator should:

  1. Reduce the filter aid polymer feed rate
  2. Increase the flash mix intensity
  3. Increase the alum dose so more floc forms and pH rises
  4. Feed a base such as lime or caustic soda Correct answer.

Explanation

Alum consumes alkalinity and releases hydrogen ions, so low-alkalinity waters can crash in pH and leave the optimum coagulation range. Adding a base restores alkalinity and holds pH where floc forms well. Increasing the alum dose would drive pH down further.

Question 44Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

For turbidity removal with alum, coagulation is generally most effective in which pH range?

  1. 6.0 to 7.5 Correct answer.
  2. 4.0 to 5.0
  3. 8.5 to 9.5
  4. 10.0 to 11.0

Explanation

Alum works best when it forms a solid aluminum hydroxide floc, which occurs near neutral pH, roughly 6.0 to 7.5 for most turbidity removal. At high pH the soluble aluminate ion forms and floc becomes poor. Optimum pH for a given water is confirmed by jar testing.

Question 45Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

Sweep floc coagulation refers to:

  1. Precipitating a bulky metal hydroxide that enmeshes particles Correct answer.
  2. Oxidizing organics with permanganate before settling
  3. Filtering fine particles out through a bed of anthracite media
  4. Neutralizing particle charge so particles repel each other less

Explanation

At higher coagulant doses the metal hydroxide precipitates in bulk and forms a sweep floc that physically entraps and enmeshes colloidal particles. This mechanism works well for low-turbidity waters where charge neutralization alone is inefficient. It typically requires enough alkalinity to support the higher dose.

Question 46Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

Flocculation basins are commonly tapered, meaning the velocity gradient decreases from inlet to outlet, because:

  1. High G early builds floc; gentle G later avoids shearing it Correct answer.
  2. It reduces the number of basins needed for floc formation
  3. It keeps the velocity gradient high throughout to prevent settling
  4. It maintains exactly the same velocity gradient in every stage

Explanation

Early stages use higher velocity gradients to maximize particle collisions while floc is small and strong. Later stages drop the gradient so the larger, fragile floc is not broken apart before settling. This tapering produces denser, more settleable floc than a single constant gradient.

Question 47Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

A sedimentation basin has a surface area of 2,000 square feet and receives a flow of 4.0 MGD. What is the surface overflow rate?

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

Explanation

Surface overflow rate equals flow divided by surface area: 4,000,000 gallons per day divided by 2,000 square feet equals 2,000 gallons per day per square foot. This value is the settling velocity a particle must exceed to be removed. Lower overflow rates generally produce better settling.

Question 48Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

A flocculation basin holds 150,000 gallons and receives a flow of 3.0 MGD. What is the detention time?

  1. 2.4 hours
  2. 0.6 hour
  3. 1.2 hours Correct answer.
  4. 4.8 hours

Explanation

Detention time equals volume divided by flow: 150,000 gallons divided by 3,000,000 gallons per day equals 0.05 day, which is 1.2 hours. This falls in the typical flocculation range of 15 to 45 minutes to an hour or more depending on design. Detention time is used together with velocity gradient to judge flocculation adequacy.

Question 49Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

A filter measures 20 feet by 20 feet and treats 1.5 MGD. What is the filtration rate in gallons per minute per square foot?

  1. 2.6 gpm/ft² Correct answer.
  2. 5.2 gpm/ft²
  3. 2.0 gpm/ft²
  4. 1.3 gpm/ft²

Explanation

Convert flow to gallons per minute: 1,500,000 divided by 1,440 minutes per day equals about 1,042 gpm. Divide by the 400 square feet of media area to get roughly 2.6 gpm/ft². This is within the typical rapid sand filter loading range of about 2 to 4 gpm/ft².

Question 50Treatment Processes (Coagulation, Flocculation, Sedimentation, Filtration)

A filter is normally taken out of service for backwashing when:

  1. The filter has been on line for exactly 24 hours
  2. Finished water turbidity falls below the 0.10 NTU goal
  3. The coagulant metering pump has been running at full speed
  4. Head loss reaches the limit or turbidity breaks through Correct answer.

Explanation

Filters are backwashed on one of two triggers: terminal head loss, or turbidity breakthrough in the effluent. Either condition signals that the bed can no longer store or capture solids effectively. Run time alone is not a reliable criterion because loading varies.