Q: Regeneration in Zeolite Softening — After exhaustion, which regenerant solution is used to restore sodium-form zeolite in a domestic/industrial softener?

Introduction: Ion-exchange softeners eventually “exhaust” as their exchange sites become occupied by calcium and magnesium. Regeneration reverses the exchange, restoring the sodium form so the unit can continue removing hardness. Choosing the correct regenerant is essential for practical, economical operation. Given Data / Assumptions: Softening resin/zeolite initially in Na-form (Na-Z). Exhausted resin becomes Ca-Z and Mg-Z during service. Regeneration uses a concentrated brine to displace Ca2+/Mg2+ from the matrix. Concept / Approach: High-strength NaCl brine provides a large excess of Na+ that drives the equilibrium back to Na-Z, releasing Ca2+/Mg2+ into the waste stream. Alternative salts with Ca2+ or Mg2+ cannot regenerate a Na-form softener—indeed, they would further load the bed with hardness ions rather than remove them. Step-by-Step Solution: Write reverse exchange: Ca-Z + 2 Na+ → 2 Na-Z + Ca2+ (similarly for Mg-Z).Provide Na+ in great excess using ~10% NaCl brine.Flush and rinse to remove displaced hardness and excess salt.Return to service with restored Na-form capacity. Verification / Alternative check: Standard operating procedures for domestic and industrial softeners specify NaCl brine regeneration; the chemistry and economics are well established. Why Other Options Are Wrong: A–C would not regenerate a Na-form softener and would contaminate with hardness ions. E is not used for regeneration because it lacks the necessary driving force and can alter alkalinity. Common Pitfalls: Confusing regeneration (restoring capacity) with backwashing (re-stratifying the bed). Both are necessary steps but serve different purposes. Final Answer: 10% sodium chloride (NaCl) solution (brine)

Question 1

Wastewater treatment — Secondary treatment primarily uses which mechanism to consume and remove organic wastes?

Accepted Answer

Introduction: Municipal and many industrial wastewater plants are organized into primary, secondary, and tertiary stages. Secondary treatment targets dissolved and colloidal biodegradable organics using biological processes. This question checks recognition of the core mechanism used in secondary treatment. Given Data / Assumptions: Conventional activated sludge, trickling filters, oxidation ditches, or aerated lagoons. Influent contains biodegradable organics measured as BOD/COD. Goal is to reduce BOD and suspended solids. Concept / Approach: Secondary treatment relies on microbial consortia to oxidize organic matter to CO2 and water, and to assimilate nutrients into biomass. Processes include suspended-growth (activated sludge) and attached-growth (trickling filters). Physical and chemical steps can assist but biology is the primary engine of secondary treatment. Step-by-Step Solution: Identify the stage: “secondary” implies biological conversion of organics.Map to unit processes: aeration basins plus clarifiers (activated sludge) or biofilm reactors (trickling filters).Select the choice that explicitly references micro-organisms. Verification / Alternative check: Performance metrics (BOD removal, sludge yield) depend on microbial kinetics and oxygen transfer, not solely on chemicals or sieving. Why Other Options Are Wrong: (b) Pure chemical oxidation is tertiary or advanced treatment in many designs. (c) Filtration alone is primary/tertiary clarification, not the heart of secondary treatment. (d) and (e) omit the fundamental biological aspect. Common Pitfalls: Confusing primary clarification with secondary biology; assuming disinfection belongs to secondary (it is typically post-secondary). Final Answer: Micro-organisms (biological oxidation and assimilation)

Question 2

Activated sludge recirculation — The activated sludge process consists of returning a portion of the clarifier underflow (settled sludge) to the aeration basin influent. Which option most closely captures this concept?

Accepted Answer

Introduction: The activated sludge process (ASP) is a suspended-growth biological treatment in which mixed liquor from the aeration basin flows to a secondary clarifier. Settled biomass (sludge) is then returned to the aeration basin as return activated sludge (RAS) to maintain sufficient microbial concentration. This question focuses on that hallmark recirculation step. Given Data / Assumptions: Aspiration: maintain target mixed liquor suspended solids (MLSS) in aeration. Process flow: aeration basin → secondary clarifier → sludge return to aeration influent. Wasting (WAS) is a separate, controlled withdrawal. Concept / Approach: RAS is drawn from the clarifier underflow and is returned to the aeration basin inlet (influent stream). Although the provided options use the word “effluent,” the intended idea is the clarifier return to the reactor feed line. Among the choices, the most accurate representation is returning a portion from the clarifier back to the stream entering the reactor (i.e., to the influent of the aeration tank). Step-by-Step Solution: Identify the ASP loop: aeration → clarification → RAS to aeration influent.Select the option that describes sending a portion of the clarifier outflow/return to the reactor inlet.Note: “Effluent” in the option should be understood as the clarifier return line rather than final plant effluent. Verification / Alternative check: Process and instrumentation diagrams (P&IDs) show RAS pumps routing settled sludge from the clarifier back to the front of the aeration basin, confirming the recirculation concept. Why Other Options Are Wrong: (b) and (d) refer to streams exiting units, not the required recycle. (c) merely names the influent stream without indicating recycle from the clarifier. (e) No recycle would collapse MLSS and treatment performance. Common Pitfalls: Confusing final plant effluent with clarifier underflow; overlooking that RAS is biomass-rich, not just water. Final Answer: Effluent (clarifier return) sent to the stream entering the reactor

Question 3

Sanitizers for rubber equipment — For cleaning and sanitizing equipment made of rubber components (e.g., hoses, gaskets), which sanitizer class is generally used?

Accepted Answer

Introduction: Sanitizer selection must consider material compatibility and effectiveness. Rubber components are ubiquitous in food and bioprocess plants (gaskets, hoses, diaphragms). This question asks which sanitizer class is commonly used for rubber while avoiding undue corrosion or degradation. Given Data / Assumptions: Standard elastomers used in processing (e.g., EPDM, Buna-N). Routine clean-in-place (CIP) or manual cleaning conditions. Rinse steps available after caustic washing. Concept / Approach: Alkaline cleaning solutions (e.g., sodium hydroxide) are widely used to saponify and remove fats and proteins while being generally compatible with common elastomers when used at appropriate concentrations and temperatures. Strong mineral acids can swell or degrade some rubbers and are reserved for specific mineral-scale removal with material checks. Therefore, strong alkalis are typically preferred for routine sanitization of rubber equipment surfaces. Step-by-Step Solution: Identify the soil load (organic residues) and material (rubber/elastomer).Select a sanitizer/cleaner with good organic removal and acceptable compatibility: strong alkali.Rinse thoroughly to remove residues and protect material longevity. Verification / Alternative check: Manufacturer compatibility charts for EPDM and NBR show good resistance to caustic at typical CIP strengths, supporting the use of strong alkalis in sanitization routines. Why Other Options Are Wrong: Strong acids can damage certain rubbers or cause swelling; not routinely preferred. Simultaneous strong acid and base application is unsafe and unnecessary. Oxidizers alone can attack polymers and require careful control; not the default. “None of these” is incorrect because caustic sanitation is standard practice. Common Pitfalls: Overconcentrated caustic or high temperature leading to premature gasket aging; failing to follow with adequate rinsing. Final Answer: Strong alkalis (e.g., caustic soda solutions)

Question 4

Wastewater metrics — In environmental engineering and bioprocessing, BOD stands for what?

Accepted Answer

Introduction: BOD is one of the most widely used measures of water and wastewater quality. It quantifies the amount of dissolved oxygen that microorganisms will consume while stabilizing biodegradable organic matter under specified conditions (often 5 days at 20 °C, called BOD5). Given Data / Assumptions: Standard lab test conditions (BOD5). Sample contains biodegradable organics and active microbes or seed. Oxygen uptake reflects biochemical oxidation, not just chemical reactions alone. Concept / Approach: “Biochemical” emphasizes the role of microbial metabolism in oxygen consumption. BOD correlates with the organic load requiring treatment and is a key design parameter for secondary treatment units like activated sludge systems. Step-by-Step Solution: Recognize the acronym components: Bio + chemical + oxygen + demand.Link to test method: measure DO decline over time due to microbial action.Select the correct expansion: Biochemical oxygen demand. Verification / Alternative check: BOD results are often compared with COD (chemical oxygen demand); BOD is typically lower because it excludes non-biodegradable oxidation. Why Other Options Are Wrong: “British” expansions are incorrect and nonstandard. “Biological oxygen depletion” is close in spirit but not the accepted term. Common Pitfalls: Confusing BOD with COD; overlooking nitrification (which can increase BOD unless inhibited or corrected). Final Answer: Biochemical oxygen demand

Question 5

Filter media in water treatment — Which of the following substances are commonly used within a filter bed or cartridge?

Accepted Answer

Introduction: Water and process-liquid filtration often employs granular or fibrous media to remove particulates and, in some cases, dissolved organics. Sand and activated carbon are among the most common media used in municipal and industrial systems. Given Data / Assumptions: Gravity or pressure filters in treatment trains. Objectives: turbidity removal (sand) and adsorption of organics/odors (carbon). No specialized membrane processes implied. Concept / Approach: Sand filters (single or dual media) remove suspended solids via straining and depth filtration. Activated carbon (charcoal) filters adsorb organic molecules and chlorine/chloramine, improving taste and odor and protecting downstream processes. Aluminum chloride is a coagulant added upstream to form flocs, not a bed medium itself. Step-by-Step Solution: Identify bed media: sand and carbon are standard.Distinguish coagulants (aluminum salts) from media.Select “Both (a) and (b)”. Verification / Alternative check: Treatment schematics show coagulation–flocculation (aluminum salts) preceding sand/carbon filtration stages, confirming roles. Why Other Options Are Wrong: (a) or (b) alone ignores the other widely used medium. (d) Misclassifies a chemical additive as a filter medium. (e) Polyethylene pellets are not standard universal media for potable-water filters. Common Pitfalls: Assuming carbon alone clarifies suspended solids efficiently; in practice, sand handles particulates while carbon adsorbs dissolved compounds. Final Answer: Both (a) and (b)

Question 6

Material-specific sanitation — A sanitizer used specifically for vitreous enamel (glass-lined) surfaces is typically which of the following?

Accepted Answer

Introduction: Glass-lined (vitreous enamel) reactors and tanks are used for their excellent corrosion resistance and cleanability. However, aggressive chemicals can craze or attack the enamel over time. Selecting compatible cleaners/sanitizers is therefore essential to preserve the lining. Given Data / Assumptions: Surface is vitreous enamel/glass-lined steel. Routine sanitation rather than heavy-scale removal. Desire to minimize enamel damage while achieving cleanliness. Concept / Approach: Mild alkaline cleaners, especially those formulated with sodium silicate, help buffer the solution and deposit a protective silicate film that is compatible with glass surfaces. Strong acids or highly caustic solutions can exacerbate microcracks or leach components from the enamel, so gentler alkalinity with silicate is preferred for regular sanitation. Step-by-Step Solution: Identify the sensitive substrate (vitreous enamel).Select a compatible sanitizer/cleaner: weak alkali plus sodium silicate.Avoid harsh extremes that may degrade the lining over time. Verification / Alternative check: Manufacturer care guidelines for glass-lined equipment recommend mild alkaline cleaners with silicates and caution against strong acids/bases, especially at high temperatures. Why Other Options Are Wrong: Strong alkalis or strong acids can damage enamel with repeated exposure. “None of these” is incorrect because an accepted option exists. Unstabilized hypochlorite at low pH can release chlorine gas and is unsafe for routine sanitation. Common Pitfalls: Using descalers too frequently; neglecting thorough rinsing that leaves residues affecting product quality. Final Answer: Weak alkali with sodium silicate (to protect the enamel)

Question 7

Terminology — The activated sludge process (ASP) is sometimes referred to as which type of biological oxidation system?

Accepted Answer

Introduction: Biological wastewater processes can be broadly categorized as suspended-growth (cells freely suspended in mixed liquor) or attached-growth (biofilms on media). The activated sludge process is the archetypal suspended-growth system, sustained in a well-mixed, aerated basin. Given Data / Assumptions: Aeration provides mixing and oxygen to keep flocs suspended. Secondary clarifier returns biomass to maintain MLSS. Terminology emphasizes hydrodynamic state of the biomass. Concept / Approach: Because the microbial solids are suspended and “fluidized” by aeration-induced mixing, ASP is sometimes described (informally) as a fluid-bed biological oxidation system. In contrast, fixed-bed systems (e.g., trickling filters, rotating biological contactors) rely on biofilms attached to media and differ in hydrodynamics and design equations. Step-by-Step Solution: Classify ASP as suspended growth (cells in the fluid phase).Relate suspended state to a “fluid-bed” notion sustained by aeration.Select the option that reflects suspended/fluidized behavior rather than fixed media. Verification / Alternative check: Design texts contrast ASP (completely mixed, suspended growth) with fixed-film processes; the descriptive phrase “fluid bed” reflects the maintained suspension of flocs by air-induced mixing. Why Other Options Are Wrong: (b) Fixed bed refers to attached-growth biofilms, not ASP. (c) “Turning bed” is not a standard term for ASP. (d) Incorrect because a descriptive choice exists. (e) Trickling filters are attached-growth, not ASP. Common Pitfalls: Assuming “fluidized bed” strictly means granular media fluidization; here the phrase is descriptive of suspended growth, not a literal sand-fluidized reactor. Final Answer: Fluid bed biological oxidation system (suspended growth kept in suspension by aeration)

Question 8

Sanitizers in Food and Water Microbiology — Among the following, which compound is classified as a surface-active agent (surfactant/detergent) used as a sanitizer for equipment and hard surfaces?

Accepted Answer

Introduction: In sanitation programs for food processing, dairy, and drinking-water facilities, chemicals are chosen based on how they remove soils and inactivate microbes. Surface-active agents (surfactants) reduce surface tension, wetting equipment surfaces and helping lift organic films so that subsequent disinfection is more effective. This question checks recognition of which listed chemical is actually a surfactant used as a sanitizer/cleaner. Given Data / Assumptions: We are comparing common names used in industry and older texts. Only one option is a true surface-active agent (detergent). Other salts listed act mainly as builders or sequestrants, not as primary surfactants. Concept / Approach: Surfactants possess hydrophilic and hydrophobic moieties that allow them to emulsify fats and disrupt microbial membranes (to a limited degree), assisting sanitation. Trade names like Teepol historically refer to synthetic detergents (for example, alkylbenzene sulfonates), widely used as wetting/cleaning agents prior to or along with disinfectants such as chlorine, iodophors, or quaternary ammonium compounds. Step-by-Step Solution: Identify the surfactant among the choices: Teepol is a synthetic detergent brand; hence a surface-active agent.Recognize tetra phosphate and metaphosphate: these are phosphate salts used as builders/water softeners that enhance detergency but are not themselves primary surfactants.Exclude non-surfactant materials (for example, mineral additives) that lack amphiphilic structure.Conclude that Teepol is the correct surface-active sanitizer/cleaner. Verification / Alternative check: Sanitation protocols commonly use a detergent (surfactant) wash step followed by rinsing and then a disinfectant. Product data sheets for Teepol-type detergents emphasize wetting, emulsification, and cleaning of stainless steel and polymeric surfaces in dairies and breweries. Why Other Options Are Wrong: A: Tetra phosphate is a builder that sequesters hardness but is not a surfactant. C: Metaphosphate acts similarly as a sequestrant/water softener. D: Incorrect because a valid surfactant (Teepol) is listed. E: Quenched lime is an alkali, not a surfactant. Common Pitfalls: Confusing “builder” salts with “detergents.” Builders improve surfactant action but do not replace the amphiphilic molecules that provide wetting and emulsification. Final Answer: Teepol (synthetic detergent/surfactant)

Question 9

Hydrologic Cycle Basics — What is the ultimate natural source of freshwater that feeds rivers, lakes, and groundwater?

Accepted Answer

Introduction: Understanding the hydrologic cycle is fundamental to water resources and environmental engineering. While many water bodies store or convey freshwater, the ultimate natural input that replenishes them is atmospheric precipitation. This question tests whether you can distinguish ultimate sources from derived or secondary reservoirs. Given Data / Assumptions: “Ultimate source” means the primary input that drives the freshwater system. Surface waters (rivers, lakes) and groundwater are replenished by precipitation and snowmelt. Evaporation and transpiration return water vapor to the atmosphere, closing the cycle. Concept / Approach: Precipitation (rain and snow) results from condensation of atmospheric moisture and provides net inputs on land. Rivers, lakes, and aquifers are pathways and storages; they do not create water but receive it. Dew and forests influence microclimates and interception but are not ultimate generators of freshwater. Hence, precipitation is the correct answer. Step-by-Step Solution: Define “ultimate source” as the original input to terrestrial hydrology.Identify precipitation (rain and snow) as that input.Classify other options as storages or modifiers, not sources.Select rain and snow as the correct response. Verification / Alternative check: Water budgets consistently treat precipitation as the inflow term; surface and subsurface flows are redistribution of that input across the landscape. Why Other Options Are Wrong: A and D: Rivers/lakes and “surface/underground” are sinks/reservoirs fed by precipitation. B: Dew and forests affect micro-scale processes but do not create net freshwater. E: Ocean currents move saline water, not the freshwater source for terrestrial systems. Common Pitfalls: Equating “where we get water from” with “ultimate natural source.” Your tap may draw from a river, but that river was replenished by precipitation. Final Answer: Rain and snow (precipitation)

Question 10

Zeolite (Ion-Exchange) Softening — The process removes both temporary and permanent hardness. In this method, the calcium and magnesium in water are effectively removed as:

Accepted Answer

Introduction: Water hardness arises from dissolved calcium and magnesium salts. The zeolite (also called sodium-zeolite) softening process removes hardness via ion exchange, not by bulk precipitation. Recognizing the correct removal mechanism helps distinguish it from lime–soda softening (a precipitation process). Given Data / Assumptions: Sodium-form zeolite (Na-Z) exchanges Na+ for Ca2+/Mg2+ in hard water. Process targets both temporary (bicarbonate) and permanent (sulfate/chloride) hardness. Regeneration is later accomplished with brine (NaCl) to restore Na-Z. Concept / Approach: In ion exchange, calcium and magnesium ions are captured on the solid zeolite matrix as Ca-Z and Mg-Z while sodium ions are released to the water. No insoluble carbonate, sulfate, or chloride precipitates in the bulk solution during this step; instead, the ions are immobilized on the resin/zeolite. Hence, they are “removed as insoluble zeolite forms.” Step-by-Step Solution: Write exchange reaction: 2 Na-Z + Ca2+ → Ca-Z + 2 Na+ (analogous for Mg2+).Note that hardness ions leave the solution phase and bind to the solid.Conclude removal occurs by solid-phase capture (insoluble zeolite form), not precipitation as carbonates/sulfates/chlorides.Select the option describing “insoluble zeolites.” Verification / Alternative check: Operationally, the bed exhausts as exchange sites become Ca/Mg-loaded; regeneration with NaCl brine reverses the reaction, restoring Na-Z and releasing Ca/Mg in the brine waste. Why Other Options Are Wrong: A, C, D: These describe precipitation mechanisms typical of lime–soda treatment, not zeolite exchange. E: No gaseous product removes hardness in this process. Common Pitfalls: Assuming all softening involves precipitation. Ion exchange is distinct and often used as a polishing step after clarification/filtration. Final Answer: Insoluble zeolites (Ca/Mg form) retained on the ion-exchange matrix

Question 11

Drinking-Water Quality (BIS) — What is the maximum permissible turbidity limit for potable water (in the absence of an alternate source), expressed in NTU?

Accepted Answer

Introduction: Turbidity reflects light-scattering by suspended particles and colloids. It is a key aesthetic and operational parameter in drinking-water treatment because high turbidity can protect microorganisms from disinfection and degrade consumer acceptability. Standards specify both desirable and permissible limits to guide utilities. Given Data / Assumptions: BIS (IS 10500) specifies a desirable turbidity of about 1 NTU and a higher permissible value when no alternative source is available. Units are Nephelometric Turbidity Units (NTU). Question asks for the maximum permissible limit in typical practice. Concept / Approach: Where the desirable target (1 NTU) cannot be met, the standard allows a maximum permissible limit of 5 NTU. Water meeting this limit is generally clear to consumers and amenable to effective disinfection, though utilities are encouraged to achieve the lower desirable level whenever feasible. Step-by-Step Solution: Identify the parameter: turbidity in NTU.Recall BIS hierarchy: desirable < permissible (in absence of alternate source).Match the maximum permissible value: 5 NTU.Select 5 NTU as the correct option. Verification / Alternative check: Utility monitoring and regulatory documents commonly reference 1 NTU (desirable) and 5 NTU (permissible) for treated water entering the distribution system. Why Other Options Are Wrong: B–D exceed the permissible level and would be considered substandard. E reflects the desirable target, not the maximum permissible limit referenced in the question. Common Pitfalls: Confusing raw-water turbidity (often much higher) with treated-water limits; always apply standards to the correct sampling point. Final Answer: 5 NTU

Question 12

Regeneration in Zeolite Softening — After exhaustion, which regenerant solution is used to restore sodium-form zeolite in a domestic/industrial softener?

Accepted Answer

Introduction: Ion-exchange softeners eventually “exhaust” as their exchange sites become occupied by calcium and magnesium. Regeneration reverses the exchange, restoring the sodium form so the unit can continue removing hardness. Choosing the correct regenerant is essential for practical, economical operation. Given Data / Assumptions: Softening resin/zeolite initially in Na-form (Na-Z). Exhausted resin becomes Ca-Z and Mg-Z during service. Regeneration uses a concentrated brine to displace Ca2+/Mg2+ from the matrix. Concept / Approach: High-strength NaCl brine provides a large excess of Na+ that drives the equilibrium back to Na-Z, releasing Ca2+/Mg2+ into the waste stream. Alternative salts with Ca2+ or Mg2+ cannot regenerate a Na-form softener—indeed, they would further load the bed with hardness ions rather than remove them. Step-by-Step Solution: Write reverse exchange: Ca-Z + 2 Na+ → 2 Na-Z + Ca2+ (similarly for Mg-Z).Provide Na+ in great excess using ~10% NaCl brine.Flush and rinse to remove displaced hardness and excess salt.Return to service with restored Na-form capacity. Verification / Alternative check: Standard operating procedures for domestic and industrial softeners specify NaCl brine regeneration; the chemistry and economics are well established. Why Other Options Are Wrong: A–C would not regenerate a Na-form softener and would contaminate with hardness ions. E is not used for regeneration because it lacks the necessary driving force and can alter alkalinity. Common Pitfalls: Confusing regeneration (restoring capacity) with backwashing (re-stratifying the bed). Both are necessary steps but serve different purposes. Final Answer: 10% sodium chloride (NaCl) solution (brine)

Question 13

Drinking-Water Metals (BIS) — What is the maximum desirable limit for lead (Pb) in potable water as per typical BIS specifications?

Accepted Answer

Introduction: Lead is a toxic heavy metal with serious health impacts. Drinking-water standards set stringent limits to protect public health. This question asks for the maximum desirable limit commonly cited in legacy BIS-aligned exam material, recognizing that jurisdictions periodically update values based on risk assessments. Given Data / Assumptions: We refer to widely used values historically taught in many Indian water-quality courses. Units are mg/L (milligrams per liter). “Maximum desirable limit” denotes the target value for treated water. Concept / Approach: Legacy BIS references frequently cite 0.05 mg/L as the limit for lead in potable water in exam contexts. Many modern guidelines (for example, WHO) have moved toward tighter limits; however, within the scope of this question bank and options provided, 0.05 mg/L is the recognized answer. Always consult the latest standard for design and compliance, as regulatory values may change with new evidence. Step-by-Step Solution: Identify parameter and units: lead in mg/L.Select the historically referenced BIS value among the given choices.Confirm that other options are either too lax (unsafe) or atypical.Conclude with 0.05 mg/L for this item set. Verification / Alternative check: Past syllabi and solved papers often use 0.05 mg/L; engineering design should verify current BIS/IS 10500 editions for the most up-to-date requirement. Why Other Options Are Wrong: B–D are higher than typical health-protective limits. E is also too high and not aligned with potable standards. Common Pitfalls: Memorizing a single number without checking the latest code revision. Standards evolve; designers must confirm current values before specification. Final Answer: 0.05 mg/L

Question 14

Temporary Hardness on Boiling — When temporarily hard water (bicarbonate hardness) is boiled, which of the following species is formed/released as part of the decomposition?

Accepted Answer

Introduction: Temporary hardness arises from bicarbonates of calcium and magnesium. Heating or boiling decomposes bicarbonates, removing hardness by precipitating carbonates and releasing carbon dioxide gas. Recognizing the reaction products is important for distinguishing temporary from permanent hardness behaviors. Given Data / Assumptions: Feed water contains Ca(HCO3)2 and/or Mg(HCO3)2. Boiling drives off CO2 and shifts equilibria. No chemical additives are considered (thermal softening only). Concept / Approach: On boiling, bicarbonate decomposes: Ca(HCO3)2 → CaCO3(s) + CO2(g) + H2O. Thus, the process both forms a solid precipitate (calcium carbonate) and releases carbon dioxide gas. The key identifiable gaseous product is CO2, which can be observed as effervescence during boiling. Step-by-Step Solution: Start with temporary hardness species: metal bicarbonates.Apply thermal decomposition: bicarbonate → carbonate + CO2 + H2O.Identify the gas formed: CO2.Select carbon dioxide as the correct option. Verification / Alternative check: Kettle scaling (CaCO3) and bubbling CO2 are common household evidence of this reaction. Analytical tests confirm lowered alkalinity after boiling. Why Other Options Are Wrong: A: Calcium bicarbonate is the reactant, not the product. B: Calcium sulfate relates to permanent hardness; boiling doesn’t remove it. C: Hydrogen chloride is not involved. E: Sodium nitrate is unrelated. Common Pitfalls: Assuming boiling removes all hardness; permanent hardness (sulfates/chlorides) remains and needs chemical treatment or ion exchange. Final Answer: Carbon dioxide (gas released)

Question 15

Wastewater Strength — What does the Chemical Oxygen Demand (COD) parameter measure for a water/wastewater sample?

Accepted Answer

Introduction: Chemical Oxygen Demand (COD) is a widely used aggregate measure of wastewater strength. It estimates the oxygen equivalent of organics (and some oxidizable inorganics) by chemical oxidation, typically using a strong oxidant under acidic conditions. This metric helps design treatment processes and monitor compliance. Given Data / Assumptions: Standard dichromate COD test under reflux, acidic conditions, with catalysts. COD reported in mg O2/L. Sample may contain biodegradable and non-biodegradable organics. Concept / Approach: COD represents the oxygen that would be consumed if the oxidizable materials in the sample were fully oxidized. Unlike BOD (Biochemical Oxygen Demand), which measures oxygen uptake by microorganisms over time, COD is a faster chemical test and often yields higher values because it oxidizes a broader spectrum of compounds. Step-by-Step Solution: Identify the test principle: chemical oxidation of organics using a strong oxidant (e.g., dichromate).Relate measured oxidant consumption to oxygen equivalent.Understand that COD approximates the total oxidizable load, informing process sizing and control.Select the option that best captures this definition. Verification / Alternative check: Comparisons of COD and BOD in plant data show COD ≥ BOD in most cases; the ratio guides biodegradability assessments and process selection. Why Other Options Are Wrong: A confuses COD with BOD related to microbial growth. C focuses on calcium oxidation, which is not the essence of COD. D and E do not describe COD at all. Common Pitfalls: Equating COD directly with BOD; always interpret together for biodegradability insights. Ensure chloride interference is addressed during testing. Final Answer: The amount of oxygen that would be consumed to chemically oxidize pollutants (primarily organic matter) present in the sample

Question 16

Impacts of Water Hardness — Which statement correctly identifies what hardness does NOT do?

Accepted Answer

Introduction: Hardness in water is due to calcium and magnesium ions. While it creates several operational and aesthetic problems, moderate hardness is generally not a direct health hazard. This question asks you to identify what hardness does not do among the listed effects. Given Data / Assumptions: Hardness levels typical of raw ground- and surface-waters. Boiler operation is sensitive to scale formation. Soaps form insoluble salts with Ca2+/Mg2+, reducing lathering efficiency. Concept / Approach: In boilers, hardness precipitates as scale (for example, CaCO3, CaSO4), which lowers heat-transfer efficiency and can cause damage—so it certainly has adverse effects. In cooking, hardness prolongs softening of foods like legumes. In laundering, soap scum forms and detergent demand increases. However, moderate hardness by itself does not make water unfit for drinking; many guidelines set acceptable ranges for hardness for taste and scaling control, not for toxicity. Step-by-Step Solution: Evaluate each effect based on known impacts.Confirm that boilers, cooking, and washing are adversely affected.Recognize that potable acceptance is not invalidated solely by hardness; it is mainly an aesthetic/operational issue.Select the statement that claims unfitness for drinking as the one that is not correct. Verification / Alternative check: Standards provide guideline values for hardness primarily for scaling/taste; there is no acute toxicity at typical environmental concentrations of Ca2+ and Mg2+ in drinking-water. Why Other Options Are Wrong: A: False—hardness clearly causes boiler scale. B and D are well-documented operational issues. E is a corollary of D in detergent systems. Common Pitfalls: Assuming “hard water is unsafe.” Safety concerns relate more to contaminants such as heavy metals, pathogens, or nitrates, not calcium/magnesium hardness. Final Answer: Hardness makes water unfit for drinking on health grounds alone.

Question 17

Softening Permanent Hardness — Which chemical treatment can soften permanent hard water by sequestration/complexation?

Accepted Answer

Introduction: Permanent hardness stems from non-carbonate salts (for example, CaSO4, MgCl2) that do not decompose on boiling. Such waters can be softened by precipitation (lime–soda), ion exchange, or sequestration using polyphosphates. This question focuses on a classical sequestration method used historically in domestic and small-scale applications. Given Data / Assumptions: Target is calcium and magnesium ions contributing to permanent hardness. We are considering chemical addition routes rather than thermal softening. Answer choices include common salts used in water conditioning. Concept / Approach: Sodium hexametaphosphate (often marketed as “Calgon”) complexes Ca2+ and Mg2+, converting them into soluble, non-precipitating forms that do not form scale or soap scum. This is sequestration, not removal; the hardness ions remain in solution but are “tied up.” For true removal, precipitation or ion exchange would be required. Step-by-Step Solution: Identify the agent known for complexing hardness ions: sodium hexametaphosphate.Distinguish sequestration (complexation) from precipitation and exchange.Recognize that other listed salts do not reliably sequester hardness across conditions.Select sodium hexametaphosphate as the correct answer. Verification / Alternative check: Detergent formulations historically included polyphosphates to control hardness. Modern environmental constraints have reduced their use due to eutrophication concerns, but the chemistry remains valid. Why Other Options Are Wrong: A: Sodium silicate may aid corrosion control but is not a standard sequestering softener. B: Bicarbonate alters alkalinity but does not neutralize permanent hardness. D: Sodium phosphate variants can precipitate under some conditions but are not the classic sequestrant. E: Ferric chloride is a coagulant, not a softener. Common Pitfalls: Confusing “softening” by removal with “conditioning” by sequestration; Calgon softening is conditioning that prevents scale formation without reducing total dissolved hardness. Final Answer: Sodium hexametaphosphate (Calgon)

Question 18

In municipal and industrial wastewater treatment, sedimentation (gravity settling) is a physical unit operation used primarily to remove which type of suspended particles from water?

Accepted Answer

Introduction / Context: Sedimentation, also called gravity settling, is one of the most common primary treatment steps in water and wastewater engineering. It exploits density differences between suspended solids and the surrounding fluid to separate solids without chemical reaction or biological conversion. Understanding what types of particles sedimentation can remove clarifies where it fits in a treatment train and why flotation and filtration are used for other particle classes. Given Data / Assumptions: Sedimentation tanks (clarifiers) operate under quiescent to mildly turbulent flow. Particles are discrete or flocculent, with densities relative to water. No chemical coagulation or air addition is assumed for the basic mechanism. Concept / Approach: Gravity acts on particles more dense than water, giving them a positive settling velocity according to Stokes’ law for small Reynolds numbers: v_s ∝ (ρ_p − ρ_w) * d_p^2. Such particles can be removed by settling in clarifiers if the hydraulic surface overflow rate is low enough. Particles less dense than water have negative apparent settling velocity (they rise) and are typically removed by flotation rather than sedimentation. Step-by-Step Solution: Identify the mechanism: sedimentation separates on the basis of density difference and gravity.Relate settling velocity to particle–fluid density contrast: more dense than water ⇒ positive settling velocity.Link to design: if v_s ≥ surface overflow rate, the particle is captured.Conclude the class of particles removed: those more dense than water.Recognize alternative processes: flotation for lighter-than-water particles; filtration for very fine colloids. Verification / Alternative check: Plant experience shows grit, sand, and heavy organic flocs are readily removed in primary clarifiers, whereas fats, oils, and greases float and are skimmed, not settled. Why Other Options Are Wrong: Particles less dense than water: they rise, calling for flotation/skimming. “Pertinacious material”: vague and not a standard engineering category. None of the above: incorrect because gravity settling targets denser particles. Common Pitfalls: Expecting sedimentation to remove true colloids; these usually require coagulation–flocculation prior to settling. Also, excessive hydraulic loading reduces capture even for dense particles. Final Answer: remove particles that are more dense than water

Question 19

In the conventional wastewater-treatment train, biological oxidation processes (commonly called “biological treatment”) are most accurately classified as which stage of treatment?

Accepted Answer

Introduction / Context: Wastewater treatment is typically organized into primary, secondary, and tertiary (advanced) stages. Knowing where biological oxidation processes fit is essential for understanding plant layouts (e.g., activated sludge, trickling filters, oxidation ditches) and for linking performance targets with regulatory effluent limits (BOD, TSS, nutrients). Given Data / Assumptions: Primary treatment emphasizes physical removal (screens, grit, primary sedimentation). Biological processes oxidize dissolved and colloidal organics (BOD removal). Tertiary/advanced treatment targets nutrient polishing and microcontaminants. Concept / Approach: Secondary treatment is defined by biological oxidation and clarification. Microorganisms convert soluble and colloidal organics to biomass, carbon dioxide, and water, after which secondary clarifiers remove biomass solids. This stage achieves major BOD and TSS reductions compared with primary treatment alone. Tertiary processes (e.g., filtration, nutrient removal, advanced oxidation, membranes) follow as needed for stringent limits. Step-by-Step Solution: List treatment stages and their dominant mechanisms.Match “biological oxidation” to the stage focused on BOD removal via microbes.Confirm typical unit operations: aeration basins plus secondary clarifiers.Exclude primary (physical) and tertiary (polishing) stages.Select “secondary treatment” as the correct classification. Verification / Alternative check: Design manuals specify that meeting secondary treatment standards (e.g., 85% BOD and TSS removal) relies on biological oxidation systems like activated sludge or trickling filters. Why Other Options Are Wrong: Primary: Mostly gravity-based solids removal; minimal BOD oxidation. Tertiary: Polishing and nutrient removal beyond basic biological oxidation. All of these: The processes are not simultaneously primary and tertiary. Common Pitfalls: Confusing disinfection with tertiary treatment; disinfection is often a post-secondary step but not itself “biological oxidation.” Final Answer: secondary treatment

Question 20

During coagulation–flocculation for drinking water or wastewater clarification, which chemical is commonly added as a coagulant to destabilize colloids and promote floc formation?

Accepted Answer

Introduction / Context: Coagulation–flocculation is a crucial step for removing turbidity and natural organic matter. A coagulant neutralizes particle charges so that short-range attractive forces allow aggregation into settleable or filterable flocs. Identifying the typical coagulant helps operators and students understand jar testing and dosage optimization. Given Data / Assumptions: Standard surface water treatment train with rapid mixing and slow flocculation. Goal is to reduce turbidity and remove colloids. Common coagulants include aluminum and iron salts. Concept / Approach: Aluminum sulphate (alum) is one of the world’s most widely used coagulants. In water, alum hydrolyzes to form aluminum hydroxide precipitates that enmesh particles and neutralize negative charges. Alternatives include ferric chloride or polyaluminum chloride; however, aluminum sulphate remains a canonical example, especially in introductory questions. Step-by-Step Solution: Identify the function: charge neutralization and sweep floc formation.Select a chemical that hydrolyzes to hydroxide flocs (Al(OH)3).Aluminum sulphate fits this role and is ubiquitous in treatment practice.Reject inert oxides (e.g., Al2O3) that are not added as soluble coagulants.Confirm that CaCl2 is mainly for hardness or chloride addition, not primary coagulation. Verification / Alternative check: Jar tests demonstrate turbidity removal with alum dose optimization; sludge contains amorphous Al(OH)3 with captured impurities. Why Other Options Are Wrong: Aluminum oxide: Insoluble; not used as a coagulant. Calcium chloride: Adjusts ionic strength/hardness but not a standard coagulant. None of these: Incorrect because alum is the classic coagulant. Common Pitfalls: Confusing alum with polymeric coagulant aids; both can be used, but alum is the primary coagulant in many plants. Final Answer: Aluminum sulphate

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