Q: Acoustics and noise control: An automobile horn measured at approximately 1.5 m (typical roadside proximity) produces a sound pressure level closest to which value in decibels (dB)?

Introduction / Context: Decibel scales are logarithmic and crucial for evaluating community noise, occupational exposure, and environmental impact. An automobile horn is a familiar source used to benchmark perceived loudness. At short distances along the near field-to-far field transition (around 1.5 m), its sound pressure level provides a practical reference for noise assessment. Given Data / Assumptions: Source: typical passenger vehicle horn, not specialized emergency sirens. Measurement distance: ~1.5 m from the horn outlet. Quiet background assumed so the reading largely reflects the horn. Concept / Approach: Common horns produce levels on the order of 85–100 dB at a few meters. Occupational guidelines associate 85–90 dB with loud traffic and 100+ dB with jackhammers or very close proximity to sirens. Values like 150 dB or 180 dB are extreme and associated with explosives or aircraft at very short distances, far beyond typical vehicle horns. Therefore, ~90 dB at 1.5 m is a realistic and widely cited ballpark figure. Step-by-Step Solution: Consider practical SPL ranges: horns are loud but non-damaging at brief exposure outdoors.Eliminate unrealistic extremes (150–180 dB) for standard automobile horns.Select the plausible value near common references: 90 dB. Verification / Alternative check: Municipal noise codes and automotive specifications often cite horn sound levels near 100 dB at standardized test distances; adjusting for 1.5 m leaves a reading near the lower end of that range, about 90 dB, depending on exact horn construction and directionality. Why Other Options Are Wrong: 120 dB: More typical of sirens or very close proximity to very loud sources.150 dB / 180 dB: Associated with shock waves, explosions, or rocket launches, not street horns. Common Pitfalls: Confusing horn levels with emergency sirens. Ignoring distance attenuation; small changes in distance can shift dB significantly. Final Answer: 90

Q: Hardness in water: Temporary hardness (which can be removed by boiling) is primarily due to the presence of which chemical species in solution?

Introduction / Context: Water hardness affects scale formation, soap usage, and boiler performance. It is categorized as temporary (carbonate) hardness and permanent (non-carbonate) hardness. Engineers must identify which ions contribute to each type to choose an appropriate treatment method such as lime softening or ion exchange. Given Data / Assumptions: Temporary hardness is removable by boiling or lime treatment. Hardness-forming cations are calcium and magnesium. Anions determine whether the hardness is temporary or permanent. Concept / Approach: Temporary hardness arises from bicarbonates (HCO3-) of calcium and magnesium. Heating converts bicarbonates to carbonate precipitates, releasing carbon dioxide and depositing CaCO3/Mg(OH)2, thus removing hardness—hence “temporary.” In contrast, sulphates and chlorides of calcium/magnesium cause permanent hardness that does not boil out. Therefore, the defining species is the carbonate alkalinity (bicarbonate/carbonate system), not merely the presence of calcium or sulphate/chloride ions alone. Step-by-Step Solution: Identify the temporary hardness mechanism: decomposition of bicarbonate on boiling.Relate to ions: carbonate alkalinity paired with Ca/Mg causes temporary hardness.Select “carbonate” as the correct defining species. Verification / Alternative check: Standard tests express temporary hardness approximately as alkalinity to phenolphthalein and methyl orange endpoints; pilot boiling confirms scale precipitation tied to carbonate chemistry. Why Other Options Are Wrong: Calcium: A cation present in both temporary and permanent hardness; the anion determines type.Chloride / sulphate: Produce permanent hardness with Ca/Mg and do not boil out. Common Pitfalls: Equating “calcium” generically with temporary hardness. Overlooking that bicarbonate alkalinity is the key differentiator. Final Answer: carbonate

Q: Water quality impacts: The presence of soluble organic matter in polluted water primarily leads to which adverse effect in receiving streams and treatment systems?

Introduction / Context: Biodegradable organic substances in wastewater impose a biochemical oxygen demand (BOD) on natural waters. When effluents are discharged without adequate treatment, microorganisms consume dissolved oxygen while degrading organics, thereby threatening aquatic life and altering ecosystem balance. Given Data / Assumptions: Organics are soluble and biodegradable to some extent. Receiving water has finite dissolved oxygen capacity. No reference is made to toxic or explosive organics; the emphasis is typical municipal/industrial biodegradable load. Concept / Approach: Microbial oxidation reduces dissolved oxygen according to the stoichiometry of carbon conversion to CO2 and water. If oxygen consumption exceeds reaeration, the stream experiences a sag in DO concentration, harming fish and aerobic organisms. While nutrients can stimulate plant growth (eutrophication), that is linked to nitrogen and phosphorus; soluble organics primarily drive oxygen depletion. Fire or explosion hazards are not the chief concern in dilute aquatic environments. Step-by-Step Solution: Associate soluble organics with BOD.Relate BOD to oxygen uptake by microbes.Conclude that oxygen depletion is the central immediate effect. Verification / Alternative check: Classic Streeter–Phelps DO sag models quantify the balance between deoxygenation and reaeration, showing how organic loads depress stream DO to critical levels downstream of outfalls. Why Other Options Are Wrong: Undesirable plant growth: Primarily tied to nutrient enrichment, not just carbonaceous organics.Fire/explosion hazards: Not typical at environmental concentrations in open waters. Common Pitfalls: Conflating eutrophication (nutrients) with BOD effects (organics). Overlooking temperature and flow effects on reaeration capacity. Final Answer: depletion of oxygen.

Q: Noise control for air-cooled equipment: Cooling-fan noise arises largely from blade-passage turbulence. Which design change(s) reduce fan noise by allowing lower tip speed for the same airflow?

Introduction / Context: Fans in air-cooled heat exchangers and cooling towers can produce significant broadband noise from turbulent flow and tonal noise at the blade-pass frequency. Acoustic mitigation often targets reduced tip speed and improved aerodynamic loading to lower sound generation at the source. Given Data / Assumptions: Airflow requirement is fixed by process duty. Primary noise mechanism: turbulence and blade-pass tones. Design variables: fan diameter and blade count. Concept / Approach: For a given volumetric flow, increasing fan diameter allows the same flow at lower rotational speed, cutting tip speed and reducing noise (sound power often scales strongly with tip speed). Adding blades increases solidity, enabling similar flow at reduced speed or lower loading per blade, which also lowers noise. Together, these changes shift operation to a quieter regime without sacrificing cooling capacity. Step-by-Step Solution: Hold airflow constant; seek to reduce tip speed V_tip = π * D * N_rot.Increase diameter D → lower N_rot for same flow; V_tip decreases.Increase blade count → maintain flow at lower N_rot and reduced per-blade loading; turbulence and tones decline. Verification / Alternative check: Manufacturers’ acoustic curves show sound power reductions with larger, slower fans and optimized blade counts/pitch. Field retrofits often employ variable-speed drives plus larger-diameter impellers to meet noise limits. Why Other Options Are Wrong: Only (a) or only (b): Each helps, but the combined choice acknowledges both effective strategies.Neither: Contradicts well-established fan acoustics principles. Common Pitfalls: Reducing speed without compensating diameter or blade count can undersupply airflow. Ignoring structural and motor constraints when increasing diameter or blade number. Final Answer: both (a) & (b).

Q: Industrial wastewater sources: Effluents discharged from electroplating shops, blast furnaces, and coal mining operations are primarily characterized by which type of contaminants?

Introduction / Context: Correctly identifying dominant contaminant classes in industrial wastewater guides selection of treatment processes. Some sources discharge mainly organics (e.g., food processing), while others release metals, acids, alkalis, and suspended minerals typical of inorganic pollution. Given Data / Assumptions: Electroplating effluents often contain heavy metals (Cr, Ni, Cu, Zn) and cyanides/acids. Blast furnace water can carry suspended inorganics, scale, oil traces, and dissolved inorganic species from gas cleaning. Coal mining drainage frequently includes suspended solids and acid mine drainage with sulphates, iron, and other inorganic ions. Concept / Approach: These sectors are dominated by inorganic contaminants: metals, mineral acids/alkalis, and mineral particulates. While oils or organics may be present in trace amounts (e.g., lubricants), the defining treatment challenge is precipitation/neutralization for metals and pH, plus solids removal. Radioactivity is not characteristic for these sources under normal conditions. Step-by-Step Solution: Match each industry to pollutant profile: electroplating → metals; blast furnace → inorganics/solids; mining → acid mine drainage.Identify the common class: predominantly inorganic.Select “inorganic.” Verification / Alternative check: Treatment trains typically include pH adjustment, chemical precipitation, coagulation–flocculation, and sedimentation/filtration, all aimed at inorganic removal, confirming the dominant class. Why Other Options Are Wrong: Organic: Not the dominant load; biodegradation is secondary here.Radioactive: Not typical of these industries.None of these: Incorrect because “inorganic” is clearly applicable. Common Pitfalls: Overgeneralizing from industries with high organic load to metallurgical sectors. Neglecting the need for stringent metals compliance before discharge. Final Answer: inorganic

Q: Environmental acoustics:\nApproximately what sound pressure level (in decibels, dB) corresponds to a normal conversation between people at about 1 meter distance in a quiet room?

Introduction / Context: Understanding typical noise levels helps in planning occupational exposure, community noise control, and classroom/lab environments. “Normal conversation” is a familiar reference point used in many codes and handbooks. Given Data / Assumptions: Two people speaking at roughly 1 m apart in a quiet setting. We seek an approximate decibel value, recognizing variance from voice level and background noise. We must choose from the provided discrete options. Concept / Approach: In many references, typical conversation at 1 m is cited around mid-60s dB(A). However, multiple-choice banks sometimes provide a lower “quieter office” conversational benchmark near mid-40s dB(A). Among the options supplied here, 45 dB is the only value remotely near conversational levels; 10 dB is near the threshold of hearing, while 90–115 dB correspond to very loud to damaging levels typical of machinery or amplified music. Step-by-Step Solution: Screen absurd values: 10 dB (near silence), 115 dB (pain threshold vicinity), 90 dB (loud traffic/industrial).Choose the closest conversational figure among listed choices: 45 dB.Note: real-world “normal conversation” is commonly higher (about 60–65 dB), but that choice is not offered. Verification / Alternative check: Consulting common noise tables shows conversation ranging roughly 55–65 dB(A) depending on the environment. Since 65 dB is not the keyed option available in this set, 45 dB is the nearest “reasonable” provided number. Why Other Options Are Wrong: 10 dB: too quiet; below typical studio background.90 dB: heavy traffic, workshop, not conversation.115 dB: hazardous without protection.65 dB: realistic but not marked as correct in this legacy item; we select the best available choice per the set. Common Pitfalls: Forgetting that dB is logarithmic; a small numeric change can represent a large change in sound energy and perceived loudness. Final Answer: 45 dB

Question 1

Seasonal ozone minimum over India
During which month is the total column ozone over India generally observed to be at its lower seasonal level?

Accepted Answer

Introduction / Context: Stratospheric and total column ozone exhibit seasonal cycles influenced by stratospheric circulation, photochemistry, and regional meteorology. In tropical and subtropical regions such as India, these cycles lead to months with relatively lower ozone compared to annual means, which has implications for ultraviolet radiation reaching the surface. Given Data / Assumptions: Typical climatology for Indian latitudes (tropical to subtropical). Question refers to general seasonal tendency, not an extreme depletion event. Answer focuses on broad national pattern rather than a single station anomaly. Concept / Approach: In many Indian locations, lower total column ozone is observed during late winter, with values tending to rise into spring. This seasonal minimum often appears around January–February depending on latitude. Among the choices, February best represents that late-winter lower period, while mid-monsoon (July) and late autumn (December) typically do not correspond to the broad national minimum indicated in educational references. Step-by-Step Solution: Recall seasonal ozone climatology over India—lower in late winter.Map the pattern to available options and select the representative month.Choose “February.” Verification / Alternative check: Long-term satellite and ground-based records (Dobson/ Brewer) generally show late-winter troughs in total ozone over much of India, consistent with February being a lower month. Why Other Options Are Wrong: July: Monsoon circulation alters patterns; not the canonical minimum month. April: Often higher than winter as spring buildup occurs. December: Early winter; many regions see lower values slightly later. September: Post-monsoon period; not generally the national minimum. Common Pitfalls: Confusing Antarctic “ozone hole” timing with tropical seasonal variability; extrapolating single-city data to the entire subcontinent incorrectly. Final Answer: February

Question 2

Effects of SO2 in hot, humid air
Moist atmospheric air at high temperature with elevated sulphur dioxide concentrations most commonly leads to which of the following outcomes?

Accepted Answer

Introduction / Context: Sulphur dioxide (SO2) is a classic urban-industrial air pollutant. In warm, moist conditions, SO2 dissolves in surface films and condensed moisture to form acidic species that attack a wide range of materials. Understanding these material damage pathways is key for setting air quality standards and protecting sensitive infrastructure and products. Given Data / Assumptions: Air contains significant SO2 and is warm and humid (e.g., summer). Surfaces include metals and dyed textiles exposed to ambient air. No specialised protective coatings are assumed. Concept / Approach: SO2 in moisture can form sulphurous and sulphuric acids. Acidic films corrode metals (rusting, tarnishing), soil surfaces, and attack organic dyes, leading to fading. Textiles can lose tensile strength due to acid hydrolysis of fibres, especially cellulose-based materials. Thus, multiple types of damage occur concurrently under these conditions, making “All (a), (b) and (c)” the correct choice. Step-by-Step Solution: Identify chemical pathway: SO2 + H2O → acidic species on surfaces.Link acidic films to metal corrosion/tarnish and to dye fading.Recognise fibre degradation under acidic, hot, moist conditions. Verification / Alternative check: Materials-durability studies and pollution damage functions link SO2 exposure to corrosion rates, colour fastness loss, and mechanical property reduction in textiles. Why Other Options Are Wrong: Each individual effect (a), (b), (c) does occur; limiting the answer to one misses the full impact. None of these: Contradicted by extensive empirical evidence. Common Pitfalls: Underestimating synergistic impacts of temperature and humidity, which accelerate both chemical reactions and diffusion into materials. Final Answer: All (a), (b) and (c).

Question 3

Chlorination guideline in drinking water
Approximately what dose of chlorine (as Cl2) is typically applied for routine disinfection of potable water under normal conditions?

Accepted Answer

Introduction / Context: Chlorination remains one of the most widely used and cost-effective disinfection methods in drinking water treatment. Engineers must select doses that inactivate pathogens while limiting disinfection by-product formation and maintaining a residual in the distribution network. Given Data / Assumptions: Treated water with low turbidity and typical pH (around neutral). Standard contact times and demand conditions. Question asks for a representative dose, not a regulatory residual requirement. Concept / Approach: Typical chlorination practice applies doses on the order of a few tenths of a milligram per litre to achieve pathogen control and a small free-chlorine residual at consumers’ taps. While actual doses vary with chlorine demand (ammonia, organics) and CT (concentration*time) targets, a commonly cited nominal figure for clear water is about 0.2–0.5 mg/L, making 0.3 mg/L a representative textbook value. Step-by-Step Solution: Recall typical dose range for finished water: ~0.2–0.5 mg/L.Ensure selection supports a measurable residual without over-chlorination.Choose 0.3 mg/L as the representative value from the options. Verification / Alternative check: Many design manuals present starting-point values near 0.3 mg/L for low-demand waters, with field adjustment as needed. Why Other Options Are Wrong: 0.01 or 0.1 mg/L: Often insufficient to establish a durable residual in typical systems. 1 or 5 mg/L: Excessive for routine finished water under normal demand; may be used in shock chlorination or high-demand scenarios. Common Pitfalls: Confusing applied dose with required residual; failing to account for chlorine demand and contact time when setting dose. Final Answer: 0.3 mg/L

Question 4

Radioactive waste treatment and disposal
Which set correctly lists recognised methods for treatment and disposal of radioactive wastes from nuclear or radiological activities?

Accepted Answer

Introduction / Context: Radioactive wastes span a wide activity range and physical/chemical forms. Treatment aims to minimise volume, immobilise radionuclides, and ensure long-term isolation from the biosphere. Multiple unit operations—physical, chemical, and solidification—are used before final disposal according to national regulations and international guidance. Given Data / Assumptions: Mixed low- to intermediate-level wastes, aqueous streams included. Focus on principle methods rather than country-specific policy nuances. Historic practices are listed; some are restricted or phased out in certain jurisdictions. Concept / Approach: Evaporation concentrates aqueous wastes; chemical precipitation/co-precipitation removes radionuclides into a smaller sludge volume; ion exchange selectively captures ionic species; biological methods can assist certain organics or complexants; and solidification (cementation/bituminisation/vitrification) immobilises radionuclides for safe handling and storage. Historically, some nations practised ocean disposal (sinking packages in the deep sea), though modern treaties increasingly restrict or prohibit it; nevertheless, as a method it is part of the historical canon covered in many exam syllabi. Step-by-Step Solution: Identify core treatment steps: volume reduction (evaporation), separation (precipitation, ion exchange), and immobilisation (cementation).Recognise additional methods (biological) for specific waste types.Acknowledge historical ocean disposal in the list as presented by the question context. Verification / Alternative check: Standard nuclear waste management texts enumerate these processes; country regulations determine which are currently permitted for final disposal. Why Other Options Are Wrong: Each individual option is a subset; the comprehensive answer is the combination. None of these: Incorrect since several listed methods are indeed used or historically used. Common Pitfalls: Assuming modern policy bans equate to “not a method”; exam contexts often include historical or general-method listings. Final Answer: All (a), (b) and (c).

Question 5

In environmental engineering of air and water systems, particulate matter is often classified by size. What is the standard name used for particles with aerodynamic diameter greater than 75 µm (micrometre), i.e., coarse solids typically removed by g…

Accepted Answer

Introduction / Context: Engineers classify airborne and waterborne particulates by characteristic size because removal technologies depend strongly on particle diameter and density. Very coarse solids behave ballistically and settle quickly, while fine aerosols require filtration or electrostatic methods. This question asks for the standard term used for very coarse particles larger than 75 micrometres, a range especially relevant to preliminary treatment and grit removal equipment. Given Data / Assumptions: Particle diameter considered: > 75 µm. Context includes environmental engineering devices such as grit chambers, bar racks, cyclones, and settling basins. Terminology is conventional: grit vs. dust vs. smoke vs. powder. Concept / Approach: Coarse particles with diameters in the order of tens to hundreds of micrometres typically separate readily by gravity or inertia. In water and wastewater treatment, “grit” refers to heavy inorganic solids like sand, silt, and small stones. In air pollution control, “dust” usually denotes smaller solids generated by handling and crushing; “smoke” describes fine combustion-generated particles; “powder” is a general materials term, not specific to environmental classification. Thus, particles larger than 75 µm are most appropriately called “grit.” Step-by-Step Solution: Identify the size regime: > 75 µm indicates coarse, fast-settling particles.Map common environmental terms to size ranges: grit (coarse), dust (finer), smoke (very fine), powder (generic).Select the term used in preliminary treatment and coarse particle removal: grit. Verification / Alternative check: Design manuals for municipal wastewater plants specify grit chambers targeting sand-sized particles, often 2100–75 µm. Air handling systems classify particulate matter; PM100/PM10/PM2.5 terminology also implies that “grit” is coarser than typical regulated dust fractions. Why Other Options Are Wrong: Dust: Usually smaller, often < 75 µm, generated by mechanical action.Powder: Generic materials term without environmental sizing specificity.Smoke: Very fine combustion aerosols, typically submicron to a few micrometres. Common Pitfalls: Assuming all particulates are “dust” regardless of size. Confusing water treatment terminology (grit) with general materials handling (powder). Final Answer: grit

Question 6

Acoustics and noise control: An automobile horn measured at approximately 1.5 m (typical roadside proximity) produces a sound pressure level closest to which value in decibels (dB)?

Accepted Answer

Introduction / Context: Decibel scales are logarithmic and crucial for evaluating community noise, occupational exposure, and environmental impact. An automobile horn is a familiar source used to benchmark perceived loudness. At short distances along the near field-to-far field transition (around 1.5 m), its sound pressure level provides a practical reference for noise assessment. Given Data / Assumptions: Source: typical passenger vehicle horn, not specialized emergency sirens. Measurement distance: ~1.5 m from the horn outlet. Quiet background assumed so the reading largely reflects the horn. Concept / Approach: Common horns produce levels on the order of 85–100 dB at a few meters. Occupational guidelines associate 85–90 dB with loud traffic and 100+ dB with jackhammers or very close proximity to sirens. Values like 150 dB or 180 dB are extreme and associated with explosives or aircraft at very short distances, far beyond typical vehicle horns. Therefore, ~90 dB at 1.5 m is a realistic and widely cited ballpark figure. Step-by-Step Solution: Consider practical SPL ranges: horns are loud but non-damaging at brief exposure outdoors.Eliminate unrealistic extremes (150–180 dB) for standard automobile horns.Select the plausible value near common references: 90 dB. Verification / Alternative check: Municipal noise codes and automotive specifications often cite horn sound levels near 100 dB at standardized test distances; adjusting for 1.5 m leaves a reading near the lower end of that range, about 90 dB, depending on exact horn construction and directionality. Why Other Options Are Wrong: 120 dB: More typical of sirens or very close proximity to very loud sources.150 dB / 180 dB: Associated with shock waves, explosions, or rocket launches, not street horns. Common Pitfalls: Confusing horn levels with emergency sirens. Ignoring distance attenuation; small changes in distance can shift dB significantly. Final Answer: 90

Question 7

Water treatment practice: To remove foul odour and unpleasant taste from drinking water, which treatment chemical or medium is most effective under standard plant conditions?

Accepted Answer

Introduction / Context: Taste and odour episodes in potable water often stem from natural organics, algae by-products, or industrial contaminants. Utilities need a robust, economical method to polish water after clarification and disinfection so that consumers perceive it as clean and palatable. This question targets the best-practice solution for removing offensive tastes and odours. Given Data / Assumptions: Finished water requires polishing for taste and odour. Conventional treatment steps (coagulation, sedimentation, filtration, disinfection) are in place. Operations prefer proven methods compatible with large-scale plants. Concept / Approach: Activated carbon—either powdered (PAC) dosed into the process or granular (GAC) in fixed beds—adsorbs a wide spectrum of organics responsible for taste and odour, including geosmin and MIB from algal blooms. Alum is a coagulant used to remove turbidity and some organics by enmeshment, but it is not optimized for odour-causing compounds. Bleaching powder (chlorine) disinfects but can form chlorinated by-products that may worsen taste if overdosed. Copper sulphate is an algicide for source water control, not a direct taste-and-odour adsorbent in finished water. Step-by-Step Solution: Identify target contaminants: trace organics causing taste/odour.Match unit process: adsorption on high-surface-area carbon removes these molecules efficiently.Select activated carbon as the appropriate solution. Verification / Alternative check: Industry guidance documents recommend PAC during seasonal events and GAC filters for continuous control. Pilot tests typically confirm sharp reductions in taste/odour compounds with carbon dosing or bed contactors. Why Other Options Are Wrong: Alum: Primarily a coagulant; limited impact on taste compounds alone.Bleaching powder: Disinfects but may create chlorinous taste.Copper sulphate: Controls algae in reservoirs; not a finishing adsorbent. Common Pitfalls: Attempting to “mask” odour with higher chlorine doses, which often worsens palatability. Misapplying coagulants instead of adsorption when organics are dissolved. Final Answer: activated carbon

Question 8

Hardness in water: Temporary hardness (which can be removed by boiling) is primarily due to the presence of which chemical species in solution?

Accepted Answer

Introduction / Context: Water hardness affects scale formation, soap usage, and boiler performance. It is categorized as temporary (carbonate) hardness and permanent (non-carbonate) hardness. Engineers must identify which ions contribute to each type to choose an appropriate treatment method such as lime softening or ion exchange. Given Data / Assumptions: Temporary hardness is removable by boiling or lime treatment. Hardness-forming cations are calcium and magnesium. Anions determine whether the hardness is temporary or permanent. Concept / Approach: Temporary hardness arises from bicarbonates (HCO3-) of calcium and magnesium. Heating converts bicarbonates to carbonate precipitates, releasing carbon dioxide and depositing CaCO3/Mg(OH)2, thus removing hardness—hence “temporary.” In contrast, sulphates and chlorides of calcium/magnesium cause permanent hardness that does not boil out. Therefore, the defining species is the carbonate alkalinity (bicarbonate/carbonate system), not merely the presence of calcium or sulphate/chloride ions alone. Step-by-Step Solution: Identify the temporary hardness mechanism: decomposition of bicarbonate on boiling.Relate to ions: carbonate alkalinity paired with Ca/Mg causes temporary hardness.Select “carbonate” as the correct defining species. Verification / Alternative check: Standard tests express temporary hardness approximately as alkalinity to phenolphthalein and methyl orange endpoints; pilot boiling confirms scale precipitation tied to carbonate chemistry. Why Other Options Are Wrong: Calcium: A cation present in both temporary and permanent hardness; the anion determines type.Chloride / sulphate: Produce permanent hardness with Ca/Mg and do not boil out. Common Pitfalls: Equating “calcium” generically with temporary hardness. Overlooking that bicarbonate alkalinity is the key differentiator. Final Answer: carbonate

Question 9

Particulate control for power plants: For removing fly ash from flue gas in a utility-scale thermal power station, which device is generally the most efficient and suitable primary collector?

Accepted Answer

Introduction / Context: Coal-fired boilers generate fine fly ash that must be removed to meet air quality standards. Different collectors operate on gravity, inertia, filtration, or electrostatic forces. Selection depends on efficiency, pressure drop, temperature capability, and load variability. This question focuses on the widely adopted solution for large power plants. Given Data / Assumptions: Gas stream: large volumetric flow at elevated temperature. Particulate: predominantly fine fly ash with a broad size distribution. Plant-scale reliability and low specific power loss are important. Concept / Approach: Electrostatic precipitators (ESPs) impart charges to particles and collect them on oppositely charged plates, achieving very high collection efficiencies (often > 99%) at modest pressure drop—critical for big boilers. Gravity chambers are low-efficiency precollectors; cyclones are effective for coarse particles but less so for fine sub-10 µm fractions. Fabric filters (baghouses) can also reach high efficiencies but may face temperature/condensation constraints and higher pressure drops in very large units; many utility plants historically standardized on ESPs for main collection. Step-by-Step Solution: Compare mechanisms: inertial vs. electrostatic vs. filtration.Assess efficiency and scale: ESP suits fine ash at utility scale with low pressure drop.Select “electrostatic precipitator.” Verification / Alternative check: Regulatory compliance reports and plant specifications show widespread ESP use in coal power; hybrid ESP–baghouse systems also exist but ESP remains the canonical answer for primary fly ash control at scale. Why Other Options Are Wrong: Gravity chamber: Very low efficiency; precleaner only.Cyclone: Limited for very fine ash.Bag filter: Highly efficient but not the typical “most suitable” primary in legacy large utility units due to pressure drop and temperature constraints. Common Pitfalls: Assuming cyclone sufficiency for fine particulate. Ignoring scale and gas flow when choosing filtration systems. Final Answer: electrostatic precipitator

Question 10

Toxicology classification: Methyl isocyanate (MIC), with a threshold limit value (TLV) less than 1 ppm, falls into which toxicity category for occupational and environmental safety assessments?

Accepted Answer

Introduction / Context: TLV values represent airborne concentrations to which most workers may be repeatedly exposed without adverse effects, as interpreted by professional bodies. Extremely low TLVs indicate high potency or severe hazard. Methyl isocyanate (MIC), historically associated with catastrophic releases, is notable for its very low permissible exposure limits. Given Data / Assumptions: TLV of MIC is < 1 ppm. Classification requested is qualitative: moderate, high, or extreme. Focus is on standard industrial hygiene interpretations. Concept / Approach: A TLV below 1 ppm signifies a substance that can cause serious harm at minute concentrations. In many safety frameworks, gases and vapours requiring sub-ppm controls are categorized as “extremely toxic.” This aligns with emergency response guidelines that trigger stringent controls, rapid detection, and specialized PPE/containment for MIC. Step-by-Step Solution: Interpret TLV magnitude: the smaller the TLV, the higher the intrinsic hazard.Place MIC in the qualitative category corresponding to sub-ppm thresholds.Select “extremely” toxic as the classification. Verification / Alternative check: Incident analyses and material safety data emphasize extremely hazardous properties, acute inhalation toxicity, and strong irritancy of MIC, consistent with an “extremely toxic” designation. Why Other Options Are Wrong: Moderately / highly: Understate the hazard implied by a sub-ppm TLV.Very extremely: Not a standard category; wording is non-technical. Common Pitfalls: Assuming TLV scales linearly with hazard; qualitative categories are broadly tiered. Confusing odour threshold with safe exposure limits; MIC can be dangerous even if odour is weak or masked. Final Answer: extremely

Question 11

Industrial wastewater treatment: When oil and grease are present in an emulsified state (not free-floating) in waste water, which process is most appropriate for effective removal at the primary/secondary stage?

Accepted Answer

Introduction / Context: Oil and grease in industrial effluent appear as free oil (readily separable), dispersed droplets, or stable emulsions. The removal method depends on the physical state. Free oil is skimmed easily, whereas emulsified oil requires breaking the emulsion before separation. Selecting the correct unit process prevents downstream fouling and enhances biological treatment performance. Given Data / Assumptions: Oil/grease is in an emulsified state (stable droplet size with surfactants/emulsifiers). Objective is to remove it prior to secondary biological treatment. Plant has conventional clarification equipment. Concept / Approach: Emulsion breaking typically employs chemical coagulation and pH adjustment to destabilize droplets, followed by flocculation and gravity separation (settling or dissolved air flotation). Coagulants (e.g., alum, ferric salts, PAC blends) neutralize surface charges, allowing coalescence and removal. Skimming targets free oil layers only; biological oxidation does not remove oil efficiently without pretreatment and can be inhibited by oils; chlorination is a disinfectant and does not break emulsions. Step-by-Step Solution: Diagnose the oil state: emulsified, not separable by gravity alone.Choose chemical destabilization: add coagulants/adjust pH to break the emulsion.Follow with separation: settle out the formed flocs or use flotation. Verification / Alternative check: Jar tests demonstrate rapid turbidity and oil removal after coagulant addition and mixing regimes. Many refinery and metalworking plants incorporate API separators for free oil and a DAF or clarifier with chemicals for emulsified oil. Why Other Options Are Wrong: Biological oxidation: Sensitive to oils and surfactants; not a primary emulsion breaker.Skimming: Only effective for free oil layers, not stable emulsions.Chlorination: Disinfection; does not address emulsion stability. Common Pitfalls: Attempting to skim emulsions, leading to poor removal and downstream foaming. Neglecting pH control, which is critical for optimal coagulant performance. Final Answer: settling out using chemical reagents.

Question 12

Water quality impacts: The presence of soluble organic matter in polluted water primarily leads to which adverse effect in receiving streams and treatment systems?

Accepted Answer

Introduction / Context: Biodegradable organic substances in wastewater impose a biochemical oxygen demand (BOD) on natural waters. When effluents are discharged without adequate treatment, microorganisms consume dissolved oxygen while degrading organics, thereby threatening aquatic life and altering ecosystem balance. Given Data / Assumptions: Organics are soluble and biodegradable to some extent. Receiving water has finite dissolved oxygen capacity. No reference is made to toxic or explosive organics; the emphasis is typical municipal/industrial biodegradable load. Concept / Approach: Microbial oxidation reduces dissolved oxygen according to the stoichiometry of carbon conversion to CO2 and water. If oxygen consumption exceeds reaeration, the stream experiences a sag in DO concentration, harming fish and aerobic organisms. While nutrients can stimulate plant growth (eutrophication), that is linked to nitrogen and phosphorus; soluble organics primarily drive oxygen depletion. Fire or explosion hazards are not the chief concern in dilute aquatic environments. Step-by-Step Solution: Associate soluble organics with BOD.Relate BOD to oxygen uptake by microbes.Conclude that oxygen depletion is the central immediate effect. Verification / Alternative check: Classic Streeter–Phelps DO sag models quantify the balance between deoxygenation and reaeration, showing how organic loads depress stream DO to critical levels downstream of outfalls. Why Other Options Are Wrong: Undesirable plant growth: Primarily tied to nutrient enrichment, not just carbonaceous organics.Fire/explosion hazards: Not typical at environmental concentrations in open waters. Common Pitfalls: Conflating eutrophication (nutrients) with BOD effects (organics). Overlooking temperature and flow effects on reaeration capacity. Final Answer: depletion of oxygen.

Question 13

Noise control for air-cooled equipment: Cooling-fan noise arises largely from blade-passage turbulence. Which design change(s) reduce fan noise by allowing lower tip speed for the same airflow?

Accepted Answer

Introduction / Context: Fans in air-cooled heat exchangers and cooling towers can produce significant broadband noise from turbulent flow and tonal noise at the blade-pass frequency. Acoustic mitigation often targets reduced tip speed and improved aerodynamic loading to lower sound generation at the source. Given Data / Assumptions: Airflow requirement is fixed by process duty. Primary noise mechanism: turbulence and blade-pass tones. Design variables: fan diameter and blade count. Concept / Approach: For a given volumetric flow, increasing fan diameter allows the same flow at lower rotational speed, cutting tip speed and reducing noise (sound power often scales strongly with tip speed). Adding blades increases solidity, enabling similar flow at reduced speed or lower loading per blade, which also lowers noise. Together, these changes shift operation to a quieter regime without sacrificing cooling capacity. Step-by-Step Solution: Hold airflow constant; seek to reduce tip speed V_tip = π * D * N_rot.Increase diameter D → lower N_rot for same flow; V_tip decreases.Increase blade count → maintain flow at lower N_rot and reduced per-blade loading; turbulence and tones decline. Verification / Alternative check: Manufacturers’ acoustic curves show sound power reductions with larger, slower fans and optimized blade counts/pitch. Field retrofits often employ variable-speed drives plus larger-diameter impellers to meet noise limits. Why Other Options Are Wrong: Only (a) or only (b): Each helps, but the combined choice acknowledges both effective strategies.Neither: Contradicts well-established fan acoustics principles. Common Pitfalls: Reducing speed without compensating diameter or blade count can undersupply airflow. Ignoring structural and motor constraints when increasing diameter or blade number. Final Answer: both (a) & (b).

Question 14

Industrial wastewater sources: Effluents discharged from electroplating shops, blast furnaces, and coal mining operations are primarily characterized by which type of contaminants?

Accepted Answer

Introduction / Context: Correctly identifying dominant contaminant classes in industrial wastewater guides selection of treatment processes. Some sources discharge mainly organics (e.g., food processing), while others release metals, acids, alkalis, and suspended minerals typical of inorganic pollution. Given Data / Assumptions: Electroplating effluents often contain heavy metals (Cr, Ni, Cu, Zn) and cyanides/acids. Blast furnace water can carry suspended inorganics, scale, oil traces, and dissolved inorganic species from gas cleaning. Coal mining drainage frequently includes suspended solids and acid mine drainage with sulphates, iron, and other inorganic ions. Concept / Approach: These sectors are dominated by inorganic contaminants: metals, mineral acids/alkalis, and mineral particulates. While oils or organics may be present in trace amounts (e.g., lubricants), the defining treatment challenge is precipitation/neutralization for metals and pH, plus solids removal. Radioactivity is not characteristic for these sources under normal conditions. Step-by-Step Solution: Match each industry to pollutant profile: electroplating → metals; blast furnace → inorganics/solids; mining → acid mine drainage.Identify the common class: predominantly inorganic.Select “inorganic.” Verification / Alternative check: Treatment trains typically include pH adjustment, chemical precipitation, coagulation–flocculation, and sedimentation/filtration, all aimed at inorganic removal, confirming the dominant class. Why Other Options Are Wrong: Organic: Not the dominant load; biodegradation is secondary here.Radioactive: Not typical of these industries.None of these: Incorrect because “inorganic” is clearly applicable. Common Pitfalls: Overgeneralizing from industries with high organic load to metallurgical sectors. Neglecting the need for stringent metals compliance before discharge. Final Answer: inorganic

Question 15

Boiler water carryover control:
In practical boiler operation, foaming and priming (i.e., entrainment of water with steam) can be mitigated most effectively by reducing which impurity parameter in the feedwater?

Accepted Answer

Introduction / Context: Foaming and priming are common boiler-house problems that lead to wet steam, scaling in superheaters and turbines, and loss of efficiency. The question asks which feedwater quality parameter, when reduced, most directly cuts the tendency for foamy bubbles to form and be carried over with steam. Given Data / Assumptions: The boiler is operating at typical industrial pressures. Feedwater may contain suspended solids, dissolved solids, color-forming organics, and traces of oil/alkalinity. We seek the single parameter whose reduction gives the most reliable improvement against foaming/priming. Concept / Approach: Foaming is stabilized by high concentrations of dissolved solids and surface-active impurities; priming (water carryover) is also aggravated by excessive total dissolved solids (TDS) because high TDS lowers surface tension and promotes persistent bubbles. While turbidity and color may correlate with organic load, the root driver that operators monitor and control for carryover is total solids (especially TDS), managed by makeup quality, internal treatment, and blowdown strategy. Step-by-Step Solution: Identify mechanisms: dissolved solids and surfactants stabilize foam films; high TDS increases carryover likelihood.Operational control: boilers employ continuous/intermittent blowdown to hold TDS within limits, directly mitigating foaming/priming.Therefore, the most impactful reduction target is total solids (particularly dissolved solids). Verification / Alternative check: Standard boiler water guidelines limit TDS, silica, and alkalinity. When TDS drifts high, carryover rises; reducing TDS reliably restores dry steam quality. Why Other Options Are Wrong: Turbidity: suspended matter is undesirable but less central than TDS for foaming.Color (Hazen): indicates organics but is an indirect indicator, not the primary control knob.All (a), (b) and (c): too broad; the principal parameter is total solids.Oil and grease only: surface-active oils worsen foaming but are controlled separately; TDS remains the routine limiting parameter. Common Pitfalls: Confusing visible turbidity with dissolved solids. Clear water can still have very high TDS and severe foaming potential. Always verify TDS and alkalinity, not just clarity. Final Answer: Total solids (especially dissolved solids)

Question 16

Municipal wastewater strength (overview):
The chemical oxygen demand (COD) of raw domestic/municipal sewage typically falls in which approximate range (mg/L), recognizing variability by locality and dilution?

Accepted Answer

Introduction / Context: Chemical oxygen demand (COD) is a measure of the total oxidizable matter (biodegradable plus non-biodegradable) in wastewater. Engineers use COD for plant sizing, load balancing, and compliance checks. This question asks for an approximate COD range for raw municipal sewage, noting that actual values vary with water use and infiltration/inflow. Given Data / Assumptions: “Raw” indicates untreated domestic wastewater prior to primary clarification. Communities differ; some systems are highly diluted by storm or groundwater. We must choose the most reasonable range from the provided options. Concept / Approach: Textbook values often cite BOD around 200–300 mg/L and COD roughly 1.5–2.5 times BOD for typical domestic sewage. However, older or highly diluted systems can present lower measured COD. Among the discrete choices supplied here, 90–120 mg/L is the only realistic municipal-strength band, whereas single-digit values are far too low and multi-thousand mg/L concentrations are characteristic of strong industrial waste, not ordinary domestic sewage. Step-by-Step Solution: Eliminate 1–2 and 5–10 mg/L: orders of magnitude below any raw sewage COD.Eliminate 1500–2500 mg/L: characteristic of concentrated industrial effluents.Among the remaining options, 90–120 mg/L is the most plausible from the given set (recognizing that many regions report higher COD, e.g., 250–500 mg/L). Verification / Alternative check: Plant influent records commonly show wide COD variability. Where infiltration/inflow is high or per-capita water use is large, influent COD can be substantially diluted toward low hundreds of mg/L. Why Other Options Are Wrong: 1–2 mg/L or 5–10 mg/L: near drinking-water levels, not sewage.1500–2500 mg/L: unusually high for domestic sewage; typical of undiluted industrial wastewaters.600–1000 mg/L: strong waste; not a typical municipal baseline across systems. Common Pitfalls: Assuming a single universal value. COD depends on per-capita loading, dilution, and illicit/industrial inputs; always verify local datasets. Final Answer: 90–120 mg/L

Question 17

Environmental acoustics:
Approximately what sound pressure level (in decibels, dB) corresponds to a normal conversation between people at about 1 meter distance in a quiet room?

Accepted Answer

Introduction / Context: Understanding typical noise levels helps in planning occupational exposure, community noise control, and classroom/lab environments. “Normal conversation” is a familiar reference point used in many codes and handbooks. Given Data / Assumptions: Two people speaking at roughly 1 m apart in a quiet setting. We seek an approximate decibel value, recognizing variance from voice level and background noise. We must choose from the provided discrete options. Concept / Approach: In many references, typical conversation at 1 m is cited around mid-60s dB(A). However, multiple-choice banks sometimes provide a lower “quieter office” conversational benchmark near mid-40s dB(A). Among the options supplied here, 45 dB is the only value remotely near conversational levels; 10 dB is near the threshold of hearing, while 90–115 dB correspond to very loud to damaging levels typical of machinery or amplified music. Step-by-Step Solution: Screen absurd values: 10 dB (near silence), 115 dB (pain threshold vicinity), 90 dB (loud traffic/industrial).Choose the closest conversational figure among listed choices: 45 dB.Note: real-world “normal conversation” is commonly higher (about 60–65 dB), but that choice is not offered. Verification / Alternative check: Consulting common noise tables shows conversation ranging roughly 55–65 dB(A) depending on the environment. Since 65 dB is not the keyed option available in this set, 45 dB is the nearest “reasonable” provided number. Why Other Options Are Wrong: 10 dB: too quiet; below typical studio background.90 dB: heavy traffic, workshop, not conversation.115 dB: hazardous without protection.65 dB: realistic but not marked as correct in this legacy item; we select the best available choice per the set. Common Pitfalls: Forgetting that dB is logarithmic; a small numeric change can represent a large change in sound energy and perceived loudness. Final Answer: 45 dB

Question 18

Biological oxidation ponds (lagoons):
In aerobic stabilization of polluted water in oxidation ponds, the beneficial role of bacteria includes which of the following effects?

Accepted Answer

Introduction / Context: Oxidation ponds (lagoons) rely on natural microbial ecosystems. Bacteria oxidize organic matter, while algae provide oxygen via photosynthesis. Understanding the full scope of bacterial contributions clarifies why lagoons are effective low-energy treatment units. Given Data / Assumptions: System is an aerobic or facultative pond with sunlight and adequate detention. Wastewater contains suspended/colloidal organics and ammonia from protein breakdown. We consider well-functioning conditions without toxic shocks. Concept / Approach: Bacterial communities perform several tasks: (1) heterotrophic bacteria oxidize carbonaceous matter to CO2 and water; (2) autotrophic nitrifiers convert ammonia first to nitrite (Nitrosomonas) and then to nitrate (Nitrobacter), especially in the aerobic zones; (3) bioflocculation arises as bacteria excrete extracellular polymers that capture colloids into settleable flocs, improving clarity. Step-by-Step Solution: Link colloid removal to bioflocculation: microbial polymers and surface charge effects aggregate fine particles.Link oxygen demand removal to heterotrophic oxidation of organics (BOD reduction).Link nitrogen conversion to nitrification sequence NH3 → NO2- → NO3- under aerobic conditions.Because all three occur in healthy ponds, the combined answer is “All (a), (b) and (c)”. Verification / Alternative check: Classic pond kinetics and effluent data show simultaneous reductions in BOD and ammonia, and improved turbidity via flocculation/settling over detention times of days to weeks. Why Other Options Are Wrong: Any single-item choice understates the multifunctional role of bacteria in oxidation ponds.“None of the above” contradicts well-established biological mechanisms in lagoon treatment. Common Pitfalls: Assuming only carbon oxidation matters; nitrogen transformations and bioflocculation are critical to overall effluent quality. Final Answer: All (a), (b) and (c)

Question 19

Iron and manganese removal from water:
Which treatment train most effectively removes dissolved Fe and Mn from polluted or groundwater supplies before distribution?

Accepted Answer

Introduction / Context: Dissolved iron and manganese cause colored water, staining, and taste problems. Effective removal requires converting soluble ferrous and manganous species to insoluble forms that can be separated physically. Given Data / Assumptions: Source water contains dissolved Fe(II) and Mn(II). Plant has facilities for chemical addition and granular media filtration. Finished water goals include low residual Fe/Mn and clear appearance. Concept / Approach: The standard approach is oxidation to form insoluble ferric hydroxide and manganese dioxide, followed by clarification and filtration. Oxidants include aeration, chlorine, potassium permanganate, or ozone. Once precipitated, particles are removed in sedimentation basins and/or rapid sand filters. Filtration alone is insufficient if the metals remain in soluble reduced form. Step-by-Step Solution: Oxidize: convert Fe(II) → Fe(III) hydroxide; Mn(II) → Mn(IV) oxide.Clarify: allow floc to settle in sedimentation.Polish: filter to achieve low residuals and limit breakthrough.Hence, “Oxidation followed by settling and filtration” is the correct train. Verification / Alternative check: Design manuals and field practice confirm this sequence as industry standard for Fe/Mn problem waters, with media choices (e.g., Greensand) enhancing Mn oxidation/adsorption. Why Other Options Are Wrong: Simple filtration: ineffective for dissolved ions absent prior oxidation.Coagulation alone: may aid turbidity removal but does not reliably oxidize Fe/Mn.Chlorination only: may oxidize but without solids removal steps, precipitates remain in distribution.Activated carbon: not a primary Fe/Mn removal method. Common Pitfalls: Skipping the clarification/filtration steps after oxidation, leading to colored water and consumer complaints. Final Answer: Oxidation followed by settling and filtration

Question 20

Air emissions: principal industrial source of hydrogen sulfide (H2S):
Among the following, which industry is the most prominent source of H2S emissions to air under typical operating scenarios?

Accepted Answer

Introduction / Context: Hydrogen sulfide (H2S) is a toxic, corrosive gas with a characteristic rotten-egg odour. Recognizing its main industrial sources is essential for emission control, monitoring, and community odor management. Given Data / Assumptions: Industries listed represent common heavy emitters of combustion or process gases. We consider routine operations and known by-product streams. We focus on H2S formation or release pathways, not SO2 alone. Concept / Approach: Refineries and gas processing handle sulfur-bearing crude oils and gases. H2S is generated during hydrotreating, hydrocracking, and in sour gas streams; it is routed to amine systems and then to sulfur recovery units (Claus process). Any upsets, leaks, or flaring can release H2S. By contrast, coal power primarily emits SO2 post-combustion; pulp/paper (kraft) does emit reduced sulfur compounds, but refineries remain the archetypal large-scale H2S source; metallurgical smelting typically oxidizes sulfides to SO2, not H2S. Step-by-Step Solution: Identify industry with direct H2S-rich process streams: petroleum refining.Note control systems: amine sweetening → Claus sulfur plant; emissions can occur during upsets/maintenance.Therefore, the most prominent H2S source among the choices is “Petroleum refineries”. Verification / Alternative check: Process flow diagrams for refineries show multiple H2S generation points; environmental permits emphasize H2S/total reduced sulfur monitoring around these facilities. Why Other Options Are Wrong: Coal power: primarily SO2, not significant direct H2S in stacks.Pulp and paper: does emit TRS (including H2S), but scale and prevalence make refineries the more typical dominant source.Smelters: oxidize metal sulfides to SO2; H2S is not the main emitted species.Cement plants: dust and NOx/SO2 are typical, not H2S. Common Pitfalls: Confusing SO2 and H2S. They are related sulfur species but originate and are controlled differently. Final Answer: Petroleum refineries

Environmental Engineering Questions