INTRODUCTION

The Biochemical Oxygen Demand (B.O.D.) of sewage or of polluted water is the amount of oxygen required for the biological decomposition of organic matter to occur under aerobic conditions and at the standardised time and temperature. Usually, the time is taken as 5 days and the temperature 20°C as per the global standard. The B.O.D. test is among the most important method in sanitary analysis to determine the polluting power, or strength of sewage, industrial wastes or polluted water. It serves as a measure of the amount of clean diluting water required for the successful disposal of sewage by dilution. The test has its widest application in measuring waste loading to treatment plants and in evaluating the efficiency of such treatment systems.

The test consists in taking the given sample in suitable concentrations in dilute water in B.O.D. bottles. Two bottles are taken for each concentration and three concentrations are used for each sample. One set of bottles is incubated in a B.O.D. incubator for 5 days at 20°C; the dissolved oxygen (initial) content (D1) in the other set of bottles will be determined immediately. At the end of 5 days, the dissolved oxygen content (D2) in the incubated set of bottles is determined. Starch is a indicator in the iodometric titration and it turns deep dark blue when iodine is present in a solution. Then,

P = Dilution Factor

Dilution Factor = Bottle Volume (300 ml) / Sample Volume

Where,

P = Decimal fraction of sample used

D₁ = Dissolved oxygen of diluted sample (mg/l), immediately after preparation

D₂ = Dissolved oxygen of diluted sample (mg/l), at the end of 5 days incubation

BOD is based on the principle that if sufficient oxygen is available, aerobic biological decomposition (i.e., stabilization of organic waste) by microorganisms will continue until all waste has been consumed.

The BOD is an empirical biological test. This test may be considered as a wet oxidation procedure in which the living organisms serve as the medium for oxidation of organic matter to carbon dioxide and water.

On the basis of the above relation, it is possible to interpret BOD data in terms of organic matter as well as the amount of oxygen used during its oxidation.

Prerequisite for BOD Test:

DO measurement:

1. Make dilution water by adding 2mL/L of the following reagents in distilled water:
   a. Phosphate buffer solution
   b. Magnesium sulfate solution
   c. Calcium chloride solution
   d. Ferric chloride solution
   e. Sodium Sulfite solution
2. For a given sample bottle, add 1 mL of alkali azide and then 1 mL manganous sulfate solution. Shake the bottle well and keep it open for 5 minutes to settle the precipitate. Add 2 mL of concentrated H2SO4 and place the cap on the bottle. Shake the bottle well till all the precipitate is dissolved.
3. Take 203 mL of sample in a conical flask and titrate with standard sodium thiosulfate solution (0.025N) till the colour changes from dark yellow to light yellow. Then add a few drops of starch indicator and continue to titrate till the colour of the solution becomes either colourless or changes to its original sample colour. Note down the volume of 0.025N sodium thiosulfate consumed.
4. Calculate DO value of the sample. Remember that in a 200 mL sample, 1 mL of sodium thiosulfate of 0.025N equals to 1 mg/L dissolved oxygen: =>Dissolved oxygen (DO) (in mg/L) = mL of sodium thiosulfate (0.025N) consumed.

Among the three values of B.O.D. obtained for a sample select that dilution showing the residual dissolved oxygen of at least 1mg/l and a depletion of at least 2mg/l. If two or more dilutions are showing the same condition then select the B.O.D. value obtained by that dilution in which the maximum dissolved oxygen depletion is obtained. Waste water from sewage treatment plants often contains organic materials that are decomposed by microorganisms, which use oxygen in the process. (The amount of oxygen consumed by these organisms in breaking down the waste is known as the Biochemical Oxygen Demand or BOD).

Objectives that we accomplish from BOD determination :

• To assess the quality of surface water and waste water
• To establish the amount of organic matter in samples
• To understand the significance of Dissolved Oxygen in sample

Relevant Indian Standard for Biological Oxygen Demand Test :

IS 3025 (Part 44)-1993: Method of Sampling and Test (Physical and Chemical) for Water and Wastewater, Biochemical Oxygen demand, First Revision.