Test 20 - COD and Wastewater Strength

Question 1 ప్రశ్న 1

In wastewater engineering, which statement best defines chemical oxygen demand (COD) as determined by the standard dichromate method?

A The concentration of dissolved oxygen immediately available to aquatic organisms. B The oxygen consumed solely by living microorganisms during a fixed incubation period. C The oxygen equivalent of substances chemically oxidized under specified test conditions, including some inorganic reducing substances. D The mass concentration of suspended solids that settle in one hour.

Question 2 ప్రశ్న 2

Which pairing correctly identifies the current BIS COD testing standard and the principal oxidant in its open-reflux method?

A IS 3025 (Part 58):2023 and potassium dichromate in acidic medium. B IS 3025 (Part 44):2023 and dissolved oxygen supplied by an aerator. C IS 10500 and sodium chloride in alkaline medium. D IS 3025 (Part 15):2023 and potassium permanganate as the prescribed COD oxidant.

Question 3 ప్రశ్న 3

Which comparison of COD and BOD testing is technically most accurate for routine wastewater assessment?

A COD and BOD are both obtained solely from the decrease in dissolved oxygen after five days. B COD requires a viable microbial seed, while BOD is determined only by chemical reflux. C COD invariably measures only biodegradable carbon, whereas BOD measures every nonbiodegradable compound. D COD is determined chemically in hours, whereas the conventional BOD test requires a stated multiday incubation such as 3 days at 27°C or 5 days at 20°C.

Question 4 ప్రశ్న 4

A wastewater has BOD5/COD = 0.45, measured on comparable samples without apparent toxicity. Which interpretation is most defensible?

A Exactly 45% of the wastewater mass is biodegradable and exactly 55% is inert. B The ratio suggests appreciable biological treatability, but it is not an exact measurement of the biodegradable COD fraction. C The wastewater is guaranteed to achieve 45% COD removal in any biological reactor. D The COD value must be smaller than the BOD5 value because the ratio is below one.

Question 5 ప్రశ్న 5

A treatment plant operator needs a same-day indication of changing influent oxidizable load while retaining biological-process performance monitoring. Which approach is most appropriate?

A Replace all BOD testing permanently with a dissolved-oxygen meter reading. B Use total suspended solids as an exact substitute for COD and BOD. C Use COD for relatively rapid chemical oxygen-demand tracking and maintain BOD or respirometric checks for biological interpretation. D Assume that each COD result is equal to the five-day BOD without establishing a relationship.

Question 6 ప్రశ్న 6

A high-chloride industrial wastewater is tested for COD using acidic dichromate. Which interference and control are correctly paired?

A Chloride can cause a positive COD interference; mercuric sulfate is used in specified methods to complex chloride. B Chloride always lowers measured COD; silver sulfate is added solely to remove suspended solids. C Chloride is the microbial seed; sodium hydroxide is added to prevent nitrification. D Chloride has no possible effect on dichromate COD, even at high concentrations.

Question 7 ప్రశ్న 7

What is the principal function of silver sulfate included in the sulfuric-acid reagent of a conventional dichromate COD test?

A It is the primary oxidant consumed in place of potassium dichromate. B It supplies the microbial organisms needed for BOD exertion. C It neutralizes the wastewater so that the digestion proceeds at pH 7. D It acts as a catalyst that promotes oxidation of certain organic compounds.

Question 8 ప్రశ్న 8

In an open-reflux COD test, the blank and digested sample receive identical dichromate doses and the remaining oxidant is titrated with ferrous ammonium sulfate (FAS). For a positive COD result, which titration relationship normally holds?

A The sample requires more FAS than the reagent blank because it has consumed more dichromate. B The reagent blank requires more FAS than the sample because the sample has consumed part of the dichromate. C Blank and sample must require identical FAS volumes for every valid wastewater. D The FAS titration volumes are unrelated to the amount of oxidant consumed.

Question 9 ప్రశ్న 9

In a titrimetric COD determination, FAS consumption is 18.0 mL for the reagent blank and 10.0 mL for a 40.0 mL sample. If FAS normality is 0.100 N and COD = (A - B)N(8000)/V, what is the COD?

A 80 mg/L B 320 mg/L C 160 mg/L D 640 mg/L

Question 10 ప్రశ్న 10

The reagent blank needs 25.0 mL of 0.100 N FAS, while a digested 20.0 mL wastewater aliquot needs 13.0 mL. Using COD = (A - B)N(8000)/V, determine COD of the aliquot before any further dilution correction.

A 480 mg/L B 240 mg/L C 960 mg/L D 120 mg/L

Question 11 ప్రశ్న 11

An industrial effluent is diluted by making 1 volume of original sample up to 5 total volumes before COD digestion. The diluted sample gives COD = 130 mg/L with negligible dilution-water COD. What is the original effluent COD?

A 26 mg/L B 130 mg/L C 520 mg/L D 650 mg/L

Question 12 ప్రశ్న 12

A wastewater sample has BOD5 = 180 mg/L and COD = 500 mg/L under comparable testing conditions. What is its BOD5/COD ratio?

A 0.64 B 0.36 C 2.78 D 0.18

Question 13 ప్రశ్న 13

Two comparable wastewaters have the following results: X, BOD5 = 210 mg/L and COD = 420 mg/L; Y, BOD5 = 80 mg/L and COD = 400 mg/L. Which conclusion is most defensible?

A Y has a greater BOD5/COD ratio than X. B X and Y have identical BOD5/COD ratios. C X has the higher BOD5/COD ratio and may be more amenable to biological treatment, subject to treatability checks. D X is guaranteed to be completely biodegradable because its BOD5/COD ratio is 0.50.

Question 14 ప్రశ్న 14

A wastewater treatment plant receives 3.5 MLD at an influent COD concentration of 400 mg/L. What is the COD mass loading, assuming the stated average flow and concentration represent the same period?

A 1400 kg/day B 400 kg/day C 3500 kg/day D 14000 kg/day

Question 15 ప్రశ్న 15

Two streams mix without reaction or loss: stream 1 is 2 MLD at COD 300 mg/L, and stream 2 is 3 MLD at COD 500 mg/L. What is the mixed COD concentration?

A 380 mg/L B 400 mg/L C 480 mg/L D 420 mg/L

Question 16 ప్రశ్న 16

A treatment unit reduces COD concentration from 720 mg/L to 180 mg/L. Assuming equal influent and effluent flow rates, what percentage of COD mass is removed?

A 25% B 75% C 80% D 60%

Question 17 ప్రశ్న 17

A receiving-water sample has measured DO of 1.5 mg/L and COD of 400 mg/L. Which interpretation correctly distinguishes the two results?

A The COD value proves that 400 mg/L of dissolved molecular oxygen is present. B The dissolved oxygen must equal COD minus BOD under all field conditions. C DO is the oxygen currently dissolved in the water, whereas COD represents potential chemical oxygen-equivalent demand under the test. D The DO value must be divided by COD to obtain the exact organic biodegradability fraction.

Question 18 ప్రశ్న 18

An unfiltered wastewater sample has total COD of 560 mg/L, while a representative filtered aliquot has COD of 210 mg/L. Assuming comparable test performance and a defined filtration cutoff, what is the estimated particle-associated COD?

A 350 mg/L B 770 mg/L C 210 mg/L D 0 mg/L

Question 19 ప్రశ్న 19

An industrial wastewater has high dichromate COD but an unexpectedly low BOD5 result. Which investigation is technically justified before declaring the wastewater non-biodegradable?

A Assume that the COD test must be wrong because biological and chemical tests are interchangeable. B Conclude that the wastewater contains no oxidizable organic matter. C Assume the BOD bottle always develops sufficient biomass even when toxicants are present. D Check for toxicity, pH effects, microbial seeding problems and slowly biodegradable compounds that may suppress short-term BOD.

Question 20 ప్రశ్న 20

Why can a comparison of total BOD5 and COD be misleading when nitrification is significant during BOD incubation?

A The standard COD reflux measures ammonia oxidation by nitrifying microorganisms in the same way as BOD incubation. B Total BOD5 may include nitrogenous microbial oxygen demand, whereas dichromate COD does not measure that biological nitrification demand in the same way. C Nitrification necessarily reduces BOD5 to zero regardless of organic pollution. D Nitrification affects only the suspended-solids test and never the BOD result.

Question 21 ప్రశ్న 21

Which statement about COD, total suspended solids (TSS) and dissolved solids in wastewater is correct?

A Every milligram of TSS contributes exactly one milligram of COD. B COD measures the same physical property as total dissolved solids. C COD indicates chemical oxygen-equivalent demand, whereas TSS and dissolved-solids tests classify material by operational physical separation and residue methods. D A perfectly clarified wastewater must have zero COD because no suspended solids remain.

Question 22 ప్రశ్న 22

A sewer carries 2.0 MLD with COD = 600 mg/L and BOD5 = 240 mg/L. What are the COD and BOD5 loads, respectively?

A 1200 kg/day and 480 kg/day B 480 kg/day and 1200 kg/day C 600 kg/day and 240 kg/day D 1200 kg/day and 720 kg/day

Question 23 ప్రశ్న 23

In a titrimetric COD test, blank FAS consumption is 18.0 mL, sample aliquot is 20.0 mL, FAS normality is 0.100 N, and the sample COD is 320 mg/L. What sample FAS consumption is consistent with COD = (A - B)N(8000)/V?

A 6.0 mL B 8.0 mL C 12.0 mL D 10.0 mL

Question 24 ప్రశ్న 24

A plant's influent has BOD5 = 200 mg/L and COD = 500 mg/L, while its effluent has BOD5 = 30 mg/L and COD = 150 mg/L. Which ratio comparison is correct?

A BOD5/COD increases from 0.20 to 0.40. B BOD5/COD decreases from 0.40 to 0.20, suggesting that the residual demand is relatively less exerted biologically over five days. C The two ratios remain at 0.40 because both measures decreased. D The effluent COD necessarily consists of exactly 80% nonbiodegradable organic mass.

Question 25 ప్రశ్న 25

An industrial plant's discharge changes from 3.0 MLD at COD 450 mg/L to 2.0 MLD at COD 700 mg/L. What does a COD mass-load comparison show?

A The new COD load is 700 kg/day, substantially below the old load. B The load stays at 1350 kg/day because the reduction in flow offsets the concentration increase exactly. C The COD load rises slightly from 1350 kg/day to 1400 kg/day despite the reduced flow. D The COD load falls from 1400 kg/day to 1350 kg/day because the new flow is lower.

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