Question 1

In water softening, what fraction of hardness does the sodium–zeolite (cation-exchange) process remove under normal operation?

Accepted Answer

Introduction / Context: Hardness in water arises from divalent cations, primarily calcium and magnesium. Different softening processes target different hardness fractions. The sodium–zeolite process (ion exchange) is widely used in domestic and industrial softening systems; understanding its scope avoids misapplication and sets expectations for downstream scaling control. Given Data / Assumptions: Zeolite or synthetic resin in sodium form exchanges Na+ for Ca2+ and Mg2+. Temporary hardness relates to bicarbonates; permanent hardness to chlorides/sulfates. Regeneration uses brine (NaCl) to restore Na+ form. Concept / Approach: Cation exchange is non-selective with respect to the associated anion. Whether calcium or magnesium is present as bicarbonate (temporary hardness) or as sulfate/chloride (permanent hardness), the resin removes the cation and replaces it with sodium. Therefore, both temporary and permanent hardness are reduced to very low levels until resin exhaustion. Step-by-Step Solution: Recognize that “hardness” is due to Ca2+/Mg2+ regardless of anions.Sodium–zeolite exchanges Na+ for these ions across the board.Hence, it removes both temporary and permanent fractions.Compare with lime–soda softening, which chemically precipitates fractions depending on dose.Conclude that ion exchange comprehensively softens the water. Verification / Alternative check: Softened water analyses show near-zero Ca2+/Mg2+ until breakthrough; alkalinity/anions pass unchanged, confirming cation-specific removal independent of hardness type. Why Other Options Are Wrong: Only temporary or only permanent: Misstates the broad cation-exchange action. Dissolved gases: Not a hardness issue and not removed by zeolite softening. Common Pitfalls: Confusing TDS with hardness; ion exchange swaps cations but does not lower total dissolved solids significantly, as Na+ increases while Ca2+/Mg2+ decrease. Final Answer: both temporary and permanent hardness of water

Question 2

For process water used in dairy plants, what is the recommended microbiological limit for proteolytic and/or lipolytic organisms to avoid product spoilage and enzyme-mediated defects?

Accepted Answer

Introduction / Context: In the dairy industry, process water quality directly influences product safety and shelf life. Proteolytic and lipolytic microorganisms produce enzymes that degrade proteins and fats, causing bitterness, rancidity, or texture defects. Setting strict microbiological limits for these organisms in incoming and utility water helps maintain product quality in milk, cheese, yogurt, and ice cream plants. Given Data / Assumptions: Water is used for equipment rinsing, ingredient dilution, and CIP steps. Proteolytic/lipolytic organisms are measured as counts per milliliter. Industry guidance sets low allowable counts to minimize enzyme carryover. Concept / Approach: Even small populations of protease- and lipase-producing microbes can release thermostable enzymes that survive pasteurization and later degrade dairy products. Practical standards therefore constrain these counts to very low levels. Among the listed choices, the commonly cited criterion is not more than 10 per ml, balancing practical monitoring with stringent quality control to prevent spoilage issues and off-flavors. Step-by-Step Solution: Define the risk: proteases and lipases remain active even after thermal steps.Adopt a conservative numerical limit consistent with dairy QA programs.From the options, select 10 per ml as the upper allowable count.Implement monitoring via selective media and periodic verification.Tie water quality results to preventive actions (e.g., sanitation checks, filtration). Verification / Alternative check: Historical QA/QC guidelines and plant SOPs frequently prescribe very low allowable counts for these functional groups due to their outsized impact on flavor and texture stability. Why Other Options Are Wrong: Counts of 15 or 20 per ml increase risk of enzyme carryover and spoilage. Count of 5 per ml is more stringent than typical practice and may be unnecessarily restrictive given monitoring variability. Common Pitfalls: Relying solely on total plate count; functional groups like psychrotrophs with proteolysis/lipolysis can cause defects even when overall counts are modest. Final Answer: 10 proteolytic and/or lipolytic organisms per ml

Question 3

Which reagent, when boiled with hard water, can remove both temporary (bicarbonate) and permanent (non-bicarbonate) hardness by precipitating calcium and magnesium salts?

Accepted Answer

Introduction / Context: Hardness removal is a classic water-treatment topic. Temporary hardness arises from bicarbonates of calcium and magnesium, while permanent hardness is due to chlorides, sulfates, and nitrates of these ions. Different reagents target different fractions. Identifying a reagent that can address both types when heated clarifies laboratory and field dehardening practices. Given Data / Assumptions: Objective: convert dissolved Ca2+/Mg2+ into insoluble precipitates. Boiling conditions or hot contact with the reagent are available. No ion exchange resins are used (chemical precipitation approach). Concept / Approach: Sodium carbonate (washing soda) supplies carbonate ions that precipitate calcium as CaCO3 and, after subsequent reactions on heating, can also help precipitate magnesium as MgCO3 and Mg(OH)2. In contrast, lime (calcium hydroxide) primarily removes temporary hardness by converting bicarbonates to carbonates; permanent hardness typically requires soda ash addition. Thus, boiling hard water with sodium carbonate can address both fractions more comprehensively among the listed choices. Step-by-Step Solution: Introduce CO3^2− via Na2CO3 to precipitate Ca2+ as CaCO3.Heat promotes MgCO3 instability, leading to Mg(OH)2 formation that removes Mg2+.Temporary hardness is removed as bicarbonates convert to carbonates and precipitate.Permanent hardness (chlorides/sulfates) is reduced as Ca2+/Mg2+ are still captured by CO3^2− and OH−.Decant or filter to remove precipitates, reducing total hardness. Verification / Alternative check: Lime–soda ash softening in municipal practice employs both Ca(OH)2 and Na2CO3; among the single reagents listed, sodium carbonate is the more generally effective precipitant for both hardness types under hot conditions. Why Other Options Are Wrong: Calcium hydroxide: Best for temporary hardness; often needs soda ash for permanent hardness. Calcium oxide: Quicklime that forms Ca(OH)2; same limitation as lime alone. Calcium carbonate: Sparingly soluble; not an effective reagent for removal. Common Pitfalls: Expecting complete deionization; chemical softening reduces hardness but does not remove all dissolved solids like ion exchange or RO. Final Answer: sodium carbonate

Question 4

Within the conventional “tertiary” (advanced) treatment stage of a wastewater plant, which of the following unit operations is most representative of tertiary polishing of secondary effluent?

Accepted Answer

Introduction / Context: Tertiary or advanced treatment refers to processes applied after secondary biological treatment to polish effluent and meet stringent limits for solids, turbidity, nutrients, or emerging contaminants. Clarifying which unit operations typify this stage helps in selecting appropriate technologies for reuse and discharge permits. Given Data / Assumptions: Secondary clarifier effluent still contains fine suspended solids and colloids. Tertiary processes aim to reduce TSS/turbidity and sometimes nutrients. Conventional tertiary trains often include media filtration or membranes. Concept / Approach: Filtration (e.g., rapid sand, dual-media, cloth disk, or membrane filtration) is a hallmark tertiary operation that removes fine particles not captured by sedimentation. While chemical coagulation and flocculation can precede filtration as aids, filtration itself provides the polishing step that characterizes tertiary treatment in many plants. Step-by-Step Solution: Define tertiary treatment as post-secondary polishing.Identify filtration as the unit that directly lowers turbidity and TSS.Note that coagulation/flocculation is preparatory when used; sedimentation is primary/secondary.Select filtration as the representative tertiary operation.Acknowledge optional nutrient removal steps (e.g., denitrification filters). Verification / Alternative check: Plant P&IDs and process schematics commonly show rapid sand filters or membranes after secondary clarifiers, confirming filtration’s role in tertiary polishing. Why Other Options Are Wrong: Chemical coagulation/flocculation: Often used as a pretreatment for filtration but not, by itself, the polishing step. Sedimentation: Typically primary or secondary clarification, not tertiary. None of these: Incorrect because filtration is a standard tertiary unit. Common Pitfalls: Assuming disinfection alone is tertiary; while post-treatment, it does not remove particulates and may be impaired by high turbidity. Final Answer: filtration

Question 5

As per Bureau of Indian Standards (BIS) drinking-water specifications, what is the maximum permissible limit for total dissolved solids (TDS) when no alternate source is available?

Accepted Answer

Introduction / Context: Total dissolved solids (TDS) reflect the concentration of inorganic salts and small amounts of organic matter dissolved in water. BIS standards specify both desirable limits and higher permissible limits in the absence of alternative sources, guiding utilities in assessing palatability and corrosion/scaling tendencies. Given Data / Assumptions: BIS provides a desirable limit and a maximum permissible limit. When alternate sources are unavailable, a higher TDS may be accepted. Units are in mg/l (equivalent to ppm for dilute aqueous solutions). Concept / Approach: While 500 mg/l is the desirable TDS limit for aesthetics and taste, BIS allows up to 2000 mg/l as the maximum permissible value if no better source exists. This acknowledges regional variability while maintaining health and usability considerations. Above this level, taste and scaling often become unacceptable without treatment (e.g., RO, ion exchange). Step-by-Step Solution: Identify desirable vs permissible categories in BIS standard.Recall the desirable TDS = 500 mg/l.Recognize maximum permissible TDS = 2000 mg/l in absence of alternatives.Select 2000 mg/l from the options provided.Tie to practical implications (taste, scaling, appliances). Verification / Alternative check: Publicly available BIS 10500 tables list 500 mg/l (desirable) and 2000 mg/l (permissible) for TDS, confirming the selection. Why Other Options Are Wrong: 500 mg/l: Desirable, not the maximum permissible. 1000 or 1500 mg/l: Not the BIS-stated permissible limit. Common Pitfalls: Confusing hardness with TDS; they are related but not identical. RO reduces TDS but may require remineralization for taste and corrosivity control. Final Answer: 2000 mg/l

Question 6

Permanent hardness in natural waters is primarily caused by which dissolved salts of calcium and magnesium?

Accepted Answer

Introduction / Context: Water hardness affects scaling in boilers, heat exchangers, and household appliances. Distinguishing temporary from permanent hardness guides the selection of treatment processes (e.g., lime softening versus ion exchange). Permanent hardness persists on boiling and is associated with specific salts. Given Data / Assumptions: Hardness refers to divalent cations, mainly Ca2+ and Mg2+. Temporary hardness comes from bicarbonates that decompose on boiling. Permanent hardness remains after boiling due to non-bicarbonate anions. Concept / Approach: Permanent hardness is due to chlorides and sulfates of calcium and magnesium. These salts do not decompose upon heating and therefore require chemical precipitation (soda ash) or ion exchange for removal. In contrast, bicarbonates (temporary hardness) release CO2 and precipitate as carbonates during boiling or lime treatment. Step-by-Step Solution: Identify relevant cations: Ca2+ and Mg2+.Classify salts: bicarbonates (temporary) vs chlorides/sulfates (permanent).Select “chlorides and sulfates of calcium and magnesium.”Reject sodium/potassium salts; these do not contribute to hardness.Tie back to treatment: soda ash or ion exchange is required. Verification / Alternative check: Analytical hardness calculations separate carbonate and non-carbonate hardness; the latter correlates with chloride and sulfate concentrations of Ca and Mg. Why Other Options Are Wrong: Bicarbonates: Temporary hardness, removable by boiling. Sodium/potassium salts: Monovalent cations do not cause hardness. Phosphates of Na/K: Not a standard cause of hardness in potable waters. Common Pitfalls: Equating TDS with hardness; high TDS water may have low hardness if dominated by sodium salts. Final Answer: chlorides and sulfates of calcium and magnesium

Question 7

Among naturally occurring sources, which water type is generally considered the purest at the point of formation due to atmospheric distillation before potential contamination during fall or collection?

Accepted Answer

Introduction / Context: Naturally occurring waters vary in dissolved solids, gases, particulates, and microbes. Identifying the source that is inherently purest at its moment of formation helps explain why certain waters require less mineral removal but more attention to atmospheric and surface contamination pathways. Given Data / Assumptions: “Purest” refers to minimal dissolved salts at the moment of formation. All natural waters can acquire contaminants from air, soil, and biota. Surface and groundwater pick up minerals through contact with earth materials. Concept / Approach: Rainwater forms through atmospheric distillation: evaporation leaves most dissolved salts behind, and condensation yields low-mineral water. Although rain can absorb gases (CO2, SO2, NOx) and pick up particulates during descent, its baseline mineral content is typically lower than river, pond, or well waters, which have prolonged contact with soils and rocks that contribute ions via weathering. Step-by-Step Solution: Recognize distillation-like cycle (evaporation–condensation) for rain.Compare with river/pond water that contact sediments and biota.Compare with well water that dissolves minerals while percolating through aquifers.Conclude that rainwater is the purest at formation, despite possible airborne acquisitions.Emphasize need for collection safeguards to keep rainwater potable. Verification / Alternative check: Conductivity/TDS measurements show rainwater typically has very low ionic strength relative to surface or groundwater, barring pollution events like acid rain or dust storms. Why Other Options Are Wrong: River/pond water: Higher dissolved and suspended loads due to catchment runoff. Well water: Often mineral-rich (hard) due to rock–water interactions. Common Pitfalls: Assuming rainwater is automatically potable; while low in minerals, it may require filtration and disinfection to remove particulates and microbes collected during harvesting. Final Answer: rain water

Question 8

Drinking-water quality (India, BIS): What is the maximum desirable limit for total hardness, expressed as CaCO3, in potable water supplied for human consumption?\r (Choose the closest standard value used for routine compliance checks.)

Accepted Answer

Introduction: Total hardness in drinking water is primarily due to calcium and magnesium salts and is conventionally expressed as milligrams per liter (mg/L) of calcium carbonate (CaCO3), often abbreviated as ppm. Regulatory bodies publish guideline values to protect plumbing, taste, and household equipment while also considering health and practicality. This question checks recognition of the typical Indian Bureau of Standards (BIS) desirable limit used in many water-supply assessments. Given Data / Assumptions: Total hardness is reported as CaCO3 equivalent. We are asked for the commonly cited maximum desirable limit used in routine monitoring. Units: mg/L is numerically equal to ppm in dilute aqueous solutions. Concept / Approach: Hardness affects soap consumption, scaling in pipes and heaters, and user acceptability. Standards typically present two tiers: an ideal or desirable limit for consistent aesthetic/operational quality, and a higher permissible limit in the absence of an alternative source. In many exam and practice contexts, the desirable limit is represented as 300 mg/L as CaCO3, with a higher permissible ceiling for challenging supply situations. Step-by-Step Solution: Identify the requested tier: “maximum desirable limit.”Recall the widely taught value associated with Indian potable water guidelines.Match to options: 300 ppm (mg/L) is the standard desirable level frequently cited. Verification / Alternative check: Practical field guides for municipal water operations commonly set routine targets at the desirable level, with operational allowances up to a higher permissible limit when supply constraints exist. This is consistent with consumer acceptability and infrastructure longevity considerations. Why Other Options Are Wrong: 600 ppm: Often referenced as a higher permissible value, not the desirable target. 500 ppm and 1000 ppm: Exceed typical aesthetic and operational recommendations. 200 ppm: A stricter target than the commonly taught desirable value in many syllabi; not the expected answer here. Common Pitfalls: Confusing “desirable” with “permissible” limits; always check which tier the question asks for. Also, ensure units are correctly recognized as mg/L or ppm in dilute water. Final Answer: 300 ppm

Question 9

Dairy sanitation practice: Which acid is most commonly used for removal of “milk stone” (calcium and magnesium scale) from dairy equipment and pipelines during acidic CIP cleaning?

Accepted Answer

Introduction: “Milk stone” is a mineral deposit consisting mostly of calcium and magnesium salts, often combined with protein residues, that forms on dairy equipment surfaces. Effective clean-in-place (CIP) regimes alternate alkaline and acid steps. The acid step must dissolve mineral scale rapidly without excessive corrosion or residue formation. Given Data / Assumptions: The goal is to remove calcium/magnesium carbonate and phosphate deposits. Chemicals must be compatible with stainless steel and elastomers commonly used in dairies. Industry norms favor fast-acting, rinse-friendly acids. Concept / Approach: Nitric acid is widely used in dairy CIP as the primary acidic cleaner for descaling. It dissolves carbonate/phosphate scales efficiently and passivates stainless steel surfaces, reducing corrosion risk when used at prescribed concentrations and temperatures. While phosphoric acid is also used, nitric acid remains the benchmark in many dairy standards for consistent, rapid scale removal. Step-by-Step Solution: Identify the deposit (milk stone) as mineral scale.Select an acid that both dissolves scale and is equipment-compatible.Nitric acid best matches speed, effectiveness, and passivation benefits for stainless systems. Verification / Alternative check: Standard dairy CIP procedures typically specify an alkaline wash for protein/fat followed by a nitric acid rinse to remove mineral scale and re-passivate metal surfaces. Why Other Options Are Wrong: Phosphoric acid: Useful in some plants but generally secondary to nitric for milk stone removal. Gluconic/tartaric/acetic acids: Weaker organic acids; slower and less common as primary descalers in dairy CIP. Common Pitfalls: Overusing strong acids without inhibitors or passivation control can damage gaskets or cause pitting; always follow validated CIP recipes. Final Answer: nitric acid

Question 10

Wastewater stabilization ponds: Lagoons used for biological treatment can be categorized by operating mode and oxygen regime.\r Which description best captures the types encountered in practice?

Accepted Answer

Introduction: Wastewater lagoons (stabilization ponds) are large, shallow basins designed to remove pollutants through a combination of sedimentation, microbial metabolism, and, in some cases, aeration. Their classification depends on oxygen availability and energy input. Understanding lagoon types helps in process selection for municipal and agro-industrial effluents. Given Data / Assumptions: Lagoons may be natural (no mechanical aeration) or mechanically assisted. Microbial ecology varies with dissolved oxygen and depth. Design targets include BOD removal, pathogen reduction, and sludge stabilization. Concept / Approach: Three widely recognized categories are: (1) Anaerobic lagoons for high-strength wastes where oxygen is absent and methane/CO2 form; (2) Facultative lagoons with an aerobic surface layer and an anaerobic bottom; and (3) Aerated lagoons that use mechanical aerators or diffusers to increase oxygen transfer and mixing. Each has specific loading rates, depths, and performance expectations. Step-by-Step Solution: Identify the oxygen regime options for lagoon operation.Match the listed categories to real lagoon types: anaerobic, facultative, and aerated are all standard.Therefore select the inclusive option “all of these.” Verification / Alternative check: Design manuals illustrate typical depths: anaerobic (2–5 m), facultative (1–2 m), and aerated lagoons with mixers/aerators to boost kLa and allow higher areal loadings. Why Other Options Are Wrong: Any single category alone is incomplete. “None of these” contradicts established lagoon typology. Common Pitfalls: Confusing aerated lagoons with oxidation ditches; while both are mechanically aerated, hydraulics and configuration differ. Final Answer: all of these

Question 11

Drinking-water toxic metals (India, BIS): What is the maximum desirable limit for mercury (Hg) in potable water supplied for human consumption, reported in mg/L?

Accepted Answer

Introduction: Mercury is a toxic heavy metal with severe neurological and developmental impacts, especially in methylmercury form. Strict limits in potable water reduce chronic exposure. Water standards specify low microgram-per-liter levels to protect public health and to guide treatment plant monitoring and corrective actions. Given Data / Assumptions: Concentration units: mg/L (milligrams per liter). We seek the typical drinking-water limit used in Indian compliance contexts. Values are selected to be protective across a broad population with consumption variability. Concept / Approach: The standard limit for mercury in drinking water is set at the microgram-per-liter scale. A widely recognized compliance value is 0.001 mg/L (which equals 1 μg/L). This threshold reflects achievable analytical detection, treatment capability, and health-based risk assessments. Step-by-Step Solution: Interpret units: 0.001 mg/L = 1 μg/L.Compare listed options to typical regulatory limits in potable water.Select 0.001 mg/L as the maximum allowable/desirable concentration. Verification / Alternative check: Public health guidance documents and water-quality manuals consistently specify mercury limits at or below 1 μg/L for treated drinking water, aligning with this numeric selection. Why Other Options Are Wrong: 0.05 or 0.1 mg/L: Orders of magnitude too high for drinking water. 0.9 mg/L: Extremely high and unsafe. 0.0001 mg/L: Stricter than the commonly enforced BIS limit; not the expected answer here. Common Pitfalls: Misreading mg/L versus μg/L; always convert correctly to interpret toxicity thresholds. Final Answer: 0.001 mg/l

Question 12

Sanitizers and corrosion control: In food and beverage plants, why are corrosion inhibitors added along with chemical sanitizers during surface sanitation?

Accepted Answer

Introduction: Chemical sanitizers (e.g., chlorine compounds, peracetic acid, quaternary ammonium compounds) are used to reduce microbial loads on equipment. However, many sanitizers are oxidizing or otherwise aggressive to metals and elastomers. Corrosion inhibitors are often formulated into sanitizing solutions to protect equipment and may also help stabilize sanitizer efficacy under operating conditions. Given Data / Assumptions: Surfaces include stainless steel, valves, and gaskets that can corrode or degrade. Sanitizer solutions may be acidic or oxidizing. Formulation additives can mitigate corrosion and help maintain sanitizer performance. Concept / Approach: Inhibitors adsorb onto metal surfaces or scavenge reactive species, reducing pitting or general corrosion. Some formulations also improve sanitizer stability (e.g., buffering pH to keep active species in the most effective form) and wetting, thereby improving antimicrobial action. As a result, inhibitors serve dual roles: asset protection and performance enhancement. Step-by-Step Solution: Identify the risks: chemical attack on equipment and potential sanitizer instability.Recognize inhibitor functions: surface passivation, pH buffering, chelation, and wetting improvements.Conclude that inhibitors both prevent corrosion and can improve sanitizer action. Verification / Alternative check: Product data sheets for commercial sanitizers commonly list corrosion inhibitors and stabilizers that extend compatible contact times without damaging equipment. Why Other Options Are Wrong: Single-purpose answers are incomplete; both benefits are recognized. “None of these” ignores well-known formulation practice. Increasing product viscosity is unrelated to sanitizer performance on hard surfaces. Common Pitfalls: Overdosing or using incompatible inhibitors can leave residues; validate rinse steps and compatibility with downstream processes. Final Answer: both (a) and (b)

Question 13

Effect of quaternary ammonium compounds (QACs) on surface microbiota: Routine use of QAC sanitizers on utensils tends to shift the surviving microbial profile toward which group?

Accepted Answer

Introduction: Quaternary ammonium compounds (QACs) are cationic surfactants widely used as sanitizers because they disrupt cell membranes and denature proteins. Their antimicrobial spectrum, however, is not uniform across microorganisms, and repeated use can shift the residual surface flora toward less susceptible groups. Given Data / Assumptions: QACs are generally more active against Gram-positive bacteria than many Gram-negative rods. Environmental Gram-negative rods (e.g., Pseudomonas) can exhibit higher tolerance to QACs. Sanitizing reduces total counts but relative proportions can change. Concept / Approach: If a sanitizer preferentially kills Gram-positive organisms, then, after treatment, the fraction of Gram-negative rods among survivors may rise (even if their absolute numbers also decline). Over time, this selection pressure increases the relative prevalence of Gram-negative rods on utensils and surfaces, unless rotation of sanitizers or additional controls are employed. Step-by-Step Solution: Identify susceptibility pattern: Gram-positives are typically more susceptible to QACs.Infer ecological outcome: survivors skew toward Gram-negative rods.Select the option reflecting an increased percentage of Gram(-) rods after QAC use. Verification / Alternative check: Hygiene monitoring programs often note shifts toward QAC-tolerant Gram-negative flora, prompting sanitizer rotation or adjunct methods (e.g., oxidizing agents) to maintain control. Why Other Options Are Wrong: “Favor gram positive bacteria”: Opposite of typical QAC selectivity. “Decrease gram positive bacteria”: True but incomplete; the question asks about the resulting increase in a group. “Yeasts and molds only”: Fungal susceptibility varies; not the dominant observed shift. “None of the above”: Contradicts observed selection patterns. Common Pitfalls: Assuming total count reduction equals equal efficacy across all groups; selection can still occur and should inform sanitizer rotation strategies. Final Answer: increase the percentage of gram(-)ve rods on utensils

Question 14

Water treatment basics: Which method effectively removes both temporary (carbonate) and permanent (non-carbonate) hardness from water in a single step?

Accepted Answer

Introduction: Hardness in water arises from calcium and magnesium salts. Temporary hardness is mainly due to bicarbonates and can be reduced by boiling, while permanent hardness arises from chlorides, sulfates, and nitrates. Selecting a treatment that removes both forms is important for laboratory and certain process waters. Given Data / Assumptions: Temporary hardness is removable by precipitation of carbonates upon boiling. Permanent hardness requires ion exchange, chemical softening, or phase-change methods. We need a single-step method that removes both types effectively. Concept / Approach: Distillation vaporizes water and condenses pure steam, leaving nonvolatile dissolved salts behind, thereby removing both temporary and permanent hardness in one operation. While ion exchange can also remove both hardness types, simple filtration or decantation cannot, and boiling alone addresses primarily temporary hardness. Step-by-Step Solution: Identify methods that physically separate water from dissolved ions: distillation qualifies.Exclude partial methods: boiling addresses temporary hardness only.Exclude mechanical separation methods: filtration/decantation do not remove dissolved ions.Select “distillation” as the comprehensive single-step solution listed. Verification / Alternative check: Laboratory-grade water production lines rely on distillation or reverse osmosis plus polishing resins to remove dissolved hardness completely. Why Other Options Are Wrong: Boiling: Precipitates carbonates (temporary hardness) but leaves permanent hardness. Filtration/decantation: Do not remove dissolved ions. Ion exchange softening only: Effective, but not provided as the correct comprehensive option here compared with distillation. Common Pitfalls: Assuming any thermal treatment removes all ions; only vapor–condensation (distillation) achieves near-complete nonvolatile salt removal in one step. Final Answer: distillation

Question 15

Softening chemistry: Permanent hardness (non-carbonate hardness) in water can be removed most directly by adding which chemical reagent during lime–soda softening or similar treatment?

Accepted Answer

Introduction: Permanent hardness stems from calcium and magnesium salts of strong acids (chlorides, sulfates, nitrates) that do not decompose on boiling. Chemical softening targets these ions to form insoluble carbonates or hydroxides removable by clarification/filtration. Understanding reagent roles is essential in water-treatment design. Given Data / Assumptions: Permanent hardness must be precipitated or exchanged. Chemicals should convert dissolved Ca2+/Mg2+ into insoluble forms. We focus on common reagents used in conventional softening. Concept / Approach: In lime–soda softening, lime (Ca(OH)2) removes temporary hardness by converting bicarbonates, while soda ash (Na2CO3) provides carbonate ions that precipitate calcium as CaCO3 from non-carbonate salts. Magnesium often requires additional lime to form Mg(OH)2. Thus, soda ash is the key chemical for tackling permanent hardness directly. Step-by-Step Solution: Identify the target: Ca2+/Mg2+ from chlorides/sulfates/nitrates.Select a reagent that supplies CO3^2− to precipitate CaCO3.Soda ash (Na2CO3) fulfills this role in softening processes. Verification / Alternative check: Standard water-treatment texts show calculation steps where soda ash dosage is based on non-carbonate hardness equivalents, confirming its specific role. Why Other Options Are Wrong: Lime: Primarily addresses temporary hardness; also aids Mg removal but not the main reagent for Ca from permanent hardness. Potassium permanganate: Oxidant for iron/manganese; not a softener. Sodium bicarbonate: Adds alkalinity; does not precipitate hardness effectively. Alum: Coagulant; not a hardness-precipitating agent. Common Pitfalls: Confusing alkalinity adjustment (lime) with carbonate supply (soda ash); correct dosing requires hardness speciation. Final Answer: soda ash

Question 16

Wastewater disinfection methods: Which options list the common and widely implemented disinfection approaches used at municipal and industrial wastewater treatment plants?

Accepted Answer

Introduction: Disinfection in wastewater treatment reduces pathogens before discharge or reuse. Practical plant-scale methods must be effective, safe, and economical at high flow rates. Chlorination and ultraviolet (UV) irradiation are the two most widely applied options globally due to proven performance and established design guidance. Given Data / Assumptions: Wastewater effluents contain suspended solids that can shield microbes. Systems must be scalable and continuously operable. Residual control and by-product formation are considered in method choice. Concept / Approach: Chlorination (gas chlorine or hypochlorite) provides a measurable residual and broad-spectrum efficacy but can form chlorinated by-products; dechlorination may be needed. UV disinfection inactivates microorganisms by damaging nucleic acids without leaving a chemical residual, avoiding formation of halogenated by-products but requiring low UV transmittance attenuation and clean lamp sleeves. Many facilities use either method depending on effluent quality and regulatory goals. Step-by-Step Solution: Identify proven, scalable methods: chlorination and UV are both mainstream.Exclude nonstandard or impractical options for high flows (e.g., phenolic solvents, boiling).Select the inclusive option listing both chlorination and UV. Verification / Alternative check: Design manuals provide dosage–contact time (CT) guidelines for chlorine and UV dose–response curves, underscoring their prevalence in plant design. Why Other Options Are Wrong: Phenolic solvent: Not a practical or permitted method for large-scale wastewater disinfection. Boiling: Impractical at plant scale due to enormous energy demand. Common Pitfalls: Ignoring upstream solids removal; poor clarification can reduce UV efficacy and chlorine demand estimates. Final Answer: both (a) and (b)

Question 17

Public-health indicator organisms: The detection of coliform bacteria in a drinking-water sample is most directly an indication of the potential presence of which contamination source?

Accepted Answer

Introduction: Coliform bacteria (especially fecal coliforms and E. coli) are used as indicator organisms in water quality monitoring. Their presence signals potential fecal contamination and the possible presence of enteric pathogens such as viruses, protozoa, and pathogenic bacteria that originate from human or warm-blooded animal waste. Given Data / Assumptions: Indicator organisms are not necessarily harmful themselves but correlate with sanitary quality. Standard tests target total coliforms, fecal coliforms, or E. coli specifically. The context is potable water safety and public health protection. Concept / Approach: Detecting coliforms implies a route of fecal entry into the water supply (e.g., sewage intrusion, failing septic systems, cross-connections, or inadequate disinfection). While other contaminants may coexist, coliform detection specifically guides investigation of sanitary breaches and immediate corrective actions, such as boil-water advisories and system flushing with elevated disinfectant residuals. Step-by-Step Solution: Recognize coliforms as fecal indicator bacteria.Link positive results to contamination pathways from human/animal waste.Choose “human feces” as the most directly relevant contamination source among options. Verification / Alternative check: Public health frameworks require repeat sampling and, if confirmed positive for E. coli, immediate public notification because of the close linkage to fecal contamination and pathogen risk. Why Other Options Are Wrong: Radioactive wastes, fertilizer, heavy metals: Not indicated by coliform counts. Decaying plants/animals: Can affect taste/odor but are not specifically indicated by coliform presence. Common Pitfalls: Assuming total coliform positives always mean fecal contamination; distribution system biofilms can sometimes harbor coliforms—hence confirmatory tests for fecal coliforms or E. coli are crucial. Final Answer: human feces

Question 18

Environmental engineering – wastewater treatment overview: Which of the following are recognized biological treatment methods used in municipal and industrial wastewater treatment plants?

Accepted Answer

Introduction / Context: Biological wastewater treatment relies on microorganisms to remove dissolved and particulate organic matter, nutrients, and some toxic compounds. Common engineered systems include stabilization lagoons, the activated sludge process, and oxidation ditches. This question checks recognition of the major categories used in practice. Given Data / Assumptions: Municipal or industrial wastewater containing biodegradable organics. Operating under aerobic conditions for carbon removal. Typical secondary treatment step following primary clarification. Concept / Approach: All three listed options are biological processes. Lagoons (oxidation ponds) use large basins with long detention times, sunlight, algae–bacteria symbiosis, and natural aeration. Activated sludge uses mechanical aeration to maintain a suspended floc of biomass in a reactor with a secondary clarifier for solids separation and recycle. Oxidation ditches are extended-aeration variants with a looped channel providing long sludge age and stable nitrification/denitrification when configured properly. Step-by-Step Solution: Identify whether each listed technology depends on microbial metabolism.Lagoon: biological removal with natural/aerated oxygen supply → yes.Activated sludge: aerobic suspended growth with recycle → yes.Oxidation ditch: extended-aeration form of activated sludge → yes.Therefore, the correct selection that includes all true methods is 'All of these'. Verification / Alternative check: Typical design textbooks classify secondary biological treatment into suspended growth (activated sludge, oxidation ditch) and attached growth (trickling filters, MBBR). Lagoons are also categorized as biological treatment with long detention times. Why Other Options Are Wrong: Each individual option (a, b, or c) is correct by itself, but choosing only one would omit the other valid biological methods. Hence, only 'All of these' captures the full set. Common Pitfalls: Confusing biological processes with purely physical–chemical methods (e.g., coagulation or air stripping). Assuming lagoons are not 'engineered' because they are passive; they are standardized biological systems. Final Answer: All of these

Question 19

Water chemistry – temporary hardness removal: Which chemical addition can remove temporary hardness (bicarbonates of Ca^2+ and Mg^2+) from water by precipitation?

Accepted Answer

Introduction / Context: Hardness in water arises from multivalent cations, primarily calcium and magnesium. Temporary hardness is associated with bicarbonates and can be removed by boiling or by chemical softening. This question focuses on the classic lime softening reaction used in water treatment. Given Data / Assumptions: Temporary hardness species: Ca(HCO3)2 and Mg(HCO3)2. Goal: convert dissolved species into insoluble precipitates that can be settled/filtered. Ambient conditions typical of conventional water treatment basins. Concept / Approach: Adding calcium hydroxide [Ca(OH)2] raises pH and converts bicarbonates to carbonate, precipitating hardness as CaCO3 and Mg(OH)2. The net effect is removal of temporary hardness by forming low-solubility solids. Other salts listed either do not remove hardness or can increase it. Step-by-Step Solution: For Ca(HCO3)2: Ca(OH)2 + Ca(HCO3)2 → 2 CaCO3(s) + 2 H2O.For Mg(HCO3)2: Ca(OH)2 + Mg(HCO3)2 → MgCO3 + CaCO3 + 2 H2O; further: MgCO3 + Ca(OH)2 → Mg(OH)2(s) + CaCO3(s).Precipitates (CaCO3, Mg(OH)2) settle, reducing temporary hardness.Thus, calcium hydroxide is the appropriate reagent. Verification / Alternative check: Jar tests in water plants demonstrate turbidity rise from precipitates and reduced alkalinity post-lime dosing, consistent with hardness removal by precipitation. Why Other Options Are Wrong: Calcium carbonate: sparingly soluble; does not drive precipitation when added directly. Calcium chloride: adds Ca^2+, increasing hardness. Sodium bicarbonate: adds bicarbonate alkalinity, potentially increasing temporary hardness. Common Pitfalls: Confusing temporary vs permanent hardness; soda ash (Na2CO3) targets permanent hardness. Overdosing lime leading to excessive pH and post-stabilization needs. Final Answer: Calcium hydroxide (lime)

Question 20

Water hardness fundamentals: Temporary hardness in natural waters is primarily caused by the presence of which dissolved salts?

Accepted Answer

Introduction / Context: Understanding the chemical origin of hardness helps select the correct treatment method. 'Temporary' hardness refers to species that decompose upon boiling or alkali addition, whereas 'permanent' hardness persists without chemical softening. Given Data / Assumptions: Common cations: Ca^2+, Mg^2+. Common anions: HCO3^−, SO4^2−, Cl^−, CO3^2−. Definition: temporary hardness is removable by boiling (decomposition of bicarbonates). Concept / Approach: Bicarbonates of calcium and magnesium decompose upon heating to form insoluble carbonates and release CO2, eliminating hardness. Sulfates and chlorides remain dissolved and therefore contribute to permanent hardness. Sodium and potassium carbonates do not create hardness because Na^+ and K^+ are monovalent and do not form scale in the same way. Step-by-Step Solution: Temporary hardness salts: Ca(HCO3)2, Mg(HCO3)2.Upon boiling: Ca(HCO3)2 → CaCO3(s) + CO2 + H2O; Mg(HCO3)2 → MgCO3(s)/Mg(OH)2(s) + CO2 + H2O.Sulfates/chlorides do not decompose on boiling → permanent hardness.Therefore choose 'Bicarbonates of calcium and magnesium'. Verification / Alternative check: Classical titration of hardness (EDTA method) shows reductions after boiling due to precipitation of carbonate species only when bicarbonate hardness is present. Why Other Options Are Wrong: Chlorides/sulfates: do not cause temporary hardness; they contribute to permanent hardness. Na/K carbonates: monovalent cations; do not impart hardness in the standard definition. Common Pitfalls: Assuming all carbonate species are 'temporary'; the key is bicarbonate decomposition. Confusing alkalinity with hardness; they are related but distinct measures. Final Answer: Bicarbonates of calcium and magnesium

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