1. Biochemical Oxygen Demand
Dissolved Oxygen
“The Breath of Life”

2. Dissolved oxygen essential for fish and other aquatic animals
Solubility low
O2 is non-polar : water is polar
10oC 11.3 mg/L
20oC mg/L
As water is heated bubbles of air are seen to come out of solution

3. If Organic Waste is dumped into water
Micro-organisms use it as food
Multiply rapidly
As food broken down
Organic matter + O2 = CO2 + H2O
If a lot of organic matter and so many micro-organisms then much or even all the O2 might be used up

4. Trout need 5 p.p.m. dissolved O2 to survive
If all O2 used up anaerobic bacteria will thrive
Water becomes foul due to H2S - damages the environment
Release of substances that damage the environment is called Pollution
How do we measure pollution?.

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

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

7. Method Fill 2 bottles with water to be tested
Measure dissolved O2 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

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