Question 1

Disinfection choice worldwide: The most widely used and practically ideal disinfectant for public drinking-water supplies is

Accepted Answer

Introduction / Context: Disinfection is the final barrier in drinking-water treatment. The disinfectant should be effective, economical, and provide a measurable residual in the distribution system to guard against recontamination. Given Data / Assumptions: Common municipal practice worldwide. Focus on primary/secondary disinfection and residual maintenance. Concept / Approach: Chlorine is the global workhorse disinfectant. It is effective against many pathogens, relatively inexpensive, and leaves a residual. Alternatives like ozone or UV are powerful but leave no residual; they are often paired with chlorine/chloramines downstream. Step-by-Step Solution: Evaluate options for microbial efficacy and residual.Identify chlorine as the agent meeting both effectiveness and residual requirements.Select chlorine as the most practical, widely used choice. Verification / Alternative check: WHO and many national standards reference chlorine/chloramine residual targets in distribution, confirming routine reliance on chlorine-based disinfectants. Why Other Options Are Wrong: Alum: a coagulant, not a disinfectant. Lime: used for pH/softening control, not primary disinfection. Nitrogen/ammonia: not disinfectants (ammonia forms chloramines with chlorine but is not used alone for disinfection). Common Pitfalls: Confusing coagulants and disinfectants. Assuming ozone/UV are “ideal” without considering lack of residual. Final Answer: Chlorine.

Question 2

Economic and hydraulic considerations in pipe sizing: Which statement is correct when comparing smaller-diameter pipes at the same flow velocity and service conditions?

Accepted Answer

Introduction / Context: Choosing pipe diameter balances capital cost against operating cost. Friction losses strongly depend on diameter, so smaller pipes tend to increase head loss and energy use at a given flow rate. Exam questions often test qualitative understanding of these trends. Given Data / Assumptions: Same flow velocity or same discharge as stated. Comparable roughness and material. Pumping is required to overcome friction where relevant. Concept / Approach: Darcy–Weisbach shows head loss h_f ∝ (L/D) * V^2; at equal velocity, smaller D produces higher h_f. At equal discharge, V increases in a smaller pipe, further raising losses. Pumping power ∝ Q * head; therefore, smaller pipes typically raise operating cost. Capital cost generally increases with diameter, not vice versa. Step-by-Step Solution: Assess (a): Smaller D at equal V → larger head loss, so (a) is false.Assess (b): With higher friction in smaller D for same Q, pumping cost increases, not decreases → false.Assess (c): Smaller pipes are cheaper in capital cost, not more expensive → false.Thus “None of these” is correct. Verification / Alternative check: Manufacturer curves and lifecycle cost analyses consistently show a trade-off: larger initial cost for larger pipes but savings in pumping energy over time; the statements (a)–(c) contradict this basic principle. Why Other Options Are Wrong: (a) and (b) reverse friction/power trends. (c) contradicts typical material/installation pricing. (e) cannot be true if any are false. Common Pitfalls: Comparing at equal velocity vs. equal discharge without care—either way, smaller D increases losses. Ignoring roughness/age effects that further penalize smaller pipes. Final Answer: None of these.

Question 3

Raw-water intake siting: Pick the incorrect requirement for locating a water-supply intake on a river, lake, or reservoir.

Accepted Answer

Introduction / Context: Proper intake location is critical to raw-water quality and operational reliability. Intakes should minimize contamination risk and sediment/debris intake while ensuring access for operation and maintenance across hydrologic conditions. Given Data / Assumptions: Surface-water source used for municipal supply. Typical river hydraulics with upstream–downstream gradients. Concept / Approach: To reduce contamination, raw-water intakes are placed upstream of wastewater outfalls and other pollution sources. They should also be close to the plant to limit pipeline cost, located in good-quality water zones, and accessible in flood events. Debris/sediment fields are avoided to reduce wear and clogging. Step-by-Step Solution: Identify the statement that contradicts standard siting practice: locating the intake downstream of an outfall—this is incorrect.All other statements align with accepted siting guidance. Verification / Alternative check: Design manuals and environmental regulations emphasize buffer distances upstream of discharges where feasible, or require additional treatment safeguards if hydraulic constraints exist. Why Other Options Are Wrong (as answers): (a), (b), (d), (e) are correct siting principles; they are not the “incorrect” statement. Common Pitfalls: Confusing “upstream” and “downstream” relative to flow direction. Ignoring seasonal changes in water quality zones in stratified reservoirs. Final Answer: It should be located downstream of the wastewater disposal (outfall) point.

Question 4

Purpose of filtration in conventional water treatment: What is the primary contaminant class targeted by filtration (after sedimentation and before disinfection)?

Accepted Answer

Introduction / Context: Filtration (e.g., rapid sand filters, dual-media filters) follows clarification to polish the water. Its core purpose is to remove the remaining fine particles that cause turbidity and can shield microorganisms from disinfectants. Given Data / Assumptions: Conventional treatment sequence: coagulation–flocculation–sedimentation–filtration–disinfection. Use of rapid gravity filters with appropriate backwashing. Concept / Approach: Filtration physically strains and adsorbs fine suspended matter and microflocs, reducing turbidity and particle counts. While it does reduce microbial counts significantly, complete pathogen inactivation is achieved by subsequent disinfection. Colour and odour may be partly reduced but typically require activated carbon or oxidation for reliable removal. Step-by-Step Solution: Identify the principal target: turbidity and particulates.Acknowledge secondary benefits: some colour/odour reduction and microbial reduction.Conclude that turbidity removal is the primary design objective. Verification / Alternative check: Turbidity performance goals (e.g., NTU limits) are standard compliance metrics for filters, confirming their primary role. Why Other Options Are Wrong: (a) and (b) are not primary filtration objectives; rely on carbon/oxidation. (d) claims complete pathogen removal; final inactivation is via disinfection. (e) incorrectly suggests equal removal of all classes. Common Pitfalls: Assuming filtration alone ensures microbial safety without disinfection. Overlooking headloss development and backwash scheduling that affect turbidity capture. Final Answer: Turbidity (fine suspended and colloidal particles).

Question 5

Permanent hardness in water treatment In municipal water-supply engineering, which of the following processes is specifically used to remove permanent hardness (caused by calcium and magnesium salts of sulphates, chlorides, etc.)?

Accepted Answer

Introduction / Context: Hardness of water is of two types: temporary (carbonate hardness) and permanent (non-carbonate hardness). Permanent hardness arises mainly from calcium and magnesium sulphates and chlorides. It does not respond to simple boiling and needs specific softening techniques used in water-treatment practice. Given Data / Assumptions: Permanent hardness is due to Ca^2+ and Mg^2+ salts of sulphates/chlorides/nitrates. Municipal-scale treatment options are being compared. Goal is hardness removal, not turbidity removal or disinfection. Concept / Approach: Temporary hardness can be removed by boiling or by lime softening as bicarbonates decompose. Permanent hardness requires ion exchange or advanced chemical softening. The zeolite (sodium ion-exchange) process exchanges Ca^2+/Mg^2+ with Na^+ on the resin/zeolite, effectively removing non-carbonate hardness without forming scale. Step-by-Step Solution: Identify the target: remove permanent hardness (non-carbonate).Evaluate options: boiling and simple lime treatment are ineffective for sulphates/chlorides unless combined with soda ash.Select the standard method: zeolite (sodium ion-exchange) softening. Verification / Alternative check: Plant design manuals list sodium zeolite softeners and lime-soda ash processes for permanent hardness. Boiling or alum/chlorine addition do not remove non-carbonate hardness. Why Other Options Are Wrong: Adding alum: coagulant for turbidity/colour, not hardness. Adding lime only: primarily addresses temporary hardness unless combined with soda ash. Chlorination: for disinfection, no hardness reduction. Boiling: precipitates bicarbonates only (temporary hardness). Common Pitfalls: Confusing turbidity removal with hardness removal, and assuming boiling treats all hardness types. Final Answer: Zeolite (sodium ion-exchange) process

Question 6

Filter media specification for rapid gravity filters The recommended uniformity coefficient (U.C.) for good quality filter sand used in rapid gravity filters should be approximately:

Accepted Answer

Introduction / Context: In rapid gravity filtration, the filter media gradation critically affects hydraulic performance, head loss, and particle capture. The uniformity coefficient (U.C.) indicates how uniformly graded the sand is and is defined as D60/D10, where D60 and D10 are particle sizes at 60% and 10% finer by weight. Given Data / Assumptions: Filter media is silica sand used in rapid gravity filters. Goal is a practical U.C. that balances run length and backwash behaviour. Concept / Approach: A lower U.C. indicates a more uniform gradation. Standard practice for rapid gravity sand filters targets effective size (D10) around 0.45–0.70 mm and a U.C. not exceeding about 1.7–2.0. This ensures predictable hydraulic gradients and efficient backwashing without excessive media loss. Step-by-Step Solution: Recall typical specification range: U.C. ≤ about 1.7–2.0.Among given discrete options, the closest acceptable value is 2.Hence, choose U.C. ≈ 2 as the recommended figure. Verification / Alternative check: Design manuals consistently indicate U.C. less than about 1.7–2.0 for rapid filters; higher values imply broad gradation, reducing performance. Why Other Options Are Wrong: 3, 4, 5: too broadly graded; lead to non-uniform flow paths and poorer performance. None of these: incorrect, because U.C. ≈ 2 is a recognized practical limit. Common Pitfalls: Confusing slow sand filter specs (finer media) with rapid filters and tolerating overly broad gradation. Final Answer: 2

Question 7

Terminology in hydrology The total flow in a stream channel is commonly referred to as:

Accepted Answer

Introduction / Context: Hydrologic terms can overlap depending on context. Engineers must correctly interpret phrases like runoff, streamflow, and discharge when estimating water availability, designing structures, or analyzing floods. Given Data / Assumptions: “Total flow in a stream” means the volumetric rate passing a cross-section. Steady conditions are not assumed; terminology is generic. Concept / Approach: Runoff represents water from precipitation and other inputs reaching the channel (surface and subsurface contributions). When measured within the channel, that volumetric rate is termed streamflow or discharge (commonly expressed in m^3/s or cumecs). In many practical contexts, these terms are used interchangeably for the total channel flow at a section. Step-by-Step Solution: Identify equivalent descriptors: streamflow and discharge both denote volumetric flow rate.Runoff, when reaching and conveyed by the channel, contributes to that same total flow.Therefore, the inclusive option “All of the above” is appropriate. Verification / Alternative check: Hydrology texts define discharge/streamflow synonymously and connect runoff to measured flow at the outlet of a basin. Why Other Options Are Wrong: Each single term alone is not “more correct” than the others in this broad wording; they collectively represent the same concept in this context. Common Pitfalls: Restricting “runoff” only to surface contribution and excluding interflow/baseflow, which are routinely included in catchment runoff definitions. Final Answer: All of the above

Question 8

Role of alum in water treatment In conventional water-treatment trains, alum (aluminium sulphate) is primarily used as a:

Accepted Answer

Introduction / Context: Alum is one of the most widely used chemicals in water treatment. Understanding its function ensures proper jar testing, dosing control, and integration with subsequent sedimentation and filtration steps. Given Data / Assumptions: Chemical is aluminium sulphate (“alum”). Applied at rapid mix stage before flocculation. Concept / Approach: As a coagulant, alum destabilizes colloids by neutralizing their surface charges, enabling particle agglomeration. Coagulation is followed by flocculation (gentle mixing) to grow settleable flocs. Alum is not a disinfectant or catalyst; and while it aids floc formation, the distinct term “flocculant” often refers to polymer aids. Step-by-Step Solution: Identify treatment step: coagulation precedes flocculation and sedimentation.Alum neutralizes colloidal charges → microfloc formation.Therefore, alum’s primary role is as a coagulant. Verification / Alternative check: Design manuals and jar test protocols classify alum under primary coagulants, often aided by polymers (flocculants) for improved settling. Why Other Options Are Wrong: Flocculant: usually refers to polymer aids, not alum’s primary classification. Catalyst: not applicable. Disinfectant: chlorination/ozonation are used instead. Scale inhibitor: unrelated to coagulation. Common Pitfalls: Using the terms coagulant and flocculant interchangeably; in practice they are distinct steps/chemicals. Final Answer: Coagulant

Question 9

Per-capita water requirement for municipal design Which of the following is a typical per-capita water requirement adopted for urban planning (inclusive of domestic use and reasonable losses) in litres per capita per day (LPCD)?

Accepted Answer

Introduction / Context: Estimating demand is the first step in sizing water-supply systems. Per-capita water requirement (LPCD) depends on city size, service level, climate, and policy. A representative value is often used for preliminary design, refined later with category-wise breakdowns. Given Data / Assumptions: Urban system with house connections and standard amenities. Losses and non-revenue water within reasonable bounds. Concept / Approach: Commonly adopted planning figures for Indian urban areas range around 135–150 LPCD for residential and allied uses (excluding major industries). For a single best choice among rounded values, 150 LPCD is widely used for preliminary design where explicit guidance prescribes this nominal figure. Step-by-Step Solution: Compare options to typical norms: 150 LPCD matches standard practice.90 LPCD is low for full service levels; 250–400 LPCD reflect high-demand categories or special facilities.Select 150 litres as the representative planning value. Verification / Alternative check: Municipal planning manuals often list 135–150 LPCD; final value is tailored to local policies. Why Other Options Are Wrong: Too low (90, 60) for urban amenities; too high (250, 400) for typical municipal residential design without special demands. Common Pitfalls: Ignoring category-wise additions (institutional, firefighting, losses), which can alter the total system demand beyond purely domestic use. Final Answer: 150 litres

Question 10

Population forecasting with the logistic curve In the logistic (S-shaped) population growth model for a developing city, the initial portion of the curve represents a period of:

Accepted Answer

Introduction / Context: Population forecasting methods guide demand estimation for water-supply systems. The logistic curve portrays growth that accelerates initially, then decelerates as environmental and infrastructural constraints appear, approaching a saturation level (carrying capacity). Given Data / Assumptions: Developing city undergoing early expansion. Logistic curve with lower asymptote, inflection point, and upper asymptote. Concept / Approach: The logistic function starts with relatively small numbers but an increasing rate of growth because resources and space are initially abundant. Growth rate peaks near the inflection point (mid-curve), then slows as limiting factors (space, economy, resources) reduce the net growth rate, approaching a steady upper limit. Step-by-Step Solution: Identify the early phase: resources plentiful → accelerating growth.Recognize the middle phase: maximum growth rate around inflection.Recognize the late phase: growth decelerates, approaching capacity. Verification / Alternative check: Fitting historical census data to a logistic model typically shows upward curvature initially (increasing growth), validating the interpretation. Why Other Options Are Wrong: Decreasing or constant growth contradict the S-curve’s initial acceleration. Random fluctuations or sudden decline do not reflect the systematic logistic behaviour. Common Pitfalls: Confusing “growth rate” with “population level.” The early phase has both increasing population and increasing growth rate. Final Answer: Increasing growth

Question 11

Incrustation and corrosion in wells and pipelines Which of the following statements is correct regarding well/pipeline problems due to water chemistry?

Accepted Answer

Introduction / Context: Operational issues in wells and pipelines commonly stem from two mechanisms: incrustation (chemical deposition) and corrosion (electrochemical attack). Both impair system performance and service life if not controlled through water chemistry management and maintenance. Given Data / Assumptions: Groundwater with dissolved minerals like calcium and bicarbonate. Steel or iron components exposed to varying pH, dissolved oxygen, and carbon dioxide. Concept / Approach: Incrustation results from precipitation of mineral scales (e.g., CaCO3) when conditions favour reduced solubility (e.g., CO2 loss, pH rise). Scale narrows flow passages and clogs screens, lowering yield. Corrosion is driven by acidity, dissolved oxygen, CO2, and galvanic effects, leading to metal loss and leaks. Sand pumping erodes mechanically but does not cause chemical corrosion. Step-by-Step Solution: Link scaling (CaCO3 deposition) to incrustation and lower discharge.Differentiate chemical corrosion causes (acidity, O2/CO2) from mechanical sand abrasion.Reject statements that contradict established mechanisms. Verification / Alternative check: Well redevelopment records often show yield recovery after descaling/acid treatment, confirming scale-driven loss. Corrosion coupons and pH/DO monitoring correlate with corrosion rates. Why Other Options Are Wrong: Incrustation does not increase capacity; it reduces effective area. Corrosion is strongly linked to acidity and oxidants; high alkalinity is generally protective. Sand withdrawal causes abrasion, not chemical corrosion. “All the above” cannot be true because multiple statements are false. Common Pitfalls: Assuming any deposition is beneficial; ignoring that slight pH shifts can drastically change scaling tendencies (Langelier index). Final Answer: Deposition of calcium carbonate inside well screens and pipes leads to incrustation, which reduces discharge over time

Question 12

Typical porosities of geologic materials Which of the following statements about porosity values is incorrect?

Accepted Answer

Introduction / Context: Porosity is the ratio of void volume to total volume and governs storage of groundwater and the compressibility of soils/rocks. Typical ranges vary by material and compaction state. Given Data / Assumptions: Values quoted are indicative typicals, not absolutes. Materials considered: clayey sand, clean sand, sandstone, and crystalline rocks. Concept / Approach: Clayey mixtures can exhibit relatively high porosity (often 40–50% or more). Clean sands commonly range 25–40% depending on gradation/packing. Sandstones may exhibit porosity from very low to over 20%, but “up to about 15%” is a reasonable indicative figure for compact sandstone. Dense crystalline rocks like granite and quartz generally have very low primary porosity. Step-by-Step Solution: Review each statement versus typical ranges.Find no clear contradiction with representative values.Hence, choose “None of these” as none is incorrect. Verification / Alternative check: Hydrogeology references list similar ranges: sands 25–40%, sandstones often 5–20% (variable), and crystalline rocks near-zero primary porosity. Why Other Options Are Wrong: Each individual statement reflects plausible typical values; therefore singling any one as incorrect would be unjustified. Common Pitfalls: Confusing porosity with permeability; high porosity (clays) may still have very low permeability. Final Answer: None of these

Question 13

Chloramination chemistry and pH control During disinfection by chloramination, maintaining the pH slightly below 7 favors the formation of which chloramine species?

Accepted Answer

Introduction / Context: Chloramination forms combined chlorine species by reacting free chlorine with ammonia. The distribution among monochloramine, dichloramine, and nitrogen trichloride depends strongly on pH, chlorine-to-ammonia ratio, temperature, and contact time. Utilities set pH targets to favour desired species and minimize taste/odour complaints. Given Data / Assumptions: Objective: determine which species is favoured when pH is slightly below 7. Ammonia is present and chlorine is dosed appropriately. Concept / Approach: General speciation trend with decreasing pH is: monochloramine (dominant around pH 7.5–8.5) → dichloramine (favoured as pH drops below ~7) → nitrogen trichloride (favoured at still lower pH, typically <4.5), which causes strong odour issues. Therefore, maintaining pH slightly below 7 shifts equilibrium toward dichloramine. Step-by-Step Solution: Recall speciation: higher pH favours monochloramine.As pH decreases below ~7, dichloramine becomes significant.At very low pH, nitrogen trichloride predominates. Verification / Alternative check: Operational guidance notes recommend pH control (often 7.2–8.5) to prioritise monochloramine and avoid dichloramine/trichloramine taste/odour. Why Other Options Are Wrong: Monochloramine (NH2Cl): favoured at higher pH, not slightly below 7. Nitrogen trichloride (NCl3): requires much lower pH. “No chloramines” and “free chlorine only”: incorrect when ammonia is present under typical chloramination conditions. Common Pitfalls: Using incorrect formulas for chloramines; the correct species are NH2Cl (mono), NHCl2 (di), and NCl3 (tri). Final Answer: Dichloramine (NHCl2)

Question 14

Optimal fluoride concentration for dental health To ensure proper growth and protection of children’s teeth, what is the typical fluoride dosage maintained in potable water supplies?

Accepted Answer

Introduction / Context: Controlled fluoridation of potable water is practiced in many regions to reduce dental caries in children. The dosage must be carefully set to achieve benefits while avoiding fluorosis risks. Given Data / Assumptions: Typical temperate climate and average water consumption. Standard guidance for community water fluoridation. Concept / Approach: Historically, about 1 mg/L (1 ppm) fluoride was considered optimal for many climates. Some modern guidelines adjust the target based on temperature and intake, but 1 mg/L remains a widely recognized benchmark in textbook problems addressing caries prevention without exceeding aesthetic/toxicity limits. Step-by-Step Solution: Identify the safe and effective range for caries prevention.Select 1 mg/L as the canonical textbook dosage.Higher concentrations increase risk of dental/skeletal fluorosis. Verification / Alternative check: Public health references historically cite ~0.7–1.2 mg/L depending on conditions; 1 mg/L is a standard teaching value. Why Other Options Are Wrong: 2–5 mg/L exceed typical recommended levels and risk fluorosis and aesthetic issues. Common Pitfalls: Ignoring natural background fluoride which may already meet or exceed targets; blending or defluoridation may be necessary. Final Answer: 1 mg/L

Question 15

Hydrostatic pressure testing of pipelines (water supply engineering): What do we call the maximum internal pressure that a pipe is required to safely withstand without leakage during a hydrostatic pressure test?

Accepted Answer

Introduction / Context: During commissioning of a new water main or a repaired pipeline, engineers carry out a hydrostatic pressure test to confirm structural integrity and tightness. The objective is to pressurize the line to a specified value that is higher than routine operating conditions and then verify that there is no leakage or unacceptable pressure drop. Understanding the correct terminology helps avoid confusion between design, working, and test conditions. Given Data / Assumptions: The pipeline is filled, vented, and pressurized with water. The specified target pressure is above normal service (working) pressure. The acceptance criterion is no visible leakage and pressure decay within allowable limits. Concept / Approach: Standards define several pressures: working (routine operating), design (basis for thickness and class), and test pressure (pressure applied during a hydrostatic test to prove tightness and strength). The correct term for “maximum pressure which the pipe can withstand without leakage during the hydrostatic test” is test pressure. This is not necessarily the ultimate strength; it is the verification pressure used during testing, commonly expressed as a factor of the rated pressure class or the maximum allowable operating pressure (often 1.25 to 1.5 times, depending on the code and material). Step-by-Step Solution: Identify the context: formal hydrostatic testing (not day-to-day service).Relate the definition to pipeline QA/QC procedures.Match the term used by standards for that pressure: “test pressure.” Verification / Alternative check: Commissioning checklists and standard specifications (for example, municipal water specifications) always label the applied proof pressure during hydro-testing as the test pressure, distinct from the working/design values. Why Other Options Are Wrong: Working pressure: the everyday operating level, typically lower. Design pressure: basis for design calculations, not necessarily the applied test level. Hydrostatic pressure: a generic phrase; not the specific acceptance pressure. Proof stress: a material property term, not a pipeline test term. Common Pitfalls: Assuming test pressure equals burst pressure; the test is far below failure. Confusing “no leakage” with “no pressure drop”; some standards allow a small pressure decay due to temperature or trapped air as long as leakage is absent. Final Answer: Test pressure

Question 16

Surface overflow rate (SOR) for sedimentation with coagulants: Normal design values for overflow rate in sedimentation tanks using coagulants are typically within which range, expressed in L/hour/m²?

Accepted Answer

Introduction / Context: Surface overflow rate (SOR), also called surface loading rate, is a fundamental parameter for sizing sedimentation (clarification) basins. When coagulation–flocculation precedes sedimentation, flocs settle faster than discrete particles, allowing higher SORs compared to plain sedimentation. Designers must choose an SOR that balances removal efficiency, footprint, and cost. Given Data / Assumptions: Conventional treatment train with coagulation–flocculation. Clarifier is designed for steady average flow, with allowance for peak factors. SOR is expressed as L/hour/m² for clarity and quick checks. Concept / Approach: In the classical Hazen framework, particle removal depends on the ratio of settling velocity to overflow rate. With coagulant-aided flocculation, typical practice adopts SORs in the order of 1000–1250 L/hour/m² (equivalent to roughly 24,000–30,000 L/m²/day). Values much lower waste area; much higher values risk floc carryover and turbidity breakthrough. Step-by-Step Solution: Convert common daily units: 24,000–30,000 L/m²/day ≈ 1000–1250 L/m²/hour.Compare with offered ranges and select the matching interval.Choose 1000 to 1250 L/hour/m² as the normal design range for coagulated waters. Verification / Alternative check: Plant experience and standard design manuals list conventional clarifier SORs in this band for coagulant-aided settling; plate settlers and lamella units may use higher surface loadings due to increased projected area. Why Other Options Are Wrong: 250–750 L/hour/m²: overly conservative for coagulated waters; excessive tank area. 750–1000 L/hour/m²: feasible but on the lower edge; many designs target higher values. 1250–2000 L/hour/m²: appropriate for high-rate lamella units, not conventional basins. Common Pitfalls: Ignoring peak flow conditions; SOR should be checked at peak and average. Overlooking inlet/outlet hydraulics which can cause short-circuiting even at acceptable SOR. Final Answer: 1000 to 1250

Question 17

Quality characteristics of groundwater from artesian wells: Which of the following statements about artesian groundwater quality and treatment needs are generally valid?

Accepted Answer

Introduction / Context: Artesian wells tap confined aquifers under pressure. Compared to surface sources, such groundwater is naturally filtered through geologic strata, which typically reduces suspended matter but can dissolve significant mineral content. Understanding these traits informs the selection and sizing of treatment processes for public supply. Given Data / Assumptions: Typical confined aquifer conditions with long residence times. Groundwater emerges under artesian pressure through a well. Regional mineralogy influences dissolved solids and hardness. Concept / Approach: Groundwater turbidity is usually very low because pore media act as physical filters. However, extended water–rock contact leads to dissolution of calcium, magnesium, bicarbonate, chloride, and sulphate ions, causing hardness and higher TDS. As a result, treatment may focus on softening, iron–manganese removal, or aeration rather than extensive clarification needs typical of surface water. Step-by-Step Solution: Assess suspended matter: generally minimal in artesian supplies.Assess dissolved salts: commonly present; may cause hardness or salinity.Compare treatment intensity: often lower than surface water (less coagulation/sedimentation), though conditioning may still be needed.Therefore, all statements collectively are valid. Verification / Alternative check: Utilities relying on artesian aquifers often operate minimal pretreatment (aeration, filtration, disinfection) relative to surface water plants that require full clarification, confirming the generalizations above. Why Other Options Are Wrong: Each individual statement is true but incomplete; only “All the above” captures the typical overall picture for artesian groundwater. Common Pitfalls: Assuming all groundwater is soft; many aquifers produce hard water needing softening. Overlooking iron/manganese which may oxidize to cause discoloration if not treated. Final Answer: All the above

Question 18

Sedimentation in reservoirs and dead storage life: A reservoir has total capacity of 10 million m³. If the average silt deposition is 0.1 million m³ per year, and assuming (as is common in preliminary design) that dead storage is 25% of total capaci…

Accepted Answer

Introduction / Context: Reservoir capacity is commonly partitioned into dead storage (intended to trap sediment and unusable by outlet works), live or useful storage, and sometimes surcharge. Estimating the time for dead storage to silt up is essential for planning operational life and for scheduling desilting or dredging interventions. This problem tests basic sediment budgeting using simple proportional reasoning. Given Data / Assumptions: Total capacity, C_total = 10 million m³. Annual sediment deposition, S = 0.1 million m³ per year. Assumption for preliminary design: dead storage is 25% of total capacity (a common textbook simplification stated explicitly here). All incoming sediment is trapped in dead storage (conservative assumption for life estimation). Concept / Approach: Dead storage volume V_dead is allocated to accommodate long-term siltation without affecting live storage. The time to fill dead storage is simply the ratio V_dead / S when trapping efficiency is high and routing effects are ignored. This is a first-approximation method suitable for exam problems and initial feasibility checks. Step-by-Step Solution: Compute dead storage volume: V_dead = 0.25 * C_total = 0.25 * 10 = 2.5 million m³.Annual silt deposition S = 0.1 million m³/year.Time to fill dead storage: t = V_dead / S = 2.5 / 0.1 = 25 years. Verification / Alternative check: If dead storage were not specified, filling the entire reservoir would take 10 / 0.1 = 100 years. With the explicit 25% allocation, the dead storage life becomes one quarter of that value, i.e., 25 years, confirming the result. Why Other Options Are Wrong: 10, 15, 20 years: underestimate the dead storage volume or overestimate siltation rate. 50 years: double the correct time; would correspond to dead storage of 5 million m³ (50%). Common Pitfalls: Using total capacity instead of dead storage capacity. Ignoring the stated assumption and choosing 100 years (for total fill), which does not answer the dead-storage question. Final Answer: 25 years

Question 19

Proportion of domestic use in total city water demand: For planning purposes, what fraction of the total municipal water supplied is typically assumed to be domestic consumption?

Accepted Answer

Introduction / Context: Municipal demand is divided into domestic, industrial/commercial, public uses (parks, schools, street washing), firefighting provision, and losses (leakage and unaccounted-for water). Concept-level planning often applies empirical percentage splits when local metered data are unavailable. This question checks a commonly adopted share for domestic use. Given Data / Assumptions: Medium-sized Indian city without dominant heavy industry. Conventional planning values from textbook practice. Domestic fraction is intended as a representative planning number. Concept / Approach: Many syllabi and standard references treat domestic consumption as the single largest component but not the entirety of demand. Typical ranges are 35–55%, with 40% being a widely used assumption in preliminary estimates when detailed sectoral data are absent. Step-by-Step Solution: List typical components: domestic, industrial/commercial, public uses, firefighting, losses.Adopt the representative split: domestic ≈ 40% of total supply.Select the option that matches the standard assumption. Verification / Alternative check: Where utilities have metered data, domestic often falls near 40–50%. The 40% value is conservative when industry is moderate and system losses are significant (common in older systems). Why Other Options Are Wrong: 20% and 30%: usually too low for most Indian municipal systems. 60% and higher: may occur in dormitory towns with low non-domestic use, but are not typical planning defaults. Common Pitfalls: Treating these splits as universal; actual shares should be refined with local data. Confusing domestic consumption with per-capita allocation, which is a separate parameter (e.g., LPCD). Final Answer: 40%

Question 20

Air release and vacuum breaking in water distribution: At which locations on distribution mains should air valves be provided to discharge accumulated air effectively?

Accepted Answer

Introduction / Context: Entrained air collects at high points in pressurized pipelines, causing air pockets that reduce carrying capacity, increase headloss, and risk flow interruptions or water hammer. Air valves (air-release and vacuum-break types) are installed to manage this trapped air and to admit air during draining to prevent collapse. Given Data / Assumptions: Closed-conduit pressurized water distribution main with undulating profile. Air enters during filling, from dissolved air release, or via pump suction. Objective is to expel accumulated air and prevent vacuum conditions. Concept / Approach: Since air rises to the highest elevations, strategic placement is at summits (local high points). Automatic air-release valves discharge small pockets under pressure; vacuum-breakers admit air during draining or sudden negative pressure events. Locating them at summits ensures the trapped air is vented promptly. Step-by-Step Solution: Map pipeline profile and identify summits.Install suitable air valves at these high points and, as needed, near pump discharges and dead ends.Confirm connectivity of valve chambers for maintenance access and drainage. Verification / Alternative check: Hydraulic transient analyses and operational experience show headloss spikes and flow instabilities where air pockets persist at high points; properly sited air valves mitigate these issues. Why Other Options Are Wrong: Lower points: air does not accumulate there; scour valves are located at low points for drainage. Junctions only: air can accumulate between junctions at summits. Anywhere: indiscriminate placement is inefficient; profile-driven siting is required. Immediately after every valve: unnecessary and costly unless a summit coincides. Common Pitfalls: Failing to consider undulating terrain profiles. Using only manual air cocks where automatic release is needed. Final Answer: Higher points (summits) of the profile

Water Supply Engineering Questions