Biological Oxygen Demand(BOD), laboratory analysis

BOD value is the important factor for designing effluent treatment plants. It is taken as the controlling variable during balance equation development and flowrate determination between the equipments. Even though the modern instrumental methods are available, their cost of operation is higher than analytical lab methods. In brief BOD value is obtained by comparing the dissolved oxygen value of the sample between five days. 

Analytical procedure: 5 days method

Testing Apparatus:

Bottle with stopper (250ml)
Chemical Reagents:

  • NaOH solution 1N.
  • HCl 1N.
  • Sodium sulphite solution :– Dissolve 1.5g to liter water. (Note: Donot mix with the previous test solution)
  • Phosphate buffer solution: –Dissolve 8.5g KH3PO4, 21.75g K2HPO4 , 33.4g of Na2HPO4.H2O and 1.7g of NH4Cl in water(500 ml) and make 1 liter solution from it. 7.2 pH must be maintained.
  • MgSO4.7H2O solution – Dissolve 22.5g in water and make up to a liter in volumetric flask.
  • CaCl2 solution – Dissolve 27.5g in water added to 1 liter volumetric flask.
  • FeCl36H2O solution – Add water to 0.25g  and make it to 1 liter solution.
  • Seeding material – Effluent water from the sample point.

Test Procedure:
  1. Sample treatment:  pH of the sample is adjusted to 7 using acid and base solutions.
  2. Residual chlorine removed by adding sodium sulphite solution.
  3. Store the dilution water at 20 deg C and use it at approximate temperature. For 1 liter of water 1ml each of phosphate buffer solution, magnesium sulphate solution, calcium chloride solution and ferric chloride solution are added. 0.1to 1 percent seeding material added to dilution. 
  4. 5 percent of dilution is done for the sample
  5. 500 ml of seeded dilution water taken in 1liter jar and make up with dilution water to 1 lit. Pour the 1 liter solution in to two glass stoppered bottles. Lower concentration samples also prepared as above said. 
  6. Initial DO concentration is tested for one of the two sample bottles and blank one. Incubate the  bottles at 20 deg C for 5 days. After 5 days, DO is estimated for the dilution and the blank. BOD can be estimated by the following formula:

BOD = (D1-D2)-(C1-C2) F/P


  • D1 initial DO sample.
  • D2 DO after incubation of sample.
  • C1 Seeded dilution water initial DO
  • C2 Seeded dilution water after incubation DO 
  • F =ratio of the seed in the sample to that in the control, that is percent seed in D1 divided by percent seed in C1 and 
  • P= decimal fraction of the sample used.