Testing Biological Oxygen Demand

Remember, organic material (pulp, cattle manure, vegetable peels, blood, leaves, grass) can be a form of pollution entering rivers from pulp mills, feedlots, dairies, food-processing plants, meatpacking plants, forests and lawns. When bacteria decompose this material, oxygen is removed from the water and aquatic life may be harmed.

To perform the B.O.D. Test, the first sample of river water is tested on-site for dissolved oxygen. Then, a second sample of river water is sealed without bubbles, covered to prevent sunlight access and taken back to school.

• After five days without light, the second water sample is tested for dissolved oxygen concentration.

• The second DO concentration is subtracted from the first.

• If the difference is zero, the water did not contain organic material.

• If the difference is more than zero, the water did contain organic material that was decomposed by oxygen-using (aerobic) bacteria.

• Judge the water quality by referring to the chart below...

  B.O.D. mg/L	Water Quality
  < 2	Excellent
  2 - 4	Good
  4.1 - 10	Fair
  > 10	Poor

  Adapted from "Field Manual for Global Low-Cost Water Quality Monitoring".