4.4 Biological Oxygen Demand

Investigating Biological Demand

This practical took me quite a few tries to work and requires a little bit of care but it really makes the point that added organic matter increases the BOD of a sample. I usually do this activity with the class together with each person taking responsibility for a different part of the method although you could easily have it set up for small groups and compare the measurements.

This method only works with a dissolved oxygen probe. Calculating BOD using the Winker Method can be quite laborious.

Definition: BOD is a measure of the amount of dissolved oxygen required to break down the organic material in a given volume of water through aerobic biological activity.

The test measures the mass (in milligrams) of dissolved oxygen consumed per litre of water when a water sample is incubated in a dark chamber at 20°C for five days. The greater the amount of organic pollution (sewage / organic matter) the more oxygen consuming microbes are required to break it down and therefore the greater the BOD.

Useful links:

A Citizen's Guide to Understanding and Monitoring Lakes and Streams - Department of Ecology, State of Washington

Vernier Dissolved Oxygen Probe guide and information

Procedure:

All samples have to be incubated at 20 degrees for 5 days. Measurements to be taken using dissolved O2 probe, following calibriation.

  1. Opaque container (sealed) containing tap water.
  2. Opaque container (sealed) containing fish tank water from classroom.
  3. Opaque container (sealed) containing fish tank water from prep room.
  4. Opaque container (sealed) containing fish tank water from tropical tank.
  5. Opaque container (sealed) contained tap water and small measured volume of milk.
  6. Opaque container (sealed) containing tap water and small measured volume of yeast solution.
  7. Opaque container (sealed containing tap water and small measured volume of soil.
  8. Opaque container (sealed) containing tap water and some grass / leaves.

Results:

Conditions

d. O2 mgL-1 on day 0

d. O2 mgL-1 on day 5

Tips:

Make sure the dissolved measurements are taken immediately after preparing the samples.

Ensure the containers are completely sealed - use parafilm or similar.

Ensure the containers are completely wrapped in Aluminum foil to prevent light entering the samples.

Keep the containers in a cupboard (not a fridge).

These are the samples I use but you could substitute other water samples from your local surroundings.

The Vernier dissolved oxygen probe requires that you agitate the probe in the water.

Emphasise the need for a repeatable method.

The samples need to be left for 5 days so plan your timing carefully.

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