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.
A Citizen's Guide to Understanding and Monitoring Lakes and Streams - Department of Ecology, State of Washington
All samples have to be incubated at 20 degrees for 5 days. Measurements to be taken using dissolved O2 probe, following calibriation.
- Opaque container (sealed) containing tap water.
- Opaque container (sealed) containing fish tank water from classroom.
- Opaque container (sealed) containing fish tank water from prep room.
- Opaque container (sealed) containing fish tank water from tropical tank.
- Opaque container (sealed) contained tap water and small measured volume of milk.
- Opaque container (sealed) containing tap water and small measured volume of yeast solution.
- Opaque container (sealed containing tap water and small measured volume of soil.
- Opaque container (sealed) containing tap water and some grass / leaves.
d. O2 mgL-1 on day 0
d. O2 mgL-1 on day 5
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.