Biochemical Oxygen Demand (BOD)

When organic matter decomposes, it is fed upon by aerobic bacteria. In this process, organic matter is broken down and oxidized (combined with oxygen). Biochemical oxygen demand is a measure of the quantity of oxygen used by these microorganisms in the aerobic oxidation of organic matter.

When aquatic plants die, they are fed upon by aerobic bacteria. The input of nutrients into a river, such as nitrates and phosphates, stimulates plant growth. Eventually, more plant growth leads to more plant decay. Nutrients, then, can be a prime contributor to high biochemical oxygen demand in rivers.

Sources of Organic Matter

There are natural sources of organic material which include organic matter entering lakes and rivers from swamps, bogs, and vegetation along the water, particularly leaf fall.

There are also human sources of organic material. When these are identifiable points of discharge into rivers and lakes, they are called point sources. Point sources of organic pollution include:

1. pulp and paper mills;
2. meat-packing plants;
3. food processing industries;
4. wastewater treatment plants.

Nonpoint pollution comes from many sources that are difficult to identify. Nonpoint sources of organic pollution include:

1. Urban runoff of rain and melting snow that carries sewage from illegal sanitary sewer connections into storm drains; pet wastes from streets and sidewalks; nutrients from lawn fertilizers; leaves, grass clippings, and paper from residential areas;
2. Agricultural runoff that carries nutrients, like nitrogen and phosphates, from fields;
3. Runoff from animal feedlots that carries fecal material into rivers.

Changes in Aquatic Life

In rivers with high BOD levels, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live. Organisms that are more tolerant of lower dissolved oxygen may appear and become numerous, such as carp, midge larvae, and sewage worms. Organisms that are intolerant of low oxygen levels, such as caddisfly larvae, mayfly nymphs, and stonefly nymphs, will not survive. As organic pollution increases, the ecologically stable and complex relationships present in waters containing a high diversity of organisms is replaced by a low diversity of pollution-tolerant organisms.

Sampling Procedure

A dissolved oxygen bottle strapped to the extended rod sampler can be used to take a BOD sample. Remember, samples taken near the river bottom may hold more oxygen-demanding materials and organisms; therefore, to get a representative sample it is best to sample between the surface and river bottom, and away from the shore.

One of the dissolved oxygen bottles should be blackened or purchased as a "dark bottle." One approach is to wrap the bottle with black electrical tape. It is always a good idea if several bottles are available to run several BOD samples.

Like the dissolved oxygen tests, it is important to run all tests for comparison at the same time of day.

Biochemical Oxygen Demand Testing Procedures

1. Fill two dissolved oxygen bottles (one clear and one black) with sample water, holding them for two to three minutes between the surface and the river bottom. If sampling by hand remember to use gloves.
2. Prepare the clear sample bottle according to the directions for the dissolved oxygen test. Determine the DO value for this sample in mg/L.
3. Place the black sample bottle in the dark and incubate for five days at 68'F (20'C). This is very close to room temperature in many buildings. If there is no incubator, place the blackened sample bottle in a "light-tight" drawer or cabinet.
4. After five days, determine the level of dissolved oxygen (in mg/L) of this sample by repeating steps four through eleven of the DO testing procedure.
5. The BOD level is determined by subtracting this DO level from the DO level found in the original sample taken five days previously: