Biochemical Oxygen Demand Dissolved Oxygen The Breath of

Biochemical Oxygen Demand Dissolved Oxygen “The Breath of Life”

n Dissolved oxygen essential for fish and other aquatic animals n Solubility low n O 2 is non-polar : water is polar o n Solubility @ 10 C 11. 3 mg/L n Solubility @ 20 o. C 9. 2 mg/L n As water is heated bubbles of air are seen to come out of

n If Organic Waste is dumped into water n Micro-organisms use it as food n Multiply rapidly n As food broken down n Organic matter + O 2 = CO 2 + H 2 O n If a lot of organic matter and so many micro-organisms then much or even all the O 2 might be used up

n Trout need 5 p. p. m. dissolved O 2 to survive n If all O 2 used up anaerobic bacteria will thrive n Water becomes foul due to H 2 S damages the environment n Release of substances that damage the environment is called Pollution n How do we measure pollution? .

n Royal Commission on Sewage Disposal n Early 1900’s n Measure of Organic Pollution n Demand water has for dissolved oxygen. n Biochemical Oxygen Demand [B. O. D. ]

B. O. D. n The amount of dissolved oxygen n Consumed by biological action n When a sample of water is kept at 20 o. C n In the dark n For 5 days

Method n n n Fill 2 bottles with water to be tested Measure dissolved O 2 of one at once Incubate second bottle for 5 days Test for oxygen content BOD is the difference in oxygen levels in p. p. m. [mg/L] If first bottle shows no oxygen or BOD > 9. 2 mg/L dilute sample with a fixed amount of fully oxygenated distilled water

Some Examples B. O. D. [mg / L] Example 1– 2 20 – 40 100 300 500 30, 000 54, 000 Clean water Treated sewage Polluted water [fish die] Raw sewage Brewery effluent Pig slurry Silage effluent