If water contains a lot of biodegradable organic wastes, it probably also contains a lot of bacteria. Organic matter is decomposed or broken down when it is fed upon by aerobic bacteria. Aerobic bacteria need oxygen in order to digest or decompose organic wastes. In this process, organic material is broken down and oxidized (combined with oxygen). This test measures how much oxygen is being used by these microscopic organisms. The more oxygen used by microscopic organisms, the less available for fish and other aquatic life forms.

Humans can have a bad effect on streams by increasing the amount of organic wastes. This can happen when humans allow runoff from certain industries (such as pulp and paper mills, meat-packing plants, food processing industries or sewage treatment plants) to enter a stream. It can happen when streams are disturbed by construction which stirs up organic wastes trapped in the stream bed. It can also happen when agricultural runoff, such as fertilizers or manure, enters a stream.

Fertilizers contain phosphates and nitrates. When it rains, fertilizers that have been applied to lawns and fields may run off into a stream causing algae and other aquatic plants to grow rapidly. Plants that grow fast die fast. Dead plants increase the amount of organic wastes in the water. Bacteria take oxygen out of the water in order to decompose organic wastes. This lowers the oxygen content of the water which may cause fish and other aquatic animals to die. This condition is called eutrophication. Whenever you have seen a pond or lake covered with a layer of slimy, green algae, you have seen the beginning of cultural eutrophication. The last stage of eutrophication has been reached when you notice a strong rotten egg odor.

Although specialized equipment is used by water technologists to measure BOD, a close approximation of BOD values can be obtained by using a dissolved oxygen test kit and waiting five days.

1. Begin by determining the amount of dissolved oxygen in the stream water by using a Hach Dissolved Oxygen (DO) test kit as described on the "How to do a Dissolved Oxygen Test" page. Record this amount.

2. Clean and rinse the 60ml D.O. sample bottle. Carefully. refill the sample bottle with stream water. Stopper the bottle. Be sure there are no air bubbles in the sample.

3. Wrap the bottle completely with aluminum foil.

4. Place the bottle in a dark room at about 20^oC or 68^oF (room temperature) for five days (120 hours).

5. At the end of the five day period, measure the amount of dissolved oxygen left in the water by using the Hach DO test kit.

6. Subtract the DO value obtained on day five from that obtained on day one. The difference is the five-day BOD.