This webpage was created by Chad Harris


Biochemical oxygen demand (BOD), sometimes referred to as biological oxygen demand, is a quantitative expression of microbes ability to deplete the oxygen content of a wastewater. This depletion takes place due to the microbes consuming organic matter in the water via aerobic respiration. This type of respiration uses oxygen as an electron acceptor, and the organic material being consumed provides the energy source. This organic matter also undergoes oxidation without the aid of microbes, which can be measured using the chemical oxygen demand (COD) procedure.

We are rarely concerned with the BOD content of soils. However, in the land-based treatment industry BOD can be a parameter of concern. In extreme cases, high concentrations of organic matter could result in almost complete depletion of oxygen in the soil-water matrix. If this were to happen aerobic microbes would begin to die off, and if this wastewater were discharged to a stream, local ecology could be severely damaged. High levels of BOD in streams cause the dissolved oxygen (DO) content of the water to drop. It is this DO that fish, and zooplankton use to survive. If the DO drops to below a critical level the ecology of the stream could begin to die off as well. This condition can lead to an increase in anaerobic bacteria (species that can live in the absence of oxygen) that leads to the production of foul-smelling, and possibly toxic gases. These gases may include methane, hydrogen sulfide, and ammonia. If this water were to enter into groundwater sources one can imagine the potential problems with toxicity to humans.

Land-based treatment systems would make use of BOD monitoring to ensure that DO levels in the system and receiving streams remain in a safe range. This would be done by measuring BOD of all applied wastewater, and the final effluent. In some cases BOD would be tested at certain points in the soil system by collecting water from monitoring wells. This would be done to make sure DO levels are not falling to a critical level thus harming bacteria within the treatment system.


BOD monitoring is accomplished by methods set forth in the standard methods for the examination of wastewater. The procedure is easily accomplished following these procedures. The most important concepts are the pretreatment, dilution, seeding, and proper incubation of the sample.

All samples must be kept at or below 4C until the sample is ready to be tested. Because microbes are most active between pH values of 6.5 and 7.5 all samples must be neutralized to a pH within this range. It is also important to remove any residual chlorine from the sample before running the experiment. Failing to do so would kill your valuable bacterial source.

A seed source must be prepared from a commercial product, or from a diluted portion a wastewater. This seed source will be the source of bacteria added to the sample. The bacteria are added to oxidize the biodegradable organic matter in the sample. This is the backbone of the experiment. If your bactria fail to oxidize the organic matter, then the test will not work. It is also important to run a seed control to account for any BOD in the seed material.

Because the BOD concentration in most wastewater exceeds the concentration of DO available in air saturated samples, it is necessary to dilute the samples being tested. This is done by using properly prepared dilution water. This dilution water contains nutrients such as nitrogen, phosphorus, and other trace metals necessary for microbial growth. The dilution water is also buffered to ensure that the pH of the sample remains in a suitable range for microbial activity.

Most BOD tests are run over a 5 day period. The diluted samples are prepared and the initial DO is measured on day 1. The samples are then incubated at 20?C for 5 days. On day 5 the samples are removed from the incubator and the final DO concentration is measured. Most laboratories are equipped with special BOD machines that utilize computer software to make all necessary calculations based on dilution factors, seed control, and incubation time. Otherwise standard methods provides formulas for using the DO, dilution factors, and seed control results to calculate the BOD.

Pictured from left to right: BOD probe w/ automatic stirrer, BOD bottles, YSI BOD meter, BOD incubator. (Pictures from Fisher Scientific)


As already mentioned it is critical that all samples are handled properly before BOD testing and are pretreated properly. Failing to keep samples in a cool place and adjust pH before testing will severely limit the accuracy of the test. It is also very important that the dilution water and seed material be prepared properly, and consistently. As with any laboratory procedures it is important to practice consistency with the method. Failing to do so can lead to varying results.

It is also important to understand that nitrogenous demand can interfere with the BOD test. This interference comes about through the inclusion of ammonia in the dilution water and high levles of nitrogenous compounds in wastewater. While it is important to know the nitrogenous demand of the wastewater, it is considered an interference when included with the BOD test. To prevent this problem from occurring inhibitory chemicals can be added to keep nitrogenous chemicals from interfering. Results in which these chemicals have been added are usually reported as CBOD. Ammonia and organic nitrogen demand can be determined using the TKN method.

Even though we are not usually concerned with BOD in the soil system it is very important when dealing with wastewater treatment systems. Whether it be land-based systems or municipal treatment, BOD is an important parameter that must be watched very closely. The procedure is very straight forward and easy to perform with the correct equipment, and compared to other parameters, BOD is fairly cheap to perform. Failing to realize the importance of BOD in applied wastewater, effluent water, and within the soil treatment system can lead to devastating effects on the local aquatic ecology, and quality of underlying groundwater.


Fisher Scientific Web Page. 1999.

Hyland, Margaret, C. and Charles E. Kupchella. Environmental Science. 3rd Ed. Living Within the System of Nature. Prentice Hall. Englewood Cliffs, NJ. 1993. p. 342.

Madigan, M.T., John M. Martinko and Jack Parker. Biology of Microorganisms. 8th Ed. Prentice Hall. Upper Saddle River, NJ. 1997. p.551.

Standard Methods for the Examination of Water and Wastewater. 24th Ed. 1992. Method 5210 B. 5-Day BOD Test.


EMT- Environmental Monitoring Technologies, Inc. provides this web site that talks about their parameter testing procedures, including BOD.

NCSU- North Carolina State University water quality group developed this site, which talks about organic pollutants and their sources.