Question 1

Most efficient equipment for fine dust removal
Considering industrial air pollution control devices, which option generally offers the highest overall efficiency for fine particulate capture in large-scale service?

Accepted Answer

Introduction / Context: Fine particulate control is a cornerstone of air pollution engineering. The selection of a device depends on particle size distribution, gas temperature, flow rate, pressure drop tolerance, and maintenance. Among common devices, two contenders dominate fine dust service: fabric filters and electrostatic precipitators (ESPs). Given Data / Assumptions: “Most efficient” refers to overall capability on fine particles at large scales. Devices considered: bag filter (fabric filter), scrubber, ESP, cyclone. Industrial-scale context, not a benchtop or specialty niche. Concept / Approach: ESPs use electrical charging to collect both coarse and fine particles at very high efficiencies (often 99%+), with low pressure drop and suitability for large gas volumes. Bag filters can also reach excellent capture (including sub-micron) but can face temperature limits, pressure drop penalties, and fabric degradation depending on service. Cyclones excel on coarse particles; wet scrubbers are versatile for gas absorption and coarser dust but may be less efficient on the finest fractions without enhancements. Step-by-Step Solution: Compare devices by fine-particle collection and scalability.Recognize ESPs’ high efficiency and low pressure drop at large flows.Select “Electrostatic precipitator” as the broadly correct choice in classic industrial MCQs. Verification / Alternative check: Utility boilers and large kilns frequently employ ESPs for fine particulate control; performance data support their top-tier capture. Why Other Options Are Wrong: Bag filter: Highly efficient but not universally preferred at very large, hot gas flows; ESP is the canonical answer.Scrubber: Better for soluble gases and larger droplets/particles unless specialized.Cyclone: Primarily a precleaner for coarse dust; limited fine capture. Common Pitfalls: Assuming “filter” always equals highest efficiency without considering pressure drop and temperature limitations at scale. Final Answer: Electrostatic precipitator

Question 2

Unsuitable contents for polyvinyl chloride (PVC) containers
PVC containers are generally considered unsuitable for long-term storage of which category, due to potential leaching and food safety concerns?

Accepted Answer

Introduction / Context: Choice of container material affects product safety and shelf life. Polyvinyl chloride (PVC) is a common plastic with additives such as plasticizers and stabilizers. For critical applications, regulatory and food-contact standards govern whether a given grade of PVC is acceptable. The question concerns general suitability for storage. Given Data / Assumptions: Generic, non-specialty PVC container (not specifically certified food-grade). Long-term storage, at ambient conditions, with typical additives present. We must identify which content is generally unsuitable from a safety standpoint. Concept / Approach: Many PVC formulations contain plasticizers (e.g., phthalates) and stabilizers that can migrate into fatty, oily, or alcoholic foods. Unless specifically formulated and certified as food-grade, PVC containers are not recommended for storing foodstuffs. For non-food items like textiles or inert powders, general PVC storage is more acceptable (subject to static, contamination, or corrosion concerns depending on the powder). Step-by-Step Solution: Consider migration risk: additives can leach into foods, especially fatty products.Food safety regulations typically require certified food-contact materials; generic PVC may not qualify.Thus “foodstuffs” are unsuitable in the general case. Verification / Alternative check: Food packaging commonly uses polyethylene, polypropylene, PET, or specially certified PVC grades; non-certified PVC is avoided for broad food-contact use. Why Other Options Are Wrong: Woolen clothes: PVC containers can store textiles; no inherent toxic transfer issue comparable to food contact.Metallic powder: Storage may raise static or contamination issues, but not the consumer health risk posed by food-contact leaching.None of these: Incorrect because foodstuffs are generally unsuitable. Common Pitfalls: Assuming all plastics are interchangeable for food contact; certification and formulation matter. Final Answer: foodstuffs

Question 3

Exclusion zone (green belt) radius around hazardous chemical industries
What is the typical radius prescribed for an exclusion zone with a protective green belt where public access is restricted around hazardous/polluting chemical facilities?

Accepted Answer

Introduction / Context: Land-use planning and buffer zoning reduce public exposure to accidental releases, routine emissions, and nuisance impacts (noise, odour). Many siting guidelines recommend an exclusion zone—often with a planted green belt—around hazardous or highly polluting chemical facilities. Given Data / Assumptions: The question asks for a commonly cited radial distance for general MCQ purposes. Exact regulatory values can vary by country, hazard category, and risk assessment. We select a standard planning benchmark used in conventional exam settings. Concept / Approach: A 1000 m (1 km) exclusion/green belt buffer is a widely referenced planning value in many teaching texts for hazardous installations, providing dispersion distance, visual screening, and land-use separation. While site-specific quantitative risk assessment may refine this distance, the 1 km figure is the canonical single-choice answer. Step-by-Step Solution: Identify need for a defensible general value.Recognize 1 km as the commonly taught planning distance for high-hazard sites.Choose 1000 m from the options. Verification / Alternative check: Teaching materials and example siting problems often apply 1 km buffers before more detailed risk-based calculations. Why Other Options Are Wrong: 100 m / 400 m: Typically too small to ensure effective public separation for major hazard facilities.4000 m: Four kilometres is a large exclusion that exceeds common planning baselines absent special hazards. Common Pitfalls: Treating the buffer as a universal legal requirement; in practice, risk-based, topography, and meteorology-specific evaluations refine distances. Final Answer: 1000

Question 4

Occupational exposure guideline for carbon monoxide (CO)
What is the commonly cited threshold limit value (TLV-TWA) for CO in workplace air, expressed in parts per million (ppm)?

Accepted Answer

Introduction / Context: Carbon monoxide (CO) is a colourless, odourless gas produced by incomplete combustion. It binds to haemoglobin to form carboxyhaemoglobin, reducing oxygen delivery to tissues. Workplace exposure limits aim to prevent acute toxicity and protect worker health during an 8-hour shift. Given Data / Assumptions: TLV-TWA refers to an 8-hour time-weighted average limit. We are selecting a single, widely taught benchmark value for MCQs. Short-term exposure limits and ceiling values are not requested. Concept / Approach: A commonly cited occupational guideline is a TLV-TWA of 50 ppm for carbon monoxide. Although various jurisdictions and agencies can publish slightly different limits, 50 ppm appears frequently in educational materials and historic ACGIH references, making it the standard exam answer unless a specific regulatory source is specified. Step-by-Step Solution: Identify the metric: TLV-TWA for an 8-hour working day.Select the canonical value used in many training texts: 50 ppm.Confirm that this aligns with conservative protection against CO’s hypoxic effects. Verification / Alternative check: Safety manuals and industrial hygiene handbooks frequently present 50 ppm as the TWA benchmark for instructional purposes. Why Other Options Are Wrong: 5 ppm: Unusually low for an occupational TWA benchmark; closer to indoor air quality goals, not typical TLV.2000 / 5000 ppm: These are dangerously high concentrations associated with acute poisoning, not permissible exposure limits. Common Pitfalls: Confusing TWA with short-term or emergency exposure guidance; mixing up CO with CO2 limits. Final Answer: 50

Question 5

Gas cleaning in air pollution control
Which of the following materials acts as an effective adsorbent for removing nitrogen oxides (NO and NO2) from gas or air streams in industrial pollution-control practice?

Accepted Answer

Introduction / Context: Nitrogen oxides (NO and NO2, collectively NOx) are key gaseous pollutants emitted from combustion sources. Industrial gas cleaning often uses adsorption as a polishing step, especially for intermittent or low-flow exhausts. This question asks which common sorbent is actually used for capturing NOx from air/gas streams. Given Data / Assumptions: Target pollutants: NO and NO2 (NOx) in an industrial exhaust. Single-step adsorption choice among common media. Standard temperature ranges; no catalytic reduction specified unless media are impregnated. Concept / Approach: Unmodified activated carbon has limited capacity for NO; however, activated carbon (active carbon) is widely used when appropriately impregnated (e.g., with alkali or oxidants) to oxidise/convert NO to NO2 or nitrate/nitrite species that are retained. In practice, “activated carbon” is the correct umbrella choice among listed materials for adsorptive NOx control in gas cleaning. Other listed media are either unsuitable or typically used for other contaminants (moisture, H2S, SO2). Step-by-Step Solution: Identify the pollutant class: NOx (oxidised, low polarity).Match to common industrial sorbents: activated carbon with impregnants is standard for NOx polishing.Eliminate media that target other species (e.g., silica gel for water vapour, limestone for SO2).Select the best available option: Active carbon. Verification / Alternative check: Chemisorptive carbons are specified in vendor literature for NOx service; field systems combine oxidation on the carbon surface with adsorption of reaction products, confirming practical effectiveness. Why Other Options Are Wrong: Silica gel: A desiccant for water vapour; not a NOx sorbent. Bog iron (iron oxide): Commonly used for H2S removal, not NOx. Pulverised limestone: Used for SO2 neutralisation, not NOx adsorption. Molecular sieves only: Zeolites are not standard NOx adsorbents for typical stacks without special chemisorption routes. Common Pitfalls: Assuming any porous solid will adsorb NOx effectively; NO is poorly adsorbed unless oxidised or chemically bound. Activated carbon impregnations solve this gap. Final Answer: Active carbon

Question 6

Metals as water/air pollutants
There are several metals classed as priority pollutants for health and environmental reasons. Which metal listed below is generally not treated as a toxic priority pollutant in typical regulations?

Accepted Answer

Introduction / Context: Environmental regulations often identify certain metals as toxic pollutants because they bioaccumulate, persist, or cause acute/chronic toxicity. Understanding which metals are prioritised helps engineers focus monitoring and treatment on substances of greatest concern. Given Data / Assumptions: “Pollutant metals” refers to toxic priority metals commonly cited in water/air regulations. We compare toxicity, persistence, and regulatory emphasis. We select the one not typically listed as a priority toxic metal. Concept / Approach: Mercury, arsenic, lead, and cadmium are classic toxic metals with strict limits due to neurotoxicity, carcinogenicity, or kidney damage. Aluminium, while abundant and regulated for aesthetic/operational issues in water treatment, is not usually grouped with the “toxic priority metals” like Hg, Pb, Cd, As. Hence, aluminium is the correct choice here. Step-by-Step Solution: List traditional priority toxic metals: Hg, Pb, Cd, As, Cr, Ni, etc.Compare with provided options.Identify aluminium as not typically in the toxic-priority list. Verification / Alternative check: Regulatory frameworks consistently treat Hg, Pb, Cd, and As as hazardous with very low permissible limits, whereas aluminium often has secondary (aesthetic/operational) guideline values in drinking water. Why Other Options Are Wrong: Mercury, Arsenic, Lead, Cadmium: All are well-known toxic priority pollutants with stringent controls. Common Pitfalls: Confusing “commonly occurring” with “non-pollutant”; abundant presence does not imply low concern, but in aluminium’s case, regulatory focus is typically different from heavy toxic metals. Final Answer: Aluminium

Question 7

Sludge gas from Imhoff tank digestion
During sewage treatment in an Imhoff tank (anaerobic digestion of settled sludge), the principal combustible gas evolved is:

Accepted Answer

Introduction / Context: An Imhoff tank separates and digests settled sludge under anaerobic conditions. The biological conversion of organics produces “biogas.” Knowing its main constituents is essential for odour control, energy recovery, and safety planning in wastewater plants. Given Data / Assumptions: Anaerobic digestion occurs in the lower compartment. Typical biogas composition is considered. Focus is on the principal combustible component. Concept / Approach: Under anaerobic digestion, complex organics are converted to simpler compounds by a microbial consortium, culminating in methanogenesis. The resulting gas is primarily methane (combustible) and carbon dioxide (non-combustible), with traces of H2S and others. Among the options, methane (CH4) is the main combustible gas of interest. Step-by-Step Solution: Recognise digestion type: anaerobic.Recall typical biogas composition: ~55–70% CH4, ~30–45% CO2.Select the principal combustible gas: CH4. Verification / Alternative check: Plant data and textbooks confirm methane as the dominant energy carrier in digester gas; CO2 is present but not a fuel. Why Other Options Are Wrong: CO2: Major but non-combustible component. CO, H2: Not produced in significant amounts in properly operating digesters. N2: Not a digestion product; may appear only as contamination. Common Pitfalls: Confusing gas proportions; while CO2 can be significant, the “principal combustible” is methane. Final Answer: CH4

Question 8

Boiler-water corrosion control
Which combination of treatments is appropriate to prevent corrosion in boiler systems handling feedwater?

Accepted Answer

Introduction / Context: Steel corrosion in boilers is accelerated by dissolved oxygen, low alkalinity, and poor chemical control. A robust water-treatment programme must address oxygen removal, pH/alkalinity, and residual oxygen scavenging to protect the system and maintain efficiency. Given Data / Assumptions: Standard low- to medium-pressure boiler service is considered. Materials of construction are typical carbon steel. Conventional treatment chemicals are available. Concept / Approach: Corrosion is mitigated by: (1) deaeration (thermal or vacuum) to remove most dissolved oxygen; (2) alkalinity/pH control to passivate steel and minimise acid corrosion; and (3) oxygen scavengers (e.g., sodium sulphite, hydrazine-based products) to mop up residual oxygen after deaeration. Together, these reduce the driving forces for corrosion and promote protective magnetite films on steel surfaces. Step-by-Step Solution: Remove bulk oxygen via deaerator.Adjust pH/alkalinity to recommended ranges.Dose scavenger to eliminate residual oxygen.Select the option that includes all three measures. Verification / Alternative check: Industry standards and boiler vendors prescribe this three-part programme; failure in any part correlates with increased corrosion, pitting, and tube failures. Why Other Options Are Wrong: Each single-measure option is incomplete; residual oxygen or improper pH would still permit corrosion. Using only phosphate without deaeration/pH control is inadequate and risky. Common Pitfalls: Assuming oxygen scavenger alone replaces proper deaeration; scavengers are polishing steps, not primary oxygen removal methods. Final Answer: All (a), (b) and (c)

Question 9

Biological oxidation ponds in wastewater treatment
How do oxidation ponds remove organic matter from polluted water under normal design and operating conditions?

Accepted Answer

Introduction / Context: Oxidation ponds (stabilisation ponds) are low-cost, land-intensive systems that treat municipal wastewater using natural processes. Understanding their removal mechanisms helps in sizing, performance prediction, and seasonal operation planning. Given Data / Assumptions: Ponds operate as facultative/aerobic systems depending on design and loading. Sunlight and algae provide oxygen; bacteria degrade organics. We focus on the essence of how organics are removed. Concept / Approach: In oxidation ponds, microorganisms (bacteria, algae, protozoa) drive removal. Organic matter is biologically oxidised primarily under aerobic conditions near the surface, with possible facultative zones below. Thus, both statements—microbial activity and aerobic oxidation—are jointly correct and complementary, making “Both (a) & (b)” the best choice. Step-by-Step Solution: Recognise biological nature of treatment.Identify oxygen supply via photosynthesis and aeration by wind.Conclude removal mechanism: aerobic biological oxidation. Verification / Alternative check: Design texts describe BOD reduction via aerobic heterotrophs supported by algal oxygen production; facultative layers allow some settling and further stabilisation. Why Other Options Are Wrong: Only microorganisms or only aerobic oxidation: Incomplete alone; the processes are intertwined. Neither / purely physical sedimentation: Misses the biological core of oxidation ponds. Common Pitfalls: Overlooking the role of algae in oxygen supply; ignoring temperature dependence that affects microbial rates. Final Answer: Both (a) & (b)

Question 10

Toxic-gas thresholds (TLV) comparison
Among the following, which substance has the lowest threshold limit value (TLV), approximately 0.02 ppm, indicating extreme toxicity at very low concentrations?

Accepted Answer

Introduction / Context: Threshold limit values (TLVs) or guideline exposure limits help engineers design ventilation, monitoring, and emergency response plans. Some gases are hazardous at extremely low concentrations due to acute toxicity or strong irritancy. Given Data / Assumptions: Compare typical occupational/environmental guidance values. Order-of-magnitude selection rather than an exact standard from a specific year. Lowest TLV implies most toxic at trace levels. Concept / Approach: Methyl isocyanate (MIC) is infamous for its extreme acute toxicity and very low permissible exposure limits, on the order of a few hundredths of a ppm in many references. Phosgene, bromine, and ozone are also dangerous but typically have higher guideline values (often around 0.1 ppm for ceiling or TWA, depending on the standard). Therefore, MIC is the correct choice for the lowest approximate TLV near 0.02 ppm among the options listed. Step-by-Step Solution: Recall relative toxicity rankings for listed gases.Identify MIC as possessing exceptionally low allowable exposure levels.Select MIC as the lowest TLV candidate (~0.02 ppm). Verification / Alternative check: Historical guidelines and emergency response data sheets consistently flag MIC as requiring the lowest exposure thresholds among common industrial toxics. Why Other Options Are Wrong: Phosgene, Bromine, Ozone: Highly toxic/irritant but with TLVs typically higher than MIC. Chlorine dioxide: Very irritating oxidant, yet common limits exceed MIC’s extreme sensitivity level. Common Pitfalls: Mixing ceiling, STEL, and TWA values across different agencies; the key here is the relative order—MIC is the lowest among the listed substances. Final Answer: MIC (methyl isocyanate)

Question 11

Measuring visible smoke from a chimney
The Ringelmann (Ringlemann) chart is used to determine which parameter in stack emissions?

Accepted Answer

Introduction / Context: Before sophisticated opacity meters became widespread, inspectors used simple visual standards to judge smoke emissions. The Ringelmann chart provides a reference scale of grey shades to estimate smoke density (opacity) from chimneys and stacks. Given Data / Assumptions: Visual observation of stack plume against a contrasting background. Use of Ringelmann numbered shades (0 to 5). Objective: qualitative quantification of smoke density, not chemical composition. Concept / Approach: The Ringelmann chart correlates perceived darkness (opacity) of a plume to a number. It does not directly measure temperature, gas density, or mass concentration, though smoke density may relate indirectly to particulate loading. Inspectors compare the plume darkness to chart shades to report compliance with visible emission standards. Step-by-Step Solution: Identify the instrument: Ringelmann chart (visual opacity reference).Define the parameter: visible smoke density/opacity from a chimney.Select the matching option. Verification / Alternative check: Regulatory visible emission limits are often expressed as Ringelmann equivalent (e.g., “not to exceed Ringelmann 2 for more than X minutes per hour”). Why Other Options Are Wrong: Combustibles in auto exhaust, gas density, temperature: Not measured by a visual greyscale. Particulate mass directly: Requires isokinetic sampling; Ringelmann is only an opacity estimate. Common Pitfalls: Assuming Ringelmann numbers equal particulate mass; relationships are site- and fuel-specific and not directly interchangeable. Final Answer: Smoke density from a chimney

Question 12

Active atmospheric layer for weather
Which region of the atmosphere is the most meteorologically active, where rain, storms, and lightning predominantly occur?

Accepted Answer

Introduction / Context: Weather phenomena such as cloud formation, precipitation, and thunderstorms are governed by convection, moisture, and temperature gradients. Identifying the atmospheric layer where these drivers are strongest is fundamental to environmental engineering and meteorology. Given Data / Assumptions: Standard vertical structure: troposphere near the surface, stratosphere above, then mesosphere and thermosphere. Most water vapour is concentrated near the surface. Temperature generally decreases with altitude in the troposphere, enabling convection. Concept / Approach: The troposphere contains the bulk of atmospheric mass and virtually all water vapour. Its temperature lapse rate fosters buoyant convection, cloud growth, and storms. Higher layers (stratosphere and beyond) are more stable; the stratosphere has a temperature inversion due to ozone absorption, which suppresses vertical mixing, thus limiting weather. Step-by-Step Solution: Recall vertical layering and water vapour distribution.Recognise convection and instability in the troposphere.Conclude: Troposphere is the active weather layer. Verification / Alternative check: Observational meteorology confirms that cloud systems and storms are almost entirely tropospheric, with tops sometimes reaching the tropopause. Why Other Options Are Wrong: Stratosphere/Thermosphere/Mesosphere: Not the principal layers for rain and thunderstorms; vertical mixing is limited compared with the troposphere. None of these: Incorrect; troposphere is correct. Common Pitfalls: Confusing the ozone-rich stratosphere with the weather layer; stratospheric processes are important but not where routine weather occurs. Final Answer: Troposphere

Question 13

Lower atmosphere mass distribution
Because gas density decreases with altitude, about 70% of the total atmospheric mass is found in the lowest ~5 km. This lower region is known as the:

Accepted Answer

Introduction / Context: Atmospheric mass is concentrated near Earth’s surface because pressure and density decrease rapidly with height. Understanding naming and properties of layers is essential for dispersion modelling and environmental assessments. Given Data / Assumptions: Approximate statement: a significant majority of atmospheric mass is in the lowest several kilometres. We need the correct name for this layer. Concept / Approach: The lowest layer of the atmosphere is the troposphere. It extends from the surface to the tropopause (height varies with latitude and season). Most mass and almost all weather reside here due to gravitational compression and strong mixing. Step-by-Step Solution: Associate “lowest few kilometres” with the troposphere.Recall that stratosphere lies above the tropopause; ionosphere is a high-altitude ionised region.Select: Troposphere. Verification / Alternative check: Standard atmospheric models attribute the majority of mass to the troposphere; pressure halves approximately every 5.5 km, demonstrating rapid mass decline with altitude. Why Other Options Are Wrong: Ionosphere/Exosphere: Far above the bulk mass region. Stratosphere: Begins above the troposphere; contains much less mass than the troposphere. None of these: Incorrect because “troposphere” is correct. Common Pitfalls: Mixing up layer names or assuming uniform density with height; the atmosphere is highly stratified. Final Answer: Troposphere

Question 14

Natural sources of air pollution Which of the following is not a natural source of air pollution (i.e., the odd one out)?

Accepted Answer

Introduction / Context: Air pollution has both natural and anthropogenic sources. Recognising natural contributors is important for background level estimation, air quality modelling, and setting realistic regulatory targets. The task is to identify which listed item is not natural. Given Data / Assumptions: We assess each option for natural origin. Processes such as volcanoes, lightning, biogenic emissions, and sunlight-driven reactions can occur without human activity. Concept / Approach: Volcanic eruptions emit gases and particulates naturally. Lightning fixes nitrogen and forms oxidants episodically. Biological decay releases methane, VOCs, and other compounds. Photochemical oxidation of organic matter occurs when sunlight drives reactions among naturally emitted VOCs and background oxidants. Therefore, all listed items are indeed natural sources or processes. The correct answer is “None of these,” meaning none is exclusively non-natural. Step-by-Step Solution: Evaluate each listed process for a natural mechanism.Confirm that volcanoes, lightning, decay, and photochemical reactions all occur naturally.Conclude that all are natural; choose “None of these.” Verification / Alternative check: Baseline atmospheric measurements in remote regions show natural contributions from sea spray, dust, biogenic VOCs, volcanic sulphur, and lightning-produced nitrogen oxides, even in the absence of human activity. Why Other Options Are Wrong: Each individual option (a), (b), (c), and (e) describes a process that naturally emits pollutants or precursors. Common Pitfalls: Assuming “photochemical oxidation” is strictly anthropogenic; in reality, biogenic VOCs undergo sunlight-driven chemistry naturally, though human emissions can intensify it. Final Answer: None of these

Question 15

The average thickness of the ozone layer in the stratosphere is approximately 300 Dobson Units (DU).

Accepted Answer

Introduction / Context: The ozone layer in the stratosphere absorbs harmful ultraviolet radiation and is commonly quantified using Dobson Units (DU). A global, long-term climatological average is often cited near 300 DU, though it varies with latitude and season. Given Data / Assumptions: We are asked for the typical average stratospheric ozone column. Units are in Dobson Units (DU). Accept reasonable global mean around 300 DU. Concept / Approach: 1 DU corresponds to a 0.01 mm thick layer of pure ozone at standard temperature and pressure. Climatological studies place the global mean near 300 DU, higher at mid to high latitudes and lower near the tropics. Step-by-Step Solution: 1) Identify what DU measures: integrated ozone in the atmospheric column.2) Recall the accepted global average: about 300 DU.3) Select the matching option. Verification / Alternative check: Satellite and ground-based data (e.g., Dobson spectrophotometers) routinely report climatological means near 300 DU, with normal variability ±50–100 DU by region/season. Why Other Options Are Wrong: 150 DU: too low for a global average; might occur in extreme depletion events.230 DU: low compared to global mean; occurs in ozone hole conditions.750 DU and 1500 DU: unrealistically high for Earth’s stratosphere. Common Pitfalls: Confusing local episodic values with global means; misinterpreting DU as a height or pressure level rather than a column amount. Final Answer: 300 DU

Question 16

Disinfection of water is done to remove disease-causing microorganisms (pathogenic bacteria) rather than color, taste, or odor issues.

Accepted Answer

Introduction / Context: Disinfection is a targeted step in drinking water treatment that aims to inactivate or kill pathogenic microorganisms. Aesthetics such as color, taste, and odour are addressed by other treatment stages (coagulation, filtration, activated carbon). Given Data / Assumptions: Treated water must be microbiologically safe. Disinfection focuses on bacteria, viruses, and protozoa. Common agents: chlorine, chloramines, ozone, UV. Concept / Approach: Disinfection achieves pathogen log-reduction. It does not primarily remove color/taste/odour, which are addressed earlier or by adsorptive processes. Step-by-Step Solution: 1) Define objective: protect public health by removing pathogens.2) Identify appropriate disinfectant and dose for required log inactivation.3) Provide adequate contact time (CT = concentration * time) for effectiveness.4) Maintain residual (for chlorine/chloramine) in distribution to prevent regrowth. Verification / Alternative check: Microbiological tests (e.g., total coliform absence) verify disinfection; aesthetic fixes alone cannot ensure safety. Why Other Options Are Wrong: Color-causing compounds: addressed by coagulation/filtration/adsorption, not disinfection.Bad taste: often tackled by activated carbon or aeration.Foul odour: likewise treated by adsorption/aeration, not pathogen kill.Dissolved minerals: require softening/ion exchange, not disinfection. Common Pitfalls: Assuming chlorine will fix taste/odour/minerals; underestimating required CT and residual maintenance; ignoring by-product control (THMs, HAAs). Final Answer: Bacteria (pathogens)

Question 17

Threshold Limit Value (TLV) denotes the maximum acceptable concentration for workplace exposure; the TLV (ceiling) for phosgene in air is about 0.1 ppm (parts per million).

Accepted Answer

Introduction / Context: Phosgene (COCl2) is a highly toxic industrial gas. Occupational exposure guidelines define maximum allowable concentrations to protect worker health. TLV values are typically expressed as TWA (time-weighted average), STEL (short-term exposure limit), or ceiling limits that should never be exceeded. Given Data / Assumptions: We are interested in an accepted, practical TLV for phosgene. Many standards cite a very low ceiling value around 0.1 ppm due to high toxicity. Concept / Approach: Relate health risk to dose: even brief exposures to phosgene can cause severe respiratory injury. Therefore, limits are set extremely low, and a ceiling (instantaneous) cap is appropriate for control. Step-by-Step Solution: 1) Identify phosgene as a pulmonary irritant causing delayed-onset edema.2) Recall standard occupational guidance: limit near 0.1 ppm as a ceiling.3) Choose the option that matches 0.1 ppm. Verification / Alternative check: Cross-reference safety datasheets and occupational hygiene references that list a 0.1 ppm ceiling; different jurisdictions may use closely related values, but 0.1 ppm is widely recognized. Why Other Options Are Wrong: 0.002 ppm and 0.05 ppm: more conservative than typical regulatory or consensus TLVs.0.2 ppm: double the recognized ceiling; risk of underprotection.1.2 ppm: orders of magnitude too high. Common Pitfalls: Confusing TWA with ceiling limits; ignoring the need for immediate alarms and leak detection for phosgene. Final Answer: 0.1 ppm

Question 18

Photochemical smog is generated from automobile exhaust when hydrocarbons and nitrogen oxides react in the presence of strong sunlight; it typically appears on sunny days and harms crops, trees, and human health (eye irritation, asthma).

Accepted Answer

Introduction / Context: Photochemical smog is a secondary air pollution phenomenon found in urban areas with heavy traffic and strong solar radiation. It involves sunlight-driven reactions of precursors from vehicle exhaust. Given Data / Assumptions: Primary precursors: hydrocarbons (HC/VOCs) and nitrogen oxides (NOx). Intense sunlight drives reactions. Products include ozone (O3), PANs, and oxidants. Concept / Approach: Sunlight catalyzes reactions of NOx and VOCs to form oxidants; these oxidants cause health and vegetation damage. Step-by-Step Solution: 1) Vehicle exhaust emits NO, NO2, and hydrocarbons.2) Under sunlight, NO2 photolyzes, generating O atoms.3) O combines with O2 to form O3; VOC radicals sustain NO2 regeneration and ozone accumulation.4) Resulting oxidants cause eye irritation, asthma exacerbation, and plant damage. Verification / Alternative check: Observationally strongest on clear, sunny, stagnant days; declines with reduced NOx/VOC emissions and after sunset. Why Other Options Are Wrong: Formed by HC and NO in sunlight: correct.Appears only on sunny days: correct in practical terms—sunlight is required.Harmful to vegetation and causes irritation: correct.Mainly a winter fog phenomenon: incorrect—classical photochemical smog is summer/sunny-day related. Common Pitfalls: Confusing photochemical smog with reducing (London) smog; assuming fog is necessary; overlooking VOC controls. Final Answer: All of the above

Question 19

Fluorosis (dental and skeletal) is caused by an excessive concentration of fluoride ions in drinking water.

Accepted Answer

Introduction / Context: Fluorosis is a public health issue arising from prolonged ingestion of high levels of fluoride in drinking water. It affects teeth (dental fluorosis) and bones (skeletal fluorosis). Given Data / Assumptions: Problem concerns the specific contaminant responsible for fluorosis. Other heavy metals cause different toxicities. Concept / Approach: Associate each contaminant with its characteristic health effect and identify which one uniquely causes fluorosis. Step-by-Step Solution: 1) Define fluorosis: mottling of enamel, joint pain, bone deformities at higher exposure.2) Link cause: chronic intake of excess fluoride (above guideline levels).3) Conclude contaminant: fluoride ions in drinking water. Verification / Alternative check: Public health guidelines set fluoride limits; areas with geogenic fluoride show higher fluorosis prevalence. Why Other Options Are Wrong: Mercury: neurotoxicity, kidney damage; not fluorosis.Lead: neurodevelopmental toxicity; anemia; not fluorosis.Arsenic: skin lesions, cancers; not fluorosis.Iron: aesthetic/operational issues (staining); not fluorosis. Common Pitfalls: Confusing fluoride with arsenic issues; assuming any heavy metal causes fluorosis; ignoring exposure duration. Final Answer: Fluoride

Question 20

In rapid sand filtration, the typical water filtration rate ranges around 10 to 15 kilolitres per square metre per hour (equivalent to 10–15 m/h).

Accepted Answer

Introduction / Context: Rapid sand filters (RSF) are widely used in municipal plants for turbidity removal after coagulation–flocculation and sedimentation. Their hallmark is a comparatively high filtration rate versus slow sand filters. Given Data / Assumptions: Medium: sand (effective size typically 0.45–0.7 mm). Operation: periodic backwashing restores headloss. Units: 1 kL/m2/hr equals 1 m/h. Concept / Approach: Standard design guidance places RSF rates roughly in the 5–15 m/h band; many plants adopt 10–15 m/h for balanced performance and run time between backwashes. Step-by-Step Solution: 1) Recognize RSF are designed for higher throughput than slow sand filters.2) Convert units: kL/m2/hr is numerically equal to m/h.3) Select the band aligning with common practice: 10–15 kL/m2/hr. Verification / Alternative check: Check headloss buildup and effluent turbidity; design manuals cite similar ranges for conventional RSF. Why Other Options Are Wrong: 0.1–1: too low; typical of slow sand filters.3–6: low for RSF; used in some specialized media.15–20 or 25–40: too high for standard RSF; risk turbidity breakthrough. Common Pitfalls: Confusing RSF with slow sand rates; ignoring unit equivalence; overdriving filters causing short runs. Final Answer: 10 to 15 kL/m2/hr

Environmental Engineering Questions