Question 1

Skimming tanks in wastewater treatment are primarily provided for what purpose?

Accepted Answer

Introduction / Context: Grease and oil can foul downstream processes and cause scum accumulation. Skimming tanks (or grease traps) are used in preliminary treatment to remove floating matter. Given Data / Assumptions: Municipal sewage with fats, oils, and grease (FOG). Tank equipped with baffles and skimming mechanisms. Concept / Approach: Skimming relies on gravity separation: lower-density oils/grease rise to the surface and are collected by scrapers or weirs, preventing carryover. Step-by-Step Solution: Step 1: Recognize FOG removal as the skimming tank function.Step 2: Distinguish from grit chambers (remove heavy inorganic grit) and screens (remove coarse debris).Step 3: Select the option describing grease/oil removal. Verification / Alternative check: Plant schematics show skimming tanks downstream of screening/grit removal or integrated with primary clarifiers as scum removal systems. Why Other Options Are Wrong: Sludge removal is a function of sludge hoppers/valves, not skimming. Screens handle coarse solids, not oils. Grit chambers remove sand/gravel; skimmers handle floatables. Common Pitfalls: Confusing grit removal with skimming; neglecting grease control leads to scum blankets and odor issues. Final Answer: Removal of grease and oil from sewage

Question 2

The main biological process responsible for sewage treatment inside septic tanks is predominantly which of the following?

Accepted Answer

Introduction / Context: Septic tanks are onsite primary treatment units where wastewater is detained to allow solids to settle and undergo biological stabilization. Given Data / Assumptions: Enclosed tank with no aeration system. Typical detention times of 24–48 hours or more. Sludge undergoes slow stabilization. Concept / Approach: Without aeration, the dominant process is anaerobic digestion: hydrolysis, acidogenesis, and methanogenesis convert organics to biogas (methane and carbon dioxide) with volume reduction of sludge. Step-by-Step Solution: Identify septic environment: low oxygen, quiescent conditions.Conclude anaerobic pathways dominate.Therefore, select anaerobic decomposition. Verification / Alternative check: Design manuals describe septic tanks as anaerobic primary treatment units; aerobic mechanisms require aeration which is absent. Why Other Options Are Wrong: Aerobic decomposition: Not supported in a sealed, unaerated tank. Parasitic decomposition: Not a primary engineered mechanism. Chemical oxidation/Photolysis: Minor or negligible in septic conditions. Common Pitfalls: Assuming septic tanks provide complete treatment; effluent typically needs soil absorption or further treatment. Final Answer: Anaerobic decomposition (anaerobic digestion)

Question 3

Hydraulically equivalent full-flow sewers: A circular pipe (diameter D) and a square conduit (side b) discharge the same on the same grade and roughness. Which relation holds good?

Accepted Answer

Introduction / Context: “Hydraulically equivalent” full-flow conduits discharge the same flow for the same slope and roughness. For gravity sewers, Manning/Chézy formulations show discharge depends on cross-sectional area and hydraulic radius. Equating these appropriately yields a relationship between characteristic dimensions of different shapes. Given Data / Assumptions: Both conduits run full, same slope S and roughness n. Discharge relation: Q ∝ A * R^(2/3) for Manning's form. Circle: A_c = π D^2 / 4, P_c = π D, R_c = A_c / P_c = D/4. Square: A_s = b^2, P_s = 4 b, R_s = b/4. Concept / Approach: Equate capacity terms A R^(2/3) for the two shapes. Because R = A/P, the capacity term becomes A * (A/P)^(2/3) = A^(5/3) / P^(2/3), which leads to a simple power relation between D and b. Step-by-Step Solution: For circle: capacity term = (π D^2 / 4)^(5/3) / (π D)^(2/3) = [π D^(8/3)] / [4^(5/3)].For square: capacity term = (b^2)^(5/3) / (4b)^(2/3) = b^(8/3) / 4^(2/3).Equate and simplify: π D^(8/3) / 4^(5/3) = b^(8/3) / 4^(2/3) ⇒ π D^(8/3) = 4 b^(8/3). Verification / Alternative check: Dimensional consistency and monotonic relation: larger D corresponds to larger b, as expected for equal capacity. Why Other Options Are Wrong: Other exponents do not arise from the A^(5/3)/P^(2/3) equivalence; they would imply incorrect dependence on shape parameters. Common Pitfalls: Using R instead of A^(5/3)/P^(2/3) directly; mixing running full with part-full relations. Final Answer: π D^(8/3) = 4 b^(8/3)

Question 4

Before discharging foul sewage to a natural water body, which stages of treatment are generally applied in municipal practice?

Accepted Answer

Introduction / Context: Surface water quality protection requires that sewage be treated to remove floatables, settleable solids, organic load (BOD), pathogens, and stabilized sludge prior to discharge. Treatment trains combine several unit operations and processes sequentially. Given Data / Assumptions: Municipal wastewater with typical solids and BOD. Compliance with discharge standards requiring multiple barriers. Conventional plant layout. Concept / Approach: Preliminary treatment (screening, grit removal) protects downstream units. Primary treatment (sedimentation) removes settleable solids and some BOD. Secondary treatment (oxidation via biological processes) reduces soluble and colloidal organics. Sludge digestion stabilizes solids for safe disposal, while disinfection reduces pathogens to acceptable levels before discharge or reuse. Step-by-Step Solution: First remove large debris by screening.Then remove settleable solids by sedimentation (primary clarifier).Reduce organic load biologically (oxidation in secondary treatment).Stabilize sludge (digestion) and disinfect effluent before discharge. Verification / Alternative check: National standards typically mandate secondary treatment and, where required, tertiary disinfection—affirming the multi-stage approach. Why Other Options Are Wrong: Options a–d describe single stages and are insufficient alone to meet discharge norms. Common Pitfalls: Assuming primary treatment alone is adequate; overlooking pathogen removal needs. Final Answer: All the above (screening, sedimentation, oxidation, sludge digestion and disinfection)

Question 5

In wastewater oxidation processes (natural or engineered), which compounds are typically formed as the end products of oxidizing organics and reduced species?

Accepted Answer

Introduction / Context: Biochemical and chemical oxidation in wastewater treatment converts reduced compounds into more oxidized, stable forms. Recognizing the typical products helps in interpreting plant performance and effluent characteristics. Given Data / Assumptions: Presence of organic carbon, ammonia, and sulfide species in sewage. Oxidation can be biological (aerobic) or chemical (e.g., with oxidants). Complete oxidation end products considered. Concept / Approach: Carbon in organics is oxidized to carbon dioxide; ammonia is converted to nitrite and then nitrate (nitrification); sulfides/sulfur are oxidized to sulfate. These transformations lower BOD and toxic odors and stabilize the wastewater. Step-by-Step Solution: Carbonaceous oxidation → CO2 and water.Autotrophic nitrifiers oxidize NH3 → NO2- → NO3-.Sulfur species (H2S, S2-) oxidize to SO4^2-. Verification / Alternative check: Stoichiometric equations and process monitoring (CO2 evolution, nitrate increase, reduction in sulfide odors) support these end products. Why Other Options Are Wrong: Each individual option describes a real oxidation product; therefore the aggregate choice (all the above) is correct. Common Pitfalls: Confusing denitrification (which reduces nitrate to nitrogen gas) with oxidation; ignoring partial oxidation intermediates. Final Answer: All the above

Question 6

Household branch sewer (100 mm diameter): What minimum gradient is typically adopted to achieve self-cleansing velocity under standard design practice?

Accepted Answer

Introduction / Context: Self-cleansing velocity is the minimum velocity required to keep solids from depositing in gravity sewers. For small diameter household laterals (100 mm), codes recommend sufficiently steep gradients to maintain adequate shear stress even at low flows. Given Data / Assumptions: Domestic wastewater with typical solids and flush patterns. Standard design references targeting about 0.6–0.75 m/s self-cleansing. Small diameter branch sewer: 100 mm. Concept / Approach: The velocity V depends on slope S via Manning’s relation. For 100 mm lines, a common recommended minimum slope is about 1 in 57 to 1 in 60 to reach self-cleansing velocities at part-full conditions. Step-by-Step Solution: Identify minimum slope range for 100 mm laterals.Select the nearest standard ratio available among the options.Therefore, adopt 1 in 60 as the typical minimum gradient. Verification / Alternative check: Multiple codes and manuals list 1:57–1:60 for 100 mm domestic drains, confirming the selection. Why Other Options Are Wrong: Flatter slopes like 1:100 or 1:120 may not consistently achieve self-cleansing at low flows; “None of these” (option d) is incorrect, and 1:200 (option e) would be too flat. Common Pitfalls: Applying trunk sewer slopes to house drains; ignoring part-full hydraulics and roughness impacts. Final Answer: 1 in 60

Question 7

Relative dissolved oxygen (DO): Compared with fresh river water at the same temperature and pressure, seawater typically contains how much dissolved oxygen?

Accepted Answer

Introduction / Context: Dissolved oxygen (DO) solubility guides aeration, ecology, and outfall design. Salinity reduces oxygen solubility relative to freshwater, which matters when estimating reaeration and oxygen sag in marine discharges. Given Data / Assumptions: Equal temperature and barometric pressure for comparison. Typical ocean salinity (~35 g/L). Focus on equilibrium DO solubility. Concept / Approach: Gas solubility decreases with increasing salinity (the “salting out” effect). Empirical correlations and tables show seawater DO solubility is roughly 15–25% lower than that of freshwater over common environmental temperatures, making “about 20% less” a practical rule of thumb. Step-by-Step Solution: Hold temperature and pressure constant to isolate salinity effect.Consult typical solubility differences; seawater shows substantial reduction.Choose the closest rounded value: about 20% less. Verification / Alternative check: Standard DO tables (fresh vs. sea) confirm the reduction near 20% across common temperatures. Why Other Options Are Wrong: 10% less underestimates the salinity effect; “more DO” contradicts the salting-out principle; 5% less is too small. Common Pitfalls: Comparing DO concentration measurements influenced by biological demand rather than equilibrium solubility; ignoring temperature/pressure dependence. Final Answer: About 20% less DO

Question 8

Which pipe materials and structures offer better resistance to hydrogen sulphide (H₂S) corrosion in sewers carrying septic sewage?

Accepted Answer

Introduction / Context: Hydrogen sulphide in septic sewage oxidizes to sulphuric acid on moist pipe crowns, causing severe crown corrosion. Material choice is crucial for durability. Given Data / Assumptions: Presence of H₂S and associated sulphuric acid generation. Gravity sewers with air space and moist surfaces at the crown. Concept / Approach: Ceramic and vitrified (glazed) materials resist acid attack significantly better than unprotected concrete or steel. Glazed stoneware (vitrified clay) has a dense, acid-resistant surface ideal for H₂S environments. Step-by-Step Solution: Identify materials: concrete and steel are vulnerable without protective linings.Glazed ceramic surfaces exhibit excellent acid resistance.Select glazed stoneware as the best resistance among listed options. Verification / Alternative check: Many legacy sewers use vitrified clay precisely for corrosion resistance; modern systems may use PVC/HDPE or lined concrete for similar reasons. Why Other Options Are Wrong: R.C.C. (unlined): Prone to acid attack; crown corrosion common. Asbestos cement: Better than plain concrete but inferior to vitrified ceramic. Glazed ware (generic ceramic): Overlaps with vitrified clay; the specific standard product is glazed stoneware pipe. Unlined mild steel: Highly susceptible to corrosion. Common Pitfalls: Ignoring need for linings/coatings or ventilation in long flat sewers with septicity. Final Answer: Glazed stoneware pipes

Question 9

Dry-weather flow (DWF) in sewer design: Which of the following best describes dry-weather flow for an urban catchment, excluding any contribution from rainfall or storm runoff?

Accepted Answer

Introduction / Context: Dry-weather flow (DWF) is a foundational term in sanitary sewer design and operation. It represents the sewage generated from households, institutions, and industries under normal conditions without rainfall contribution. Understanding DWF ensures proper sizing of pipes, appurtenances, and treatment units for routine operation and capacity checks. Given Data / Assumptions: DWF excludes stormwater or infiltration due to rainfall. Represents typical sanitary and trade wastewater over a day. Used as a basis for minimum-velocity checks and treatment plant loading. Concept / Approach: In practice, DWF is treated as the average daily sewage flow under non-rainfall conditions. Peak factors (e.g., hourly or diurnal variations) are later applied to this base value to determine peak DWF. Separately, wet-weather flow (WWF) includes DWF plus infiltration/inflow from storms. Step-by-Step Solution: Identify what is excluded: rainfall-derived infiltration/inflow.Identify the period basis for DWF: daily sanitary generation.Therefore, DWF is the average daily rate of flow under dry conditions. Verification / Alternative check: Design manuals define DWF as the average sanitary sewage discharged per day (before applying peaking factors and without storm contributions). This aligns with utility practice worldwide. Why Other Options Are Wrong: Average monthly/annual rates: used for longer-term accounting, not the operational definition of DWF.Per-capita water supply allowance: a planning input, not the definition of actual sewage flow.None of these: incorrect because the daily basis is correct. Common Pitfalls: Confusing DWF with peak DWF (after peaking factor). Including storm infiltration in DWF; that belongs to WWF. Final Answer: Average daily rate of flow

Question 10

Typical total solids in domestic sewage: Approximately how many kilograms of total solids are present in 1000 kg of typical domestic sewage?

Accepted Answer

Introduction / Context: Total solids (TS) in domestic sewage are a key indicator for process design and loading of primary settling and biological treatment units. Knowing the approximate fraction of solids helps estimate sludge production and clarifier performance. Given Data / Assumptions: Typical municipal sewage (mainly domestic) without undue industrial loads. Representative TS concentration around 1000–2000 mg/L (0.1–0.2% by mass). 1000 kg of sewage is roughly 1000 L for estimation because the density is close to water. Concept / Approach: Domestic sewage often contains about 0.1% by weight of solids, sometimes up to 0.2% depending on local conditions. Converting this mass fraction to kilograms per 1000 kg gives an easy estimate of total solids present. Step-by-Step Solution: Assume TS mass fraction ≈ 0.1–0.2%.Compute solids in 1000 kg: 0.001–0.002 of 1000 kg = 1.0–2.0 kg.Hence the best approximate range is 1.0 to 2.0 kg. Verification / Alternative check: Standard texts report TS ≈ 500–1200 mg/L (volatile + fixed) for domestic sewage; many systems see near 1000 mg/L, supporting ~1 kg per 1000 kg, and up to ~2 kg where stronger wastewater occurs. Why Other Options Are Wrong: 0.5–1.0 kg: can be low for typical domestic sewage.5.0–10.0 kg: implies 0.5–1.0% solids, which is far higher than usual domestic sewage.15–20 kg: much too high for typical cases. Common Pitfalls: Confusing total solids with suspended solids only. Using industrial-strength wastewater values for domestic sewers. Final Answer: 1.0 to 2.0 kg

Question 11

Stream pollution zones: If dissolved oxygen (D.O.) in a natural stream falls to essentially zero, the affected reach corresponds to which classical zone of a polluted stream?

Accepted Answer

Introduction / Context: After a wastewater discharge enters a river, the receiving water experiences characteristic changes in dissolved oxygen (D.O.) and quality along the flow path. Recognizing these classical zones helps diagnose pollution severity and the self-purification process. Given Data / Assumptions: D.O. concentration is observed to drop to ~0 mg/L. Typical river self-purification model with zones: degradation, active decomposition (septic), recovery, and clean water. Steady discharge and conservative understanding of oxygen demand. Concept / Approach: In the septic or active decomposition zone, biochemical oxygen demand (BOD) far exceeds oxygen reaeration, driving D.O. to near zero. Putrefaction and anaerobic processes dominate until downstream reaeration and reduced BOD allow D.O. to recover. Step-by-Step Reasoning: Immediately after discharge: degradation begins; D.O. starts falling.In the active decomposition zone: oxygen deficit peaks; D.O. ~ 0 mg/L.Farther downstream: reaeration and oxidation reduce deficit; D.O. increases (recovery zone). Verification / Alternative check: The D.O. sag curve analysis (Streeter–Phelps framework) mirrors these zones, with the minimum D.O. typically inside or near the active decomposition zone depending on kinetics. Why Other Options Are Wrong: Degradation zone: D.O. is declining but not necessarily zero.Recovery/cleaner water zones: D.O. is rising or near saturation, not zero.None: incorrect because a standard zone fits the observation. Common Pitfalls: Equating first drop in D.O. with zero; true zero is characteristic of the septic zone. Ignoring reaeration effects which eventually restore D.O. Final Answer: Zone of active decomposition (septic zone)

Question 12

Volume reduction during sludge dewatering: In a digestion/thickening tank, if the sludge volume is V1 at moisture content p1% and is reduced to moisture content p2% (solids mass constant), what is the resulting volume V2 ?

Accepted Answer

Introduction / Context: Sludge handling often involves thickening or dewatering to reduce volume. When moisture content changes while the mass of dry solids remains the same, we can relate initial and final volumes using simple mass-fraction bookkeeping. Given Data / Assumptions: Initial sludge volume V1 at moisture content p1% (water by mass). Final moisture content p2% after thickening/dewatering. Dry solids mass is constant; density approximated close to water for volume–mass proportionality. Concept / Approach: Let Ms be the dry solids mass. Initially, solids fraction by mass is (100 - p1)%. Finally, solids fraction is (100 - p2)%. With Ms constant, the total mass (and hence volume) scales inversely with the solids fraction, yielding a direct relation between V1 and V2. Step-by-Step Solution: Let Ms = constant.Initial total mass ∝ V1, with solids fraction (100 - p1)/100.Final total mass ∝ V2, with solids fraction (100 - p2)/100.Equal solids mass gives: V2 / V1 = (100 - p1) / (100 - p2).Therefore: V2 = V1 * (100 - p1) / (100 - p2). Verification / Alternative check: Edge check: If p2 < p1 (drier sludge), denominator increases, so V2 < V1, which is physically correct. If p2 → 100%, volume would tend to infinity for constant Ms, which also aligns with the limiting behavior (pure water with vanishing solids fraction is not meaningful for fixed Ms). Why Other Options Are Wrong: (p2/p1) or (p1/p2) assume proportionality to moisture, not solids fraction.Expressions with (100 + p) mix up solids and water fractions.Linear difference (p1 - p2)/100 lacks mass balance basis. Common Pitfalls: Using moisture %, instead of solids % = (100 - moisture %). Forgetting that solids mass, not total mass, remains constant. Final Answer: V2 = V1 * (100 - p1) / (100 - p2)

Question 13

Self-cleansing velocity criterion: When a sewer is designed for self-cleansing velocity, what should happen at the invert (bottom) of the conduit under design flow?

Accepted Answer

Introduction / Context: Self-cleansing velocity is the minimum velocity (or shear stress) in a sewer that prevents deposition of suspended and bed materials while avoiding damage to the pipe. It is a key check for minimum-flow conditions in sanitary and combined systems. Given Data / Assumptions: Design target is to prevent deposition during low flows. Pipe material should not be eroded by routine operation. Velocity or shear stress is set based on sediment characteristics. Concept / Approach: At self-cleansing velocity, the hydraulic forces at the invert are just adequate to keep solids from settling; they are not so high as to scour the pipe wall or protective coatings. Thus, the desired condition is absence of both silting and scouring under design flow. Step-by-Step Reasoning: Set a threshold for bed shear/velocity based on sediment size and specific gravity.Check velocity at minimum flow; ensure it meets or exceeds this threshold.Confirm that the selected velocity is still below erosive limits for the pipe material. Verification / Alternative check: Standards often recommend minimum velocities around 0.6–0.75 m/s for sanitary sewers (varies by code, sediment load, and slope), specifically to maintain self-cleansing without erosion. Why Other Options Are Wrong: Silting or continuous scouring indicates the velocity is too low or too high, respectively.Simultaneous silting and scouring contradicts a stable operating point.Selective movement of only gross solids does not guarantee a clean invert. Common Pitfalls: Using a single velocity value irrespective of sediment characteristics. Ignoring pipe roughness and slope, which affect shear stress. Final Answer: Neither silting nor scouring occurs at the bottom

Question 14

Surface overflow concept: For a sedimentation basin of length L and width B treating a discharge Q, the settling (terminal) velocity required for a particle to be fully removed in ideal plug flow is

Accepted Answer

Introduction / Context: Clarifier sizing often uses the concept of surface overflow rate (SOR), also called overflow velocity. Under ideal plug-flow assumptions, removal of a particle requires its settling velocity to be at least equal to the upward (overflow) velocity across the plan area. Given Data / Assumptions: Basin plan dimensions: length L and width B; depth D does not affect ideal SOR. Flow rate: Q (steady, uniform distribution). Ideal settling without short-circuiting or turbulence. Concept / Approach: The theoretical criterion is v_s ≥ Q / A_plan, where A_plan = L * B. This means a particle whose settling velocity equals or exceeds Q / (L * B) will reach the sludge zone before escaping with the effluent. Step-by-Step Solution: Compute plan surface area: A_plan = L * B.Overflow (upward) velocity: v_o = Q / A_plan.Removal criterion: v_s ≥ v_o = Q / (L * B). Verification / Alternative check: Depth D mainly affects detention time and sludge storage but not the ideal surface loading criterion; hence it is absent from the ideal v_s expression. Why Other Options Are Wrong: Formulas involving D (depth) contradict the surface loading logic.Expressions multiplying L/B or using LBD do not represent plan overflow. Common Pitfalls: Including depth in the ideal SOR calculation. Ignoring short-circuiting; real designs often apply safety factors. Final Answer: v_s = Q / (L * B)

Question 15

Hume steel pipes (also called cement-mortar lined and coated steel): Which description best matches Hume steel pipes used in water/wastewater conveyance?

Accepted Answer

Introduction / Context: Hume steel pipes are widely used where the strength of steel is desirable but corrosion resistance and hydraulic smoothness of cement mortar are also needed. The combined system improves durability and water quality while maintaining manageable weight and constructability. Given Data / Assumptions: Pipe consists of a steel cylinder for structural strength. Cement mortar lining inside improves hydraulics and corrosion resistance. Cement mortar coating outside enhances external corrosion protection. Concept / Approach: The defining feature of Hume steel pipes is the cement-mortar lining and coating applied to a steel shell. This sandwich approach reduces corrosion, limits tuberculation, and provides a smoother interior surface for reduced headloss. Step-by-Step Reasoning: Identify material core: steel shell.Identify internal protection: cement-mortar lining.Identify external protection: cement-mortar coating.Thus, correct description includes both internal lining and external coating. Verification / Alternative check: Manufacturer catalogs and design manuals list Hume steel pipes as steel cylinders with cement-mortar lining and coating; sometimes additional wrappings or coatings are added for aggressive soils. Why Other Options Are Wrong: Steel only / R.C.C. only: incomplete or incorrect.Only inside or only outside coating: partial description, not the standard system. Common Pitfalls: Confusing Hume steel with purely R.C.C. (spun) pipes. Assuming only internal lining without external protection in corrosive soils. Final Answer: Both (c) and (d)

Question 16

Surface loading terminology: The ratio of design discharge to the plan surface area of a sedimentation tank is called what?

Accepted Answer

Introduction / Context: In clarifier design, the parameter Q/A (where A is the plan surface area) is fundamental. It appears under various interchangeable names in the literature, and recognizing this equivalence helps in reading diverse references and codes. Given Data / Assumptions: Q = design flow. A = plan surface area (length × width). Ideal clarification theory where removal is related to surface overflow. Concept / Approach: Q/A is commonly termed surface loading rate (SLR), overflow rate, or overflow velocity. Under ideal conditions, a particle with settling velocity equal to or greater than Q/A will be removed. These terms are synonymous in most water and wastewater design texts. Step-by-Step Reasoning: Compute Q/A as a design check.Use it to compare with settling velocities of particle classes.Recognize that different sources may label it as surface loading, overflow rate, or overflow velocity. Verification / Alternative check: Multiple handbooks and standards cross-reference these terms, confirming their equivalence for conventional basins. Why Other Options Are Wrong: Each of (a), (b), and (c) is correct; selecting only one is incomplete.Detention coefficient is not a standard name for Q/A. Common Pitfalls: Mistaking Q/A for detention time; detention is V/Q and involves depth. Using SLR without considering short-circuiting and flocculation behavior. Final Answer: All of these

Question 17

Hydraulic checks for sewers: During design, sewer pipes should be checked for which of the following hydraulic conditions?

Accepted Answer

Introduction / Context: Sanitary and combined sewers must function reliably across a wide range of flows. Hydraulic checks at both ends of the operating spectrum ensure prevention of siltation at low flows and avoidance of excessive velocities or surcharge at high flows. Given Data / Assumptions: Minimum flow occurs during off-peak hours or dry seasons. Maximum flow occurs during peak sanitary loads and/or storm events (combined systems). Pipe material and grade selected to meet both criteria. Concept / Approach: Two principal checks are required: (1) Minimum-velocity (self-cleansing) check to prevent deposition and odor problems; (2) Maximum-velocity/headroom check to prevent scouring, erosion, and structural or operational issues such as surcharge and manhole flooding. Step-by-Step Reasoning: At minimum flow: verify velocity ≥ self-cleansing threshold (e.g., about 0.6–0.75 m/s, depending on standards and sediments).At maximum flow: verify velocity ≤ allowable limit for the pipe material and that hydraulic grade line stays within design constraints (no unacceptable surcharge).Adjust slope/diameter as needed to satisfy both checks. Verification / Alternative check: Design manuals require both checks and provide recommended velocity ranges and depth-of-flow criteria for partial-full conditions in circular pipes. Why Other Options Are Wrong: Minimum-only or maximum-only ignores one side of the operational envelope.Structural-only check is insufficient; hydraulics govern serviceability.Average-only ignores extremes that cause most failures. Common Pitfalls: Using one diameter for all reaches without confirming local grades and flows. Neglecting infiltration/inflow in maximum-flow checks. Final Answer: Both minimum and maximum flow conditions

Question 18

Rational method in stormwater design: The widely used rational formula for estimating peak drainage discharge (Q = C * i * A) was evolved by which engineer?

Accepted Answer

Introduction / Context: The rational method is one of the oldest and most widely applied tools in urban drainage design for estimating peak runoff from small catchments. The question checks historical attribution, reinforcing foundational knowledge used in sewer and stormwater sizing. Given Data / Assumptions: Peak discharge is estimated using Q = C * i * A (consistent units). Method is typically applied to small, relatively uniform urban catchments. Historical inquiry: who formally evolved and popularized the formula in modern practice? Concept / Approach: While early ideas about runoff predate the 19th century, Emil Kuichling (late 1800s) is credited with evolving and systematizing the rational formula for American practice, including guidance on selection of runoff coefficients and critical storm duration (time of concentration). Recognizing the name helps link design practice to its origins and literature. Step-by-Step Solution: Identify the formula: Q = C * i * A.Recall the historical figure associated with formalizing the method: Emil Kuichling (1889 era).Select the corresponding option. Verification / Alternative check: Standard hydrology texts cite Kuichling for the rational method’s evolution and adoption in urban drainage design, noting assumptions such as uniform rainfall intensity over the duration equal to time of concentration. Why Other Options Are Wrong: Lloyd-Davies: Associated with runoff coefficients and UK practice, but not the original evolution of the rational formula.Fruhling: Not credited with evolving the method.Mulvaney: An early contributor in runoff estimation history, but the rational method in current form is attributed to Kuichling.“All of these”: Historically inaccurate. Common Pitfalls: Confusing early contributors with the engineer who evolved the method in modern design context. Assuming “All of these” when multiple familiar names appear. Final Answer: Kuichling

Question 19

Plumbing in buildings: An anti-siphonage (anti-syphonage) pipe is fitted where to prevent trap seal loss by siphonage?

Accepted Answer

Introduction / Context: Fixture traps must retain a water seal to block sewer gases. Siphonage during flushing can pull the seal away unless a vent (anti-siphonage pipe) is correctly installed. Understanding where to place this vent is central to sanitary plumbing design and codes. Given Data / Assumptions: Conventional water-closet (W.C.) with a P- or S-trap. Risk of self-siphonage or induced siphonage during high discharge. Vent piping can equalize pressure across the trap. Concept / Approach: An anti-siphonage (vent) pipe connects near the trap arm to equalize pressure, preventing negative pressure that could suck water out of the trap. This venting is part of standard building plumbing systems and ensures hygienic conditions by maintaining the trap’s seal depth. Step-by-Step Solution: Identify the problem: trap seal loss due to pressure drop.Mitigation: provide a vent path to atmosphere via anti-siphonage pipe.Location: connected with the W.C. trap (or close on the trap arm) per code. Verification / Alternative check: Plumbing codes (e.g., IPC) require trap venting; anti-siphonage pipes or vents are connected near traps, not at manholes or main sewers. Why Other Options Are Wrong: Septic tank/manhole/street sewer: Venting there does not protect the individual fixture trap from local pressure fluctuations.Gully traps only: W.C. traps specifically need anti-siphonage protection. Common Pitfalls: Assuming any vent in the system protects every trap; vents must be correctly located. Ignoring maximum trap-arm length limits which affect siphonage risk. Final Answer: With a W.C. trap (vent connection)

Question 20

Allowance for future and uncertainties in sewers: Engineers keep one-quarter to one-third of the cross-section vacant at “maximum discharge”. This spare space is primarily to account for which factors?

Accepted Answer

Introduction / Context: Sewer design incorporates a margin beyond the computed maximum flow. Leaving roughly 25% to 33% vacant space at the design “maximum” discharge provides resilience against uncertainty and future growth. Given Data / Assumptions: Conventional gravity sewer design practice. Allowance magnitude: about 1/4 to 1/3 empty space at design maximum. Potential uncertainties in flow estimates and future conditions. Concept / Approach: Uncertainties arise from hydrologic estimation, infiltration/inflow (I/I), and demographic growth. The vacant fraction offers hydraulic headroom so the system can continue to function without immediate surcharge when real-world flows exceed design projections. Step-by-Step Solution: Recognize key drivers: estimate error, I/I surges, and demand growth.Relate design margin to these drivers: vacant space acts as buffer storage/conveyance.Choose the comprehensive option that covers all drivers. Verification / Alternative check: Design manuals recommend such margins to handle uncertainties and to delay costly upsizing when populations or inflow exceed forecasts. Why Other Options Are Wrong: Single-cause options ignore the multifactor nature of uncertainty.Sediment-only explanation is incomplete; deposition is controlled by self-cleansing velocities rather than large vacant fractions. Common Pitfalls: Assuming the margin is primarily for sediment; it is for future capacity and uncertainties. Misinterpreting “maximum discharge” as leaving no freeboard. Final Answer: All of the above

Waste Water Engineering Questions