Effluent testing – BOD, COD, contaminants MCQs With Answer

Effluent testing – BOD, COD, contaminants MCQs With Answer

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Effluent testing – BOD, COD, contaminants MCQs With Answer

This quiz set is designed for M.Pharm students to strengthen understanding of effluent testing focused on BOD, COD and common contaminants. It covers principles, standard methods, sample handling, interferences, calculations and interpretation of results in the context of pharmaceutical effluent management. Questions emphasize practical laboratory considerations—preservation, reagents, seed corrections, nitrification inhibition—and regulatory relevance such as biodegradability indicators and impacts on receiving waters. Use these MCQs to test conceptual depth, prepare for examinations, and support safe, compliant effluent monitoring in pharmaceutical manufacturing and wastewater treatment settings.

Q1. What does BOD5 represent in effluent testing?

  • The amount of oxygen required for complete chemical oxidation of sample
  • The oxygen consumed by microorganisms in 5 days at 20°C
  • The theoretical oxygen demand calculated from molecular formula
  • The oxygen required to oxidize inorganic salts in sample

Correct Answer: The oxygen consumed by microorganisms in 5 days at 20°C

Q2. Which reagent is the primary oxidant in the standard COD closed reflux method?

  • Potassium permanganate
  • Potassium dichromate
  • Sodium hypochlorite
  • Hydrogen peroxide

Correct Answer: Potassium dichromate

Q3. Why is mercuric sulfate added during COD analysis?

  • To catalyze oxidation of organic matter
  • To oxidize ammonia to nitrate
  • To remove chloride interference by forming chloride complexes
  • To act as a color indicator for titration

Correct Answer: To remove chloride interference by forming chloride complexes

Q4. Which statement correctly describes the relationship between COD and BOD for most wastewaters?

  • BOD is always higher than COD for all effluents
  • COD is typically higher than BOD because it measures chemically oxidizable materials including non‑biodegradables
  • BOD and COD measure identical portions of pollutants using different units
  • COD only measures volatile organics while BOD measures dissolved organics

Correct Answer: COD is typically higher than BOD because it measures chemically oxidizable materials including non‑biodegradables

Q5. What is the purpose of adding a seed to some BOD tests?

  • To increase sample conductivity for titration
  • To provide a microbial inoculum when the sample is low in active bacteria
  • To chemically neutralize acids in the sample
  • To precipitate suspended solids before incubation

Correct Answer: To provide a microbial inoculum when the sample is low in active bacteria

Q6. Which compound is commonly used as a nitrification inhibitor in BOD testing to prevent oxygen consumption by nitrifying bacteria?

  • Ammonium chloride
  • Allylthiourea (ATU)
  • Potassium dichromate
  • Mercuric chloride

Correct Answer: Allylthiourea (ATU)

Q7. What is the correct unit for reporting BOD and COD concentrations?

  • mg/L as O2
  • ppm as CaCO3
  • % v/v
  • mg/kg

Correct Answer: mg/L as O2

Q8. How should wastewater samples for BOD and COD be preserved before analysis?

  • Kept at ambient lab temperature and analyzed within 72 hours
  • Frozen at −20°C until analysis
  • Cooled to 4°C and analyzed within 24 hours
  • Acidified to pH 2 and stored at room temperature

Correct Answer: Cooled to 4°C and analyzed within 24 hours

Q9. What does ThOD (theoretical oxygen demand) represent?

  • The oxygen required for biological degradation in 5 days
  • The stoichiometric oxygen required for complete chemical oxidation to CO2, H2O and mineral forms
  • The measured oxygen uptake during COD test
  • The oxygen demand caused only by nitrogenous compounds

Correct Answer: The stoichiometric oxygen required for complete chemical oxidation to CO2, H2O and mineral forms

Q10. Which parameter primarily differentiates TSS from TDS in effluent analysis?

  • TSS refers to dissolved salts; TDS refers to volatile solids
  • TSS are particles retained on a filter (suspended); TDS are dissolved after filtration
  • TSS measures oxygen demand; TDS measures conductivity
  • TSS is measured gravimetrically while TDS is always measured spectrophotometrically

Correct Answer: TSS are particles retained on a filter (suspended); TDS are dissolved after filtration

Q11. Which of the following contaminants is most likely to cause acute inhibition of activated sludge microorganisms and reduce BOD removal?

  • Simple carbohydrates
  • Phenolic compounds
  • Inorganic bicarbonate
  • Low concentrations of nitrate

Correct Answer: Phenolic compounds

Q12. A sample shows BOD5/COD ratio of 0.75. How would you classify its biodegradability?

  • Non‑biodegradable
  • Moderately biodegradable (poor)
  • Readily biodegradable
  • Only chemically oxidizable, not biologically

Correct Answer: Readily biodegradable

Q13. During COD titration, what is the role of silver sulfate?

  • To neutralize excess acid after digestion
  • To act as an indicator for titration end point
  • To catalyze oxidation of certain organic chlorides and improve oxidation efficiency
  • To precipitate dissolved solids prior to digestion

Correct Answer: To catalyze oxidation of certain organic chlorides and improve oxidation efficiency

Q14. Which interference can lead to overestimation of COD if not properly controlled?

  • Excess dissolved oxygen in sample
  • High chloride concentration
  • Low alkalinity below 50 mg/L as CaCO3
  • Presence of biodegradable proteins

Correct Answer: High chloride concentration

Q15. Why is dilution water buffered and supplemented with nutrients in BOD tests?

  • To neutralize sample color for spectrophotometric reading
  • To supply essential nutrients and maintain pH for microbial activity during incubation
  • To inactivate toxic compounds present in the sample
  • To chemically oxidize organic matter prior to incubation

Correct Answer: To supply essential nutrients and maintain pH for microbial activity during incubation

Q16. Which statement about BOD5 measurement is true?

  • BOD5 measures ultimate biodegradability achieved over decades
  • BOD5 underestimates ultimate BOD because some organics degrade slowly beyond 5 days
  • BOD5 includes oxygen demand from complete chemical oxidation of inorganics
  • BOD5 is independent of incubation temperature

Correct Answer: BOD5 underestimates ultimate BOD because some organics degrade slowly beyond 5 days

Q17. Which effluent characteristic is most directly associated with causing dissolved oxygen depletion in receiving waters?

  • High conductivity
  • High biochemical oxygen demand (BOD)
  • High pH above 9.0
  • High turbidity from inorganic clays

Correct Answer: High biochemical oxygen demand (BOD)

Q18. In the closed reflux COD method, why are digestion tubes sealed during heating?

  • To ensure volatile organics escape and are not measured
  • To maintain reflux conditions and prevent loss of volatile oxidation products, improving oxidation completeness
  • To allow air exchange and supply oxygen for oxidation
  • To keep the sample anaerobic during digestion

Correct Answer: To maintain reflux conditions and prevent loss of volatile oxidation products, improving oxidation completeness

Q19. Which contaminant type is more likely to contribute to COD but not significantly to BOD?

  • Readily biodegradable sugars
  • Recalcitrant chlorinated organic compounds resistant to biological degradation
  • Simple amino acids
  • Volatile fatty acids

Correct Answer: Recalcitrant chlorinated organic compounds resistant to biological degradation

Q20. When conducting BOD analyses with seeded samples, how is the final BOD result typically corrected?

  • By multiplying by the seed volume fraction
  • By subtracting the oxygen uptake measured in a seed blank from the seeded sample result
  • By adding the seed blank value to the sample result
  • No correction is needed because seed contribution is negligible

Correct Answer: By subtracting the oxygen uptake measured in a seed blank from the seeded sample result

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