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ABC Class II Water Treatment study materials
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ABC Class II Water Treatment practice questions

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Question 1Source Water Characteristics and Protection

Which set of characteristics is most typical of a confined groundwater source compared with a surface water source?

  1. Wide temperature swings, high turbidity, and low dissolved minerals
  2. Steady temperature, low turbidity, and very low dissolved minerals
  3. Steady temperature, high turbidity, and elevated dissolved minerals
  4. Steady temperature, low turbidity, and elevated dissolved minerals Correct answer.

Explanation

Confined aquifers are shielded from direct surface runoff and sunlight, so their water stays cool and clear and does not swing with weather. Long contact time with aquifer minerals dissolves calcium, magnesium, iron, and other constituents, raising mineral content. Surface supplies show the opposite pattern: variable temperature, variable turbidity, and generally lower mineral content.

Question 2Source Water Characteristics and Protection

Apparent color in a raw water sample is best described as the color that

  1. is produced solely by dissolved iron and manganese
  2. is measured only after the sample has been chlorinated
  3. includes both dissolved and suspended material in the unfiltered sample Correct answer.
  4. remains after the sample is filtered through a 0.45-micron filter

Explanation

Apparent color is measured on the unfiltered sample and therefore includes both dissolved substances and suspended particles such as clay, algae, and precipitated iron. True color is measured after the sample is filtered to remove those particles. Comparing the two tells the operator how much of the color is particle-related.

Question 3Source Water Characteristics and Protection

Earthy, musty taste and odor in a surface water supply is most commonly associated with which compound?

  1. Hydrogen sulfide
  2. Sodium fluoride
  3. Calcium carbonate
  4. Geosmin Correct answer.

Explanation

Geosmin and 2-methylisoborneol (MIB) are metabolic byproducts of certain algae and actinomycetes, and they are detectable at extremely low concentrations. They cause the classic earthy, musty odor in raw water. Hydrogen sulfide produces a rotten-egg odor instead.

Question 4Source Water Characteristics and Protection

Turbidity in a raw surface water sample is a measurement of

  1. suspended and colloidal particles that scatter light Correct answer.
  2. dissolved mineral concentration
  3. pH buffering capacity
  4. total organic carbon content

Explanation

A turbidimeter measures how much light is scattered by particles suspended in the water, reported in nephelometric turbidity units. Dissolved substances do not scatter light appreciably, so they contribute little to the reading. Turbidity therefore serves as a quick index of particle load, including clay, silt, and microorganisms.

Question 5Source Water Characteristics and Protection

A wellhead protection area is the land surface area

  1. through which contaminants can migrate to a public well Correct answer.
  2. located within one mile of any public water system intake
  3. owned by the utility and lying inside the fenced well enclosure
  4. where groundwater is known to be more than 30 meters deep

Explanation

A wellhead protection area is defined by groundwater flow and travel time: it is the surface area through which water, and any contaminant it carries, can move to the well. Delineation uses aquifer characteristics such as flow direction, gradient, and pumping rate. Ownership, distance alone, and aquifer depth do not define the boundary.

Question 6Source Water Characteristics and Protection

Hardness in a raw water source is caused primarily by the presence of

  1. iron and aluminum ions
  2. calcium and magnesium ions Correct answer.
  3. chloride and sulfate ions
  4. sodium and potassium ions

Explanation

Hardness is defined as the sum of polyvalent metal ions, dominated in most supplies by calcium and magnesium. These ions react with soap and form scale when heated. Sodium, potassium, chloride, and sulfate are associated with other water quality issues but not with hardness.

Question 7Source Water Characteristics and Protection

A water treatment plant draws from a river, and a heavy thunderstorm passes over the upper watershed. The operator should expect the raw water to show

  1. increased turbidity and increased organic load Correct answer.
  2. decreased turbidity and decreased organic load
  3. decreased turbidity and increased organic load
  4. increased turbidity and decreased organic load

Explanation

Storm runoff scours soil, streambanks, and land surfaces, carrying clay and silt that raise turbidity. The same runoff carries leaf litter, soil organic matter, and other debris that raise total organic carbon. Both changes together increase coagulant demand and disinfection by-product risk.

Question 8Source Water Characteristics and Protection

A community well in a farming area shows steadily rising nitrate concentrations. Which source is the most likely cause?

  1. Corrosion of the submersible pump windings
  2. Decay of naturally occurring radon in bedrock
  3. Disinfection of the well with chlorine tablets
  4. Fertilizer and manure leaching into the aquifer Correct answer.

Explanation

Nitrate is highly soluble and moves readily through soil into groundwater. Nitrogen fertilizer applications and animal manure are the dominant anthropogenic sources in agricultural areas. Chlorination, pump corrosion, and radon decay do not produce nitrate.

Question 9Source Water Characteristics and Protection

Under what condition are dissolved iron and manganese most commonly found in well water?

  1. When the aquifer pH is above 10
  2. When the groundwater is anoxic, with little dissolved oxygen Correct answer.
  3. When the well is chlorinated continuously
  4. When the groundwater is saturated with dissolved oxygen

Explanation

In oxygen-poor groundwater, iron and manganese remain in the reduced, soluble form and stay in solution. Once the water is exposed to air or an oxidant, they convert to insoluble oxides that cause staining and complaints. Oxygenated or high-pH water favors the insoluble forms instead.

Question 10Source Water Characteristics and Protection

In a thermally stratified lake, the hypolimnion is the

  1. warm, well-mixed upper layer
  2. cold, isolated bottom layer Correct answer.
  3. shoreline region where mixing is continuous
  4. narrow zone where temperature drops rapidly

Explanation

Summer heating leaves a warm, circulating upper layer (epilimnion) over a cold, dense bottom layer (hypolimnion). The two are separated by the thermocline, a zone of rapid temperature change. Because the hypolimnion does not mix with the atmosphere, it can lose dissolved oxygen over the season.

Question 11Source Water Characteristics and Protection

Why is Cryptosporidium a particular concern for plants using surface water?

  1. It is destroyed by normal free chlorine residuals in the clearwell
  2. Its oocysts resist chlorine and can pass through filters Correct answer.
  3. It multiplies inside finished-water storage tanks within hours
  4. It occurs only in groundwater beneath agricultural fields

Explanation

Cryptosporidium forms thick-walled oocysts that survive standard free chlorine doses at practical contact times. The oocysts are also small enough to penetrate a filter bed that is not properly operated. Treatment therefore relies on physical removal by coagulation, sedimentation, and filtration, plus inactivation by UV or ozone.

Question 12Source Water Characteristics and Protection

Raw water TOC rises from 2 mg/L to 6 mg/L after a runoff event. What is the main treatment concern?

  1. Lower coagulant demand and longer filter runs
  2. Higher coagulant demand and more disinfection by-product potential Correct answer.
  3. Lower chlorine demand and fewer disinfection by-products
  4. Higher alkalinity and reduced corrosion potential

Explanation

Organic matter consumes coagulant and oxidant, so both coagulant and chlorine doses usually must rise. Organics also react with chlorine to form trihalomethanes and other disinfection by-products. More organics reaching the filters also shortens filter runs.

Question 13Source Water Characteristics and Protection

Which organism is the most specific indicator that a source water has been contaminated by human or animal feces?

  1. Cyanobacteria
  2. Escherichia coli Correct answer.
  3. Iron-oxidizing bacteria
  4. Nitrate-reducing bacteria

Explanation

E. coli lives in the intestinal tract of warm-blooded animals and is shed in feces, so its presence signals fecal contamination and possible enteric pathogens. Iron bacteria, cyanobacteria, and nitrate reducers occur naturally in water and do not indicate fecal pollution.

Question 14Source Water Characteristics and Protection

A utility wants to reduce sediment and nutrient loading to its reservoir from adjacent farmland. Which practice best addresses this at the source?

  1. Adding powdered activated carbon at the raw water intake
  2. Establishing a vegetated riparian buffer along tributaries Correct answer.
  3. Raising the chlorine residual in the distribution system
  4. Increasing the coagulant dose at the treatment plant

Explanation

A vegetated buffer slows runoff, traps sediment, and takes up nutrients before they reach the stream, so it reduces loading at the watershed level. That is a source water protection measure. The other choices are treatment or distribution actions that address symptoms after the contamination has already arrived.

Question 15Source Water Characteristics and Protection

The primary purpose of a sanitary survey of a public water system is to

  1. verify the deadlines for the consumer confidence report
  2. identify conditions that could contaminate the water system Correct answer.
  3. check the calibration of the plant's turbidimeters
  4. measure disinfectant residual throughout the distribution system

Explanation

A sanitary survey is an on-site inspection of the source, facilities, equipment, and operations to find conditions that could allow contamination or cause water quality failure. It is preventive rather than compliance-sampling in nature. Instrument calibration, residual monitoring, and reporting deadlines are separate program activities.

Question 16Source Water Characteristics and Protection

A well near a river produces water whose turbidity and temperature closely track the river's. Under the federal Surface Water Treatment Rule, this source should be classified as

  1. an emergency source usable only during drought
  2. groundwater under the direct influence of surface water Correct answer.
  3. a confined aquifer exempt from filtration
  4. a groundwater source needing only chlorination

Explanation

When a well responds quickly and closely to a nearby surface water body, it is considered groundwater under the direct influence of surface water and must meet surface water treatment requirements, including filtration and disinfection. Monitoring results must be reviewed to verify the classification. A true confined aquifer behaves independently of the river.

Question 17Source Water Characteristics and Protection

A stratified lake mixes completely during fall turnover. Which raw water change should the operator anticipate?

  1. A sharp drop in turbidity with stable dissolved oxygen
  2. A temporary rise in iron, manganese, and turbidity Correct answer.
  3. Fewer taste and odor complaints from customers
  4. A permanent decrease in raw water alkalinity and pH

Explanation

During stratification the bottom layer accumulates reduced iron and manganese and anoxic decay products. Turnover lifts that material into the whole water column, so metals, turbidity, and taste and odor problems can rise for days to weeks. Treatment adjustments such as oxidation and increased coagulation are usually needed until the lake re-stratifies.

Question 18Source Water Characteristics and Protection

A coastal utility has steadily increased pumping from its well field, and chloride and TDS in the wells have risen for several years. The most likely cause is

  1. decomposition of organic matter in the aquifer
  2. increased chlorination of the wells themselves
  3. saltwater intrusion from the nearby ocean Correct answer.
  4. corrosion of the well casing releasing chloride

Explanation

Over-pumping lowers the freshwater head and allows denser seawater to migrate landward and upward into the well field, raising chloride and total dissolved solids. The gradual, years-long trend following increased pumping is the signature of intrusion. Casing corrosion and chlorination cannot produce a sustained chloride and TDS rise of this kind.

Question 19Source Water Characteristics and Protection

A reservoir experiences a dense cyanobacterial bloom. Which combination of problems should the operator anticipate?

  1. Taste and odor compounds plus cyanotoxins Correct answer.
  2. Fewer filter backwashes and lower chlorine demand
  3. Lower raw water pH and reduced coagulant demand
  4. Higher dissolved oxygen and lower TOC

Explanation

Cyanobacteria release geosmin and MIB that cause earthy and musty odors, and some species produce cyanotoxins such as microcystin that are hazardous to consumers. Bloom material also raises chlorine demand, clogs filters, and can raise pH through photosynthesis. Control requires source management, powdered activated carbon or ozone, and cyanotoxin monitoring.

Question 20Source Water Characteristics and Protection

A utility drawing from a river learns that an industrial solvent has spilled upstream. The most appropriate immediate action is to

  1. wait for the next scheduled bacteriological sample before acting
  2. flush the distribution system to waste once the spill reaches the plant
  3. continue normal treatment and rely on chlorine to destroy the chemical
  4. close the intake and use stored or alternate water while sampling Correct answer.

Explanation

Source water protection planning calls for an alternate supply or stored raw water so the intake can be shut before the plume arrives. Concurrent sampling identifies the compound and its concentration so treatment decisions and public notice can be based on data. Chlorination is not a reliable safeguard against organic solvents and can sometimes create additional byproducts.

Question 21Coagulation, Flocculation, and Sedimentation

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

  1. To destabilize colloidal particles Correct answer.
  2. To strip dissolved gases such as carbon dioxide from the water
  3. To disinfect pathogenic organisms present in the raw water
  4. To soften hard water by precipitating calcium and magnesium

Explanation

Coagulation neutralizes the negative surface charge that keeps colloids apart, allowing them to collide and stick together. The coagulant does not by itself remove the particles; settling and filtration do the physical removal. Disinfection, gas stripping and softening are separate processes.

Question 22Coagulation, Flocculation, and Sedimentation

Which chemical is the most widely used primary coagulant in North American water treatment plants?

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

Explanation

Alum is the most common primary coagulant because it is effective, inexpensive, and readily available. Ferric salts and polyaluminum chloride are used as alternatives. Alum reacts with natural alkalinity to form an aluminum hydroxide floc that enmeshes particles.

Question 23Coagulation, Flocculation, and Sedimentation

A jar test is performed primarily to determine which of the following?

  1. The chlorine demand of the finished water
  2. The filter run length before backwash
  3. The solids concentration of the sludge
  4. The optimum coagulant dose and pH Correct answer.

Explanation

A jar test simulates coagulation, flocculation, and settling on a bench scale using several doses. The operator selects the dose and pH that produce the clearest supernatant and best floc. This lets the plant change chemical feed rates with confidence.

Question 24Coagulation, Flocculation, and Sedimentation

Compared with a rapid mix basin, a flocculation basin is designed to provide which of the following?

  1. The same mixing intensity but with coagulant added continuously
  2. A lower velocity gradient applied for a longer detention time Correct answer.
  3. No mixing at all so that particles can settle out
  4. A much higher velocity gradient applied for a longer time

Explanation

Flocculation uses gentle, slow mixing with a low velocity gradient and a longer detention time so particles can collide and grow into settleable flocs. Rapid mix is short and intense so the coagulant is dispersed quickly. Too much energy in flocculation would shear the flocs apart.

Question 25Coagulation, Flocculation, and Sedimentation

A typical rapid mix detention time in a conventional water treatment plant is which of the following?

  1. Ten to fifteen minutes
  2. Two to four hours
  3. A few seconds to about one minute Correct answer.
  4. Thirty to forty-five minutes

Explanation

Rapid mix must disperse the coagulant almost immediately, before hydrolysis of the metal salt is complete. Detention times are therefore short, from a few seconds up to about one minute, at a high velocity gradient. Longer times are characteristic of flocculation or sedimentation basins.

Question 26Coagulation, Flocculation, and Sedimentation

In a rectangular sedimentation basin, the zone where settled solids are collected, concentrated, and removed is the:

  1. Flocculation zone
  2. Inlet zone
  3. Effluent zone
  4. Sludge zone Correct answer.

Explanation

A settling basin is commonly described in four zones: inlet, settling, sludge, and outlet. The sludge zone at the bottom collects and thickens the solids for removal by scraping or blowdown. Flocculation occurs in a separate basin upstream.

Question 27Coagulation, Flocculation, and Sedimentation

When alum is added to water that contains natural bicarbonate alkalinity, what typically happens to alkalinity and pH?

  1. Alkalinity increases and pH rises
  2. Alkalinity is consumed and pH decreases Correct answer.
  3. Alkalinity is consumed and pH increases
  4. Alkalinity is unchanged and pH is unchanged

Explanation

Alum reacts with bicarbonate alkalinity to form aluminum hydroxide floc, consuming alkalinity and releasing hydrogen ions. As a result, pH drops unless lime or another base is fed. If raw alkalinity is too low, coagulation becomes poor and pH must be supplemented.

Question 28Coagulation, Flocculation, and Sedimentation

The surface overflow rate of a sedimentation basin is calculated by dividing the:

  1. Basin surface area by the flow rate
  2. Flow rate by the basin volume
  3. Flow rate by the basin surface area Correct answer.
  4. Basin volume by the flow rate

Explanation

Overflow rate equals flow divided by surface area and is expressed in gpd/ft². It represents the settling velocity a particle must exceed to be removed. Dividing volume by flow gives detention time instead.

Question 29Coagulation, Flocculation, and Sedimentation

A sedimentation basin holds 240,000 gallons and receives a flow of 1,200 gpm. What is the detention time?

  1. 6.7 hours
  2. 2.0 hours
  3. 3.3 hours Correct answer.
  4. 4.8 hours

Explanation

Detention time equals basin volume divided by flow rate. Here 240,000 gal divided by 1,200 gpm gives 200 minutes. Two hundred minutes equals 3.3 hours.

Question 30Coagulation, Flocculation, and Sedimentation

A clarifier with a surface area of 2,500 square feet receives a flow of 1.5 million gallons per day. What is the overflow rate?

  1. 600 gpd/ft² Correct answer.
  2. 900 gpd/ft²
  3. 750 gpd/ft²
  4. 450 gpd/ft²

Explanation

Overflow rate is flow divided by surface area. Converting 1.5 MGD to 1,500,000 gpd and dividing by 2,500 ft² gives 600 gpd/ft². This is the settling velocity, in gpd/ft², that a particle must exceed to be captured.

Question 31Coagulation, Flocculation, and Sedimentation

Why are flocculation basins often designed with tapered, decreasing mixing intensity from inlet to outlet?

  1. To promote floc growth without breaking up the larger flocs Correct answer.
  2. To raise the pH gradually as water moves through the basin
  3. To keep the coagulant from dissolving completely
  4. To increase the flow velocity toward the clarifier

Explanation

Early flocculation benefits from more mixing energy because particles are small and collisions are needed to build flocs. As flocs grow they become fragile, so the velocity gradient is reduced to avoid shearing them apart. The result is larger, denser flocs that settle well.

Question 32Coagulation, Flocculation, and Sedimentation

A colloidal particle whose zeta potential is very close to zero is best described as:

  1. Fully oxidized and ready for filtration
  2. Strongly stabilized and unlikely to settle
  3. Likely to dissolve into true solution
  4. Destabilized and able to agglomerate Correct answer.

Explanation

Zeta potential is the potential at the shear plane around a particle and indicates the strength of electrostatic repulsion. A high magnitude means particles repel each other and stay dispersed. Near zero, repulsion is overcome by van der Waals attraction so particles can stick together.

Question 33Coagulation, Flocculation, and Sedimentation

Sweep floc coagulation with alum depends mainly on which condition?

  1. Keeping the water above pH 9.5 throughout treatment
  2. Feeding a strong oxidant just ahead of rapid mix
  3. A high coagulant dose that forms a bulky hydroxide precipitate Correct answer.
  4. A very low coagulant dose that only neutralizes charge

Explanation

At higher doses and near the optimum pH, alum forms a large volume of aluminum hydroxide precipitate. This gelatinous sweep enmeshes and carries particles down as it settles. Charge neutralization alone occurs at much lower doses.

Question 34Coagulation, Flocculation, and Sedimentation

Short-circuiting in a sedimentation basin most directly causes which effect?

  1. Increased sludge compaction at the basin bottom
  2. Higher alkalinity in the finished water
  3. Faster floc formation in the inlet zone
  4. Reduced detention time and poorer particle removal Correct answer.

Explanation

Short-circuiting lets a portion of the flow travel a shortened path straight to the outlet. The actual detention time is less than the design value, so particles have less time to settle and effluent quality suffers. Baffling and well-designed inlets and outlets distribute flow and reduce short-circuiting.

Question 35Coagulation, Flocculation, and Sedimentation

Inclined tube or plate settlers improve clarifier performance mainly by doing which of the following?

  1. Reducing the total flow through the basin
  2. Adding coagulant chemicals directly to the water
  3. Increasing the depth of the settling zone
  4. Increasing the effective settling area within the basin Correct answer.

Explanation

Closely spaced inclined tubes or plates create many shallow settling surfaces, greatly increasing the effective area available for settling. A particle has only a short distance to fall before contacting a surface and sliding down. Capacity increases without enlarging the basin footprint.

Question 36Coagulation, Flocculation, and Sedimentation

A high weir loading rate on a clarifier tends to:

  1. Increase the detention time in the basin
  2. Reduce the depth of the sludge blanket
  3. Carry floc over the weir into the filter influent Correct answer.
  4. Improve the stability of the floc blanket

Explanation

Weir loading rate is the flow per unit length of weir and governs the upward velocity of water leaving the basin. Excessive rates create high local velocities near the outlet that drag floc over the weir. Keeping weir loading low protects the filters from solids carryover.

Question 37Coagulation, Flocculation, and Sedimentation

Cold raw water tends to slow the settling of floc primarily because:

  1. The overflow rate of the basin increases
  2. Viscosity increases and floc formation is slower Correct answer.
  3. Alkalinity drops sharply at low temperature
  4. Particles become denser and resist settling

Explanation

As water temperature falls, viscosity rises and the settling velocity of a given particle decreases. Cold water also slows the chemical reactions of coagulation and produces lighter, more fragile flocs. Operators often increase coagulant dose or adjust mixing during winter.

Question 38Coagulation, Flocculation, and Sedimentation

A polymer fed as a coagulant aid typically does which of the following?

  1. It raises the alkalinity of the settled water
  2. It replaces the need for a separate rapid mix step
  3. It disinfects the water as it passes through the clarifier
  4. It builds larger, stronger flocs Correct answer.

Explanation

Polymer molecules bridge between small flocs, producing larger, denser, and stronger flocs that settle faster. Because the flocs are more robust, the primary coagulant dose can often be reduced. Polymers do not disinfect and do not eliminate rapid mix.

Question 39Coagulation, Flocculation, and Sedimentation

In Type II (flocculant) settling, the settling velocity of a particle typically:

  1. Decreases as the particle grows larger
  2. Is independent of particle size and density
  3. Increases as particles collide and grow larger Correct answer.
  4. Remains constant throughout the settling depth

Explanation

In flocculant settling, particles coalesce as they fall, so their mass and diameter increase with depth and time. Larger flocs settle faster, so velocity is not constant. This contrasts with Type I discrete settling, where particles do not change size.

Question 40Coagulation, Flocculation, and Sedimentation

Which condition is most likely to cause a rising sludge blanket in a clarifier?

  1. Insufficient sludge wasting combined with high solids loading Correct answer.
  2. Low-turbidity raw water treated with a low coagulant dose
  3. A sudden decrease in plant flow rate
  4. Warmer water temperatures in the basin

Explanation

The blanket rises when the mass of solids entering the clarifier exceeds the mass leaving through wasting and underflow. High solids loading or inadequate wasting both cause this imbalance. Operators control it by matching wasting rates to solids loading and monitoring blanket depth.

Question 41Coagulation, Flocculation, and Sedimentation

A discrete particle settles at 0.60 mm/s in water at 20°C. If the water temperature drops to 5°C and all other conditions stay the same, the settling velocity will most nearly:

  1. Decrease to about 0.15 mm/s
  2. Remain at about 0.60 mm/s
  3. Decrease to about 0.40 mm/s Correct answer.
  4. Increase to about 0.90 mm/s

Explanation

Stokes' law shows settling velocity is inversely proportional to the dynamic viscosity of the water. Viscosity rises from about 1.0 cP at 20°C to about 1.5 cP at 5°C, so velocity falls to roughly two-thirds of its original value. That gives about 0.40 mm/s.

Question 42Coagulation, Flocculation, and Sedimentation

The dimensionless product G times t (velocity gradient multiplied by detention time) is used in flocculation design to:

  1. Compare the mixing energy applied in flocculation basins Correct answer.
  2. Estimate the chlorine CT credit for disinfection
  3. Determine the sludge production per million gallons treated
  4. Calculate the surface overflow rate of a clarifier

Explanation

G is the root-mean-square velocity gradient, a measure of mixing intensity, and multiplying by detention time gives a measure of total mixing energy applied, often called the Camp number. Designers use it to compare flocculation performance among basins. It is unrelated to disinfection CT, which uses disinfectant residual and contact time.

Question 43Coagulation, Flocculation, and Sedimentation

A plant feeds 30 mg/L of alum. Approximately how much alkalinity, as CaCO3, will that dose consume?

  1. About 14 mg/L Correct answer.
  2. About 7 mg/L
  3. About 3 mg/L
  4. About 30 mg/L

Explanation

Each milligram per liter of commercial alum consumes roughly 0.45 to 0.5 mg/L of alkalinity as CaCO3. Multiplying 30 mg/L by about 0.45 gives approximately 13.5 mg/L, or about 14 mg/L. If raw alkalinity is less than this demand, pH will fall and lime must be added.

Question 44Coagulation, Flocculation, and Sedimentation

Density currents in a sedimentation basin are most commonly caused by:

  1. Temperature differences between the influent and the basin water Correct answer.
  2. Algae growth on the basin walls
  3. Variations in the elevation of the effluent weirs
  4. Excessive sludge scraper speed

Explanation

Water that is colder and therefore denser than the basin contents will dive and flow along the bottom as a density current. This undercuts the settling zone and short-circuits the basin. Wind, inlet design, and dissolved solids differences can also create density currents.

Question 45Coagulation, Flocculation, and Sedimentation

A clarifier produces clear effluent at low flow but turbid effluent with a rising blanket during high-flow periods. The most likely cause is:

  1. Excessive sludge wasting
  2. Insufficient flocculation mixing energy
  3. A coagulant dose that is too low at all flows
  4. Hydraulic overload of the clarifier Correct answer.

Explanation

Performance that tracks flow indicates a hydraulic limitation: at high flow the overflow rate exceeds the settling velocity of the floc, so solids are carried out and the blanket rises. A chemical problem such as a low dose or poor flocculation would degrade effluent at all flows, not only during peaks. Restoring the design overflow rate or adding settling area corrects it.

Question 46Coagulation, Flocculation, and Sedimentation

What is the primary purpose of rapid mixing immediately after a coagulant such as alum is added to water?

  1. To build large, dense floc particles that will settle rapidly in the clarifier
  2. To concentrate and remove the solids that have already settled to the basin floor
  3. To provide the contact time needed for disinfection of the water
  4. To disperse the coagulant quickly so it contacts particles and destabilizes their charges Correct answer.

Explanation

Rapid mix, or coagulation, is a short, high-intensity mixing step (usually well under a minute at a high velocity gradient) whose job is to spread the coagulant throughout the water so it can neutralize the surface charge on colloids. Building floc is the job of the later, gentler flocculation step.

Question 47Coagulation, Flocculation, and Sedimentation

A jar test is performed at a water treatment plant primarily to:

  1. Calculate the sludge volume index of the settled solids
  2. Measure the chlorine demand of the finished water
  3. Estimate how long a filter run will last before backwashing is needed
  4. Determine the optimum coagulant dose and coagulation pH for the raw water Correct answer.

Explanation

A jar test simulates coagulation, flocculation, and settling on a bench scale so the operator can compare several coagulant doses and pH values side by side. The dose that produces the clearest supernatant in the shortest time is selected for the plant.

Question 48Coagulation, Flocculation, and Sedimentation

In a solids-contact clarifier that operates with a suspended sludge blanket, the principal mechanism for removing particles from the water is:

  1. Flotation of the particles to the surface by dissolved air
  2. Adsorption of the particles onto beds of granular activated carbon
  3. Simple gravity settling in a large, still basin with no upward flow
  4. Contact with and entrapment in the suspended blanket of previously formed solids Correct answer.

Explanation

In a sludge-blanket clarifier, incoming coagulated water is passed upward through a suspended mass of solids that was formed earlier in the process. Small floc particles collide with, stick to, and are filtered out by this blanket, so the blanket acts as both a contact medium and a filter.

Question 49Coagulation, Flocculation, and Sedimentation

A treatment plant processes 2.0 million gallons per day and feeds alum at a dose of 25 mg/L. Using 8.34 lb/gal as the weight of water, how many pounds of alum are required per day?

  1. 209 lb/day
  2. 417 lb/day Correct answer.
  3. 50 lb/day
  4. 1,250 lb/day

Explanation

The standard feed calculation is pounds per day = flow in MGD multiplied by dose in mg/L multiplied by 8.34. Here, 2.0 times 25 times 8.34 equals about 417 lb/day of alum.

Question 50Coagulation, Flocculation, and Sedimentation

A clarifier has a surface overflow rate of 0.50 gallons per minute per square foot. For discrete particles of a size that settle at 0.30 gallons per minute per square foot, what fraction of those particles is expected to be removed?

  1. 60 percent Correct answer.
  2. 167 percent
  3. 100 percent
  4. 30 percent

Explanation

For discrete particles, the fraction removed equals the settling velocity divided by the surface overflow rate whenever the settling velocity is lower than the overflow rate. Here 0.30 divided by 0.50 equals 0.60, or 60 percent. Particles that settle faster than the overflow rate are removed completely.