Question 1

Design sewage flow for a separate system: A township has 40 hectares at a density of 250 persons/hectare. Water demand is 200 L/person/day and sewage flow is 80% of water supply. What peak sewage discharge (assuming peak factor ≈ 3 for separate sani…

Accepted Answer

Introduction / Context: Sanitary sewers in a separate system are sized for peak sanitary flows, often using a peaking factor to account for diurnal variations. Converting population and per-capita water demand to peak sewage discharge is a standard design exercise. Given Data / Assumptions: Area = 40 ha; density = 250 persons/ha → population = 10000. Water demand = 200 L/person/day. Sewage flow = 80% of water use (average). Use peak factor ≈ 3 for separate sanitary sewer design. Concept / Approach: Compute average sewage flow from population and per-capita demand, then apply a peaking factor to obtain design peak discharge. Finally, convert daily volume to flow rate in cubic meters per second. Step-by-Step Solution: Population P = 40 * 250 = 10000 persons.Average water use = 10000 * 200 L/day = 2,000,000 L/day = 2000 m^3/day.Average sewage = 0.80 * 2000 = 1600 m^3/day.Peak sewage (daily basis with factor 3) = 3 * 1600 = 4800 m^3/day.Convert to cumec: Q = 4800 / 86400 = 0.05556 m^3/s ≈ 0.0556 cumec. Verification / Alternative check: Back-calculation: 0.0556 * 86400 ≈ 4800 m^3/day → matches peak volume computed. Why Other Options Are Wrong: 0.05552 and 0.05554 are rounding variations but 0.0556 is the conventional rounded value; 0.0558 overstates; 0.0450 underestimates (would correspond to a smaller peaking factor). Common Pitfalls: Forgetting to multiply by the peaking factor; mixing average and peak flows; omitting the liters-to-cubic-meters or day-to-seconds conversions. Final Answer: 0.0556 cumec

Question 2

Where are “sludge banks” most likely to form if raw or insufficiently treated sewage is disposed of into natural water bodies?

Accepted Answer

Introduction / Context: Sludge banks are accumulations of settleable solids on the bed near outfalls. They are an environmental hazard, causing odor, anaerobic conditions, and ecological damage. Understanding where they most commonly occur informs outfall design and treatment requirements. Given Data / Assumptions: Discharge of raw/partially treated sewage via marine outfalls. Settling and deposition are possible near the outfall zone. Shallow coastal conditions with limited dispersion may exist. Concept / Approach: Marine (sea) outfalls, especially in shallow or low-energy coastal waters, are prone to deposition if outfall length, diffuser design, or treatment level are inadequate. Persistent deposition forms “sludge banks.” While lakes and rivers can also experience deposits, classical sanitary engineering terminology frequently associates “sludge banks” with marine outfalls that fail to provide adequate dilution and dispersion. Step-by-Step Solution: Consider receiving body hydrodynamics (tides, waves, currents).Evaluate settleable solids fraction and near-field mixing.Identify most cited context for sludge bank formation: marine outfalls in seas. Verification / Alternative check: Historical cases of coastal pollution show sludge accumulations down-current of inadequately designed ocean outfalls. Why Other Options Are Wrong: Rivers and lakes can accrue sediments, but the term “sludge bank” is most commonly applied to marine coastal deposits near sewage outfalls. Common Pitfalls: Confusing general sedimentation in lakes/rivers with specific sewage sludge banks; ignoring the role of diffuser depth and current regime. Final Answer: seas

Question 3

Safety testing in sewer manholes: The lead acetate paper test is conducted to detect the presence of which hazardous gas before entry and maintenance operations?

Accepted Answer

Introduction / Context: Confined-space entry into sewer manholes requires pre-entry atmospheric testing for toxic and asphyxiating gases. A classic field indicator is the lead acetate paper test, commonly referenced in public health engineering and maintenance manuals as a quick screen for certain sewer gases. Given Data / Assumptions: Sewer manhole atmosphere may contain a mixture of gases (H₂S, CH₄, CO₂, low O₂). Lead acetate paper darkens in the presence of hydrogen sulphide. The test is qualitative, not a replacement for calibrated gas detection meters. Concept / Approach: Hydrogen sulphide (H₂S) produced by anaerobic decomposition reacts with lead acetate to form lead sulphide, causing a characteristic brown/black discoloration. This quick check alerts workers to a toxic environment requiring ventilation and continuous monitoring before entry. Step-by-Step Solution: Expose lead acetate strip to manhole air near the opening without entering.Observe for darkening indicating PbS formation due to H₂S.If darkening occurs, ventilate thoroughly and verify with a multi-gas meter (O₂, H₂S, LEL, CO).Proceed only after readings are within safe limits and PPE/permits are in place. Verification / Alternative check: Confirm using an electronic multi-gas detector with specific H₂S sensor and alarms; cross-check oxygen levels and explosive limits to ensure comprehensive safety. Why Other Options Are Wrong: Methane (CH₄) is flammable but does not blacken lead acetate; carbon dioxide (CO₂) is asphyxiant without this reaction; diesel vapours are not targeted by the lead acetate test. Common Pitfalls: Relying solely on the paper test; ignoring oxygen deficiency; skipping ventilation; entering without a confined-space permit and attendant. Final Answer: Hydrogen sulphide (H₂S)

Question 4

Resource recovery in wastewater management: After appropriate treatment and drying, what is a common beneficial use of dried sewage (biosolids)?

Accepted Answer

Introduction / Context: Modern sanitation emphasizes circular economy principles where treated wastewater and solids can be beneficially reused. Dried, stabilized sewage sludge (biosolids) can return organic matter and nutrients to soils when processed to meet safety standards. Given Data / Assumptions: Biosolids are properly stabilized (e.g., anaerobic digestion), dewatered, and dried. Pathogen reduction and vector attraction reduction meet local regulations. Land application aligns with agronomic nutrient rates and soil needs. Concept / Approach: Biosolids contain nitrogen, phosphorus, potassium, and organic carbon that improve soil fertility and structure. When processed, they can be applied as fertilizer/soil conditioner for agriculture, landscaping, and land reclamation, replacing a portion of synthetic fertilizers. Step-by-Step Solution: Stabilize sludge to reduce pathogens and odors (e.g., digestion, composting, thermal drying).Dewater and dry to produce a granular or cake product.Test for metals, pathogens, and nutrient content; classify per standards.Apply to land at rates matching crop nutrient uptake and soil constraints. Verification / Alternative check: Soil and tissue testing verifies nutrient uptake and confirms no exceedance of metal limits; audit application records and environmental monitoring. Why Other Options Are Wrong: (b) Structural building materials require different mechanical properties and certification; (c) biosolids are not dosing chemicals for BOD control; (d) paints/coatings do not use biosolids as base material. Common Pitfalls: Applying without nutrient management planning; using unstabilized sludge; ignoring local regulations and public acceptance. Final Answer: Fertilizer for soil conditioning and nutrient return

Question 5

Sewer design practice: What is the typical upper limit of sewer diameter generally adopted in urban designs before transitioning to multiple barrels or alternative conveyance methods?

Accepted Answer

Introduction / Context: Very large sewers become difficult to construct, ventilate, and maintain. Designers often cap single circular sewers at a practical maximum and then consider parallel barrels, boxes, or tunnels for additional capacity and redundancy. Given Data / Assumptions: Urban gravity sewer systems with man entry considerations. Economic and safety constraints drive practical diameter limits. Beyond certain sizes, special tunnel construction methods are preferred. Concept / Approach: A diameter around 3.0 m is a common practical upper bound in many municipal design guides, after which multiple conduits or tunnel solutions are evaluated. This balances hydraulics, constructability, ventilation, and maintenance access, though local practice may vary. Step-by-Step Solution: Estimate peak flows and select a trial diameter via Manning’s equation.Check velocities, self-cleansing, and surcharge limits.If diameter approaches 3.0 m, assess alternative configurations: twin barrels, boxes, or tunnel sewers.Evaluate O&M access, ventilation, and safety for confined spaces. Verification / Alternative check: Value engineering compares cost, risk, and lifecycle considerations for a single oversized barrel versus dual conduits or tunnels. Why Other Options Are Wrong: 1.0 m and 2.0 m are routinely exceeded in trunk sewers; 4.0 m is uncommon as a standard circular sewer due to constructability and O&M challenges. Common Pitfalls: Ignoring maintenance access; oversizing a single barrel instead of providing redundancy; inadequate ventilation strategies. Final Answer: 3.0 m

Question 6

Pipeline joints in sanitary engineering: Lead-caulked bell-and-spigot joints are characteristically used with which type of pipe during sewer or water main installation?

Accepted Answer

Introduction / Context: Choosing the correct jointing method is critical for the watertightness and durability of pressure and gravity pipelines. Traditional cast iron mains frequently used bell-and-spigot ends sealed with lead and oakum, known as lead-caulked joints. Given Data / Assumptions: Traditional municipal water/sewer installations. Compatibility between joint material and pipe material is required. Lead-caulked joints are historical but still referenced in legacy systems. Concept / Approach: Lead-caulked joints involve packing oakum and pouring molten lead into the bell space, then caulking to seal. This method is specific to metallic bell-and-spigot joints—principally cast iron—providing flexibility and watertightness. Stoneware/earthenware generally use cement or rubber-ring joints; G.I. uses threaded or flanged joints. Step-by-Step Solution: Identify pipe type: cast iron with bell-and-spigot ends.Insert oakum, pour lead, and caulk to form a tight seal.Test for leaks and ensure alignment.Apply corrosion protection as required. Verification / Alternative check: Hydrostatic pressure testing confirms joint integrity; modern alternatives include push-on rubber gasket joints (Tyton) in ductile iron pipes. Why Other Options Are Wrong: Stoneware/earthenware use cement/rubber-ring joints; G.I. employs threaded/flanged joints—lead caulking is not used. Common Pitfalls: Poor caulking causing leaks; inadequate safety when handling molten lead; lack of flexibility in non-metallic materials. Final Answer: Cast iron (C.I.) pipes

Question 7

Planning horizon in wastewater projects: The typical design period adopted for sewage treatment works (process units and appurtenances) is generally in which range?

Accepted Answer

Introduction / Context: Infrastructure planning uses a rational design period (planning horizon) that balances capital cost, population growth, technology change, and O&M. For sewage treatment plants (STPs), civil works may last longer, but process units are typically sized for medium-term projections with modular expansion capability. Given Data / Assumptions: Urban STP with staged expansion possible. Population and flow projections carry uncertainty beyond two decades. Equipment life cycles are often shorter than civil structures. Concept / Approach: Design periods around 15–20 years are widely adopted for STPs, allowing cost-effective phasing, incorporation of improved treatment technologies, and adaptation to regulatory changes. Trunk sewers and outfalls may use longer horizons; pumping and process equipment often have shorter replacement cycles. Step-by-Step Solution: Project population and flows for 20-year horizon with interim stages.Size primary, secondary, and tertiary units to meet future standards.Plan modular expansion to defer capital until needed.Schedule equipment renewals within the design horizon. Verification / Alternative check: Compare with national/state design manuals for recommended horizons; run sensitivity analyses for growth scenarios. Why Other Options Are Wrong: (a) 5–10 years is too short for major civil investments; (c) and (d) are long and risk overbuilding or technological obsolescence. Common Pitfalls: Ignoring modularity; not allowing for stricter effluent norms; neglecting infiltration/inflow growth. Final Answer: 15 to 20 years

Question 8

Water quality and stream sanitation basics: Identify the correct statements about dilution, self-purification, photosynthesis, and grab sampling in wastewater studies.

Accepted Answer

Introduction / Context: Sanitation engineering combines laboratory testing with stream processes and basic biology. Clear terminology ensures correct interpretation of field and lab results when assessing wastewater impacts and treatment performance. Given Data / Assumptions: Dilution and receiving water behavior influence observed concentrations. Self-purification integrates biodegradation and re-aeration. Photosynthesis by algae/macrophytes requires light. Grab sampling aims for a representative snapshot with adequate mixing. Concept / Approach: Each statement reflects standard practice: dilution factor compares volumes; self-purification describes natural recovery; photosynthesis is light-dependent; and for quickly collected grab samples, locations with turbulence (surface/baffles/mixers) are often chosen to better represent the bulk flow at that instant (site-specific judgment still applies). Step-by-Step Solution: Define dilution factor = V_diluent / V_sewage for bioassays or mixing studies.Recognize stream self-purification via BOD decay and atmospheric re-aeration.Acknowledge photosynthesis depends on solar radiation; depth and turbidity limit it.Select grab sample point where turbulence improves representativeness; avoid dead zones. Verification / Alternative check: Use composite sampling for time-averaged quality; apply DO sag modeling to validate self-purification behavior. Why Other Options Are Wrong: In this set, all individual statements are valid; therefore “All the above” is correct. Common Pitfalls: Confusing dilution with decay; sampling in unmixed zones; interpreting photosynthetic DO spikes as sustained recovery. Final Answer: All the above

Question 9

Peaking factors for sewer design: For sewer mains of 0.5 m to 1.0 m diameter, what ratio is commonly assumed for maximum daily sewage flow to average daily sewage flow?

Accepted Answer

Introduction / Context: Sewer hydraulics must accommodate diurnal and seasonal peaks. A simple way to reflect daily peaking is to multiply the average daily flow by a peaking factor that depends on catchment size and network characteristics. Given Data / Assumptions: Medium-sized mains (0.5–1.0 m diameter) serving urban populations. Average daily flow known from per capita contributions. Daily peaking factor chosen per local guidelines. Concept / Approach: For the stated mains, a commonly adopted planning value is 2.0 for max daily/average daily. Larger systems with population averaging may have lower ratios; smaller systems may experience higher peaks. Final design should also consider hourly peaking and infiltration/inflow allowances. Step-by-Step Solution: Compute average daily flow from population and unit rates.Apply daily peaking factor PF_day ≈ 2.0 → Q_max,day = 2.0 * Q_avg,day.Check hourly peak factor for capacity and surcharge risk.Verify velocities for self-cleansing under peak and low flows. Verification / Alternative check: Compare with local standards (e.g., Harmon/Metcalf recommendations) and adjust based on observed flow monitoring where available. Why Other Options Are Wrong: 1.5 may understate peaks; 2.5 or 3.0 are more typical for smaller basins or for hourly peaking rather than daily. Common Pitfalls: Applying hourly factors to daily sizing; ignoring I/I contributions; neglecting future growth. Final Answer: 2.0

Question 10

Sewer network terminology: The conduit that receives and conveys combined flows from two or more main sewers toward the outfall is called a…

Accepted Answer

Introduction / Context: Consistent terminology helps in planning and communicating sewer layouts. Networks generally progress from laterals to mains, then to trunks, and finally to the outfall that discharges to treatment or disposal. Given Data / Assumptions: Two or more main sewers discharge into a larger downstream conduit. The downstream conduit is not yet the final outfall to the treatment works. Gravity system with hierarchical branching. Concept / Approach: A trunk sewer is the large collector receiving flows from several main sewers and conveying them toward the system’s outfall. The outfall sewer is the terminal conduit that carries sewage to the treatment plant. An intercepting sewer diverts combined sewer overflows or intercepts existing outfalls to a centralized treatment facility. Step-by-Step Solution: Identify upstream conduits as mains collecting from laterals.Recognize the larger downstream collector as the trunk.Trace the trunk to the outfall that connects to the treatment works.Note special roles of interceptors for CSO control. Verification / Alternative check: Check system maps and design notes; the outfall is typically singular and directly connected to the plant headworks. Why Other Options Are Wrong: (b) Outfall is the final conduit to treatment; (c) a main is smaller and upstream; (d) interceptor serves diversion/control functions. Common Pitfalls: Using “outfall” generically for any large sewer; confusing interceptor with trunk in separate sewer systems. Final Answer: Trunk sewer

Question 11

Stoneware (vitrified clay) sewers: Identify the correct statements about uses, manufacture, firing, and glazing treatments applied during production.

Accepted Answer

Introduction / Context: Stoneware (vitrified clay) pipes are widely used for small-diameter gravity sewers because of their chemical resistance, smooth glazed interior, and longevity when properly installed and bedded. Given Data / Assumptions: Small to medium house connections and laterals in sanitary sewers. Pipe manufacturing includes forming, drying, firing, and glazing steps. Glazing improves hydraulic smoothness and reduces infiltration/exfiltration risks. Concept / Approach: Selected clays/shales are formed into pipe shapes, dried to remove free moisture, and fired to achieve vitrification. Salt glazing (introducing sodium chloride into the kiln atmosphere) creates a hard, glossy, impervious finish. These properties make stoneware well-suited to house drainage where corrosion resistance to domestic sewage is essential. Step-by-Step Solution: Form the pipe (extrusion or molding) to required size with socket/spigot ends.Dry at low temperatures (~150°C) to remove moisture and avoid cracking.Fire through intermediate (~700°C) to high temperatures (approaching ~1200°C) for vitrification.Apply salt glazing to achieve a durable, impermeable surface as required. Verification / Alternative check: Quality control includes dimensional checks, ring crush tests, absorption limits, and hydrostatic leak tests to verify vitrification and glazing integrity. Why Other Options Are Wrong: Options (a) through (d) are accurate descriptors; therefore the comprehensive correct choice is “All the above”. Common Pitfalls: Improper bedding and backfilling causing cracking; using inappropriate jointing materials; ignoring differential settlement at house connections. Final Answer: All the above

Question 12

Egg-shaped sewers (new section): If D denotes the diameter of the upper circular portion, what is the overall vertical depth of the “new egg-shaped” sewer section expressed as a multiple of D?

Accepted Answer

Introduction / Context: Egg-shaped sewer sections are used where wide flow variation is expected, because the shape gives better velocity at low flows and adequate capacity at peak flows. The “new egg-shaped” profile has standardized proportions, and designers often need the overall depth in terms of the upper circular diameter D. Given Data / Assumptions: Section is the standardized “new egg-shaped” sewer. D denotes the diameter of the upper circular portion (the crown). We seek the overall vertical depth as a multiple of D. Concept / Approach: Standard proportion charts (commonly used in sewer design handbooks) provide fixed geometric ratios for old and new egg-shaped sections. The new egg-shaped section is slightly “deeper” than the old type to enhance low-flow hydraulics. The overall depth is tabulated relative to the crown diameter D and does not vary with hydraulic conditions; it is purely geometric. Step-by-Step Solution: Identify that the problem asks a geometric ratio, independent of flow.Refer to the standardized new egg-shaped geometry, where overall depth is set by proportion to the crown diameter.From standard proportions, overall depth ≈ 1.625 D. Verification / Alternative check: Comparing with the old egg-shaped section (shallower), the new egg-shaped overall depth is larger, in line with the 1.625 D standard ratio used in many civil engineering references and competitive examination keys. Why Other Options Are Wrong: 1.250 D, 1.350 D, and 1.425 D are too small for the new egg-shaped profile; 1.500 D corresponds to other egg-shaped conventions or rules of thumb, not the standardized new egg-shaped geometry. Common Pitfalls: Confusing “old” versus “new” egg-shaped proportions; mixing up the crown diameter with equivalent circular diameter for capacity; or assuming ratios vary with hydraulic slope (they do not—these are geometric). Final Answer: 1.625 D

Question 13

Grit chamber operations: For a plant handling about 4.5 million litres per day (MLD), what is the preferred method of removing accumulated grit?

Accepted Answer

Introduction / Context: Grit chambers remove heavy inorganic particles (sand, silt, gravel) to protect pumps and downstream units. The method of grit removal depends on plant size and flow. At higher flows, manual removal becomes unsafe and inefficient, so mechanical methods are preferred. Given Data / Assumptions: Design flow ≈ 4.5 MLD (borderline of small-to-medium works). Standard municipal grit (specific gravity around 2.65) and conventional chambers. Continuous operation with regular cleaning requirements. Concept / Approach: Design guidelines generally recommend manual removal only for very small works (lower flows). Around 4.5 MLD and above, mechanical grit removal—such as conveyor screws, bucket elevators, or air-lift systems integrated with classifiers—is commonly adopted for reliability, safety, and labor efficiency. Step-by-Step Solution: Assess plant size: ~4.5 MLD is beyond very small systems.Balance labor/safety with throughput: mechanical removal is favored at this scale.Select mechanical removal as the appropriate method. Verification / Alternative check: Operation and maintenance manuals show that consistent grit quantities at these flows require mechanized handling to avoid accumulation and odor issues, and to maintain stable hydraulics. Why Other Options Are Wrong: Manual removal becomes impractical at this throughput; “electrical” as a separation method is not applicable; purely hydraulic flushing does not remove grit from the facility; air-lift pumps by themselves are components of mechanical systems, not the complete method in lieu of mechanical handling. Common Pitfalls: Underestimating grit volumes; ignoring safety; deferring cleaning until deposition affects velocities and retention time. Final Answer: Mechanically (mechanical grit removal)

Question 14

Circular sewer at partial flow: If the central angle at the pipe's centre subtended by the water surface is α (radians), what is the ratio of flow depth at partial flow (y) to full depth (D)?

Accepted Answer

Introduction / Context: For partially flowing circular sewers, geometric relations link depth, wetted perimeter, and cross-sectional area to the central angle at the center of the pipe. These relationships are crucial for computing hydraulic radius and velocity by Manning’s or Chezy’s equations. Given Data / Assumptions: Circular pipe, radius R = D/2. Liquid segment subtends central angle α (radians) at the pipe centre. We require the ratio y/D, where y is flow depth and D is the full diameter. Concept / Approach: The geometric relation for a circular segment gives the depth from crown or invert using the half-angle. If we denote the half-angle by θ, then α = 2θ. The depth measured from the lowest point (invert) is y = R(1 - cos θ). Substituting θ = α/2 gives y = R[1 - cos(α/2)]. Because D = 2R, the required ratio is y/D = [1 - cos(α/2)] / 2. Step-by-Step Solution: Let R = D/2 and α = 2θ.Depth from invert: y = R(1 - cos θ).Replace θ with α/2: y = R[1 - cos(α/2)].Divide by D = 2R: y/D = (1 - cos(α/2)) / 2. Verification / Alternative check: At α = π (half-full), α/2 = π/2 → cos(π/2) = 0, so y/D = 1/2 (correct). At α approaching 0, y/D → 0 (consistent). Why Other Options Are Wrong: Using α instead of α/2 or adding instead of subtracting cos terms leads to incorrect depth ratios. The sine expression does not represent the vertical depth relation for a circular segment. Common Pitfalls: Mixing degrees and radians; confusing α with its half-angle; using diameter in place of radius in intermediate steps. Final Answer: y/D = (1 - cos(α/2)) / 2

Question 15

Residential colony sewers of 100 mm, 150 mm, and 225 mm diameters are all laid at a gradient of 1 in 120. Which size is least likely to choke in due course (i.e., maintains better self-cleansing at this slope)?

Accepted Answer

Introduction / Context: Sewer self-cleansing depends on achieving sufficient shear stress at the pipe invert to transport solids. For a given slope, larger diameters tend to have larger hydraulic radii, affecting velocity and shear. Designers choose sizes and slopes to limit deposition and choking. Given Data / Assumptions: Common gradient: S = 1/120 for all three pipes. Same roughness and similar flow regime expectations. Comparative question: which size is least likely to choke at this slope. Concept / Approach: With Manning’s relation, V ∝ R^(2/3) * S^(1/2) for a given roughness. At the same slope, increasing diameter increases hydraulic radius R for partially full flows typical in sewers. Greater R generally gives higher velocity and bed shear, improving self-cleansing conditions and reducing choking risk, all else equal. Step-by-Step Solution: Compare hydraulics at constant slope: larger D → larger R.Higher R increases V for the same n and S.Therefore the 225 mm pipe is least likely to choke. Verification / Alternative check: Practical experience confirms that very small sewers (e.g., 100 mm) are prone to deposition unless steeper slopes or frequent connections (creating turbulence) are present. Why Other Options Are Wrong: 100 mm and 150 mm diameters have lower hydraulic radii and are more vulnerable to deposition at the same slope; “All of these” is incorrect because size influences self-cleansing; “None” is also incorrect. Common Pitfalls: Assuming velocity is independent of pipe size at the same slope; ignoring partial-flow behavior and solids transport criteria. Final Answer: 225 mm diameter

Question 16

Select the correct statements about circular sewer sections and hydraulic equivalence.

Accepted Answer

Introduction / Context: Choosing sewer cross-sections involves hydraulic efficiency, constructability, and operation at part-full conditions. Understanding key properties of circular sections and the concept of hydraulic equivalence helps make informed design decisions. Given Data / Assumptions: Gravity flow, turbulent regime is typical. Standard Manning/Chézy design practices. Comparative statements on shape efficiency and equivalence. Concept / Approach: (a) For a given perimeter, the circle maximizes area and thus hydraulic mean depth R = A/P; it is highly efficient hydraulically. (b) In separate systems (only sewage), circular pipes are ubiquitous due to ease of manufacture and laying. (c) For part-full operation, circular pipes perform well above about half depth, ensuring adequate velocities for self-cleansing. (d) Hydraulic equivalence means that if two different shapes, running full, have the same discharge under the same slope and roughness, they can be considered equivalent in capacity for that condition. Step-by-Step Solution: Evaluate each statement on accepted hydraulic principles and practice.All four statements match conventional design knowledge.Therefore, “All the above” is correct. Verification / Alternative check: Design manuals and standard texts endorse circular pipes for separate systems; they also define hydraulic equivalence and the advantages of circular geometry for R and capacity. Why Other Options Are Wrong: Selecting any one item would ignore the broader truth of the others; there is no contradiction among the statements. Common Pitfalls: Confusing “running full” capacity with “part-full” performance; overlooking that egg-shaped sections can outperform circular at very low flows but are costlier. Final Answer: All the above

Question 17

Wastewater analysis: Kjeldahl nitrogen (TKN) is defined as the sum of which nitrogenous forms present in the sample?

Accepted Answer

Introduction / Context: Kjeldahl nitrogen (often reported as TKN) is a key parameter in sewage and sludge analysis. It is used to estimate the nitrogen that will exert oxygen demand during biological treatment and to size nutrient removal processes. Given Data / Assumptions: Standard digestion and analysis procedures for TKN. Oxidized forms (nitrite, nitrate) are considered separately. Concept / Approach: TKN includes organic nitrogen and ammonia nitrogen. During Kjeldahl digestion, organic nitrogen converts to ammonium, which is then measured. Nitrite and nitrate are not included in TKN unless reduced to ammonia prior to analysis (modified methods); molecular nitrogen (N2) is not part of TKN. Step-by-Step Solution: Identify the nitrogenous species quantified by TKN.Exclude oxidized nitrogen (NO2-, NO3-) unless reduced in special methods.Therefore, TKN = organic N + ammonia N. Verification / Alternative check: Standard Methods for the Examination of Water and Wastewater defines TKN accordingly, with nitrite and nitrate covered by separate analyses. Why Other Options Are Wrong: (a) includes N2 gas, which is not measured by TKN; (b) is vague and includes nitrogen gas; (d) cannot be correct since (a) and (b) are incorrect; (e) refers to oxidized forms not part of TKN. Common Pitfalls: Confusing TKN with total nitrogen (TN), which includes nitrate and nitrite; overlooking pre-reduction steps in modified methods. Final Answer: Organic nitrogen and ammonia nitrogen

Question 18

Operational criterion for sewer velocities: Which statement best describes the lower limit that should not be violated during design to avoid deposition?

Accepted Answer

Introduction / Context: Self-cleansing velocity is the minimum flow velocity needed to prevent sedimentation of solids in gravity sewers. Maintaining velocities below this threshold leads to deposition, odor, and reduced capacity, while extremely high velocities risk abrasion and structural damage. Given Data / Assumptions: Gravity sewers carrying domestic wastewater with some grit. Typical target minimum velocities around 0.6–0.75 m/s at half-full (varies by code and context). Upper velocities limited by scouring/erosion constraints. Concept / Approach: Design aims to maintain V ≥ V_sc (self-cleansing). Excessively high velocities are also undesirable, but the key “must not violate” lower bound is the self-cleansing limit to avoid chronic deposition and maintenance issues. Step-by-Step Solution: Identify the critical lower bound—self-cleansing velocity.Ensure minimum design velocity at representative flow conditions meets or exceeds this value.Hence, “should not be less than the self-cleansing velocity” is correct. Verification / Alternative check: Sewer design manuals specify minimum slopes or minimum velocities consistent with self-cleansing criteria derived from settling and shear considerations. Why Other Options Are Wrong: (a) reverses the concept; (c) and (d) provide unrealistic absolute numbers; (e) is overly prescriptive and not universally correct across sizes/flows. Common Pitfalls: Designing only for peak flow velocity and ignoring low-flow conditions; neglecting solids characteristics and grit load. Final Answer: Velocity should not be less than the self-cleansing velocity

Question 19

Plumbing basics: In building drainage, what is the principal function of a trap installed on a fixture connection?

Accepted Answer

Introduction / Context: Fixture traps are mandatory in sanitary plumbing to protect indoor air quality. They form a water seal between the fixture and the drainage system, blocking sewer gases that may be hazardous or malodorous from entering living areas. Given Data / Assumptions: Standard P-, S-, or bottle-traps filled with water after each use. Proper venting is provided to prevent siphonage of the trap seal. Domestic sanitary installations. Concept / Approach: A trap relies on a standing water seal of adequate depth (often around 50–75 mm) to resist pressure fluctuations and seal against gas flow. It is not a mechanical pest barrier nor a gas scrubber; instead, it is purely a hydraulic seal that must be maintained and protected from siphonage, back pressure, or evaporation. Step-by-Step Solution: Identify the purpose: prevent sewer gas ingress.Water seal acts as a physical barrier to gas flow.Therefore option describing gas prevention via water seal is correct. Verification / Alternative check: Plumbing codes specify minimum trap seal depths and venting requirements to ensure seals are not lost, confirming the functional role of traps. Why Other Options Are Wrong: Traps do not capture rodents, do not chemically dissolve gases, and are not intended to create siphonic action; air admittance valves are different components used in venting, not substitutes for trap seals. Common Pitfalls: Allowing trap seals to evaporate in seldom-used fixtures; improper venting leading to siphonage and loss of seal. Final Answer: It is a water seal that prevents the entry of foul sewer gases into occupied spaces

Question 20

Septic tank design: Freeboard of about 0.3 m is provided above the top sewage line. Does this freeboard primarily serve to accommodate scum, and is that the correct explanation?

Accepted Answer

Introduction / Context: Septic tanks provide primary settling and anaerobic digestion. A freeboard space is left above the liquid level to accommodate operational contingencies. Understanding why freeboard is specified helps ensure reliable and odor-free operation. Given Data / Assumptions: Freeboard ≈ 0.3 m above the operating liquid level. Domestic septic systems with typical scum formation. Ventilation is provided to avoid gas buildup. Concept / Approach: During operation, floating scum (oils, fats, vegetative matter) accumulates on the surface. Freeboard prevents scum from contacting the roof slab or flowing into the outlet tee, and provides buffer for surge, foaming, and gas release. Thus, stating that 0.3 m freeboard is provided and that it accommodates scum is accurate and causally linked. Step-by-Step Solution: Assertion (A): Freeboard of 0.3 m is provided above the top sewage line — True.Reason (R): It serves to accommodate scum and operational surges — True.R correctly explains A — choose the combined true/true with correct explanation. Verification / Alternative check: Standard septic tank design guides specify freeboard for scum and foam accommodation and to allow safe ventilation space under the cover slab. Why Other Options Are Wrong: Any option denying the truth of A or R, or denying the causal relation, conflicts with standard design rationale. Common Pitfalls: Confusing freeboard with working depth; ignoring outlet tee/inlet baffle elevations relative to scum layer. Final Answer: Both A and R are true and R is the correct explanation of A

Waste Water Engineering Questions