BIOCHEMICAL OXYGEN DEMAND (BOD)

BOD is a measure of the amount of "food" for bacteria found in river water. It is an aggregate measure of organic matter that includes all sorts of things. It depends on the fact that bacteria in the water will utilize organic matter in their respiration and thereby remove oxygen from the river.

INSTRUCTIONS FOR USING TEST KIT:

1. Collect at least two water samples in BOD bottles by totally submerging the bottles in the water, preferably in the middle of the channel. Stopper the bottles tightly, using the stoppers that come with them. Make sure that no air bubbles are found in the bottles! If you collect your river water in a collecting bottle and fill the BOD bottles from that collecting bottle, be sure that you do not shake the collecting bottles so that air would get introduced into it! It is very important that the water in the bottle be an exact sample of the water in the river and that no air has been introduced either by shaking or by bubbles. Note also that it is best to take the sample from mid-depth in the river, between the surface and the bottom.

2. Run the dissolved oxygen test on one of these samples at once. This can be the same sample used to find DO for class purposes.

3. Take the other sample and wrap it completely, so that absolutely no light can enter the bottle and enable algae trapped in the water sample to carry out photosynthesis. This can be done most easily by wrapping the bottle in aluminum foil, electrical tape, or some such, and placing it in a closed cardboard box. Incubate the sample for 5 days at 20øC (room temperature). This will enable bacteria in the river water to metabolize normally, using up dissolved oxygen in the process.

4. Run the dissolved oxygen test on the incubated sample(s).

5. Calculate the biochemical oxygen demand -- that is, the oxygen that has been demanded (i.e. used up) during the 5 days' incubation using the following formula: BOD = DO[original] - DO[after incubation]

*Note that the schedule for the GREEN project will make it difficult to follow this procedure precisely. In order to measure BOD on the testing day, teachers will need to get the sample to be incubated 5 days before the river sampling day, so that the DO in the river on sampling day can be compared with the DO of a sample that has been incubated for 5 days. Note that this depends on the assumption that the DO on sampling day is the same as on the day when the teacher took the original sample to be incubated. This is not always a good assumption. Alternatively, and much more accurately, the BOD sample can be taken on the collecting day and measured 5 days later. This will delay data reporting somewhat, but that is a small price to pay.

QUESTIONS TO THINK ABOUT:

If BOD is above about 5 ppm, you have a problem. Also, if there appears to be no BOD in your sample, you have a problem! Why is this?

EFFECTS OF HIGH OXYGEN DEMAND

• You have a great many aerobic bacteria, which rob other organisms of oxygen. They may even remove all oxygen from the water in extreme circumstances.
• Lower oxygen tolerant organisms will appear (Carp, Sewage worms)
• Diversity of Aquatic life is lowered
• River quality is lowered

WHAT IS CREATING THE POOR CONDITIONS?

• Where are the nearest swamps and bogs?
• Is there erosion in the fields that might bring in large amounts of topsoil?
• Does any nearby plant make paper?
• Are there any meat packing plants nearby?
• What happens to the pet waste in your area?
• Are there any sewage plants and/or combined sewer overflows in the area?
• Are there any nearby plants discharging large amounts of organic water?
• How much organic matter is in the road and street runoff?

*WHY IS THERE SO MUCH ORGANIC MATTER IN YOUR AREA? HOW CAN IT BE CONTROLLED?

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