Biochemical Oxygen Demand (BOD)

December 02, 2016 10:25 pm

Biochemical Oxygen Demand is the oxygen required by microorganisms to degrade the organic matter present in the sample

Definition

Biochemical Oxygen Demand (BOD) is defined as the amount of oxygen required by bacteria while stabilizing decomposable organic matter under aerobic conditions.

The BOD test may be considered as a wet oxidation procedure in which the living organisms serve as the medium for oxidation of the organic matter to carbon dioxide and water.

Range

Reduction of Sample and Increase

Neutralizations

Neutralize the sample to pH around 7.0 using alkali or acid of such strength that the quantity of reagent does not dilute the sample by more than 0.5 percent.

Sample Volume and Reduction Technique

Sample volumes in ml, per liters dilutions = X / expect BOD X 1000

• For keeping 2 dilution takes X = 2.5 and 4.0
• For single dilutions, takes X= 3.0 or 3.5
• In case of high BOD sample, preparing primary dilution with distilled water and then made the finally dilutions.

1. Take requisite quantity of sample in one litre volumetric flask.
2. Dilute to the mark with the dilution water by siphoning from the container.
3. Mix well. Rinse three BOD bottles with the diluted sample and fill up these bottles with the diluted sample.
4. Stopper the bottles immediately after removing the air bubbles.

Determination of Initial Dissolved Oxygen (DO) [IS 3025 (Part 38 ) : 1989.]

1. Determine initial DO for one bottle and keep two bottles for incubation at 27°C ± 1°C for 3 days. Prepare six blanks by siphoning out dilution water directly into the bottles.
2. Determine initial DO in two bottles and incubate remaining four bottles at 27°C ± l°C for 3 days.

Determination of Final DO [IS 3025 (Part 38 ) : 1989.]

After 3 days incubation at 27°C ± 1°C, determine final DO in incubated bottles.

Calculation

1. When Sample is Undiluted
   BOD, mg/L = DO before incubation - DO after incubation.
2. When Dilution Water is Seeded
   BOD, mg/L = (D1 – D2 / P ) X 1000
3. When Dilution Water is Seeded
   BOD, mg/L = [D1 – D2 – (B1 – B2)f / P] X 1000
   Where,
   D1 : Initial DO of sample in mg/L
   D2 : DO of sample after incubation in mg/L
   B1 : DO of seed control before incubation in mg/L
   B2 : DO of seed control after incubation in mg/L

Application of BOD data

BOD data have wide application in environmental engineering practice. It is the principal test applied to domestic and industrial wastes to determine strength in terms of oxygen required for stabilization.

It is the only test applied that gives a measure of the amount of biologically oxidisable organic matter present that can be used to determine the rates at which oxidation will occur, or BOD will be exerted, in receiving bodies of water. BOD is therefore a major criterion used in stream pollution control where organic loading must be restricted to maintain desired dissolved oxygen levels.

The determination is used in studies to measure the purification capacity of streams and serves regulatory authorities as a means of checking on the quality of effluents discharged to such waters.

BOD information is used in determination of the size of certain treatment units, particularly trickling filters, and activated sludge units.

After treatment plants are placed n operation, the test is used to evaluate the efficiency of various units.

Conclusion

BOD test is thus an important factor in determining the pollutional strength of domestic & industrial wastes in terms of the oxygen they will require if discharged into naturally watercourses in which aerobic condition exists.

Disclaimer

The details given are for general understanding. References should be taken from respective authorized agencies.