Question 1

For designing circular sewer pipes having diameters from 0.4 m to 0.9 m at maximum design flow, what depth of flow is generally assumed in practice so that some freeboard and ventilation are available?

Accepted Answer

Introduction / Context: Sanitary sewers are gravity conduits that also function as open channels when not flowing full. Designers rarely size them to run full at peak because some freeboard is desirable for ventilation, surcharge allowance, and to reduce odor and pressurization risks. A common rule-of-thumb is to assume a fraction of the diameter at peak design flow. Given Data / Assumptions: Circular sewers with diameters between 0.4 m and 0.9 m. Conventional municipal design practice for peak (maximum) design flow. Need to choose a practical depth fraction for design capacity. Concept / Approach: Although a circular section gives maximum discharge when running full from strict hydraulics, sewer design includes serviceability concerns. Operating “part-full” at design peak leaves headroom for inflow/infiltration, surges, and ensures air movement. Many practice manuals adopt two-thirds depth (about 0.67D) as the standard design assumption for peak flow sizing of medium sewers. Step-by-Step Solution: Recognize the need for non-full-flow design to allow freeboard and ventilation.Identify widely used practice value: depth ≈ 2/3 of diameter at design peak.Select the closest option: two-thirds full. Verification / Alternative check: Two-thirds full corresponds to a large proportion of full-flow capacity while leaving headspace. Numerous standard problems and exam syllabi reference this assumption for circular sewers in the 0.4–0.9 m range. Why Other Options Are Wrong: Full depth sacrifices freeboard; half or one-third full are overly conservative; three-fourths full leaves less ventilation margin than the common standard. Common Pitfalls: Confusing “hydraulic maximum discharge” with “design practice”; ignoring operational needs like aeration and surcharge allowance. Final Answer: two-thirds full

Question 2

Field practice in sewer construction and access structures: identify the correct statements about boning rods, manhole cover shape, and depth-based manhole classification.

Accepted Answer

Introduction / Context: Sewer construction relies on simple tools and standardized appurtenances to ensure alignment, levels, and safe access. Understanding boning rods, manhole cover shapes, and depth-based classifications is foundational for site engineers and inspectors. Given Data / Assumptions: Statements concern typical municipal practice. Depth thresholds are approximate and widely used for classification. Focus is on practicality and safety in operations. Concept / Approach: Boning rods sight invert lines during trench excavation. Circular manhole covers cannot fall through their own opening and are easier to roll and handle. Depth classification (shallow, normal, deep) helps decide laddering, ventilation, and construction details. Commonly, depth < 0.9 m is shallow; > 1.5 m is deep. Step-by-Step Solution: Check each statement versus standard practice.(a) Boning rod → correct use for invert levels.(b) Circular cover → correct for safety and convenience.(c) & (d) → depth limits align with common classifications. Verification / Alternative check: These practices are cited in standard public works manuals and frequently tested in diploma/degree examinations in civil engineering. Why Other Options Are Wrong: Each individual statement is correct, so “All the above” is the only comprehensive choice. Common Pitfalls: Memorizing exact depths without noting they are guidelines; overlooking the safety rationale for circular covers. Final Answer: All the above.

Question 3

In wastewater analysis, what does an Imhoff cone directly measure during the settleability test (typically reported in mL/L after 60 minutes)?

Accepted Answer

Introduction / Context: An Imhoff cone is a simple tapered vessel used in wastewater laboratories and field settings to evaluate how much material will settle by gravity in a set time. The result helps operators anticipate primary settling performance and sludge production rates. Given Data / Assumptions: Measured parameter is the settled volume after a defined time (often 60 minutes). Reported typically as millilitres per litre (mL/L). Sample is raw sewage or primary effluent as appropriate. Concept / Approach: Settlable solids are the fraction that will settle out under quiescent conditions. The Imhoff cone allows direct reading of the settled volume at the bottom after the test period. This differs from suspended/dissolved solids which require filtration/evaporation and gravimetric analysis. Step-by-Step Solution: Fill cone with a known volume of sample.Allow to stand undisturbed for the specified time (e.g., 60 min).Read the settled volume → “settlable solids”. Verification / Alternative check: Process control charts in primary treatment commonly track mL/L of settleable solids via Imhoff cones. Why Other Options Are Wrong: Suspended and dissolved solids require filtration/evaporation and drying; volatile/fixed solids are subdivisions based on ignition, not directly observed as a settled volume. Common Pitfalls: Shaking or disturbing the cone; misreading the meniscus; confusing settleable with suspended solids. Final Answer: settlable solids

Question 4

In septic tanks used for on-site sanitation, the principal biological decomposition of organic matter is carried out by which type of microorganisms?

Accepted Answer

Introduction / Context: Septic tanks are closed, non-aerated sedimentation-cum-digestion units commonly used for individual houses and small communities. Understanding the dominant biological regime informs maintenance and expected effluent quality. Given Data / Assumptions: Septic tanks are covered tanks without aeration equipment. Scum and sludge layers form; gas vents exist. Objective: identify dominant microorganisms responsible for digestion. Concept / Approach: In the absence of oxygen, organic matter is decomposed by anaerobic bacteria producing biogas (chiefly methane and carbon dioxide). Aerobic processes are limited to thin layers near inlets/outlets if any air enters, but the overall tank digestion is anaerobic. Step-by-Step Solution: Recognize the closed, low-oxygen environment of a septic tank.Identify anaerobic digestion as the principal pathway.Select “anaerobic bacteria”. Verification / Alternative check: Septic tank gas composition (methane and CO2) confirms anaerobic activity. Why Other Options Are Wrong: Aerobic bacteria require oxygen; simultaneous aerobic-anaerobic action with aeration does not occur in sealed septic tanks; algae do not drive digestion in the dark, enclosed tank. Common Pitfalls: Assuming aerobic stabilization like in activated sludge; overlooking the role of scum/sludge blanket and gas evolution in mixing. Final Answer: anaerobic bacteria

Question 5

Sludge digestion gas composition: Identify the incorrect statement about anaerobic digestion and the biogas produced during sludge stabilization.

Accepted Answer

Introduction / Context: Anaerobic digestion stabilizes sewage sludge, producing a combustible gas mixture called biogas. Knowing the approximate composition is essential for energy recovery, safety, and process monitoring. Given Data / Assumptions: Municipal sludge digestion under mesophilic conditions. Typical steady-state performance without toxic shocks. Statements mix process definition and gas composition. Concept / Approach: Standard biogas composition is roughly 60–75% methane, 25–40% carbon dioxide, with traces of hydrogen sulfide and moisture. The incorrect claim is that CO2 is about 75%; this reverses the major constituent and would imply a very low energy content gas. Step-by-Step Solution: Evaluate each statement against common data.(a) and (b) are definitional and correct.(d) reflects accepted methane range; (e) is also true due to volatile solids destruction.(c) is incorrect because CO2 is not the predominant constituent. Verification / Alternative check: Plant digester gas analyses and handbooks confirm methane as the principal combustible fraction, not CO2. Why Other Options Are Wrong: They describe accepted practice and outcomes of digestion; only (c) misstates the gas composition. Common Pitfalls: Confusing landfill gas (which is similar but site-dependent) with digester gas; forgetting that methane provides the calorific value. Final Answer: Biogas from digestion contains about 75% carbon dioxide.

Question 6

Stream dilution and nuisance potential: In conventional stream sanitation design, a nuisance is likely to be experienced if the dilution factor (ratio of stream flow to wastewater flow) is less than which threshold?

Accepted Answer

Introduction / Context: When untreated or partially treated wastewater is discharged to a receiving stream, adequate dilution is necessary to minimize odor, septic conditions, and visible pollution. The dilution factor is a simple indicator in preliminary assessments. Given Data / Assumptions: Simple screening-level criterion for nuisance avoidance. Typical domestic wastewater without unusual industrial toxins. Moderate temperatures and normal re-aeration conditions. Concept / Approach: Nuisance conditions (odor, septic sludge deposits, anoxic zones) are more likely at low dilution ratios. Many classical references cite that if dilution falls below roughly 20:1, the chance of nuisance increases markedly unless treatment and reaeration are provided. Step-by-Step Solution: Interpret dilution factor as stream flow / wastewater flow.Identify the common threshold of concern: around 20.Select the smallest acceptable threshold that aligns with practice: 20. Verification / Alternative check: Heuristic rules in older design manuals use ratios of 20:1 or higher to limit nuisance for raw or lightly treated wastes. Why Other Options Are Wrong: 10 is too low; 40, 60, and 100 are more conservative than the commonly cited minimum threshold. Common Pitfalls: Using dilution alone without accounting for BOD decay, reaeration, and temperature; ignoring modern effluent standards that often require secondary treatment regardless of dilution. Final Answer: 20

Question 7

Provision of drop manholes: At what condition at a junction should a drop connection be provided to avoid turbulence and excessive velocities in the manhole?

Accepted Answer

Introduction / Context: Where a higher-level branch sewer joins a lower-level main, the incoming jet can cause erosion and turbulence in a standard manhole. A drop manhole introduces a vertical drop pipe to dissipate energy and protect the structure. Given Data / Assumptions: Gravity sewers with manhole junctions. Significant invert level difference between incoming branch and main sewer. Desire to prevent splashing, odor, and structural damage. Concept / Approach: Practice commonly sets a threshold vertical difference (often around 0.6 m) above which a drop connection is used. The drop pipe conveys the branch flow down to near the main invert smoothly, limiting kinetic energy at the manhole benching. Step-by-Step Solution: Assess invert level difference: if >≈ 0.6 m, adopt drop connection.Provide drop pipe along manhole wall with appropriate fittings.Ensure adequate ventilation and benching at the base. Verification / Alternative check: Municipal standards and exam references consistently cite the ≈ 60 cm criterion for selecting drop manholes. Why Other Options Are Wrong: Road location, space constraints, or mere intersection do not automatically require a drop; level difference is the key driver. Common Pitfalls: Ignoring erosive effects of high-velocity jets; providing drops for trivial level differences unnecessarily increasing cost. Final Answer: When the invert level of a branch sewer is more than about 60 cm above the invert of the main sewer.

Question 8

Typical total solids concentration in raw municipal sewage is approximately within which range (mg/L)?

Accepted Answer

Introduction / Context: Total solids (TS) in sewage include both suspended and dissolved constituents. Knowledge of typical ranges informs preliminary design of sedimentation, aeration, and sludge handling facilities. Given Data / Assumptions: Domestic (municipal) sewage without heavy industrial contribution. Raw influent values (before primary settling). Standard mg/L units (same as ppm by mass in dilute water systems). Concept / Approach: Typical raw municipal sewage often has total solids around 1000–1200 mg/L, with suspended solids commonly 200–350 mg/L and the remainder dissolved. Hence, the commonly cited TS range closest to practice is 1000–1500 mg/L. Step-by-Step Solution: Recall commonly reported TS values ≈ 1 g/L.Map to the option ranges; 1000–1500 mg/L best fits.Select that range. Verification / Alternative check: Numerous textbooks and municipal data sets report TS near 1000 mg/L for domestic influents, varying with infiltration and lifestyle factors. Why Other Options Are Wrong: 100–500 or 500–1000 are low for TS; 1500–2000 and above are high for typical domestic sewage without industrial input. Common Pitfalls: Confusing suspended solids with total solids; relying on a single sample without accounting for diurnal variability. Final Answer: 1000 to 1500

Question 9

Quality assurance tests for newly laid sewer lines: which combination correctly describes common tests carried out on site?

Accepted Answer

Introduction / Context: Commissioning sewer lines requires simple but effective field tests to ensure structural integrity, alignment, and freedom from blockages. These tests avoid early failures and maintenance complaints after backfilling and road restoration. Given Data / Assumptions: Newly laid gravity sewers of typical municipal sizes. Standard field equipment available: plugs, lamps, mirrors, test balls. Need to verify joints, alignment, and clear bore. Concept / Approach: The water test checks leakage by plugging ends and observing loss of head or visible leakage. The straightness test confirms alignment by checking a continuous line of sight with a lamp and mirror. The obstruction test verifies no sags or debris by ensuring a ball of specified size passes through freely. Step-by-Step Solution: Specify water test procedure for watertightness.Describe lamp-and-mirror sighting for straightness.State typical smooth ball diameter (often ~75 mm for small-to-medium pipes; scaled for larger diameters). Verification / Alternative check: Public works manuals outline these as standard acceptance tests before handover. Why Other Options Are Wrong: Limiting to only (a) and (b) would omit the crucial obstruction check; hence (d) is most complete. Common Pitfalls: Using undersized balls that miss partial blockages; performing tests before adequate curing/backfill support; not documenting test conditions. Final Answer: All the above.

Question 10

Assertion–Reason on sewer operation: Assertion (A): Minimum self-cleansing velocity should occur in a sewer at least once per day. Reason (R): If initial deposition is not flushed, it obstructs flow and promotes further deposition, eventually blocki…

Accepted Answer

Introduction / Context: Self-cleansing velocity is the minimum flow velocity needed to keep solids in suspension or move them along without deposition. Designing and operating sewers to achieve this velocity periodically is essential for long-term reliability. Given Data / Assumptions: Normal domestic sewers carrying settleable solids and grit. Daily diurnal peaks provide higher velocities than nighttime base flows. Standard thresholds (e.g., around 0.6–0.75 m/s for sanitary sewage) apply depending on grit content and pipe roughness. Concept / Approach: If solids deposit and remain, they reduce the effective flow area and increase roughness, thereby causing additional energy loss and further deposition. This positive feedback can culminate in blockages and sanitary overflows. Ensuring at least once-daily attainment of the self-cleansing velocity flushes sediments out. Step-by-Step Solution: Assess daily flow variation and confirm peak periods achieve the target velocity.Select gradients and diameters in design to meet this criterion.Link cause–effect: lack of flushing → initial deposition → increased resistance → more deposition → potential blockage. Verification / Alternative check: Operational records show that lines failing to hit cleansing velocities develop slime, grit banks, and odors, necessitating jetting or flushing trucks. Why Other Options Are Wrong: (b) denies the causal link; (c) and (d) contradict tested practice; (e) conflicts with field evidence. Common Pitfalls: Ignoring peak attenuation in large trunks; choosing grades from tables without checking resulting velocities for actual design flows. Final Answer: Both A and R are true and R is the correct explanation of A

Question 11

BOD₅ calculation for a diluted sample: 3.0 mL of raw sewage is diluted to 300 mL. The initial dissolved oxygen (DO) of the diluted sample is 8 mg/L, and after 5 days at 20°C the DO is 5 mg/L. What is the BOD₅ of the raw sewage (in mg/L)?

Accepted Answer

Introduction / Context: Biochemical Oxygen Demand over 5 days at 20°C (BOD₅) is a standard test in environmental engineering to estimate biodegradable organic pollution. When samples are diluted, the oxygen depletion measured on the diluted bottle is scaled back by the dilution factor to obtain the BOD of the original sewage. Given Data / Assumptions: Raw sewage volume used = 3.0 mL. Total diluted volume = 300 mL. Initial DO (diluted) = 8 mg/L. Final DO after 5 days (diluted) = 5 mg/L. Incubation temperature = 20°C; nitrification not considered. Concept / Approach: BOD₅ of the original sample = (DO initial − DO final) * dilution factor. The dilution factor equals total volume / sample volume when the seed correction is negligible and blank is acceptable. Step-by-Step Solution: Dilution factor = 300 / 3 = 100.Oxygen depletion in bottle = 8 − 5 = 3 mg/L.BOD₅ (raw) = 3 * 100 = 300 mg/L. Verification / Alternative check: If a seed/blank correction were required, it would be subtracted before multiplying by the dilution factor. Here none is indicated, so 300 mg/L stands. Why Other Options Are Wrong: 100 mg/L and 200 mg/L understate the effect of the 100× dilution. 400 mg/L overstates depletion beyond the observed 3 mg/L. Common Pitfalls: Forgetting to apply the dilution factor; mixing up mL and L; neglecting seed correction when present; using temperature other than 20°C for BOD₅. Final Answer: 300 mg/L

Question 12

Terminology for depressed sewer crossings: The arrangement used to carry a sewer below an obstruction and beneath the hydraulic grade line is commonly called what?

Accepted Answer

Introduction / Context: Sewer alignments sometimes must pass beneath roads, streams, or utilities. When the crown of the sewer must dip below the hydraulic grade line to cross such obstacles, special depressed arrangements are used and referred to by several equivalent terms in practice. Given Data / Assumptions: The sewer crosses below an obstruction. The pipe invert is below the hydraulic gradient. Gravity flow conditions with flowing full at the depressed reach are possible. Concept / Approach: Such depressed sections are widely called inverted siphons, sag pipes, or depressed sewers. Although “siphon” suggests suction, in sewerage it simply denotes a depressed pressure-flow reach conveying flow under a head differential between upstream and downstream manholes. Step-by-Step Solution: Identify need to dip the alignment below the obstruction.Provide inlet and outlet chambers to distribute and collect flows to multiple barrels if required.Size the depressed barrels for self-cleansing velocities at design flows.Provide air release, flushing, and access provisions for maintenance. Verification / Alternative check: Hydraulic checks confirm that energy grade line allows passage without surcharge at design flow; CFD or steady-state models may be used. Why Other Options Are Wrong: Options (a), (b), and (c) are not wrong—they are synonymous; therefore the comprehensive correct choice is “All of these”. Common Pitfalls: Underestimating headloss; omitting flushing arrangements; allowing deposition in low-velocity conditions. Final Answer: All of these

Question 13

Confined-space gas check: A lead acetate paper kept for about 5 minutes at a manhole turns black. Which gas is indicated?

Accepted Answer

Introduction / Context: Before manhole entry, simple qualitative field tests are sometimes used alongside instrumented monitoring. Lead acetate paper is a classic indicator of hydrogen sulphide (H₂S), a highly toxic sewer gas produced by anaerobic decomposition of sulphates and organic matter. Given Data / Assumptions: Paper is moistened with lead acetate solution. Exposure time ≈ 5 minutes. Blackening of the paper is observed. Concept / Approach: H₂S reacts with lead acetate to form black lead sulphide (PbS). This visual reaction is specific enough for a quick qualitative alert but must be complemented by calibrated multi-gas meters for safe entry decisions (O₂, H₂S, LEL, CO, etc.). Step-by-Step Solution: Expose lead acetate strip at manhole opening.Observe for darkening (PbS formation) due to H₂S.If darkening occurs, ventilate and re-test with an electronic detector.Proceed only if gas readings are within permitted exposure limits. Verification / Alternative check: Electronic sensors for H₂S provide quantitative ppm readings and alarms; cross-check oxygen percentage and LEL for methane/volatile hazards. Why Other Options Are Wrong: CO₂ does not blacken lead acetate; methane is flammable but non-reactive in this test; oxygen is not a contaminant gas and does not cause blackening. Common Pitfalls: Relying solely on paper tests; not accounting for oxygen deficiency; inadequate ventilation before entry. Final Answer: Hydrogen sulphide

Question 14

Manhole safety indication: A safety lamp flame intensifies or ignites near the upper portion of a manhole. Which contaminant is most likely present?

Accepted Answer

Introduction / Context: Methane (CH₄) is lighter than air and tends to accumulate near the crown of sewers and the upper part of manholes. A safety lamp (or flame tube) flaring or igniting at the top region suggests a combustible gas concentrated aloft—commonly methane in sewer atmospheres. Given Data / Assumptions: Flame behavior is observed in the upper manhole zone. Environment is a sewer where CH₄ generation is possible. No strong petroleum sources indicated. Concept / Approach: Because CH₄ has a relative density < 1, it stratifies above air; thus flames may enlarge or ignite near the top. While petrol vapours are heavier-than-air and tend to settle lower, methane signals hazard at the top. Always use intrinsically safe gas detectors rather than open flames for real testing. Step-by-Step Solution: Consider gas density and stratification in confined spaces.Identify methane as a common light combustible in sewers.Confirm with an LEL sensor and CH₄ channel on a multi-gas detector.Ventilate and control ignition sources per confined-space protocols. Verification / Alternative check: Use calibrated LEL and CH₄ sensors; continuous monitoring during entry confirms safe conditions. Why Other Options Are Wrong: CO₂ is non-flammable; H₂S, though toxic, is not typically concentrated at the top and is dangerous at low ppm; petrol vapours usually accumulate low due to higher density. Common Pitfalls: Using a flame test (unsafe); ignoring stratification; failing to ventilate adequately. Final Answer: Methane gas

Question 15

Chemical Oxygen Demand (COD) test chemistry: In the standard COD test, organic matter is oxidized by potassium dichromate in the presence of which acid catalyst medium?

Accepted Answer

Introduction / Context: The COD test measures the oxygen equivalent of organic matter oxidizable by a strong chemical oxidant. The classical method uses potassium dichromate in an acidic medium with catalysts to ensure near-complete oxidation of organics in wastewater samples. Given Data / Assumptions: Oxidant: potassium dichromate. Reflux digestion is performed. Acid medium and catalyst required. Concept / Approach: Potassium dichromate requires a strongly acidic medium—concentrated sulphuric acid—often with silver sulfate as a catalyst and mercuric sulfate to complex chlorides. Under heating, dichromate oxidizes organics; the consumed dichromate is then determined titrimetrically to compute COD. Step-by-Step Solution: Add known dichromate volume to sample in sulphuric acid.Digest under reflux for specified time.Back-titrate remaining dichromate with ferrous ammonium sulfate.Calculate COD from dichromate consumed per liter. Verification / Alternative check: Run blanks and standards; verify chloride interference is suppressed by mercuric sulfate if present. Why Other Options Are Wrong: HCl and HNO₃ are not used as the standard digestion medium; citric acid is not suitable for strong oxidation conditions. Common Pitfalls: Ignoring chloride interference; insufficient digestion; unsafe handling of concentrated H₂SO₄ and dichromate. Final Answer: Sulphuric acid

Question 16

Detritus (grit) tanks in preliminary treatment: Which statements correctly describe their operating targets and function?

Accepted Answer

Introduction / Context: Detritus or grit tanks are preliminary treatment units designed to remove fine inorganic particles (sand, silt, grit) that would otherwise abrade pumps and reduce downstream capacity. They operate at controlled velocities and short detention times to favor settling of heavier inorganics over organics. Given Data / Assumptions: Very short detention (order of minutes). Carefully controlled low horizontal velocity. Target removal: inorganics, not organics. Concept / Approach: By maintaining a low approach velocity (often cited near 0.09 m/s for detritus tanks and about 0.3 m/s for conventional grit channels) and a detention of a few minutes, heavier grit settles while lighter organics are carried forward, limiting septic conditions and odors. Step-by-Step Solution: Set velocity control baffles to maintain ~0.09 m/s.Provide detention of roughly 3–4 minutes at average flow.Design hopper or scraped bottom to collect settled grit separately.Prevent undue removal of organics to preserve downstream biological treatment efficiency. Verification / Alternative check: Field measurements of velocity and grit capture efficiency validate design; adjust weirs/baffles to tune velocities. Why Other Options Are Wrong: In this set each individual statement describes a recognized design/operation feature; hence “All the above” is correct. Common Pitfalls: Velocities too high causing carryover of grit; too low causing organic settling and odor; long detention times promoting septic conditions. Final Answer: All the above

Question 17

pH in wastewater engineering: Evaluate the statements—(A) Determining the pH of sewage is important. (R) The efficiency of several treatment methods depends on pH. What is the correct A–R relationship?

Accepted Answer

Introduction / Context: pH fundamentally influences chemical equilibria and biological activity in wastewater systems. Accurate pH measurement guides process control for coagulation, digestion, nitrification, disinfection, and corrosion control in sewers and plants. Given Data / Assumptions: A: pH determination is important. R: Process efficiency depends on pH availability/control. Typical unit processes are pH sensitive. Concept / Approach: Many unit operations have optimum pH windows: alum/iron coagulation, biological nitrification (around neutral), anaerobic digestion stability (alkalinity and pH), and chlorine disinfection efficacy. Therefore, knowing and adjusting pH is integral to achieving treatment goals, explaining why pH measurement is important. Step-by-Step Solution: Identify pH-sensitive processes (coagulation, biological steps, disinfection).Relate target performance to pH control bands.Conclude that pH measurement is essential because process efficiency relies on it. Verification / Alternative check: Process control charts show reduced efficiency outside pH setpoints; jar tests and upstream alkalinity additions demonstrate dependence. Why Other Options Are Wrong: Options (b)–(d) contradict the well-established causal link between pH and treatment efficiency. Common Pitfalls: Calibrating pH meters infrequently; ignoring temperature effects; relying on grab pH only when diurnal variation is significant. Final Answer: Both A and R are true and R is the correct explanation of A

Question 18

Hydraulics of self-cleansing velocity in sewers: At the self-cleansing velocity threshold, what happens at the sewer invert (bottom)?

Accepted Answer

Introduction / Context: Self-cleansing velocity is the minimum mean flow velocity required to prevent sediment deposition in sewers. Design manuals specify values (often around 0.6–0.75 m/s for sanitary sewers) to ensure that shear stress at the invert is sufficient to mobilize and remove settleable solids. Given Data / Assumptions: Steady uniform flow condition considered. Granular settleable solids are present. Velocity at or just above the self-cleansing limit. Concept / Approach: At the self-cleansing threshold, boundary shear equals the critical value needed to initiate motion of typical sewer solids. This produces scouring sufficient to prevent accumulation. Below this velocity, silting begins; well above it, excessive scouring may risk pipe wear. Step-by-Step Solution: Relate shear stress τ₀ ≈ γw * R * S to critical shear for particles.Select design velocity to meet or exceed initiation-of-motion criteria.Conclude that at threshold, invert experiences scouring action preventing deposition. Verification / Alternative check: Field observation of clean invert and absence of deposits during peak dry-weather flows validates design; computational checks use Shields parameter for initiation of motion. Why Other Options Are Wrong: (a) Silting indicates velocity below threshold; (c) simultaneous strong silting and scouring is inconsistent; (d) some scouring is needed to remain self-cleansing. Common Pitfalls: Confusing self-cleansing with average velocity only; neglecting low-flow periods; ignoring grit sizes and slopes that affect required velocity. Final Answer: Scouring occurs at bottom

Question 19

Stoneware (vitrified clay) pipes: Which of the following is NOT a correct characteristic for stoneware pipes used in gravity sewers?

Accepted Answer

Introduction / Context: Stoneware (vitrified clay) pipes are a traditional choice for small gravity sewers and house connections because of chemical resistance and a smooth glazed bore. However, their brittle nature and jointing make them unsuitable as pressure pipes. Given Data / Assumptions: Application is primarily gravity sewerage. Glazed, low-absorption vitrified body. No internal pressure service expected. Concept / Approach: In gravity flows, hydraulic roughness is low and sulphide corrosion resistance is high, favoring stoneware. Pressure service demands tensile strength, flexible joints, and pressure-rated integrity; materials like ductile iron, PVC, HDPE are typically chosen for pressure duties. Step-by-Step Solution: List typical attributes: impervious, glazed, corrosion resistant, smooth bore.Contrast with pressure-pipe requirements: hoop strength, leak-tight flexible joints.Identify the incorrect statement: “specially suited to pressure pipes”. Verification / Alternative check: Standards for vitrified clay pipes specify non-pressure applications; pressure ratings for clay are not standard practice. Why Other Options Are Wrong: (a)–(c) are correct traits of stoneware in gravity service; only (d) mischaracterizes usage. Common Pitfalls: Using stoneware in areas prone to differential settlement; inadequate flexible joints; assuming it can withstand internal pressure surges. Final Answer: Specially suited to pressure-pipe applications

Question 20

Tractive force (boundary shear) in a flowing sewer: If γw is the unit weight of water, r is the hydraulic mean depth (hydraulic radius), and S is the bed slope, what is the shear (tractive) force per unit area at the invert?

Accepted Answer

Introduction / Context: Boundary shear stress (tractive force) governs the ability of flow to move sediments in open channels and partially full pipes. In uniform steady flow, it depends on the weight component of water acting downslope and is a key parameter for self-cleansing sewer design. Given Data / Assumptions: Uniform steady flow in a channel/pipe flowing as an open channel. γw = unit weight of water. r = hydraulic mean depth (area/wetted perimeter). S = slope of energy grade line ≈ bed slope for uniform flow. Concept / Approach: The standard relationship for boundary shear stress is τ₀ = γw * r * S. This comes from balancing the downslope component of the water weight with the resisting shear along the wetted perimeter in a control volume under uniform conditions. Step-by-Step Solution: Assume uniform flow: energy slope equals bed slope S.Hydraulic radius r = A / P (area / wetted perimeter).Shear stress per unit area τ₀ = γw * r * S. Verification / Alternative check: Derive via force balance on a reach of length L: downslope force = γw * A * L * S; resisting force = τ₀ * P * L; equate and simplify to τ₀ = γw * (A/P) * S = γw * r * S. Why Other Options Are Wrong: Exponents 1/2 or 2/3 do not arise in τ₀; S/r is dimensionally inconsistent for stress. Common Pitfalls: Confusing τ₀ with Manning/Chézy velocity formulas; using γ (specific weight) units inconsistently; mixing hydraulic radius with diameter. Final Answer: γw * r * S

Waste Water Engineering Questions