BOD measures the oxygen consumed by microorganisms as they decompose organic matter and includes any chemical oxidation of inorganic compounds. The BOD test measures the amount of oxygen consumed during a specified period of time, usually 5 days at 20 °C and so is called BOD5.

Biochemical Oxygen Demand

Biochemical Oxygen Demand, or BOD, is an “ancient” water quality that has been used for more than a century to estimate the amount of biodegradable organic matter in a waste stream effluent – either from a wastewater treatment plant or a factory or other point source.

The test assumes that the organic matter in the water serves as “food” for bacteria and that the faster oxygen levels decrease in a sealed bottle, the higher the initial organic matter concentration.

High BOD values in a stream usually indicate a source of wastewater that shouldn’t be there. If values are high enough, the rate of O2 consumption from bacteria consuming this organic matter may exceed the rate of O2 that diffuses in from the air (mostly from bubbling riffles and falls, and from aquatic plant and algae photosynthesis. The result is that O2 levels fall. If values go below about 5 mg/L (parts-per-million) trout and many other organisms become very stressed. In summer, warm water worsens the problem because they need more O2 at higher temperatures and because gases are less soluble in water that is warm. Warm fizzy drinks go flat faster than cold drinks. Flows are usually much lower in summer also and in droughts fish can be trapped in small pools with much reduced inputs of O2. Fish kills can result.

More about Oxygen

MN and WI wastewater standards

Measuring BOD