Question 1

In wastewater microbiology, facultative bacteria are organisms that can survive and remain active under which environmental condition(s)?

Accepted Answer

Introduction / Context: Understanding the ecological roles of microbes is central to designing biological treatment units and stabilization ponds. Facultative organisms bridge the gap between strict aerobes and strict anaerobes. Given Data / Assumptions: Typical municipal sewage with variable oxygen conditions across unit processes. Focus is on oxygen availability, not other nutrients. Concept / Approach: Facultative bacteria can switch metabolic pathways depending on oxygen availability. In aerated environments they respire aerobically; in oxygen-deficient zones they shift to nitrate or fermentative pathways as electron-acceptor availability allows. This adaptability is particularly important in facultative lagoons, which have an aerobic upper layer and an anaerobic bottom layer. Step-by-Step Solution: Identify the defining characteristic: survival and activity with or without oxygen.Exclude strict categories (only aerobic or only anaerobic).Select the option describing dual tolerance and adaptability. Verification / Alternative check: Operational experience shows facultative populations persisting across diurnal DO cycles and depth stratification, sustaining treatment performance despite fluctuating oxygen levels. Why Other Options Are Wrong: Strictly aerobic or strictly anaerobic descriptions contradict the definition of facultative organisms. Oxygen is the key variable; sunlight is not the defining requirement for bacteria discussed here. Common Pitfalls: Confusing facultative bacteria with anoxic denitrifiers or phototrophs; the term specifically addresses flexibility regarding oxygen presence. Final Answer: Both with oxygen and without oxygen (can adapt to either condition).

Question 2

In sludge digestion at about 21°C (mesophilic, cool conditions), the approximate duration of the acid regression stage is typically closest to which value?

Accepted Answer

Introduction / Context: Sludge digestion progresses through stages: hydrolysis, acid formation (acidification), acid regression, and methane fermentation. Temperature strongly affects reaction rates and stage durations. At lower mesophilic temperatures near 20–21°C, digestion slows markedly compared with operation near 35°C. Given Data / Assumptions: Temperature ≈ 21°C. Conventional single-stage anaerobic digestion regime. We seek a representative duration for the acid regression period. Concept / Approach: During acid regression, volatile acids formed earlier are consumed as methanogens establish dominance, alkalinity recovers, and pH stabilizes. At ~20–21°C, both acidogens and methanogens are slower, extending stage durations; typical practice quotes on the order of months rather than weeks for cool digestion. Step-by-Step Solution: Relate temperature to kinetics: cooler → slower → longer stage times.Industry experience puts acid regression roughly at multiple weeks under ~21°C conditions.Among the choices, 60 days is the closest representative duration. Verification / Alternative check: Design texts often show total digestion times at ~20°C roughly two to three times those at ~35°C. A 60-day regression period is consistent with elongated stabilization under cool operation. Why Other Options Are Wrong: 15 or 30 days: more typical of warmer digestion or partial stages. 90 or 120 days: excessively long for the regression stage alone in standard practice. Common Pitfalls: Confusing overall retention time with a single stage; not all plants operate at cool mesophilic conditions—some are heated to 35°C to shorten digestion times. Final Answer: 60 days.

Question 3

In a municipal trickling filter used for secondary treatment, which process is fundamentally responsible for the removal of organic matter?

Accepted Answer

Introduction / Context: Despite the name “trickling filter,” the dominant removal mechanism in this unit process is biological. Wastewater is sprinkled over a media bed that supports biofilm growth, effecting oxidation of dissolved and colloidal organics. Given Data / Assumptions: Trickling filters are fixed-film biological reactors. No chemical oxidants are normally dosed for core BOD removal. Concept / Approach: The attached biomass utilizes organic substrates and oxygen supplied by natural draft or forced aeration. While some incidental straining can occur, sustained BOD and ammonia removal arise from microbial metabolism across the biofilm thickness with periodic sloughing that is captured in secondary clarifiers. Step-by-Step Solution: Identify primary mechanism → biological oxidation by biofilm.Acknowledge minor filtration effects but note they are not dominant.Therefore, select biological action as the best answer. Verification / Alternative check: Performance correlations (e.g., NRC/Metcalf & Eddy equations) relate loading and biofilm kinetics, not straining capacity—further evidence that biology is primary. Why Other Options Are Wrong: Pure mechanical filtration does not explain soluble BOD removal. “Neither” conflicts with decades of operating experience. “Both in equal measure” overstates the role of physical straining. Chemical oxidation is not the basis of standard trickling filter operation. Common Pitfalls: Taking the word “filter” literally and assuming granular filtration is the key mechanism; it is an attached-growth bioreactor. Final Answer: Biological action by attached microbial biofilm.

Question 4

When establishing sewer alignments for a new drainage district, the recommended setting-out procedure generally starts from which location and proceeds upstream?

Accepted Answer

Introduction / Context: Accurate layout of sewer lines requires hydraulic continuity and feasible gradients. A practical field procedure is to start where the system must ultimately discharge and then work progressively upstream. Given Data / Assumptions: Gravity sewer system with a defined outfall to a treatment plant or water body. Gradients and cover must be maintained while avoiding deep excavations. Concept / Approach: Starting from the outfall ensures that downstream constraints (invert elevations, receiving works) are honored first. Designers can then establish feasible slopes and depths for upstream reaches, minimizing costly re-grading and conflict with utilities. Step-by-Step Solution: Fix the outfall invert and grade based on receiving structure requirements.Set upstream invert levels to maintain self-cleansing velocities and cover.Iterate through branches to maintain hydraulic continuity and avoid excessive depths. Verification / Alternative check: Field practice and surveying workflows commonly begin from benchmarks at the outfall, ensuring cumulative fall is adequate and conflicts at the discharge are avoided. Why Other Options Are Wrong: Starting at the city center or tail ends risks mis-matching levels at the outfall. “Any convenient point” ignores critical boundary conditions. Highest ground is not necessarily hydraulically controlling. Common Pitfalls: Neglecting downstream constraints first; this can force expensive redesigns or deep excavation later. Final Answer: From the outfall (discharge point) upstream toward the tail ends.

Question 5

For plain primary sedimentation tanks, what is a typical range of surface overflow rate (also called overflow loading) expressed in litres per square metre per day?

Accepted Answer

Introduction / Context: The surface overflow rate (SOR) is a primary sizing parameter for sedimentation tanks. It represents the flow per unit surface area and is linked to the theoretical particle settling velocity required for removal. Given Data / Assumptions: Plain primary sedimentation for municipal wastewater. Typical design at average temperatures and normal solids loading. Concept / Approach: For primary clarifiers, typical SOR values fall roughly in the 25,000–35,000 L/m²/day range (about 25–35 m³/m²/day). Designing within this band helps achieve reliable removal of settleable solids without excessive tankage or sludge blanket disturbance. Step-by-Step Solution: Select the range reflecting common practice for primary clarifiers.Avoid higher SORs that risk insufficient settling or scouring under peak flow.Thus, 25,000–35,000 L/m²/day is appropriate. Verification / Alternative check: Operational data across utilities show primary clarifiers meeting performance criteria with SORs near this band, adjusting for temperature and solids loading. Why Other Options Are Wrong: 40,000–60,000 and higher: more typical of secondary clarifiers under certain conditions or risk poor primary solids capture. 80,000–100,000: too high for primary sedimentation in most cases. None of these: incorrect because a standard range is provided. Common Pitfalls: Confusing primary and secondary clarifier SORs; neglecting peak-flow checks that may require additional area or flow equalization. Final Answer: 25,000 to 35,000 litres/sq m/day.

Question 6

Regarding asbestos-cement (AC) sewers, which statements accurately describe their characteristics and limitations in wastewater conveyance?

Accepted Answer

Introduction / Context: Material selection for sewers must consider weight, structural capacity, and corrosion resistance. Asbestos-cement (AC) piping has been used historically, but it has notable constraints in modern wastewater environments. Given Data / Assumptions: Sewer environment can produce hydrogen sulphide, which oxidizes to sulphuric acid on moist surfaces. Structural loads may include soil cover, traffic, and trench conditions. Concept / Approach: AC pipes are relatively lightweight, easing handling. However, their compressive and ring strengths are limited compared with reinforced concrete or ductile iron. In aggressive sewer atmospheres, biogenic sulphuric acid attacks cementitious matrices, leading to material degradation, softening, or loss of section over time. Step-by-Step Solution: Assess weight → AC is lighter than many alternatives, aiding installation.Assess structural capacity → limited ability to withstand high compressive and external loads.Assess corrosion resistance → susceptible to sulphuric acid in crown corrosion scenarios. Verification / Alternative check: Case histories document corrosion at the pipe crown where H₂S oxidizes to H₂SO₄; standards often recommend protective linings or alternative materials in high-sulphide systems. Why Other Options Are Wrong: Each individual statement (a–c) is accurate; hence (d) is correct. “None of the above” contradicts well-established material behavior. Common Pitfalls: Ignoring crown corrosion risk; assuming light weight implies adequate strength under deep burial or traffic loads without proper bedding and design checks. Final Answer: All of the above.

Question 7

Sewer network terminology: The sewer that carries the combined discharge from an entire collecting system and conveys it to the treatment plant is called what?

Accepted Answer

Introduction / Context: In municipal drainage and wastewater engineering, each conduit in a network has a specific function. Understanding names like house, lateral, branch, trunk, and outfall sewers is essential for planning, hydraulic design, and operations. This question checks recognition of the main carrier that finally delivers flow to the treatment plant. Given Data / Assumptions: A collecting system feeds progressively larger conduits. Flow is ultimately conveyed to a wastewater treatment plant (WWTP). Standard civil engineering terminology is used. Concept / Approach: A house sewer connects a building to a street sewer. A lateral sewer collects several house connections along a street. A branch or sub-main receives several laterals. A trunk/main carries multiple branches. The final large conduit that conveys all collected wastewater to the WWTP or point of disposal is termed the outfall sewer. Step-by-Step Solution: Trace the flow path: house sewer → lateral → branch/main → trunk.Identify the last link that leaves the collection system toward treatment.That terminal conduit is called the outfall sewer. Verification / Alternative check: Network schematics in standard texts consistently label the terminal conveyance to the plant as the outfall sewer, distinct from in-plant influent lines or force mains associated with pumping. Why Other Options Are Wrong: House, lateral, and branch sewers serve upstream collection; none is the terminal carrier to the WWTP. Common Pitfalls: Confusing the term “outfall sewer” with “outfall” (the discharge point). The outfall sewer is the pipe; the outfall is the location. Final Answer: Outfall sewer

Question 8

Sewer maintenance methods: Which statements about cleaning small, medium, and large sewers are correct?

Accepted Answer

Introduction / Context: Sewer operation and maintenance requires different methods depending on pipe size, access, and nature of deposits. Proper technique selection maintains hydraulic capacity, prevents blockages, and protects public health. Given Data / Assumptions: “Small”, “medium”, and “large” refer to typical municipal diameters. Deposits include grit, grease, roots, and organic solids. Access via manholes is available. Concept / Approach: Small sewers are often flushed using water jets or periodic surges. Medium sewers can be cleared using cane rodding or wooden pills (also called cleaning balls or pigs) that push or entrain deposits. Large sewers typically require entry or remote equipment to remove deposits using buckets, jetting trucks, or mechanical scrapers. Step-by-Step Solution: Classify sewer by size and accessibility.Match cleaning method: flushing for small; rodding or pills for medium; mechanical removal for large.Confirm that each statement matches standard practice. Verification / Alternative check: Utility SOPs and O&M manuals align with this tiered approach, adding modern tools like high-pressure jetting and CCTV inspections. Why Other Options Are Wrong: Options (a) to (d) are each correct individually; the best consolidated choice is (e) “All of the above”. Common Pitfalls: Using excessive flushing where surcharge risk exists; rodding fragile pipes; entering large sewers without proper confined-space procedures. Final Answer: All of the above

Question 9

Pressure in sewers: Identify the correct statements about internal pressure and its structural implications for pressure sewers.

Accepted Answer

Introduction / Context: Sewers are commonly gravity conduits, but some reach segments or rising mains operate under pressure. Understanding internal pressure and corresponding stresses is crucial to material selection and structural safety, especially for ductile iron, steel, or pressure-rated PVC/HDPE pipes. Given Data / Assumptions: Pipe runs full and is pressurized (e.g., a rising main). Internal pressure acts radially on the pipe wall. Pipe design follows tensile (hoop) stress criteria. Concept / Approach: For a thin-walled cylinder, hoop stress sigma_h ≈ p * D / (2 * t), where p is internal pressure, D diameter, t wall thickness. This tensile stress governs pressure rating. Materials and joints must resist tension; thrust blocks or restrained joints resist axial forces at bends and fittings. Step-by-Step Solution: Recognize that “internal pressure” is the pressure of sewage inside a full, pressurized pipe.Translate pressure to hoop (tensile) stress in the pipe wall.Conclude that pressure sewers are designed for tensile stresses, using pressure-rated pipes and joints. Verification / Alternative check: Manufacturers provide pressure classes tied to allowable hoop stress; design checks verify safety under surge and steady-state pressures. Why Other Options Are Wrong: Options (a), (b), and (c) are each correct; thus the comprehensive correct choice is (d). Common Pitfalls: Ignoring surge/transient pressures; underestimating axial forces at bends; using non-pressure-rated joints in pressure segments. Final Answer: All of the above

Question 10

Design conventions for gravity sewers: Which of the following statements are typically adopted in practice for size and velocity criteria?

Accepted Answer

Introduction / Context: Gravity sewers are sized not only for capacity but also for maintenance. Designers conventionally avoid flowing “just full” in larger sewers to leave air space, reduce odor issues, and accommodate surges, while ensuring self-cleansing velocities to limit sediment deposition. Given Data / Assumptions: Small pipes: < 0.4 m diameter. Large pipes: ≥ 0.4 m diameter. Target minimum velocity near 0.8 m/s for self-cleansing (textbook value ranges 0.6–0.9 m/s). Concept / Approach: At design peak, small pipes are often checked “running full” due to practical constraints. Larger pipes are checked at a fraction (about 2/3–3/4) of full depth to provide reserve capacity and ventilation. A minimum velocity criterion reduces grit accumulation and maintenance burden. Step-by-Step Solution: Select pipe diameter from discharge using Manning’s formula.Confirm depth of flow fraction (e.g., 0.67–0.75) for larger sewers at peak.Check velocity ≥ self-cleansing threshold (≈ 0.8 m/s) at design flow. Verification / Alternative check: Utility standards commonly list similar criteria; local codes may specify exact minimum velocities and depth fractions. Why Other Options Are Wrong: All three statements reflect standard practice; hence (d) “All of the above” is the most inclusive and correct option. Common Pitfalls: Ignoring future peak factors; undersized ventilation leading to septicity; relying on too low self-cleansing velocities for gritty catchments. Final Answer: All of the above

Question 11

Stoneware (vitrified clay) sewer sizes: Which of the following nominal sizes is NOT commonly adopted for stoneware sewers?

Accepted Answer

Introduction / Context: Stoneware (vitrified clay) pipes are widely used for house connections and smaller gravity sewers because of corrosion resistance. However, there are practical manufacturing and handling limits, so very large diameters are uncommon; larger mains are typically reinforced concrete, ductile iron, or other materials. Given Data / Assumptions: Standard nominal sizes often increase in steps (e.g., 75 mm or 7.5 cm). Upper practical limit for stoneware in many standards is around 60 cm internal diameter. Very large sizes become impractical to manufacture, transport, and lay. Concept / Approach: While 52.5 cm can occur in some step series, 67.5 cm and 82.5 cm are already atypical and 90 cm is generally beyond standard stoneware ranges, where other materials take over. Among the listed options, 90 cm is the clearest non-standard/unsuitable choice for stoneware in common practice, hence the best answer. Step-by-Step Solution: Recall typical stoneware diameter range (up to ≈ 60 cm).Compare listed sizes to the typical upper limit.Identify 90 cm as outside practical stoneware range. Verification / Alternative check: Procurement catalogs and standards for vitrified clay confirm limited maximum diameters; larger sizes rely on alternative materials. Why Other Options Are Wrong: 52.5 cm may exist in stepped series; 67.5 and 82.5 are unusual, but 90 cm is the most clearly beyond standard practice, making it the best incorrect size among the choices. Common Pitfalls: Assuming all listed step sizes exist for every material; ignoring internal vs. external diameter conventions. Final Answer: 90 cm

Question 12

Setting out a sewer trench: For a trench width of 2.0 m, at what typical distance from the center line is the offset line pegged parallel to it for setting out?

Accepted Answer

Introduction / Context: Setting out sewer trenches requires offset pegs/lines placed away from excavation to preserve reference after digging. The offset must clear the trench edge and provide safe working space for instruments and sighting rods. Given Data / Assumptions: Trench width = 2.0 m (measured at ground level). Offset line is parallel to the sewer center line. Allowance is needed beyond half trench width for safety and sighting clearance. Concept / Approach: Half trench width is 1.0 m. A practical additional clearance (0.5–0.6 m) is typically added to place offset pegs beyond collapse or interference zones. Thus, many field practices adopt ≈ 1.6 m from the centre line for a 2.0 m trench, ensuring stability and clear line-of-sight after excavation. Step-by-Step Solution: Compute half width = 2.0 / 2 = 1.0 m.Add typical clearance ~0.6 m for safe offset.Offset distance ≈ 1.0 + 0.6 = 1.6 m = 160 cm. Verification / Alternative check: Agency manuals often prescribe offset distances based on trench widths, soil conditions, and anticipated side slopes; values near 1.5–1.6 m are common for a 2.0 m trench. Why Other Options Are Wrong: 100–140 cm may be too close for safe preservation of pegs; 180 cm is workable but not the typical value indicated in many standard MCQs. Common Pitfalls: Placing offsets too close leading to loss during excavation; not recording offsets and references in the field book. Final Answer: 160 cm

Question 13

Manhole spacing vs. pipe size: If manholes are spaced about 300 m apart on a straight reach, the sewer diameter is most likely

Accepted Answer

Introduction / Context: Manhole spacing depends on sewer diameter, maintenance requirements, and equipment reach. Smaller sewers require frequent access due to cleaning needs; very large sewers permit longer spacing and sometimes human entry for inspection. Given Data / Assumptions: Straight alignment with no junctions or gradient changes. Typical utility guidance for spacing vs. size. 300 m is relatively long for routine access on small pipes. Concept / Approach: Typical spacing: 30–100 m for small sewers, increasing with size. A spacing of ~300 m suggests a very large conduit where maintenance can occur from fewer access points without compromising service. Hence, diameters exceeding 1.5 m are consistent with such spacing on straight reaches. Step-by-Step Solution: Compare given spacing (300 m) to usual ranges for small and large sewers.Infer that long spacing implies large diameter with reduced blockage risk.Select the size category “> 1.5 m”. Verification / Alternative check: Large combined or trunk sewers often have manholes or access shafts at wider intervals, supplemented by CCTV or walk-through inspections. Why Other Options Are Wrong: 0.9–1.5 m diameters commonly need closer spacing for cleaning access, especially in grit-laden systems. Common Pitfalls: Ignoring the need for manholes at bends/junctions regardless of straight-run spacing; confusing maximum code-allowed spacing with recommended practice. Final Answer: > 1.5 m

Question 14

Handling grit chamber output: Which use is NOT appropriate for grit and silt removed from grit chambers?

Accepted Answer

Introduction / Context: Grit chambers remove heavy inorganic particles (sand, gravel, silt) to protect downstream processes. Proper disposal or reuse must avoid structural risks and health hazards. This question asks which proposed use is unsuitable. Given Data / Assumptions: Material is mostly inorganic grit with possible organic contamination. No specialized washing or sanitization assumed. Reuse must not compromise structural integrity or hygiene. Concept / Approach: Concreting requires controlled aggregates meeting grading, cleanliness, durability, and strength requirements. Grit chamber outputs are neither graded nor clean enough and may introduce deleterious substances, compromising concrete performance. Conversely, regulated dumping to fill low-lying areas (with proper environmental controls) can be acceptable. Step-by-Step Solution: Identify characteristics of grit: variable size, contaminants.Compare to concrete aggregate specifications: washed, graded, strong, durable.Conclude that concreting is not appropriate; controlled landfilling or embankment filling may be possible under regulations. Verification / Alternative check: Concrete codes disallow dirty, organic-laden, or weak aggregates. Many utilities dispose grit at approved landfills or use it for non-structural fill under environmental safeguards. Why Other Options Are Wrong: “Raising low-lying areas” can be acceptable with permits; “both” and “neither” do not reflect the nuanced reality. Common Pitfalls: Using grit without testing; overlooking leachate control and vector attraction reduction for organic fractions. Final Answer: Concreting

Question 15

Settling under Stokes’ regime: For a spherical particle of diameter d < 0.1 mm settling in a fluid, which expression gives the terminal settling velocity according to Stokes’ law?

Accepted Answer

Introduction / Context: Primary sedimentation and grit removal design rely on predicting particle settling velocities. For very small spherical particles at low Reynolds number (laminar regime), Stokes’ law provides a simple analytical expression relating size, fluid properties, and density contrast to settling velocity. Given Data / Assumptions: Spherical particle with diameter d < 0.1 mm (small enough for laminar settling). Specific gravity of particle = Gs; fluid kinematic viscosity = nu. Flow conditions satisfy Stokes’ regime (Re <≈ 1). Concept / Approach: Stokes’ law (laminar settling) states the drag force equals weight minus buoyancy at terminal velocity. Solving the force balance yields v_s proportional to d^2 and inversely proportional to viscosity. The density term appears as (Gs − 1) because buoyancy subtracts the fluid density from particle density. Step-by-Step Solution: Write force balance at terminal velocity: drag = submerged weight.Drag (sphere) = 3 * pi * mu * d * v_s; convert mu to nu using mu = rho * nu.Submerged weight = (pi/6) * d^3 * (rho_s − rho) * g.Solve for v_s to obtain v_s = [ g * (Gs − 1) * d^2 ] / (18 * nu). Verification / Alternative check: Dimensional check: numerator has L/T^2 * L^2 = L^3/T^2; denominator has L^2/T; result is L/T (velocity), consistent. Why Other Options Are Wrong: Option (b) inverts the relation; (c) uses d^1 not d^2; (d) uses d^3, which applies to buoyant weight but not to velocity form. Common Pitfalls: Applying Stokes’ law beyond its Re limit; using kinematic vs dynamic viscosity inconsistently; neglecting flocculation that changes effective particle size and density. Final Answer: v_s = [ g * (Gs − 1) * d^2 ] / (18 * nu)

Question 16

Choice of sewerage system: In regions where light rains occur frequently and are fairly uniform through the year, which sewerage system is generally preferred?

Accepted Answer

Introduction / Context: Selection of a sewerage system (separate, combined, or partially combined) depends on rainfall pattern, topography, environmental regulations, and economics. The pattern and intensity of stormwater strongly influence the hydraulic and treatment implications of the chosen system. Given Data / Assumptions: Rainfall is light but frequent and fairly uniform through the year. Urban setting where storm runoff volumes are moderate. Historical textbook perspective on system choice (recognizing modern CSO controls). Concept / Approach: Where rainfall is frequent but light, the combined system historically offered simplicity—one network for both sanitary sewage and storm runoff—keeping velocities adequate and avoiding a dry-weather underutilized storm system. Modern practices may favor separate systems to avoid combined sewer overflows (CSOs), but the classical exam answer associates uniform, light rainfall with the combined system choice due to economy and adequate self-cleansing flows. Step-by-Step Solution: Identify rainfall regime: light and uniform → frequent small runoff events.Recognize that a combined system can efficiently convey both flows with stable velocities.Select “Combined system” per standard teaching references. Verification / Alternative check: Where heavy, sporadic storms occur, separate systems reduce peak loads to WWTPs; where storms are light and frequent, combined systems maintain flow in one conduit but must manage CSO impacts. Why Other Options Are Wrong: Separate system is favored for intense or infrequent storms; partially combined is a compromise but not the classical recommendation for the stated rainfall regime; “none” is incorrect. Common Pitfalls: Ignoring environmental constraints; overlooking CSO consent decrees; applying a one-size-fits-all rule without local analysis. Final Answer: Combined system

Question 17

In municipal hydrology for storm-water design, the intensity of rainfall should be reported in which standard unit? (Assume conventional practice for intensity–duration–frequency curves and sewer design.)

Accepted Answer

Introduction / Context: Rainfall intensity is a fundamental input for storm-water drainage, roof drain sizing, and combined sewer overflow analysis. Designers typically read intensity from intensity–duration–frequency (IDF) curves. Using a standard and consistent unit avoids conversion errors that can drastically affect calculated peak flows and conduit sizes. Given Data / Assumptions: Conventional civil engineering practice for urban drainage. Intensity is defined as rainfall depth divided by the time over which it falls. No special regional unit system is imposed in the question; metric is used. Concept / Approach: Intensity I is depth/time. In metric hydrology, intensity is most commonly expressed as millimetres per hour or centimetres per hour. This aligns with typical storm durations (5–180 minutes) and allows easy integration with runoff methods (e.g., rational method where I is in consistent units with C and A). Step-by-Step Solution: Define intensity: I = rainfall depth / duration.Select a practical time base: hours are commonly used for IDF curves.Choose a depth unit suited to typical storm depths: millimetres or centimetres. The option provided that matches standard practice is cm/hour. Verification / Alternative check: Most IDF tables are given in mm/h (equivalent to cm/h divided by 10). Converting between mm/h and cm/h is straightforward and keeps dimensional consistency in runoff equations. Why Other Options Are Wrong: cm/minute: Too fine a time base; uncommon and invites errors. cm/day: Appropriate for hydrology at basin scale, not urban drainage peaks. mm/second and litres/m²·hour: Nonstandard or awkward for IDF use in sewer design. Common Pitfalls: Mixing mm/h and mm/min in the rational formula; misreading IDF charts drawn for specific return periods; failing to convert if rainfall records are in different units. Final Answer: cm/hour

Question 18

In onsite sanitation planning, what is the typical rate of accumulation of digested sludge per person per year to be used for preliminary sizing (at ambient Indian conditions)?

Accepted Answer

Introduction / Context: Accurate estimates of sludge accumulation are essential for sizing septic tanks, sludge drying beds, and desludging schedules. Underestimating accumulation causes premature tank failure and overflows; overestimating wastes capital and land. Given Data / Assumptions: Domestic wastewater with typical per-capita wastewater generation. Digested (stabilized) sludge volume, not raw sludge. Indian climate with warm temperatures aiding digestion. Concept / Approach: Standard design handbooks recommend using a per-capita annual digested sludge volume for preliminary sizing. For warm climates, a typical planning value is around 0.02 m³/person·year (that is, 20 litres/person·year). This incorporates solids reduction during anaerobic digestion and excludes excessive scum or grit allowances, which are handled separately. Step-by-Step Solution: Identify recommended planning value for digested sludge accumulation.Convert cubic metres to litres: 0.02 m³ = 20 L.Select the option matching 20 litres/person·year. Verification / Alternative check: Cross-check with typical design ranges (15–30 L/person·year). The central tendency of 20 L/person·year is widely used for first-cut design and then refined with local data. Why Other Options Are Wrong: 10 or 15 L: May under-size tanks, risking frequent desludging. 25 or 30 L: Conservative; may be used in colder climates or high solids loads but not the common baseline. Common Pitfalls: Confusing raw sludge volume with digested sludge; ignoring allowances for scum storage and grit; not adjusting desludging interval assumptions, which changes required storage volume. Final Answer: 20 litres

Question 19

For computing mean velocity of flow in sanitary sewers with known slope and roughness, which empirical relation is most widely used in modern practice?

Accepted Answer

Introduction / Context: Hydraulic design of gravity sewers requires a reliable formula linking velocity, hydraulic radius, slope, and roughness. Among several historical relations, one has become the de facto standard in sewer design manuals. Given Data / Assumptions: Open-channel (partially full) or full-flow gravity conditions. Steady, uniform flow assumption for sizing. Use of a roughness parameter calibrated for pipe material. Concept / Approach: Manning's formula expresses velocity as V = (1/n) * R^(2/3) * S^(1/2), where n is Manning's roughness coefficient, R is hydraulic radius, and S is slope of energy grade line. It is simple, dimensionally consistent in SI, and supported by abundant tabulations of n for sewer materials. Step-by-Step Solution: Identify the design need: mean velocity under steady, uniform flow.Select formula with widely available roughness data: Manning's.Apply for partial flow by using actual wetted area and perimeter to compute R. Verification / Alternative check: Chezy, Kutter, and Bazin are predecessors and can be interrelated, but modern codes almost universally present charts/tables in terms of Manning's n for pipes and channels. Why Other Options Are Wrong: Chezy/Kutter/Bazin: Historically important but less convenient; require coefficients that vary with R and S. Williams–Hazen: Suited to pressurized water mains; not ideal for open-channel sewer hydraulics. Common Pitfalls: Using inappropriate n values; forgetting that sediment or slime layers increase effective roughness; applying full-flow hydraulic radius to partial-flow conditions. Final Answer: Manning's formula

Question 20

For a separate sewerage system carrying only sanitary sewage under gravity, which cross-sectional shape is generally the most hydraulically efficient and economical to construct and maintain?

Accepted Answer

Introduction / Context: Choosing a sewer cross-section affects construction cost, self-cleansing ability, and maintenance. In separate systems (without storm flow), the shape must perform well over a broad range of partial-flow depths. Given Data / Assumptions: Separate sewerage system (no storm runoff). Gravity flow, steady uniform design condition. Standard construction methods and materials. Concept / Approach: Circular sections are structurally efficient (uniform ring compression under earth loads), economical to fabricate, and hydraulically satisfactory. They provide reasonable velocities at partial depths and are easy to clean and inspect. Egg-shaped sections are advantageous for combined systems with large flow variation; however, for separate sanitary sewers, circular pipes dominate. Step-by-Step Solution: Assess hydraulic efficiency: circular sections have high hydraulic radius for a given area, especially near half-full.Consider structural and construction aspects: circular pipes are easy to produce and strong in ring compression.Select the widely adopted option: circular. Verification / Alternative check: Most municipal standards specify circular PVC, vitrified clay, concrete, or ductile iron pipes for sanitary sewers; noncircular shapes appear mainly in large combined sewers or special cases. Why Other Options Are Wrong: Rectangular/parabolic/trapezoidal: Rare for buried sanitary sewers due to structural and maintenance drawbacks. New-egg: Useful for combined flow variation, not necessary for separate sanitary flows. Common Pitfalls: Assuming egg-shaped is always superior; in separate systems it adds cost/complexity with limited benefit. Final Answer: Circular

Waste Water Engineering Questions