Question 1

Chloride limit in treated potable water (public supplies): What is the maximum permissible chloride content that should not be exceeded in treated water supplied to the public (expressed as mg/L or PPM)?

Accepted Answer

Introduction / Context: Chloride in drinking water is primarily an aesthetic parameter affecting taste and potential corrosion behavior. Standards specify desirable or permissible limits to ensure consumer acceptability and long-term system integrity. Engineers should know common reference values used in design and compliance checks. Given Data / Assumptions: Treated water distributed for public consumption. Chloride expressed as mg/L (PPM). Typical Indian/International guideline band for taste acceptability. Concept / Approach: Many standards list a desirable limit around 200–250 mg/L for chloride due to taste thresholds, with a higher permissible limit in the absence of an alternative source. In textbook multiple-choice contexts for public water supplies, 250 mg/L (PPM) is the widely cited ceiling for acceptability. Step-by-Step Solution: Identify parameter: chloride concentration in finished water.Recall common acceptability limit: approximately 250 mg/L.Select the option matching 250 PPM. Verification / Alternative check: Consumer taste panels and long-standing guideline values correlate noticeable salty taste increases above roughly 250 mg/L, supporting this benchmark for municipal supplies. Why Other Options Are Wrong: 50–200 PPM: within acceptable range; not the typical maximum cited for public supplies. Values below 250 PPM are often desirable, but the question asks for the maximum permissible limit. Common Pitfalls: Confusing chloride with residual chlorine (used for disinfection and measured in mg/L but at far lower levels). Assuming 250 PPM is a health-based limit; it is primarily aesthetic. Final Answer: 250 PPM

Question 2

Wastewater microbiology terminology: Bacteria that can survive and metabolize with or without free dissolved oxygen are known as

Accepted Answer

Introduction / Context: Biological treatment processes rely on microbial communities with differing oxygen requirements. Clarifying the terms aerobic, anaerobic, and facultative is crucial for understanding stabilization ponds, activated sludge, trickling filters, and septic processes. Given Data / Assumptions: “With or without free oxygen” means bacteria can function under both oxic and anoxic/anaerobic conditions. Focus is on metabolic flexibility rather than a strict requirement. Concept / Approach: Aerobic bacteria require oxygen as an electron acceptor. Anaerobic bacteria cannot tolerate free oxygen and use other acceptors (such as CO2, nitrates, sulphates). Facultative bacteria adapt to the environment: in the presence of oxygen they respire aerobically; without oxygen they switch to anaerobic pathways. This property makes facultative organisms valuable in pond systems where oxygen availability varies diurnally and by depth. Step-by-Step Solution: Identify the defining phrase “with or without oxygen”.Match to microbial class with metabolic flexibility: facultative bacteria.Select Option C accordingly. Verification / Alternative check: Design guidelines for facultative stabilization ponds explicitly rely on such organisms to stabilize BOD in both surface aerobic and deeper anoxic layers, corroborating the term usage. Why Other Options Are Wrong: Aerobic: require oxygen; do not fit “without oxygen”. Anaerobic: cannot tolerate oxygen; do not fit “with oxygen”. Obligate bacteria: term indicates strict requirement (obligate aerobes or anaerobes), not flexible. Photosynthetic bacteria: classification by energy source, not by oxygen tolerance. Common Pitfalls: Confusing “facultative” with “optional presence”; it specifically refers to metabolic capability in both conditions. Assuming facultative bacteria will perform identically under both regimes; kinetics differ. Final Answer: Facultative bacteria

Question 3

Most widely used type of tube well in India: Which type of tube well is considered the most important and widely used for Indian alluvial aquifers?

Accepted Answer

Introduction / Context: In alluvial formations common across large parts of India, tube wells are constructed to tap unconfined or confined aquifers of sand and gravel. The intake section design (screen/strainer) governs entrance losses, sand pumping, and longevity. Among the historical and widely cited types is the “strainer well,” used extensively in practice and examinations. Given Data / Assumptions: Alluvial aquifer with significant sand content. Standard well construction using a screen/strainer to admit water and exclude sand. Focus on commonly adopted, textbook-cited type. Concept / Approach: Strainer wells employ metal or PVC strainers with slots or wire-wound screens in the intake zone opposite water-bearing layers. Proper slot size selection and gravel packing (if applied) control sand entry. Legacy terminology in Indian water-resources texts frequently labels such installations “strainer wells,” reflecting their prevalence. Step-by-Step Solution: Identify the most common intake arrangement for alluvial formations.Relate to standard practice: strainer/screen opposite aquifer layers.Select “Strainer well.” Verification / Alternative check: State groundwater departments and numerous case studies describe strainer-screened tube wells as the default method in alluvial belts, supporting the choice. Why Other Options Are Wrong: Cavity wells: suited to clayey formations; not the primary alluvial solution. Slotted/perforated pipe: generic terms; without proper strainer design, sand exclusion is poor. Gravel-packed well: a construction detail that can also be used with strainer screens but is not the historical “type” referenced by the question. Common Pitfalls: Selecting a generic term instead of the established nomenclature in Indian textbooks. Ignoring slot size and grading, which are critical to performance. Final Answer: Strainer well

Question 4

Choice of distribution network pattern for irregular towns: For a city developed haphazardly with many dead-ends and irregular street patterns, which pipe layout is generally adopted?

Accepted Answer

Introduction / Context: Water distribution layout must conform to urban morphology. Regular, planned towns favor looped or grid systems that provide redundancy and balanced pressures. In contrast, organically grown or haphazard cities often lack continuous loops, leading to branch-like terminal lines, i.e., dead ends. Recognizing the appropriate layout helps in realistic hydraulic modeling and valve placement. Given Data / Assumptions: Street network is irregular, with many cul-de-sacs and discontinuous links. Limited right-of-way or budget to retrofit full loops. Objective is a feasible initial layout rather than the theoretically ideal one. Concept / Approach: Dead end (tree) systems branch from trunk mains to sub-mains and service connections, terminating without forming loops. They are straightforward to build where streets do not form closed blocks. However, they suffer from stagnation and poor redundancy, thus require blowoffs and careful valve placement. In planned upgrades, conversion toward grid or ring systems is desirable. Step-by-Step Solution: Assess urban pattern: irregular and non-rectilinear.Match to feasible hydraulic layout: dead end (branch) system.Select the option “Dead end system.” Verification / Alternative check: Standard texts and municipal practice consistently associate haphazard urban fabric with tree/dead-end layouts, later retrofitted with interconnections where possible to improve fire flows and reliability. Why Other Options Are Wrong: Grid iron/ring: require planned, interconnected streets. Radial: suits cities with a central storage feeding radial mains. Hybrid: while practically used, the question asks the generally adopted pattern for such towns; the canonical answer is dead end. Common Pitfalls: Assuming dead-end systems are acceptable without flushing; water quality can degrade in terminal lines. Overlooking the need for future looping to improve resilience. Final Answer: Dead end system

Question 5

Drinking water quality – maximum permissible colour (Cobalt–Platinum scale) For domestic water supplies, what is the maximum permissible colour value on the Cobalt–Platinum (Pt–Co) colour scale, assuming no alternate source is available?

Accepted Answer

Introduction / Context: Colour in drinking water is an important aesthetic parameter that affects consumer acceptance and can indicate the presence of natural organic matter, corrosion products, or industrial contaminants. Standards often specify a desirable limit and a maximum permissible limit for colour measured on the Cobalt–Platinum (Pt–Co) scale. Given Data / Assumptions: Measurement uses the standard Pt–Co (Cobalt–Platinum) colour scale. Question asks for maximum permissible value used in many public water supply standards when no better source is available. Domestic (potable) supplies are intended. Concept / Approach: Typical guidance distinguishes between a desirable limit (often about 5 Pt–Co units) and a permissible limit in the absence of an alternate source (commonly about 15 Pt–Co units). Above the permissible limit, water can appear visibly coloured, affecting acceptability and potentially interacting with disinfection by-product formation if associated with organics. Step-by-Step Solution: Identify the standard metric: Pt–Co units (sometimes called Hazen units).Recall typical limits: desirable ≈ 5, permissible (in absence of alternate) ≈ 15.Select the option matching the maximum permissible value: 15 Pt–Co units. Verification / Alternative check: Many national standards and design manuals list colour desirable 5 and permissible 15 Pt–Co units. Utilities strive to be at or below the desirable value to minimize consumer complaints. Why Other Options Are Wrong: 5 and 10: These reflect desirable/stricter targets but not the upper permissible limit. 20 and 25: These exceed typical permissible limits and are likely to be objectionable. Common Pitfalls: Confusing “desirable” with “permissible in absence of alternate source.” Also, mixing up Pt–Co colour with turbidity; they are different parameters measured by different methods. Final Answer: 15 PPM (Pt–Co units)

Question 6

Centrifugal pump installation – essential practices and precautions Which of the following statements together describe standard good practice for starting and operating a centrifugal pump in a water-supply installation?

Accepted Answer

Introduction / Context: Centrifugal pumps are the workhorses of municipal water systems. Safe and efficient operation depends on correct priming, sensible piping practice, and appropriate starting procedures that avoid overloading and loss of prime. Given Data / Assumptions: A standard end-suction centrifugal pump serving a water-supply line. Conventional suction foot valve (or a check at the wet well) used to retain water in the suction line. Operators can start the pump with the discharge valve throttled as needed to limit load. Concept / Approach: Before start, the casing and suction must contain water to prevent dry running and to allow the impeller to generate head—hence priming is mandatory. A foot valve (non-return valve) prevents backflow and loss of prime when the pump stops. Piping at the pump nozzles is often the same as or slightly smaller than the downstream distribution main; the overall delivery main is frequently larger to reduce velocity and friction losses. Starting against a wide-open discharge on an empty line can produce high flow at low head, increasing power draw and risking motor overload; throttling the discharge at start-up is standard practice. Step-by-Step Solution: Confirm (a): Priming is essential to avoid air binding and damage.Confirm (c): The foot valve retains water in the suction line, maintaining prime.Interpret (b): While site-specific, the delivery main is commonly larger than the pump nozzles to limit friction; the statement captures that relation.Evaluate (d): Starting with open discharge and an empty line can cause excessive flow and motor overloading; throttling is safer.Therefore, the comprehensive correct choice is “All the above.” Verification / Alternative check: Commissioning manuals recommend priming, check/foot valves for priming retention, and starting with a throttled discharge to control load and avoid water hammer. Why Other Options Are Wrong: Choosing any single statement ignores interdependent best practices. Common Pitfalls: Dry starting, suction leaks that admit air, and starting with fully open discharge on an empty, short line—all of which can damage the pump or trip the motor. Final Answer: All the above

Question 7

Gravitational distribution systems – cities traditionally supplied by gravity In which of the following cities is water distribution historically carried out predominantly by a gravitational system (i.e., high-level storages feeding zones by gravity…

Accepted Answer

Introduction / Context: Topography strongly influences the selection of distribution schemes in urban water supply. Upland sources allow storages at elevations that can feed consumers by gravity, which improves energy efficiency and reliability. Given Data / Assumptions: Traditional, widely cited examples in Indian water-supply texts. Gravity distribution implies primary reliance on head available from source and service reservoirs, with limited pumping within zones. Concept / Approach: Hilly or coastal-upland cities with high-level sources or service reservoirs often employ gravity systems. Dehradun, located in the foothills, and Mumbai (Bombay), supplied from upland reservoirs, are classic examples. In contrast, Delhi and Kolkata rely extensively on river intakes and pumping to convey and distribute water across flat terrain. Step-by-Step Solution: Identify cities with natural elevation advantage → Dehradun and Mumbai.Recognize Delhi and Kolkata as predominantly pump-fed due to flat topography.Choose the combined option reflecting the gravity tradition in Dehradun and Mumbai. Verification / Alternative check: Classic design examples cite gravity-fed supplies from high-elevation lakes/reservoirs to Mumbai and hill-fed sources to Dehradun. Why Other Options Are Wrong: Single-city answers omit the other well-known case. Delhi and Kolkata are largely pump-operated networks. Common Pitfalls: Equating gravity conveyance from source with gravity distribution everywhere; many systems use a mix of both, but the question targets the predominant historical approach. Final Answer: Both Dehradun and Bombay (Mumbai)

Question 8

Chlorination – what does chlorination NOT remove in a water-treatment context? Identify the parameter that is not removed (i.e., not significantly reduced) by routine chlorination of water in conventional treatment practice.

Accepted Answer

Introduction / Context: Chlorination is primarily a disinfection step designed to inactivate pathogenic microorganisms. It can also participate in side reactions with certain reduced species, but it is not a substitute for upstream processes that remove organic carbon or oxygen-demanding substances. Given Data / Assumptions: Routine post-filtration chlorination for potable water. Reasonable doses used for disinfection rather than chemical oxidation of organics. Concept / Approach: BOD represents biodegradable organic load that exerts oxygen demand biologically. Typical chlorination doses do not appreciably reduce BOD; removal of BOD occurs primarily by coagulation/flocculation, sedimentation, and filtration, or by biological processes. Chlorine can react with free ammonia to form chloramines (thereby reducing free ammonia), and it can slightly affect measured DO via chemical reactions, but the aim is not DO removal. Step-by-Step Solution: Identify the principal purpose → disinfection, not organic removal.Assess each parameter: free ammonia reacts to form chloramines, organics may be partially oxidized but not reliably “removed,” DO is not a target and only marginally affected, and BOD is not substantially reduced by chlorination.Select BOD as the parameter that chlorination does not remove. Verification / Alternative check: Design manuals treat BOD removal upstream of disinfection; disinfectant contact tanks are sized for CT (concentration × time) to achieve microbial inactivation, not for organic oxidation. Why Other Options Are Wrong: (a) Ammonia can be combined by chlorination (breakpoint chlorination concepts). (c) Some reactive organics may be oxidized partially, though not a controlled “removal” step. (d) DO is not removed as a treatment objective; any change is incidental. Common Pitfalls: Expecting chlorination to fix high-organic or high-BOD waters; proper clarification and filtration are required before disinfection. Final Answer: Biochemical oxygen demand (BOD)

Question 9

Well hydraulics – flow fields and controlling areas in different well types Choose the correct overall statement about flow patterns and controlling flow areas for common well types used in water-supply practice.

Accepted Answer

Introduction / Context: Understanding how water approaches a well screen or a developed cavity is essential for predicting head losses and sand control. Introductory groundwater texts present idealized patterns for different well designs. Given Data / Assumptions: Strainer (screen) wells in unconsolidated formations. Cavity wells in coarse formations where a natural cavity is developed. Steady radial flow assumptions around a vertical well in a homogeneous aquifer. Concept / Approach: For a vertical screened section in a uniform aquifer, far-field flow to the well is radial in plan. For a localized cavity, approach flow is often visualized as quasi-spherical near the cavity. The hydraulic entrance area that governs entrance losses depends on the active screen length for strainers and on cavity surface area for cavity wells. Step-by-Step Solution: Evaluate (a): Radial inflow to screen → conceptually correct.Evaluate (b): Cavity well inflow → frequently idealized as spherical near the cavity.Evaluate (c): Entrance area is proportional to active screen length → correct.Evaluate (d): Entrance area controlled by cavity size → correct.Since all (a)–(d) are correct, the only consistent choice for a single-answer question is “None of these” (none are incorrect). Verification / Alternative check: Classical well-loss relationships include a term associated with entrance velocity over the screen or cavity area, highlighting dependence on length (screen) or surface (cavity). Why Other Options Are Wrong: Selecting any one of (a)–(d) would imply the others are wrong, which is not the case. Common Pitfalls: Confusing far-field radial flow with near-well entrance effects; both views are used at different spatial scales. Final Answer: None of these

Question 10

Urban population forecasting – key demographic components When predicting the future population of a city for water-supply design, which demographic factors must be considered together?

Accepted Answer

Introduction / Context: Population forecasts drive source sizing, treatment-plant capacity, and distribution design. Errors in demographic assumptions can lead to under- or over-sizing with financial and service-level consequences. Given Data / Assumptions: Standard municipal planning framework. Projection horizon typically 20–30 years. Multiple methods (arithmetical, geometrical, incremental, logistic) rely on demographic drivers. Concept / Approach: Net population change = natural increase (births − deaths) + net migration (inflow − outflow). Any robust prediction explicitly or implicitly accounts for all three. Even methods that extrapolate historical totals inherently embody these drivers. Step-by-Step Solution: Identify components of change → births, deaths, migrants.Recognize forecasting methods incorporate these directly (cohort) or indirectly (trend-based).Hence, the comprehensive correct choice is “All the above.” Verification / Alternative check: Planning manuals admonish designers to validate projections against known housing, industry, and migration trends, not merely rely on one historical curve. Why Other Options Are Wrong: Births or deaths alone omit the critical migration term; migrants alone ignores natural increase. Common Pitfalls: Transposing national growth rates to a local city without accounting for migration shocks from new industries or transport links. Final Answer: All the above

Question 11

Pipe materials – common joint for asbestos-cement (AC) pressure pipes Which joint is most commonly used to join asbestos-cement (AC) water pipes in distribution systems?

Accepted Answer

Introduction / Context: Selection of a pipe joint depends on pipe material and the need to accommodate misalignment, expansion, and field assembly speed. Asbestos-cement (AC) pipes historically used mechanical couplings suited to their material characteristics. Given Data / Assumptions: Legacy AC pressure mains in water distribution (now often replaced by DI/HDPE). Field assembly with minimal hot-works. Concept / Approach: Mechanical couplings such as dresser couplings provide a flexible, gasketed connection between plain-ended pipes. They suit AC pipes because they avoid excessive localized stresses and allow small angular deflections. Flanged joints are more typical with metallic pipes and appurtenances; lead-caulked spigot-and-socket joints are associated with cast iron; simplex-specific joints are uncommon in AC practice. Step-by-Step Solution: Match pipe material (AC) with a widely used mechanical joint → dresser coupling.Exclude flanged and lead-caulked options as less common for AC distribution lines.Select “Dresser coupling joint.” Verification / Alternative check: Older design manuals and vendors list dresser-type couplings and AC collars as typical for AC pipes, offering ease of repair and insertion. Why Other Options Are Wrong: (a) Flanged joints used mainly at valves/equipment, not long runs of AC pipe. (b) Gland-type flexible joints are more typical for DI/steel. (e) Lead-caulked spigot-socket is a cast-iron tradition. Common Pitfalls: Applying high bolt torque or misalignment on brittle AC pipes; always follow torque and insertion limits of the coupling manufacturer. Final Answer: Dresser coupling joint

Question 12

Threshold odour number (TON) – limit for public water supplies The dilution ratio at which odour is just detectable is the threshold odour number (TON). For public water supplies, what maximum TON is generally acceptable?

Accepted Answer

Introduction / Context: Odour is a key aesthetic parameter in drinking water. The threshold odour number (TON) represents the dilution factor at which a panel can just detect an odour. Maintaining a low TON is vital to consumer acceptability and confidence in potable water. Given Data / Assumptions: TON determined by standard panel dilution methods. Question asks for a typical maximum limit recommended for public supplies. Concept / Approach: Many design guides adopt a conservative TON limit to ensure that even sensitive consumers do not detect unpleasant odours. A commonly cited target is TON ≤ 3 for distributed water, reflecting high acceptability. Larger numbers indicate stronger odours and deteriorating aesthetic quality. Step-by-Step Solution: Recognize that lower TON values correspond to better quality.Recall typical target values for distribution systems → not exceeding 3.Select the smallest threshold consistent with standard recommendations: 3. Verification / Alternative check: Utilities often adopt internal objectives stricter than regulatory maxima to avoid consumer complaints; a TON of 3 is widely used as an operational criterion. Why Other Options Are Wrong: 5, 7, 9, 10 indicate increasingly stronger odours and are typically unacceptable for finished water. Common Pitfalls: Confusing TON with taste thresholds; both are sensory but evaluated separately. Also, odour can be episodic due to algal metabolites; control requires upstream treatment, not just chlorination. Final Answer: 3

Question 13

Source selection – which source generally yields the least impurities? Considering typical natural sources, which source usually provides water with the least impurities for public supply (before any treatment)?

Accepted Answer

Introduction / Context: The initial quality of source water influences the complexity and cost of treatment. Waters originating from subsurface pathways in unpolluted upland areas are commonly the clearest and least contaminated. Given Data / Assumptions: Sources considered: upland springs, reservoirs, plain streams, lowland lakes. Generalized impurity levels prior to significant anthropogenic impacts. Concept / Approach: Hill springs are often fed by groundwater that has undergone natural filtration through soil and rock, removing suspended matter and many pathogens. In contrast, plain streams and lowland lakes collect runoff, sediments, and pollutants, typically exhibiting higher turbidity and variable quality. Reservoirs may stratify and experience algal growth, necessitating more treatment steps. Step-by-Step Solution: Identify the source with natural filtration and isolation → hillside springs.Contrast with surface impoundments and lowland systems subject to runoff and algal issues.Choose “Springs along hill slopes.” Verification / Alternative check: Many historic gravity-supply systems leveraged upland springs due to their clarity and low microbial counts, though modern practice still disinfects all potable water. Why Other Options Are Wrong: (a) Reservoirs can have algae, manganese release, and turbidity spikes. (b) Plain streams carry silt and pollutants. (c) Lowland lakes may be eutrophic with taste and odour issues. Common Pitfalls: Assuming spring water needs no treatment; disinfection is still required, and protection of the recharge area is essential. Final Answer: Springs along hill slopes

Question 14

Water-supply design for industrial townships: When estimating total water demand for an industrial township, what percentage share is generally assumed for combined industrial and commercial water demand (as a fraction of the total supply)?

Accepted Answer

Introduction / Context: In municipal and industrial water-supply planning, engineers must break down the total demand into domestic, institutional, commercial, industrial, firefighting, and loss components. A typical planning task is to assign a reasonable percentage to the industrial and commercial portion when specific factory data are not yet available. Using standard ranges prevents under-designing transmission mains, reservoirs, and treatment facilities. Given Data / Assumptions: Preliminary design for an industrial township with mixed commercial activity. No detailed water-use audit from individual industries is available. Use conventional benchmark percentages taught in water-supply engineering. Concept / Approach: Textbook practice and many state design manuals suggest a planning allowance for industrial plus commercial demand in the range of roughly one-fifth to one-quarter of the total system demand for industrial townships. This assumes domestic use remains important but is not the sole driver. The exact split is refined later with metered consumption and category-wise connections, but the preliminary sizing benefits from a robust envelope. Step-by-Step Solution: Identify category: industrial township (industry is a major consumer).Apply standard percentage range used at planning stage: 20% to 25% of total supply.Select the option that matches the conventional range. Verification / Alternative check: If later surveys show heavier industrial loads (e.g., steel, textiles, or refineries), the percentage can be recalibrated. Conversely, in service-sector towns, the commercial/industrial fraction may be closer to 10–15%. Why Other Options Are Wrong: 10% and 10–15%: more typical of non-industrial towns; risk underestimation for industrial townships. 15–20%: borderline low for explicitly industrial settings. 30–35%: high for preliminary sizing and can lead to costly oversizing without evidence. Common Pitfalls: Copying percentages from purely residential towns; double-counting losses within commercial/industrial categories; ignoring diurnal variation and peak factors which must be addressed separately. Final Answer: 20 to 25%

Question 15

Groundwater abstraction terminology: An Abyssinian tube well is a special type of which well construction?

Accepted Answer

Introduction / Context: Abyssinian tube wells are small-diameter, shallow wells used widely for rural and emergency water supply. Understanding how they differ from other well types (cavity, slotted, dug-cum-bore, artesian) is important for source selection, screen design, and maintenance planning. Given Data / Assumptions: Driving depth is limited to shallow aquifers with adequate yield. Well consists of a small-diameter pipe with a screened (strainer) section at the bottom. Often installed by driving or jetting with a hand pump attached. Concept / Approach: An Abyssinian well employs a strainer (screen) to admit water while excluding sand. The assembly is driven into a permeable formation. The screen aperture and slot size are selected relative to the aquifer grain-size curve to minimize sand pumping. This is characteristic of strainer type wells, not cavity type wells (which rely on a developed cavity in coarse strata) or slotted terminology used differently in large-diameter tubewells. Step-by-Step Solution: Identify defining feature: a screened or perforated section at the intake.Note installation method: light equipment, driving/jetting feasible.Classify accordingly: strainer type well. Verification / Alternative check: Field practice confirms that maintenance involves screen cleaning and ensuring the foot-valve and hand pump remain airtight to maintain suction. Why Other Options Are Wrong: Slotted type well: generic phrasing, but Abyssinian specifically denotes strainer use with small-diameter driven pipes. Cavity type well: depends on developing a cavity in coarse formations; not the Abyssinian method. Dug-cum-bore well and artesian well: different construction and hydraulic behavior. Common Pitfalls: Using too fine a screen (excessive headloss) or too coarse (sand carryover); trying to install in tight clays with negligible yield. Final Answer: Strainer type well

Question 16

Pipeline appurtenances — location of air valves: In pressure pipelines of a water-supply system, where are air valves generally installed to ensure safe and efficient operation?

Accepted Answer

Introduction / Context: Air valves (air release and vacuum break valves) are essential for long pressure mains. Trapped air can reduce capacity, cause inaccurate flow measurements, induce water hammer, and accelerate corrosion. Proper siting of these valves is a core part of transmission main design. Given Data / Assumptions: Closed-conduit, pressurized flow. Topography with rises and falls (undulating profile). Desire to avoid air pockets and vacuum conditions. Concept / Approach: Air naturally migrates to high points. At summits, air pockets collect and constrict the cross-section, increasing headloss and potentially causing vapor pockets. Air valves release accumulated air during operation and admit air rapidly during sudden draining or column separation, preventing excessive negative pressure. Step-by-Step Solution: Identify hydraulic profile and summits along the alignment.Place air-release/vacuum valves at each summit and, if needed, at long grades to limit accumulation spacing.Provide isolation valves and bypasses for maintenance. Verification / Alternative check: Surge analysis confirms that properly located air valves limit transient pressures. Constructability checks ensure accessibility at summits. Why Other Options Are Wrong: Pipe junctions and service connections: not systematic locations for trapped air. Low points: suited for scour/sluice valves to remove sediment, not for air release. Pump discharge only: insufficient for long undulating mains. Common Pitfalls: Undersizing or omitting vacuum-breaking capacity; installing at low points where air will not accumulate. Final Answer: At summits (high points)

Question 17

Aesthetic water-quality parameter — color limit: For domestic water supplies, what is the maximum permissible color based on the cobalt–platinum (Hazen) color scale?

Accepted Answer

Introduction / Context: Color in drinking water is an aesthetic quality that influences consumer acceptance and indicates potential organic matter or dissolved metals. The cobalt–platinum (Hazen) color scale is a traditional reference for setting limits. Given Data / Assumptions: Public potable supply standards aimed at good appearance and taste. Color measured against the Hazen scale. Concept / Approach: Most guidance sets a limit around 5 Hazen units for supplied water to avoid noticeable tint. While raw surface water may have higher color, treatment via coagulation, activated carbon, or oxidation is used to meet distribution standards. Step-by-Step Solution: Identify parameter: true color on Hazen scale.Recall common acceptability limit: ≈ 5 Hazen units.Choose the matching option. Verification / Alternative check: At values above ≈ 15 Hazen units, consumers typically notice visible tint; utilities target ≤ 5 for consistency. Why Other Options Are Wrong: 10–25 Hazen units may still be safe health-wise but can trigger complaints and signal inadequate treatment. Common Pitfalls: Confusing color (Hazen) with turbidity (NTU); relying solely on chlorine for organic color removal instead of coagulation or adsorption. Final Answer: 5 ppm (≈ 5 Hazen units)

Question 18

Hydraulic structures — retardation of flow: In which of the following treatment units is the flow of water intentionally retarded to promote settling and clarification?

Accepted Answer

Introduction / Context: Clarification processes in water treatment rely on reducing water velocity so that suspended particles can settle under gravity, or can be captured by flocculation and subsequent settling. Various terms are used for these units, but the hydraulic principle is the same: retard the flow to achieve settling. Given Data / Assumptions: Conventional treatment train including coagulation–flocculation and sedimentation. Goal is to remove settleable solids and reduce turbidity prior to filtration. Concept / Approach: Settling tanks, sedimentation tanks/basins, and clarifiers are functionally similar. They provide quiescent conditions by enlarging plan area (lower surface overflow rate) and increasing detention time, allowing particles with settling velocity greater than the critical velocity to be removed. Step-by-Step Solution: Recognize that each named unit performs sedimentation.All such units slow the flow to promote gravitational settling.Therefore, the collective answer is “All of the above”. Verification / Alternative check: Design parameters such as surface overflow rate and detention time are specified for each of these unit types and share overlapping ranges depending on process configuration. Why Other Options Are Wrong: Each individual unit listed indeed retards flow; selecting only one would be incomplete. Common Pitfalls: Confusing mixing/flocculation basins (which intentionally maintain gentle velocity gradients) with sedimentation, which favors very low velocities and uniform flow distribution. Final Answer: All of the above

Question 19

Population forecasting in water-supply planning: Which graphical or mathematical form can conveniently represent population growth for demand estimation over time?

Accepted Answer

Introduction / Context: Water-supply systems are sized for future demand, so population growth modeling is a foundational step. Different mathematical models and plotting techniques are used depending on the city’s development stage, constraints, and historical data behavior. Given Data / Assumptions: Historical population data at decadal intervals. Urban growth possibly influenced by economy, land availability, and policy. No single universal model fits all cities and periods. Concept / Approach: The arithmetical increase method assumes a constant increment per period (linear growth). The geometric (semi-log) method assumes a constant percentage growth, suitable for early fast-growing towns. The logistic curve accounts for a carrying capacity, leveling off as resources or land become limiting. Engineers often plot data in multiple ways to see which model best fits and then select the projection that aligns with planning narratives. Step-by-Step Solution: Plot population vs time on arithmetic scale and on semi-log paper.Check residuals; if growth percentage is roughly constant, a semi-log straight line is appropriate.If saturation is evident, fit a logistic curve with inflection and carrying capacity. Verification / Alternative check: Compare projections against known drivers (industry projects, zoning, transit). Adjust for policy constraints and sensitivity-test infrastructure capacities. Why Other Options Are Wrong: Each method is valid under specific conditions; therefore, limiting to a single method would be misleading. Hence, “All of the above” is best. Common Pitfalls: Extrapolating exponential growth indefinitely; ignoring migration shocks; failing to reconcile model outputs with land-use plans. Final Answer: All of the above

Question 20

Water distribution networks — preferred layout in well-planned cities: Which distribution network layout is generally adopted in a well-planned city to ensure redundancy, circulation, and pressure uniformity?

Accepted Answer

Introduction / Context: A city’s pipe network layout influences reliability, pressure maintenance, water age, and fire-fighting performance. In modern, well-planned urban areas with regular street patterns, looped networks are preferred over branched dead-ends. Given Data / Assumptions: City with planned blocks and intersecting streets. Objective: redundancy, uniform pressures, and ease of isolation without widespread outages. Concept / Approach: The grid-iron system forms loops so that water can reach any node from multiple directions, limiting stagnation and allowing sections to be shut off for maintenance while maintaining service via alternate paths. Terms like interlaced or reticulation are often used synonymously with looped networks, but “grid-iron system” is the classic designation for well-planned cities laid out on a rectangular pattern. Step-by-Step Solution: Identify network objective: looping and redundancy.Match to common terminology: grid-iron (looped) layout.Select the corresponding option. Verification / Alternative check: Hydraulic modeling shows looped networks maintain pressures better under peak and fire flows and exhibit lower water age compared to dead-ends. Why Other Options Are Wrong: Interlaced/reticulation often mean looped too, but the canonical choice taught is “grid-iron system”. Dead-end systems suit unplanned growth and are prone to stagnation. Pure radial systems are more typical around large reservoirs or zones, not whole-city coverage. Common Pitfalls: Confusing terminology; assuming any loop suffices without considering valve placement and sectorization. Final Answer: Grid-iron system

Water Supply Engineering Questions