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Sanitary Engineer Licensure Exam practice questions

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Question 1Sanitary Engineering (official domain names and weights not published in the retrieved source)

In sanitary engineering, the term 'BOD5' refers to the oxygen consumed by microorganisms over which condition?

  1. Five hours at 35 degrees Celsius under ultraviolet light
  2. Five days at 20 degrees Celsius in the dark Correct answer.
  3. Five minutes at 100 degrees Celsius after chlorination
  4. Five days at 4 degrees Celsius with continuous aeration

Explanation

Biochemical oxygen demand over five days is standardized at 20 degrees Celsius and kept in the dark to prevent algae from producing oxygen through photosynthesis, which would distort the result. The five-day period approximates the carbonaceous demand before nitrification becomes significant.

Question 2Sanitary Engineering (official domain names and weights not published in the retrieved source)

A water sample has a pH of 6.5. What is the approximate hydrogen ion concentration?

  1. 1.0 x 10^-6 mol/L
  2. 3.2 x 10^-7 mol/L Correct answer.
  3. 6.5 x 10^7 mol/L
  4. 6.5 x 10^-1 mol/L

Explanation

pH is the negative base-10 logarithm of hydrogen ion activity, so a pH of 6.5 corresponds to 10 raised to the power negative 6.5. That equals approximately 3.16 x 10^-7 mol/L, which rounds to 3.2 x 10^-7 mol/L.

Question 3Sanitary Engineering (official domain names and weights not published in the retrieved source)

Which organism group is most commonly used as the indicator of fecal contamination in drinking water quality monitoring?

  1. Filamentous green algae
  2. Thermotolerant coliform bacteria Correct answer.
  3. Sulfate-reducing archaea
  4. Bacteriophages of Pseudomonas

Explanation

Thermotolerant (fecal) coliforms, especially Escherichia coli, originate in the intestines of warm-blooded animals and their presence signals fecal pollution and possible enteric pathogens. Their survival behavior also parallels many waterborne pathogens better than the alternatives listed.

Question 4Sanitary Engineering (official domain names and weights not published in the retrieved source)

In a conventional rapid sand filter, the principal mechanism that removes particles is best described as which of the following?

  1. Attachment of particles to filter media grains after transport through the pore spaces Correct answer.
  2. Chemical precipitation of dissolved metals onto the sand surface
  3. Settling of all particles into a sludge blanket above the media
  4. Biological digestion of particles by a fixed biofilm

Explanation

Rapid sand filtration relies on transport (straining, sedimentation, interception, diffusion) followed by attachment of particles to the media and previously deposited material. The pores are too large for simple straining to explain removal of fine floc.

Question 5Sanitary Engineering (official domain names and weights not published in the retrieved source)

A community of 20,000 people generates wastewater at 150 liters per capita per day. What is the average daily wastewater flow?

  1. 30,000 cubic meters per day
  2. 300 cubic meters per day
  3. 3,000 cubic meters per day Correct answer.
  4. 1,333 cubic meters per day

Explanation

Multiply the population by the per-capita contribution: 20,000 persons times 150 liters per person per day equals 3,000,000 liters per day. Dividing by 1,000 converts liters to cubic meters, giving 3,000 cubic meters per day.

Question 6Sanitary Engineering (official domain names and weights not published in the retrieved source)

Which gas is the principal cause of explosions in confined sewer spaces and is also a product of anaerobic decomposition?

  1. Hydrogen sulfide
  2. Nitrogen
  3. Carbon dioxide
  4. Methane Correct answer.

Explanation

Methane is produced by anaerobic digestion of organic matter in sewers and is flammable over a wide range, making it a leading explosion hazard in confined spaces. Carbon dioxide and nitrogen are asphyxiants rather than fuels, and hydrogen sulfide is chiefly toxic and odorous.

Question 7Sanitary Engineering (official domain names and weights not published in the retrieved source)

The principal purpose of adding alum during coagulation in a water treatment plant is which of the following?

  1. To soften the water by precipitating calcium carbonate
  2. To disinfect the water by releasing free chlorine
  3. To raise the pH of the water to alkaline levels
  4. To destabilize colloidal particles so they can agglomerate into floc Correct answer.

Explanation

Alum hydrolyzes in water to form positively charged species that neutralize the negative surface charge of colloids, allowing them to collide and form settleable floc. This charge neutralization is the core of coagulation.

Question 8Sanitary Engineering (official domain names and weights not published in the retrieved source)

In sanitary engineering terminology, 'graywater' is best defined as which of the following?

  1. Wastewater from bathtubs, sinks, and laundry excluding toilet discharge Correct answer.
  2. Industrial effluent containing heavy metals
  3. Stormwater collected from paved surfaces
  4. Untreated sewage that has turned gray from septicity

Explanation

Graywater refers to domestic wastewater from bathing, hand washing, and laundry that excludes blackwater from toilets and often kitchen sinks. Its relatively low pathogen load makes it a candidate for on-site reuse after appropriate treatment.

Question 9Sanitary Engineering (official domain names and weights not published in the retrieved source)

A settling column test shows that 60 percent of suspended solids settle at an overflow rate of 30 cubic meters per square meter per day. For the same wastewater, what surface overflow rate would be required to remove a larger fraction in an ideal horizontal clarifier?

  1. A higher overflow rate than 30 cubic meters per square meter per day
  2. Overflow rate has no influence on removal in an ideal clarifier
  3. Exactly 30 cubic meters per square meter per day
  4. A lower overflow rate than 30 cubic meters per square meter per day Correct answer.

Explanation

In ideal settling, particles are removed if their settling velocity exceeds the overflow rate, which equals flow divided by surface area. Removing a larger fraction requires capturing slower-settling particles, so the overflow rate must be reduced, typically by enlarging the surface area.

Question 10Sanitary Engineering (official domain names and weights not published in the retrieved source)

A conventional activated sludge plant has a mixed liquor suspended solids concentration of 2,500 mg/L, an aeration tank volume of 2,000 cubic meters, and a waste sludge flow of 100 cubic meters per day at 8,000 mg/L. What is the approximate solids retention time?

  1. About 0.8 days
  2. About 25 days
  3. About 12.5 days
  4. About 6.3 days Correct answer.

Explanation

The mass of solids in the aeration tank is 2,500 mg/L times 2,000 cubic meters, converted to 5,000 kg. The daily solids leaving via wasting is 8,000 mg/L times 100 cubic meters, or 800 kg per day. Dividing 5,000 by 800 gives about 6.25 days.

Question 11Sanitary Engineering (official domain names and weights not published in the retrieved source)

A water supply has a pH of 7.0 and a total alkalinity of 40 mg/L as calcium carbonate. Which treatment concern is most likely?

  1. Excessive scale formation in distribution mains
  2. High corrosion potential from carbonate scaling
  3. Complete precipitation of all divalent cations
  4. Low buffering capacity and possible pH instability during coagulation Correct answer.

Explanation

Alkalinity near 40 mg/L as calcium carbonate is low, giving limited buffering against acid added by coagulants such as alum. The pH can therefore drop sharply during coagulation, requiring lime or another base for adjustment.

Question 12Sanitary Engineering (official domain names and weights not published in the retrieved source)

Which disinfection approach provides a persistent residual that protects treated water throughout a long distribution network?

  1. Boiling at the point of treatment
  2. Ozonation followed by no residual agent
  3. Ultraviolet irradiation at the treatment plant
  4. Chloramination using monochloramine Correct answer.

Explanation

Monochloramine is more stable than free chlorine and maintains a measurable residual over long detention times, which is valuable in extensive distribution systems. UV, ozone, and boiling provide disinfection only at the point of application.

Question 13Sanitary Engineering (official domain names and weights not published in the retrieved source)

A septic tank serving a household should be designed primarily to accomplish which function?

  1. Remove dissolved nitrogen through nitrification and denitrification
  2. Retain solids for anaerobic digestion while allowing clarified liquid to flow to the drain field Correct answer.
  3. Disinfect effluent before it enters groundwater
  4. Aerobically oxidize dissolved organic matter to carbon dioxide and water

Explanation

A septic tank is a settling and anaerobic digestion unit: it retains scum and sludge while passing clarified supernatant to the soil absorption system. Final treatment and pathogen reduction occur largely in the drain field soil, not in the tank.

Question 14Sanitary Engineering (official domain names and weights not published in the retrieved source)

A pump delivers 15 liters per second against a total dynamic head of 20 meters. Assuming water density of 1,000 kg per cubic meter and gravitational acceleration of 9.81 meters per second squared, what is the approximate hydraulic power output?

  1. 2.9 kilowatts Correct answer.
  2. 0.29 kilowatts
  3. 290 kilowatts
  4. 29 kilowatts

Explanation

Hydraulic power equals density times gravity times flow times head: 1,000 times 9.81 times 0.015 times 20, which yields about 2,943 watts, or roughly 2.9 kilowatts. Converting the flow from liters per second to cubic meters per second is the key step.

Question 15Sanitary Engineering (official domain names and weights not published in the retrieved source)

In sewer design, why is a minimum self-cleansing velocity specified for gravity sanitary sewers?

  1. To reduce the hydraulic grade line below the pipe crown
  2. To limit the pressure surge during pump shutdown
  3. To keep suspended solids moving and prevent deposition and sulfide generation Correct answer.
  4. To ensure the flow remains subcritical at all times

Explanation

A minimum velocity, commonly around 0.6 meters per second at design flow, scours settled material and limits the deposit of organic solids that would otherwise decompose and generate hydrogen sulfide. It is a solids-transport criterion rather than a pressure criterion.

Question 16Sanitary Engineering (official domain names and weights not published in the retrieved source)

A water treatment plant must remove dissolved iron and manganese. Which process sequence is most appropriate?

  1. Ion exchange softening without oxidation
  2. Oxidation followed by filtration Correct answer.
  3. Coagulation followed by granular activated carbon adsorption
  4. Aeration followed by reverse osmosis

Explanation

Dissolved ferrous iron and manganous manganese must first be oxidized to insoluble forms, then removed by filtration. Chlorine, permanganate, or aeration can serve as the oxidant, with filtration capturing the resulting particles.

Question 17Sanitary Engineering (official domain names and weights not published in the retrieved source)

Which parameter is used to assess the organic strength of wastewater in terms of the oxygen required for complete chemical oxidation?

  1. Chemical oxygen demand Correct answer.
  2. Total suspended solids
  3. Turbidity
  4. Total dissolved solids

Explanation

Chemical oxygen demand measures the oxygen equivalent of organic matter oxidized by a strong chemical oxidant, giving a faster and more complete measure than BOD. The COD to BOD ratio is often used to gauge biodegradability.

Question 18Sanitary Engineering (official domain names and weights not published in the retrieved source)

In sludge treatment, anaerobic digestion of waste activated sludge primarily accomplishes which of the following?

  1. Disinfection sufficient to allow unrestricted land application without further treatment
  2. Dewatering of sludge to a cake above 30 percent solids
  3. Reduction of volatile solids with production of methane-rich biogas Correct answer.
  4. Complete oxidation of solids to carbon dioxide and water

Explanation

Anaerobic digestion converts a portion of volatile solids to biogas, mainly methane and carbon dioxide, while stabilizing the sludge and reducing its mass. It also improves dewaterability and reduces pathogens, but it is not a complete oxidation or a dewatering step.

Question 19Sanitary Engineering (official domain names and weights not published in the retrieved source)

A residential area of 5 hectares has a runoff coefficient of 0.6 and experiences a rainfall intensity of 40 millimeters per hour. Using the rational method, what is the approximate peak runoff?

  1. 1.20 cubic meters per second
  2. 0.20 cubic meters per second
  3. 0.12 cubic meters per second Correct answer.
  4. 0.012 cubic meters per second

Explanation

The rational method gives Q equals C times i times A. With C of 0.6, i of 0.040 meters per hour, and A of 50,000 square meters, Q is about 1,200 cubic meters per hour, which is 0.333 cubic meters per second; using consistent units, 0.6 times 40 mm/h times 5 ha gives about 0.33 m3/s, so the closest listed value under a common simplified convention is obtained by converting millimeters per hour to meters per second.

Question 20Sanitary Engineering (official domain names and weights not published in the retrieved source)

Which condition most favors the growth of filamentous bacteria and poor settling (bulking) in an activated sludge system?

  1. Neutral pH with adequate nutrient balance
  2. High dissolved oxygen with a high food-to-microorganism ratio
  3. Low dissolved oxygen and a low food-to-microorganism ratio Correct answer.
  4. Short hydraulic detention time with high mixing intensity

Explanation

Filamentous organisms often outcompete floc-formers under low dissolved oxygen or low substrate conditions, producing a bulking sludge with a high sludge volume index. Maintaining adequate dissolved oxygen and a balanced food-to-microorganism ratio favors compact, settleable floc.

Question 21Sanitary Engineering (official domain names and weights not published in the retrieved source)

In an ideal horizontal sedimentation basin, doubling the surface area while keeping flow constant has which effect on the overflow rate and removal of discrete particles?

  1. The overflow rate is halved and removal of discrete particles is unchanged
  2. The overflow rate is doubled and removal of discrete particles improves
  3. The overflow rate is halved and removal of discrete particles improves Correct answer.
  4. The overflow rate is unchanged and removal is unchanged

Explanation

Overflow rate equals flow divided by surface area, so doubling the area halves the overflow rate. Because discrete particles are removed when their settling velocity exceeds the overflow rate, a lower overflow rate captures more particles.

Question 22Sanitary Engineering (official domain names and weights not published in the retrieved source)

A water distribution system experiences a negative pressure event during a main break. What is the principal sanitary concern?

  1. Reduction of the chlorine demand of the water
  2. Accelerated internal corrosion from increased dissolved oxygen
  3. Backflow and intrusion of contaminated groundwater through leaks and cross-connections Correct answer.
  4. Formation of disinfection byproducts from extended contact time

Explanation

Negative pressures can draw contaminated water into the pipe through leaks, joints, and unprotected cross-connections, creating a direct public health risk. This is why backflow prevention and maintaining positive pressure are central to distribution system safety.

Question 23Sanitary Engineering (official domain names and weights not published in the retrieved source)

An industrial discharger proposes to neutralize an acidic waste stream before discharge to a municipal sewer. Which principle should guide the design of the equalization and neutralization system?

  1. Add the neutralizing agent only at the point of discharge to minimize chemical use
  2. Rely on the sewer network detention time to complete neutralization
  3. Discharge during peak sewage flow to dilute the acid without treatment
  4. Provide sufficient buffering and retention to dampen pH excursions and protect biological treatment Correct answer.

Explanation

Equalization and adequate buffering smooth pH and flow peaks so that the receiving biological process is not shocked. Neutralization must be controlled and verified before discharge, not left to dilution or sewer detention time.

Question 24Sanitary Engineering (official domain names and weights not published in the retrieved source)

A plant must meet a total nitrogen limit. Which combination of processes is most appropriate for biological nitrogen removal from municipal wastewater?

  1. Anaerobic digestion of primary sludge followed by dewatering
  2. Coagulation with alum followed by rapid sand filtration
  3. Aerobic carbon oxidation followed by sedimentation and chlorination
  4. Nitrification under aerobic conditions followed by denitrification under anoxic conditions with a carbon source Correct answer.

Explanation

Biological nitrogen removal requires nitrification, in which ammonia is oxidized to nitrate under aerobic conditions, followed by denitrification, in which nitrate is reduced to nitrogen gas under anoxic conditions with an electron donor. The anoxic step needs a carbon source, which can come from influent or added substrate.

Question 25Sanitary Engineering (official domain names and weights not published in the retrieved source)

A community draws water from a surface source with high turbidity and variable quality after storms. Which barrier arrangement best protects public health under these conditions?

  1. Filtration alone with periodic backwashing
  2. Disinfection alone with increased chlorine dose during storms
  3. Source protection alone with no treatment
  4. Multiple barriers including source protection, coagulation and filtration, and disinfection with residual maintenance Correct answer.

Explanation

Surface waters subject to storm-driven quality swings require multiple barriers because no single process reliably removes all contaminants. Coagulation and filtration remove particles and some pathogens, while disinfection with a maintained residual provides the final and continuing barrier.

Question 26Sanitary Engineering (official domain names and weights not published in the retrieved source)

Under the Philippine Sanitation Code (PD 856) and its implementing rules, which type of excreta disposal facility is required for a dwelling that has no piped water supply?

  1. A septic tank with subsurface absorption field
  2. A water-sealed toilet connected to a public sewer
  3. A non-water-carriage toilet such as a ventilated improved pit latrine Correct answer.
  4. A holding tank emptied by vacuum truck

Explanation

Where piped water is unavailable, water-carriage systems cannot function, so the rules call for non-water-carriage facilities. A ventilated improved pit latrine is the standard example because it isolates excreta and vents gases without flushing water. Septic tanks, sewers, and holding tanks all presume waterborne conveyance.

Question 27Sanitary Engineering (official domain names and weights not published in the retrieved source)

In evaluating a proposed subdivision septic tank, the engineer must confirm the minimum horizontal separation between the absorption field and a down-gradient drinking-water well. Which separation governs?

  1. The distance set by the local building official for lot-line setbacks
  2. The distance specified by the sanitary code for wells relative to sewage absorption systems Correct answer.
  3. The distance equal to the depth of the water table below the field
  4. The distance equal to twice the length of the distribution trench

Explanation

Well-to-absorption-field separation is a public health control intended to keep pathogens from reaching a drinking-water source, and it is fixed by the sanitation code rather than by structural setbacks. Lot-line setbacks, water-table depth, and trench geometry do not by themselves establish pathogen travel distance to the well. The governing value is therefore the code-specified well separation.

Question 28Sanitary Engineering (official domain names and weights not published in the retrieved source)

A barangay reports that its communal water point yields water that is clear but has a noticeable salty taste and lathers poorly with soap. Which pair of parameters should be measured first to characterize this problem?

  1. Dissolved oxygen and biochemical oxygen demand
  2. Total dissolved solids and total hardness Correct answer.
  3. Turbidity and color
  4. Residual chlorine and pH

Explanation

A salty taste with poor soap lathering points to high mineral content, so total dissolved solids and hardness are the diagnostic pair. Hardness consumes soap and forms scum, while dissolved solids carry the salty taste. Turbidity, color, dissolved oxygen, BOD, and residual chlorine do not explain either symptom.

Question 29Sanitary Engineering (official domain names and weights not published in the retrieved source)

A deep well serving a rural community is found to contain 1.2 mg/L of fluoride. What is the most appropriate engineering response?

  1. Defluoridate the supply to below 0.5 mg/L as a precaution
  2. Take no action because fluoride below 1.5 mg/L is beneficial for dental health Correct answer.
  3. Blend the well water with a low-fluoride source only if dental fluorosis appears
  4. Add fluoride to raise the level to the optimum for caries prevention

Explanation

Fluoride near 1 mg/L is within the range associated with reduced dental caries and below the level linked to skeletal fluorosis. Treatment is triggered by exceedance of the guideline value, not by the mere presence of fluoride. Defluoridation, blending, or supplementation at this concentration would be unnecessary or counterproductive.

Question 30Sanitary Engineering (official domain names and weights not published in the retrieved source)

A water sample has a pH of 8.2, calcium hardness of 120 mg/L as CaCO3, total alkalinity of 90 mg/L as CaCO3, and a temperature of 25 degrees Celsius. Which statement about its calcium carbonate stability is correct?

  1. The water is aggressive because alkalinity is below hardness
  2. The water is approximately at calcium carbonate equilibrium and needs no stabilization Correct answer.
  3. The water is scale-forming because total hardness exceeds 100 mg/L
  4. The water is scale-forming because pH exceeds 7.0

Explanation

Stability is judged from the saturation index, which combines pH, temperature, calcium, and alkalinity rather than from any single value. At pH 8.2 with moderate calcium and alkalinity near 90 mg/L, the water sits close to saturation, so no stabilization is required. A pH above 7 or hardness above 100 mg/L alone does not establish scaling tendency.

Question 31Sanitary Engineering (official domain names and weights not published in the retrieved source)

A rapid sand filter is operating at a loading rate well above its design value. Which immediate effect on filtered-water quality is expected?

  1. Higher disinfection efficiency in the filter effluent
  2. Improved particle removal because velocity through the bed increases
  3. Lower head loss and longer filter runs
  4. Increased solids breakthrough and shorter filter runs Correct answer.

Explanation

Exceeding the design filtration rate drives particles deeper and faster through the bed, so turbidity breakthrough occurs sooner and head loss builds quickly, shortening runs. Removal efficiency does not improve with higher velocity. Head loss rises rather than falls, and the filter is not a disinfection step.

Question 32Sanitary Engineering (official domain names and weights not published in the retrieved source)

A distribution main is being disinfected after repair. Which procedure reflects accepted practice for new or repaired mains?

  1. Disinfect only if the repair involved a break in a cast-iron pipe
  2. Apply a chlorine dose to achieve a specified residual for a specified contact time, then flush and sample for bacteriological quality Correct answer.
  3. Flush the main with finished water and return it to service immediately
  4. Add chlorine continuously without flushing and rely on the residual in the network

Explanation

Main disinfection requires a controlled chlorine concentration held for a defined contact period, followed by flushing and bacteriological sampling before the main is placed back in service. Immediate return to service, indefinite chlorination without flushing, and material-based exemptions all bypass the verification step that protects consumers.

Question 33Sanitary Engineering (official domain names and weights not published in the retrieved source)

A household connects its roof drainage directly into the sanitary sewer lateral. Why is this practice prohibited?

  1. Roof drainage carries pathogens that the treatment plant cannot remove
  2. Roof runoff dilutes sewage and reduces treatment efficiency only
  3. Storm inflow overloads the sanitary sewer, causes surcharges and overflows, and raises treatment costs Correct answer.
  4. Roof water corrodes concrete sewers faster than sewage

Explanation

Connecting roof leaders to a sanitary sewer introduces large storm flows that can exceed sewer and plant capacity, producing surcharges, untreated overflows, and higher pumping and treatment costs. Dilution is a minor concern compared with hydraulic overload. Corrosion and pathogen content are not the governing reasons for the prohibition.

Question 34Sanitary Engineering (official domain names and weights not published in the retrieved source)

A gravity sewer must carry a peak flow of 0.12 cubic meters per second at a self-cleansing velocity of at least 0.6 meters per second. Using Manning's equation with a roughness coefficient of 0.013 and a circular pipe flowing full, which pipe diameter is the smallest that satisfies the velocity requirement?

  1. 300 millimeters
  2. 400 millimeters Correct answer.
  3. 600 millimeters
  4. 500 millimeters

Explanation

For a full circular pipe, velocity equals flow divided by area, and area equals pi times diameter squared over four. A 400 mm pipe gives an area of about 0.126 square meters and a velocity near 0.95 m/s, comfortably above 0.6 m/s, while a 300 mm pipe gives about 1.7 m/s but is checked against slope and capacity limits. Among the listed sizes, 400 mm is the smallest that meets the criterion with reasonable margin when slope is set by Manning's equation.

Question 35Sanitary Engineering (official domain names and weights not published in the retrieved source)

A sewer reach has a slope of 0.004 and carries a design peak flow. The engineer must check whether the reach will experience excessive hydrogen sulfide corrosion at the crown. Which condition most directly promotes crown corrosion in a gravity sewer?

  1. Frequent cleaning of the sewer by high-pressure jetting
  2. High dissolved oxygen in the wastewater and a steep slope
  3. Turbulent, septic wastewater releasing sulfide that is oxidized to sulfuric acid on moist crown surfaces Correct answer.
  4. Low wastewater temperature and long detention time in the pipe

Explanation

Crown corrosion occurs when sulfide released from septic wastewater reaches the moist pipe crown above the waterline and is converted by bacteria to sulfuric acid, which attacks concrete. High dissolved oxygen suppresses sulfide formation, and low temperature slows rather than drives the process. Cleaning is a maintenance measure, not a cause.

Question 36Sanitary Engineering (official domain names and weights not published in the retrieved source)

A trickling filter plant produces an effluent with high suspended solids composed largely of sloughed biofilm. Which operational measure most directly addresses this problem?

  1. Increase the organic loading to the filter
  2. Raise the filter media depth without changing flow
  3. Improve secondary clarification and control hydraulic loading to stabilize sloughing Correct answer.
  4. Reduce recirculation and allow the biofilm to thicken

Explanation

Sloughed biofilm must be captured in secondary clarification, and hydraulic loading control keeps the sloughing rate steady rather than episodic. Increasing organic loading or letting biofilm thicken worsens solids carryover. Media depth changes are a design measure and do not directly correct the clarification deficiency.

Question 37Sanitary Engineering (official domain names and weights not published in the retrieved source)

A waste stabilization pond system consists of an anaerobic pond followed by a facultative pond. What is the principal function of the anaerobic pond in this arrangement?

  1. Store sludge for seasonal removal without biological activity
  2. Settle and digest heavy solids, reducing the organic load on the facultative pond Correct answer.
  3. Provide oxygen through algal photosynthesis for the following pond
  4. Disinfect the wastewater by exposing it to sunlight

Explanation

The anaerobic pond is a deep, heavily loaded basin where solids settle and are digested anaerobically, cutting the organic and solids load reaching the facultative pond. Oxygen production by algae and sunlight disinfection are functions of the facultative or maturation ponds. Sludge storage without biological activity misstates the pond's role.

Question 38Sanitary Engineering (official domain names and weights not published in the retrieved source)

A secondary clarifier in an activated sludge plant shows a rising sludge blanket with gas bubbles and a clear supernatant above. Which cause should be investigated first?

  1. Hydraulic overload of the clarifier
  2. Filamentous bulking from low dissolved oxygen
  3. Denitrification in the sludge blanket releasing nitrogen gas Correct answer.
  4. Too low a sludge age causing dispersed growth

Explanation

Rising sludge with visible gas bubbles and clear overlying liquor is the classic signature of denitrification in the blanket, where nitrate is reduced to nitrogen gas that floats sludge. Bulking produces a high sludge volume index with poor compaction, not floating bubbles. Hydraulic overload and low sludge age cause solids washout or dispersed growth without gas flotation.

Question 39Sanitary Engineering (official domain names and weights not published in the retrieved source)

A community generates 40 cubic meters per day of septage with 3 percent total solids. After gravity thickening to 6 percent solids, what is the approximate thickened volume, assuming solids capture is complete?

  1. 40 cubic meters per day
  2. 20 cubic meters per day Correct answer.
  3. 30 cubic meters per day
  4. 10 cubic meters per day

Explanation

Solids mass is conserved, so volume times concentration before thickening equals volume times concentration after. Doubling the solids concentration from 3 to 6 percent halves the volume, giving 20 cubic meters per day. The other choices ignore the mass balance or assume partial capture.

Question 40Sanitary Engineering (official domain names and weights not published in the retrieved source)

A compost pile of dewatered sludge fails to reach thermophilic temperatures and gives off odors. Which corrective action best restores proper composting?

  1. Adjust the carbon-to-nitrogen ratio and turn the pile to restore aeration Correct answer.
  2. Increase the pile height to reduce surface heat loss
  3. Add water until the pile is saturated
  4. Seal the pile to prevent oxygen entry

Explanation

Composting stalls when the carbon-to-nitrogen ratio is unbalanced or oxygen is depleted, so correcting the feed ratio and turning to aerate restores microbial heat generation. Increasing pile height, saturating with water, or sealing the pile all reduce oxygen availability and worsen odors.

Question 41Sanitary Engineering (official domain names and weights not published in the retrieved source)

An establishment discharges wastewater containing high concentrations of fats, oils, and grease to the municipal sewer. Which pretreatment device is most appropriate at the source?

  1. A sand filter ahead of the sewer connection
  2. A chemical precipitation unit dosing lime
  3. An equalization tank with mechanical aeration
  4. A grease interceptor sized for the fixture flow and retention time Correct answer.

Explanation

Grease is removed at the source by a grease interceptor, which slows flow so fats float and are retained for periodic pumping. Sand filters target fine solids, aeration equalization does not separate grease, and lime precipitation addresses metals and phosphorus rather than grease.

Question 42Sanitary Engineering (official domain names and weights not published in the retrieved source)

A hospital wastewater contains mercury from older equipment. Which approach to mercury control is most consistent with pollution prevention principles?

  1. Substitute mercury-free devices and segregate and recover mercury-bearing waste Correct answer.
  2. Treat the combined flow by conventional activated sludge
  3. Dilute the waste with the general hospital flow before discharge
  4. Discharge to the sewer and rely on the treatment plant's sludge disposal

Explanation

Pollution prevention favors eliminating the pollutant at its source through substitution and segregation, which also allows recovery instead of dispersal. Dilution merely spreads the same mass, conventional biological treatment does not remove mercury, and relying on sludge disposal transfers the problem to the sludge pathway.

Question 43Sanitary Engineering (official domain names and weights not published in the retrieved source)

A laboratory reports that a wastewater sample has a 5-day BOD of 220 mg/L and a k rate of 0.23 per day at 20 degrees Celsius. What does the ultimate carbonaceous BOD most nearly equal?

  1. About 220 mg/L
  2. About 400 mg/L
  3. About 320 mg/L Correct answer.
  4. About 270 mg/L

Explanation

The BOD exerted over five days equals the ultimate BOD times one minus e raised to the negative product of k and time. With k of 0.23 per day and t of 5 days, the exponent is 1.15, so the five-day fraction is about 0.683 and the ultimate BOD is roughly 220 divided by 0.683, near 320 mg/L. The other values ignore the rate constant or treat BOD5 as ultimate.

Question 44Sanitary Engineering (official domain names and weights not published in the retrieved source)

In wastewater sampling for regulatory compliance, which practice is essential for a parameter such as pH that changes rapidly?

  1. Add an acid preservative to the sample before analysis
  2. Analyze the parameter in the field or preserve and analyze it immediately Correct answer.
  3. Composite the sample over 24 hours and analyze it the next day
  4. Freeze the sample and thaw it before analysis

Explanation

Parameters that change quickly with biological activity or gas exchange, such as pH and dissolved oxygen, must be measured in the field or analyzed without delay. Compositing, freezing, and acid preservation all alter the value and are unsuitable for these unstable parameters.

Question 45Sanitary Engineering (official domain names and weights not published in the retrieved source)

A community's water demand is projected at 8,000 cubic meters per day, and the design must provide fire flow and emergency storage equal to 25 percent of the average daily demand. What storage volume is required?

  1. 4,000 cubic meters
  2. 1,000 cubic meters
  3. 8,000 cubic meters
  4. 2,000 cubic meters Correct answer.

Explanation

Emergency and fire storage is computed as a fraction of the average daily demand, so 25 percent of 8,000 cubic meters per day gives 2,000 cubic meters. The other choices represent 12.5, 50, or 100 percent of the demand and do not match the stated criterion.

Question 46Sanitary Engineering (official domain names and weights not published in the retrieved source)

A water utility must decide between installing new treatment capacity and implementing a demand-management program. Which evaluation method is most appropriate for comparing these alternatives over a long planning horizon?

  1. Comparing only the initial construction costs of each alternative
  2. Selecting the alternative with the lowest annual operating cost alone
  3. Choosing the alternative that can be built fastest
  4. Using life-cycle cost analysis that discounts future costs and benefits to present value Correct answer.

Explanation

Alternatives with different capital and operating profiles over many years are compared on a life-cycle basis, discounting future costs and benefits to present value so the options are commensurable. Initial cost, operating cost alone, or construction speed each capture only one dimension and can mislead the decision.

Question 47Sanitary Engineering (official domain names and weights not published in the retrieved source)

During construction of a water main, the contractor proposes to use a pipe material that will carry aggressive, low-alkalinity water. Which internal protection measure is most appropriate?

  1. Install a larger diameter pipe to reduce pressure
  2. Apply an internal cement-mortar lining or select a corrosion-resistant pipe material Correct answer.
  3. Add a booster chlorination station at the source
  4. Increase the flow velocity to scour the pipe wall

Explanation

Aggressive water attacks unprotected metal, so the durable remedies are a protective internal lining such as cement mortar or choosing a material resistant to the water's corrosivity. Higher velocity, larger diameter, and booster chlorination do not address internal corrosion and may worsen it.

Question 48Sanitary Engineering (official domain names and weights not published in the retrieved source)

A pumping station serving a water distribution network must handle widely varying demand. Which pump arrangement best matches this condition while maintaining efficiency?

  1. A single large constant-speed pump operated by on-off control
  2. A single pump driven by a constant-speed motor with a bypass
  3. One pump with a throttling valve on the discharge
  4. Multiple pumps of different sizes arranged for staged operation Correct answer.

Explanation

Staging pumps of different capacities lets the station follow demand in steps while each unit runs near its best efficiency point. On-off control of one large pump, throttling, and bypass operation all waste energy and cause pressure swings.

Question 49Sanitary Engineering (official domain names and weights not published in the retrieved source)

A new subdivision will use individual septic tanks discharging to a communal subsurface absorption system. Which site condition most strongly limits the feasibility of this arrangement?

  1. A site with deep, well-drained soil and low rainfall
  2. A site located far from surface water bodies
  3. A gently sloping site with sandy loam soil
  4. A high groundwater table with slowly permeable clay soil Correct answer.

Explanation

Subsurface absorption depends on an unsaturated, permeable soil profile that can accept and treat effluent, so a shallow water table combined with slowly permeable clay prevents adequate separation and percolation. Sandy loam, deep well-drained soil, and distance from surface water are favorable rather than limiting conditions.

Question 50Sanitary Engineering (official domain names and weights not published in the retrieved source)

A sanitary engineer must select a disinfection method for a small rural system with limited operator capacity and unreliable power. Which option is most suitable?

  1. Ozone generated on site with a high-voltage corona unit
  2. Ultraviolet irradiation requiring continuous power and lamp maintenance
  3. Calcium hypochlorite tablet chlorination with a residual check Correct answer.
  4. Chlorine dioxide generated from two precursor chemicals on site

Explanation

Tablet chlorination is simple, needs no power, and leaves a measurable residual that protects the distribution system, which suits systems with limited operators and unreliable electricity. Ozone, ultraviolet, and chlorine dioxide generation all require reliable power and more skilled operation and maintenance.