Wednesday, September 23, 2020

Biochemical oxygen demand

Determination the Biochemical oxygen demand of a soil sample.

Biochemical Oxygen Demand

BOD title image

The biochemical oxygen demand  is a chemical procedure, which may be defined as the amount of dissolved oxygen needed by aerobic organisms in a water body to break the organic materials being present in the given water sample at specific temperature and a period of time. Biochemical oxygen demand (BOD) of a water sample can be determined as the amount of oxygen required for the biological decomposition of dissolved organic matter to occur under standard condition at a standardized time and temperature.  According to the BIS the recommended time specifically taken as 5 days and the temperature is 20°C. In the Biochemical oxygen demand (BOD) test during a specified incubation period it is measured the molecular oxygen utilized for the biochemical degradation of organic material (carbonaceous demand) and oxidize inorganic material such as sulfides and ferrous ion. The test also measures the amount of oxygen used to oxidize reduced forms of nitrogen (nitrogenous demand). The oxygen demand of typical waste is measured as the mg/litter. The saturated value of Dissolved oxygen (DO) for water may be  9.1 mg/L at 20°C. During the 5 days incubation period of the Biochemical oxygen demand (BOD) test the Dissolved oxygen (DO) should not drop to Zero, if so then the experiment may be concluded as invalid. 


For the biochemical oxygen demand the sample may be filled in an airtight bottle and incubated at specific temperature for 5 days. The dissolved oxygen (DO) content of the sample is determined before and after five days of incubation at 20°C and the BOD is calculated from the difference between initial and final DO. The initial DO can be determined shortly after the dilution is made, molecular oxygen uptake occurring after this measurement is included in the BOD measurement.

The five-day BOD of a diluted sample is given by, 

BOD Formula image

DF formula image

In some Special cases, it may necessary to seed the dilute water with microorganisms to ensure that there maybe an adequate bacterial population to carry out the biodegradation. In such cases, two sets of BOD bottles must be prepared, one for just the seeded dilution water (called the "blank") and the other for the mixture of wastewater and dilution wader. The changes in DO in both are measured. The oxygen demand of wastewater (BODw) is then determined from the following relationship:

BODm formula image
BOD description image


  • BOD bottle
  • Beaker (250 ml)
  • Measuring cylinder
  • Dropper
  • Stirrer


  • Manganous sulfate solution
  • Alkaline potassium iodide solution
  • 0.025N sodium thiosulfate
  • Starch solution (indicator)
  • Concentrated sulfuric acid.


In the procedure of biochemical oxygen demand need to test out a couple set of Dissolved oxygen (DO) test. Fill two BOD bottles with sample (or diluted sample); the bottles should be completely filled. Then immediately determine initial DO (DOi) in one bottle. Keep the other bottle in dark at 20°C and after particular days (usually 5-days) determine DO (DOf) in the sample (or diluted sample). Dissolved oxygen (DO) can be  determined by the following procedure:
  1. Add 1 mL of manganous sulfate solution to the BOD bottle by means of pipette, dipping in end of the pipette just below the surface of the water. 
  2. Add 1 mL of alkaline potassium iodide solution to the BOD bottle in a similar manner.
  3. Insert the stopper and mix by inverting the bottle several times. 
  4. Allow the "precipitates" to settle halfway and mix again.
  5. Again allow the "precipitates" to settle halfway. 
  6. Add 1 mL of concentrated sulfuric acid. Immediately insert the stopper and mix as before. 
  7. Allow the solution to stand at least 5 minutes. 
  8. Take out 100 mL of solution into an Erlenmeyer flask from the solution, and immediately add 0.025N sodium thiosulfate drop by drop from a burette until the yellow color almost disappears.
  9. Now Add about 1 mL of starch solution indicator to the flask and continue the titration process until the blue color just disappears. Note the amount of thiosulfate solution used (disregard any return of the blue color).


Dissolved oxygen, DO (mg/L) = mL of 0.025N sodium thiosulfate added x Multiplying Factor (M.F.)

The five-day BOD of a diluted sample is given by,

BOD Formula image

 BODm formula image


BOD table image

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