Question 1

Airborne particulates terminology — what are submicron particles called? Particles smaller than 1 micron that can remain suspended in air for very long periods and be transported by wind currents are known as:

Accepted Answer

Introduction / Context: Air pollution control relies on correct classification of particulates because control devices target different size ranges. Understanding terms like fumes, mists, smoke, and aerosols helps in selecting proper filtration and electrostatic precipitation methods. Given Data / Assumptions: Particle size in question is less than 1 micron. These particles can stay airborne for a long time due to low settling velocity. Transport by wind indicates minimal gravitational settling. Concept / Approach: “Aerosol” is a generic term for a suspension of fine solid particles or liquid droplets in a gas. Submicron particles exhibit very low terminal velocities, remain suspended, and travel with airflow. While “smoke” typically refers to combustion-generated fine solids, and “fumes” are often condensed metal oxides from high-temperature processes, “aerosols” encompasses both solids and liquids and captures the key behavior highlighted here: persistent suspension and long-range transport. Step-by-Step Solution: Map the description (submicron, long suspension) to terminology.Recognize “aerosols” as the broad, accurate term for such particles.Select “Aerosols” as the best answer. Verification / Alternative check: Environmental engineering references define aerosols by size-dependent atmospheric residence time and transport characteristics consistent with the prompt. Why Other Options Are Wrong: Fumes: often specific to condensed vapors (e.g., metal fumes), not the most general term. Mists: generally liquid droplets; the question does not specify liquids. Smoke: subset related to combustion; “aerosols” is broader and more accurate here. Common Pitfalls: Assuming “smoke” for all fine particles; smoke excludes many non-combustion aerosols. Final Answer: Aerosols

Question 2

Water quality index — what does turbidity indicate? Turbidity in a water sample is primarily an indication of the presence of:

Accepted Answer

Introduction / Context: Turbidity is a routine water-quality parameter measured to assess the clarity of water. Elevated turbidity can impact filtration, disinfection efficiency, and aesthetic quality. Interpreting turbidity correctly is essential for treatment design and monitoring. Given Data / Assumptions: Turbidity measured by nephelometric methods correlates with light scattering. Typical contributors include clay, silt, organic colloids, and microorganisms. Dissolved substances do not generally scatter light strongly enough to cause turbidity. Concept / Approach: Turbidity arises from suspended and colloidal particles that scatter light. Inorganic fines (clays, silts) and organic colloids increase scattering, resulting in higher NTU values. Dissolved solids pass through filters and usually do not cause turbidity, while floating solids (large debris) are often removed by screening and are not the main driver of turbidity readings. Step-by-Step Solution: Identify what scatters light: suspended/colloidal particles.Link elevated NTU to fine solids rather than purely dissolved species.Conclude that turbidity primarily indicates suspended inorganic matter (and colloids). Verification / Alternative check: Jar tests and filtration reduce turbidity by removing particulate matter; conductivity or TDS measurements, not turbidity, track dissolved ions. Why Other Options Are Wrong: Dissolved solids: can make water hard or saline but do not significantly increase turbidity. Floating solids: too large; turbidity meters are sensitive to fine particles. Dissolved gases: have negligible effect on turbidity. Common Pitfalls: Confusing color (from dissolved organics) with turbidity; color is different from light-scattering turbidity. Final Answer: Suspended inorganic matter and colloids

Question 3

Industrial hygiene — threshold limit values (TLV) of highly toxic gases What is the typical TLV (time-weighted average) in air for ozone (O3) and for phosgene (COCl2)?

Accepted Answer

Introduction / Context: TLV values guide safe workplace exposures to toxic gases. Ozone and phosgene are both highly hazardous even at very low concentrations, so engineers must recognize the correct order of magnitude when designing monitoring and ventilation systems. Given Data / Assumptions: Common TLV figures used in safety training for industrial environments. Values are order-of-magnitude indicators; exact limits may vary by standard-setting body. Both gases are toxic/irritant at low ppm levels. Concept / Approach: Ozone is a powerful oxidant with TLVs in the fraction of a ppm. Phosgene is a severe pulmonary irritant (choking gas) historically used as a chemical warfare agent; its occupational limits are likewise very low. Among the options, 0.1 ppm correctly reflects the low permissible exposure levels for both gases. Step-by-Step Solution: Recall that both O3 and COCl2 have limits far below 1 ppm due to acute toxicity.Compare with options: 25, 100, and 1000 ppm are orders of magnitude too high for these gases.Select 0.1 ppm as the appropriate TLV order. Verification / Alternative check: Safety datasheets and industrial hygiene references list TLV/PEL values in tenths of ppm or less for these agents. Why Other Options Are Wrong: 25, 100, 1000 ppm: These would represent dangerously high exposures for O3 or phosgene and are not acceptable workplace limits. Common Pitfalls: Confusing alarm setpoints (which may be even lower) with TLVs. Final Answer: 0.1 ppm

Question 4

Valve noise in flow control — dominant aerodynamic source In control valves handling gases, the most prevalent aerodynamic noise originates primarily from which mechanisms in turbulent flow?

Accepted Answer

Introduction / Context: Noise generated by control valves can be a major occupational and environmental concern. Aerodynamic noise arises when compressible gas flows accelerate through restrictions and dissipate turbulent energy as acoustic waves. Given Data / Assumptions: Gas flow through throttling elements (orifices, trims) causes high turbulence. Mach numbers and pressure ratios may be significant, enhancing noise generation. Structural-borne noise is not the main focus here. Concept / Approach: In turbulent gas flow, fluctuating velocity fields lead to Reynolds stresses (correlations of velocity fluctuations) and intense shear layers at jets and boundaries. These turbulent structures interact with geometry and compressibility effects to radiate sound. Thus, both Reynolds-stress fluctuations and shear forces are principal contributors to aerodynamic valve noise. Step-by-Step Solution: Recognize noise source: conversion of turbulent kinetic energy to acoustic energy.Identify mechanisms: Reynolds stress fluctuations and shear layers near the vena contracta and downstream jets.Conclude that both mechanisms contribute significantly. Verification / Alternative check: Empirical noise-prediction correlations for control valves include terms linked to turbulence intensity and shear, aligning with this explanation. Why Other Options Are Wrong: Only one mechanism: incomplete representation of aerodynamic noise generation. Neither: contradicts established aeroacoustic theory. Common Pitfalls: Attributing noise solely to mechanical vibration; aerodynamic sources dominate in high-speed gas throttling. Final Answer: Both Reynolds stresses and shear forces

Question 5

Electrostatic precipitators (ESP) — typical ionization voltage In industrial ESPs used for particulate control, the typical ionization (applied) voltage level is on the order of:

Accepted Answer

Introduction / Context: Electrostatic precipitators remove fine particles by charging them in a high-voltage field and collecting them on oppositely charged plates. The selection of voltage magnitude and polarity is vital for corona formation and charging efficiency. Given Data / Assumptions: Industrial-scale ESP with rigid or wire discharge electrodes. Standard power supplies with transformer–rectifier sets. Gas composition and particulate load typical of boilers or process furnaces. Concept / Approach: Stable corona discharge for particle charging is achieved using high-voltage direct current in the tens of kilovolts range. Typical setpoints are 30–70 kV DC depending on gas properties and spacing. AC would not maintain a steady unidirectional field for effective charging and collection, and household-level voltages are far too low to sustain corona in industrial gaps. Step-by-Step Solution: Identify the need for a steady electric field and corona: favors DC.Recall typical industrial ranges: tens of kilovolts.Select 30–70 kV DC as the appropriate magnitude and type. Verification / Alternative check: Manufacturer datasheets and commissioning records list transformer–rectifier outputs in this DC range. Why Other Options Are Wrong: AC at 30–70 kV: not standard for ESP corona fields. 230 V AC/DC: insufficient for corona over industrial electrode gaps. Common Pitfalls: Assuming higher frequency AC would suffice; ESPs conventionally use rectified DC for stable charging. Final Answer: 30 to 70 kV DC

Question 6

Refinery wastewater treatment — removal of free oil In petroleum refinery effluents, oil is present in both free and emulsified forms. Which unit process is used specifically to remove free oil from wastewater?

Accepted Answer

Introduction / Context: Refinery wastewater typically contains oil in free, dispersed, and emulsified states. Treatment trains are staged so that free oil is removed early to prevent interference with downstream biological or physicochemical processes. Given Data / Assumptions: Influent contains separable free oil droplets larger than typical emulsion droplet sizes. Gravity separation is feasible due to density difference between oil and water. Subsequent steps may include DAF, API separators, and biological treatment. Concept / Approach: Free oil separates by gravity because its droplet size is large enough for buoyant rise at practical residence times. API or parallel-plate separators create quiescent zones and short settling paths; skimmers collect the oil layer. Biological units (oxidation ponds, lagoons, trickling filters) are not designed for primary free-oil removal and can be upset by oil carryover. Step-by-Step Solution: Classify oil: free vs. emulsified.Match mechanism: gravity separation for free oil; coalescers or chemical treatment for emulsions.Select “gravity separator with skimming” as the proper primary unit. Verification / Alternative check: Standard refinery wastewater flow diagrams show API separators upstream of biological treatment precisely for free oil removal. Why Other Options Are Wrong: Oxidation pond / aerated lagoon / trickling filter: biological systems targeting dissolved and colloidal organics, not primary oil separation. Common Pitfalls: Confusing dispersed/emulsified oil (needing DAF or emulsion breaking) with free oil removable by simple gravity. Final Answer: Gravity separator with oil skimming devices

Question 7

Environmental health – Fluorosis due to air contaminants: Fluorosis is a bone disease observed in communities exposed to high concentrations of which contaminant in the ambient air?

Accepted Answer

Introduction / Context: Fluorosis is a public-health problem linked to excessive fluoride exposure. While groundwater with high fluoride is a common source, certain industrial emissions also contribute. Understanding which air pollutant causes skeletal and dental fluorosis is essential in environmental engineering and occupational hygiene. Given Data / Assumptions: The question concerns atmospheric exposure pathways. Bone disease (skeletal fluorosis) is explicitly referenced. Candidate pollutants include acidic gases and typical urban/industrial species. Concept / Approach: Fluoride ions accumulate in calcified tissues. In air, the relevant species is gaseous hydrogen fluoride (HF) or particulate fluorides emitted from aluminum smelters, phosphate processing, brick/ceramic kilns, and glass manufacture. Chronic exposure can lead to dental mottling progressing to skeletal fluorosis marked by bone thickening and joint stiffness. Step-by-Step Solution: Identify pollutant that contains fluorine capable of yielding fluoride ions in the body: hydrogen fluoride.Link exposure to health endpoint: HF inhalation leads to systemic fluoride burden and fluorosis with chronic exposure.Exclude other listed pollutants whose toxicology does not include fluorosis. Verification / Alternative check: Industrial hygiene references list fluoride compounds (HF, particulate fluorides) as causes of fluorosis; NO2 and H2S cause respiratory and neurological effects respectively, not fluorosis. Why Other Options Are Wrong: Nitrogen dioxide: primarily causes airway inflammation and contributes to photochemical smog; no fluorosis. Hydrogen sulfide: neurotoxic at high doses; no link to fluoride-related bone changes. Unburned hydrocarbons: participate in smog formation; not fluoride. Sulfur dioxide: respiratory irritant and acid rain precursor; does not cause fluorosis. Common Pitfalls: Confusing HF with HCl or SO2 because they are all acid gases; only HF provides fluoride ions that accumulate in bone. Final Answer: Hydrogen fluoride (HF)

Question 8

Air quality standards – Threshold Limit Value (TLV) for sulfur dioxide: What is the commonly cited safe concentration (TLV) for SO2 in workplace or ambient air, expressed in parts per million (ppm)?

Accepted Answer

Introduction / Context: Threshold Limit Values (TLVs) and similar guideline values set safe upper bounds for pollutant concentrations to protect health. Sulfur dioxide (SO2) is a pungent, water-soluble gas arising from combustion of sulfur-bearing fuels and certain industrial processes. Given Data / Assumptions: A single-value multiple-choice recall of a widely taught TLV for SO2. Older exam and textbook conventions often reference a 5 ppm ceiling/short-term level in simple questions. Concept / Approach: While modern occupational limits vary by jurisdiction and can be lower, traditional engineering exam keys frequently use 5 ppm as the indicative safe upper bound for short exposures. Extremely high values like hundreds or thousands of ppm are dangerous and not representative of a TLV. Step-by-Step Solution: Compare options against typical occupational hygiene scales (single-digit ppm vs. hundreds/thousands).Select the historically taught TLV level used in many problem sets: 5 ppm. Verification / Alternative check: Contemporary standards (e.g., short-term exposure limits) often fall at or below a few ppm. Values in the hundreds or thousands indicate acute hazard conditions, not permissible limits. Why Other Options Are Wrong: 500/1000/2000 ppm: dangerously high; these are emergency/accident levels, not TLVs. 0.5 ppm: seen in some modern guidelines but not the classic value targeted by this question style. Common Pitfalls: Confusing historic exam values with the latest jurisdiction-specific standards; always check the context and syllabus year. Final Answer: 5 ppm

Question 9

Industrial hygiene – Maximum safe working concentration of carbon dioxide: What is the maximum allowable concentration of CO2 for safe working conditions, expressed in ppm?

Accepted Answer

Introduction / Context: Carbon dioxide (CO2) is a normal constituent of air and a marker of ventilation adequacy in occupied spaces. At elevated concentrations it can cause headaches, drowsiness, and impaired cognitive function; very high levels can be dangerous. Given Data / Assumptions: The question asks for a standard, widely taught safe working limit. Traditional engineering exams cite 5000 ppm (0.5% by volume) as an 8-hour time-weighted average for occupational exposure. Concept / Approach: Different organizations publish guidance, but 5000 ppm is the conventional classroom value for maximum allowable workplace concentration over a full shift. Lower thresholds (e.g., 1000 ppm) are comfort/ventilation targets, not occupational exposure limits. Step-by-Step Solution: Eliminate very low values used for indoor air quality comfort (50–1000 ppm).Recognize 5000 ppm as the widely referenced occupational limit.Exclude 10,000 ppm as a short-term/upper bound value not used as the primary safe working limit. Verification / Alternative check: Occupational hygiene texts commonly list 0.5% CO2 as the accepted full-shift limit; values beyond that are used for short-duration exposure guidelines only. Why Other Options Are Wrong: 50/1000/2000 ppm: indicate good ventilation benchmarks, not legal exposure maxima. 10,000 ppm: too high for an 8-hour general limit. Common Pitfalls: Confusing indoor air comfort targets with occupational exposure limits; mixing up ppm and percent units. Final Answer: 5000 ppm

Question 10

Health effects of automobile exhaust – Leaded petrol: Inhalation of lead compounds emitted from vehicles using leaded gasoline can cause which of the following health effects?

Accepted Answer

Introduction / Context: Leaded gasoline historically introduced inorganic lead into urban air via exhaust. Although phased out in many countries, understanding the health impacts remains crucial for environmental health education and legacy site management. Given Data / Assumptions: Exposure pathway: inhalation of airborne lead compounds. Health endpoints: hematologic, neurologic, and systemic toxicity are recognized for lead. Concept / Approach: Lead interferes with heme synthesis causing anaemia, accumulates in bone, and is neurotoxic. Chronic exposure leads to cognitive deficits, peripheral neuropathy, and in severe cases encephalopathy. The umbrella term plumbism encompasses systemic lead poisoning, including gastrointestinal and renal effects. Step-by-Step Solution: Map toxicological pathways to symptoms: heme pathway inhibition → anaemia; neuronal disruption → neurological disorders; systemic toxicity → blood poisoning.Recognize that all listed effects are established outcomes of lead exposure. Verification / Alternative check: Epidemiological studies show reduced IQ and neurobehavioral changes in children with elevated blood lead; workers exposed to lead fumes/dust present anaemia and neuropathy. Why Other Options Are Wrong: Only respiratory irritation: lead’s primary chronic harms are systemic; simple irritation understates risk. Common Pitfalls: Assuming only children are at risk; adults also suffer hematologic and neurologic effects. Confusing short-term irritation with chronic systemic toxicity. Final Answer: All of the above

Question 11

Particulate control devices – Working principle of a cyclone separator: The operating principle of a cyclone dust collector is primarily based on which action exerted on the particles?

Accepted Answer

Introduction / Context: Cyclone separators are rugged, low-maintenance devices used to remove medium-to-coarse particles from gas streams in boilers, kilns, and process exhausts. Knowing their governing mechanism guides selection and performance prediction. Given Data / Assumptions: Swirling gas flow is generated tangentially in a conical/cylindrical body. Particles have inertia relative to the carrier gas. Concept / Approach: As gas swirls, particles experience an outward radial force proportional to mass, tangential velocity squared, and inversely to radius (centrifugal force). Particles migrate to the wall, lose momentum, and drop into a hopper, while cleaned gas exits near the centerline. Step-by-Step Solution: Induce tangential entry → create vortex flow.Centrifugal force drives particles radially outward to the wall.Axial flow carries the gas upward through a vortex finder; particles fall by gravity into the dust leg. Verification / Alternative check: Empirical grade-efficiency curves show improved capture with higher inlet velocity and larger particle size, consistent with centrifugal separation physics. Why Other Options Are Wrong: Diffusion/Brownian motion: dominate for ultrafines, not the cyclone’s main mechanism. Gravitational settling only: gravity helps once at wall, but the key separation is centrifugal. Electrostatic attraction: principle of ESPs, not cyclones. Common Pitfalls: Using cyclones for submicron particles expecting high efficiency—electrostatic precipitators or fabric filters are better for very fine dust. Final Answer: Centrifugal force

Question 12

Coal washing wastewater treatment: Coal washeries discharge wastewater with about 3% suspended solids (clay, slate, stone). Which primary unit is most appropriate for removing these solids at scale?

Accepted Answer

Introduction / Context: Coal washing produces large volumes of slurry containing dense mineral particles and clays. Designing cost-effective primary treatment requires distinguishing bulk solids separation (settling/thickening) from polishing or dewatering steps. Given Data / Assumptions: Suspended solids ≈ 3% by mass, dominated by mineral fines. High flow rates common in washeries. Goal: primary removal and thickening for recycle/disposal. Concept / Approach: Gravity settling in circular thickeners or sedimentation tanks with rotating scrapers is the industry standard to concentrate underflow to high solids, while producing overflow for further treatment or reuse. Scrapers move settled sludge to a central sump for pumping, ensuring continuous operation. Step-by-Step Solution: Select a robust, continuous primary separator: a thickener/clarifier with scrapers.Mechanics: hindered settling followed by sludge consolidation; rake mechanism transports solids.Downstream steps (e.g., vacuum filters) can further dewater the thickener underflow if needed. Verification / Alternative check: Process flowsheets from washeries show thickeners as the mainstay for bulk solids removal, with flocculant addition when fines are high to improve settling. Why Other Options Are Wrong: Chemical coagulation as sole step: often used as an aid, but not a standalone substitute for the primary gravity unit. Vacuum filtration first: uneconomical for dilute slurries; used after thickening. Clarifiers without scrapers: impractical at these loadings; sludge removal is essential. Membrane ultrafiltration: cost-prohibitive and prone to fouling for 3% solids. Common Pitfalls: Sending dilute slurry directly to filters; ignoring need for rake torque and underflow density control in thickener design. Final Answer: Sedimentation tanks with mechanical scrapers (thickeners)

Question 13

Sewage stabilization – Definition of relative stability: The ratio of oxygen available to the oxygen required for stabilization of sewage is known as what?

Accepted Answer

Introduction / Context: Sewage stabilization reflects the degree to which biodegradable organics have been oxidized. Classical sanitary engineering introduced the concept of relative stability to quickly assess whether the oxygen present (or supplied) is sufficient to meet the biochemical demand. Given Data / Assumptions: Aerobic stabilization is considered. Oxygen available and oxygen required are compared as a ratio. Concept / Approach: Relative stability = oxygen available / oxygen required. If this value approaches 1 (or 100%), the sewage is considered stable, meaning minimal further oxygen demand remains. This is a different concept from BOD, which measures oxygen required; relative stability compares requirement to availability. Step-by-Step Solution: Identify that the ratio (available/required) is being asked.Recall the term used historically in wastewater texts: relative stability. Verification / Alternative check: Old-standard assays (e.g., 5-day BOD alongside relative stability tests) collectively indicate oxygen balance in receiving waters. Why Other Options Are Wrong: BOD/COD: measure oxygen demand of organics; not a ratio to available oxygen. Bacterial stability factor/oxygen ion concentration: non-standard or incorrect terminology. Common Pitfalls: Conflating BOD (demand) with stability (balance of supply versus demand). Final Answer: Relative stability

Question 14

Boiler water treatment – Controlling scale formation: Which measure most directly reduces scale formation in boilers supplied by natural waters?

Accepted Answer

Introduction / Context: Scale forms when sparingly soluble salts (e.g., CaCO3, CaSO4, Mg(OH)2) and silica precipitate on heat-transfer surfaces, increasing fuel use and causing overheating. The cornerstone of scale control is managing the constituents that form scale. Given Data / Assumptions: Natural feedwaters often contain calcium, magnesium, silica, and suspended alumina-bearing clays. Boiler conditions promote precipitation. Concept / Approach: Softening (lime-soda, zeolite, ion exchange) and silica reduction (lime softening, magnesium dosing, demineralization) remove scale formers before the boiler. Proper alkalinity and phosphate programs in high-pressure units further condition residual hardness to produce non-adherent sludge for blowdown. Step-by-Step Solution: Target the root cause: remove hardness and silica to reduce deposit precursors.Maintain appropriate internal treatment and blowdown to control any remaining solids. Verification / Alternative check: Boiler performance data show lower heat-flux penalties and tube metal temperatures when hardness and silica are minimized. Why Other Options Are Wrong: Preheating alone: aids deaeration and some precipitation but insufficient by itself. pH just below 7: incorrect; boilers typically operate alkaline (pH > 8.5) to prevent acid corrosion. Eliminating H2S: addresses odor/corrosion, not principal scale formation. Adding oxygen: increases corrosion risk. Common Pitfalls: Overlooking silica’s role in high-pressure units; relying solely on blowdown instead of proper pretreatment. Final Answer: Reducing hardness, silica, and alumina in the feedwater

Question 15

Atmospheric layers and pollution distribution: In which layer of the atmosphere are most air pollutants found in the highest concentrations near the Earth's surface?

Accepted Answer

Introduction / Context: Understanding where pollutants accumulate helps in modeling dispersion and designing air-quality control strategies. The atmosphere is divided into layers based on temperature gradients and composition. Given Data / Assumptions: Emission sources (traffic, industry, biomass burning) are surface-based. Weather phenomena (winds, mixing, precipitation) that disperse pollutants occur in the lowest layer. Concept / Approach: The troposphere extends from the surface to about 8–15 km and contains most of the atmospheric mass and water vapor. Turbulence, boundary layer dynamics, and temperature inversions control pollutant concentrations. Consequently, primary pollutants and secondary smog constituents largely reside and react within the troposphere. Step-by-Step Solution: Identify the layer with active weather and turbulence: troposphere.Associate surface emissions and limited vertical mixing with higher near-ground concentrations. Verification / Alternative check: Air quality monitoring networks measure pollutants at ground level within the planetary boundary layer, a sub-part of the troposphere. Why Other Options Are Wrong: Stratosphere: contains ozone layer; far fewer anthropogenic primary pollutants. Mesosphere/Thermosphere/Exosphere: extremely thin air; pollutant concentrations are negligible relative to the troposphere. Common Pitfalls: Confusing stratospheric ozone chemistry with urban tropospheric pollution. Final Answer: Troposphere

Question 16

Self-reoxygenation in polluted streams: In a watercourse receiving industrial wastewater, how is dissolved oxygen (DO) naturally replenished?

Accepted Answer

Introduction / Context: When organic-laden effluents enter a stream, microorganisms consume dissolved oxygen, potentially creating an oxygen sag. Natural processes can counterbalance this depletion and are central to stream self-purification models used in environmental design. Given Data / Assumptions: Open stream with sunlight exposure and turbulence. Presence of phytoplankton and periphyton typical of natural waters. Concept / Approach: Two principal mechanisms restore DO: (1) Reaeration, where oxygen transfers from air to water driven by a concentration gradient, enhanced by turbulence (riffles, falls, wind). (2) Photosynthesis, where algae and aquatic macrophytes produce oxygen during daylight, increasing DO locally and diurnally. Step-by-Step Solution: Acknowledge reaeration as a continuous process determined by gas-transfer coefficients and deficit.Recognize photosynthesis as a daylight process adding oxygen and offsetting BOD.Combine both mechanisms as contributors to DO replenishment. Verification / Alternative check: Streeter–Phelps models include a reaeration term; field DO profiles show diurnal swings with midday peaks due to photosynthesis and nighttime declines when respiration dominates. Why Other Options Are Wrong: Single-mechanism answers ignore the well-established dual contribution. Nitrification: consumes oxygen rather than producing it. Common Pitfalls: Ignoring nighttime DO minima and the risk of fish kills when respiration exceeds reaeration. Final Answer: Both (a) and (b)

Question 17

In industrial gas cleaning, which adsorbent is most traditionally used to remove hydrogen sulfide (H2S) from a gaseous stream for odor control and sulfur capture?

Accepted Answer

Introduction / Context: Hydrogen sulfide (H2S) is a toxic, corrosive gas with a characteristic rotten-egg odor. In gas purification and odor abatement, choosing the right solid medium for H2S removal is critical for safety, environmental compliance, and protection of downstream equipment. Given Data / Assumptions: The feed is a gaseous stream containing H2S. Goal: remove H2S primarily by adsorption/chemisorption on a solid bed. Ambient or moderate temperatures typical of dry scrubbing are assumed. Concept / Approach: Classical “dry box” purification uses iron-oxide-impregnated media (often called bog iron or iron sponge). H2S reacts with hydrated iron oxides to form iron sulfide, which can be air-regenerated to elemental sulfur. This chemisorption route is robust, simple, and widely used for town gas, biogas polishing, and small natural-gas streams. Step-by-Step Solution: Identify removal mechanism: chemisorption of H2S on iron oxide is favored.Select medium: bog iron (iron oxide on wood chips or pellets) provides reactive sites.Reaction pathway (schematic): Fe2O3·H2O + H2S → Fe2S3 + H2O (followed by regeneration with air to form S).Confirm practicality: widely proven at scale with manageable pressure drop and regenerability. Verification / Alternative check: Activated carbon can remove H2S effectively when impregnated (e.g., with KOH or KI), but unimpregnated carbon is much less effective for sustained H2S loading. Silica gel and limestone powder are not standard dry H2S adsorbents for gas streams. Why Other Options Are Wrong: Silica gel: suited for moisture removal, poor H2S uptake. Active carbon: effective mainly when chemically impregnated; generic carbon is inferior to bog iron for H2S. Limestone powder: used in wet scrubbing (slurry) for SO2/H2S neutralization, not as a dry adsorbent. Common Pitfalls: Confusing dry chemisorption media with wet alkaline scrubbers; ignoring the need for impregnated grades when specifying activated carbon for H2S. Final Answer: bog iron (iron oxide on support)

Question 18

For wastewater treatment, which statements about aerobic biological oxidation ponds (stabilization ponds) are correct regarding pathogen removal, non-biodegradable contaminants, and chlorination?

Accepted Answer

Introduction / Context: Aerobic oxidation ponds (stabilization ponds) are low-cost, low-energy treatment units that rely on sunlight, algae, and aerobic bacteria to reduce biochemical oxygen demand (BOD) and pathogens. Understanding what they can and cannot remove informs process selection and polishing steps like disinfection. Given Data / Assumptions: Aerobic pond with adequate detention time and mixing by wind action. Target outcomes: BOD reduction and pathogen suppression. Presence of some non-biodegradable organics (e.g., alkylbenzene sulfonates, ABS). Concept / Approach: Oxidation ponds reduce BOD via aerobic microbial metabolism supported by algal oxygen production. Sunlight (UV) and long detention times help inactivate pathogens. However, persistent non-biodegradable substances pass largely untreated. Disinfection (e.g., chlorination) after pond treatment enhances pathogen removal before discharge or reuse. Step-by-Step Solution: Relate process to outcomes: aerobic conditions favor BOD removal and pathogen die-off.Account for influent composition: non-biodegradables are not significantly degraded.Add a polishing step: chlorination further reduces fecal coliforms and viruses.Conclude: all three statements (pathogen removal, poor performance on non-biodegradables, enhanced removal with chlorination) are correct. Verification / Alternative check: Performance data from maturation ponds show high log reductions for fecal indicators with sufficient retention time and subsequent chlorination, while surfactants and other refractory organics often require advanced treatments. Why Other Options Are Wrong: Choosing any single statement ignores the combined reality of capabilities (pathogen reduction) and limitations (non-biodegradables), plus the benefit of chlorination. Common Pitfalls: Overestimating pathogen kill without adequate retention; assuming oxidation ponds can remove recalcitrant contaminants; skipping disinfection prior to reuse. Final Answer: all (a), (b) & (c)

Question 19

For municipal wastewater with low biochemical oxygen demand (low-BOD), which treatment unit is generally the most economical primary option for stabilization and polishing?

Accepted Answer

Introduction / Context: When BOD levels are relatively low, capital- and energy-intensive processes may be unnecessary. Oxidation ponds provide an economical, natural-treatment alternative that uses solar energy and microbial ecology to polish effluents before discharge or reuse. Given Data / Assumptions: Influent has low BOD (e.g., partially treated municipal wastewater). Adequate land availability and favorable climate for pond operation. Objective: cost-effective polishing and pathogen reduction. Concept / Approach: Oxidation ponds provide long detention times, allowing sedimentation of fine particulates, aerobic oxidation of organics, and photolytic/UV inactivation of pathogens. For low-BOD influent, the required area decreases, making ponds particularly economical versus mechanical aeration or advanced processes. Step-by-Step Solution: Compare units: sedimentation tanks/clarifiers primarily remove settleable solids, not dissolved organics.Sludge digesters are anaerobic units for concentrated sludges, not low-BOD wastewater.Oxidation ponds address residual soluble BOD and pathogens at low cost, matching the need.Conclusion: oxidation ponds are the most economical option under the given conditions. Verification / Alternative check: Design manuals consistently recommend stabilization ponds for small communities and low-strength wastewater due to low OPEX and simplicity, provided land is available. Why Other Options Are Wrong: Sedimentation/clarifier: insufficient for dissolved BOD removal. Sludge digester: intended for sludge stabilization, not dilute wastewater. Common Pitfalls: Underestimating land requirement or seasonal performance; not providing disinfection where reuse requires pathogen limits. Final Answer: oxidation ponds

Question 20

For a cyclone separator handling dusty gas, the overall dust-collection efficiency most strongly depends on which geometric and operating factors?

Accepted Answer

Introduction / Context: Cyclone separators remove particulates by imparting a high tangential velocity, creating a centrifugal field that drives particles to the wall. Geometry and operating conditions together determine cut size and fractional efficiency curves. Given Data / Assumptions: Standard reverse-flow cyclone. Goal: maximize overall collection efficiency at a given pressure drop. Particles span a distribution of sizes and densities. Concept / Approach: Smaller body diameter increases angular velocity for a given volumetric flow, improving separation of fine particles. Higher inlet gas velocity raises centrifugal acceleration. Adequate height and proper proportions increase residence time and reduce short-circuiting, improving capture across the size spectrum. Step-by-Step Solution: Relate separation to centrifugal force: F_c ∝ m * v_tangential^2 / r.Decrease r (smaller diameter) → higher F_c at same v_tangential.Increase v_tangential (via inlet velocity) → higher F_c and better fine capture, mindful of erosion/pressure drop.Set proportions/height to avoid re-entrainment and ensure adequate residence time for particles to migrate to the wall and drop out. Verification / Alternative check: Empirical models (e.g., Stairmand, Lapple) show explicit dependencies of cut size on gas velocity and characteristic dimensions, validating the multi-factor influence on efficiency. Why Other Options Are Wrong: Any single factor alone does not guarantee high efficiency; cyclone performance is a coupled function of geometry and velocity. Common Pitfalls: Over-increasing inlet velocity causing erosion and excessive pressure drop; using oversized cyclones that reduce centrifugal acceleration and degrade fine capture. Final Answer: all (a), (b) & (c)

Environmental Engineering Questions