Question 1

Industrial mercury pollution sources:
Which industry has historically been a significant source of mercury discharge to air and water, especially where the mercury cell process was used?

Accepted Answer

Introduction / Context: Mercury is a potent neurotoxin with well-documented environmental and health impacts. Identifying principal industrial sources is key to regulatory control and remediation strategies. Given Data / Assumptions: Focus on historical and legacy discharges rather than modern closed-loop plants. Mercury cell chlor-alkali processes used mercury as a cathode in chlorine and caustic soda production. Other listed industries may emit different pollutants but are not classic mercury sources. Concept / Approach: In the mercury cell process, losses of elemental mercury could occur via fugitive emissions, brine sludge, and wastewater, leading to contamination of local environments. Many jurisdictions have phased out or retrofitted such plants, but the historical association remains strong. Step-by-Step Solution: Match pollutant to process: mercury → mercury cell chlor-alkali.Eliminate distractors: tanneries (chromium), nitric acid (NOx), phosphoric acid (fluoride, phosphogypsum), beverages (minimal pollutants).Select “Chlor-alkali industry.” Verification / Alternative check: Environmental case studies of riverine mercury hotspots often trace back to legacy chlor-alkali plants using mercury technology. Why Other Options Are Wrong: Tanneries: concerns include chromium and sulfide, not mercury.Beverage plants: sanitary wastewater, not heavy metals.Phosphoric acid: fluoride and radionuclide issues dominate.Nitric acid: NOx emissions, not mercury. Common Pitfalls: Confusing mercury with chromium or lead sources; overlooking the specific electrochemical process that used mercury metal. Final Answer: Chlor-alkali industry

Question 2

Removing oily impurities from electroplating effluents:
Which method is normally not used for routine removal of oily or greasy impurities from electroplating wastewater at typical plant scales?

Accepted Answer

Introduction / Context: Electroplating facilities may discharge wastewater containing oils and greases from cleaning and surface-prep steps. Selecting practical, cost-effective unit operations for oil removal is essential before metal precipitation and polishing treatment stages. Given Data / Assumptions: Plant handles continuous or batch wastewater flows of modest to large volume. Oils may be free, dispersed, or partially emulsified. We seek what is “normally not” used in routine practice due to cost/complexity. Concept / Approach: Common front-end oil removal methods include gravity separation (API separators), dissolved air flotation with skimming, coalescers, and chemical coagulation to break emulsions. Ultrafiltration is also widely applied for stable emulsions in metal finishing. Centrifugation can separate immiscible liquids but is uncommon for bulk wastewater oil removal because of high energy use, maintenance, and poor economics at large flow rates. Step-by-Step Solution: List standard practices: gravity separation, DAF, skimming, chemical emulsion breaking.Note advanced membrane options: ultrafiltration is viable for emulsified oil streams.Identify the atypical choice for routine wastewater service: centrifugation. Verification / Alternative check: Industry guidelines favor DAF/coalescers over centrifuges for economy and robustness; ultrafiltration is a recognized technology for oily wastewater when emulsions resist gravity methods. Why Other Options Are Wrong: Chemical coagulation: standard to destabilize emulsions.Flotation & skimming: a mainstay for oil removal.Ultrafiltration: commonly applied for stable emulsions in metal finishing.Gravity separation: basic primary step, not the “not used” option. Common Pitfalls: Assuming any mechanical separator is practical; overlooking lifecycle cost and scaling issues associated with centrifuges. Final Answer: Centrifugation

Question 3

Ambient air quality perspective:
From a pollution control viewpoint, a typical maximum permissible concentration for sulfur dioxide (SO2) in ambient air is closest to which value (order of magnitude)?

Accepted Answer

Introduction / Context: Sulfur dioxide is a regulated ambient air pollutant due to its respiratory effects and role in acid deposition. Standards vary by country and averaging time, but exam questions often test order-of-magnitude literacy for safe levels versus obviously dangerous concentrations. Given Data / Assumptions: We compare broad ppm-level choices to identify a reasonable upper bound for “permissible” ambient air concentrations. Regulatory details differ, but values in the tens to hundreds of ppm are acutely hazardous, not permissible. We seek a realistic magnitude used in legacy problem banks. Concept / Approach: Ambient SO2 standards are in the sub-ppm to low-ppm range depending on averaging time. Values like 50 or 500 ppm are far beyond ambient limits and would represent severe accidental releases. Around 1 ppm serves as a recognizable upper bound magnitude for short periods in many historical contexts, which is the intended exam-style answer among the provided options. Step-by-Step Solution: Eliminate 500 and 50 ppm as implausible for “permissible” ambient air.Between 5 ppm and 1 ppm, select the lower value as the defensible upper bound magnitude.Note that some modern standards are lower, but option sets constrain the choice. Verification / Alternative check: Comparing international standards shows typical limits well below 5 ppm; thus 1 ppm is the closest reasonable magnitude offered. Why Other Options Are Wrong: 5 ppm: too high for ambient compliance.50 or 500 ppm: emergency exposure levels, not ambient limits.0.01 ppm: may be closer to modern micro-averages but is not the keyed value in many traditional question banks. Common Pitfalls: Memorizing one jurisdiction’s exact limit and overlooking the exam’s intention to test approximate magnitude recognition. Final Answer: 1 ppm

Question 4

Primary treatment unit operations:
In municipal sewage treatment, the function of a skimming tank is to remove which category of materials from the wastewater surface before downstream processing?

Accepted Answer

Introduction / Context: Primary treatment aims to remove easily separable materials from raw sewage to protect downstream biological units. Skimming tanks (or grease traps) specifically target floatable matter that accumulates at the water surface and can foul equipment or cause odors. Given Data / Assumptions: Unit placed upstream of biological treatment and often adjacent to primary clarifiers. Influent contains soaps, fats, oils, and grease from kitchens and commercial sources. Mechanisms available include gravity separation, scum baffles, and mechanical skimmers. Concept / Approach: Oil and fats are less dense than water and tend to agglomerate and float, forming a surface scum. A skimming tank provides quiescent conditions so these materials can collect and be removed mechanically. This improves aesthetics, reduces oxygen demand, and prevents interference with aeration systems and digesters. Step-by-Step Solution: Identify the physical property exploited: density difference leading to flotation.Relate to equipment feature: baffles and skimmers that capture floating scum.Choose the option stating “Oil and fatty substances.” Verification / Alternative check: Plant process flow diagrams show grease removal stages labeled as skimming or scum removal, validating the intended function. Why Other Options Are Wrong: Dissolved solids: require processes like ion exchange or RO; not removed by skimming.Suspended mineral solids and grit: removed in primary clarifiers and grit chambers via settling, not skimming.Dissolved gases: managed by aeration/stripping, not skimming. Common Pitfalls: Confusing skimming with sedimentation; one targets floatables, the other settles heavy particles. Final Answer: Oil and fatty substances (grease and scum)

Question 5

Lead emitted from vehicles burning leaded petrol (tetraethyl lead additive):
Which health effects are most commonly associated with chronic or sub-acute exposure to airborne lead particles released by automobile exhaust?

Accepted Answer

Introduction / Context: For decades, many countries used tetraethyl lead as a petrol additive to increase the octane number. Combustion of leaded petrol emitted fine lead particulates into the atmosphere. Recognising the spectrum of health effects from such exposure is crucial for environmental health, occupational hygiene, and public policy. Given Data / Assumptions: Automobiles using leaded petrol release inorganic lead aerosols. Human exposure occurs primarily via inhalation and secondary ingestion (settled dust). Question asks for health outcomes linked with atmospheric lead exposure. Concept / Approach: Lead is a cumulative toxin that interferes with heme synthesis, neural transmission, and gastrointestinal function. Chronic exposure typically presents with neurological symptoms (irritability, depression, cognitive deficits), gastrointestinal distress (abdominal pain, nausea, diarrhea, anorexia leading to weight loss), and neuromuscular impacts (weakness, wrist/foot drop in severe cases). Therefore an inclusive option capturing neurological, muscular, and GI outcomes is correct. Step-by-Step Solution: Identify the toxicant: inorganic lead compounds in exhaust.Map organ systems affected: nervous system, GI tract, neuromuscular function, hematopoietic system.Select the option that aggregates these effects. Verification / Alternative check: Classic clinical descriptions of lead toxicity include abdominal colic, anorexia, peripheral neuropathy with muscle weakness, mood changes, and anemia, aligning with the composite choice. Why Other Options Are Wrong: Paralysis & appetite loss: plausible but incomplete alone.Nervous depression: true but too narrow.Gastritis & diarrhea: possible but not the full picture. Common Pitfalls: Assuming only hematologic effects; lead has multi-system impacts, especially on developing nervous systems of children. Final Answer: all (a), (b) and (c)

Question 6

Disinfection of water to destroy water-borne pathogens:
Which of the following is regularly used as a primary disinfectant in municipal water treatment practice?

Accepted Answer

Introduction / Context: Disinfection is the critical step that inactivates pathogenic microorganisms in drinking water. An effective disinfectant must be potent, economical, and able to maintain a residual throughout the distribution system to prevent recontamination. Given Data / Assumptions: Goal: kill bacteria, viruses, and protozoa to safe levels. Options include common chemicals with varied uses. Municipal scale and routine practice are implied. Concept / Approach: Chlorine and its derivatives (sodium hypochlorite, chloramines) are the most widely used disinfectants worldwide due to strong pathogen inactivation, formation of a measurable residual, and relatively low cost. Alkalis adjust pH but are not disinfectants. Benzene hexachloride is a pesticide, not a potable water disinfectant. Alkyl benzene sulphonate (ABS) is a synthetic detergent and surfactant, not used to disinfect municipal water. Step-by-Step Solution: Define disinfection: targeted microbial inactivation.Evaluate each option for germicidal use in potable water.Select chlorine because it provides both primary inactivation and distribution residual. Verification / Alternative check: Alternative disinfectants (ozone, UV) are also used but typically require chlorination or chloramination for residual maintenance in the network. Why Other Options Are Wrong: Alkalis: pH control only; do not reliably kill pathogens.Benzene hexachloride: agricultural insecticide; unsafe/unsuitable for drinking water disinfection.ABS: detergent causing foaming; not a disinfectant. Common Pitfalls: Confusing water softening/conditioning chemicals with disinfectants or assuming any biocide is appropriate for potable water. Final Answer: Chlorine

Question 7

Biogas from anaerobic sludge digestion:
Which gas is the principal component evolved from a sludge digestion tank operating under anaerobic conditions?

Accepted Answer

Introduction / Context: Sludge digestion stabilises sewage biosolids via anaerobic microbial pathways. The process generates a combustible gas mixture known as biogas, which can be recovered for energy. Identifying the main constituent is central to energy recovery calculations and safety considerations. Given Data / Assumptions: Process: anaerobic digestion of primary/secondary sewage sludge. Stable mesophilic or thermophilic operation. Typical digester performance with no unusual feed contaminants. Concept / Approach: Anaerobic digestion converts complex organics to methane and carbon dioxide through hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The resulting biogas typically contains about 55–70% methane (CH4) and 30–45% CO2, with traces of H2S, H2, and moisture. Methane is therefore regarded as the principal gas due to its highest energy content and fraction. Step-by-Step Solution: Recognise the process stages ending with methanogenic archaea producing CH4.Compare typical volume fractions: CH4 fraction exceeds other components.Select methane as the principal digester gas. Verification / Alternative check: Energy balances and flare/engine specifications for digesters assume methane as the main fuel component, confirming its dominance. Why Other Options Are Wrong: CO2: Significant but secondary by volume and has no calorific value.CO: Present only in trace amounts if at all.N2: Not a typical digestion product; may appear only from air ingress. Common Pitfalls: Confusing landfill gas composition with digester gas or assuming CO is a major product of biological processes (it is not). Final Answer: CH4

Question 8

Nutrients in wastewater and eutrophication:
Discharge of nitrogen and phosphorus into lakes and ponds primarily leads to which adverse outcome?

Accepted Answer

Introduction / Context: Excess nutrients in receiving waters trigger eutrophication, a process that disrupts aquatic ecosystems and water use. Understanding the dominant symptom guides nutrient removal strategies in wastewater treatment and watershed management. Given Data / Assumptions: Wastewater contains elevated nitrogen (ammonia, nitrate) and phosphorus (orthophosphate, polyphosphates). Recipient is a lentic water body (lake/pond) with longer residence times. Concept / Approach: N and P are key limiting nutrients for algal and aquatic plant growth. When their concentrations rise, biomass blooms rapidly, forming algal mats and excessive macrophyte growth. This leads to diurnal oxygen swings, hypoxia on decay, fish kills, and taste/odour episodes. Hence the most direct and defining consequence is undesirable plant (algal) growth. Step-by-Step Solution: Link nutrient input to primary productivity increase.Recognise eutrophication symptoms: algal blooms and weed proliferation.Select the option that explicitly captures this growth response. Verification / Alternative check: Lake trophic state indices track chlorophyll-a and nutrient concentrations; control measures target nutrient reduction to curb blooms. Why Other Options Are Wrong: Foaming: More associated with surfactants and industrial discharges.Odour: Can occur secondarily from decay, but growth is the primary effect.Turbidity: May increase, but it is a symptom secondary to bloom growth. Common Pitfalls: Confusing the immediate cause (nutrient enrichment) with downstream effects like hypoxia; the hallmark sign is explosive plant/algal growth. Final Answer: undesirable plant growth

Question 9

Microbial safety in water treatment:
Which process step is specifically intended to kill infective bacteria present in water supplied for public use?

Accepted Answer

Introduction / Context: Drinking water treatment trains are designed to remove particles, reduce organics, and ensure microbiological safety. It is essential to distinguish between steps that condition water and the step that specifically inactivates pathogens before distribution. Given Data / Assumptions: Typical plant includes coagulation/flocculation, sedimentation, filtration, and a final chemical or physical disinfectant step. Goal is to achieve regulatory microbial standards. Concept / Approach: Disinfection refers to the targeted inactivation of pathogens to safe levels, commonly using chlorine/chloramines, ozone, or ultraviolet irradiation. Sterilisation implies complete destruction of all forms of microbial life (including spores), which is not required or practical for municipal water. Aeration mainly strips gases and oxidises some species; it is not a reliable pathogen inactivation step. Step-by-Step Solution: Define each term by function.Match function to objective: pathogen control → disinfection.Select the process aligned with public health protection in distribution. Verification / Alternative check: Regulatory frameworks require measurable disinfectant residuals and log reductions of specified microbes, confirming disinfection as the correct step. Why Other Options Are Wrong: Sterilisation: Overly stringent and not implemented at scale for potable water.Aeration: Physical gas exchange, not assured microbiological kill.None of these: Incorrect; disinfection is standard. Common Pitfalls: Assuming filtration alone guarantees pathogen safety; filtration and disinfection act in series for robust protection. Final Answer: disinfection

Question 10

Air emissions from fertiliser manufacturing:
Which pollutant is generally absent or negligible in direct process emissions from a typical nitrogen fertiliser (ammonia/urea) plant?

Accepted Answer

Introduction / Context: Different fertiliser plants (nitrogenous vs. phosphatic) have distinct emission profiles. Identifying typical pollutants informs stack monitoring, scrubber selection, and compliance planning. Given Data / Assumptions: Plant considered is a nitrogen fertiliser complex (ammonia/urea) rather than a phosphatic acid/superphosphate unit. Focus is on process emissions rather than auxiliary boilers. Concept / Approach: Ammonia/urea plants feature synthesis loops and urea prilling/granulation. Fugitive and stack emissions often include unreacted NH3 (ammonia slip) and NOx from fired reformers. Phosphatic fertiliser and sulphuric acid units would emit SOx, but in a pure nitrogen fertiliser facility significant SO2/SO3 is not characteristic unless sulphur-based utilities are present. Carbon monoxide (CO) is typically minimal in properly operated reformers and is largely converted to CO2 via shift before synthesis; residual CO in process vents is negligible compared to other pollutants. Step-by-Step Solution: Contrast nitrogen vs. phosphatic pathways to infer pollutant species.Identify common emissions: NH3 (from vents), NOx (from combustion), particulate urea dust.Note CO is not a dominant process emission; select CO as generally absent/negligible. Verification / Alternative check: Plant permits typically list NH3 and NOx as primary controlled pollutants for urea units; CO limits are often applicable only to utility boilers rather than process. Why Other Options Are Wrong: SO2 & SO3: Can be significant for phosphatic plants; may also appear if sulphur-fired utilities exist.NO2: Present as part of NOx from fired heaters/reformers.NH3: Characteristic slip/fugitive pollutant in nitrogen fertiliser production. Common Pitfalls: Conflating boiler stack pollutants with process vents; always distinguish plant type and emission point. Final Answer: CO

Question 11

Acute toxicity threshold for phosgene (COCl2):
Approximate threshold limit value (TLV) / maximum permissible safe level for occupational exposure to phosgene gas is in which range (ppm)?

Accepted Answer

Introduction / Context: Phosgene is a highly toxic pulmonary irritant historically used as a chemical warfare agent and currently encountered in certain chemical manufacturing contexts. Knowing its very low permissible exposure limit is critical for worker safety and emergency planning. Given Data / Assumptions: Question asks for order-of-magnitude TLV in ppm. Standard 8-hour time-weighted exposure is implied; ceiling limits for phosgene are extremely low. Concept / Approach: Phosgene reacts with lung tissue, causing delayed pulmonary edema; symptoms can be insidious. Occupational limits are set well below 1 ppm, often at tenths of a ppm as ceiling values. Therefore, the correct selection is the less than 1 ppm category, reflecting the extreme toxicity and the need for rapid detection and stringent containment. Step-by-Step Solution: Recall that phosgene permissible limits are in the 0.1 ppm order.Compare offered ranges; only “<1” captures that magnitude.Select “<1” as the appropriate TLV range. Verification / Alternative check: Material safety data and regulatory tables list phosgene with very low ceiling values, typically around 0.1 ppm, confirming the selection. Why Other Options Are Wrong: 10–100 / 100–200 / 100–1000 ppm: These are orders of magnitude too high and would be life-threatening. Common Pitfalls: Underestimating delayed onset; even small exposures require medical observation due to potential late-developing edema. Final Answer: <1

Question 12

Meaning of Biological Oxygen Demand (BOD):
In water quality and wastewater engineering, the term BOD is primarily used in relation to which type of water?

Accepted Answer

Introduction / Context: Biological Oxygen Demand (BOD) quantifies the biodegradable organic load in a water sample by measuring the oxygen consumed by microorganisms over a standard period. It is a cornerstone metric for wastewater characterisation and treatment design. Given Data / Assumptions: BOD is measured in mg/L, typically over 5 days at 20°C. The question contrasts clean water uses with wastewaters. Concept / Approach: BOD is most relevant for waste streams—municipal sewage and industrial effluents—where biodegradables are present. While potable, cooling, and distilled waters should have extremely low BOD, engineers apply BOD to evaluate and regulate discharges from industries and sewage works. Thus, among the choices, industrial effluents best matches the context in which BOD is routinely referenced, specified, and limited by permits. Step-by-Step Solution: Define BOD as a wastewater strength parameter.Match to stream types: effluents contain organics requiring biological oxidation.Select industrial effluents as the most appropriate association. Verification / Alternative check: Discharge consents typically stipulate BOD limits; drinking water standards do not rely on BOD as a core parameter. Why Other Options Are Wrong: Potable: Very low organics; BOD not a defining spec.Cooling: Primarily thermal concerns; BOD is minimal unless contaminated.Distilled: Essentially zero organics and BOD. Common Pitfalls: Confusing BOD with COD; COD is a complementary, faster chemical test but measures all oxidisable matter, not just biodegradable. Final Answer: industrial effluents

Question 13

Human biomonitoring for mercury exposure:
What is the commonly cited maximum permissible limit of mercury in human blood, expressed in micrograms per 100 c.c. (mL)?

Accepted Answer

Introduction / Context: Mercury exposure arises from occupational settings, environmental contamination, and diet (notably methylmercury in fish). Blood mercury concentration is a standard biomonitoring index. Recognising a typical upper permissible limit aids screening and clinical triage. Given Data / Assumptions: Units are micrograms per 100 c.c. (mL), a traditional clinical convention. Question targets a widely referenced guideline value rather than a specific jurisdiction’s most recent threshold. Concept / Approach: Historical public health references often cite a screening threshold near 7 micrograms per 100 mL for total mercury, above which further evaluation is warranted. While modern guidance may use micrograms per litre and distinguish organic/inorganic species, the offered choices reflect classic textbook values; hence 7 µg/100 mL is the best match. Step-by-Step Solution: Convert framing: 7 µg/100 mL equals 70 µg/L.Compare with typical action levels found in clinical toxicology texts.Select 7 as the widely cited limit among the options. Verification / Alternative check: Many screening programs flag results in this vicinity for further assessment of exposure source and neurological symptoms, especially in pregnant individuals due to fetal susceptibility. Why Other Options Are Wrong: 1: Unduly strict relative to older clinical screening references in these units.19 or 82: Excessively high; would typically prompt urgent intervention. Common Pitfalls: Confusing units (µg/L vs µg/100 mL) and speciation (methylmercury vs inorganic) when interpreting laboratory results. Final Answer: 7

Question 14

Most widely used coagulant for clarifying turbid water:
Which chemical coagulant is most commonly applied in conventional water treatment plants to destabilise suspended particles and enhance settling/filtration?

Accepted Answer

Introduction / Context: Coagulation–flocculation is a core step for removing turbidity and natural organic matter in surface waters. Selecting the right coagulant affects effluent clarity, sludge production, and chemical costs. Given Data / Assumptions: Treatment objective: reduce turbidity and improve filter performance. Common, commercially available inorganic coagulants are considered. Concept / Approach: Alum (aluminium sulphate) has long been the default coagulant in conventional plants owing to its effectiveness across a wide pH range, availability, and predictable sludge characteristics. Ferric salts are also effective and sometimes preferred for cold water or high organic loads, but globally alum remains the most widely used. Lime primarily adjusts pH/alkalinity and can aid softening, while sodium aluminate is typically used as an adjunct to boost aluminium content or alkalinity, not as the predominant coagulant by itself. Step-by-Step Solution: Identify which candidate is a primary coagulant vs. pH/alkalinity adjuster.Recall widespread adoption history and dosing practice.Select alum as the most widely used coagulant. Verification / Alternative check: Design manuals commonly present alum dose–response curves and jar-test procedures as the standard baseline, with ferric salts as alternatives. Why Other Options Are Wrong: Lime: Used for softening and pH control, not primary coagulation of typical surface waters.Ferric chloride: Widely used but less universally than alum.Sodium aluminate: Supplementary additive, not primary choice. Common Pitfalls: Assuming one coagulant fits all raw waters; jar testing remains essential to optimise dose and pH. Final Answer: Alum (aluminium sulphate)

Question 15

Wastewater lagoons (oxidation/maturation ponds) for purifying polluted water
Which statements about lagoon systems used for biological treatment are accurate regarding construction, operating principle, and achievable effluent quality when multiple…

Accepted Answer

Introduction / Context: Lagoon systems, also called oxidation ponds or maturation ponds, are widely used for municipal and industrial wastewater polishing. They exploit natural biological processes in a controlled, shallow basin to reduce biochemical oxygen demand (BOD), suspended solids (SS), pathogens, and odour. This question checks your understanding of what lagoons are, how they work, and what level of effluent quality they can approach when multiple cells are arranged in series. Given Data / Assumptions: Lagoons are engineered, shallow, earthen basins with long detention time. They host mixed communities of bacteria, algae, and protozoa. Oxygen is supplied by surface re-aeration and algal photosynthesis; optional mechanical aeration can be added. Series operation (facultative followed by maturation cells) improves clarity and stability. Concept / Approach: Lagoons achieve treatment by biological oxidation of organics. In facultative zones, bacteria oxidize carbonaceous matter while algae generate oxygen. Settling of biological flocs and algal-bacterial aggregates lowers suspended solids. Series ponds provide staged treatment: primary reduction, polishing, and pathogen die-off. Step-by-Step Solution: Recognize that lagoon = large shallow artificial basin; many synonymous names are used in literature.Identify the core mechanism: micro-organisms oxidize organics in the presence of dissolved oxygen; algae support oxygen balance.Acknowledge that placing 3–4 lagoons in series markedly improves effluent clarity and BOD removal, approaching very low SS and BOD values under favourable design and loading.Hence, statements (a), (b), and (c) together are correct. Verification / Alternative check: Typical designs report progressively lower BOD and SS from cell to cell. Warm climates, adequate area, and proper hydraulic retention time yield excellent polishing, confirming the combined statements. Why Other Options Are Wrong: (a) alone or (b) alone or (c) alone: Each is true, but the question asks which statements are accurate overall; the correct comprehensive choice is all (a), (b), and (c). Common Pitfalls: Assuming lagoons are purely sedimentation devices (they are biological); underestimating the benefit of series cells; forgetting the role of algal photosynthesis in oxygen supply. Final Answer: All (a), (b) & (c).

Question 16

Typical salt content of seawater
Approximately what percentage of dissolved salts does open ocean seawater contain on average?

Accepted Answer

Introduction / Context: The salinity of seawater is a foundational parameter in environmental and marine engineering. It influences density, buoyancy, osmotic pressure, and the performance of desalination processes. A common benchmark is the average ocean salinity expressed as mass percent of dissolved salts. Given Data / Assumptions: A single representative percentage for open ocean conditions is sought. Salinity varies modestly with geography, evaporation, precipitation, and river inflow. Answer choices span low to very high percentages. Concept / Approach: Average seawater salinity is classically cited near 35 grams of salts per kilogram of water. Converting 35 g/kg to a percentage gives approximately 3.5 percent by mass. This value is used in standard textbook problems, oceanographic references, and as a baseline for desalination design calculations. Step-by-Step Solution: Recall the canonical salinity: about 35 g/kg.Convert to percent: 35 g per 1000 g ≈ 3.5%.Check against options and select 3.5. Verification / Alternative check: Many oceanographic datasets cluster around 34–36 g/kg; slight regional deviations do not change the widely accepted ~3.5% figure for general-use MCQs. Why Other Options Are Wrong: 0.5% or 1%: Far below typical ocean salinity; closer to brackish water ranges.10%: Much higher than open ocean; would imply hypersaline conditions (e.g., enclosed salt lakes) rather than oceans. Common Pitfalls: Confusing parts per thousand (ppt) with percent; 35 ppt equals 3.5%, not 35%. Final Answer: 3.5

Question 17

Typical pH of oxidized (stabilized) sewage
After adequate aeration and biological oxidation, the pH of treated sewage is generally around which value?

Accepted Answer

Introduction / Context: pH is a key control parameter in wastewater treatment. During biological oxidation (secondary treatment), acids formed initially are buffered and consumed, and the effluent typically trends toward neutral to slightly alkaline values, depending on alkalinity and process conditions. Given Data / Assumptions: “Oxidized sewage” implies stabilized effluent post-aeration/secondary treatment. Municipal sewage has natural alkalinity; nitrification can slightly acidify unless alkalinity is adequate. We seek the most representative pH value of treated effluent. Concept / Approach: Typical secondary effluent pH lies close to neutral. Values near 7.0–7.5 are common when alkalinity buffers acids and the process is well operated. Extremely acidic (≈2) or strongly alkaline (≈13) conditions are incompatible with biological treatment and regulatory discharge norms. Step-by-Step Solution: Classify options: 1.8 (strongly acidic), 6.2 (slightly acidic), 7.3 (near neutral), 13.4 (strongly alkaline).Select the near-neutral value that matches oxidized municipal effluent: 7.3.Confirm that 7.3 aligns with routine plant data for properly buffered systems. Verification / Alternative check: Design manuals show typical effluent pH between 6.5 and 8.5; 7.3 sits in the middle of this band. Why Other Options Are Wrong: 1.8 / 13.4: Outside biological viability; would denature biomass.6.2: Possible in unusual low-alkalinity or nitrification-heavy cases, but less representative than 7.3 for generic “oxidized sewage.” Common Pitfalls: Assuming influent pH carries through; biological processes and buffering shift pH toward neutrality in well-run systems. Final Answer: 7.3

Question 18

Best device for removing very fine particulates from power station flue gas
In large thermal power plants, which equipment is preferred for high-efficiency removal of fine fly ash from flue gases?

Accepted Answer

Introduction / Context: Controlling particulate emissions from coal-fired and similar thermal power stations is crucial for air quality compliance. Flue gas contains a broad particle size distribution, with significant fractions in the fine (sub-micron to a few microns) range, often called fly ash. The question focuses on the most common, highly efficient industrial choice for large-scale fine particulate control. Given Data / Assumptions: Large, continuous-duty thermal power station service. Target: very fine particulates (fly ash) at high flow rates and temperatures. Need: proven, high-collection efficiency with acceptable pressure drop and operability. Concept / Approach: Electrostatic precipitators (ESPs) charge particles using corona discharge and then collect them on oppositely charged plates. ESPs can achieve very high collection efficiencies (often exceeding 99%) for a wide size range, with relatively low pressure drop—making them a mainstay in power generation. While fabric filters (baghouses) can also achieve excellent fine-particle removal, ESPs have historically been the default in many large units due to thermal tolerance, capacity, and low pressure losses. Step-by-Step Solution: Identify required performance: very high efficiency on fine fly ash at large gas volumes.Match technology: ESPs provide high efficiency with low pressure drop.Confirm common industry practice: ESPs are widely installed on utility boilers.Select “Electrostatic precipitators.” Verification / Alternative check: Regulatory performance reports and plant configurations around the world corroborate ESP use for fine ash control in large boilers. Why Other Options Are Wrong: Wet scrubber: Effective for soluble gases and coarser dust; fine particle capture can be lower without specialized designs.Bag filter: Very efficient but historically less common on very large, high-temperature utility boilers (though increasingly used); the canonical answer for big power stations is ESP.Dust catcher: A simple inertial device, not suitable for fine particles at utility scale. Common Pitfalls: Equating all “filters” with best fine capture regardless of scale; process constraints and pressure drop matter in utility service. Final Answer: Electrostatic precipitators

Question 19

Dominant constituents in wastewater from textile, pulp & paper, tanning, distillery, dairy, and meat packing industries
Which group best describes the primary type of contaminants in the combined wastewaters from these sectors?

Accepted Answer

Introduction / Context: Understanding the typical contaminant profile of industrial wastewaters guides process selection for treatment. The sectors listed—textile, pulp and paper, tanning, distillery, dairy, and meat packing—have characteristic organic loads and specific pollutants that dictate biological treatment, equalization, and sometimes advanced steps. Given Data / Assumptions: Industries listed are largely agro-based or use organic feedstocks and process chemicals. Wastewater contains biodegradable organics, suspended solids, colour, fats, oils, and greases in varying proportions. We are to choose the major contaminant group across them collectively. Concept / Approach: Pulp and paper effluents carry lignin derivatives, cellulose fines, and BOD/COD. Textile effluents include dyes and sizing agents (organic), though inorganics may occur. Tanneries release organic matter (proteins, fats), plus specific inorganics (chromium) but the bulk oxygen demand is organic. Distilleries and dairies discharge high-BOD organics (sugars, alcohols, whey proteins). Meat packing effluents are rich in proteins and fats. Overall, natural organic products dominate the oxygen demand and treatment design basis. Step-by-Step Solution: Aggregate the sectors’ waste characteristics and identify the common denominator: high organic loads.Note exceptions (e.g., chromium in tanning) but recognize they do not outweigh the primary organic nature overall.Select “Natural organic products.” Verification / Alternative check: Biological treatment (aerobic/anaerobic) is standard for these industries, confirming organic dominance. Why Other Options Are Wrong: Radioactive substances: Not characteristic of these sectors.Inorganic pollutants: Present in some cases (e.g., salts, chromium), but not the primary overall constituent driving treatment.None of these: Incorrect because “Natural organic products” is appropriate. Common Pitfalls: Overgeneralizing from a single industry (e.g., chromium in tanning) rather than the collective profile. Final Answer: Natural organic products

Question 20

Key reaction in biochemical treatment of sewage effluents
Which fundamental process best describes the main transformation occurring in secondary (biological) wastewater treatment?

Accepted Answer

Introduction / Context: Secondary wastewater treatment relies on microbial communities to convert dissolved and colloidal organic matter into stable products. Understanding the core biochemical pathway helps in selecting and operating activated sludge, trickling filters, and aerated lagoons. Given Data / Assumptions: “Biochemical treatment” here refers to conventional secondary treatment in public works (activated sludge, trickling filters). Influent contains biodegradable organics measured as BOD/COD. Aeration or passive oxygen transfer is available in aerobic systems. Concept / Approach: The principal reaction in aerobic biological treatment is oxidation of organic carbon to CO2 and water, with a fraction assimilated into new biomass. While some systems include anoxic/anaerobic zones (denitrification or digesters), the mainstream carbon removal step is oxidative. “Fermentation” is part of anaerobic digestion and occurs without oxygen, but typical secondary treatment for municipal sewage emphasizes aerobic oxidation. Step-by-Step Solution: Identify mainstream process: activated sludge and trickling filters are aerobic.State core reaction: organics + O2 → CO2 + H2O + biomass.Select “Oxidation” as the overarching mechanism. Verification / Alternative check: Plant operation targets sufficient dissolved oxygen and aeration capacity, directly reflecting an oxidative process. Why Other Options Are Wrong: Reduction: Applies to specific pollutants (e.g., nitrate to nitrogen gas under anoxic conditions), not the primary carbon removal step.Dehydration: A physical sludge thickening/drying step, not the biochemical conversion in secondary treatment.Fermentation: Characteristic of anaerobic digestion; not the main reaction in conventional aerobic secondary treatment of sewage. Common Pitfalls: Confusing anaerobic digestion of sludge with the aerobic secondary treatment of the liquid stream. Final Answer: Oxidation

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