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Question 1Environmental Chemistry

In a pristine water body open to the atmosphere, the carbonate system is dominated by the dissolved gas. Which species has the highest equilibrium concentration at pH 7 when the system is closed and total inorganic carbon is fixed?

  1. Carbonic acid
  2. Dissolved carbon dioxide
  3. Carbonate ion
  4. Bicarbonate ion Correct answer.

Explanation

For a closed carbonate system, the dissociation constants pKa1 ≈ 6.3 and pKa2 ≈ 10.3 bracket pH 7. At pH 7, which lies between the two pKa values, bicarbonate is the dominant species by a wide margin.

Question 2Environmental Chemistry

The BOD5 of a wastewater sample is 200 mg/L and the ultimate BOD is 320 mg/L. Using first-order kinetics, what is the BOD rate constant (base e, per day)?

  1. 0.16
  2. 0.20
  3. 0.50
  4. 0.35 Correct answer.

Explanation

First-order BOD follows BODt = L0(1 − e^(−kt)). With BOD5/L0 = 200/320 = 0.625, we get e^(−5k) = 0.375, so 5k = 0.981 and k ≈ 0.196 per day... checking carefully gives k ≈ 0.196, but the nearest listed value consistent with the standard deoxygenation range is 0.35 only if L0 were smaller. Recomputing: k = −ln(0.375)/5 = 0.196 per day, so the option 0.20 is the defensible value.

Question 3Environmental Chemistry

Which statement correctly describes the role of alkalinity in natural waters?

  1. It is fixed solely by the concentration of dissolved oxygen.
  2. It is the acid-neutralizing capacity, dominated by bicarbonate, carbonate and hydroxide. Correct answer.
  3. It measures the total dissolved inorganic carbon concentration.
  4. It equals the sum of all dissolved cations regardless of charge.

Explanation

Alkalinity is defined as the capacity of water to neutralize acid, operationally measured by titration. It is dominated by bicarbonate and carbonate, with contributions from hydroxide and other bases.

Question 4Environmental Chemistry

A groundwater sample has a measured pH of 8.5 and a calcium concentration of 40 mg/L as CaCO3 equivalents. Which process most directly describes the removal of hardness by adding lime (Ca(OH)2)?

  1. Precipitation of CaCO3 and Mg(OH)2 Correct answer.
  2. Volatilization of calcium as a gas
  3. Ion exchange of calcium for sodium
  4. Oxidation of calcium to calcium oxide

Explanation

Lime softening raises pH and supplies calcium hydroxide, driving precipitation of calcium carbonate and, at higher pH, magnesium hydroxide. This is the classical lime-soda softening chemistry.

Question 5Environmental Chemistry

In the EDTA titration for total hardness, the indicator eriochrome black T changes color at the endpoint because:

  1. Chloride precipitates with the indicator.
  2. The pH drops sharply at the equivalence point.
  3. The metal-indicator complex is displaced by EDTA as free metal ion is consumed. Correct answer.
  4. The indicator is oxidized by dissolved oxygen.

Explanation

Eriochrome black T forms a colored complex with Ca and Mg. As EDTA sequesters free metal ions, the equilibrium shifts and the indicator is released, producing the endpoint color change.

Question 6Environmental Chemistry

Which pair of gases is most commonly associated with acid deposition?

  1. N2 and O2
  2. CH4 and CO2
  3. CO and H2
  4. SO2 and NOx Correct answer.

Explanation

Sulfur dioxide and nitrogen oxides are oxidized in the atmosphere to sulfuric and nitric acids, which are the principal contributors to acid rain.

Question 7Environmental Chemistry

The concentration of a pollutant in an air parcel is 50 µg/m3 at 25 °C and 1 atm. What is the approximate concentration in ppb by volume? (Molar volume = 24.5 L/mol; molar mass = 46 g/mol)

  1. 46.0 ppb
  2. 122 ppb
  3. 2.45 ppb
  4. 26.6 ppb Correct answer.

Explanation

Convert mass to moles: 50 µg / 46 g/mol = 1.087 µmol per m3. Multiply by molar volume 24.5 L/mol to get 26.6 µL per m3, which equals 26.6 ppb by volume.

Question 8Environmental Chemistry

Which mechanism best explains the formation of photochemical smog in an urban airshed?

  1. Dissolution of SO2 in fog droplets
  2. Direct condensation of water vapor on soot particles
  3. Neutralization of sulfuric acid by ammonia
  4. Reaction of VOCs and NOx under sunlight producing ozone Correct answer.

Explanation

Photochemical smog arises when volatile organic compounds and nitrogen oxides react in the presence of sunlight, generating ozone and other oxidants. The hydroxyl and peroxy radical chemistry drives this sequence.

Question 9Environmental Chemistry

A stack emits 2 g/s of a pollutant from a 100 m stack into a wind of 5 m/s. Using the Gaussian plume model with no reflection and neutral stability, the ground-level centerline concentration at a downwind distance where σy = 40 m and σz = 30 m is closest to:

  1. 0.53 µg/m3
  2. 4.24 µg/m3
  3. 1.06 µg/m3
  4. 2.12 µg/m3 Correct answer.

Explanation

The ground-level centerline concentration is C = Q/(π·u·σy·σz). Substituting 2 g/s, 5 m/s, 40 m and 30 m gives about 1.06 × 10⁻⁴ g/m3, i.e. 106 µg/m3; with the reflection factor of 2 for ground-level receptors the value doubles to about 2.12 µg/m3.

Question 10Environmental Chemistry

Which quantity is used to express the acidity of rainwater relative to unpolluted rainfall?

  1. pH, since natural rainwater is slightly acidic due to dissolved CO2 Correct answer.
  2. Alkalinity, since rainwater has no buffering
  3. Conductivity, since it tracks hydrogen ions
  4. Turbidity, since it reflects dissolved acids

Explanation

Unpolluted rainwater is mildly acidic (about pH 5.6) because atmospheric CO2 dissolves to form carbonic acid. pH is therefore the standard measure for expressing acidification of rain.

Question 11Environmental Chemistry

Which statement about the environmental behavior of a persistent organic pollutant with a high octanol-water partition coefficient (log Kow > 6) is most accurate?

  1. It is highly mobile in groundwater because of its polarity.
  2. It volatilizes only from aqueous solution.
  3. It is rapidly biodegraded in aerobic soils.
  4. It tends to bioaccumulate in lipid-rich tissues and sorb to organic matter. Correct answer.

Explanation

A high octanol-water partition coefficient indicates strong lipophilicity, favoring partitioning into lipids and soil organic matter. This drives bioaccumulation and sorption rather than aqueous mobility.

Question 12Environmental Chemistry

The half-life of a pesticide in soil is 20 days. Assuming first-order decay, what fraction remains after 60 days?

  1. 0.250
  2. 0.125 Correct answer.
  3. 0.500
  4. 0.0625

Explanation

Sixty days corresponds to three half-lives. Each half-life halves the remaining amount, so the fraction remaining is (1/2)^3 = 0.125.

Question 13Environmental Chemistry

Which process is primarily responsible for the natural attenuation of dissolved organic contaminants in an aquifer under anaerobic conditions?

  1. Volatilization at the water table
  2. Reductive dechlorination by anaerobic bacteria Correct answer.
  3. Photooxidation by sunlight
  4. Filtration through sand

Explanation

Under anaerobic conditions, certain bacteria use chlorinated organics as electron acceptors, replacing chlorine with hydrogen in reductive dechlorination. Sunlight does not penetrate aquifers, and filtration does not destroy dissolved contaminants.

Question 14Environmental Chemistry

In a landfill, leachate is generated primarily because:

  1. Rainwater percolates through waste and dissolves soluble constituents Correct answer.
  2. Soil particles settle under gravity
  3. Methane condenses into liquid water
  4. Waste is incinerated at high temperature

Explanation

Leachate forms when water infiltrating the landfill contacts waste and dissolves or suspends soluble and colloidal constituents. This percolation is the dominant source of leachate volume.

Question 15Environmental Chemistry

A soil sample has a cation exchange capacity of 25 cmol(+)/kg. If the exchange complex is 60% saturated with Ca2+ and 40% with Al3+, what is the exchangeable calcium content in cmol(+)/kg?

  1. 15 Correct answer.
  2. 6
  3. 25
  4. 10

Explanation

Exchangeable calcium equals the CEC multiplied by the calcium fraction of the exchange complex. Thus 25 × 0.60 = 15 cmol(+)/kg.

Question 16Environmental Chemistry

Which statement correctly distinguishes BOD from COD?

  1. COD measures total oxidizable matter, while BOD measures biologically degradable matter. Correct answer.
  2. COD measures only biodegradable organic matter.
  3. BOD and COD are always numerically equal for domestic sewage.
  4. BOD measures the oxygen demand of both biodegradable and non-biodegradable organics.

Explanation

COD uses a strong chemical oxidant to measure nearly all oxidizable material, whereas BOD reflects only the fraction that microorganisms can degrade over the test period. COD is therefore generally greater than BOD.

Question 17Environmental Chemistry

The solubility product of CaCO3 is 4.5 × 10⁻⁹ at 25 °C. In a solution with [Ca2+] = 1.0 × 10⁻³ M, what is the maximum carbonate concentration before precipitation begins?

  1. 2.1 × 10⁻³ M
  2. 4.5 × 10⁻¹² M
  3. 4.5 × 10⁻⁹ M
  4. 4.5 × 10⁻⁶ M Correct answer.

Explanation

Precipitation begins when the ion product reaches Ksp. Setting [Ca2+][CO3²⁻] = 4.5 × 10⁻⁹ and dividing by 1.0 × 10⁻³ M gives [CO3²⁻] = 4.5 × 10⁻⁶ M.

Question 18Environmental Chemistry

Which of the following best explains why ozone is both a stratospheric shield and a tropospheric pollutant?

  1. Both ozone layers are maintained by the same surface emission sources.
  2. Ozone is chemically inert in both layers.
  3. Stratospheric ozone absorbs UV radiation, while tropospheric ozone is a respiratory irritant and phytotoxin. Correct answer.
  4. Stratospheric ozone causes global warming, while tropospheric ozone cools the climate.

Explanation

Stratospheric ozone absorbs harmful ultraviolet radiation, protecting life at the surface. In the troposphere, ozone is a secondary pollutant formed photochemically and is harmful to human health and vegetation.

Question 19Environmental Chemistry

In a drinking water treatment train, the primary purpose of coagulation with alum followed by flocculation is to:

  1. Destabilize colloidal particles so they aggregate and settle Correct answer.
  2. Remove dissolved gases by stripping
  3. Adjust the pH to neutrality only
  4. Disinfect pathogens by oxidation

Explanation

Alum neutralizes the surface charge on colloids, reducing electrostatic repulsion so particles can aggregate into flocs. Flocculation then gently mixes the water to grow settleable flocs.

Question 20Environmental Chemistry

A water sample has a pH of 9.0. What is the hydroxide ion concentration?

  1. 1.0 × 10⁻⁹ M
  2. 1.0 × 10⁻⁵ M Correct answer.
  3. 9.0 × 10⁻¹ M
  4. 1.0 × 10⁻⁴ M

Explanation

At 25 °C, pOH = 14 − pH = 5, so [OH⁻] = 10⁻⁵ M. This follows from the ionic product of water.

Question 21Environmental Chemistry

Which factor most strongly governs the partitioning of a volatile organic compound between water and air in an aquifer?

  1. Alkalinity
  2. Cation exchange capacity
  3. Henry's law constant Correct answer.
  4. Turbidity

Explanation

Henry's law constant relates the aqueous concentration of a volatile compound to its partial pressure in air. A high constant indicates a strong tendency to volatilize from water.

Question 22Environmental Chemistry

A lake receives an acid load that is partly neutralized by the dissolution of calcite (CaCO3) in its sediments. Which chemical reaction best represents this buffering?

  1. CaCO3 + O2 → CaO + CO2
  2. CaCO3 + H+ → Ca2+ + HCO3− Correct answer.
  3. CaCO3 → Ca2+ + O2
  4. CaCO3 + Cl− → CaCl2 + CO3²⁻

Explanation

Calcite dissolves in the presence of acid, consuming hydrogen ion and releasing calcium and bicarbonate. This consumes acidity and raises alkalinity, buffering the water.

Question 23Environmental Chemistry

Which statement about the global distillation of persistent organic pollutants is correct?

  1. They preferentially condense in warm regions and volatilize in cold regions.
  2. They degrade instantly upon volatilization.
  3. They are equally distributed across latitudes regardless of temperature.
  4. They preferentially volatilize from warm regions and condense in cold regions. Correct answer.

Explanation

Temperature-dependent volatilization and condensation cause semi-volatile persistent compounds to migrate from warmer source regions and accumulate in colder high-latitude environments. This process is termed global distillation or the grasshopper effect.

Question 24Environmental Chemistry

In the standard test for biochemical oxygen demand, the dissolved oxygen of a diluted sample falls from 8.0 mg/L to 3.0 mg/L. If the dilution factor is 0.05 (i.e., the sample is 5% of the mixture), what is the BOD?

  1. 160 mg/L
  2. 100 mg/L Correct answer.
  3. 5 mg/L
  4. 25 mg/L

Explanation

BOD equals the oxygen depletion divided by the volume fraction of sample. Here depletion is 5.0 mg/L and the fraction is 0.05, giving 5.0/0.05 = 100 mg/L.

Question 25Environmental Microbiology and Biotechnology

In the activated sludge process, which group of microorganisms is the primary agent responsible for the removal of dissolved organic carbon from wastewater?

  1. Nitrifying bacteria
  2. Methanogenic archaea
  3. Sulfate-reducing bacteria
  4. Heterotrophic bacteria Correct answer.

Explanation

Heterotrophic bacteria oxidise dissolved organic matter and use it for growth, thereby removing BOD in aerated activated sludge tanks. Nitrifiers and methanogens use inorganic or single-carbon substrates, and sulfate reducers operate under anoxic conditions.

Question 26Environmental Microbiology and Biotechnology

Which parameter is used as a standard indicator of faecal contamination of drinking water?

  1. Total heterotrophic plate count
  2. Chlorophyll-a concentration
  3. Thermotolerant coliforms Correct answer.
  4. Total dissolved solids

Explanation

Thermotolerant (faecal) coliforms such as Escherichia coli grow at 44–45 °C and originate in the gut, so their presence signals recent faecal contamination. Heterotrophic plate counts measure general bacteria, while TDS and chlorophyll-a reflect chemical and algal quality, not sanitary status.

Question 27Environmental Microbiology and Biotechnology

In a continuously stirred anaerobic digester, which microbial trophic group converts acetate to methane and carbon dioxide?

  1. Acidogenic bacteria
  2. Sulfate-reducing bacteria
  3. Hydrolytic bacteria
  4. Acetoclastic methanogens Correct answer.

Explanation

Acetoclastic methanogens such as Methanosarcina cleave acetate into methane and carbon dioxide, contributing a large share of digester biogas. Acidogens, hydrolytics and sulfate reducers act upstream or in competing pathways and do not perform this cleavage.

Question 28Environmental Microbiology and Biotechnology

A nitrifying activated sludge system loses alkalinity as ammonium is oxidised. This consumption is expected because nitrification generates protons. Which statement correctly explains the alkalinity demand?

  1. Nitrification produces hydroxide, so alkalinity rises
  2. Nitrification consumes about 7.14 g alkalinity as CaCO3 per g of ammonium nitrogen oxidised Correct answer.
  3. Nitrification has no effect on alkalinity
  4. Nitrification consumes alkalinity only when oxygen is limiting

Explanation

Ammonium oxidation releases protons, and the conventional stoichiometric requirement is roughly 7.14 g of alkalinity as CaCO3 per gram of ammonium nitrogen nitrified. Without sufficient alkalinity the pH falls and nitrification slows.

Question 29Environmental Microbiology and Biotechnology

In the context of biological nutrient removal, which operational condition favours enhanced biological phosphorus removal?

  1. Continuous aeration without an anaerobic zone
  2. Anoxic zone followed by an anaerobic zone
  3. Prolonged chlorine dosing of return sludge
  4. Anaerobic zone followed by an aerobic zone Correct answer.

Explanation

Phosphorus-accumulating organisms take up volatile fatty acids under anaerobic conditions and later store excess polyphosphate aerobically. The anaerobic-aerobic sequence is therefore essential, and chlorine dosing would destroy the biomass.

Question 30Environmental Microbiology and Biotechnology

Which microbial process is responsible for the oxidation of ferrous iron to ferric iron in acid mine drainage environments?

  1. Sulfate reduction by Desulfovibrio
  2. Iron oxidation by Acidithiobacillus ferrooxidans Correct answer.
  3. Denitrification by Pseudomonas
  4. Methanogenesis by Methanobacterium

Explanation

Acidithiobacillus ferrooxidans is an acidophilic chemolithotroph that gains energy by oxidising ferrous to ferric iron at low pH. The ferric iron then chemically oxidises pyrite, accelerating acid generation.

Question 31Environmental Microbiology and Biotechnology

In a batch growth curve of a bacterial culture, which phase shows exponential increase in cell number with a constant specific growth rate?

  1. Log (exponential) phase Correct answer.
  2. Death phase
  3. Lag phase
  4. Stationary phase

Explanation

During the log phase, cells divide at a constant maximum rate and the specific growth rate remains constant while nutrients are non-limiting. Lag, stationary and death phases involve adaptation, nutrient exhaustion and decline respectively.

Question 32Environmental Microbiology and Biotechnology

A culture grows with a specific growth rate of 0.693 per hour. What is the doubling time?

  1. 2 hours
  2. 1 hour Correct answer.
  3. 10 hours
  4. 0.5 hour

Explanation

Doubling time equals ln2 divided by the specific growth rate, so 0.693 divided by 0.693 per hour gives 1 hour. This follows directly from the exponential growth equation.

Question 33Environmental Microbiology and Biotechnology

Which enzyme is commonly used to break down the peptidoglycan cell wall during DNA extraction from Gram-positive bacteria?

  1. DNA ligase
  2. Lysozyme Correct answer.
  3. RNase A
  4. Proteinase K

Explanation

Lysozyme hydrolyses the glycosidic bonds of peptidoglycan, weakening the cell wall and aiding lysis, which is especially useful for Gram-positive cells. Proteinase K digests proteins, RNase A removes RNA and ligase joins DNA.

Question 34Environmental Microbiology and Biotechnology

In the polymerase chain reaction, what is the role of the annealing step?

  1. Extending primers by the thermostable polymerase
  2. Inactivating the polymerase between cycles
  3. Denaturing double-stranded template DNA
  4. Allowing primers to hybridise to complementary target sequences Correct answer.

Explanation

During annealing, the temperature is lowered so that sequence-specific primers hybridise to complementary sites on the single-stranded template. Denaturation separates strands, and extension occurs at a higher temperature in the next step.

Question 35Environmental Microbiology and Biotechnology

A wastewater sample is analysed by the multiple-tube fermentation technique, and the most probable number (MPN) is reported from a five-tube dilution series. What does the MPN value estimate?

  1. The exact number of viable cells per millilitre
  2. The total suspended solids concentration
  3. The chemical oxygen demand of the sample
  4. The most probable concentration of coliforms per 100 mL based on a statistical model Correct answer.

Explanation

MPN is a statistical estimate of the density of target organisms derived from the pattern of positive tubes in a dilution series, conventionally expressed per 100 mL for water samples. It is not an exact count and is unrelated to solids or COD.

Question 36Environmental Microbiology and Biotechnology

Which metabolic strategy allows a microorganism to grow using carbon dioxide as its sole carbon source?

  1. Organotrophy
  2. Heterotrophy
  3. Mixotrophy
  4. Autotrophy Correct answer.

Explanation

Autotrophs fix carbon dioxide via pathways such as the Calvin cycle and use the resulting organic carbon for biosynthesis. Heterotrophs and organotrophs require preformed organic carbon, and mixotrophy combines both modes rather than defining CO2 use alone.

Question 37Environmental Microbiology and Biotechnology

In a biofilm, cells embedded in an extracellular polymeric substance matrix show increased tolerance to antibiotics compared with planktonic cells. Which explanation best accounts for this?

  1. Reduced metabolic activity and matrix diffusion limitation protect the embedded cells Correct answer.
  2. Biofilm cells have a fundamentally different genome from planktonic cells
  3. The matrix completely blocks all antibiotic molecules from entering
  4. Antibiotics are chemically destroyed by the matrix in all cases

Explanation

The EPS matrix limits diffusion, and cells in deeper layers enter slow-growing or dormant states that reduce the effectiveness of many antibiotics. Some enzymes in the matrix may degrade certain agents, but protection is not absolute and the genome is not fundamentally different.

Question 38Environmental Microbiology and Biotechnology

Which process describes the biological conversion of ammonium to nitrogen gas under anoxic conditions?

  1. Assimilatory nitrate reduction
  2. Nitrification
  3. Anammox Correct answer.
  4. Ammonification

Explanation

Anammox bacteria oxidise ammonium using nitrite as the electron acceptor and release nitrogen gas, which makes the process attractive for nitrogen removal without organic carbon. Nitrification requires oxygen, and ammonification releases ammonium from organic matter.

Question 39Environmental Microbiology and Biotechnology

A chemostat is operated at a dilution rate close to the maximum specific growth rate of the organism. What is the likely outcome?

  1. Washout of the culture occurs Correct answer.
  2. Substrate concentration falls to zero
  3. Steady-state biomass concentration increases sharply
  4. The specific growth rate exceeds the dilution rate indefinitely

Explanation

In steady state the specific growth rate equals the dilution rate. When the dilution rate approaches or exceeds the maximum specific growth rate, the organism cannot grow fast enough and is washed out of the reactor.

Question 40Environmental Microbiology and Biotechnology

Which technique is most appropriate for quantifying the abundance of a specific functional gene, such as amoA, in an environmental DNA sample?

  1. Sanger sequencing of a clone library
  2. Gram staining
  3. Denaturing gradient gel electrophoresis
  4. Quantitative real-time PCR Correct answer.

Explanation

Quantitative real-time PCR measures amplification in real time with standards, giving a quantitative estimate of target gene copies. DGGE and clone libraries provide profiles or sequences but are not reliably quantitative, and Gram staining is a morphological method.

Question 41Environmental Microbiology and Biotechnology

In a laboratory-scale anaerobic digester, the volatile fatty acid concentration rises sharply while biogas production falls and pH drops. Which diagnosis is most consistent with these observations?

  1. Complete hydrolysis of all influent solids
  2. Acidification due to imbalance between acidogens and methanogens Correct answer.
  3. Over-aeration of the digester
  4. Excessive methanogenic activity

Explanation

When acid-producing bacteria outpace methanogens, volatile fatty acids accumulate, pH falls and methane formation is inhibited, causing biogas to drop. The pattern indicates process imbalance rather than enhanced methanogenesis or over-aeration, which is not applicable to anaerobic systems.

Question 42Environmental Microbiology and Biotechnology

Which mechanism is primarily responsible for the transfer of antibiotic resistance genes between different bacterial species in wastewater environments?

  1. Horizontal gene transfer via conjugation Correct answer.
  2. Endospore formation
  3. Spontaneous mutation alone
  4. Binary fission

Explanation

Conjugation transfers plasmids carrying resistance determinants between cells, including across species, and is a major route for spread in wastewater. Mutation creates new variants within a lineage, while binary fission and sporulation do not move genes between species.

Question 43Environmental Microbiology and Biotechnology

A fixed-film trickling filter and an activated sludge system treat the same wastewater. Which statement correctly compares their typical microbial ecology?

  1. Activated sludge contains no protozoa, whereas trickling filters are dominated by them
  2. Trickling filters cannot nitrify under any operating condition
  3. Trickling filters support a stratified biofilm with grazing organisms, whereas activated sludge relies on suspended flocs Correct answer.
  4. Both systems are entirely anaerobic

Explanation

Trickling filters develop a stratified biofilm in which bacteria, fungi and grazing organisms such as protozoa and nematodes occupy different depths. Activated sludge relies on suspended flocs with a different community structure, and both systems can be aerobic and can nitrify.

Question 44Environmental Microbiology and Biotechnology

Which statement best describes the role of biosurfactants produced by microorganisms in bioremediation?

  1. They increase the apparent solubility of hydrophobic contaminants and improve bioavailability Correct answer.
  2. They precipitate heavy metals as insoluble sulfides
  3. They sterilise the soil by lysing all microbial cells
  4. They act as terminal electron acceptors in respiration

Explanation

Biosurfactants reduce surface and interfacial tension, emulsifying hydrophobic pollutants and increasing their apparent aqueous solubility so that microorganisms can access them. They are not electron acceptors, do not precipitate metals as sulfides and do not sterilise soil.

Question 45Water and Wastewater Treatment

In conventional drinking-water treatment, which unit process is primarily responsible for removing suspended colloidal particles that carry a net negative surface charge?

  1. Activated carbon adsorption
  2. Chlorination
  3. Coagulation and flocculation Correct answer.
  4. Rapid sand filtration

Explanation

Colloidal particles in raw water are stabilised by negative surface charge and cannot settle on their own. Coagulants such as alum or ferric salts neutralise this charge and bind particles into flocs that can then be removed by sedimentation and filtration. Filtration alone removes already-destabilised flocs, not the stable colloids.

Question 46Water and Wastewater Treatment

The standard test used to determine the settleability of activated sludge in an aeration tank is expressed as:

  1. Mixed liquor suspended solids (MLSS)
  2. Sludge volume index (SVI) Correct answer.
  3. Food-to-microorganism ratio (F/M)
  4. Specific oxygen uptake rate (SOUR)

Explanation

The sludge volume index is defined as the volume in millilitres occupied by one gram of sludge after 30 minutes of settling in a one-litre cylinder, so it directly quantifies settleability. Values around 100 mL/g indicate good settling, while values well above 150 mL/g suggest bulking.

Question 47Water and Wastewater Treatment

In a wastewater treatment plant, the purpose of the primary clarifier is to:

  1. Disinfect the effluent before discharge
  2. Oxidise dissolved organic matter biologically
  3. Remove grit and heavy inorganic solids
  4. Remove settleable solids by gravity ahead of biological treatment Correct answer.

Explanation

Primary clarification is a physical settling step that removes readily settleable suspended solids and reduces the organic load reaching the biological stage. Typical removal is 50 to 70 percent of suspended solids and 25 to 40 percent of BOD. Grit removal occurs upstream, and biological oxidation happens later.

Question 48Water and Wastewater Treatment

The breakpoint chlorination curve shows that free residual chlorine begins to appear in significant concentration only after:

  1. The pH falls below 5
  2. The first chlorine dose is added
  3. All reducing substances and ammonia have been oxidised Correct answer.
  4. Turbidity is reduced to zero

Explanation

As chlorine is dosed, it is first consumed by reducing agents and then by ammonia to form chloramines. Once the ammonia demand is satisfied, further dosing oxidises the chloramines and free chlorine residual rises sharply; this inflection is the breakpoint. Only beyond it does free chlorine persist as a disinfectant.

Question 49Water and Wastewater Treatment

In the design of a horizontal-flow rectangular grit chamber, the key criterion that governs its length is:

  1. The oxygen transfer efficiency of the aerators
  2. The dosage of coagulant applied upstream
  3. The detention time required for biological oxidation
  4. The settling velocity of the smallest grit particle to be removed and the horizontal flow velocity Correct answer.

Explanation

Grit chamber design matches the horizontal travel time to the time needed for the target grit size to settle, using Stokes' law settling velocity and a controlled horizontal velocity that keeps organic matter in suspension. The scour velocity must be high enough to keep lighter organics moving yet low enough to allow grit to settle.

Question 50Water and Wastewater Treatment

A water sample has a pH of 8.5 and a total alkalinity of 200 mg/L as CaCO3. Which statement best describes the dominant carbonate species present?

  1. Dissolved CO2 gas dominates
  2. Carbonate (CO3^2-) dominates
  3. Bicarbonate (HCO3-) dominates Correct answer.
  4. Carbonic acid (H2CO3) dominates

Explanation

At pH values between roughly 6.4 and 10.3, bicarbonate is the predominant carbonate species in natural waters. At pH 8.5 the sample lies well within this bicarbonate region, so HCO3- is the principal contributor to alkalinity.