Bio oxygen demand (B.O.D) is the amount of oxygen required for the microorganisms (bacteria) present in the waster water to convert the organic substance to stable compounds such as CO2 and H2O,
Organic substance + oxygen bacteria --> CO2 + H2O
Bacteria placed in contact with organic materials will utilize it as a food source in the utilization the organic material will be oxidized to CO2 H2O. B.O.D is considered to be the measure of organic content of the waste, the B.O.D determination has been done by measuring the amount of oxygen utilized by the micro-organic has in the stabilization of waste water for 5 days at 20 C. For domestic sewage the 5 days B.O.D value (B.O.D) is represent approximately 2/3 of the demand to be consumed of all the oxidization materials were in fact oxidized for measurement of high B.O.D values the waste is required to be dilute the diluted water is carefully manufactured and contains a mixture of salts necessary for biological activities plus a phosphate buffer to maintain neutral PH.
The water is activated before mixing with sewage.
Bottle burette, pipette, pipette filter, graduated cylinder
Manganese sulphate alkali iodide acid concentrated sulphate acid standard hio sulphate and star itch indicator.
Take two B.O.D tubes and half fill it with distilled water.
Add 3ml of waste water (polluted water) to the B.O.D tubes with the help of pipit.
Now filled the tubes with distilled water and fix stopper on it.
Put one of the tubes in incubator at 20 C for 5 days.
Add 2ml of alkali iodide oxide and shake well if oxygen is present the color will be brown otherwise while)
Add 2ml of concentrated H2SO and shake well which will give a color which is in resemblance to mustard oil.
Take 200ml from this solution in a graduted cylinder and add 1ml of strach indicator to it which will give a yellowish color.
Put the gragraduated cylinder below the burette containing standard solution of sodium this sulphate and note the initial reading.
Fill dissolved oxygen of the first tube the dissolved oxygen is found in similar way.
- Find the B.O.D by using the formula
B.O.D (mg/lit) = (zero day D.O - 5 days D.O ) x 300/ml of sample
The BRCES (British Royal Commission Effluent Standard) allows a B.O.D of 20 mg/lit in a treated sewagr to be discharged to body of water.