Biochemical oxygen demand

Oxygen helps liberate biochemical energy from food by acting as the electron acceptor for the reaction that metabolizes adenosine triphosphate , ATP, one of the body's major chemical energy sources. Metabolic processes that require oxygen are called aerobic . Naturally occurring oxygen is in the form of molecular oxygen, O2. Atmospheric oxygen is obtained by the body in the lungs in tiny air sacs called alveoli. Within the alveoli, red blood cells (rbc's) in narrow blood vessels absorb oxygen and carry it to cells throughout the body. Respiration , inhaling and exhaling of air, is subconsciously controlled by the brain in response to fluctuations in carbon dioxide levels. The average human body of 139 lb (63 kg) consumes 250 ml of O2 each minute. The major single-organ oxygen consumers are the liver, brain, and heart (consuming 20.4%, 18.4%, and 11.6%, respectively), while the sum total of all the body's skeletal muscles consume about 20%. In addition, the kidneys use up about 7.2%, and the skin uses 4.8% The rest of the body consumes the remaining 17.6% of the oxygen. Oxygen use can also be measured per 100 gm of an organ to indicate concentrations of use; as such, heart usage is highest, followed by the kidneys, then the brain, and then the liver. During exercise , the biochemical oxygen demand increases for active tissues including the heart and skeletal muscles.

Oxygen is the molecule used by animals as a final electron acceptor for metabolism . Two electrons (one at a time) from metabolic products can chemically bind each oxygen molecule. While numerous molecules combine with oxygen in the human body, one of the major chemical reactions involving oxygen is the synthesis of the high-energy phosphate bonds in ATP. ATP is the cell's currency for generating muscle contractions and driving certain ions through membrane-bound ion channels. Oxygen facilitates aerobic ATP production in mitochondria of cells throughout the body. Aerobic production of 36 molecules of ATP from one glucose molecules occurs in the citric acid metabolic cycle. About 1 L of oxygen can release the chemical energy stored in 1 g of food.

Oxygen is carried through the body in a number of chemical forms including simple O, water (H2O), carbon dioxide (CO2), and oxyhemoglobin. Unbound oxygen radicals can be highly toxic to cells. Allowingrandom oxidation reactions to occur throughout the cell , these radicals can be very destructive, and cellular defenses have evolved to combat them. In fact, the oxygen radical H2O2 is highly toxic to cells and can be used as a bactericidal agent. H2O and CO2 are end products for several aerobic reactions. And oxyhemoglobin is the oxygen shuttle complex that carries oxygen to needy cells. One rbc contains around 350 million hemoglobin molecules. Hence, one rbc can carry about 1.5 billion oxygen molecules.

Hemoglobin is a large globular protein made up of four polypeptide chains (two alpha and two beta hemoglobins, in adults) that each contain one heme complex. Heme complexes are sophisticated ring structures that contain a central ferrous iron atom. The iron atoms can each bind one O2 molecule. Hence, one hemoglobin molecule can bind four O2 molecules. This is conventionally represented as Hb4O8. The four Hb components can alter their orientation to favor uptake or release of the oxygen. When the Hb bonds are relaxed, they favor uptake, and when they are tense they favor release. Affinity is the chemical term used to indicate how eager multiple units are to interact with one another. The chemical affinity for the first oxygen to bind is lower than the affinity for the later oxygens to bind. In other words, once one oxygen has bound to the hemoglobin, the binding of the other three oxygen molecules is more favorable. In addition, the amount of oxygen bound or released depends on the concentration of oxygen in two locations (where it is coming from and where it is being absorbed).

Oxygen flow is greatly determined by local partial pressure gradient. Just like it is more difficult to push water up a waterfall, so it is difficult to absorb oxygen into an area that already has more oxygen than the place it is coming from. In both cases, a certain amount of pressure is causing something to flow one way. As rbcs travel through arteries and veins around the human body, they collect oxygen in the alveoli where the partial pressure of oxygen is higher than it is in the rbcs. Usual average atmospheric pressure is measured as 1 atmosphere (1 atm) or 14.7 pounds per square inch (14.7 psi). Since oxygen makes up about 21% of atmospheric air, the partial pressure of oxygen is 0.21 atm or 3.09 psi. Because venous oxygen partial pressure is less than 3.09 psi, oxygen is driven into the blood in the lungs. Although a small amount of O2 gas dissolves into the plasma (the fluid surrounding the blood cells), most is bound by hemoglobin. The reverse process occurs as capillaries supply tissues with oxygen. The partial pressure of oxygen in the tissue is lower than in the blood, so oxygen flows into the tissue. CO2 travels a reverse course where high tissue partial pressures push CO2 out into the veins that carry it to the lungs for release into the atmosphere. The CO2 partial pressure of the atmosphere is significantly lower than that of body tissues. The relationship of gaseous absorption to atmospheric pressure makes it crucial for mountain climbers and scuba divers to calculate their expected partial pressure gaseous exposure before climbing or diving. Miscalculations could lead to death.

Body tissues vary in their oxygen dependency. Hypoxia is the condition of existing with a lowered oxygen supply. The brain and heart are the two most hypoxia sensitive organs. A severe drop in available oxygen can cause brain death in five minutes. Less severe hypoxia can lead to other mental problems such as dizziness, headache, disorientation, drowsiness, or impaired judgment. Although basic brain functions may recover fully from a short hypoxic period, higher neural functions can be severely impaired.

During rigorous exercise, oxygen demand may increase to up to 15 times the normal demand. As muscles deplete their oxygen supplies, the lowered muscular oxygen partial pressure steepens the pressure gradient, so that even more oxygen leaves the blood and enters the muscles. If aerobic metabolism is unable to supply enough ATP to muscle cells, then anaerobic metabolism can provide some ATP. However, anaerobic metabolism temporarily adds lactic acid to the muscle creating an oxygen debt whereby the muscle fatigues and requires a recovery period to get rid of the lactic acid. An initial oxygen debt is also always present for about the first 30 seconds of exercise until circulation can accelerate to provide additional oxygen.

Various life forms are classified on the basis of their tolerance or requirement of oxygen. Different types of bacteria are aerobic, facultatively aerobic, or anaerobic. Aerobes use oxygen to generate energy. Facultative aerobes can use oxygen but survive without it. Oxygen is highly toxic to anaerobes which die rapidly when exposed to it.

See also Cellular respiration; Circulatory system.

Resources

books

Guyton & Hall. Textbook of Medical Physiology. 10th ed. New York: W. B. Saunders Company, 2000.

Rhoads R., and R. Pflanzer, eds. Physiology. 2nd ed. New York: Saunders College Publishing, 1992.

Louise Dickerson

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Hemoglobin

—An iron-containing, protein complex carried in red blood cells that binds oxygen for transport to other areas of the body.

Hypoxia

—State of deficient oxygen supply.

Biochemical oxygen demand

The biochemical oxygen demand (BOD) test is an indirect measure of the biodegradable organic matter in an aqueous sample. The test is indirect because oxygen used by microbes as they degrade organic matter is measured, rather than the depletion of the organic materials themselves. Generally, the test is performed over a five-day period (BOD₅) at 68°F (20°C) in 10 fl oz (300 ml) bottles incubated in the dark to prevent interference from algal growth. Dissolved oxygen (DO) levels (in mg/l) are measured on day zero and at the end of the test. The following equation relates how the BOD₅ of a sample is calculated:

Note that in the above equation "ml sample" refers to the amount of sample that is placed in a 300 ml BOD bottle. This is critical in doing a test because if too much sample is added to a BOD bottle, microbes will use all the DO in the water (i.e., DOafter 5 days=0) and a BOD₅ cannot be calculated. Thus, the DO uptake at various dilutions of a given sample are made. One way of making the dilutions is to add different amounts of sample to different bottles. The bottles are then filled with dilution water which is saturated with oxygen, contains various nutrients, is at a neutral pH , and is very pure so as not to create any interference. Researchers attempt to identify a dilution which will provide for a DO uptake of greater than or equal to 2 mg/l and a DO residual of greater than or equal to 0.5–1.0 mg/l.

In some cases compounds or ions may be present in a sample that will inhibit microbes from degrading organic materials in the sample, thereby resulting in an artificially low or no BOD. Other times, the right number and/or types of microbes are not present, so the BOD is inaccurate. Samples may be "seeded" with microbes to compensate for these problems. The absence of a required nutrient will also make the BOD test invalid.

Thus, a number of factors can influence the BOD, and the test is known to be rather imprecise (i.e., values obtained vary by 10–15% of the actual value in a good test). However, the test continues to be a primary index of how well waters need to be treated or are treated and of the quality of natural waters. If wastes are not treated properly and contain too much BOD, the wastes may create serious oxygen deficits in environmental waters. The saturation concentration of oxygen in water varies somewhat with temperature and pressure but is often only in the area of 8–10 mg/l or less. To serve as a reference, the BOD₅ of raw, domestic wastewater ranges from about 100–300 mg/l. The demand for oxygen by the sewage is therefore clearly much greater than the amount of oxygen that water can hold in a dissolved form. To determine allowable BOD levels for a wastewater discharge , one must consider the relative flows/volumes of the wastewater and receiving (natural) water, temperature, reaeration of the natural water, biodegradation rate, input of other wastes, influence of benthic deposits and algae, DO levels of the wastewater and natural water, and condition/value of the receiving water.

[Gregory D. Boardman ]

RESOURCES

BOOKS

Corbitt, R. A. Standard Handbook of Environmental Engineering. New York: McGraw-Hill, 1990.

Davis, M. L., and D. A. Cornwell. Introduction to Environmental Engineering. New York: McGraw-Hill, 1991.

Tchobanoglous, G., and E. D. Schroeder. Water Quality. Reading, MA: Addison-Wesley, 1985.

Viessman Jr., W., and M. J. Hammer. Water Supply and Pollution Control. 5th ed. New York, Harper Collins, 1993.

BIOLOGICAL OXYGEN DEMAND

Biological Oxygen Demand (BOD) is one of the most common measures of pollutant organic material in water. BOD indicates the amount of putrescible organic matter present in water. Therefore, a low BOD is an indicator of good quality water, while a high BOD indicates polluted water. Dissolved oxygen (DO) is consumed by bacteria when large amounts of organic matter from sewage or other discharges are present in the water. DO is the actual amount of oxygen available in dissolved form in the water. When the DO drops below a certain level, the life forms in that water are unable to continue at a normal rate. The decrease in the oxygen supply in the water has a negative effect on the fish and other aquatic life. Fish kills and an invasion and growth of certain types of weeds can cause dramatic changes in a stream or other body of water. Energy is derived from the oxidation process. BOD specifies the strength of sewage. In sewage treatment, to say that the BOD has been reduced from 500 to 50 indicates that there has been a 90 percent reduction.

The BOD test serves an important function in stream pollution-control activities. It is a bioassay procedure that measures the amount of oxygen consumed by living organisms while they are utilizing the organic matter present in waste, under conditions similar in nature. The other traditional tests or indicators for water quality are chemical oxygen demand (COD) and pH.

For results of the BOD test to be accurate, much care must be taken in the actual process. For example, additional air cannot be introduced. Temperature must be 20°C, which is the usual temperature of bodies of water in nature. A five-day BOD test is used in environmental monitoring. This test is utilized as a means of stating what level of contamination from pollutants is entering a body of water. In other words, this test measures the oxygen requirements of the bacteria and other organisms as they feed upon and bring about the decomposition of organic matter. Time and temperature, as well as plant life in the water, will have an effect on the test. High BOD burdens or loads are added to wastewater by food processing plants, dairy plants, canneries, distilleries and similar operations, and they are discharged into streams and other bodies of water.

Mark G. Robson

(see also: Ambient Water Quality; Dissolved Oxygen; Ecosystems; Water Quality )

Bibliography

Shelton, T. (1991). Interpreting Drinking Water Quality Analysis—What Do the Numbers Mean? New Brunswick, NJ: Rutgers Cooperative Extension.

Wallace, R. (2000). Maxcy-Rosenau-Last Public Health and Preventive Medicine. Stamford, CT: Appleton & Lange.

Oxygen Demand, Biochemical

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Biochemical oxygen demand (BOD) is a measure of how much organic pollution is in water. The BOD test measures the amount of dissolved oxygen in water that is used up due to the breakdown of organic pollutants, such as sewage, in a certain number of days. Raw sewage has a BOD of forty to 150 milligrams per liter, whereas drinking water has a BOD of less than 0.5 milligrams per liter.

Engineers and scientists measure the BOD of a lake or river to see how healthy the water is. The lower the BOD, the healthier the water. Water needs to have oxygen in it to support aquatic life such as fish and plants. Oxygen in the water is replenished from the atmosphere through aeration, but if it is used up faster than it is replenished, the water becomes anaerobic (or hypoxic)—existing in the deficiency or absence of free oxygen. Anaerobic water cannot support life.

see also Fresh Kills; Hypoxia; Water Treatment.

Bibliography

peavey, howard s.; rowe, donald r.; and tchobanoglous, george. (1985). environmental engineering. new york: mcgraw-hill.

Julie Hutchins Cairn

biochemical oxygen demand (biological oxygen demand, BOD) An indicator of the pollution of water by organic matter where the decomposition of the organic matter by micro-organisms depletes the water of dissolved oxygen. It is measured as the mass (mg) of oxygen per litre of water that is taken up when a sample of water containing a known amount of oxygen is kept in darkness at 20°C, usually for five days (when the result is described as the BOD₅).

biological oxygen demand (BOD, biochemical oxygen demand) An indicator of the polluting capacity of an effluent where pollution is caused by the take-up of dissolved oxygen by micro-organisms that decompose the organic material present in the effluent. It is measured as the weight (mg) of oxygen used by one litre of sample effluent stored in darkness at 20°C for five days.

biochemical oxygen demand (BOD) The amount of oxygen taken up by microorganisms that decompose organic waste matter in water. It is therefore used as a measure of the amount of certain types of organic pollutant in water. BOD is calculated by keeping a sample of water containing a known amount of oxygen for five days at 20°C. The oxygen content is measured again after this time. A high BOD indicates the presence of a large number of microorganisms, which suggests a high level of pollution.

biological oxygen demand (BOD, biochemical oxygen demand) An indicator of the polluting capacity of an effluent where pollution is caused by the take-up of dissolved oxygen by micro-organisms that decompose the organic material present in the effluent. It is measured as the weight (mg) of oxygen used by one litre of sample effluent stored in darkness at 20°C for five days.

biological oxygen demand (BOD, biochemical oxygen demand) Indicator of the polluting capacity of an effluent, where pollution is caused by the take-up of dissolved oxygen by micro-organisms that decompose the organic material present in the effluent. It is measured as the weight (mg) of oxygen used by 1 litre of sample effluent stored in darkness at 20°C for five days.