Question 1

Sanitary sewer design: if q is the average sewage flow from a city of population P, how is the maximum sewage flow (design peak) generally expressed for sizing main sewers?

Accepted Answer

Introduction / Context: Sewer capacities must accommodate short-term peaks above the daily average. Designers apply a population-dependent peak factor (PF) to the average flow to obtain the design peak discharge for pipes and pumping stations. Given Data / Assumptions: q represents average sewage flow; P is contributing population. Diurnal, seasonal, and infiltration variations cause peaks. Empirical PF relationships (e.g., Harmon, Babbit, or local codes) are used. Concept / Approach: The general relationship is Q_max = PF * q. PF decreases with increasing P because larger populations “average out” individual usage variability. Additional allowances may be added for infiltration/inflow (I/I) based on local experience and standards. Step-by-Step Solution: Estimate base average flow q from per capita contributions and P.Select PF from the governing standard as a function of P (and sometimes land use).Compute design peak Q_max = PF * q and check velocities and slopes for self-cleansing.Account for I/I and future growth as required. Verification / Alternative check: Compare against monitored flow data where available; confirm that resulting velocities meet minimum and maximum criteria under peak and low flows. Why Other Options Are Wrong: (a) Ignores peaking; (b) a fixed factor is not universally valid; (d) inverts the relationship and would underestimate the peak. Common Pitfalls: Using an inappropriate PF for population size; omitting infiltration allowances; not checking pump cycles or storage needs during extreme peaks. Final Answer: Q_max = PF * q, where PF is a peak factor dependent on P and local criteria

Question 2

Rainfall characterization for hydrologic design: which parameters are used to describe rainfall at a place for runoff estimation and stormwater design?

Accepted Answer

Introduction / Context: Hydrologic design of drains, culverts, and detention requires quantifying storms by how hard it rains, for how long, and how often such storms occur. These three descriptors underpin IDF (Intensity–Duration–Frequency) curves used worldwide. Given Data / Assumptions: Local IDF data or design storms available from agencies. Stationarity assumed for design, unless climate-adjusted factors are prescribed. Single event design for small urban works; continuous simulation for larger systems may be used. Concept / Approach: Intensity controls peak runoff; duration controls catchment response relative to time of concentration; frequency (return period) reflects risk tolerance and desired level of service. Together, they define the design storm used in Rational or hydrograph-based methods. Step-by-Step Solution: Choose target return period based on facility class (e.g., 2–10 years for minor drainage).From IDF curves, read intensity corresponding to the chosen duration (often Tc).Apply the selected intensity in runoff calculations (e.g., Rational Q = C * i * A).Check sensitivity for nearby durations to ensure conservative sizing. Verification / Alternative check: Cross-check with historical storm records or regional guidelines; ensure consistency with floodplain and overland flow pathways. Why Other Options Are Wrong: Each of (a)–(c) is part of standard rainfall characterization, hence “All the above” is correct. Common Pitfalls: Confusing storm “direction” with “duration”; selecting intensity for the wrong duration; ignoring temporal distribution within the storm hyetograph. Final Answer: All the above.

Question 3

Sludge digestion and stabilization: during the alkaline regression stage of sludge treatment, what is the typical pH range observed?

Accepted Answer

Introduction / Context: Sludge digestion proceeds through biochemical stages. After initial acidogenesis (acid formation), the process enters methanogenic phases where acids are consumed and pH rises—commonly termed alkaline regression—leading toward a stable, less odorous end product. Given Data / Assumptions: Anaerobic digestion under controlled conditions. Monitoring parameters include pH, volatile acids, alkalinity, gas production. Operational goal is to maintain conditions favorable to methanogens. Concept / Approach: In early stages, volatile fatty acids accumulate and the pH may drop. As methanogenesis consumes acids and produces biogas, alkalinity recovers and pH drifts above neutral, typically in the 7.0–7.5 (or slightly higher) range, indicating alkaline regression and process stabilization. Step-by-Step Solution: Observe initial acid phase: pH drifts downward as acids rise.Adjust feed/alkalinity to avoid souring; maintain temperature and mixing.During regression, acids are converted; alkalinity increases.Record pH trending upward beyond 7.0, confirming stabilization progress. Verification / Alternative check: Correlate rising pH with reduced volatile acids-to-alkalinity ratio and steady methane-rich gas production. Why Other Options Are Wrong: (b) and (c) describe the acid phase, not the regression stage; (d) “more than 6” is ambiguous and may still be acidic. Common Pitfalls: Assuming high pH alone guarantees stability; ignoring buffering capacity; failing to monitor temperature, which strongly affects microbial activity. Final Answer: More than 7 (alkaline)

Question 4

In municipal wastewater engineering (Indian conditions), what is the typical average temperature of domestic sewage used for design and process calculations?

Accepted Answer

Introduction / Context: Sewage temperature directly affects biochemical reaction rates, viscosity, oxygen transfer, and gas generation. For Indian design problems and exam standards, a representative average temperature is used when site-specific data are unavailable. This question checks that reference value. Given Data / Assumptions: Domestic sewage in tropical/subtropical Indian climate. Average, not peak summer or winter extremes. Steady-state assumptions for textbook calculations (e.g., BOD rate constants, oxygen solubility). Concept / Approach: Design manuals tabulate typical sewage temperatures by climate. For India, many standard texts take about 20°C as a reasonable annual-average for calculations. This anchors temperature correction factors for reaction rates (e.g., k_T = k_20 * θ^(T-20)). Step-by-Step Solution: Identify typical Indian ambient-water temperatures for municipal systems.Select the widely adopted average value used to normalize kinetic parameters.Hence choose 20°C as the standard average figure. Verification / Alternative check: At 20°C, oxygen solubility and viscosity values match common tables used for aeration and hydraulics, confirming consistency with design norms. Why Other Options Are Wrong: 10–15°C are too low for most Indian cities; 25–30°C represent warm-season conditions but are not used as a single average baseline. Common Pitfalls: Confusing wastewater temperature with air temperature; using peak temperatures for all-year kinetic design without correction factors. Final Answer: 20°C

Question 5

In the natural (biological) process of anaerobic sludge digestion, which stage occurs first in the sequence of transformations?

Accepted Answer

Introduction / Context: Anaerobic digestion stabilizes sludge by converting complex organics into biogas. Understanding the sequential stages is essential for diagnosing digester upsets and setting loading/temperature controls. Given Data / Assumptions: Conventional mesophilic anaerobic digestion of municipal sludge. Mixed microbial consortia operate without oxygen. Staged view: hydrolysis/acidogenesis followed by acetogenesis/methanogenesis. Concept / Approach: The initial breakdown is hydrolysis and acidogenesis (commonly grouped as acid fermentation): polymers become monomers; fermentative bacteria produce volatile fatty acids, alcohols, CO2, NH3, and H2. Subsequent steps convert these acids into acetate and eventually methane and carbon dioxide under methanogens, often termed alkaline or methane fermentation because pH tends to recover upward as acids are consumed. Step-by-Step Solution: Recognize that complex organics must first be hydrolyzed/acidified.Identify this as the acid fermentation stage.Note that methane (alkaline) fermentation is the later stabilizing stage. Verification / Alternative check: Process monitoring shows initial pH drop and VFA increase, characteristic of acidogenesis, followed by pH recovery and gas production as methanogenesis dominates. Why Other Options Are Wrong: “Acid regression” is not a standard stage name; “Alkaline/methane fermentation” is subsequent; aerobic nitrification is unrelated to anaerobic sludge digestion. Common Pitfalls: Confusing acid build-up (temporary) with process failure; ignoring alkalinity buffering needed to keep pH in the methanogenic range. Final Answer: Acid fermentation (acidogenesis and hydrolysis)

Question 6

In a digestion tank, the sludge moisture content decreases from 90% to 80%. Assuming solids mass remains constant, what is the percentage decrease in sludge volume?

Accepted Answer

Introduction / Context: Sludge conditioning aims to reduce water content. When solids mass is constant, volume change is governed by moisture change. This type of computation is common in sizing digesters, thickeners, and storage. Given Data / Assumptions: Initial moisture = 90% → solids fraction = 10%. Final moisture = 80% → solids fraction = 20%. Solids mass remains constant; sludge density approximated uniform for mass–volume proportionality. Concept / Approach: Let solids mass be S. Total mass (and volume, by proportionality) initially Mi = S/(solids fraction) = S/0.10 = 10S. Finally Mf = S/0.20 = 5S. The percentage decrease is (Mi − Mf)/Mi * 100. Step-by-Step Solution: Mi = 10S.Mf = 5S.Decrease = 10S − 5S = 5S.Percentage decrease = (5S / 10S) * 100 = 50%. Verification / Alternative check: Doubling the solids fraction from 10% to 20% halves the volume for constant solids mass, confirming 50% reduction. Why Other Options Are Wrong: 25%, 10%, and 5% underestimate the strong effect of halving the water fraction; 40% still underestimates. Common Pitfalls: Using moisture percentages directly on water mass rather than on total mass; forgetting that constant solids mass means volume is inversely proportional to solids fraction. Final Answer: 50%

Question 7

In preliminary treatment, coarse screens can sometimes be omitted if which maceration device is provided to handle large solids in sewage?

Accepted Answer

Introduction / Context: Coarse screening protects downstream pumps and basins from rags and debris. Where screening and manual raking are undesirable, in-line maceration can be used to reduce solids size so they pass safely through the system. Given Data / Assumptions: Domestic/municipal wastewater with rags and fibrous materials. Desire to avoid separate screenings handling and disposal. Use of in-line mechanical devices. Concept / Approach: Comminutors and shredders grind/coarsely macerate solids to small particles that continue with the flow, reducing the need for coarse bar screens. Plant choice depends on headloss, maintenance, and grit content. Step-by-Step Solution: Identify equipment that replaces coarse screening by size reduction.Comminutor and shredder both perform maceration/grinding.Therefore, selecting both (a) and (b) is correct. Verification / Alternative check: Vendor literature and design guides list comminutors/grinders as alternatives to coarse screens in specific layouts. Why Other Options Are Wrong: Venturi aerator does not macerate solids; “neither” is incorrect because macerators clearly can replace coarse screens in many designs. Common Pitfalls: Ignoring grit abrasion on cutters; placing macerators upstream of grit removal leading to wear; not providing bypass arrangements. Final Answer: Both (a) and (b)

Question 8

A circular sewer flows at a depth equal to one-quarter of its diameter (y = D/4). What is the wetted perimeter of the flowing segment, expressed in terms of D?

Accepted Answer

Introduction / Context: Hydraulic computations for partially full circular sewers require geometric elements such as wetted perimeter P and area A. From these, hydraulic radius R = A/P and velocity by Manning or Chezy can be obtained. Given Data / Assumptions: Circular sewer of diameter D, radius R = D/2. Flow depth y = D/4. Rigid geometry; no deformation. Concept / Approach: For a circular segment: if 2θ is the central angle (in radians) subtended by the water surface at the pipe centre, the relation between depth and θ is y = R(1 − cos θ). The wetted perimeter equals the arc length P = 2θR. Step-by-Step Solution: Set y = D/4 = R/2. Use y = R(1 − cos θ) → R/2 = R(1 − cos θ).Solve for cos θ = 1 − 1/2 = 1/2 → θ = π/3.Compute P = 2θR = 2*(π/3)*(D/2) = πD/3. Verification / Alternative check: At half-full, P would be πR = πD/2. Our depth is less than half-full, so P = πD/3 is consistent (smaller than πD/2). Why Other Options Are Wrong: πD/6 underestimates the arc; πD/2 corresponds to half-full; πD and 2πD/3 are too large for this shallow depth. Common Pitfalls: Confusing θ with 2θ; using degrees in place of radians when multiplying by radius; substituting diameter for radius in arc-length formula. Final Answer: πD/3

Question 9

For large-diameter municipal sewers, what is the typical maximum spacing adopted between manholes along straight reaches (subject to local codes and access needs)?

Accepted Answer

Introduction / Context: Manholes provide access for inspection, cleaning, connection, and ventilation. Spacing depends on pipe size, equipment reach, and alignment geometry, with larger sewers allowing longer spacing. Given Data / Assumptions: Straight alignments without junctions or gradient changes. Large-diameter gravity sewers (not small house laterals). Typical standards used in many municipalities. Concept / Approach: Rules of thumb: small sewers 80–120 m, medium up to ~150–200 m, and large trunks up to ~300 m depending on cleaning technology and safety. Hence a maximum of about 300 m is commonly cited for large sewers, barring bends, junctions, or slope changes which require closer spacing. Step-by-Step Solution: Identify that larger sewers permit longer access intervals.Recall typical code limits and operational needs.Select 300 m as the practical upper spacing for large trunks. Verification / Alternative check: Many design guides list 250–300 m maxima for trunks, aligning with jetting and CCTV ranges. Why Other Options Are Wrong: 50–200 m are typical for smaller pipes or constrained sites; 400 m often exceeds accepted access limits for maintenance and safety. Common Pitfalls: Ignoring the need for manholes at grade breaks, changes in diameter, junctions, or at every bend; overextending spacing beyond cleaning equipment capability. Final Answer: 300 m

Question 10

Select the correct statement regarding water-supply mains versus gravity sewers in planning and layout.

Accepted Answer

Introduction / Context: Network planning differs for pressurized water mains and gravity sewers. Understanding hydraulics, gradients, and solids behavior is crucial for reliable service and maintenance. Given Data / Assumptions: Water-supply mains are pressurized, conveying treated water. Gravity sewers flow partially full and need sufficient slope for self-cleansing. Topography may vary across the corridor. Concept / Approach: Water mains are not “pure” in the absolute sense but contain low solids; they do not rely on a self-cleansing velocity concept like sewers. Sewers must maintain grade to transport settleable solids; their alignment may follow undulating ground using appropriate slopes, siphons, or aqueducts to cross valleys while maintaining hydraulic requirements. Step-by-Step Solution: Reject absolute claims about purity or clogging rules for water mains.Recognize gravity sewer design depends on slope, not “level” laying.Select the statement acknowledging sewers can traverse varied topography with proper design. Verification / Alternative check: Design manuals show long sewer profiles rising and falling with terrain, incorporating manholes and siphons where necessary. Why Other Options Are Wrong: (a) “Perfectly pure” is inaccurate; (b) self-cleansing velocity pertains to sewers; (d) sewers cannot be laid level—minimum slopes are required to avoid deposition; (e) is false because one statement is correct. Common Pitfalls: Applying sewer criteria to water mains; assuming sewer lines must mirror flat planes rather than hydraulic grade. Final Answer: Gravity sewers may be routed up and down hills and valleys while maintaining hydraulic gradient and adequate slope

Question 11

In a standard-rate trickling filter followed by secondary settling, by what percentage is the influent BOD typically reduced under normal operation?

Accepted Answer

Introduction / Context: Trickling filters are attached-growth biological reactors. Their performance is often reported as percentage biochemical oxygen demand (BOD) removal after secondary clarification. Knowing typical ranges aids in process selection and compliance estimates. Given Data / Assumptions: Domestic sewage, standard-rate trickling filter. Proper distribution, ventilation, and media depth. Settled effluent measured after secondary clarification. Concept / Approach: Under well-operated conditions, trickling filter systems commonly achieve about 80–90% BOD removal overall. Loadings, temperature, and recirculation affect performance; high-rate configurations may have different ranges but still target secondary-quality effluent. Step-by-Step Solution: Identify typical secondary-treatment performance criteria.Match with established removal efficiencies of trickling filter + clarifier.Select 80–90% as the representative range. Verification / Alternative check: EPA and standard texts list secondary processes (TFs and activated sludge) in roughly the same overall BOD removal bracket when properly designed. Why Other Options Are Wrong: Lower ranges (30–60%) correspond to preliminary/primary removal or poorly loaded filters; 95% is typical of tertiary polishing, not standard TF systems. Common Pitfalls: Confusing primary settling removal (about 25–35%) with biological secondary removal; ignoring the contribution of the secondary clarifier. Final Answer: 80 to 90%

Question 12

Primary treatment of sewage mainly targets the removal of which class of contaminants before biological secondary treatment?

Accepted Answer

Introduction / Context: Primary treatment prepares wastewater for biological processes by reducing settleable and floatable load. Clarifying its principal objective helps in sizing sedimentation tanks and anticipating downstream loads. Given Data / Assumptions: Municipal sewage with typical solids spectrum. Conventional preliminary units (screens, grit chambers) are upstream. Primary settling tanks follow preliminary treatment. Concept / Approach: Primary treatment comprises sedimentation to remove settleable suspended solids (a large fraction of which are organic) and scum skimming. While oil/grease and grit are addressed too, the main quantitative removal is of suspended solids by gravity settling, cutting BOD by about 25–35% prior to secondary treatment. Step-by-Step Solution: Recognize the stage sequence: screening/grit → primary sedimentation.Identify the dominant mass removed: settleable suspended solids (largely organic).Choose the option reflecting this principal function. Verification / Alternative check: Mass balances show primary sedimentation removes significant TSS and a portion of BOD; dedicated API separators are used when oil/grease is the main concern. Why Other Options Are Wrong: (b), (c), and (d) each cover only a subset; primary treatment is broader but quantitatively centered on suspended solids removal via settling. Common Pitfalls: Equating primary treatment solely with screening or grit removal; overlooking scum removal as ancillary to sedimentation. Final Answer: Large suspended organic solids by sedimentation

Question 13

Which instrument is typically used to determine the pH value of sewage accurately during laboratory or field testing?

Accepted Answer

Introduction / Context: pH strongly influences biological activity, chemical dosing, and corrosion in sewers. Accurate measurement is required for process control and compliance reporting. Given Data / Assumptions: Routine wastewater testing in a treatment plant or laboratory. Standard methods for pH measurement. Concept / Approach: pH is measured electrometrically using a potentiometric device (pH meter) with glass/reference electrodes. The meter reports hydrogen ion activity as pH. Other instruments serve different purposes: Imhoff cone measures settleable solids, turbidimeter measures turbidity, and jar tester evaluates coagulation/flocculation performance. Step-by-Step Solution: Identify the property: pH → requires electrometric measurement.Match the instrument: potentiometer (pH meter) with electrodes.Select the corresponding option. Verification / Alternative check: Calibration with buffer solutions pH 4, 7, and 10 ensures accurate readings—standard practice in wastewater labs. Why Other Options Are Wrong: Imhoff cone, turbidimeter, and jar tester do not measure pH; they target settleability, turbidity, and coagulation studies respectively. Common Pitfalls: Using indicator papers for approximate pH when precision is required; neglecting temperature compensation on pH meters. Final Answer: Potentiometer (electronic pH meter)

Question 14

Stoneware sewers (vitrified clay pipes) are commonly manufactured in a range of small diameters. Which of the following nominal sizes are commercially available for municipal house connections and laterals?

Accepted Answer

Introduction / Context: Stoneware (vitrified clay) sewers are widely used for house connections and short laterals because they are resistant to corrosion from sulfides and domestic sewage. A frequent exam point is recognising the standard nominal diameters in which these pipes are commonly supplied. Given Data / Assumptions: The question concerns commercially available nominal sizes for stoneware sewer pipes. Context is municipal wastewater systems (house connections and small laterals). Options list 10 cm, 15 cm, 20 cm, and 25 cm sizes. Concept / Approach: House service connections typically begin at small diameters due to relatively low flows. Traditional catalog sizes for vitrified clay include 100 mm, 150 mm, 200 mm, and 250 mm, matching the listed metric values. These sizes balance hydraulic capacity, maintenance practicality, and availability of fittings and joints. Step-by-Step Solution: Identify common nominal diameters for stoneware: 100, 150, 200, and 250 mm.Map to options: 10 cm, 15 cm, 20 cm, and 25 cm.Since all listed sizes are standard, the correct choice includes all items. Verification / Alternative check: Typical procurement schedules and BIS-style specifications show the same nominal diameter series for vitrified clay, confirming the completeness of the list provided in the options. Why Other Options Are Wrong: Any single-size choice omits other equally standard sizes and is therefore incomplete. Common Pitfalls: Confusing minimum permitted sewer diameters (for public mains) with commercially available pipe sizes for house connections. Assuming availability only in one or two sizes; in practice, multiple sizes exist for graded capacities. Final Answer: All of the above

Question 15

Sewer ventilation practice: What is the typical spacing (centre-to-centre distance) recommended between ventilation columns along a sewer line to control gases and odours?

Accepted Answer

Introduction / Context: Ventilation columns (vents) help dissipate corrosive and odorous gases (like hydrogen sulfide and methane) from sewers, reducing corrosion of pipes and manhole covers while improving public health and safety. Appropriate spacing is a core design decision in low- to medium-velocity gravity networks. Given Data / Assumptions: Conventional gravity sewers in urban or peri-urban settings. Passive ventilation strategy relying on natural draft. Spacing should prevent gas accumulation and promote regular air exchange. Concept / Approach: Field practice and design guides commonly recommend installing ventilating columns at intervals sufficient to avoid dead-air pockets yet not so frequent as to be uneconomical. The 150 m to 300 m range is widely cited for general conditions, with closer spacing at branch summits or near odor-sensitive receptors. Step-by-Step Solution: Identify recommended range from standard practice: 150–300 m.Compare to provided options and select the matching interval.Acknowledge that site specifics (terrain, diameter, flow) can adjust spacing but the baseline falls in this band. Verification / Alternative check: Published municipal standard details and codes frequently repeat the same spacing guidance, sometimes specifying additional vents at ends of branches or significant changes in size or slope. Why Other Options Are Wrong: Closer than 150 m: typically unnecessary and cost-ineffective unless special conditions exist.Greater than 300 m: may allow gas accumulation and odour complaints. Common Pitfalls: Ignoring additional vents at summits and branch heads. Assuming one-size-fits-all spacing without considering local odour complaints or corrosion history. Final Answer: 150 m to 300 m

Question 16

Ventilating shafts along sewers are generally provided at approximately what regular interval under standard municipal practice?

Accepted Answer

Introduction / Context: Ventilating shafts supplement manholes and ventilating columns to maintain air exchange within sewer networks. By placing shafts at regular intervals, designers reduce the accumulation of corrosive gases and balance internal air pressure with the atmosphere. Given Data / Assumptions: Conventional gravity sewers in flat to gently sloping terrain. Passive ventilation (no mechanical blowers). Regular interval design to simplify layout and construction. Concept / Approach: Common guidelines call for ventilating features approximately every 150 m, with adjustments for branch ends, high points, and points of size or slope change. The 150 m spacing tends to prevent stagnation of air pockets and supports odour control strategies. Step-by-Step Solution: Identify typical interval from practice: about 150 m.Match to the given options and select 150 m.Note that tighter spacing may be applied near sensitive areas or where turbulence is high. Verification / Alternative check: Many municipal specifications and study examples use 150 m as a standard design value for vents/shafts, with permitted variation based on local conditions. Why Other Options Are Wrong: 100 m: conservative and costlier; used only when local odour/corrosion history justifies it.200–500 m: may permit gas accumulation; not preferred for general conditions. Common Pitfalls: Overlooking venting at summits or at terminal ends of laterals. Assuming that manholes alone always provide sufficient ventilation. Final Answer: 150 m

Question 17

Resistance to sulfide corrosion in sewers: Among the following materials, which sewer type best resists hydrogen sulfide–induced corrosion in domestic wastewater environments?

Accepted Answer

Introduction / Context: Hydrogen sulfide produced under septic conditions in sewers can oxidize to sulfuric acid at moist surfaces, severely corroding many traditional materials. Material selection is therefore crucial to ensure durability and low lifecycle cost in corrosive wastewater atmospheres. Given Data / Assumptions: Environment: domestic sewage with potential for sulfide generation. Materials considered: brick, cast iron, reinforced concrete, and lead. Evaluation based on chemical resistance to sulfide attack and acidic conditions. Concept / Approach: Acidic condensates attack cement paste in concrete and mortar in brickwork, and also corrode unprotected ferrous metals. Traditionally, vitrified clay, certain plastics, and lead have shown strong chemical resistance to such attack. Among the options, lead exhibits excellent corrosion resistance to sulfuric environments compared with brick, cast iron, and ordinary R.C.C. Step-by-Step Solution: Assess brick sewer: mortar and bricks are vulnerable to acid dissolution.Assess cast iron: unlined iron suffers from acid corrosion unless protected.Assess R.C.C.: sulfate/sulfuric acid degrade cementitious matrix and steel unless specially protected.Lead: notable chemical resistance in acidic wastewater settings; thus most resistant among given choices. Verification / Alternative check: Historically, vitrified clay and lead have been used where high corrosion potential exists. Modern alternatives include PVC, HDPE, and glass-fiber reinforced pipes with appropriate linings. Within the listed legacy materials, lead is the most corrosion resistant. Why Other Options Are Wrong: Brick: mortar joints and bricks deteriorate under sulfuric acid attack.Cast iron: corrodes without protective linings or coatings.R.C.C.: cement paste is chemically vulnerable; requires liners or coatings to resist. Common Pitfalls: Assuming concrete is universally durable in all wastewater conditions without liners. Overlooking microbially induced corrosion at the air–sewage interface. Final Answer: Lead sewer

Question 18

Radiation sensitivity of human organs: Which organ group has the lowest tolerance (i.e., withstands the minimum radiation) and is most radiosensitive?

Accepted Answer

Introduction / Context: In environmental engineering and public health, knowledge of organ radiosensitivity is important for safety standards during radiological surveys, medical imaging considerations, and environmental exposure assessments. Given Data / Assumptions: Comparison among thyroid, kidneys, eyes, and gonads (ovaries/testis). Focus is on relative radiosensitivity (lower tolerance implies higher sensitivity). Concept / Approach: Rapidly dividing cells and tissues with significant reproductive potential are generally more radiosensitive. The gonads are highly sensitive because radiation-induced damage can affect reproductive cells and genetics, leading to hereditary effects and impaired fertility. While the lens of the eye is also sensitive (risk of cataracts), gonadal tissue is typically considered the most critical for protection. Step-by-Step Solution: List common sensitive organs: gonads and lens of the eye rank high.Evaluate the options: the gonads are broadly regarded as having the lowest safe exposure threshold due to genetic and fertility implications.Select ovaries/testis as the most radiosensitive among given choices. Verification / Alternative check: Radiation protection guidelines emphasize shielding of gonads and limiting exposure, reflecting their high radiosensitivity relative to many other organs. Why Other Options Are Wrong: Thyroid: sensitive but usually shielded and less critical than gonads for hereditary risk.Kidneys: more radioresistant than gonadal tissue.Eyes: lens is sensitive to cataract formation but hereditary implications make gonads the priority. Common Pitfalls: Confusing tissue radiosensitivity (biological effect) with short-term symptomatic effects. Assuming organ mass dictates sensitivity; cell type and mitotic activity are key. Final Answer: Ovaries/Testis

Question 19

Air pollution basics: Among the gases listed, which is most significant as a conventional air pollutant due to its health and environmental impacts from combustion sources?

Accepted Answer

Introduction / Context: Air pollutants are substances in the atmosphere that, even at low concentrations, harm human health, ecosystems, or materials. Identifying key pollutants assists in control strategies and regulatory frameworks for ambient air quality management. Given Data / Assumptions: Common gases present in ambient air or emitted from combustion. Evaluation focuses on direct pollutant status and health effects at typical ambient concentrations. Concept / Approach: Sulphur dioxide (SO2) is a classic criteria pollutant produced by combustion of sulfur-containing fuels. It causes respiratory irritation, contributes to acid deposition, and forms fine particulate sulfates that harm visibility and health. By contrast, oxygen and nitrogen are natural major constituents of air, and carbon dioxide, while a greenhouse gas of climate concern, is not treated as a primary toxic air pollutant in the same health-based sense as SO2 at ambient levels. Step-by-Step Solution: List candidate gases: CO2, O2, N2, SO2.Assess pollutant significance: SO2 is regulated for acute and chronic exposure impacts.Select SO2 as the most significant conventional air pollutant among the listed gases. Verification / Alternative check: Ambient air quality standards worldwide include SO2 as a criteria pollutant with prescribed limits to protect public health and ecosystems, underscoring its significance. Why Other Options Are Wrong: Carbon dioxide: addressed primarily under climate policy rather than acute ambient toxicity control.Oxygen, Nitrogen, Helium: not pollutants under normal conditions; these are essential or inert atmospheric components. Common Pitfalls: Equating greenhouse gas status with conventional air pollutant toxicity. Overlooking secondary particulate formation from SO2 emissions. Final Answer: Sulphur dioxide

Question 20

Minimum gradient for self-cleansing: For a 150 mm diameter sanitary sewer under Indian practice, the typical minimum gradient adopted to achieve self-cleansing velocity is approximately:

Accepted Answer

Introduction / Context: Self-cleansing velocity criteria ensure that solids do not settle in sewers at low flows. Designers use recommended minimum gradients by diameter to achieve these velocities without causing undue excavation or scouring risks. Given Data / Assumptions: Pipe diameter: 150 mm (house connections and small laterals). Conventional Indian practice for minimum gradients. Typical target self-cleansing velocity around 0.6–0.75 m/s. Concept / Approach: Empirical tables relate minimum gradients to diameters so that, at minimum design flows, the resulting mean velocity meets self-cleansing thresholds. For 150 mm diameter, a common recommended gradient is about 1 in 100, balancing hydraulic performance with constructability. Step-by-Step Solution: Identify recommended gradient for 150 mm from typical design tables: approximately 1 in 100.Check that this gradient yields mean velocities near the self-cleansing range at expected DWF.Select 1 in 100 from the options. Verification / Alternative check: Worked examples and standard schedules often list 1/100 for 150 mm sewers, with variations for local conditions and roughness values. Why Other Options Are Wrong: 1 in 60: steeper than necessary, potentially increasing excavation cost.1 in 120: may be marginal for self-cleansing under low flows.None/1 in 300: not representative or too flat for reliable self-cleansing. Common Pitfalls: Applying one gradient to all diameters; tables vary with size. Ignoring roughness increase over time due to slime and sediment. Final Answer: 1 in 100

Waste Water Engineering Questions