Question 1

Sewer flow variation: The ratio of minimum hourly flow to average sewage flow is typically closest to which value?

Accepted Answer

Introduction / Context: Sewer design requires knowledge of diurnal variation in sanitary flows. Minimum hourly flows govern issues such as odor control, sulfide formation, and maintenance of self-cleansing velocities during off-peak periods. Given Data / Assumptions: Domestic sanitary flow exhibits morning and evening peaks and overnight lows. Typical design guides assume empirical ratios for peak and minimum hourly flow relative to the average daily flow. Industrial contributions may alter the profile but household-dominant systems follow common rules of thumb. Concept / Approach: Common guidance sets peak hourly flow at roughly 1.5–3.0 times the average, while the minimum hourly flow can be around 0.5 times the average for typical communities. This accounts for human activity patterns and building plumbing behavior. Step-by-Step Solution: Define average sewage flow Q_avg.Adopt minimum hourly fraction f_min ≈ 0.5 based on diurnal variation data.Compute minimum hourly flow: Q_min ≈ 0.5 * Q_avg, giving a ratio of 1/2. Verification / Alternative check: Multiple design manuals and empirical curves for domestic systems show minimum-hour factors near 0.4–0.6. Selecting 1/2 matches the central tendency and is widely used for preliminary design. Why Other Options Are Wrong: 1/4 or 1/3: Too low for typical domestic diurnal curves, may apply only to very small or intermittent populations. 3/4 or 2/3: Too high; do not reflect the pronounced overnight lull commonly observed. Common Pitfalls: Using peak-to-average ratios for minimum flow checks. Ignoring infiltration/inflow which can slightly raise the minimum but is not the governing sanitary component. Final Answer: 1/2

Question 2

Dilution (disposal to water bodies): In which situation is the dilution method of sewage disposal not preferred on public-health and water-supply grounds?

Accepted Answer

Introduction / Context: Dilution refers to discharging treated (or sometimes preliminarily treated) wastewater into a natural water body, relying on mixing and assimilative capacity. While historically practiced, modern regulations require care to protect downstream uses, especially potable supply intakes. Given Data / Assumptions: Sewage disposal near a source water intake poses pathogen and contaminant risks. Receiving water quality (DO levels and currents) affects dilution, but does not eliminate risk to water supplies. Concept / Approach: The dilution method is contraindicated where the water body serves as a drinking-water source near the outfall. Even with high DO and good currents, incomplete pathogen removal or shock loads can compromise public health. Buffer distances, advanced treatment, and outfall siting are critical if any mixing zone is contemplated. Step-by-Step Solution: Identify the sensitive use: potable water supply.Assess proximity: near the sewage discharge, risk is highest.Conclude that this is the least preferred scenario for dilution-based disposal. Verification / Alternative check: Guidelines typically require stringent treatment and setback distances from drinking-water intakes; many jurisdictions prohibit such discharges in source-water protection zones. Why Other Options Are Wrong: Fresh sewage and high DO or adequate currents speak to assimilative capacity but do not outweigh the risk at drinking-water intakes. None of these is incorrect because one scenario is clearly unsuitable. Common Pitfalls: Overreliance on dilution as a substitute for adequate treatment. Ignoring seasonal and flow variability that can reduce mixing and DO. Final Answer: When the receiving water is also used for municipal supply near the disposal point

Question 3

Sewer crossing design: An inverted siphon (depressed sewer crossing) is generally designed with how many parallel barrels to ensure capacity and maintainability?

Accepted Answer

Introduction / Context: An inverted siphon conveys sewage under obstacles (e.g., rivers, road cuts) where gravity flow at grade is not feasible. Because depressed segments run full and are prone to sedimentation, redundancy and operational flexibility are essential. Given Data / Assumptions: Urban sewer crossing requiring reliable operation and maintenance. Allowance for variable flows (dry to wet weather) and cleaning. Concept / Approach: Best practice provides multiple parallel barrels. A common configuration is three pipes: one sized for dry-weather flow, a second to share load as flows increase, and a third as standby for maintenance or peak events. This arrangement maintains velocity in service barrels (improving self-cleansing) while providing redundancy. Step-by-Step Solution: Recognize inverted siphons run under pressure and cannot easily vent air or remove grit during operation.Adopt multi-barrel design to match low and high flows and allow isolation.Select three pipes as the typical standard configuration. Verification / Alternative check: Design manuals often illustrate two- or three-barrel siphons, with three preferred for large crossings because it balances hydraulics and maintenance access. Why Other Options Are Wrong: One pipe: No redundancy; poor performance at low flows due to low velocity and sedimentation risk. Two pipes: Better than one but less flexible for maintenance while maintaining capacity. Four or five: Possible on very large systems but not generally required; adds cost and complexity. Common Pitfalls: Oversizing a single barrel, which reduces velocity and promotes deposition. Omitting isolation valves and flushing connections. Final Answer: Three pipes

Question 4

Pumping equipment selection: Which pump type is commonly termed a non-clog pump because it permits passage of solids present in raw sewage?

Accepted Answer

Introduction / Context: Sewage contains rags, grit, and fibrous materials. Pumps in lift stations must pass such solids without frequent blockage. Special impeller designs and pump selections address this practical need. Given Data / Assumptions: Raw municipal sewage with suspended solids and fibrous matter. Goal: select a pump that can handle solids passage. Concept / Approach: Centrifugal sewage pumps with non-clog impellers (e.g., single-vane, two-vane, recessed/vortex impellers) are widely used. They provide sufficient passage diameter and hydraulic design to reduce ragging and clogging. Reciprocating pumps are unsuitable for debris-laden flows, and pneumatic ejectors are niche devices, not mainstream pumps. Step-by-Step Solution: Identify solids-handling requirement.Select centrifugal non-clog impeller as the standard solution for municipal lift stations.Confirm suitability via solids passage criteria (e.g., 75–100 mm sphere passage for larger stations). Verification / Alternative check: Manufacturers market “non-clog” centrifugal pumps specifically for wastewater service, corroborating the choice. Why Other Options Are Wrong: Reciprocating pump: Sensitive to solids; maintenance-intensive. Pneumatic ejector: Uses compressed air; limited capacity and not a general-purpose pump. Axial-flow only: Used for high flow/low head clean water; not the standard solids-handling sewage pump. Common Pitfalls: Ignoring solids passage diameter when selecting pumps. Overlooking ragging tendency and need for chopper/vortex options. Final Answer: Centrifugal pump (non-clog impeller)

Question 5

Sewer appurtenances: Identify the incorrect statement regarding manholes, catch basins, and inlets in sewer systems.

Accepted Answer

Introduction / Context: Sewer networks include several appurtenances—manholes for access, catch basins and inlets for street drainage interception. Understanding their correct placement helps ensure functionality and ease of maintenance. Given Data / Assumptions: Combined versus separate sewer systems have different inlet strategies. Manholes are routine access points. Catch basins separate grit/debris from roadway runoff before entering storm sewers. Concept / Approach: It is incorrect to claim that inlets are provided in all sewers. Inlets are features of stormwater collection along curbs and streets; they are not required on sanitary-only sewers that receive flows from building laterals rather than street runoff. Step-by-Step Solution: Confirm manhole usage: access, alignment changes, and junctions.Note catch basins: first-line debris traps in storm systems.Identify the incorrect universal statement about inlets across all sewer types. Verification / Alternative check: Separate sanitary systems have no street inlets by design; thus the statement claiming universality is false. Why Other Options Are Wrong: Manhole statements are true and standard practice. None of these is wrong because one statement is indeed incorrect. Common Pitfalls: Confusing combined systems (which may have inlets) with sanitary-only systems. Final Answer: Inlets are generally provided in all sewers regardless of type and location

Question 6

Biochemical oxygen demand (BOD): The BOD exerted during the first 20 days of incubation is generally referred to as which stage of oxygen demand?

Accepted Answer

Introduction / Context: BOD measures the oxygen required by microorganisms to oxidize biodegradable organic matter in wastewater. The oxygen demand has distinct phases: carbonaceous BOD (CBOD) and nitrogenous BOD (NBOD). Given Data / Assumptions: Standard BOD test is 5 days at 20°C (BOD5), but long-term BOD can be observed over 20 days or longer. Carbonaceous oxidation dominates early periods before nitrification (NBOD) becomes significant unless inhibited. Concept / Approach: The first 20 days generally capture the carbonaceous stage, especially if nitrification is inhibited or delayed. CBOD reflects oxidation of organic carbon compounds to CO2 and water, which is the principal oxygen demand in municipal sewage during the initial period. Step-by-Step Solution: Define phases: CBOD first, NBOD later (unless seeded for nitrifiers).Relate 20-day window predominantly to CBOD for typical sewage tests.Select 'Carbonaceous demand' as the recognized term. Verification / Alternative check: Analytical protocols often report CBOD5 (carbonaceous BOD at 5 days) to exclude nitrification via inhibitors (e.g., allyl thiourea), underscoring that the initial BOD is carbonaceous. Why Other Options Are Wrong: Initial demand/First-stage demand: Vague descriptors; the precise term is carbonaceous demand. All of these: Overly broad; only the specific term is correct. Nitrogenous demand: Occurs later as ammonia is oxidized to nitrites/nitrates. Common Pitfalls: Confusing CBOD with NBOD and misinterpreting BOD5 versus long-term BOD. Final Answer: Carbonaceous demand

Question 7

Process selection for a large city: For treating municipal sewage at metropolitan scale, which process train is generally recommended?

Accepted Answer

Introduction / Context: Large cities require robust, scalable, and controllable wastewater treatment capable of meeting stringent effluent standards. The process must handle variable flows and loads while allowing energy-efficient operation and nutrient removal options. Given Data / Assumptions: Metropolitan-scale flows with significant industrial and commercial contributions. Regulatory compliance for BOD, TSS, and potentially nutrients. Existing best practices in urban wastewater treatment. Concept / Approach: Primary sedimentation removes settleable solids and reduces organic loading to the biological stage. The activated sludge process (ASP) offers high-rate biological oxidation with process control (MLSS, SRT, DO), modularity (plug-flow, step-feed), and the ability to integrate nitrification-denitrification and biological phosphorus removal. Trickling filters or Imhoff tanks are typically suited to smaller plants or specific contexts and offer less process control at very large scales. Step-by-Step Solution: Select primary sedimentation to reduce TSS and protect downstream aeration.Choose activated sludge for secondary treatment to achieve low effluent BOD/TSS and enable nutrient removal if needed.Provide tertiary polishing where regulations require further reduction. Verification / Alternative check: Most large municipal WWTPs worldwide use some variant of ASP (conventional, extended aeration, step-feed, oxidation ditches, SBRs) after primary clarification, validating the selection. Why Other Options Are Wrong: Imhoff + low-rate trickling filters: Viable for small/medium towns; scaling and control are limited for megacities. High-rate trickling filters only: Can be used, but control and footprint for nutrient removal are less favorable than ASP at very large scales. None of these / Septic tanks: Not appropriate for metropolitan systems. Common Pitfalls: Underestimating the need for process control, redundancy, and flexibility in large plants. Ignoring sludge handling (thickening, digestion, dewatering) integral to the overall train. Final Answer: Primary sedimentation followed by activated sludge (secondary) treatment

Question 8

Environmental engineering — classify rainfall as “acid rain”: at what pH value or lower is precipitation considered acidic?

Accepted Answer

Introduction / Context: Acid rain is precipitation with a hydrogen ion concentration higher than that of normal, unpolluted rain. In environmental engineering and atmospheric science, classifying rainfall as “acidic” helps assess impacts on water bodies, soils, buildings, and ecosystems, and it guides mitigation strategies for emissions of sulfur oxides (SOx) and nitrogen oxides (NOx). Given Data / Assumptions: Normal clean rain equilibrated with atmospheric CO2 has a slightly acidic pH around 5.6. Question asks for a threshold pH at or below which rainfall may be classified as acidic in practical environmental monitoring. pH is defined as −log10[H+]; lower pH means greater acidity. Concept / Approach: While 5.6 is the theoretical pH of pure rainwater in equilibrium with CO2, regulatory and textbook MCQs often expect a stricter operational cutoff for “acid rain” classification. Values ≤ 5.0 clearly indicate anthropogenic acidification beyond natural carbonation. Hence, pH ≤ 5 is a commonly used simple criterion in many question banks to label precipitation as acidic beyond natural background effects. Step-by-Step Solution: Recognize natural-background pH ≈ 5.6 (from carbonic acid).Monitoring categories often flag more acidic events when pH ≤ 5.0.Among the provided options, 5 is the clear “acid rain” threshold below natural background.Therefore, select pH 5 as the classification cutoff given the choices. Verification / Alternative check (if short method exists): If an option at 5.6 were provided and explicitly tied to the CO2 equilibrium definition, one might argue for “≤ 5.6.” However, many standardized MCQs emphasize pH ≤ 5 for unequivocal acidification. Given the present options, 5 is the appropriate selection. Why Other Options Are Wrong: 6 or 6.5 are too high (near neutral to mildly acidic). A pH of 7 is neutral and not acid rain by any definition. Common Pitfalls (misconceptions, mistakes): Confusing natural slightly acidic rain (≈5.6) with anthropogenically acidified precipitation; assuming only very low pH (e.g., ≤4) counts as “acid rain.” Final Answer: 5

Question 9

Sanitary engineering — common causes of sewer clogging in urban drainage systems

Accepted Answer

Introduction / Context: Sewer networks transport wastewater reliably only when internal flow velocities are sufficient to prevent deposition. Clogging leads to surcharge, odour, sanitary hazards, and costly maintenance. Understanding the typical causes helps in design (self-cleansing velocity), operation (grease traps), and public awareness campaigns. Given Data / Assumptions: Conventional gravity sewers conveying domestic and trade wastewater. Inlet misuse (solid wastes in manholes) can occur. Hydraulic conditions may vary across dry-weather flow and peak flow. Concept / Approach: Clogging arises when the shear stress at the invert is too low to re-entrain settled particles or when immiscible substances accumulate. Low discharge reduces velocity, increasing the risk of silting. Grease adheres to pipe walls and traps solids. Inappropriate disposal of solids (textiles, plastics) causes localized blockages. Therefore, multiple mechanisms can simultaneously contribute to clogging in real sewers. Step-by-Step Solution: Identify hydraulic cause: insufficient velocity → sedimentation and silting.Identify material cause: oils/grease → fatbergs and adhesion of solids.Identify behavioural cause: dumping solids via manholes → obstructions.Synthesize: all listed items are valid causes. Verification / Alternative check (if short method exists): Field maintenance logs routinely cite combined effects (silting during low-flow seasons, grease from kitchens, and foreign objects) as drivers of blockages. Why Other Options Are Wrong: Each single-cause option is incomplete; clogging is multifactorial. Common Pitfalls (misconceptions, mistakes): Assuming satisfactory performance at design flow guarantees self-cleansing at all times; neglecting grease control and community education. Final Answer: all the above

Question 10

Wastewater treatment — dissolved oxygen (DO) in sewage: select the correct comprehensive statement

Accepted Answer

Introduction / Context: Dissolved oxygen (DO) in wastewater is a critical indicator for biological processes and receiving water quality. Adequate DO in treated effluent prevents septic conditions, odours, and fish kills when discharged into surface waters, and it reflects effective secondary treatment and aeration control. Given Data / Assumptions: “ppm” is used synonymously with mg/L in dilute aqueous systems. Fresh domestic sewage rapidly depletes DO via biochemical oxygen demand (BOD). Impurities reduce oxygen solubility relative to pure water. Concept / Approach: Properly treated secondary effluent typically targets a DO of about 4–5 mg/L leaving the plant to avoid oxygen sag in the receiving stream. Raw sewage, unless extremely fresh, usually shows negligible DO because microbes consume oxygen quickly. Solubility of oxygen in saline/impure water is modestly lower than in distilled water; “about 95%” is a reasonable instructional approximation acknowledging reduced solubility due to dissolved constituents. Step-by-Step Solution: Assess (a): Treated effluent DO target ≈ 4 mg/L → acceptable.Assess (b): Fresh sewage may retain a little DO; it quickly drops with time → true.Assess (c): Oxygen solubility decreases in wastewater compared to distilled water → broadly correct.Therefore, (d) “All the above” summarizes accurately. Verification / Alternative check (if short method exists): Standard design manuals (activated sludge) recommend maintaining DO of 1.5–3 mg/L in aeration tanks and ≥4 mg/L in final effluent to prevent septicity downstream. Why Other Options Are Wrong: Choosing one statement alone ignores the broader context; “None of these” contradicts established practice and physical chemistry. Common Pitfalls (misconceptions, mistakes): Assuming raw sewage can sustain high DO; overlooking the effect of temperature and salinity on DO solubility. Final Answer: All the above

Question 11

Preliminary treatment of sewage — what is the unit process called that removes free oil and grease from wastewater?

Accepted Answer

Introduction / Context: Preliminary treatment protects downstream processes by removing large solids, grit, and floatables. Free oils and grease can foul equipment, inhibit biological processes, and cause odours. A dedicated step is used to remove this fraction before primary settling and biological treatment. Given Data / Assumptions: Wastewater contains free (non-emulsified) oil and grease that naturally float. Flow conditions allow surface accumulation and removal. Coarse solids have already been intercepted by screens/grit chambers. Concept / Approach: Skimming is the process of removing floating matter from the surface. In skimming tanks (or API separators in industrial settings), oils accumulate at the surface and are scraped or skimmed off. Screening removes coarse debris via bar racks; filtration is a separate solids removal process typically used for tertiary polishing, not for bulk oil and grease removal in the headworks. Step-by-Step Solution: Identify contaminant: free oil/grease → lower density than water → floats.Choose unit: skimming tank or oil–water separator encourages surface accumulation.Removal mechanism: mechanical skimmer/overflow weirs collect the floating layer.Downstream benefit: reduces scum, odour, and protects aeration systems. Verification / Alternative check (if short method exists): Design references specify skimming devices in primary clarifiers and separate separators for oily wastewater—confirming the terminology. Why Other Options Are Wrong: Screening targets rags and large debris; filtration is not the primary headworks method for oil; “none” contradicts standard practice; “floatation” is a different enhanced process (e.g., DAF) but the basic term here is skimming. Common Pitfalls (misconceptions, mistakes): Confusing skimming with dissolved air flotation; attempting to remove emulsified oils by simple skimming (requires coagulation/DAF). Final Answer: skimming

Question 12

Sewer hydraulics — definition of self-cleansing velocity in sanitary/storm drains

Accepted Answer

Introduction / Context: Self-cleansing velocity is a key design parameter for sewers: it is the minimum flow velocity required to prevent deposition of suspended solids and grit so that the system remains largely free of silt under normal operation. This protects capacity and reduces maintenance costs. Given Data / Assumptions: Gravity flow in sanitary or storm sewers. Typical sediment sizes expected from domestic/urban sources. Designers select slopes to meet self-cleansing at a specified fraction of design flow (often near DWF). Concept / Approach: The critical tractive stress on the pipe invert must exceed the resisting weight of settled particles for re-entrainment. Expressed simply, if the mean velocity exceeds a threshold (commonly ~0.6–1.0 m/s depending on practice and pipe), the sewer is considered self-cleansing because settled solids are scoured and do not accumulate. Step-by-Step Solution: Define the goal: avoid deposition → maintain minimum velocity V_min.Link to shear stress: τ = ρ g R S; choose S so τ ≥ τ_critical for sediments.Translate into velocity via Manning/Chezy to ensure V ≥ V_min at low flows.Resulting definition: the velocity at which no accumulation remains. Verification / Alternative check (if short method exists): Operational data: systems designed for V_min at DWF show fewer blockages than those that only meet criteria at peak flow. Why Other Options Are Wrong: “Velocity at dry weather flow” is not necessarily self-cleansing; “flushing” velocity is a maintenance operation, not a design criterion; “pressure filter” is unrelated. Common Pitfalls (misconceptions, mistakes): Confusing peak-flow velocity with self-cleansing; ignoring sediment characteristics when selecting V_min. Final Answer: velocity at which no accumulation remains in the drains

Question 13

Water treatment — in flocculation, which property of flocculated particles does not change?

Accepted Answer

Introduction / Context: Flocculation is the gentle mixing stage after coagulation where destabilized colloids agglomerate into larger, settleable flocs. Understanding which particle characteristics evolve (and which remain effectively unchanged) helps operators interpret jar tests and optimize mixing energy and coagulant aids. Given Data / Assumptions: We consider individual primary particles combining into aggregates (flocs). Water chemistry (pH, alkalinity) and coagulant dose are appropriate for aggregation. “Weight” here is interpreted as the intrinsic material density/weight per unit solid content of the constituent particles (not the growing mass of an aggregate as it captures more solids). Concept / Approach: During flocculation, particles collide and stick, forming irregular aggregates. The aggregate’s size increases and its shape becomes more irregular (higher porosity/floc factor). However, the intrinsic material density of the constituent solids does not change; what changes is bulk (apparent) density due to entrapped water and porosity. Classical teaching phrases this as: size and shape change, but the basic particle material property (often described in questions as “weight”/“specific gravity”) does not. Step-by-Step Solution: Coagulant destabilizes colloids → reduces repulsion.Gentle mixing → particle collisions → aggregates form.Observable changes: larger, more irregular flocs (size ↑, shape varies).Underlying solid material property effectively unchanged → weight (per solid) unchanged. Verification / Alternative check (if short method exists): Jar tests show rising floc size without chemical change in mineral composition; density of solid phase remains the same, although aggregate apparent density can differ. Why Other Options Are Wrong: Size and shape clearly change during flocculation; “none of these” ignores evident aggregation effects. Common Pitfalls (misconceptions, mistakes): Confusing intrinsic solid density with aggregate bulk density; assuming flocculation alters chemistry of the solid phase. Final Answer: weight

Question 14

Hydraulically equivalent sections — for a square sewer (side = 1000 mm), find the diameter of a circular section with the same discharge capacity under equal slope and roughness

Accepted Answer

Introduction / Context: Two closed conduits are “hydraulically equivalent” if, under the same slope and roughness, they convey the same discharge. For full-flow sewers, discharge (for turbulent regime) scales approximately with A * R^(2/3), where A is area and R is hydraulic radius (A/P). Converting between shapes assists in design substitutions and catalog selection. Given Data / Assumptions: Square section: side a = 1000 mm = 1.0 m. Target: equivalent circular diameter d. Equal slope S and Manning/Chezy roughness for both sections. Use Q ∝ A * R^(2/3) for comparison (same proportionality constant). Concept / Approach: For a square running full: A_s = a^2, P_s = 4a, R_s = A_s / P_s = a/4. For a circle running full: A_c = (π d^2)/4, P_c = π d, R_c = d/4. Equate A_s * R_s^(2/3) = A_c * R_c^(2/3) and solve for d in terms of a. Step-by-Step Solution: Compute A_s * R_s^(2/3) = a^2 * (a/4)^(2/3) = a^(8/3) * 4^(−2/3).Compute A_c * R_c^(2/3) = (π d^2/4) * (d/4)^(2/3) = (π/4) * d^(8/3) * 4^(−2/3).Set equal and cancel 4^(−2/3): a^(8/3) = (π/4) * d^(8/3).Solve for d: d = a * (4/π)^(3/8) ≈ a * 1.095 → for a = 1000 mm, d ≈ 1095 mm. Verification / Alternative check (if short method exists): Using hydraulic mean depth equality alone would give d = a; however, equal discharge requires the A * R^(2/3) criterion, yielding the 1.095 factor. Why Other Options Are Wrong: 1045, 1065, and 1075 mm underestimate the required diameter; 1000 mm ignores the difference in wetted perimeter/area interplay between shapes. Common Pitfalls (misconceptions, mistakes): Equating only hydraulic radius or only area; forgetting that both A and R influence discharge capacity. Final Answer: 1095 mm

Question 15

Sewage disposal practice in large cities — which method is commonly adopted (historical MCQ perspective)?

Accepted Answer

Introduction / Context: Historically, large coastal or riverine cities often relied on the “dilution” principle—discharging treated or partially treated effluent into a large body of water with sufficient assimilative capacity. Modern practice emphasizes advanced treatment to meet discharge standards; however, this classic MCQ reflects the traditional approach where extensive land for irrigation or oxidation ponds was unavailable in dense urban areas. Given Data / Assumptions: “Large cities” with limited land resources. Access to large receiving waters (sea/major river) to dilute after treatment. Question framed in the context of conventional multiple-choice knowledge. Concept / Approach: Where vast land areas for sewage farming (irrigation) or stabilization ponds (oxidation) are impractical, outfalls to large water bodies have been commonly used, contingent on primary/secondary treatment and dilution to meet water quality criteria. Putrefaction is not a disposal method; it is an undesirable anaerobic decomposition process. Step-by-Step Solution: Identify urban constraint: limited land → irrigation/oxidation less feasible.Available alternative: outfall with mixing capacity → dilution method.Confirm that regulatory compliance still requires adequate treatment before discharge.Conclude dilution is the best answer among the options. Verification / Alternative check (if short method exists): Many legacy metropolitan systems historically discharged to coastal waters post-treatment; modern upgrades add tertiary treatment and outfall diffusers. Why Other Options Are Wrong: Irrigation/oxidation need large land tracts; putrefaction is not a planned disposal method. Common Pitfalls (misconceptions, mistakes): Assuming “dilution is the solution to pollution” without acknowledging current environmental regulations; confusing process steps with disposal methods. Final Answer: dilution

Question 16

Design peaking factor — ratio of maximum sewage flow to average flow for mains up to 1 m diameter

Accepted Answer

Introduction / Context: Sewer design uses a peaking factor to relate maximum (peak) flow to average daily flow, accounting for diurnal variation and simultaneity of use. Smaller mains typically experience higher variability, hence larger peaking factors, than very large interceptors that average many sources. Given Data / Assumptions: Mains up to 1 m diameter. Typical municipal domestic wastewater without extreme industrial shock loads. Use of standard empirical guidance (e.g., 2.5–3.0 range for small mains). Concept / Approach: Conventional design manuals adopt a factor near 3 for small to medium sewers to ensure capacity during peak hours and to sustain self-cleansing velocities. Larger trunks may use smaller factors due to flow attenuation and storage in the network. Step-by-Step Solution: Identify pipe class: up to 1 m → small/medium main.Select peaking factor from standard tables → ≈ 3.0.Compute Q_max = 3 * Q_avg during design if needed.Provide adequate slope to maintain self-cleansing at low flows. Verification / Alternative check (if short method exists): Harmon or Babbit–Babbit formulae yield similar peaks for typical populations; attenuation in larger systems lowers the factor. Why Other Options Are Wrong: 1.5–2.5 underestimate peak conditions for small mains; 4.0 is overly conservative for typical municipal conditions. Common Pitfalls (misconceptions, mistakes): Applying a single factor across an entire network regardless of size; ignoring infiltration/inflow effects that can also peak during storms. Final Answer: 3.0

Question 17

Acoustics — in SI units, the “power” of a sound source is measured in which unit?

Accepted Answer

Introduction / Context: Sound power is the total acoustic energy emitted by a source per unit time. It is a fundamental quantity in building acoustics, environmental noise control, and equipment specifications, distinct from sound pressure (a local field quantity) and from decibel-scaled levels used for comparison and regulation. Given Data / Assumptions: We are asked for the SI unit of “power,” not “energy,” “force,” or “mass.” Acoustic power can be converted to sound power level (dB) using a reference power. Concept / Approach: In SI, power is measured in watts (W), equal to joules per second. In acoustics, sound power level Lw (in dB) is defined by 10 * log10(P/P0), where P0 = 1 picowatt for airborne sound. This should not be confused with sound pressure level (SPL) measured in decibels referenced to 20 micropascals, nor with force (newton) or mass (kilogram). Step-by-Step Solution: Identify physical quantity: power → rate of energy emission.Recall SI unit: 1 watt = 1 joule/second.Relate to acoustics: Lw uses watts relative to a reference power.Hence select “watts.” Verification / Alternative check (if short method exists): Check dimensional consistency: energy (J) per time (s) → W. ISO acoustic standards confirm power in watts. Why Other Options Are Wrong: Joules are energy, not power; “neutons” appears to be “newtons,” a unit of force; “kgs” is mass; decibels are logarithmic levels, not base SI units of power. Common Pitfalls (misconceptions, mistakes): Confusing sound power (a source property) with sound pressure (a field measurement) or with subjective loudness metrics. Final Answer: watts

Question 18

Pipeline installation practice in civil engineering: Asbestos–cement (A.C.) pipes for water supply and sewer rising mains are generally installed in which orientation for proper alignment, bedding, and jointing?

Accepted Answer

Introduction / Context: Asbestos–cement (A.C.) pipes were historically used for water distribution and pump rising mains. Understanding standard laying orientation helps ensure structural adequacy, hydraulic performance, and joint integrity along a pipeline route. Given Data / Assumptions: The question asks the usual or general laying orientation of A.C. pipes. Typical pipeline works aim to maintain uniform gradients and bedding support. Joints (e.g., spigot-and-socket) are designed for axial alignment. Concept / Approach: Linear pipelines are constructed along a designed alignment and gradient. To achieve self-cleansing velocities (for sewers) and minimize headloss (for water mains), the pipeline is laid on continuous horizontal or gently graded alignments. Vertical installation is not practical for continuous flows or for jointed rigid pipes, and steep angles would induce joint stress and support issues. Step-by-Step Solution: Identify the function: a line conveying water or sewage along a route.Recognize that pipelines require continuous bedding and even support.Conclude that the standard practice is laying pipes horizontally (or along designed slight gradients), not vertically or at steep angles. Verification / Alternative check: Practical construction manuals depict trench excavation with uniform bedding; pipes are aligned horizontally with slight slopes as specified by hydraulic design. Why Other Options Are Wrong: Vertically / 30° / 60° / random incline: Not used for linear conveyance; causes joint stress, air locking, and support difficulties. Common Pitfalls: Confusing local gradient (a few per-mille) with “angle” laying. Even with a gradient, the practical orientation is essentially horizontal along the alignment. Final Answer: horizontally

Question 19

Wastewater quality basics: Select the correct statements about pH and the typical pH of fresh domestic sewage.

Accepted Answer

Introduction / Context: pH is a fundamental parameter in wastewater engineering. It influences biological activity, corrosion/scale tendencies in sewers, and the effectiveness of many treatment processes such as coagulation and disinfection. Given Data / Assumptions: The statements describe what pH represents and typical pH ranges in fresh sewage. Fresh domestic sewage commonly shows mild alkalinity (pH slightly above 7). Anaerobic decomposition over time may reduce pH (becoming acidic), but that is not “fresh.” Concept / Approach: By definition, pH < 7 is acidic, pH = 7 is neutral, and pH > 7 is alkaline. Fresh municipal sewage typically has a pH around 7.2–8.0 due to bicarbonate alkalinity and the nature of household waste streams. Step-by-Step Solution: Confirm that pH expresses hydrogen ion activity and thus acidity/alkalinity.Relate acidic sewage to pH < 7 and alkaline sewage to pH > 7.Recognize fresh sewage is usually slightly alkaline; hence all statements are correct. Verification / Alternative check: Design manuals for wastewater treatment list typical fresh sewage pH as mildly alkaline; long retention (septic conditions) often shifts pH downward. Why Other Options Are Wrong: Each individual statement (a–d) is correct; therefore “All the above” is the most complete choice. Common Pitfalls: Assuming all sewage is acidic; freshness and aeration conditions matter. Ignoring industrial inputs which can alter pH significantly. Final Answer: All the above.

Question 20

Primary settling tank sizing: A rectangular tank 24 m long, 6 m wide, and 3 m deep is designed for a detention time of 2 hours. What is the allowable sewage flow rate (m^3 per hour)?

Accepted Answer

Introduction / Context: Sedimentation tank detention time is a fundamental design check linking tank volume to flow. Given geometry and detention time, we can directly compute the allowable volumetric flow rate. Given Data / Assumptions: Length L = 24 m, Width B = 6 m, Water depth H = 3 m. Detention time t = 2 hours. No allowance for sludge storage is requested; use gross volume for this calculation check. Concept / Approach: Detention time t = V / Q. Therefore, Q = V / t. Compute the tank volume from its rectangular plan and depth. Step-by-Step Solution: V = L * B * H = 24 * 6 * 3 = 432 m^3.t = 2 hours, so Q = V / t = 432 / 2 = 216 m^3 per hour.Match with the closest option: 216 cu m. Verification / Alternative check: Unit check: m^3 divided by hours gives flow in m^3/h, which is correct. A quick sense-check shows that halving the volume would halve the detention time, matching proportional expectations. Why Other Options Are Wrong: 204, 208, 212, 220 m^3/h are numerical distractors; only 216 m^3/h equals 432/2. Common Pitfalls: Mistakenly using area instead of volume in the formula. Mixing minutes and hours, causing unit errors (e.g., 120 minutes vs 2 hours). Final Answer: 216 cu m

Waste Water Engineering Questions