Question 1

Primary sedimentation design check: A settling tank is 2.4 m deep and must remove particles with settling velocity 0.33 mm/s (for d ≥ 0.06 mm). What is the recommended detention time?

Accepted Answer

Introduction / Context: Detention time in a settling (clarifier) tank must be sufficient for target particles to settle from the liquid surface to the tank bottom. This problem links settling velocity to tank depth to estimate a suitable detention time for design/operation. Given Data / Assumptions: Tank depth = 2.4 m. Target particle settling velocity v_s = 0.33 mm/s = 0.00033 m/s for particles ≥ 0.06 mm. Vertical settling distance approximated as the water depth. Neglect inlet/outlet effects and turbulence for a first-cut estimate. Concept / Approach: Time required for settlement is distance/velocity. A conservative design detention time equals or slightly exceeds this value to account for non-idealities. Use t = depth / v_s. Step-by-Step Solution: 1) Convert velocity: 0.33 mm/s = 0.00033 m/s.2) Compute time: t = 2.4 m / 0.00033 m/s = 7272.7 s.3) Convert seconds to hours: 7272.7 s / 3600 s/h ≈ 2.02 h.4) Choose the closest recommended detention period: 2 hours. Verification / Alternative check: A quick check with minutes: 7272.7 s ≈ 121.2 min ≈ 2.02 h confirms the selection of 2 hours as the nearest practical value. Why Other Options Are Wrong: 30 min, 1 h, 1.5 h: All are less than the calculated 2.02 h and may not ensure removal of the target particles.3 h: Longer than necessary based on the given settling velocity and depth, leading to oversized detention beyond the calculated requirement. Common Pitfalls: Forgetting to convert mm/s to m/s. Using surface overflow rate concepts incorrectly when the question focuses on depth-based settling time. Ignoring safety factors; here we choose the nearest practical value rounding from 2.02 h. Final Answer: 2 hours

Question 2

Design discharge composition: If the ratio of storm drainage to sanitary sewage is 20:1, what percentage of the total design discharge flows during non-monsoon (sewage only) periods?

Accepted Answer

Introduction / Context: Combined or parallel drainage systems often consider design discharges that include large stormwater components seasonally. During dry weather (non-monsoon), only the sanitary sewage portion flows. This question evaluates proportional reasoning applied to design ratios. Given Data / Assumptions: Ratio of storm drainage to sewage = 20 : 1. Total design discharge during monsoon = storm + sewage = 20 + 1 = 21 parts. Non-monsoon discharge consists only of the sewage component. Concept / Approach: Compute the fraction of the total design discharge represented by sewage alone, then convert to percentage. Step-by-Step Solution: 1) Total design parts = 20 (storm) + 1 (sewage) = 21.2) Sewage-only fraction in dry weather = 1 / 21.3) Convert to percentage: (1 / 21) * 100 ≈ 4.76% ≈ 5%. Verification / Alternative check: Check arithmetic: 21 * 5% = 1.05 parts, very close to the 1 part sewage component; rounding explains the small difference. Why Other Options Are Wrong: 10%, 15%, 20%, 25%: These overestimate the sewage-only share relative to the given 20:1 design ratio. Common Pitfalls: Confusing the ratio 20:1 with 1:20 or misallocating which part is sewage. Forgetting to convert fraction to a percentage. Final Answer: 5%

Question 3

Quality of septic-tank effluent: Which statement best describes the characteristics of effluent leaving a septic tank?

Accepted Answer

Introduction / Context: Septic tanks provide primary treatment via sedimentation and anaerobic digestion. They remove settleable solids and some floatables but do not provide complete biochemical stabilization. Understanding the effluent quality is crucial for safe disposal in soak pits, leach fields, or subsequent treatment units. Given Data / Assumptions: Septic tank effluent is being discharged after primary treatment only. No additional secondary or tertiary treatment is assumed. Concept / Approach: Primary treatment reduces suspended solids but leaves significant dissolved organics (BOD, COD) and fine suspended particles. Therefore, effluent remains “foul” and must be infiltrated into soil or treated further to protect public health and receiving waters. Step-by-Step Solution: 1) Identify treatment level: septic tank = primary treatment.2) Recognize residual pollutants: dissolved organics, pathogens, fine suspended solids.3) Conclude that effluent requires subsurface disposal (soil absorption) or additional treatment.4) Select the option describing this state: foul and still containing dissolved and suspended solids. Verification / Alternative check: Regulatory frameworks typically prohibit direct surface discharge of septic tank effluent without further treatment, confirming the characterization of the effluent as inadequate for direct discharge. Why Other Options Are Wrong: Option A: Direct discharge into open drains is unsafe and usually illegal.Option C: Equating septic effluent with complete treatment is incorrect.Options D and E: Effluent is neither sterile nor free of organics; pathogens and BOD remain significant. Common Pitfalls: Overestimating the performance of septic tanks. Ignoring pathogen risks in partially treated effluent. Final Answer: Foul effluent containing dissolved and suspended solids, requiring soil absorption or further treatment

Question 4

Effect of sewer size on flow velocity and deposition: Which statement is correct for gravity sewers carrying the same discharge?

Accepted Answer

Introduction / Context: Gravity sewer hydraulics often involve partially full flow. For a fixed discharge and slope, cross-sectional size influences flow area, hydraulic radius, and consequently velocity. Understanding this relationship helps avoid deposition problems. Given Data / Assumptions: Compare sewers under the same slope and the same discharge. Manning equation applies: V ∝ (R^(2/3) * S^(1/2)) / n, and Q = A * V. Concept / Approach: For a given Q, increasing size generally increases area A. If slope and roughness are unchanged, the needed velocity V to carry Q reduces because Q = A * V. Lower V increases the risk of deposition unless the sewer runs at a depth that optimizes hydraulic radius and maintains self-cleansing conditions. Step-by-Step Solution: 1) Hold Q and S constant; increase diameter.2) Since A increases with size, V = Q / A decreases.3) Lower V makes it harder to keep solids in suspension, promoting deposition if below self-cleansing velocity.4) Therefore, a larger sewer for the same flow tends to have lower velocity and greater deposition risk. Verification / Alternative check: Design guidelines always check for minimum self-cleansing velocities; oversizing pipes is discouraged precisely because of the velocity drop at low flows. Why Other Options Are Wrong: Option A: Velocity does not increase merely due to larger size under the same Q.Option B: Smaller size generally increases velocity for the same Q (within capacity limits).Option C: Larger size does not guarantee freedom from deposition; it often worsens it at low flow.Option E: Velocity depends on both area and hydraulic radius; it is not independent of size. Common Pitfalls: Equating larger size with better performance without checking velocity. Ignoring partial-flow hydraulics and self-cleansing criteria. Final Answer: For a larger sewer carrying the same flow and slope, velocity tends to be lower, encouraging deposition.

Question 5

Detection of free (ammoniacal) ammonia in sewage: Which reagent/method is used to detect free ammonia in wastewater?

Accepted Answer

Introduction / Context: Rapid qualitative and quantitative tests for nitrogen species (ammonia, nitrite, nitrate) are fundamental in wastewater analysis. Free (ammoniacal) ammonia is commonly detected colorimetrically using specific reagents. Given Data / Assumptions: We are testing for free ammonia (NH3/NH4+), not nitrates or nitrites. Standard colorimetric methods are to be considered. Concept / Approach: Nesslerization uses Nessler's reagent (alkaline potassium mercuric iodide). In the presence of ammonium ions, a yellow to brown coloration (or precipitate at high concentrations) develops, enabling detection and measurement of ammonia in water and wastewater. Step-by-Step Solution: 1) Identify the target analyte: free ammonia.2) Recall the classical method: Nessler's reagent test.3) Recognize the expected response: yellow to brown color proportional to ammonia concentration.4) Select the option that matches this method. Verification / Alternative check: Analytical chemistry references list Nesslerization as a standard approach for free ammonia detection; alternative methods (e.g., ion-selective electrodes, phenate method) also exist but are not among the options provided. Why Other Options Are Wrong: Phenol-disulphonic acid: Primarily for nitrate, not free ammonia.Boiling: Not a reliable specific test for ammonia in complex matrices like sewage.Potassium permanganate: An oxidant used in various demand tests; not a specific ammonia indicator.Ammonium molybdate: Used mainly for phosphate (phosphomolybdate complex), not ammonia. Common Pitfalls: Confusing tests for nitrates, nitrites, and ammonia. Assuming any color change with a strong oxidant indicates ammonia. Final Answer: Nessler's reagent (alkaline potassium mercuric iodide) producing a yellow to brown coloration

Question 6

In wastewater engineering, what are “clarigesters”? Choose the option that correctly describes the construction and working form of a clarigester unit used for combined sedimentation and digestion.

Accepted Answer

Introduction / Context: Clarigesters are process units used in municipal sewage treatment that combine primary settling (clarification) with sludge digestion in one structure. They are often deployed when land is limited or when a compact plant layout is desired. Understanding their configuration helps operators and designers distinguish them from septic tanks, primary clarifiers, and separate sludge digesters. Given Data / Assumptions: Treats raw or settled sewage in a primary stage. Combines clarification and anaerobic sludge digestion. Geometry is typically circular with internal hoppers for sludge collection. Concept / Approach: A clarigester is essentially a circular, two-storey Imhoff-type arrangement. Settling occurs in the upper zone; heavier solids pass through slots into a lower, sealed digestion compartment. Hopper-shaped bottoms facilitate gravity movement and compaction of sludge, while gases are vented without disturbing the settling flow above. This configuration differentiates a clarigester from a simple septic tank (single compartment, rectangular or circular) or from separate clarifier–digester systems. Step-by-Step Solution: Identify if the unit is two-storey (clarification above, digestion below).Check for circular plan with hopper bottoms for sludge thickening.Confirm internal slots or openings that allow settled solids to drop into the digestion zone without short-circuiting flow.Match this description to the correct option. Verification / Alternative check: Textbook descriptions of clarigesters emphasize their circular form and the presence of bottom hoppers to store and digest sludge. Plant schematics show scrapers directing solids to hoppers and gas vents protecting the settling zone from turbulence. Why Other Options Are Wrong: (a) Lacking hoppers limits efficient sludge concentration. (b) and (d) septic tanks are not two-storey Imhoff arrangements. (e) is unnecessary because a correct description exists. Common Pitfalls: Confusing clarigesters with standard primary clarifiers (no digestion zone) or with stand-alone Imhoff tanks of different geometry; overlooking the need for gas vents to prevent carryover. Final Answer: circular Imhoff double tanks with bottom hoppers

Question 7

Preliminary treatment of sewage — which operations occur at this stage? Identify the correct grouping of unit operations typically provided before primary treatment.

Accepted Answer

Introduction / Context: Preliminary treatment protects downstream units by removing materials that would cause wear, clogging, or operational instability. It typically includes screening, grit removal, and oil/grease separation before sedimentation or biological steps. Recognizing these components is essential for design and troubleshooting of wastewater facilities. Given Data / Assumptions: Sewage contains rags, plastics, sand/grit, floatables (oils, grease), and organic matter. Objective: protect pumps, clarifiers, and biological reactors. Conventional municipal plant layout. Concept / Approach: Each listed operation targets a specific contaminant class: screening for coarse floatables, grit chambers for mineral particles with high settling velocities, and skimming for oils/grease that float. Grouping these together describes the preliminary train accurately. Step-by-Step Solution: Screening: bar racks or fine screens remove large floatables.Grit removal: horizontal, aerated, or vortex grit chambers remove sand and silt.Skimming: scum baffles or dedicated skimming tanks separate oil and grease.Therefore, all listed operations are part of preliminary treatment. Verification / Alternative check: Standard process flow diagrams in design manuals show screens → grit chambers → skimming prior to primary sedimentation. Why Other Options Are Wrong: (a), (b), and (c) each describe only part of preliminary treatment; the comprehensive correct choice is (d). Common Pitfalls: Confusing preliminary with primary treatment (sedimentation), or placing oil skimming only in primary clarifiers; overlooking maintenance needs of screens and grit systems. Final Answer: All the above.

Question 8

When is a drop manhole provided in a sewerage system? Select the condition that necessitates constructing a drop connection in a manhole.

Accepted Answer

Introduction / Context: Manholes provide access, ventilation, and flow control in sewer networks. A special form, the drop manhole, safely conveys a branch flow entering at a higher elevation down to the main invert, preventing destructive free falls and turbulence inside the chamber. Given Data / Assumptions: Branch sewer invert is substantially above the main sewer invert. Need to avoid erosion, odor release, and splashing in the manhole shaft. Urban gravity sewer with manholes at junctions. Concept / Approach: A vertical or near-vertical drop pipe is fitted to carry incoming flow down to the benching level, discharging smoothly into the main channel. This configuration is used when grade differences exceed the acceptable limit for direct junctions (commonly a drop of around 0.6 m or more, depending on standards). Step-by-Step Solution: Identify elevation difference at junction: branch invert − main invert.If difference exceeds allowable, specify a drop connection (internal or external pipe).Provide energy dissipation at outlet and adequate ventilation.Therefore, the correct triggering condition is a higher-level branch joining a lower main. Verification / Alternative check: Design guides illustrate internal/external drop manholes to manage large invert differences and to mitigate turbulence and corrosion. Why Other Options Are Wrong: (a) is vague; continuous sewer grade changes are handled by slope design, not a drop manhole. (c) relates to maintenance lighting, not design geometry. (d) is invalid; (e) pertains to outfall hydraulics, not manhole junction geometry. Common Pitfalls: Using free fall within the manhole; neglecting corrosion risks from turbulence and H2S; omitting access for maintenance of the drop pipe. Final Answer: a branch sewer joins the main sewer at a considerably higher level

Question 9

Nitrogen cycle transformations in sewage treatment Arrange the sequence of nitrogen transformations occurring from organic nitrogen to gaseous nitrogen under typical biological processes.

Accepted Answer

Introduction / Context: In wastewater treatment, nitrogen is transformed through a well-known sequence: ammonification, nitrification, and denitrification. Understanding the order is essential to design and operate biological nutrient removal systems and to interpret process monitoring data (e.g., ammonia, nitrite, nitrate concentrations). Given Data / Assumptions: Organic nitrogen in sewage is converted by microbes. Oxic and anoxic environments are available in treatment stages. Final end product of denitrification is predominantly nitrogen gas. Concept / Approach: First, organics undergo ammonification releasing ammonia (NH3/NH4+). Next, nitrifying bacteria oxidize ammonia to nitrite (NO2−) and then to nitrate (NO3−). Finally, under anoxic conditions, denitrifiers reduce nitrate to nitrogen gas (N2), which is released to the atmosphere. This is the textbook nitrogen cycle within treatment systems. Step-by-Step Solution: Ammonification: organic-N → NH4+ (liberation of ammonia).Nitrification step 1: NH4+ → NO2− (formation of nitrites).Nitrification step 2: NO2− → NO3− (formation of nitrates).Denitrification: NO3− → N2 (liberation of nitrogen gas). Verification / Alternative check: Process monitoring in BNR plants shows decreasing ammonia with corresponding increases in nitrite/nitrate in aeration, followed by nitrate decrease and nitrogen gas release in anoxic tanks. Why Other Options Are Wrong: (b) and (c) invert the sequence. (d) is nonsensical, repeating nitrates without proper steps. (e) is unnecessary since a correct option exists. Common Pitfalls: Confusing nitrification (strictly aerobic) with denitrification (anoxic), or assuming nitrite is always detectable—it is often quickly oxidized to nitrate in well-aerated systems. Final Answer: Liberation of ammonia → formation of nitrites → formation of nitrates → liberation of nitrogen

Question 10

Recommended detention period for grit (sand) chambers What is the typical design detention time used in conventional (non-aerated) grit chambers handling municipal sewage?

Accepted Answer

Introduction / Context: Grit chambers remove heavy inorganic particles such as sand, gravel, and eggshells to protect pumps and prevent abrasion. The detention time is chosen to settle grit while keeping most organics in suspension, avoiding odor and putrefaction. Selecting a practical value is central to reliable operation. Given Data / Assumptions: Municipal wastewater with typical grit size and specific gravity around 2.65. Horizontal-flow (non-aerated) grit chamber layout. Peak flow conditions inform sizing; uniform approach velocity targeted. Concept / Approach: Design practice commonly adopts a detention time of about 30–60 seconds at peak flow for horizontal units; however, many standard problems and code-based exercises round to roughly 2 minutes to ensure adequate removal under varying conditions and to account for fluctuations. Aerated grit chambers may use 2–5 minutes with controlled spiral flow, but basic horizontal chambers remain near 1–2 minutes. Step-by-Step Solution: Choose detention time so grit settles but organics do not: typical 1–2 minutes.Check corresponding chamber dimensions to maintain approach velocity (about 0.3 m/s) to prevent deposition of organics.Adopt 2 minutes as a robust standard value in many design problems and exams. Verification / Alternative check: Design manuals present detention times close to 1–2 minutes for conventional units and longer for aerated types; operator experience supports these values for reliable grit capture. Why Other Options Are Wrong: (a) can be marginal at low temperatures or variable grit loads; (c) and (d) are long for conventional chambers and risk organic settling; (e) is shorter than usual textbook practice. Common Pitfalls: Using too long detention, causing septic conditions; ignoring scouring at high velocities; forgetting provisions for grit washing and dewatering. Final Answer: 2 minutes

Question 11

Air pollution from vehicles — identify the most dangerous pollutant among the choices Which of the following is considered the most acutely dangerous pollutant in typical vehicular emissions?

Accepted Answer

Introduction / Context: Vehicle exhaust contains a mixture of gases and particulates. While carbon dioxide (CO2) drives climate change, acute toxicity to humans from short-term exposure is dominated by carbon monoxide (CO), especially in enclosed or poorly ventilated spaces (tunnels, garages). Selecting the most dangerous pollutant here focuses on immediate health risk in typical traffic environments. Given Data / Assumptions: Conventional petrol engines without perfect after-treatment. Acute toxicity perspective rather than long-term climate impact. Urban roadside exposure scenarios. Concept / Approach: CO binds to hemoglobin with much greater affinity than oxygen, forming carboxyhemoglobin and reducing oxygen delivery to tissues, which can lead to headaches, dizziness, and at high levels, death. SO2 is present mainly from sulfur in fuel but is typically lower for vehicular gasoline; ozone (O3) is a secondary pollutant formed photochemically, not directly emitted. CO2 is non-toxic at ambient levels but is a greenhouse gas. Step-by-Step Solution: Compare acute toxicity: CO >> SO2 (vehicular), CO2 (climate), O3 (secondary).Identify pollutant directly emitted in high proportion by spark-ignition engines: CO.Select CO as the most dangerous for acute health effects among options. Verification / Alternative check: CO poisoning incidents in enclosed traffic spaces are well documented; occupational and ambient air quality standards set stringent limits on CO exposure. Why Other Options Are Wrong: (b) SO2 is more characteristic of high-sulfur fuels and stationary sources. (c) CO2 is not acutely toxic at environmental concentrations. (d) O3 forms downwind via NOx and VOCs; acute exposure is harmful but it is not directly emitted by vehicles. Common Pitfalls: Confusing chronic climate impact (CO2) with acute toxicity (CO); assuming all pollutants are directly emitted by vehicles. Final Answer: CO

Question 12

What governs the longitudinal gradient (slope) selected for sanitary sewers? Pick the best statement describing the factors that primarily determine sewer grade.

Accepted Answer

Introduction / Context: The grade (slope) of a gravity sewer must be chosen to achieve self-cleansing velocities while preventing excessive velocities that could cause scouring and structural issues. Because velocity is a function of hydraulic radius and slope for a given discharge (e.g., via Manning’s equation), several variables co-determine the selected slope. Given Data / Assumptions: Gravity flow in partially full circular conduits. Use of empirical relations like Manning’s V = (1/n) R^(2/3) S^(1/2). Target self-cleansing velocity, commonly around 0.6–0.75 m/s for sanitary sewers (varies by standard). Concept / Approach: For a specified design discharge, the chosen diameter and slope together set the hydraulic radius and thus velocity. Conversely, a required self-cleansing velocity dictates a minimum slope for a chosen diameter and expected flow. Therefore, all three—discharge, diameter, and target velocity—are linked and jointly determine the gradient. Step-by-Step Solution: Start from Manning: V = (1/n) R^(2/3) S^(1/2).Given Q, choose D; compute A and R for partial flow.Solve for S that yields V ≥ V_min (self-cleansing), respecting V ≤ V_max (no scouring).Hence slope depends on velocity target, diameter, and discharge. Verification / Alternative check: Design tables present recommended minimum slopes as a function of pipe size and anticipated flow to achieve self-cleansing—confirming multi-dependence. Why Other Options Are Wrong: (a), (b), and (c) alone do not fully specify the slope; the integrated view in (d) is correct. Common Pitfalls: Choosing minimal slopes without checking low-flow velocities; ignoring roughness n changes over time due to slime or sediment deposition. Final Answer: all the above.

Question 13

Which statement about septic tanks is incorrect? Evaluate the following statements about septic tanks and choose the incorrect one (if any).

Accepted Answer

Introduction / Context: Septic tanks provide on-site primary treatment and anaerobic digestion for domestic wastewater in areas lacking centralized sewers. Understanding their essential features ensures proper design, siting, and operation, and helps distinguish them from other units like Imhoff tanks or aerated package plants. Given Data / Assumptions: Domestic sewage strength within typical ranges. Single or two-compartment septic tank with baffles/tees to promote quiescence. Ventilation via vent pipes and access through manholes; tank covered for safety and odor control. Concept / Approach: Each statement is compared against standard septic tank characteristics. Septic tanks are indeed horizontal, continuous-flow settling tanks with anaerobic digestion. Detention periods commonly fall around 12–36 hours (varies with codes and loading). Tanks are covered and vented to manage gases like methane and hydrogen sulfide safely. Therefore, none of the statements is incorrect. Step-by-Step Solution: Check (a): correct—continuous flow with quiescent conditions.Check (b): correct—detention of roughly 0.5–1.5 days is typical in many codes.Check (c): correct—tanks are covered; venting prevents gas accumulation.Check (d): correct—process relies on anaerobic decomposition of settled solids. Verification / Alternative check: Design manuals for on-site wastewater systems reflect these features; installation standards require covers and venting; performance depends on sufficient detention. Why Other Options Are Wrong: Since (a)–(d) are correct, the only valid choice for an incorrect statement is (e) “None of these,” meaning none is incorrect. Common Pitfalls: Confusing septic tank detention with primary clarifiers in centralized plants; overlooking the need for regular sludge pumping; assuming aerobic processes dominate inside the tank. Final Answer: None of these

Question 14

Building services (sanitary work): For installing an Indian-type W.C. (squatting pan) on a reinforced concrete (R.C.C.) floor slab, what should be done to the slab level in the toilet portion to accommodate the pan, trap, and proper slope to the gul…

Accepted Answer

Introduction / Context: An Indian-type W.C. (squatting pan) requires room below the finished floor to place the pan, water-seal trap, and to provide a gentle gradient for the outlet pipe. Civil engineers commonly “sink” the slab in the toilet area for proper fitting and finished levels. Given Data / Assumptions: Conventional Indian-type squatting pan with P- or S-trap. Nominal floor finishes and bedding mortar thickness. Need for adequate fall to the gully or branch sewer without raising the floor elsewhere. Concept / Approach: The sunk slab provides vertical space for the fixture and trap so the final toilet floor can be flush with adjacent rooms while still maintaining pipe gradients. A typical practice is a 20 cm sunk depth, which suits common pan depths and trap geometry without excessive excavation or step-downs. Step-by-Step Solution: Plan fixture layout and trap position relative to outlet centerline.Provide sunk slab depth ≈ 20 cm over the toilet area.Lay pan and trap, ensuring water seal and alignment.Fill and screed to achieve final floor level and required slope toward the floor drain. Verification / Alternative check: Check the finished floor level continuity with adjacent spaces and confirm the outlet pipe has the minimum gradient recommended (for example, 1 in 40 to 1 in 60, as per local practice and pipe size). Why Other Options Are Wrong: (b) Raising the slab complicates access and creates unwanted steps; (c) 50 cm is excessive and wasteful; (d) not sinking generally forces unsatisfactory pipe fall or raised platforms. Common Pitfalls: Insufficient sinking causing trap clashes; ignoring waterproofing, leading to leakage; not coordinating with tiling thickness and thresholds. Final Answer: Should be sunk by 20 cm

Question 15

Urban drainage hydrology: definitions related to peak runoff and time of concentration (Tc) when using the Rational Method for small catchments

Accepted Answer

Introduction / Context: For small urban catchments the Rational Method is widely adopted to estimate peak runoff for drainage design. Knowing the correct terminology around peak discharge and time of concentration prevents misuse and improves sizing of sewers, roadside drains, and culverts. Given Data / Assumptions: Small catchment (typically < 50 ha) with relatively uniform rainfall intensity during storm duration. Rational formula: Q = C * i * A with consistent units. Design intensity i is selected for a duration equal to Tc and chosen return period. Concept / Approach: Peak discharge occurs when the entire catchment contributes simultaneously—by definition, when storm duration equals or exceeds Tc. Hence Tc is both the travel time from the remotest point and the time after which full contributing area is effective at the outlet, making statements (c) and (d) equivalent descriptors of the same physical concept. Step-by-Step Solution: Define peak runoff: the maximum flow rate at the outlet during the storm.Identify Tc: travel time from remotest point to outlet.Set rainfall intensity i for duration = Tc to compute Q_peak via the Rational Method.Ensure catchment area A (in ha or km^2) and runoff coefficient C are appropriate to land use. Verification / Alternative check: Hydrograph methods (e.g., unit hydrograph) can be used to check peak runoff estimates for complex basins; for small, simple areas, the Rational Method is generally adequate. Why Other Options Are Wrong: Each individual statement (a–d) is correct; therefore the complete, correct choice is “All the above”. Common Pitfalls: Using i for durations much shorter than Tc (overestimation) or much longer (underestimation); applying the method to large, non-uniform basins. Final Answer: All the above.

Question 16

Operation & maintenance of sewerage: which method is typically used to clean small-diameter sewers and remove localized blockages?

Accepted Answer

Introduction / Context: Keeping small sewers free of debris is essential for reliable drainage. Different cleaning methods are chosen based on pipe size, access, blockage type, and available equipment. Given Data / Assumptions: Small-diameter branch or lateral sewers in urban areas. Access via cleanouts or manholes. Localized silt, rags, or grease causing partial blockage. Concept / Approach: Cane rodding uses flexible rods coupled together with end tools (corkscrews, cutters) to manually break, hook, or push blockages through small sewers. It is quick, targeted, and requires minimal water—well suited for small lines. Flushing is useful for larger sewers or after loosening deposits; wooden pigs are more common in larger conduits or pressure mains, not small gravity laterals. Step-by-Step Solution: Open access point and assess blockage location.Assemble cane rods with suitable tool head.Work the rods through the pipe to engage and clear the obstruction.Flush to remove loosened debris and confirm free flow. Verification / Alternative check: Post-cleaning CCTV inspection (where available) verifies clearance and pipe condition. Why Other Options Are Wrong: (a) Flushing alone may not dislodge stubborn blockages; (c) wooden pills are uncommon in small lines; (d) incorrect because a standard method exists. Common Pitfalls: Forcing tools that damage joints; ignoring safety and confined-space procedures; not addressing sources of repeated blockages (grease control). Final Answer: Cane rodding using flexible rods and tools

Question 17

Circular sewer running partially full: with diameter D and central angle α (in radians) subtended by the water surface at the sewer centre, which geometric relations are correct for hydraulic computations?

Accepted Answer

Introduction / Context: Hydraulic design of circular sewers operating partially full requires correct geometric relations for area, wetted perimeter, surface width, and hydraulic radius. These feed Manning’s or Chezy’s equations to estimate normal depth, velocity, and capacity. Given Data / Assumptions: Uniform prismatic circular conduit of diameter D. Central (subtended) angle of the water segment at the pipe centre is α radians. Steady uniform flow, gravity-driven, partially full (0 < α < 2π). Concept / Approach: The circular-segment geometry provides closed-form expressions: the wetted perimeter equals the arc length; the area equals area of sector minus area of triangle; top width equals the chord length. The hydraulic radius follows directly as R = A / P, fundamental in Manning’s equation V = (1/n) * R^(2/3) * S^(1/2). Step-by-Step Solution: Wetted perimeter: P = radius * α * 2, with radius = D/2 ⇒ P = (D * α)/2.Area: sector area = (α/2π) * area of full circle = (α/2π) * (πD^2/4) = (αD^2/8); subtract triangle area = (D^2/8) * sin α ⇒ A = (D^2/8) * (α − sin α).Top width: chord length = D * sin(α/2).Hydraulic radius: R = A / P (definition). Verification / Alternative check: At α = π (half full): P = (Dπ)/2, A = (D^2/8) * (π − 0) = (πD^2/8) = half of full area (πD^2/8), and T = D, which match known half-full relations. Why Other Options Are Wrong: In this set, (a)–(d) are all correct; choosing any single one is incomplete, so “All the above” is the correct comprehensive selection. Common Pitfalls: Mixing degrees and radians for α; using hydraulic diameter 4A/P (common in pipe flow) instead of hydraulic radius A/P in open-channel form of Manning’s equation. Final Answer: All the above.

Question 18

Self-purification of rivers receiving wastewater discharges: which factors materially contribute to the natural purification capacity?

Accepted Answer

Introduction / Context: Rivers possess a finite capacity to assimilate organic loads through dilution, decay, and re-aeration. Understanding the drivers of self-purification helps in setting discharge standards and locating outfalls to protect aquatic life and downstream users. Given Data / Assumptions: Organic wastewater discharge into a flowing river. Oxygen-demanding reactions (BOD) consume dissolved oxygen (DO). Re-aeration from atmosphere and tributary inflows replenishes DO. Concept / Approach: Hydrology governs dilution and travel time; DO level sets the immediate oxygen reserve; temperature controls biochemical kinetics and oxygen solubility; and turbulence elevates gas transfer, enhancing DO recovery. All four together determine the sag curve (DO deficit vs distance downstream) and ultimate recovery point. Step-by-Step Solution: Assess hydrology: flow rate, velocity, depth, channel geometry.Quantify oxygen demand (BOD) and existing DO.Account for temperature effects on reaction rate constants and DO saturation.Evaluate turbulence/re-aeration coefficient (k_r) influenced by velocity and depth. Verification / Alternative check: Apply a DO sag model (e.g., Streeter–Phelps) to simulate deficit and check compliance with water-quality objectives. Why Other Options Are Wrong: Each individual factor is indeed a contributor; thus the combined answer “All the above” is correct. Common Pitfalls: Ignoring temperature seasonality; overestimating re-aeration in deep, slow reaches; not considering diurnal photosynthesis/respiration effects of algae. Final Answer: All the above.

Question 19

Design check for circular sewers at low flow depth: if a circular sewer flows at only 0.2 of its full depth, how is the nominal gradient commonly adjusted to maintain self-cleansing velocity?

Accepted Answer

Introduction / Context: Sewers should achieve self-cleansing velocity to prevent deposition. At shallow depths, the hydraulic radius and velocity reduce; designers compensate by increasing the gradient (slope) relative to that required at half-full or full-flow reference conditions. Given Data / Assumptions: Circular gravity sewer designed by Manning’s equation. Operating depth is only 0.2 of full diameter. Target is to maintain adequate velocity for self-cleansing. Concept / Approach: Velocity V ∝ R^(2/3) * S^(1/2) for a given roughness n. At y/D = 0.2, both hydraulic radius and area reduce; therefore, to recover target velocity, S must be increased compared to the “nominal” slope used at more favorable depths (e.g., 0.5D). Empirical practice tables suggest multiplying slope, and a common rule-of-thumb is to treble the nominal gradient for y/D ≈ 0.2. Step-by-Step Solution: Establish base slope S0 meeting velocity at reference depth (often 0.5D).At y/D = 0.2, compute reduced R and V via partial-flow factors.Increase slope S so that V ≥ self-cleansing velocity (e.g., 0.6–0.9 m/s depending on standards).Rule-of-thumb adjustment: S ≈ 3 * S0 for y/D ≈ 0.2. Verification / Alternative check: Use partial-flow charts/tables (ratio V/V_full vs y/D) to confirm that tripling the slope yields required V for the pipe size and roughness. Why Other Options Are Wrong: (a) Keeping slope unchanged would likely underperform; (b) doubling may still be insufficient; (d) overly pessimistic—proper design can achieve self-cleansing. Common Pitfalls: Ignoring minimum velocity criteria; using nominal slope without low-flow checks; not accounting for silt-laden inflows. Final Answer: Nominal gradient is trebled.

Question 20

Wastewater laboratory practice: an Imhoff cone is used to measure which parameter during primary settling assessment?

Accepted Answer

Introduction / Context: The Imhoff cone is a simple volumetric device used in wastewater treatment plants to estimate the volume of solids that will settle out under quiescent conditions. It supports process control of primary sedimentation and grit removal operations. Given Data / Assumptions: Standard cone volume graduations (typically up to 1000 mL). Settling time commonly 1 hour for settleable solids. Mixed liquor or raw sewage sample. Concept / Approach: Settleable solids are those that settle by gravity in a specified period. Measuring their volume provides a fast field indicator of solids loading and clarifier performance without lab filtration or drying steps required for total or fixed solids tests. Step-by-Step Solution: Fill the Imhoff cone with a well-mixed wastewater sample to a known volume.Allow to settle undisturbed for 1 hour.Read the settled sludge volume (mL/L) from the cone graduations.Record and trend the data to adjust primary clarifier operation. Verification / Alternative check: Compare settleable solids with suspended solids measurements to evaluate the fraction removable by gravity vs requiring biological treatment. Why Other Options Are Wrong: (a), (b), and (c) require standard gravimetric procedures (filtration/ignition), not the Imhoff cone. Common Pitfalls: Agitating during settling; reading at incorrect time; failing to note floating scum which may bias interpretation. Final Answer: Settleable solids (volume after 1 hour)

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