Question 1

At ordinary temperatures, “fresh” domestic sewage begins to turn stale (become septic/odorous) after approximately how much detention time in a closed sewer without aeration?

Accepted Answer

Introduction / Context: Characterizing sewage freshness is important for design of pumping stations, force mains, and retention structures to avoid septic conditions, odours, and corrosion. “Stale” sewage indicates onset of anaerobic conditions and production of hydrogen sulphide. Given Data / Assumptions: Typical warm weather, domestic sewage. Minimal reaeration (closed conduit), appreciable biochemical activity. No chemical dosing (e.g., nitrate, oxygen, or iron salts). Concept / Approach: In the absence of oxygen replenishment, dissolved oxygen is quickly depleted by biochemical oxygen demand. Once DO is exhausted, septic anaerobic processes produce odorous gases. Empirical practice suggests fresh sewage can begin to turn stale in only a few hours, often 2–3 hours, depending on temperature and load. Step-by-Step Solution: Recognize DO depletion rate depends on temperature and BOD.Without aeration, DO can drop rapidly in gravity sewers and wet wells.Select the time range widely cited for onset of stale/septic conditions: ≈ 2–3 hours. Verification / Alternative check: Utility SOPs often limit detention times in rising mains and wet wells to a few hours to prevent septicity and H2S formation, consistent with the 2–3 hour guideline. Why Other Options Are Wrong: (a) One hour is typically too short for significant septicity under average conditions. (c), (d), and (e) exceed typical onset period; by then septic conditions are established unless mitigated. Common Pitfalls: Ignoring temperature effect (warmer sewage turns septic faster); assuming open channels afford the same timeline as closed sewers; neglecting industrial contributions that can accelerate septicity. Final Answer: About 2 to 3 hours

Question 2

The liquid wastes from kitchens, bathrooms, and wash basins are termed what in sanitation engineering? (Choose the option that is NOT the correct name.)

Accepted Answer

Introduction / Context: Sanitary terminology distinguishes between different domestic wastewater streams. Correct terms help in designing dual plumbing, greywater reuse, and selecting appropriate treatment units. Given Data / Assumptions: Kitchens, bathrooms, wash basins discharge without toilet wastes. Standard sanitation definitions: greywater versus blackwater. Concept / Approach: Wastewater without human excreta is commonly called sullage or greywater. The term sewage, in many texts, includes both sullage and soil water (toilet wastes). Because the question asks for the name that is NOT applicable, “sewage (inclusive of soil water)” is the incorrect specific label for the described stream. Step-by-Step Solution: Identify the described source: kitchens/baths/basins → greywater.Map terms: greywater = sullage; blackwater = soil water (toilet).Therefore “sewage (inclusive of soil water)” is NOT the specific term for this stream alone. Verification / Alternative check: Design manuals for on-site systems separate greywater (for potential reuse) from blackwater, underscoring the distinction. Why Other Options Are Wrong: (a) and (e) generically describe the stream correctly. (b) is the precise standard term. (d) “None of these” is false because one option is indeed not applicable. Common Pitfalls: Using “sewage” loosely for any wastewater; overlooking that many codes treat greywater differently from sewage containing faecal matter. Final Answer: Sewage (inclusive of soil water)

Question 3

For municipal design, the average rate of sewage flow generated in a community is generally assumed to be how relative to the average rate of water supply?

Accepted Answer

Introduction / Context: Estimating sewage flow from water supply is a first step in sizing sewers and treatment plants. Not all supplied water becomes wastewater due to losses and consumptive uses, so a fraction of supply is used for preliminary design when local data are lacking. Given Data / Assumptions: Municipal context without unusually high infiltration/inflow. Average daily conditions (not peak hour). Typical domestic water uses include outdoor irrigation, leakage, and evaporation. Concept / Approach: Empirical practice assumes sewage ≈ 70–90% of water supply on an average day, reflecting consumptive losses. The exact factor depends on climate, metering accuracy, and industrial discharges, but the sewage rate is generally less than the water supply rate. Step-by-Step Solution: Relate community water inflow to sewer outflow via loss coefficients.Adopt standard design fraction (e.g., 0.8) if site-specific data are absent.Conclude the sewage rate is less than the supply. Verification / Alternative check: Utility audits routinely find average return flows of 75–90% depending on season and outdoor usage. Codes sometimes tabulate default return fractions for planning. Why Other Options Are Wrong: (a) and (b) contradict observed losses. (d) confuses per-capita supply standards with return ratios. (e) is false because water use strongly drives sewage generation. Common Pitfalls: Ignoring infiltration/inflow during wet weather, which can temporarily increase sewer flows beyond return fractions; designing only on average rather than considering peaking factors. Final Answer: Less than the water supply rate (typically a fraction such as 0.7–0.9 of supply)

Question 4

During testing and inspection of newly laid sewer pipes, which of the following tests are standard practices to verify soundness and alignment?

Accepted Answer

Introduction / Context: Commissioning of sewer lines includes field tests to detect leakage, sags, misalignment, and blockages before acceptance. Using multiple simple tests provides assurance that the pipeline will perform as designed. Given Data / Assumptions: Gravity sewer pipe sections installed with manholes. Contract specifications require integrity, grade, and alignment verification. Concept / Approach: The water test fills a pipe section to a specified head to check for leakage. The ball test passes a ball (or mandrel) of standard diameter to confirm no obstructions and adequate bend clearances. The mirror test (or light test) confirms straightness by sighting a reflected light along the pipe barrel—any sags or offsets appear as discontinuities. Step-by-Step Solution: Identify purpose of each test: leak tightness, obstruction check, alignment/grade verification.Recognize that specifications often require all three for acceptance.Select the comprehensive option that includes all tests. Verification / Alternative check: Standard codes and municipal specs list these tests, sometimes alongside air tests and CCTV inspections for modern QA/QC. Why Other Options Are Wrong: (a), (b), or (c) alone do not cover all acceptance criteria. (e) is false since these are indeed standard tests. Common Pitfalls: Relying on a single test and missing defects; conducting tests before sufficient curing or backfilling; ignoring manhole-to-manhole alignment. Final Answer: All of these

Question 5

In open-channel hydraulics and sewer design, the hydraulic mean radius (hydraulic radius) R is defined as which of the following?

Accepted Answer

Introduction / Context: The hydraulic mean radius R = A/P (often called hydraulic radius) is a key parameter in Manning and Chezy formulations, representing an effective depth that couples area and boundary friction in open channels and partially full pipes. Given Data / Assumptions: Any prismatic channel or conduit with wetted perimeter P. Steady, uniform flow approximations. Concept / Approach: By definition, R = A/P where A is the cross-sectional flow area and P is the wetted perimeter in contact with the fluid. For a full circular pipe, R = D/4, but this is a special case; for other shapes and partial flow, R must be computed from A and P directly. Step-by-Step Solution: Recall Chezy: V = C √(R S); Manning: V = (1/n) R^(2/3) S^(1/2).Insert R = A/P in either equation for computation.Note special case: full circular pipe gives R = D/4, not a universal identity. Verification / Alternative check: Dimensions: A has L^2, P has L, so A/P has L, consistent with a length scale for hydraulic resistance. Why Other Options Are Wrong: (a) and (c) are not standard definitions. (b) confuses head difference with geometry. (e) is only valid for full circular pipes; the question asks a general definition. Common Pitfalls: Blindly using D/4 for partial-full pipes; forgetting that roughness and geometry both influence P and thus R. Final Answer: Cross-sectional flow area divided by wetted perimeter (A/P)

Question 6

In sewer design for buried conduits: A sewer running partially full and surrounded by compacted backfill can fail in compression due to which combined actions?

Accepted Answer

Introduction / Context: Sewers are often laid below ground level and are usually designed to carry flow partially full. Even though internal water pressure is low, the pipe can fail structurally due to external compressive effects. Understanding the sources of compressive loading helps in choosing pipe class, bedding, and backfill specifications for long-term performance. Given Data / Assumptions: Buried circular sewer, running partially full. Pipe is surrounded by backfill placed to finished ground level. Potential additional live loads from vehicles or construction traffic may act at ground surface. Concept / Approach: Compression in buried pipes arises from dead loads (pipe self-weight and soil prism above the pipe) and live loads (traffic). The soil-structure interaction transfers loads to the pipe barrel. If bedding support is inadequate or pipe class is insufficient, longitudinal or ring compression may exceed limits and cause cracking or buckling in brittle materials. Step-by-Step Solution: Identify dead load components: pipe weight + backfill prism weight.Identify live load component: wheel loads creating additional stress through distribution in the soil cover.Recognize that combined loading increases compressive stress in the pipe wall.Conclude that all listed actions contribute to compression and potential failure. Verification / Alternative check: Buried pipe design methods (for example trench or embankment formulas) explicitly include soil load and live load terms in the ring compression check, confirming the combined effect. Why Other Options Are Wrong: Self-weight only: ignores dominant soil prism weight.Backfill only: neglects pipe weight and traffic loads.Traffic only: not the only contributor; dead loads are always present. Common Pitfalls: Assuming partial flow means negligible structural load. Ignoring compaction-induced stresses during construction. Underestimating surface traffic effects for shallow cover depths. Final Answer: All of the above

Question 7

In on-site sanitation using septic tanks, which of the following detailing practices are recommended for reliable operation and maintenance?

Accepted Answer

Introduction / Context: Septic tanks are widely used for household wastewater treatment. Proper internal arrangements and clearances are essential to separate scum, sludge, and clarified effluent while preventing short-circuiting and blockage of the outlet. Given Data / Assumptions: A standard two-compartment septic tank serving domestic wastewater. Scum accumulation at the top and sludge at the bottom are expected. Hydraulics should promote quiescent settling and avoid carryover of scum. Concept / Approach: Key geometric features include freeboard to accommodate scum swell, inlet and outlet tees or baffles to direct flow and prevent scum escape, and suitable clearances to the cover to ease ventilation and inspection. Recommended immersion of inlet and outlet inverts also stabilizes flow paths. Step-by-Step Solution: Freeboard: about 0.3 m provides allowance for scum and foaming.Baffles/tees: extending to scum top ensures separation and prevents direct short-circuiting.Clear space below cover: approximately 7.5 cm aids gas space and inspection.Inlet and outlet elevations: submerging inlet around 0.3 m, and setting outlet dip near 40% of liquid depth helps retain scum and draw clarified effluent. Verification / Alternative check: Good practice manuals and codes give similar values and arrangements that balance constructability with performance, confirming the recommendations as accepted norms. Why Other Options Are Wrong: Selecting only one practice ignores the others; robust septic design is a combination of features working together. Common Pitfalls: Omitting inlet/outlet tees leading to scum discharge. Insufficient freeboard causing overflow of scum to the outlet chamber. Improper baffle clearances that hinder inspection or ventilation. Final Answer: All of the above

Question 8

In large sewers, the recommended self-cleaning (non-silting) velocity under Indian practice, to avoid deposition on the invert and sides, is approximately:

Accepted Answer

Introduction / Context: Sewer hydraulics aims to maintain velocities high enough to keep solids in motion, preventing deposition and odor issues. The self-cleaning velocity is a rule-of-thumb minimum velocity to achieve this at design flow conditions. Given Data / Assumptions: Relatively large municipal gravity sewers. Indian design practice for self-cleansing criteria. Typical mixed domestic sewage with suspended and settleable solids. Concept / Approach: Empirical recommendations indicate that velocities near or above about 0.6 to 0.75 m/s help prevent settlement of typical sewage solids. For large sewers and Indian conditions, 0.75 m/s is commonly adopted as a practical design value for self-cleansing under dry weather flow. Step-by-Step Solution: Identify the objective: avoid grit and organic deposition.Select empirical minimum velocity: 0.75 m/s for Indian practice for large sewers.Check that at peak factors, actual velocities will exceed this minimum, further improving scouring. Verification / Alternative check: Design guides and solved examples for sewer design repeatedly cite 0.75 m/s as a benchmark minimum to deter siltation in large conduits under DWF conditions. Why Other Options Are Wrong: 0.50–0.70 m/s: sometimes acceptable in small sewers or under specific conditions, but 0.75 m/s is the widely cited Indian recommendation for large sewers.1.00 m/s: conservative; may raise excavation depth or diameters unnecessarily. Common Pitfalls: Using peak flow velocity instead of minimum DWF velocity for self-cleaning check. Ignoring upstream flat slopes that can create local silting pockets. Final Answer: 0.75 m/s

Question 9

Terminology in sewerage: Which pipe carries sewage from an individual building or premises to the public system for immediate disposal?

Accepted Answer

Introduction / Context: Clarity in sewer nomenclature is essential for design drawings, approvals, and maintenance responsibilities. Different conduits form the hierarchy, from the property line to the wastewater treatment plant. Given Data / Assumptions: Typical urban sewerage network. Definitions follow conventional municipal engineering usage. Concept / Approach: A house sewer (also called a building sewer or house connection) conveys wastewater from a single building to the public sewer. Lateral sewers collect flows from multiple house connections. Intercepting sewers bypass combined sewer overflows or intercept lower-level flows. The main sewer is a larger trunk line; the outfall conveys final flow to treatment or disposal. Step-by-Step Solution: Identify the origin of sewage: individual building.Determine the correct conduit up to the public line: house sewer.Exclude network collectors and trunks which are downstream components. Verification / Alternative check: Standard plumbing codes and municipal standards define the building sewer as the conduit from the building drain to the public sewer or private disposal system, confirming the terminology. Why Other Options Are Wrong: Lateral sewer: serves multiple properties along a street.Intercepting and main sewers: larger downstream network elements.Outfall sewer: final conveyance to treatment or disposal. Common Pitfalls: Confusing building drain (inside property) with building sewer (from property line to public sewer). Using local jargon inconsistently on drawings. Final Answer: House sewer (building sewer)

Question 10

Manhole classification by depth: A manhole is considered “shallow” when its depth (from cover to invert) lies approximately within which range?

Accepted Answer

Introduction / Context: Manholes provide access for inspection, maintenance, and ventilation of sewers. They are commonly classified as shallow, normal, or deep based on depth, which guides structural detailing and safety measures. Given Data / Assumptions: Depth measured from finished cover level to sewer invert. Conventional municipal practice for classification. Concept / Approach: Shallow manholes serve near-surface sewers and small changes in direction or grade. Typical reference ranges place shallow manholes around 0.7 m to 0.9 m depth, normal up to about 1.5 m, and deep beyond that, subject to local codes. Step-by-Step Solution: Review common classification bands used in standard texts.Select the range that corresponds to “shallow.”Confirm that other ranges represent normal or deep manholes. Verification / Alternative check: Design handbooks and examination references repeatedly cite 0.7–0.9 m as the depth range for shallow manholes in conventional practice. Why Other Options Are Wrong: 0.4–0.7 m: too small for standard shallow manholes; often used for inspection chambers.0.9–1.5 m: more typical of normal manholes. Common Pitfalls: Confusing small inspection chambers with manholes capable of entry. Ignoring site-specific safety requirements that may redefine categories. Final Answer: 0.7 m to 0.9 m

Question 11

Biochemical Oxygen Demand (BOD₅) calculation: A 2.5 ml sewage sample is diluted to 250 ml and incubated for 5 days at 20°C. If the dissolved oxygen depletion measured is 2.5 mg/litre, what is the BOD of the sewage (mg/l)?

Accepted Answer

Introduction / Context: BOD₅ is a standard test for quantifying biodegradable organic load in wastewater. It measures the oxygen consumed by microorganisms over five days at 20°C in a diluted sample. Correctly applying the dilution factor is critical to obtain the actual BOD of the original sewage. Given Data / Assumptions: Sewage aliquot volume Vs = 2.5 ml. Total diluted volume Vt = 250 ml. Observed dissolved oxygen depletion in the diluted bottle = 2.5 mg/l. Incubation: 5 days at 20°C. Concept / Approach: The BOD of the undiluted sample equals the observed depletion multiplied by the dilution factor. Dilution factor DF = Vt / Vs. The BOD is then BOD = DO depletion * DF, provided seed corrections are not required and initial and final DO are within the acceptable range. Step-by-Step Solution: Compute dilution factor: DF = 250 / 2.5 = 100.Compute BOD: BOD = 2.5 mg/l * 100 = 250 mg/l.Select the matching option. Verification / Alternative check: If a 1% aliquot (2.5 ml of 250 ml) causes 2.5 mg/l depletion in the diluted bottle, the undiluted would demand roughly 100 times more oxygen, giving 250 mg/l, consistent with medium-strength municipal sewage ranges. Why Other Options Are Wrong: 50, 100, 150, 200 mg/l: each ignores or under-applies the dilution factor. Common Pitfalls: Forgetting to account for dilution volume precisely. Confusing mg/l depletion in the bottle with BOD of the original sewage without scaling. Neglecting seeding corrections when seed is used; here none is indicated. Final Answer: 250 mg/l

Question 12

Design basis for dry weather flow (DWF): It is customary to size sanitary sewers for a peak DWF equal to what multiple of the average dry weather flow?

Accepted Answer

Introduction / Context: Sanitary sewers must carry variable flows. Even in dry weather, diurnal patterns and stochastic variations cause peaks above the average. Designers therefore apply a peaking factor to the average DWF to obtain a practical design discharge for hydraulic sizing. Given Data / Assumptions: Sanitary sewer without significant infiltration or storm inflow. Conventional practice values for peaking factor in local conditions. Average DWF known from per capita flow and contributing population. Concept / Approach: Standard practice often uses approximately 3 times the average DWF (for small to medium systems) to cover daily peaks, upstream synchronization, and uncertainties. Larger trunk mains may justify lower factors based on empirical formulas, but 3.0 is a frequently cited benchmark in examination problems. Step-by-Step Solution: Compute average DWF from unit contributions.Apply peaking factor ≈ 3 to obtain design discharge.Check velocity criteria (self-cleansing and non-scouring) at both minimum and peak flows. Verification / Alternative check: Alternative peaking factor relations (for example population dependent) often yield values near 2.5–3.0 for many service areas, corroborating the 3.0 choice in typical exam settings. Why Other Options Are Wrong: 1.0–2.0: may under-size upstream sewers resulting in surcharge.4.0–5.0: overly conservative for DWF and could be reserved for special cases or combined systems. Common Pitfalls: Using the same peaking factor for all pipe sizes and populations without judgment. Ignoring infiltration and inflow which can change design basis. Final Answer: Thrice the average demand (3.0 times)

Question 13

Water treatment unit design: The usual detention period adopted for plain sedimentation tanks (coagulation not considered) is approximately:

Accepted Answer

Introduction / Context: Plain sedimentation tanks remove settleable solids by gravity prior to filtration. The detention period is a key parameter that influences particle settling, tank dimensions, and operational performance. Given Data / Assumptions: Plain (unassisted) sedimentation without flocculation. Typical municipal water treatment practice. Temperatures in a normal range and conventional tank depths. Concept / Approach: Design detention periods for plain sedimentation commonly fall in the range of several hours to provide sufficient time for particles with practical settling velocities to reach the bottom, considering inlet and outlet hydraulics and short-circuiting controls. Step-by-Step Solution: Select a detention time that balances tank size with settling efficiency.Standard range for plain sedimentation is about 4–8 hours.Verify with sedimentation theory (surface overflow rate) rather than detention time alone in final design. Verification / Alternative check: Surface overflow rate approach typically yields tank sizes corresponding to several hours of detention for conventional plants, validating the 4–8 hour guideline. Why Other Options Are Wrong: 8–36 hours: excessive for plain sedimentation; impractical tankage.1–2 hours: too short for reliable removal in plain sedimentation without coagulation. Common Pitfalls: Relying solely on detention time without checking surface overflow rate. Ignoring inlet and outlet baffles which affect effective detention. Final Answer: 4 to 8 hours

Question 14

Ventilation of sewers: Ventilating columns (vents) should generally be provided at which locations for odor and gas control?

Accepted Answer

Introduction / Context: Ventilation prevents the buildup of corrosive and explosive gases in sewers, protects public health, and reduces odor complaints. The placement of ventilating columns aids convective exchange with the atmosphere. Given Data / Assumptions: Conventional gravity sewers in urban settings. Passive ventilation strategy using vertical columns and manholes. Concept / Approach: Strategic placement improves air exchange across the network. Regular spacing limits dead zones; venting at high points and size transitions helps release trapped gases and equalize pressure. Step-by-Step Solution: Provide vents at 150–300 m spacing to maintain air circulation.Vent upper ends of branches to release accumulating gases and admit air.Place vents at changes of size where turbulence and gas release are likely. Verification / Alternative check: Field experience and guidelines support these locations as effective points for odor and gas management, especially when combined with drop manhole ventilation and odor control media when needed. Why Other Options Are Wrong: Single selections do not provide a complete strategy; combined placement is more effective.“None of the above” contradicts standard practice. Common Pitfalls: Ignoring ventilation in flat terrain where air movement is limited. Omitting vents at branch summits, leading to odor complaints. Final Answer: All of the above

Question 15

Imhoff tanks (two-story septic sedimentation): Which of the following statements about geometry and operating parameters is generally correct?

Accepted Answer

Introduction / Context: Imhoff tanks are two-story structures combining sedimentation in the upper compartment with anaerobic digestion in the lower compartment, separated by sloped slots to prevent gas-induced sludge rising. Proper sizing ensures effective primary treatment for small communities. Given Data / Assumptions: Typical municipal sewage with moderate strength. Classical Imhoff configuration with rectangular plan. Upper flow-through sedimentation zone and lower digestion zone. Concept / Approach: Design parameters aim to give adequate settling (detention about 2 hours for the flow-through portion), gentle velocities to avoid resuspension (around 0.30 m/min), and reasonable surface loading rates (on the order of a few 10^4 litres/m^2/day) for solids removal without short-circuiting. Step-by-Step Solution: Adopt rectangular plan as common practice for channeling and slot arrangement.Use detention time near 2 hours in the sedimentation chamber for primary removal.Limit flow velocity to roughly 0.30 m/min to maintain quiescent settling.Check surface loading at approximately 30,000 L/m^2/day as a representative limit. Verification / Alternative check: Design texts on Imhoff tanks provide similar ranges, confirming that the listed values are representative for sizing and operation under standard conditions. Why Other Options Are Wrong: Rejecting any single statement would conflict with widely cited design ranges for classical Imhoff tanks. Common Pitfalls: Confusing Imhoff tanks with conventional single-story primary clarifiers. Ignoring gas venting and scum control at the slot interface which are crucial to performance. Final Answer: All of the above

Question 16

Solid waste pre-treatment in municipal wastewater engineering: Which ONE statement about screenings handling is NOT correct?

Accepted Answer

Introduction / Context: Screenings handling is one of the first steps in municipal wastewater treatment. Coarse materials such as rags, plastics, and debris are intercepted by bar screens and must be handled and disposed of safely. This question tests your understanding of terminology and standard disposal practices for screenings in environmental engineering. Given Data / Assumptions: We are dealing with screenings from preliminary treatment. Standard options for disposal include incineration, burial, and controlled dumping depending on local regulations. Composting is a biological process applied primarily to suitable organic solids (e.g., primary sludge, biosolids), not to mixed screenings which often contain inerts and plastics. Concept / Approach: Identify which statement conflicts with accepted terminology and practice. The key distinction is between incineration (thermal oxidation/burning) and composting (aerobic biological stabilization). Screenings are unsuitable for composting because of their heterogeneity and contamination; if burned, the correct term is incineration. Step-by-Step Solution: 1) Recognize that materials caught on screens are called screenings. This is standard terminology.2) Typical disposal options for screenings include incineration, burial in sanitary landfills, or controlled dumping where permitted. These are accepted practices.3) Burning screenings is a thermal process. The correct term for burning is incineration.4) Composting refers to controlled aerobic biodegradation of putrescible organics. Mixed screenings are generally not composted.5) Therefore, the statement equating burning with composting is incorrect. Verification / Alternative check: Cross-check terminology used for sludge (which can be composted) versus screenings (usually disposed or incinerated). The mismatch confirms the incorrect statement. Why Other Options Are Wrong: Option A: Correct definition of screenings.Option B: Correct list of common disposal routes.Option C: Correct term—burning equals incineration.Option E: Accurate context—screenings removal is indeed an initial unit operation. Common Pitfalls: Confusing composting (biological) with incineration (thermal). Assuming all solids from a plant can be composted; screenings composition makes them unsuitable. Overlooking regulatory requirements for handling screenings safely. Final Answer: The process of burning screenings is known as composting.

Question 17

Self-cleansing versus scouring in sewers: In which sewer material is the highest non-scouring (erosion-safe) velocity permissible?

Accepted Answer

Introduction / Context: Sewer design balances two limits: a minimum self-cleansing velocity to prevent deposition and a maximum non-scouring velocity to prevent erosion of the pipe wall. Different materials withstand different upper velocities before scouring becomes critical. Given Data / Assumptions: Comparing typical non-scouring limits among common sewer materials. Standard practice values: cast iron has the highest abrasion resistance among options listed. Concept / Approach: Non-scouring velocity is the maximum safe velocity that does not erode the internal surface. Materials with high strength and smooth, durable interiors tolerate higher velocities. Cast iron is more resistant to abrasion and impact than masonry or concrete in sewer service. Step-by-Step Solution: 1) List materials by erosion resistance: cast iron > vitrified clay ≈ concrete > brick.2) Identify which allows the greatest non-scouring velocity: cast iron.3) Select the corresponding option. Verification / Alternative check: Design manuals commonly specify higher permissible velocities for metallic pipes relative to concrete or brick due to better abrasion resistance and surface hardness, confirming the choice. Why Other Options Are Wrong: Stoneware: Smooth but brittle; non-scouring limit lower than cast iron.Cement concrete: Prone to surface wear under high velocity, especially with grit.Glazed brick: Mortar joints and brick faces are less resistant to high-velocity abrasion.Asbestos cement: Not listed in the stem originally and generally has lower abrasion resistance than cast iron. Common Pitfalls: Confusing self-cleansing (minimum) with non-scouring (maximum) velocities. Assuming smoothness alone governs scouring; material strength also matters. Final Answer: Cast iron sewers

Question 18

Standard biochemical oxygen demand (BOD) test: Over how many days is the BOD of a water sample measured under standard conditions?

Accepted Answer

Introduction / Context: Biochemical oxygen demand (BOD) is the oxygen required by microorganisms to oxidize biodegradable organic matter. A standard test period ensures comparability of results across laboratories and systems. Given Data / Assumptions: Standard conditions are implied (most commonly 20°C in the laboratory). We are asked for the standard number of days used internationally for BOD determination. Concept / Approach: The conventional BOD test is BOD5, meaning 5-day BOD, performed at 20°C in the dark to prevent photosynthesis. This historical choice approximated river travel time in temperate climates and became a standard benchmark for wastewater strength. Step-by-Step Solution: 1) Recall the standard test designation: BOD5.2) BOD5 indicates measurement over 5 days at 20°C.3) Select the corresponding option. Verification / Alternative check: Many regulations and design references specify BOD5 as the basis for effluent limits and treatment performance, confirming the answer. Why Other Options Are Wrong: 1, 2, 3 days: Too short to represent standard BOD5.10 days: Sometimes used for ultimate BOD estimation trends but not the standard test period. Common Pitfalls: Confusing BOD5 with ultimate BOD (BODu), which extrapolates beyond 5 days. Assuming temperature other than 20°C; the standard prescribes 20°C. Final Answer: 5 days

Question 19

Hydraulically equivalent sections in open channel flow: Ratio of the diameter of a circular section to the side of a square section (hydraulically equivalent for equal conveyance).

Accepted Answer

Introduction / Context: Hydraulically equivalent sections are compared so that their conveyance capacity under the same slope and roughness is equivalent. For uniform flow, discharge is proportional to A * R^(2/3) (Manning–Strickler form), making geometry critical. Given Data / Assumptions: Uniform roughness and bed slope. Comparing a full circular section (diameter D) with a full square section (side s). Hydraulic conveyance proportional to A * R^(2/3). Concept / Approach: Set the conveyance of the circle equal to that of the square: A_c * R_c^(2/3) = A_s * R_s^(2/3). For a circle, A_c = π D^2 / 4, P_c = π D, and R_c = A_c / P_c = D/4. For a square, A_s = s^2, P_s = 4 s, and R_s = s/4. Solve for D/s. Step-by-Step Solution: 1) Write equality: (π/4) * D^2 * (D/4)^(2/3) = s^2 * (s/4)^(2/3).2) Cancel the common (1/4)^(2/3) factor to simplify algebra.3) Rearrange to D^(8/3) * (π/4) = s^(8/3).4) Raise both sides to the power 3/8 to isolate D: D * (π/4)^(3/8) = s.5) Hence, D/s = (4/π)^(3/8) ≈ 1.095. Verification / Alternative check: A quick numerical evaluation of (4/π) equals about 1.273; raising to 0.375 gives approximately 1.095, matching the selected option. Why Other Options Are Wrong: 1.085, 1.075, 1.065, 1.000: These are close but do not match the derived ratio from the conveyance equality. 1.000 would only be correct if hydraulic radius equality were the sole criterion, which it is not here. Common Pitfalls: Equating only hydraulic radius instead of full conveyance A * R^(2/3). Using perimeters incorrectly (remember P_c = π D, P_s = 4 s). Final Answer: 1.095

Question 20

Septic tank operation and maintenance: What is the maximum recommended interval for removing digested sludge from a septic tank?

Accepted Answer

Introduction / Context: Septic tanks accumulate sludge (settled solids) and scum (floating matter). Timely desludging prevents carryover of solids and system failure. Design manuals often specify a maximum interval to guide households and operators. Given Data / Assumptions: Typical domestic loading conditions. Standard design retention and sludge storage assumptions. We seek a practical upper limit rather than a minimum frequency. Concept / Approach: While desludging frequency depends on tank size and usage, many guidelines limit the interval to about 1–3 years. Beyond that, sludge volume can reduce effective capacity and increase solids carryover, leading to clogging of soak pits or drainfields. Step-by-Step Solution: 1) Recognize that sludge accumulation is continuous, with stabilization happening anaerobically.2) To maintain performance, removal at or before a typical maximum of 3 years is recommended in many contexts.3) Choose the nearest option that represents a maximum recommended interval: 3 years. Verification / Alternative check: Operational experience shows that longer intervals increase risk of solids escape. Therefore, a conservative maximum interval of about 3 years is widely adopted where not otherwise specified by local codes. Why Other Options Are Wrong: 3.5, 4, 5 years: These exceed the typical safe maximum interval and risk operational problems.Every 6 months: Possible in heavy-use scenarios but not the maximum recommended interval; it is an overly frequent schedule for many domestic tanks. Common Pitfalls: Waiting for visible system failure before desludging. Assuming anaerobic digestion eliminates the need for removal; it only reduces, not eliminates solids. Final Answer: 3 years

Waste Water Engineering Questions