Biochemical Oxygen Demand Calculation

Biochemical oxygen demand calculation procedure will tell about need of fresh air to create the water useful for the marine lifestyle. If the BOD is more, then we can say that effluent is more contaminated. Biochemical oxygen demand calculation process takes five day to complete through analysis.

Apparatus required:

  • BOD bottles -300 capacity
  • BOD incubator
  • Pipette
  • Burette
  • Measuring Jar
  • 500 ml capacity iodine flask.

 Reagents required:

  • Calcium chloride solution
  • Phosphate buffer (pH-7.2)
  • Ferric chloride solution
  • Magnesium sulfate solution
  • Alkali-iodide-azide reagent
  • Manganese sulfate solution
  • Concentrated sulfuric acid.

Biochemical oxygen demand calculation procedure:

Step 1  : Take 4 liters of aerated water.

Step 2  : For 4 liters aerated water take 2 ml of sewage water. Sewage water is for introducing the microorganisms. (For 3 liters, we have to take 1.5 ml of sewage water)

Step 3 : Add 4 ml of Calcium chloride solution, 4 ml of Phosphate buffer, 4 ml of Magnesium sulfate and 4 ml of Ferric chloride as a nutrient for the survival of micro organisms. (For 1 liter, we have to take 0.5 ml of all reagents each)

Step 4  : Mix thoroughly.

Step 5  :  Now fill the above solution in two bottles. One bottle will be used for initial DO analysis. Another bottle will be used for final DO after 5 days incubation.

Step 6  : Now calculating the initial DO. Take 1ml of sample and make up to 1 liter with dilution water. Mix and fill into 2 bottles until sample overflows and recap. Overflow is used for removing any bubbles in it or any gas present in it.

Step 7  : After taking into bottles add 2ml of manganese sulfate, 2ml of sodium azide solution. Close the lid and mix it.  Wait for 2 to 3 min to settle down. Azide eliminates disturbance due to nitrate, which is the most common disturbance in naturally handled effluents.

Step 8  : Add 2ml of concentrated sulfuric acid. Close with lid and mix until clear yellow solution forms.

Step 9  : From the above solution take 200ml, and add 2ml of starch and then titrate with 0.025M sodium thiosulfate upto colorless. Note the titration value.

Step 10 : We have to do the same procedure for the sample after 5 days present in the incubator.

Biochemical Oxygen Demand Calculation ( BOD), is done by the formula:

BOD (mg/l) = [T1-T2] – [B1-B2] ×  1000 / sample taken for 1 liter

T1= Initial D.O. in sample.

T2= After 5 days D.O. in sample.

B1= Initial D.O in blank

B2= After 5 days D.O in blank.