Determination of BOD of the given sample



To determine the 5 day Bio-Chemical oxygen demand (BOD5) of the given sewage sample with 2% dilute solution.


BOD bottles, burette, pipette, conical flask, measuring jar.


  1. Magnesium sulphate solution
  2. Alkali Iodide azide
  3. Concentrated Sulphuric acid
  4. Starch solution
  5. 025 N Sodium-thio-sulphate (Na2S203)
  6. Phosphate buffer
  7. Calcium chloride
  8. Ferric chloride


BOD is the amount of oxygen required to stabilize the organic matter present in wastewater. The degree of magnitude of BOD depends on the amount of organic matter present in the wastewater and temperature during the experiment. The rate of oxygen demand or the process of stabilization on the organic matter is very high or rapid in the initial stage i.e. 1-7 days. This is mainly due to the oxidation of carbonaceous matter present in sewage. The oxidation of the rest is done due to nitrification. The time required for complete oxidation is infinite, but it is limited to 20 days. For practical purpose the period of oxidation is taken as 5 days. Then 70%-80% of oxidation will be completed within first 5 days itself.


The standard test temperature is 20°C. This temperature is taken as the temperature of natural water. There are 2 methods for determining the BOD.

  1. Direct method: In this method BOD is directly This method is applicable only for wastewater whose BOD value is < 7mg/l.
  2. Dilution method: This test is made with various

The following dilutions are suggested,

  1. 1% to 7% – Strong trade waste
  2. 1% to 5% – for raw or settled waste
  3. 5% to 15% – for treated effluent
  4. 25% to 100% – river water

Detailed Procedure

  1. Place the desired volume of distilled water in a 5 litre flask (usually about 3 litres of distilled water will be needed for each sample).
  2. Add 1ml each of phosphate buffer, magnesium sulphate solution, calcium chloride solution and ferric chloride solution for every litre of distilled water.
  3. Seed the sample with 1–2 ml of settled domestic sewage.
  4. Saturate the dilution water in the flask by aerating with a supply of clean compressed air for at least 30 minutes.
  5. Highly alkaline or acidic samples should be neutralised to pH 7.
  6. Destroy the chlorine residual in the sample by keeping the sample exposed to air for 1 to 2 hours or by adding a few ml of sodium sulphite solution.
  7. Take the sample in the requisite concentrations as per dilution method
  8. Add the required quantity of sample (calculate for 650 ml dilution water the required quantity of sample for a particular concentration) into a 1000 ml measuring cylinder. Add the dilution water up to the 650mL mark.
  9. Mix the contents in the measuring cylinder.
  10. Add this solution into two B.O.D. bottles, one for incubation and the other for determination of initial dissolved oxygen in the mixture.
  11. Prepare in the same manner for other concentrations and for all the other samples.
  12. Lastly fill the dilution water alone into two B.O.D. bottles. Keep one for incubation and the other for determination of initial dissolved oxygen.
  13. Place the set of bottles to be incubated in a B.O.D. incubator for 5 days at 20°C. Care should be taken to maintain the water seal over the bottles throughout the period of incubation.
  14. Determine the initial dissolved oxygen contents in the other set of bottles and note down the results.
  15. Determine the dissolved oxygen content in the incubated bottles at the end of 5 days and note down the results.
  16. Calculate the B.O.D. of the given sample.

Overview of Procedure

  1. Prepare the required dilution of the sewage sample.
  2. (For 1% dilution 1/100 × 300= 3 ml of sewage is taken in the BOD bottle and diluted to 300 ml with aerated distilled water).
  3. Determine the DO of this sample, which is the zeroth day DO or initial DO.
  4. In another BOD bottle, prepare the sample of same dilution as before. Keep this sample in a BOD incubator at 20°C for determining the DO after 5 days to get D2 (final DO or DO after 5 days.)


BOD5= (D1-D2)/P mg/l

D1= Zero day or initial DO (before incubation)

D2 = DO after 5 days (final or DO after incubation) P = % dilution (1% = 1/100 = 0.01)

Observations for D1

Trial No Initial Reading Final reading Difference

 Observations for D2

Trial No Initial Reading Final reading Difference



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