This procedure will tell about need of fresh air to create it useful for the marine lifestyle. If the BOD is more then we can say the effluent is more contaminated. There is a five day process for the determination of BOD.
- BOD bottles -300 capacity
- BOD incubator
- Measuring Jar
- 500 ml capacity iodine flask.
- Calcium chloride solution
- Phosphate buffer (pH-7.2)
- Ferric chloride solution
- Magnesium sulfate solution
- Alkali-iodide-azide reagent
- Manganese sulfate solution
- Concentrated sulfuric acid.
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 sodiumthiosulfate 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.
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.