Q: Water quality – typical constituents causing hardness Which of the following dissolved species are responsible for the hardness of water commonly encountered in water-supply engineering?

Introduction / Context: Hardness of water is primarily due to dissolved polyvalent cations, especially calcium and magnesium. Hardness affects soap consumption, scaling in boilers and pipes, and behavior in softening units. Recognizing which ions contribute allows engineers to select suitable treatment processes. Given Data / Assumptions: Dissolved inorganic salts in natural waters. Hardness measured as mg/L as CaCO3 equivalent. Both temporary (carbonate) and permanent (non-carbonate) hardness considered. Concept / Approach: Temporary (carbonate) hardness arises mainly from bicarbonates of calcium and magnesium. Permanent hardness arises from sulphates, chlorides, and nitrates of calcium and magnesium. Therefore, calcium species and magnesium bicarbonate/sulphate are direct contributors to hardness. Step-by-Step Solution: Identify hardness-causing cations: Ca2+ and Mg2+ dominate.Link to anions present: HCO3- (temporary hardness), SO4^2- (permanent hardness), among others.All listed species either directly contain calcium/magnesium or are typical forms causing hardness. Verification / Alternative check: Standard water-chemistry texts define total hardness = calcium hardness + magnesium hardness, confirming the listed species contribute. Why Other Options Are Wrong: Each of (a), (b), and (c) contributes; hence the comprehensive answer is 'All the above'. 'None of these' contradicts established chemistry. Common Pitfalls: Confusing alkalinity (capacity to neutralize acid) with hardness; although bicarbonates contribute to both concepts, they are not identical measures. Final Answer: All the above

Q: Slow sand filter – ripening period after cleaning After scraping and cleaning a slow sand filter, for how long is the filtered water typically not put into supply until an adequate biological ripening is achieved?

Introduction / Context: Slow sand filters rely on a biologically active layer (schmutzdecke) formed on top of the sand to achieve high removals of bacteria and turbidity. After scraping for maintenance, this layer must re-form before the filtrate is considered safe and compliant for distribution. Given Data / Assumptions: Typical slow sand filter operation without advanced pre-treatment. Normal raw-water quality conditions. Standard practice to discard or recycle initial filtrate after cleaning until ripened. Concept / Approach: The biological ripening time depends on temperature, nutrient load, and flow rate, but a common operational rule is to wait roughly one to two days after cleaning, during which filtrate is bypassed. This period allows re-establishment of the schmutzdecke and restoration of target turbidity and bacteriological performance. Step-by-Step Solution: Recognize that immediately after scraping, bacterial removal efficiency is poor.Operationally the ripening period is commonly 24–36 hours before water is directed to supply.Among the options, '24 to 36 hours' best matches standard practice. Verification / Alternative check: Commissioning records and operating manuals for slow sand plants typically quote 24–48 hours; the 24–36 hours choice sits within this accepted window and is a frequently cited exam value. Why Other Options Are Wrong: 6–12, 12–18, and 18–24 hours are usually too short for full biological activity to return. Exactly 48 hours may be conservative in many cases; operators often resume within 24–36 hours given acceptable quality tests. Common Pitfalls: Sending post-cleaning filtrate directly to supply; failing to monitor turbidity and coliforms during ripening; running excessive rates that delay schmutz layer formation. Final Answer: 24 to 36 hours

Question 1

Population growth analysis: The transitional middle portion of a logistic population growth curve represents which type of growth behaviour?

Accepted Answer

Introduction / Context: The logistic growth curve is widely used in biology and demography. It has three phases: lag phase, exponential (geometric) phase, and stationary phase. This question tests the recognition of the middle phase. Given Data / Assumptions: Curve type: Logistic (S-shaped). Middle portion: Represents rapid growth before resource limitations act. Concept / Approach: The middle part of a logistic curve shows exponential (geometric) growth, as resources are abundant and population grows rapidly. Later, growth slows down due to limiting factors. Step-by-Step Solution: 1. Initial stage → lag (slow growth).2. Middle stage → geometric (exponential) growth due to abundance of resources.3. Final stage → constant/stabilized growth. Verification / Alternative check: Population models in civil engineering planning confirm that exponential growth precedes saturation in urban populations. Why Other Options Are Wrong: Logarithmic growth: Refers to slower, diminishing growth, not the rapid stage. First over curve: Not a standard term in growth modelling. A constant rate: Refers to arithmetic growth, not logistic mid-phase. Common Pitfalls: Confusing logarithmic with geometric growth due to similar wording. Believing population always grows linearly, ignoring saturation effects. Final Answer: Geometric growth

Question 2

Fire demand estimation: Which is considered the most satisfactory formula for estimating the fire demand (Q) for an Indian city of population P (in thousands)?

Accepted Answer

Introduction / Context: Estimating fire demand is an essential part of designing a water supply system. Different empirical formulas exist, but for Indian conditions, Freeman’s formula is considered the most satisfactory. Given Data / Assumptions: Population of city: P (in thousands). Fire demand: Q (litres per minute). Empirical formulas vary by country and conditions. Concept / Approach: Freeman’s formula is adapted for Indian urban conditions. It estimates fire demand considering both population size and density of development. Step-by-Step Solution: 1. Identify requirement → Fire demand Q.2. Consider available formulas → Kuichling, Freeman, Buston, NBFU.3. For Indian practice, the recommended formula is Freeman’s. Verification / Alternative check: CPHEEO (Central Public Health & Environmental Engineering Organisation) guidelines recommend Freeman’s formula for Indian water supply design, validating this choice. Why Other Options Are Wrong: Kuichling’s formula: Used in the USA, less suited to Indian densities. Buston’s formula: Applied in limited contexts, not standard in India. NBFU formula: A US standard, not adopted in Indian codes. None of these: Wrong because Freeman’s formula is accepted in India. Common Pitfalls: Assuming any international formula applies universally. Confusing fire demand with per capita domestic demand. Final Answer: Freeman’s formula

Question 3

Water supply system components: Which of the following correctly represents the scope of a water supply system in civil engineering?

Accepted Answer

Introduction / Context: A water supply system is not limited to one activity like digging wells or building canals. It is a complete arrangement from sourcing water to its final distribution to consumers. Given Data / Assumptions: Water must be collected, transported, treated, stored, and distributed. System scope includes all engineering works related to supply. Concept / Approach: The water supply system is a holistic infrastructure chain ensuring safe and continuous delivery of potable water. Step-by-Step Solution: 1. Water is collected from sources (wells, rivers, dams).2. Conveyed through canals or pipes to treatment plants.3. Treated to meet potable standards.4. Stored in service reservoirs.5. Distributed through mains and submains to users. Verification / Alternative check: This is consistent with IS:1172 and CPHEEO manuals defining the scope of water supply systems. Why Other Options Are Wrong: Digging a well, dams, canals: These are components, not the entire system. None of these: Incorrect since one option clearly represents the whole scope. Common Pitfalls: Confusing individual components as the whole system. Neglecting treatment and distribution as part of the system. Final Answer: Entire arrangement from source to distribution

Question 4

Biochemical Oxygen Demand (B.O.D.): What is the permissible value of B.O.D. in safe drinking water according to environmental engineering standards?

Accepted Answer

Introduction / Context: B.O.D. measures the oxygen consumed by microorganisms while decomposing organic matter in water. In safe drinking water, B.O.D. should ideally be zero to ensure purity and safety. Given Data / Assumptions: B.O.D. values indicate pollution levels. Standards prescribe safe limits for potable water. Concept / Approach: B.O.D. is critical for wastewater quality but should be absent in drinking water. Hence, safe drinking water must have nil B.O.D. Step-by-Step Solution: 1. Identify water type → drinking water.2. Standard → B.O.D. should be absent.3. Hence, permissible value = Nil. Verification / Alternative check: BIS standards confirm B.O.D. must be nil in potable water, while wastewater may allow measurable values. Why Other Options Are Wrong: 5, 10, 15, 20: These values are acceptable for wastewater, not drinking water. Common Pitfalls: Confusing wastewater B.O.D. with drinking water standards. Final Answer: Nil

Question 5

In water quality analysis, turbidity of raw water is primarily a measure of the concentration of suspended and colloidal solids that cause cloudiness (not acidity or biochemical oxygen demand).

Accepted Answer

Introduction / Context: Turbidity is one of the most routinely measured parameters in water and wastewater engineering. It reflects how “cloudy” or “murky” the water looks, which directly impacts treatment design, filter performance, and public perception of drinking water quality. Understanding what turbidity actually measures helps avoid common confusions with acidity (pH) or biochemical oxygen demand (B.O.D.). Given Data / Assumptions: We are dealing with raw (untreated) water entering a water-treatment system. Turbidity is measured with standard instruments such as nephelometers and reported typically in NTU (nephelometric turbidity units). Options include suspended solids, acidity, and B.O.D., which are distinct water-quality concepts. Concept / Approach: Turbidity quantifies the scattering of light by particles present in water. The more suspended and colloidal particles present, the higher the light scattering and thus the measured turbidity. Acidity (pH) is a chemical property indicating hydrogen ion activity, while B.O.D. is a biological test that measures oxygen consumed by microorganisms during the degradation of organic matter over a defined period (commonly 5 days). Step-by-Step Solution: Step 1: Identify what turbidity gauges: light scattering from particles in suspension.Step 2: Link light scattering to particulate content: more suspended/colloidal matter implies higher turbidity.Step 3: Differentiate from acidity: pH relates to hydrogen ion concentration, not particulate cloudiness.Step 4: Differentiate from B.O.D.: B.O.D. measures oxygen uptake by microbes, not optical clarity.Step 5: Conclude that turbidity is a measure of suspended solids (and fine colloids) in water. Verification / Alternative check: Operators often correlate turbidity spikes with events that increase particulate loads (e.g., storm runoff). Filtration steps (coagulation, flocculation, sedimentation, filtration) reduce particulates and consequently reduce turbidity, confirming the direct link with suspended matter. Why Other Options Are Wrong: Acidity of water: pH does not directly influence light scattering unless it alters particle chemistry; turbidity devices do not read pH.B.O.D.: A 5-day oxygen-demand test is unrelated to instant optical scattering read by turbidity meters.None of these: Incorrect because suspended solids is exactly what turbidity indicates. Common Pitfalls: Confusing turbidity (optical property) with total suspended solids (gravimetric mass). They are related but not identical. Assuming color alone causes turbidity; dissolved color can affect absorbance but turbidity measures scattering due to particles. Equating low pH or high B.O.D. with high turbidity; these relationships are not direct. Final Answer: suspended solids

Question 6

In municipal water supply design, the fire demand of a city may be estimated using several empirical formulas; identify the correct statement about available methods.

Accepted Answer

Introduction / Context: Fire demand is the short-duration, high-flow requirement that a water distribution system must be able to supply during firefighting events. Designers estimate this demand using empirical formulas derived from historical data and urban characteristics. Knowing which formulas exist and when they apply is essential for sizing mains, hydrants, storage, and pumps. Given Data / Assumptions: We are comparing named empirical methods: Kuichling's, Freeman, Underwriters (often called National Board of Fire Underwriters), and Buston/Bustan. Question asks which of these are used to work out city fire demand. No numerical inputs are provided; this is a recognition/recall item. Concept / Approach: Multiple empirical formulae are cited in standard water-supply texts for Indian and international practice. Each provides an estimated peak flow based on population or service area. The key idea is that different cities and codes may prefer different correlations, but all listed formulas are legitimate approaches to compute fire demand. Step-by-Step Solution: Step 1: Recall that fire demand estimation is not unique; several time-tested formulae exist.Step 2: Recognize the names Kuichling, Freeman, Underwriters, and Bustan as standard in design literature.Step 3: Conclude that all listed methods can be used to work out fire demand. Verification / Alternative check: Cross-comparing results from two or more formulas for the same population often yields values within a reasonable range. Designers then select the governing (usually higher) value considering local fire codes and risk assessments. Why Other Options Are Wrong: Kuichling's formula only: Incomplete—there are other accepted formulas.Freeman formula only: Incomplete for the same reason.Under Writers formula only: Also incomplete.Bustan's formula only: Again, not comprehensive. Common Pitfalls: Using a single formula universally without checking local standards or insurance underwriters’ requirements. Ignoring peak-hour plus fire-flow scenarios; storage and main sizes should consider concurrent demands. Not validating available residual pressure at hydrants while meeting calculated flow. Final Answer: All the above.

Question 7

In water-supply mains, identify the most commonly used pump type for lifting and conveying water under typical municipal conditions.

Accepted Answer

Introduction / Context: Selecting the appropriate pump for municipal water-supply mains is critical for reliable, energy-efficient operation. While many pump types exist, one class dominates distribution service due to its simplicity, cost-effectiveness, and ability to handle moderate to high flows at practical heads. Given Data / Assumptions: Application: lifting and conveying potable water in public water-supply mains. Operating needs: continuous or near-continuous duty, moderate heads, wide flow ranges, reasonable efficiency, and easy maintenance. Pump options provided: axial-flow, reciprocating, rotary, and centrifugal. Concept / Approach: Centrifugal pumps convert rotational energy to hydraulic energy via an impeller, producing head by dynamic action. They are robust, relatively inexpensive per unit flow, tolerant of minor solids, and available in numerous configurations (end-suction, split-case, multistage). These traits make them the standard choice for municipal mains. Other pump types excel in niche roles but are not the common workhorses for distribution mains. Step-by-Step Solution: Step 1: Match pump characteristics to municipal distribution needs: steady flow, moderate head, efficiency, maintainability.Step 2: Recognize centrifugal pumps as industry-standard for these conditions due to dynamic head generation and scalability.Step 3: Eliminate less-suited types (axial-flow for very low head/high discharge, reciprocating for small high head/positive displacement duties, rotary for viscous fluids).Step 4: Conclude the most commonly used pump type is the centrifugal pump. Verification / Alternative check: Municipal standards and manufacturer catalogs for water utilities consistently show split-case and multistage centrifugal units for transmission mains and booster stations. Life-cycle cost analyses also favor centrifugal pumps for these duty points. Why Other Options Are Wrong: Axialflow pump: Best for very low head with extremely high discharge (e.g., flood control), not typical water-supply mains.Reciprocating pump: Positive displacement, suited for high-pressure/low-flow or dosing, not bulk distribution.Rotary type pump: Often used for viscous fluids or lubrication systems, not municipal water mains.None of these: Incorrect because centrifugal pumps are widely used. Common Pitfalls: Oversizing pumps leading to operation far from best efficiency point (BEP), causing vibration and energy losses. Ignoring Net Positive Suction Head (NPSH) requirements, which may cause cavitation. Not considering variable-speed drives to match diurnal demand patterns. Final Answer: centrifugal pumps

Question 8

In hydraulic engineering of long pressure conduits, surge tanks are primarily installed to serve which operational purpose?

Accepted Answer

Introduction / Context: Surge tanks are vertical chambers connected to long pressure pipelines that feed turbines or distribution systems. Their main role is to control pressure transients—commonly called water hammer—that arise when flow accelerates or decelerates rapidly due to valve or turbine gate movements. Understanding this purpose helps designers protect pipelines from overstress and maintain stable operations. Given Data / Assumptions: We consider a long pressurized main connected to a reservoir and a demand (e.g., a turbine). Rapid flow changes can create large pressure waves. A surge tank provides a free surface connected to the main. Concept / Approach: When downstream demand suddenly decreases, momentum causes a pressure rise that travels as a surge. The surge tank provides an immediate volume buffer: water level in the tank rises, absorbing energy, and limiting peak pressure. Conversely, when demand suddenly increases, water level falls, supplying extra flow to curb negative pressures. This free-surface exchange mitigates the amplitude of transient waves, reducing risk of pipe rupture, joint failure, and equipment damage. Step-by-Step Solution: Identify the transient source: rapid valve/gate changes produce pressure waves. Recognize the surge tank's function: a nearby free surface allows inflow/outflow to absorb or supply momentum. Conclude: the device guards against water hammer by moderating pressure spikes and dips. Verification / Alternative check: Engineers also use air chambers, relief valves, and optimized control strategies. Yet, for long conduits feeding turbines, surge tanks are the classic, highly effective solution to restrain transients within allowable limits. Why Other Options Are Wrong: Storage along the line: surge tanks are not intended as primary storage reservoirs. Increase velocity: their function is not to speed up flow. Overflow valves: a tank is not a valve; it does not automatically discharge to waste. Measuring discharge: surge tanks are not flow meters. Common Pitfalls: Confusing surge tanks with service reservoirs or standpipes used for head stabilization in distribution networks. Assuming surge tanks eliminate all transients—they reduce, not fully remove, pressure waves. Final Answer: To guard the system against water-hammer (transient) pressures.

Question 9

In water-supply engineering, which statements correctly describe asbestos cement (AC) pipes used for conveyance?

Accepted Answer

Introduction / Context: Material selection for pipelines balances hydraulics, constructability, durability, and cost. Asbestos cement (AC) pipes were historically popular for low-to-moderate pressure mains and laterals due to ease of handling and good hydraulic performance. This question checks recognition of their typical properties in traditional design literature. Given Data / Assumptions: We consider conventional statements about AC pipes in civil engineering exams. Focus is on physical handling, corrosion behavior, joint flexibility, and hydraulic smoothness. Concept / Approach: AC pipes are relatively light compared with metallic pipes, allowing faster laying with simpler equipment. They are inherently non-metallic and thus immune to electrochemical corrosion that affects iron/steel pipes. Gasketed push-on or similar joints offer some angular deflection, which helps in minor alignment adjustments. The bore is smooth, which lowers frictional headloss (favorable Hazen–Williams C). Step-by-Step Solution: Confirm low mass per unit length enables easy transport and installation. Note corrosion resistance due to non-metallic composition; no galvanic action. Recognize joint flexibility allowing modest deflection (order of 10°). Acknowledge smooth internal surface yields high hydraulic efficiency. Thus, all listed statements are valid in standard references. Verification / Alternative check: Historic specifications report high C-values and routine use where soil aggressiveness challenged metal pipes. Although many utilities have phased out AC due to health and handling regulations, the classical properties in exams remain as stated. Why Other Options Are Wrong: Each single statement is correct; the most complete answer is therefore “All the above.” Common Pitfalls: Confusing corrosion resistance with complete chemical inertness; AC can still be attacked by strong acids/alkalis. Assuming large curvature capability; joint deflection is limited and must respect manufacturer limits. Final Answer: All the above.

Question 10

Public water supply – hard water:\r Why is hard water generally objectionable for municipal supply and domestic use, even when it is otherwise safe microbiologically?

Accepted Answer

Introduction / Context: Hardness in water is primarily due to dissolved calcium and magnesium salts. While hardness is not a direct health hazard, it has operational and domestic drawbacks that make it undesirable in public water supplies unless within acceptable limits. Given Data / Assumptions: “Hard water” refers to elevated Ca2+ and Mg2+ concentrations, expressed as mg/L as CaCO3. Microbiological safety is a separate issue from hardness. Turbidity is a physical parameter unrelated to hardness. Concept / Approach: The primary issues with hard water are: excessive soap consumption due to scum formation with calcium/magnesium, and scaling of pipes, heaters, and fixtures. These increase maintenance costs and reduce appliance efficiency. Step-by-Step Reasoning: Identify the definition and effects of hardness.Eliminate unrelated factors: turbidity, pathogens, taste/odour are not defining features of hardness.Conclude that the main objection is soap consumption and scaling. Verification / Alternative check: Standards and design texts note recommended hardness ranges for domestic supplies and discuss softening when hardness exceeds target levels to avoid scale and soap wastage. Why Other Options Are Wrong: Turbidity depends on suspended solids, not dissolved hardness. Pathogens are microbiological contaminants, unrelated to hardness. Taste/odour issues are more associated with organics, algae, or chlorine, not simply hardness. Common Pitfalls: Confusing hardness with turbidity or microbial contamination. Assuming hardness implies unsafe water; it is an aesthetic/operational concern. Final Answer: It consumes more soap and causes scaling in pipes and appliances

Question 11

Rapid gravity filter (RGF) operating range:\r A rapid gravity (rapid sand) filter is typically used after coagulation–flocculation–sedimentation. What influent turbidity range (expressed in NTU) can an RGF reliably handle under normal design practic…

Accepted Answer

Introduction / Context: Rapid gravity filters provide final clarification and polishing after upstream treatment. Correctly specifying the turbidity band for influent water ensures acceptable filter run lengths, headloss progression, and effluent quality. Given Data / Assumptions: Filter is used post-coagulation and sedimentation (i.e., clarified water enters the RGF). NTU (nephelometric turbidity units) is the standard turbidity measure. We seek a practical influent range routinely cited for design/operation. Concept / Approach: Design texts indicate RGFs are suited to clarified waters typically below about 30–35 NTU. Higher influent turbidity reduces run length and risks turbidity breakthrough, unless enhanced coagulation or dual-media/advanced configurations are used. Step-by-Step Reasoning: Confirm process train: coagulation → flocculation → sedimentation → RGF.Adopt a conservative, commonly accepted band for influent to RGF.Select 30 to 35 NTU as a representative normal operating range. Verification / Alternative check: Many utility practices target much lower average influent turbidities to maximize filter performance; however, 30–35 NTU serves as an upper practical limit in standard references for post-clarifier water under non-extreme conditions. Why Other Options Are Wrong: 15–25 or 25–30 NTU are possible but on the lower side; the question asks what an RGF can reliably handle, for which 30–35 NTU is a well-cited upper range. 35–40 NTU and “50 NTU or more without pre-treatment” overstate capability for conventional single-stage RGFs. Common Pitfalls: Confusing raw water turbidity limits with clarified water limits. Assuming filters can compensate for inadequate coagulation/settling. Final Answer: 30 to 35 NTU

Question 12

Units of hardness:\r One degree of hardness (Clark's degree) corresponds approximately to what concentration of CaCO3 equivalent in mg/L?

Accepted Answer

Introduction / Context: Hardness is often expressed in different traditional units. Converting among these is essential for comparing standards, plant data, and research reports. Clark's degree (°Clark) is an older British unit still encountered in practice. Given Data / Assumptions: We are asked for the CaCO3 equivalent concentration corresponding to 1 °Clark. Standard conversion: 1 °Clark ≈ 1 grain per Imperial gallon of CaCO3 equivalent. Concept / Approach: Using known conversions, 1 grain per Imperial gallon equals approximately 14.25 mg/L as CaCO3. This value is widely cited and used to translate historical data into SI-based concentrations. Step-by-Step Solution: Recall: 1 °Clark = 1 grain per Imp. gallon as CaCO3.Convert to mg/L: ≈ 14.25 mg/L as CaCO3.Therefore, select 14.25 mg/L as the correct equivalence. Verification / Alternative check: Contrast with other units: 1 °German (°dH) ≈ 17.8 mg/L as CaCO3, and 1 °French (°f) = 10 mg/L as CaCO3. The stated 14.25 mg/L aligns with °Clark, distinguishing it from these other scales. Why Other Options Are Wrong: 10.25 and 12.25 mg/L: Too low; closer to the French degree is 10 mg/L. 16.25 and 20.00 mg/L: Too high, and 17.8 mg/L corresponds to German degrees, not Clark. Common Pitfalls: Mixing up German, French, and Clark scales. Reporting hardness without specifying the unit basis (as CaCO3). Final Answer: 14.25 mg/L as CaCO3

Question 13

Coagulation in water treatment:\r Which chemical is most commonly used as a primary coagulant in conventional surface water treatment plants?

Accepted Answer

Introduction / Context: Coagulation is the first key step in conventional treatment to destabilize colloids and form settleable flocs. Selecting an appropriate coagulant is crucial for turbidity removal, natural organic matter reduction, and optimal filter performance. Given Data / Assumptions: We are dealing with standard municipal plants using coagulation–flocculation–sedimentation–filtration. Common coagulants include alum, ferric salts, and polyaluminium chloride; chlorine and bleaching powder are disinfectants, not coagulants. Concept / Approach: Alum (aluminium sulphate) has historically been the most widely used primary coagulant due to availability, effectiveness over a broad pH range with alkalinity adjustment, and predictable sludge characteristics. Alternatives are used based on raw water chemistry and cost considerations. Step-by-Step Reasoning: Identify chemical function: coagulant vs. disinfectant vs. pH modifier.Chlorine/bleaching powder are oxidants/disinfectants; lime is primarily for softening/pH/alkalinity adjustment.Select alum as the standard coagulant. Verification / Alternative check: Design manuals list alum as the default choice, with ferric salts and PACl as principal alternatives. Jar tests verify dose and performance. Why Other Options Are Wrong: Chlorine/bleaching powder: not coagulants; used for disinfection. Lime: coagulant aid/softening agent, not the primary coagulant for colloid destabilization in typical surface water treatment. Ferrous sulphate alone is less commonly the primary coagulant in modern municipal practice compared to alum or ferric chloride. Common Pitfalls: Confusing coagulation (particle destabilization) with disinfection. Overlooking the need for alkalinity when dosing alum to maintain target pH. Final Answer: Alum (aluminium sulphate)

Question 14

Thin-walled pressure conduit (cylinder) – hoop (circumferential) stress formula For a thin circular conduit carrying internal pressure, if p is the internal pressure, d is the internal diameter of the conduit, and t is the wall thickness, what is th…

Accepted Answer

Introduction / Context: Design of pipes, penstocks, and pressure conduits often assumes a thin-walled cylinder to obtain quick yet safe estimates of wall stresses. The most critical membrane stress is the circumferential or hoop stress, which tends to split the cylinder longitudinally. Understanding the correct closed-form relation among internal pressure p, diameter d, and wall thickness t is essential to size the wall and select materials. Given Data / Assumptions: Thin-walled circular cylinder (t << d), uniform thickness. Internal pressure = p (gauge). Internal diameter = d. Wall thickness = t. Linear elastic, small deformation; membrane theory holds. Concept / Approach: The hoop stress arises from equilibrium of half the cylinder split by a diametral longitudinal plane. Internal pressure produces a resultant force that is resisted by the tensile forces in the wall along the cut. For thin shells, stress is assumed uniform through thickness and along the width considered. Step-by-Step Solution: Resultant pressure on projected area = p * (d * L), where L is unit length along the axis.Resisting tensile force in two walls = 2 * (sigma_hoop * t * L).Equilibrium: p * d * L = 2 * sigma_hoop * t * L.Solve for sigma_hoop: sigma_hoop = p * d / (2 * t). Verification / Alternative check: Longitudinal (axial) stress for a closed-end thin cylinder is sigma_long = p * d / (4 * t), which is half the hoop stress. This consistent ratio verifies the derivation. Why Other Options Are Wrong: p * d / t and 2 * p * d / t overestimate stress by ignoring the factor of 2 from two wall segments sharing load. p * d^2 / (4 * t) has the wrong dimension (involves area factor). p / (2 * t) omits diameter and is dimensionally inconsistent. Common Pitfalls: Using outside diameter instead of internal diameter; applying the thin-wall formula when t/d is not small (typically t/d < 1/10); forgetting that joints/corrosion allowances must be added in real design. Final Answer: sigma_hoop = p * d / (2 * t)

Question 15

Water quality – typical constituents causing hardness Which of the following dissolved species are responsible for the hardness of water commonly encountered in water-supply engineering?

Accepted Answer

Introduction / Context: Hardness of water is primarily due to dissolved polyvalent cations, especially calcium and magnesium. Hardness affects soap consumption, scaling in boilers and pipes, and behavior in softening units. Recognizing which ions contribute allows engineers to select suitable treatment processes. Given Data / Assumptions: Dissolved inorganic salts in natural waters. Hardness measured as mg/L as CaCO3 equivalent. Both temporary (carbonate) and permanent (non-carbonate) hardness considered. Concept / Approach: Temporary (carbonate) hardness arises mainly from bicarbonates of calcium and magnesium. Permanent hardness arises from sulphates, chlorides, and nitrates of calcium and magnesium. Therefore, calcium species and magnesium bicarbonate/sulphate are direct contributors to hardness. Step-by-Step Solution: Identify hardness-causing cations: Ca2+ and Mg2+ dominate.Link to anions present: HCO3- (temporary hardness), SO4^2- (permanent hardness), among others.All listed species either directly contain calcium/magnesium or are typical forms causing hardness. Verification / Alternative check: Standard water-chemistry texts define total hardness = calcium hardness + magnesium hardness, confirming the listed species contribute. Why Other Options Are Wrong: Each of (a), (b), and (c) contributes; hence the comprehensive answer is 'All the above'. 'None of these' contradicts established chemistry. Common Pitfalls: Confusing alkalinity (capacity to neutralize acid) with hardness; although bicarbonates contribute to both concepts, they are not identical measures. Final Answer: All the above

Question 16

Slow sand filter – ripening period after cleaning After scraping and cleaning a slow sand filter, for how long is the filtered water typically not put into supply until an adequate biological ripening is achieved?

Accepted Answer

Introduction / Context: Slow sand filters rely on a biologically active layer (schmutzdecke) formed on top of the sand to achieve high removals of bacteria and turbidity. After scraping for maintenance, this layer must re-form before the filtrate is considered safe and compliant for distribution. Given Data / Assumptions: Typical slow sand filter operation without advanced pre-treatment. Normal raw-water quality conditions. Standard practice to discard or recycle initial filtrate after cleaning until ripened. Concept / Approach: The biological ripening time depends on temperature, nutrient load, and flow rate, but a common operational rule is to wait roughly one to two days after cleaning, during which filtrate is bypassed. This period allows re-establishment of the schmutzdecke and restoration of target turbidity and bacteriological performance. Step-by-Step Solution: Recognize that immediately after scraping, bacterial removal efficiency is poor.Operationally the ripening period is commonly 24–36 hours before water is directed to supply.Among the options, '24 to 36 hours' best matches standard practice. Verification / Alternative check: Commissioning records and operating manuals for slow sand plants typically quote 24–48 hours; the 24–36 hours choice sits within this accepted window and is a frequently cited exam value. Why Other Options Are Wrong: 6–12, 12–18, and 18–24 hours are usually too short for full biological activity to return. Exactly 48 hours may be conservative in many cases; operators often resume within 24–36 hours given acceptable quality tests. Common Pitfalls: Sending post-cleaning filtrate directly to supply; failing to monitor turbidity and coliforms during ripening; running excessive rates that delay schmutz layer formation. Final Answer: 24 to 36 hours

Question 17

Hardness of water – principal causative salts Hardness in potable water arises primarily from dissolved salts of calcium and magnesium. Which set below correctly reflects common hardness-causing species?

Accepted Answer

Introduction / Context: Hardness is a key design parameter for softening plants and industrial water systems. It is typically expressed as mg/L as CaCO3 and arises from calcium and magnesium salts in different anionic forms. Given Data / Assumptions: Natural groundwater or surface water sources. Both temporary (carbonate) and permanent (non-carbonate) forms present. No exotic heavy-metal contributions considered. Concept / Approach: Temporary hardness is mainly due to bicarbonates of calcium (and magnesium), removable by boiling or lime softening. Permanent hardness is caused by sulphates, chlorides, and nitrates of calcium/magnesium and needs chemical precipitation or ion exchange for removal. Step-by-Step Solution: Identify the cations: Ca2+ and Mg2+.Match forms: Ca(HCO3)2 → temporary hardness; CaSO4 and MgSO4 → permanent hardness.Thus all listed species cause hardness; select 'All the above'. Verification / Alternative check: Jar tests and standard methods demonstrate that bicarbonate alkalinity contributes to temporary hardness, whereas sulphates persist unless treated. Why Other Options Are Wrong: Single-species options (a–c) are incomplete views. 'None of these' contradicts fundamental water chemistry. Common Pitfalls: Confusing hardness with alkalinity or TDS; ignoring magnesium contributions; designing softening only for calcium and overlooking magnesium load. Final Answer: All the above

Question 18

Gravity conveyance from sources – typical conduit types In water-supply engineering, gravity conduits conveying water from the source to treatment or storage may be built as which of the following types?

Accepted Answer

Introduction / Context: Gravity conveyance systems avoid pumping energy by maintaining hydraulic grade from the source. Depending on terrain and crossings, engineers select open channels, flumes, or aqueduct structures. Given Data / Assumptions: Moderate gradients enabling gravity flow. Structural and hydraulic design adequate to prevent overtopping and erosion. Terminology follows standard water-resources practice. Concept / Approach: Canals are ground-supported open channels; flumes are above-ground troughs (timber, steel, or concrete) supported on frames; aqueducts are elevated conveyances, often bridge-like, to cross depressions or rivers. All serve gravity transport when pressurization is unnecessary or undesirable. Step-by-Step Solution: Relate each term to a structural conveyance method.All listed are recognized gravity conduit types.Therefore 'All the above' is correct. Verification / Alternative check: Historic and modern schemes (Roman aqueducts to modern lined canals and steel flumes) exemplify these variants. Why Other Options Are Wrong: Choosing a single option omits valid alternatives; 'None' is clearly incorrect. Common Pitfalls: Confusing aqueducts with pressurized pipelines; overlooking structural demands (thermal movement, supports) for flumes; ignoring freeboard in canal design. Final Answer: All the above

Question 19

Water treatment sequence – where does sedimentation occur? In a conventional water-treatment train (without direct filtration), sedimentation is carried out at which stage relative to filtration?

Accepted Answer

Introduction / Context: Conventional surface-water treatment typically includes coagulation, flocculation, sedimentation, filtration, and disinfection. Each stage has a defined purpose in removing specific classes of impurities and reducing load on subsequent units. Given Data / Assumptions: Conventional plant layout (not direct filtration or package units). Coagulant aids forming settleable floc before filtration. Gravity settling basins used. Concept / Approach: Sedimentation removes most of the flocculated suspended solids, thereby protecting filters from excessive head loss and breakthrough. This step should therefore precede filtration to ensure filters polish the clarified water, removing fine particulates that escaped settling. Step-by-Step Solution: Sequence: Rapid mix → Flocculation → Sedimentation → Filtration → Disinfection.Thus, sedimentation occurs before filtration. Verification / Alternative check: Process flow diagrams in standard texts confirm that clarifiers are upstream of filters in conventional treatment. Why Other Options Are Wrong: After filtration or simultaneously is characteristic of other designs (e.g., direct filtration or solids-contact units), not the conventional sequence asked here. Chlorination is a disinfection step and not paired-inseparably with sedimentation. Common Pitfalls: Confusing conventional plants with direct filtration systems that omit sedimentation when raw turbidity is low. Final Answer: Before filtration (after coagulation–flocculation)

Question 20

Drilling methods – formation where percussion drilling is unsuitable In which formation is percussion (cable-tool) drilling particularly unsuitable for constructing water wells?

Accepted Answer

Introduction / Context: Choice of drilling method depends on formation behavior. Percussion (cable-tool) drilling pulverizes and bails cuttings by repeatedly dropping a heavy bit. Wall stability and fluid control are crucial for safe and effective operation. Given Data / Assumptions: Standard cable-tool setup without sophisticated drilling fluids. Temporary casing may be used in loose formations. Objective: safe, efficient well construction with minimal caving and sand intrusion. Concept / Approach: Quicksand (a water-saturated, cohesionless mixture) behaves like a fluidized mass under disturbance, causing severe wall instability and continuous inflow of sand. Percussion action exacerbates these problems, making progress unsafe or impractical. While unconsolidated sands and gravels are also challenging, they can sometimes be managed with close-following casing; true quicksand conditions are notoriously unsuitable. Step-by-Step Solution: Identify the formation most prone to flowing and caving under dynamic impact → quicksand.Assess alternatives: rotary mud drilling stabilizes walls by filter cake and hydrostatic head, outperforming percussion in quicksand. Verification / Alternative check: Field guidance notes recommend rotary mud or reverse-circulation methods in very loose, water-bearing sands; percussion is reserved for stiff clays and rockier formations with casing assistance. Why Other Options Are Wrong: Unconsolidated sand and gravel are difficult but may be attempted with proper casing and control. Consolidated rock is actually well suited to percussion. 'None' is incorrect because quicksand stands out as clearly unsuitable. Common Pitfalls: Underestimating the need for immediate casing; ignoring artesian heads; persisting with percussion where rotary methods would be safer. Final Answer: Quicksand

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