Water is essential for life and makes up most of the Earth's surface and human body. It is used for drinking, industry, and agriculture but can become polluted. The biological oxygen demand (BOD) test measures pollution by determining the amount of dissolved oxygen consumed by bacteria breaking down organic wastes over 5 days. Higher BOD levels indicate more pollution and risk to aquatic life since less oxygen is available. BOD is used to monitor water quality and identify sources of pollution.

1. Environmental Chemistry-
Water

3. Importance of Water
• Water is the most important and
abundant chemical on Earth
• It makes up 80% of the Earth’s surface
• 2/3 of our body weight

4. Uses of Water
• Drinking, Washing, flushing toilet
Watering gardens etc.
• Industry uses huge amounts of water
for cooling , manufacturing, generating
electricity etc.

5. Water Pollution

15. Dissolved Oxygen
• Dissolved oxygen in rivers, lakes and the
sea is vital for fish and other forms of
life
• O2 is non polar and not very soluble in
water
• Its solubility depends on the
temperature of the water

16. • Organic wastes such as sewage, slurry,
effluent from farms, effluents from
food processing factories, milk
industrial waste etc. Can leak into
rivers, lakes etc.
• These provide nutrients to bacteria and
other organisms naturally present in
water and allows them to multiply to
large numbers

17. • These large numbers produce more carbon
dioxide and use up available oxygen meaning
fish life in the water may be reduced or
killed off completely
• If it gets very bad the dissolved oxygen
level may reach 0 and only anaerobic
bacteria will survive meaning the water can
become foul smelling due to the production
of Hydrogen Sulphide gas

18. Methods used by Chemists to
indicate levels of pollution in
water
• A test to measure the demand water
has for dissolved oxygen was introduced
in the 1900’s and is called the Biological
Oxygen Demand or B.O.D. Of the
water

19. • The Biological Oxygen Demand is
defined as
• The amount of dissolved oxygen
consumed by biological action when a
sample of water is kept at 20⁰C in the
dark for 5 days

20. Measuring the B.O.D. Of
Water
• Completely fill two bottles to be tested
with water
• Measure the dissolved oxygen of one of
the samples with a dissolved oxygen
meter or by a titration called the
Winkler Method
• Incubate the second bottle at 20⁰C in
darkness for 5 days

21. • The water is kept in darkness to
prevent photosynthesis from taking
place as this would increase the amount
of dissolved oxygen
• The temperature must be kept fixed so
that a fair comparison is made also the
amount of dissolved oxygen is
dependent on temperature

22. • After 5 days measure the dissolved
oxygen in the second bottle
• The B.O.D. Is the difference in the
two dissolved oxygen levels as this is
the amount of oxygen that has been
used up in the test
• B.O.D. Is measured in mg/L of oxygen
• The higher the B.O.D level the more
polluted the water

23. Sample Results
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

24. • The more organic waste in water the
more bacteria present to feed on it this
decreases the available oxygen