Question 1

Waterborne disease risk: Pathogens in contaminated water are associated with which of the following illnesses?

Accepted Answer

Introduction / Context: Drinking water safety is assessed partly by the presence of pathogenic microorganisms. Fecal contamination can transmit bacteria and viruses responsible for several serious diseases, emphasizing the need for effective water and wastewater treatment and robust surveillance programs. Given Data / Assumptions: The water is contaminated with pathogenic organisms. Diseases listed include bacterial and viral illnesses. No immunity or vaccination is assumed. Concept / Approach: Pathogens include Salmonella typhi (typhoid), Vibrio cholerae (cholera), Shigella species and others (dysentery), and Hepatitis A virus (infectious hepatitis). These are classically associated with fecal–oral transmission through contaminated drinking water or food washed with contaminated water. Step-by-Step Solution: Identify pathogens linked to each disease.Relate routes of transmission to water contamination.Conclude that all listed diseases can be waterborne; hence comprehensive option is correct. Verification / Alternative check: Epidemiological records and WHO guidance list these as waterborne or water-related diseases preventable by safe water, sanitation, and hygiene measures. Why Other Options Are Wrong: Each individual disease is indeed water-related; choosing any single option would be incomplete. Common Pitfalls: Equating absence of coliforms with absolute safety; overlooking viral pathogens that may escape basic chlorination; ignoring post-treatment recontamination risks. Final Answer: All of the above

Question 2

Dilution factor guidance: Under what condition (dilution factor value) is no treatment of sewage generally considered acceptable in dilution-based disposal practice?

Accepted Answer

Introduction / Context: Historically, some outfalls relied on the natural assimilative capacity of receiving waters. The dilution factor—ratio of available clean water to sewage—served as a guide to the extent of required treatment. Modern practice emphasizes treatment regardless, but the classic guideline remains a teaching reference. Given Data / Assumptions: Dilution factor = quantity of diluting water / quantity of sewage. High dilution implies rapid reduction of BOD and pathogens by natural processes. No unusual toxicity or temperature effects assumed. Concept / Approach: Older standards suggested that if dilution factor exceeded about 500, the receiving water could assimilate the load without artificial treatment, given adequate DO levels and dispersion. However, environmental regulations today typically require at least primary and often secondary treatment to protect ecosystems and human health. Step-by-Step Solution: Define dilution factor as DF = Q_water / Q_sewage.Compare DF to threshold values: 150, 200–300, 400–500, and > 500.Select DF > 500 as the classical criterion for “no treatment required”. Verification / Alternative check: Textbook charts and design manuals cite increasing required treatment as DF decreases, with the least requirement at very high DF values. Why Other Options Are Wrong: DF values below or near 500 indicate insufficient natural assimilation; some level of treatment becomes necessary to protect water quality. Common Pitfalls: Applying dilution-only logic under modern discharge permits; ignoring cumulative impacts and sensitive habitats. Final Answer: More than 500

Question 3

Composting terminology: Alternating layers of vegetable waste and night soil piled above ground to form a long mound are called what?

Accepted Answer

Introduction / Context: Organic waste stabilization can be done by composting. Windrow composting forms elongated piles (windrows) that are periodically turned to maintain aerobic conditions. Historically, alternating layers of excreta and organic bulking agents were used where sanitary infrastructure was limited. Given Data / Assumptions: Layers: vegetable waste and night soil alternated. Piled above ground in a long mound. Turning/aeration presumed for aerobic stabilization and pathogen reduction. Concept / Approach: A windrow is an elongated pile optimized for surface aeration and mechanical turning. Proper C:N ratio (about 25–30:1), moisture (about 50–60%), and temperature (thermophilic range 55–65°C) ensure effective stabilization and pathogen die-off before land application or safe disposal. Step-by-Step Solution: Form long, narrow piles with alternating layers to achieve target C:N and porosity.Monitor temperature; turn when core temperature drops or exceeds target.Maintain moisture; avoid leachate and vectors with proper site management.Cure the compost before use to stabilize and reduce phytotoxicity. Verification / Alternative check: Temperature logs and odor reduction indicate successful composting; pathogen indicators (e.g., fecal coliform) verify sanitation targets. Why Other Options Are Wrong: “Heap” is generic and lacks the process implication; “plateau” is unrelated; windrow is the specific term for long mounds. Common Pitfalls: Insufficient turning; wrong moisture; inadequate curing leading to odor and pathogen risks. Final Answer: Windrow

Question 4

Inverted siphon barrel arrangement: In practice, how many parallel pipes are commonly provided for a sewer inverted siphon to handle variable flows and maintenance?

Accepted Answer

Introduction / Context: An inverted siphon (depressed sewer) conveys sewage under pressure beneath an obstruction. Because flows vary widely over a day, multiple barrels are provided so that one or more smaller barrels can run full at low flow (maintaining self-cleansing velocities) while additional barrels are opened at higher flows. Redundancy eases maintenance. Given Data / Assumptions: Sewer experiences diurnal variation. Self-cleansing velocity is required in the depressed reach. Maintenance isolation of one barrel is desirable. Concept / Approach: Many designs use three barrels of different diameters or orificed inlets: a small barrel for dry-weather flow, a medium for normal peak, and a large for storm-infiltrated peaks. With three, operators can isolate one barrel while maintaining service in the others. Two-barrel siphons also exist; three provides greater flexibility. Step-by-Step Solution: Determine diurnal flow range and minimum self-cleansing requirement.Size 2–3 barrels; select three for operational flexibility across ranges.Provide inlet/outlet structures with control gates and flushing provisions.Plan for bypass during inspection and grit removal. Verification / Alternative check: Hydraulic modeling confirms that at low flows, a single small barrel runs full with adequate velocity; at high flows, additional barrels come online without surcharge. Why Other Options Are Wrong: One barrel cannot maintain velocities over wide ranges and lacks redundancy; two barrels are common but less flexible; four barrels are rarely necessary and raise cost/complexity. Common Pitfalls: Operating too many barrels at low flow causing deposition; omitting flushing and air release valves. Final Answer: Three pipes

Question 5

Wastewater chemistry basics: If the pH of sewage is measured to be exactly 7.0 at 25°C, how is it classified?

Accepted Answer

Introduction / Context: pH is a measure of hydrogen ion activity and is fundamental to wastewater process control. It influences biological treatment, corrosion, scaling, and disinfectant efficacy. Understanding classifications helps operators quickly assess process health and potential risks. Given Data / Assumptions: Measured pH = 7.0 at standard conditions. No temperature correction required for classification. Standard pH scale from 0 (acidic) to 14 (alkaline). Concept / Approach: On the conventional logarithmic pH scale, pH 7.0 represents neutral water at 25°C. Values below 7 indicate acidity; values above 7 indicate alkalinity. Raw sewage often falls between roughly 6.5 and 8.5 depending on source contributions. Step-by-Step Solution: Identify pH value reported: 7.0.Compare to neutral reference 7.0.Classify as neutral. Verification / Alternative check: Cross-check with calibrated pH meter and buffer solutions (pH 4, 7, 10). Minor temperature effects do not alter categorical classification at exactly 7.0. Why Other Options Are Wrong: Acidic requires pH < 7; alkaline requires pH > 7; “none of these” does not apply because 7.0 is precisely neutral. Common Pitfalls: Assuming neutrality across temperatures without calibration; relying on indicator papers which may be less precise for operational decisions. Final Answer: Neutral

Question 6

Geometry of a partially full circular sewer: If the central angle of the water-filled segment is α (in radians) for a pipe of radius r, what is the cross-sectional flow area A?

Accepted Answer

Introduction / Context: Many hydraulic calculations for sewers flowing partially full require geometric properties of a circular segment: area, wetted perimeter, and hydraulic radius as functions of the central angle α. These relationships enable evaluation of conveyance using Manning or Chézy formulas. Given Data / Assumptions: Circular pipe of radius r. Water-filled segment subtends angle α at the center (radians). Uniform cross-section; ignore crown roughness variations. Concept / Approach: The area of a circular segment equals the area of the sector minus the area of the isosceles triangle formed by the two radii and the chord. Sector area for angle α is (α * r^2) / 2. Triangle area with side 2r sin(α/2) and included angle α is (r^2 * sin α) / 2. Subtracting yields the standard formula. Step-by-Step Solution: Sector area = (α * r^2) / 2.Triangle area = (r^2 * sin α) / 2.Segment (flow) area A = sector − triangle = (r^2 / 2) * (α − sin α).Use α consistent with flow depth to compute A for hydraulics. Verification / Alternative check: Check limits: if α = 0, A = 0; if α = π (half full), A = (r^2 / 2) * (π − 0) = (π r^2)/2; if α = 2π (full), expression gives full circle area (r^2/2)*(2π − 0) = π r^2, which is correct. Why Other Options Are Wrong: Options with plus signs overestimate area by adding the triangle; divisors not equal to 2 break the correct sector-triangle relationship. Common Pitfalls: Using α in degrees; forgetting to subtract the triangle area; mixing diameter D and radius r inconsistently. Final Answer: A = (r^2 / 2) * (α − sin α)

Question 7

Placement rules recap: A manhole is generally provided at each bend, junction, change of gradient, or change of sewer diameter. Which option best summarizes this practice?

Accepted Answer

Introduction / Context: Manhole spacing and placement rules are foundational to sewer design. They ensure safe access where flow conditions and geometry are most complex, reducing maintenance time and preventing chronic blockages at hydraulic transitions and network nodes. Given Data / Assumptions: Gravity sewer network with multiple junctions and geometry changes. Need for access for inspection and cleaning equipment. Standard design practices are followed. Concept / Approach: Provide manholes at bends (direction changes), junctions (network connections), changes in gradient (invert slope), and changes in pipe diameter. On long uniform reaches, additional manholes are spaced at practical intervals to maintain access, with spacing depending on pipe size and local codes. Step-by-Step Solution: List all geometry or network change points.Assign manholes at each to accommodate cleaning and inspection tools.Verify additional spacing manholes as per standards (e.g., 30–100 m).Ensure safe benching and channeling inside manholes for smooth flow. Verification / Alternative check: Operation and maintenance guidelines from utilities confirm lower blockage incidence when manholes are provided at these points. Why Other Options Are Wrong: Choosing only one location ignores the other critical points requiring access; comprehensive practice is captured by “All the above”. Common Pitfalls: Omitting manholes at diameter transitions; excessive spacing on small sewers; poor alignment causing excessive headloss through manholes. Final Answer: All the above

Question 8

In standing baffle walls used in primary sedimentation or grit chambers, bottom openings of size 15 cm × 15 cm are typically provided at what centre-to-centre spacing for proper underflow and avoidance of clogging?

Accepted Answer

Introduction / Context: In sanitary engineering, baffle walls are provided in grit chambers and sedimentation tanks to control flow patterns and direct settled or settling solids. The opening size and spacing influence velocity distribution, headloss, and the tendency to clog. Given Data / Assumptions: Opening size considered: 15 cm × 15 cm (square). Application: standing baffle wall in preliminary/primary treatment. Objective: ensure adequate underflow without excessive headloss or debris accumulation. Concept / Approach: The spacing of orifices in a baffle wall is chosen so that the average approach velocity remains low (to avoid scouring) yet sufficient to prevent deposition within the opening. Empirical guidelines used in design manuals specify centre-to-centre spacing close to 2 times the opening dimension for small baffle orifices in municipal plants. Step-by-Step Solution: Step 1: Identify typical opening dimension a = 0.15 m.Step 2: Apply rule-of-thumb spacing S ≈ 2 * a for small orifices in baffles.Step 3: Compute S = 2 * 0.15 m = 0.30 m = 30 cm.Step 4: Select the closest standard construction spacing: 30 cm c/c. Verification / Alternative check: Field practice and standard drawings for small plants commonly show 150 mm square openings at about 300 mm c/c, balancing constructability and hydraulic performance. Why Other Options Are Wrong: 22.5 cm c/c: Too close; increases headloss and clogging risk. 15 cm c/c: Essentially touching; impractical and prone to blockage. 50 cm c/c: Too wide; yields non-uniform flow and dead zones. 45 cm c/c: Wider than typical guidance; may reduce effectiveness of underflow distribution. Common Pitfalls: Designers sometimes only size the opening, ignoring spacing and approach velocity. Ignoring trash/float control can lead to rapid fouling even if spacing is hydraulically adequate. Final Answer: 30 cm c/c

Question 9

Which statement best describes design expectations for sewer pipes used in municipal sewerage systems?

Accepted Answer

Introduction / Context: Sewer design aims to transport wastewater reliably while minimizing maintenance. Key considerations include hydraulic performance, self-cleansing velocity, and durability against abrasion by grit. Given Data / Assumptions: Gravity flow dominates municipal sewers except at pumping stations. Self-cleansing velocity threshold typically ~0.6–0.75 m/s for domestic sewers (depends on grit). Abrasive wear is a real issue due to sand, grit, and scouring during storms. Concept / Approach: Good design checks both hydraulics and materials. Hydraulics ensures adequate slopes and velocities; materials selection ensures the pipe withstands chemical and mechanical attack (including abrasion). Step-by-Step Solution: Step 1: Confirm gravity conveyance: most sewers are laid to grade to avoid energy costs.Step 2: Provide slope such that v_min ≥ v_self-cleansing at dry weather flow to reduce deposition.Step 3: Select materials/linings that resist abrasion (e.g., vitrified clay, ductile iron with liners, HDPE).Step 4: The option combining all three requirements is correct. Verification / Alternative check: Standard design manuals and codes (municipal practice) list self-cleansing velocity checks and material abrasion resistance as essential criteria, confirming the combined statement. Why Other Options Are Wrong: (a) alone omits self-cleansing and abrasion resistance. (b) alone ignores gravity layout and durability. (c) alone ignores hydraulics. (e) still omits abrasion resistance. Common Pitfalls: Undersloping pipes to save depth can cause silting; ignoring grit leads to rapid wear and increased O&M costs. Final Answer: All of the above

Question 10

To maintain aerobic biological activity during composting of municipal organic wastes, the moisture content of the composting mass should be maintained approximately at what level?

Accepted Answer

Introduction / Context: Composting efficiency depends on moisture content: too dry halts microbial activity; too wet limits air diffusion and causes anaerobic conditions. Designers therefore target an optimal moisture band for aerobic composting. Given Data / Assumptions: Organic municipal waste with typical bulking agents. Aerobic process desired (odor control, faster stabilization). Temperature and aeration are adequate if moisture is in the recommended range. Concept / Approach: Aerobic microbes thrive when water films enable transport of nutrients but pore air is preserved. Empirical practice sets an optimum moisture around mid-50% by mass for MSW windrows with bulking. Step-by-Step Solution: Step 1: Identify standard moisture band for aerobic composting: roughly 50%–60%.Step 2: Choose the mid-range value commonly recommended for control: 55%.Step 3: Confirm that at 55% moisture, porosity and oxygen transfer remain adequate with periodic turning. Verification / Alternative check: Field guidance for windrow and aerated static pile composting frequently targets 50–60%, with 55% a commonly cited setpoint. Why Other Options Are Wrong: 45%–50%: Often too dry; activity slows and dusting increases. 60%–65%: Risk of anaerobic pockets, odors, and leachate. Common Pitfalls: Not re-wetting dry feed or over-wetting with leachate recirculation; neglecting bulking agents that maintain structure at target moisture. Final Answer: 55%

Question 11

During the earliest stage of decomposition of organic matter in sewage (pre-dominantly hydrolysis and initial biological breakdown), which primary product appears first in significant quantity?

Accepted Answer

Introduction / Context: Sewage contains organic nitrogen (proteins, urea). Understanding the sequence of transformations helps in process selection and troubleshooting in wastewater treatment. Given Data / Assumptions: Raw domestic sewage with typical organic nitrogen compounds. Initial decomposition occurs under reducing or mildly anaerobic conditions before full aeration. Concept / Approach: The first biochemical step is ammonification: organic nitrogen → ammonia (NH3/NH4+). Only later, under aerobic conditions, nitrifying bacteria oxidize ammonia to nitrites (NO2−) and then to nitrates (NO3−). Carbon dioxide is also formed but ammonia is the hallmark nitrogenous product of the first stage. Step-by-Step Solution: Step 1: Recognize ammonification as the earliest nitrogen transformation.Step 2: Identify that nitrification (to nitrite/nitrate) is a later, aerobic step.Step 3: Conclude that ammonia is the earliest significant nitrogenous product. Verification / Alternative check: Process flow in conventional treatment: organic N → NH4+ (primary), then NH4+ → NO2− → NO3− in aeration. Why Other Options Are Wrong: Nitrites/Nitrates: Require aerobic nitrification, not first stage. Carbon dioxide: Produced, but question focuses on nitrogenous product; ammonia is the specific early indicator. Hydrogen sulphide: Related to sulfur cycle under anaerobic conditions, not the first nitrogen product. Common Pitfalls: Confusing nitrification with ammonification; assuming nitrates appear immediately in raw sewage. Final Answer: Ammonia

Question 12

For secondary sedimentation tanks (secondary clarifiers) following biological treatment, what is the typical range of surface overflow rate used for design (expressed as litres per square metre per day)?

Accepted Answer

Introduction / Context: Surface overflow rate (SOR) is a key criterion for sizing secondary clarifiers. It relates the allowable solids loading and clarifier settling performance to plan area. Given Data / Assumptions: Municipal secondary clarifiers after activated sludge or trickling filter. Normal temperature and solids characteristics. Concept / Approach: Design practice uses SOR values derived from experience and pilot data. Typical municipal guidance for secondary clarifiers is in the range of roughly 25–35 m³/m²·day (equivalently 25,000–35,000 L/m²·day) for average flow. Step-by-Step Solution: Step 1: Identify the standard SOR range for secondary clarifiers.Step 2: Convert units: 1 m³/m²·day = 1,000 L/m²·day.Step 3: Choose the option that matches typical practice. Verification / Alternative check: Cross-check with typical state and national manuals which list similar SOR bands for average loading; peak conditions are handled via increased allowable SOR with safety factors. Why Other Options Are Wrong: 40,000–60,000+ L/m²·day: Too high for average flow; risk of solids carryover. 80,000–100,000 L/m²·day: Characteristic of peak/overload, not design average. 10,000–20,000 L/m²·day: Too conservative for typical municipal design. Common Pitfalls: Using primary clarifier SOR values or peak hour SOR directly for average design without considering return sludge and MLSS properties. Final Answer: 25,000 to 35,000 L/m²/day

Question 13

A 2.5% dilution of a sewage sample shows an oxygen depletion of 5 mg/L after 5 days at 21°C. What is the 5-day biochemical oxygen demand (BOD₅) of the undiluted sewage (in mg/L)?

Accepted Answer

Introduction / Context: BOD testing often uses dilutions to keep the dissolved oxygen (DO) depletion within measurable limits. The observed depletion must be scaled by the dilution factor to estimate the original BOD. Given Data / Assumptions: Dilution = 2.5% = 0.025 (by volume). Observed 5-day DO depletion at 21°C = 5 mg/L. No seed correction or blank depletion specified (assume negligible). Concept / Approach: BOD of original sample = observed depletion / dilution fraction, provided no additional corrections are needed. Step-by-Step Solution: Step 1: Determine dilution fraction f = 0.025.Step 2: Compute BOD₅ = 5 mg/L / 0.025.Step 3: BOD₅ = 200 mg/L. Verification / Alternative check: Check units and logic: a 40× dilution would multiply the depletion by 40 to recover the undiluted demand; 5 × 40 = 200 mg/L matches. Why Other Options Are Wrong: 50, 100, 150 mg/L: Underestimate because they ignore the dilution factor magnitude. 250 mg/L: Overestimates beyond the simple dilution scaling. Common Pitfalls: Forgetting to subtract seed/control depletion when present, or misreading the dilution percentage. Final Answer: 200 mg/L

Question 14

Which of the following are physical characteristics of municipal sewage commonly assessed during preliminary examination?

Accepted Answer

Introduction / Context: Characterizing sewage involves physical, chemical, and biological parameters. Physical characteristics provide quick diagnostic indicators of condition and treatment needs. Given Data / Assumptions: Municipal sewage at plant inlet. Standard physical observations: appearance, smell, and temperature. Concept / Approach: Physical assessments help identify septicity (odour), solids load (turbidity), dye/industrial influence (colour), and process performance implications (temperature affects reaction rates and viscosity). Step-by-Step Solution: Confirm each parameter is a recognized physical characteristic: turbidity, colour, odour, temperature.Select the consolidated option covering all such characteristics. Verification / Alternative check: Introductory wastewater texts list these as standard physical indicators alongside settleable solids observations (e.g., Imhoff cone for settleables). Why Other Options Are Wrong: Single-parameter choices omit others that are equally standard. Common Pitfalls: Confusing physical with chemical parameters (e.g., pH, alkalinity, BOD are not physical). Final Answer: All of the above

Question 15

Select the correct statements used in design and operation of sanitary sewers for municipal systems.

Accepted Answer

Introduction / Context: Peaking factors and self-cleansing velocity are basic to sewer design. They determine pipe sizes and slopes that avoid surcharge and sediment deposition. Given Data / Assumptions: Municipal sewer system with diurnal variability. Typical peaking factors from empirical curves (e.g., Harmon, Babbit). Self-cleansing velocity threshold around 0.6–0.75 m/s (varies with grit). Concept / Approach: Designers apply peaking factors to average flows to estimate maximum daily/hourly flows. They also ensure that at minimum flow the velocity exceeds the self-cleansing limit to avoid deposition. Step-by-Step Solution: Step 1: Adopt max day ≈ 2 × average day (commonly used planning factor).Step 2: Adopt max hour ≈ 3–6 × average hour (population dependent).Step 3: Provide slopes to keep v ≥ v_self-cleansing at minimum flow.Step 4: Recognize that all three statements are applied together in design. Verification / Alternative check: Standard practice manuals and codes show comparable peaking factors and self-cleansing criteria; actual values may be refined by local data. Why Other Options Are Wrong: (a) and (c) only: Omits hourly peaking, which is critical. Common Pitfalls: Using a single peaking factor for all sizes; neglecting minimum-velocity checks resulting in chronic silting. Final Answer: All of the above

Question 16

Underground sewer pipes installed in trenches are generally subjected predominantly to which kind of internal action from surrounding loads?

Accepted Answer

Introduction / Context: Buried sewers behave as rings under earth load and superimposed surface loads. Understanding the predominant internal action informs material selection and wall thickness. Given Data / Assumptions: Pipe laid in a trench with soil backfill. Uniform soil support around the pipe. Typical municipal burial depths. Concept / Approach: Ring compression is the dominant internal action in circular buried conduits due to vertical earth load and lateral soil reaction. Proper bedding distributes load and reduces bending. Step-by-Step Solution: Step 1: Model pipe as a ring under vertical load; lateral soil provides passive resistance.Step 2: The ring develops circumferential compressive thrust (hoop compression).Step 3: Bending is secondary when bedding and side support are adequate. Verification / Alternative check: Design equations for vitrified clay, concrete, and plastic pipes use crushing strength (compressive) ratings and bedding factors, confirming compression is primary. Why Other Options Are Wrong: Tension: Pipes are poor in tension and not the primary demand in buried state. Bending primary: Can occur with poor support, but not the general design assumption. Shear/Torsion: Not governing for uniformly supported buried rings. Common Pitfalls: Ignoring bedding quality and lateral support, which can raise bending stresses beyond assumptions. Final Answer: Compressive force

Question 17

Wastewater engineering — choose the correct comprehensive statement: Which of the following statements about dissolved oxygen (DO), hydrogen sulphide (H₂S), and testing methods for sewage is correct?

Accepted Answer

Introduction / Context: Sewerage design and wastewater treatment rely on understanding dissolved oxygen (DO) behavior, corrosion risks from hydrogen sulphide (H₂S), and standard laboratory testing methods. This question integrates these fundamental concepts used by civil and environmental engineers. Given Data / Assumptions: Treated effluent is to be released to a natural water body. Concrete sewers may experience biochemical and chemical environments that produce H₂S. Standard wet-chemistry methods are applicable for DO determination. Concept / Approach: H₂S corrosion: Sulphide generation in septic sewage leads to H₂S gas in the crown of concrete sewers; bacterial oxidation to sulphuric acid causes crown corrosion. Minimum DO: Maintaining around 4 mg/L DO in receiving waters is a common conservative target to protect aquatic life. Oxygen solubility: Sewage contains dissolved solids; its oxygen solubility is slightly lower than that of pure water (roughly 95%). Winkler’s method: A standard oxidation–reduction titrimetric procedure used to determine DO. Step-by-Step Solution: Evaluate each statement for correctness based on standard practice and theory.All four statements align with accepted engineering knowledge.Therefore, the comprehensive correct choice is “All the above.” Verification / Alternative check: These points appear across standard wastewater texts and specifications: sewer corrosion guidelines discuss sulphide control; receiving-water standards require DO safeguards; Winkler’s method is a classic test; and salinity/impurities reduce oxygen solubility relative to distilled water. Why Other Options Are Wrong: Choosing any single statement omits equally valid others, so only the aggregate option is fully correct. Common Pitfalls: Confusing effluent DO with receiving-water DO objectives. Underestimating sewer crown corrosion risk from H₂S in warm climates. Assuming oxygen solubility in sewage equals that in pure water exactly. Final Answer: All the above

Question 18

Septic tank design detail — inlet chamber floor slope: The bottom of the sewage inlet chamber in a septic tank is provided with an outward slope of:

Accepted Answer

Introduction / Context: Small on-site sanitation systems such as septic tanks include inlet arrangements that prevent scum carryover and promote smooth flow. The floor of the inlet chamber is given a gentle outward slope to guide influent beneath the scum layer toward the main tank volume. Given Data / Assumptions: A standard septic tank with inlet chamber and baffles/tees. Practice aligned with common design handbooks used in building sanitation. Outward slope refers to the fall away from the wall toward the tank interior. Concept / Approach: A slope that is too steep creates unnecessary velocities and turbulence; too flat may allow deposition. Practical guidance typically specifies a modest uniform slope that reliably directs flow under the scum blanket and reduces stagnation. Step-by-Step Solution: Identify conventional value ranges used for inlet chamber floors.Among the listed ratios, 1 in 10 is a widely cited practical slope for septic inlet chambers.Thus select 1 in 10. Verification / Alternative check: Typical plumbing and public health engineering manuals show slopes around 1 in 10 for such small chambers to maintain self-cleansing at low flows without surging. Why Other Options Are Wrong: 1 in 5: overly steep for this location; may induce turbulence.1 in 15, 1 in 20, 1 in 25: flatter than customary practice, increasing risk of local deposition. Common Pitfalls: Confusing inlet chamber slope with the general sewer gradient. Placing the slope inward (toward the wall) rather than outward toward the tank. Final Answer: 1 in 10

Question 19

Building drainage pipe commissioning — pre-use checks: Before putting drainage pipes in buildings to service, which of the following tests are applied?

Accepted Answer

Introduction / Context: Plumbing and building drainage systems must be leak-tight, correctly aligned, and free from obstructions. Several standard tests are employed before commissioning to verify workmanship and system integrity. Given Data / Assumptions: Pipes, joints, and fittings have been installed as per layout. Tests are conducted prior to covering or backfilling wherever applicable. Building codes typically mandate more than one test. Concept / Approach: Water test: Confirms watertightness under hydrostatic head. Smoke test: Detects leaks and improper traps by observing smoke escape. Straightness test: Ensures alignment to minimize blockages and maintain gradient. Combining these tests provides confidence in performance during actual use. Step-by-Step Solution: Identify the standard set of tests from codes and practice.All three tests are commonly required or recommended.Therefore, the correct response is “All the above.” Verification / Alternative check: Many municipal and national plumbing codes include water and smoke tests explicitly; straightness/alignment checks are standard site QA/QC procedures. Why Other Options Are Wrong: Selecting only one omits other equally necessary tests.“None of these” contradicts accepted practice. Common Pitfalls: Skipping smoke tests in favor of only water testing. Ignoring alignment, which can lead to frequent choking. Final Answer: All the above

Question 20

Sludge digestion under normal (mesophilic) conditions: The typical period required for sludge digestion is approximately:

Accepted Answer

Introduction / Context: Sludge digestion stabilizes organic matter, reduces pathogens, and minimizes odors. Under mesophilic conditions (about 25–40°C), digestion times are selected to reach desired volatile solids reduction and gas production stability. Given Data / Assumptions: Operation near the mid-mesophilic range (around 29–35°C). Conventional single-stage digester with adequate mixing and retention. “Normal conditions” implies neither thermophilic acceleration nor unusually cold temperatures. Concept / Approach: Digestion time is governed by temperature, loading rate, and desired stabilization level. Typical mesophilic detention periods are on the order of weeks; many references cite about one month as a representative period for substantial stabilization. Step-by-Step Solution: Identify the temperature regime as mesophilic.Recall standard detention periods under mesophilic conditions.Select the representative period: 30 days. Verification / Alternative check: Design manuals often note ~20–30 days at mesophilic temperatures depending on loading and performance targets; “30 days” is a classic benchmark value for design estimation. Why Other Options Are Wrong: 10–20 days: too short for typical mesophilic single-stage stabilization.60–90 days: conservative for cold climates or low-rate digesters; longer than “normal.” Common Pitfalls: Confusing mesophilic with thermophilic digestion (40–60°C), which has shorter times. Ignoring seasonal temperature effects on required detention. Final Answer: 30 days

Waste Water Engineering Questions