Determination of Biological Oxygen Demand (BOD)

Biological Oxygen Demand (BOD):
Biological Oxygen Demand (BOD) refers to the amount of oxygen that would be consumed if all the organics in one liter of water were oxidized by bacteria and protozoa. The term also refers the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period.

Determination of Biological Oxygen Demand:

Reagents:
  1. Manganous sulfate solution
  2. Alkaline potassium iodide solution
  3. 0.025N sodium thiosulfate
  4. Starch solution
  5. Concentrated sulfuric acid.
Testing Procedure:
The two BOD bottles with sample (or diluted sample); the bottles should be completely filled. Determine initial DO (DOi) in one bottle immediately after filling with sample (or diluted sample). Keep the other bottle in dark at 20 0C and after particular days (usually 5- days) determine DO (DOf) in the sample (or diluted sample). Dissolved oxygen (DO) is determined according to the following procedure:
  1. Add 1 mol 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 21 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.
  5. Add 1 ml of concentrated sulfuric acid. Immediately insert the stopper and mix as before.
  6. Aloe the solution to stand at least 5 minutes.
  7. Withdraw 100 ml of solution into an Erlenmeyer flask and immediately add 0.025N sodium thiosulfate drop by drop from a burette until the yellow color almost disappears.
  8. Add about 1 ml of starch solution and continue the addition of the thiosulfate solution until the glue color just disappears. Record the ml of thiosulfate solution used (disregard any return of the blue color).
Calculation:
Dissolved oxygen, DO (mg/l) =ml of 0.025n sodium thiosulfate added × 2

The five- days BOD of a diluted sample is given by

BOD5 = ( DOi– DOf) × D.F

Where D.F = Dilution Factor = (Vol. of wastewater + dilution water)/ (Vol. of wastewater) 
 
Result: 
BOD Level (in ppm)
Water Quality
1 - 2
Very Good
There will not be much organic waste present in the water supply.
3 - 5
Fair: Moderately Clean
6 - 9
Poor: Somewhat Polluted
Usually indicates organic matter is present and bacteria are decomposing
this waste.
100 or greater
Very Poor: Very Polluted
Contains organic waste.

 
Author of This Article:
Apu Kumar Das
B.Sc. in Textile Technology
Chittagong Textile Engineering College
Cell: +8801825272380
Email: akdas.textile13@gmail.com
Facebook: Apu Das
 
 

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