Question 1

Reservoir operation – lowest outlet sluice level:\r At what vertical level is the lowest outlet sluice of a dam typically provided relative to the dead storage zone?

Accepted Answer

Introduction / Context: Reservoir storage is partitioned into dead storage and useful or live storage. Outlet works must deliver acceptable quality water and avoid excessive sediment withdrawal. The elevation of the lowest outlet sluice is set with these objectives in mind. Given Data / Assumptions: Dead storage near the bottom accumulates settled sediments over time. Useful storage contains better quality water for supply, irrigation, or power. Question concerns the typical level of the lowest outlet sluice. Concept / Approach: Placing the lowest outlet at the top of the dead storage minimizes sediment entry while still allowing drawdown to the designed minimum operating level. Outlets below dead storage would entrain heavily silted water and risk blockage. Outlets too high would strand part of the intended useful storage and reduce dependable yield. Step-by-Step Reasoning: Define storage zones and their purposes.Balance water quality and operating flexibility by setting the outlet just above the sediment accumulation zone.Conclude that the appropriate elevation is at the top of dead storage. Verification / Alternative check: Design drawings typically show multi level outlets or selective withdrawal structures, but the lowest service outlet aligns with the live storage floor to avoid silt laden draw, consistent with this selection. Why Other Options Are Wrong: Below dead storage increases sediment entrainment and blockage risk. At useful storage top or center of dam height would waste live storage and reduce supply reliability. Common Pitfalls: Confusing sediment sluices or bottom outlets for flushing with service outlets for regular delivery. Flushing conduits may be lower but are not the service outlet referenced here. Final Answer: At the top level of dead storage

Question 2

Filter media rehabilitation – removing mud balls:\r In rapid gravity filters, which actions are used to remove and remedy mud balls and restore proper media condition?

Accepted Answer

Introduction / Context: Mud balls are agglomerations of fines, biofilm, and precipitates that form within filter media when backwashing is inadequate or non uniform. They reduce effective porosity, cause channeling, and degrade filtrate quality. Proper corrective maintenance restores media performance. Given Data / Assumptions: Filter type: rapid gravity filter with dual or mono media. Symptoms: high headloss, localized dirty spots, turbidity breakthrough. Objective: identify effective corrective actions. Concept / Approach: Solutions involve mechanical disruption, chemical cleaning, and media rehabilitation. Breaking mud balls by hand raking or scouring during shutdown dislodges compacted masses. Chemical cleaning with caustic soda helps remove greasy films and organic deposits that bind fines. Severely cemented media or damaged grains require removal and replacement to restore gradation and uniformity coefficient. Step-by-Step Reasoning: Inspect and locate mud ball zones.Perform mechanical breakup and thorough wash to remove residues.Apply chemical cleaning where biofilm or oil binding is present.Replace media where cementation or attrition is excessive. Verification / Alternative check: Post maintenance headloss curves, media sieve analyses, and filter to waste turbidity trends confirm successful rehabilitation and restored backwash expansion behavior. Why Other Options Are Wrong: Each single measure may help, but comprehensive remediation often needs a combination to fully resolve mud balls. “None of these” contradicts standard maintenance practice. Common Pitfalls: Assuming backwash alone will remove entrenched mud balls once formed. Neglecting root causes such as inadequate air scour, improper backwash rates, or blocked laterals. Final Answer: All of the above

Question 3

Lime softening and disinfection effects in water treatment Pick the single most correct statement about adding lime to raw water during treatment.

Accepted Answer

Introduction / Context: Lime (calcium hydroxide) is widely used in conventional water treatment for softening and pH adjustment. Understanding how lime affects hardness, pH, and microbial inactivation helps operators choose coagulant and disinfectant doses responsibly. Given Data / Assumptions: Lime softening operates via precipitation reactions of calcium and magnesium. Discussion refers to treatment-plant practice with subsequent clarification and filtration. Statements address typical textbook claims about bactericidal action at high pH. Concept / Approach: Lime reacts with bicarbonate alkalinity to precipitate calcium carbonate and, at higher pH, magnesium hydroxide, thereby reducing hardness. Raising pH above neutral creates an unfavorable environment for many bacteria. Classical references note strong bacterial reduction near pH 9.5–11 under adequate contact time, although final disinfection is still achieved by chlorine/UV. Step-by-Step Solution: Hardness removal: Ca(HCO3)2 + Ca(OH)2 → 2 CaCO3 ↓ + 2 H2O; Mg(HCO3)2 + 2 Ca(OH)2 → Mg(OH)2 ↓ + 2 CaCO3 ↓ + 2 H2O.pH impact: Excess lime increases hydroxide concentration, raising pH substantially.Bactericidal effect: High pH damages cellular functions of many organisms; older exam literature generalizes near-total removal at pH ≈ 9.5 with settling/filtration.Therefore each listed statement is accepted as correct in the context of standard exam practice → choose “All the above”. Verification / Alternative check: Plant operating manuals show lime softening coupled with clarification and filtration significantly reduces microbial counts, with downstream chlorination ensuring regulatory compliance. Why Other Options Are Wrong: Choosing any one of (a), (b), or (c) ignores the broader set of true effects recognized in basic texts. (d) is quoted in many exam sources; final safety still relies on a disinfectant residual, but within this question’s framework, it is treated as correct. Common Pitfalls: Assuming lime alone is a complete disinfectant barrier; operators must verify residual disinfectant and contact time. Over-liming can cause scaling and high finished-water pH without proper recarbonation. Final Answer: All the above

Question 4

Runoff estimation using Khosla’s empirical formula For catchment runoff estimation by Khosla’s formula Qy = Py − K (1.8 Ty + 32), what is the mean recommended value of the constant K?

Accepted Answer

Introduction / Context: Several empirical relations are used for quick runoff estimation when detailed hydrologic data are scarce. Khosla’s formula introduces a constant K to account for temperature and catchment losses in converting precipitation to annual runoff. Given Data / Assumptions: Qy = Py − K (1.8 Ty + 32), where Qy is annual runoff (cm), Py is annual precipitation (cm), and Ty is mean annual temperature (°C). “Mean value” of K is requested for general use. Catchment has no extreme anomalies in infiltration/evaporation beyond empirical calibration range. Concept / Approach: K is a correction factor capturing combined effects of evapotranspiration and catchment abstractions tied to temperature. Standard references suggest a representative mean K near 1.23, used where local calibration is unavailable. Step-by-Step Solution: Identify the formula structure and the role of K.Recall the commonly adopted mean value for Indian catchments: K ≈ 1.23.Select the closest matching option: 1.23. Verification / Alternative check: Worked examples in introductory hydrology texts frequently employ K = 1.23 for preliminary estimates before basin-specific calibration. Why Other Options Are Wrong: 1.21, 1.25, 1.27, 1.31 are not the standard “mean” choice; they may apply to specific local calibrations but are not the default mean value. Common Pitfalls: Using a generic K without validating against local streamflow records; always calibrate when data are available to reduce error. Final Answer: 1.23

Question 5

Microbiology in water treatment Bacteria that require free (dissolved) oxygen for survival and metabolism are called:

Accepted Answer

Introduction / Context: Microbial classification by oxygen requirement guides process selection in wastewater and water treatment (e.g., activated sludge versus anaerobic digestion). Knowing which organisms require oxygen is fundamental to designing aeration and biological steps. Given Data / Assumptions: “Free oxygen” refers to dissolved oxygen present in water. Typical environmental engineering definitions apply. Concept / Approach: Aerobic bacteria: require dissolved oxygen for respiration. Anaerobic bacteria: thrive in absence of free oxygen; oxygen may be toxic. Facultative bacteria: can switch between aerobic and anaerobic metabolism. Therefore the name for organisms that require free oxygen is “Aerobic bacteria”. Step-by-Step Solution: Match oxygen requirement in the stem to the correct category → aerobic.Exclude anaerobic (no oxygen) and facultative (can use either condition). Verification / Alternative check: Standard microbiology texts define aerobic respiration as oxygen-dependent, distinguishing it from anaerobic pathways (e.g., nitrate/sulfate reduction, fermentation). Why Other Options Are Wrong: Anaerobic: opposite of what is described. Facultative: not strictly requiring oxygen. “None of these”: incorrect because “Aerobic bacteria” fits exactly. Common Pitfalls: Confusing “facultative” with “aerobic”; facultative organisms survive without oxygen as well. Final Answer: Aerobic bacteria

Question 6

Pump selection for deep water table If the static water table is 45 m below ground level, which pump type is appropriate for lifting water from a deep tube well in such conditions?

Accepted Answer

Introduction / Context: Pump choice depends on suction limits and submergence. Atmospheric pressure limits practical suction lift for surface pumps, so deep groundwater requires pumps located down the well (or with multistage turbines) to push rather than pull. Given Data / Assumptions: Static water level ≈ 45 m below ground. No provision for staging multiple surface pumps. Standard well construction for potable supply. Concept / Approach: Surface-mounted centrifugal or reciprocating pumps cannot lift from 45 m because maximum theoretical suction lift is about 10.3 m for water at sea level, and practical values are 6–8 m. Deep well turbine or submersible pumps are set below the water level and push water to the surface, bypassing suction limitations. Step-by-Step Solution: Compare depth (45 m) to practical suction limit (≈ 7–8 m).Eliminate surface suction pumps (centrifugal/reciprocating).Select deep well turbine or submersible arrangement. Verification / Alternative check: Manufacturers’ curves and design manuals specify vertical turbine or submersible pumps for deep tube wells beyond suction capability. Why Other Options Are Wrong: Centrifugal/reciprocating surface pumps cannot sustain suction lift at 45 m. “None of these” is wrong because deep well turbine pumps are exactly intended for this case. Common Pitfalls: Underestimating suction head losses, vapor pressure effects, and NPSH requirements; always ensure adequate submergence of the bowl assembly. Final Answer: Deep well turbine pump (lineshaft or submersible)

Question 7

Plain sedimentation tank design What is the usual detention period adopted for plain (unflocculated) sedimentation tanks in conventional water treatment?

Accepted Answer

Introduction / Context: Detention period (hydraulic residence time) in sedimentation governs settling performance and tank volume. For plain sedimentation (without chemical flocculation), a moderate detention time is chosen to settle the readily settleable fraction while balancing footprint and cost. Given Data / Assumptions: No coagulation/flocculation stage before the basin. Design flow is steady for sizing purposes. Temperature and particle sizes typical of raw surface water. Concept / Approach: Standard practice adopts 4–8 hours for plain sedimentation. Longer times can lead to septic conditions and are generally unnecessary for routine clarification goals; shorter times may underperform for typical particle sizes without floc aids. Step-by-Step Solution: Recall design range from manuals: 4–8 h for plain sedimentation.Select the matching option. Verification / Alternative check: Settling theory (surface overflow rate approach) and plant designs converge to similar volumes that translate to detention within the 4–8 h band for many municipal plants. Why Other Options Are Wrong: 8–16 h, 16–24 h, and 24–36 h are excessive for plain sedimentation, increasing volume without proportional benefit. Common Pitfalls: Confusing plain sedimentation with flocculation–clarification units, which may target different design rates but typically still yield comparable basin residence times. Final Answer: 4 to 8 hours

Question 8

Population forecasting – geometrical increase method A city has a population of 50,000 in the year 2000. The average percentage increase per decade from records is 20%. Using the geometrical increase method, estimate the population in the year 2020.

Accepted Answer

Introduction / Context: Population forecasting informs capacity planning for water supply, wastewater, and urban infrastructure. The geometrical (compound) increase method assumes population grows by a constant percentage over equal time intervals. Given Data / Assumptions: Base population P0 = 50,000 at year 2000. Average percentage increase per decade r = 20% = 0.20. Forecast year = 2020 → two decades ahead (n = 2). Concept / Approach: Geometrical increase method uses compound growth: Pn = P0 * (1 + r)^n. Here, n is the number of decades between base and forecast years. Step-by-Step Solution: Identify n = 2 decades (2000 → 2010 → 2020).Compute growth factor: (1 + 0.20)^2 = 1.2^2 = 1.44.Forecast population: P2020 = 50,000 * 1.44 = 72,000. Verification / Alternative check: Check intermediate decade: 2000→2010 = 50,000 * 1.2 = 60,000; 2010→2020 = 60,000 * 1.2 = 72,000. Matches direct computation. Why Other Options Are Wrong: 56,000; 60,000; 64,000; 70,000 correspond to lower effective growth rates or arithmetic increases, not the specified 20% compound per decade. Common Pitfalls: Applying arithmetic increase instead of geometric; using 20% per year instead of per decade; forgetting to square the factor for two decades. Final Answer: 72,000

Question 9

Per capita water requirement by building type According to common planning values, 45 litres per capita per day is typically provided for which building occupancy?

Accepted Answer

Introduction / Context: Plumbing and water-supply design uses typical per capita water allowances for different occupancies. These figures reflect usage patterns such as handwashing, toilets, cleaning, and limited pantry needs. Given Data / Assumptions: Standard planning norms for non-residential buildings. Water includes domestic and sanitary use within the building; not industrial process water. Concept / Approach: Office buildings typically have relatively low per capita consumption compared with residences, hotels, or medical accommodations. A common planning value is about 45 L/person/day for offices, whereas hotels and residential institutions (hostels, nurses’ homes) have higher demands due to bathing, laundry, and meal services. Step-by-Step Solution: Match 45 LPCD to the occupancy with minimal bathing/laundry loads → office buildings.Other occupancies usually range far higher (often >100 LPCD). Verification / Alternative check: Municipal bylaws and design manuals list ~45 LPCD for offices; hotels commonly exceed 180 LPCD; hostels are often around 135 LPCD or more. Why Other Options Are Wrong: Hotels/hostels/nurses’ homes include bathing and laundry leading to significantly higher per capita usage than 45 LPCD. Common Pitfalls: Not distinguishing between domestic institutional uses and office-only uses; failing to check latest local standards. Final Answer: Office buildings

Question 10

Plain chlorination – typical chlorine dosage For plain chlorination (without ammonia), an indicative quantity of chlorine used during treatment is closest to:

Accepted Answer

Introduction / Context: In disinfection, “plain chlorination” refers to dosing chlorine to achieve a target residual after a specified contact time. While exact dose depends on chlorine demand of the water, exam problems use representative values. Given Data / Assumptions: Potable water after clarification/filtration. Target free residual often about 0.2 mg/L after ≈ 30 minutes contact. Moderate chlorine demand in typical raw waters. Concept / Approach: The applied dose must satisfy initial demand plus desired residual. A common indicative dose for plain chlorination is around 0.5 mg/L (may range from roughly 0.2–0.5 mg/L). For a single best-choice question, 0.5 mg/L is the most representative among the discrete options provided. Step-by-Step Solution: Recognize that residual of ~0.2 mg/L typically requires a higher applied dose.Pick the commonly cited exam value closest to practice → 0.5 mg/L. Verification / Alternative check: Operator logs often show 0.3–0.8 mg/L dose adjustments depending on demand; 0.5 mg/L is a typical starting point. Why Other Options Are Wrong: 0.1–0.4 mg/L may be insufficient to overcome demand and still leave a protective residual. Common Pitfalls: Assuming a fixed dose always works; real plants measure residual and adjust dosage continuously with changing raw-water quality. Final Answer: 0.5 mg/L

Question 11

Peak factor within the maximum day The ratio of maximum hourly consumption to the average hourly consumption on the maximum day is commonly taken as:

Accepted Answer

Introduction / Context: Water-demand patterns vary through the day. Designers adopt a peak-hour factor relative to the average hour of the maximum day to size mains, pumps, and storage prudently. Given Data / Assumptions: Typical urban consumption without extreme industrial spikes. “Maximum day” refers to the day with the highest 24-h demand in the design year. Concept / Approach: Standard planning values often take the peak hour on the maximum day as roughly 1.8 times the average hour of that same day. This reflects morning/evening peaks and supports sizing to avoid excessive pressure drops during short spikes. Step-by-Step Solution: Define peak-hour factor F = q_max,h / q_avg,h(max day).Adopt the commonly used figure F ≈ 1.8.Select 1.8 from the options. Verification / Alternative check: Waterworks manuals list ranges (1.5–2.0); 1.8 is a frequently cited exam value for Indian and many international contexts. Why Other Options Are Wrong: 1.2 and 1.5 underestimate peaks; 2.4 and 2.7 are unusually high for general planning. Common Pitfalls: Confusing this factor with “maximum day to average day” ratio; both factors are used but represent different planning levels. Final Answer: 1.8

Question 12

Washouts and drainage of water mains Valves installed at low points of pipelines to quickly drain water by gravity are called:

Accepted Answer

Introduction / Context: Pipeline networks need provisions for emptying lines for maintenance, flushing sediments, and protecting against freezing. Special valves are located at low points to discharge water safely to drains or sewers. Given Data / Assumptions: Potable-water distribution or transmission main. Valve positioned at local low point. Discharge routed to a safe outfall. Concept / Approach: These valves are commonly called blow-off or washout valves. They open to allow gravity drainage and high-velocity flushing to remove deposits. While one might informally say “drain valve,” the technical term in waterworks is blow-off/washout. Step-by-Step Solution: Identify the function: fast gravity drainage and flushing at low points.Match to standard nomenclature → blow-off (washout) valve. Verification / Alternative check: Distribution standards show blow-offs at dead ends and low points sized to achieve scouring velocities while draining the main. Why Other Options Are Wrong: “Drain valves” is generic and not the standard waterworks term used in design drawings/specs. “Sewer valves” are unrelated to potable water mains. “All the above” is incorrect because not all listed choices are proper terms for this application. Common Pitfalls: Undersizing blow-offs, leading to slow drain-down and ineffective flushing; not providing adequate erosion protection at the outfall. Final Answer: Blow-off valves (washout valves)

Question 13

In water treatment design, the removal efficiency of a horizontal-flow sedimentation tank primarily depends on surface overflow rate (flow per unit plan area). Which of the following parameters does the efficiency not depend upon (i.e., has the leas…

Accepted Answer

Introduction / Context: Sedimentation (plain settling) is a fundamental unit process in water and wastewater treatment. For an ideal horizontal-flow clarifier, particle removal is controlled by the relationship between particle settling velocity and the basin’s surface overflow rate. This question checks whether you know which geometric or hydraulic parameters most strongly govern efficiency and which have only secondary or negligible influence in the ideal theory. Given Data / Assumptions: Horizontal-flow sedimentation tank operating under steady conditions. Clarifier design follows the ideal settling basin theory. Overflow rate = Q / A_plan (flow divided by plan surface area). Concept / Approach: In the ideal model, a particle is removed if its settling velocity vs ≥ overflow rate (Q/A). Therefore, the fraction removed is governed by plan surface area, not by the tank’s depth. Length, velocity distribution, and detention time influence short-circuiting and flocculation opportunities in real tanks, but the first-order determinant in the ideal analysis is surface overflow rate. Step-by-Step Solution: Define overflow rate: v0 = Q / A_plan.Removal condition: particle with vs ≥ v0 will settle before reaching the outlet.Depth (D) does not appear in v0; hence efficiency in the ideal sense does not depend on D.Length (L) and velocity profile affect approach to ideality (short-circuiting), but are not the primary determinant in the ideal equation. Verification / Alternative check: Design handbooks emphasize that enlarging plan area (length or width) reduces overflow rate and improves removal, while simply deepening a tank has little benefit for removal efficiency under ideal assumptions. Why Other Options Are Wrong: Length of tank: affects hydraulics, distribution, and approximation to plug flow. Detention period: relates to flow and volume; while not the primary ideal parameter, it correlates with performance in practical designs. Velocity of water: affects turbulence and short-circuiting; too high velocity reduces performance. Common Pitfalls: Assuming that increasing depth always improves performance; confusing storage volume (which affects detention) with the surface-governed efficiency criterion. Final Answer: depth of tank

Question 14

In pump houses and water-supply pumping stations, which pipe joint is most commonly used for connecting pumps, valves, and ancillary fittings requiring rigid, maintainable connections?

Accepted Answer

Introduction / Context: Pumping stations demand robust, leak-tight, and maintainable connections between pumps, valves, and pipelines. The joint type must allow dismantling for inspection and replacement while ensuring alignment and rigidity under pressure and vibration. Given Data / Assumptions: Pressurized service with frequent maintenance needs. Alignment accuracy and rigidity are important. Connections to pumps, valves, meters, and strainers. Concept / Approach: Flanged joints provide positive bolted connections, precise gasketed sealing surfaces, and easy dismantling for maintenance. They are standard in pump rooms for equipment nozzles and appurtenances. Other joints are used elsewhere in distribution but are less suited for equipment interfaces. Step-by-Step Solution: Identify operational needs: rigidity, sealing, maintainability.Match to joint type: flanges meet all three requirements.Conclude: flanged joint is the most common choice in pumping stations. Verification / Alternative check: Manufacturer installation manuals and plant standards specify flanged ends for pumps and valves; dismantling joints or dresser couplings may be added nearby for ease of removal, but the principal connection remains flanged. Why Other Options Are Wrong: Socket and spigot/flexible: typical for buried lines, not for equipment connections. Expansion joints: used to absorb thermal movement, not as the primary pump connection. Dresser couplings: helpful for minor misalignment or quick repairs, but not the standard pump nozzle joint. Common Pitfalls: Assuming the same joint type is ideal everywhere; overlooking maintenance requirements in pump houses. Final Answer: flanged joint

Question 15

Backwash and wash-water distribution are an essential design feature of which filtration unit in water-treatment plants?

Accepted Answer

Introduction / Context: Filtration units require periodic cleaning to restore permeability and control headloss. The method of cleaning depends on the filter type. Understanding which unit needs engineered wash-water distribution helps avoid operational issues like media loss or incomplete cleaning. Given Data / Assumptions: Slow sand filters are cleaned by scraping the top schmutzdecke. Rapid gravity filters are cleaned by hydraulic backwashing, sometimes with air scour. Sedimentation tanks do not use wash-water distribution systems. Concept / Approach: Rapid gravity filters employ an underdrain and wash-water distribution system to fluidize the media bed during backwash and to ensure uniform cleaning across the filter area. Designing proper wash-water velocities is key to effective media expansion and scouring. Step-by-Step Solution: Identify cleaning mechanism per unit: scraping (slow sand) vs backwash (rapid gravity).Backwash requires uniform distribution of upward flow → underdrain/wash-water laterals.Therefore, wash-water distribution is intrinsic to rapid gravity filters. Verification / Alternative check: Standard texts and codes specify backwash rates (e.g., 15–20 L/s·m²) and air scour rates for rapid filters, confirming the need for dedicated distribution systems. Why Other Options Are Wrong: Sedimentation tanks: use sludge removal mechanisms, not backwash systems. Slow sand filters: cleaned by manual/mechanical scraping, not hydraulic backwash. “All of the above” incorrectly generalizes the requirement. Common Pitfalls: Confusing rapid gravity filters with pressure filters; both need backwash distribution, but the classic exam context points to rapid gravity filters in open tank configuration. Final Answer: rapid gravity filter

Question 16

In urban water-supply systems, service connections to individual consumer premises are most commonly provided using which pipe material (traditional practice)?

Accepted Answer

Introduction / Context: Service connections link distribution mains to individual households or buildings. The material choice is influenced by availability, ease of threading and jointing, pressure ratings, and corrosion resistance. Historically in many regions, GI pipes have been the standard for these short runs. Given Data / Assumptions: “Traditional practice” context for exam-style questions. Short-diameter, moderate-pressure service laterals. Compatibility with fittings and meters. Concept / Approach: Galvanised iron (GI) pipes are easily threaded, readily joined to meters and stopcocks, and have adequate strength for typical service pressures. While modern systems increasingly use HDPE/uPVC for corrosion resistance and flexibility, GI remains the classic answer in many syllabi and legacy systems. Step-by-Step Solution: Assess needs: strength, fittings compatibility, installation ease.GI meets these needs in conventional service lines.Therefore, GI is the standard traditional choice. Verification / Alternative check: Municipal standards historically specified GI for service lines (often with later transitions to plastic piping in contemporary practice). Why Other Options Are Wrong: Copper: durable but costlier and less common in many regions. Hume pipes: unsuitable for small-diameter house connections. PVC/uPVC: widely used now, but the conventional answer in many exam keys remains GI. Common Pitfalls: Ignoring regional updates where HDPE/uPVC dominate; however, for standardized exams referencing classical practice, GI is expected. Final Answer: galvanised iron (GI) pipes

Question 17

Which among the following are typical causes of turbidity in raw water supplied to treatment plants?

Accepted Answer

Introduction / Context: Turbidity is an optical property that describes the clarity of water. It interferes with disinfection, affects aesthetics, and indicates the presence of particulate matter. Understanding its causes is essential for selecting adequate treatment steps (coagulation, flocculation, sedimentation, filtration). Given Data / Assumptions: Turbidity measured in NTU or JTU depending on method. Particles include inorganic and organic fractions. Typical surface-water sources. Concept / Approach: Common sources include inorganic mineral fines (clay, silt) eroded from catchments and organic debris (plankton, detritus). All scatter light and elevate turbidity. Effective treatment targets particle destabilization and removal to protect downstream disinfection. Step-by-Step Solution: Identify particle types: inorganic clay/silt and organic fines.All listed categories contribute to light scattering → turbidity.Hence the exhaustive correct option is “all the above”. Verification / Alternative check: Laboratory jar tests and particle-size analysis in raw water corroborate these typical particle sources. Why Other Options Are Wrong: Each individual option represents a valid single cause; none alone is exhaustive, so “all the above” is the best answer. Common Pitfalls: Assuming turbidity is solely mineral; ignoring organic colloids and bio-particulates that can dominate during algal blooms. Final Answer: all the above.

Question 18

Select the correct advantages typically associated with centrifugal pumps in water-supply installations.

Accepted Answer

Introduction / Context: Centrifugal pumps are the workhorse of water and wastewater systems. Their hydraulic characteristics and mechanical simplicity make them the preferred option for most municipal applications. Recognizing their typical advantages is critical in preliminary selection and design. Given Data / Assumptions: Suction-lift or flooded-suction configurations. Continuous-duty municipal service. Comparison against reciprocating/positive-displacement pumps. Concept / Approach: Centrifugal pumps impart energy by a rotating impeller, yielding smooth flow with minimal pulsation. Their construction is relatively simple, enabling compact installations and lower purchase costs for comparable capacities. They are easily controlled via speed variation and throttling within efficiency limits. Step-by-Step Solution: Review attributes: simplicity → lower capital cost.Rotodynamic principle → steady discharge (no pulsation).Footprint and layout → compact compared with PD systems. Verification / Alternative check: Pump catalogs and life-cycle cost analyses show why centrifugal pumps dominate municipal waterworks for medium-to-high flows at moderate heads. Why Other Options Are Wrong: Each of a–c is true; the comprehensive correct selection is “all the above”. Common Pitfalls: Ignoring limitations at very high heads or very low flows where PD pumps may be superior; assuming throttling is always efficient without considering curves. Final Answer: all the above.

Question 19

Why are cast iron (CI) pipes historically favored in water-supply distribution systems?

Accepted Answer

Introduction / Context: Material selection for water distribution mains has evolved, but cast iron and later ductile iron have long been standards due to their mechanical robustness and service longevity. Understanding historical reasons helps when interpreting legacy systems and making rehabilitation plans. Given Data / Assumptions: Buried distribution mains under traffic and soil loads. Pressures typical of municipal networks. Compatibility with joints and fittings of the time. Concept / Approach: CI provides good ring stiffness and compressive strength to withstand burial conditions. Mechanical joints, flanges, and bell-and-spigot connections eased installation. With protective linings and coatings, CI achieved good durability and relatively low maintenance for decades. Step-by-Step Solution: Assess service environment: soil, traffic, pressure → CI suitable.Evaluate jointing: bell-and-spigot or mechanical joints → straightforward.Lifecycle: historically robust, leading to reduced maintenance relative to alternatives then available. Verification / Alternative check: Many cities still operate CI mains installed in the early to mid-20th century, demonstrating durability under proper corrosion control. Why Other Options Are Wrong: Each individual benefit is valid; a comprehensive view recognizes all listed advantages. Common Pitfalls: Confusing CI with ductile iron (DI); modern replacements often prefer DI or HDPE depending on conditions, but the historical rationale for CI remains solid. Final Answer: all the above.

Question 20

In rapid gravity (rapid sand) filters, what is the usual maximum permitted terminal headloss (loss of head) across the filter bed before initiating backwashing?

Accepted Answer

Introduction / Context: As filtration proceeds, particulates accumulate and headloss rises. The terminal headloss is the threshold beyond which performance or safety is compromised, necessitating backwash. Knowing typical values helps operators schedule efficient filter runs and maintain effluent quality. Given Data / Assumptions: Rapid gravity filters with typical media depth and effective size. Conventional treatment pre-steps (coagulation, flocculation, sedimentation). Normal design practice values from standard references. Concept / Approach: Initial headloss is low; as solids load builds, the headloss reaches a terminal value (commonly around 2.5 to 3.0 m) at which the filter is backwashed to restore permeability. Operating beyond this increases risk of media disturbance or breakthrough during rate fluctuations. Step-by-Step Solution: Track headloss via piezometric taps.Trigger backwash when terminal headloss is approached.Standard practice accepts approximately 3 m as the upper limit for many designs. Verification / Alternative check: Codes and manuals often cite terminal headloss of about 2.5–3.0 m; the nearest option provided is 3 m. Why Other Options Are Wrong: 1–2 m: too conservative for typical designs; would shorten filter runs excessively. 4–5 m: risk undue stress and poor effluent quality. Common Pitfalls: Confusing initial with terminal headloss; ignoring temperature effects on viscosity and headloss progression; delaying backwash beyond alarms. Final Answer: 3 m

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