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The main reason for treating wastewater is to reduce the amount of organic matter present in the wastewater. When wastewater which high organic content is releases into a water system such as a river, it may increase the overgrowth of algal blooms – which reduces the amount of dissolved oxygen in the water, thus killing aquatic life. Thus, biochemical (biological) oxygen demand (BOD) is used by environmental microbiologists to assess the effectiveness of wastewater treatment processes. BOD helps us to know the amount of organic matter that has been removed from the wastewater after treatment.

Biochemical (biological) oxygen demand (BOD) is defined as the amount of dissolved oxygen that is required for the microbial oxidation (decomposition) of soluble, biodegradable organic matter in an aquatic environment. Other typical sources or types of BOD include ultimate BOD (UBOD), carbonaceous BOD (CBOD) and nitrogenous BOD (NBOD). UBOD is a parameter that quantifiers the amount of oxygen required for the total biochemical degradation of organic matter by aquatic microorganisms including bacteria and protozoa. CBOD is the amount of oxygen consumed during the oxidation of carbonaceous (carbon) compounds to carbon dioxide CO2) and other oxidized end products.

NBOD is the amount of oxygen consumed during the oxidation of nitrogenous compounds by nitrifiers or nitrifying bacteria to nitrate and other oxidized products. Both NBOD and CBOD contribute to the UBOD of a body of water that is polluted by organic or inorganic wastes. BOD is the oxygen uptake demand of contaminated water such as a river contaminated by sewage discharge and other organic and inorganic wastes. When sewage effluents are discharged to a body of water such as a river, the sewage effluents may consume large amounts of dissolved oxygen present in the river. However, if the dilution of the sewage effluent in the water body is inadequate, an anaerobic or microaerobic condition – in which oxygen is completely absent or minimal respectively, will definitely set in. In such scenarios of anaerobiosis (absence of oxygen) or microaerobic condition (low oxygen concentration), there will be a possible risk of asphyxiation (suffocation) for all the living organisms including fish present in the river. Because these organisms are dependent on appropriate levels of dissolved oxygen to continue to exist, a depletion of the available dissolved oxygen (DO) in the river will lead to the death of some aquatic lives. Sources of biological oxygendemand (BOD) in surface waters could be as a result of a natural course or from human activities.

Natural sources of BOD in surface waters include organic matters that result from the decomposition (decaying) of plant and animal wastes. Human sources of BOD in surface waters include feacal matter of humans and animals, grease, detergents, grease, fats and oil, and urine. The determination of BOD is the standard method of measuring indirectly the amount of organic pollution that can be oxidized biologically or biochemically in a given sample. BOD measurement is an important technique performed in water treatment plants or processes – in which wastewaters and other types of waters including sewage required to deplete the oxygen level of the receiving waters is measured. It is noteworthy that the higher the BOD measured the higher the amount of the pollution in the test sample. BOD can be used as a measure of the degree of water pollution; and thus BOD measurement forms part of the basis of water treatment especially in water treatment plant. It is also used for the environmental monitoring of water bodies for safety purposes.

With BOD measurement, the BOD of the influent and the BOD of the effluent can be calculated; and the results of such calculation is significant in the sense that it helps us to determine the level of dissolved oxygen in the body of the water. BOD measurement is also used to test other types of water apart from wastewaters or sewage – in order to determine their oxygen-depleting potential. Oxygen is an important parameter for the existence and sustenance of live on planet earth including living organisms found in aquatic environments. It is removed from water bodies when organic matter is consumed by bacteria; and when oxygen is used up in water bodies, aquatic lives will be negatively affected.

Oxygen is used up, depleted, consumed or assimilated by microorganisms including protozoa and bacteria as they assimilate various organic and inorganic compounds present in water and other aquatic environments. BOD test measures the amount of dissolved oxygen (mg) consumed per liter of a test sample (or a known dilution of the sample) in five (5) days at 20oC; and the values of BOD tests helps us to evaluate or determine the actual quality of the water body under investigation (Figure 1).

Figure 1. BOD levels and water quality. When BOD levels are high, there is usually a decrease in available dissolved oxygen (DO); and this is because the demand for oxygen by the microbes in the water is high and they are taking that oxygen from the oxygen that is naturally dissolved in the water body. The biochemical oxygen demand (BOD) determination is an empirical test in which standardized laboratory procedures are used to determine the relative oxygen requirements of wastewaters, effluents, and polluted waters. It is applied in measuring waste loadings to treatment plants and in evaluating the BOD-removal efficiency of such treatment systems.

In practice, BOD test is performed to determine the effect dirty water or sewage containing bacteria and organic materials will have on aquatic animal and plant life when such polluted water is released into a water body including a river, stream or lake. The BOD test is widely used to determine the pollution strength of both industrial and domestic wastes in terms of the oxygen that they will require if such wastes are finally discharged into natural waterways such as streams, rivers and lakes. BOD test is one of the most important tests in stream pollution control activities; and BOD test is very important in evaluating the purification capacity of receiving bodies of water as aforementioned. Several automated biological oxygen demand (BOD) analysis systems or equipments such as the BODTrak II manometric respirometer commercially exist for determining the BOD of samples from a particular receiving water body (Figure 2).

Figure 2. The BODTrak II manometric respirometer. The BODTrakTM II Apparatus measures Biochemical Oxygen Demand (BOD) using the respirometric method. The BOD test measures the quantity of oxygen consumed by bacteria that oxidize organic matter in a water sample. A pressure sensor in the BODTrak II manometric respirometer monitors air pressure in the sample bottle and a stir bar continually mixes the sample during the incubation period. As bacteria in the test sample consume the organic matter and oxygen present in it, the air in the bottle above the test sample replenishes the oxygen used by the bacteria. This causes a pressure drop inside the bottle. The instrument’s software converts this pressure drop to mg/L BOD. The carbon dioxide produced by the bacteria is removed by potassium hydroxide pellets placed in the seal cup of the sample bottle; and this assures that the measured pressure difference is in relation to only the quantity of oxygen used. The BODTrak II manometric respirometer is a closed system; and this closed system prevents atmospheric pressure changes from interfering with the test. The large display allows the user to monitor results at any time during the experiment. The system collects 360 data points over the user-selected 5, 7, or 10 day measurement period; and the results are displayed as a graph of BOD over time. The user can move a cursor along the curve to see the BOD value at any point in time.

When there is an abundance of bacteria and organic materials in the water body, the bacteria will take in oxygen in order to breakdown these organic molecules. If bacteria are taking in large amounts of oxygen, this will have a detrimental effect on the surrounding ecosystem. However, when there are lower levels of organic waste in the water body, there will be fewer bacteria present; and thus the BOD will be lower and the dissolved oxygen levels higher. Higher levels of dissolved oxygen and lower BOD encourage the growth and development of aquatic life. Microorganisms including bacteria, protozoa and fungi are responsible for the decomposition of organic matter in the environment including aquatic environment. When these organic materials including sewage, manure, dead plants and animal matter and food wastes are present in a particular water supply, these microbes will initiate the process of breaking down the waste. As the breakdown or oxidation of the organic matter proceeds, the available dissolved oxygen will be consumed by aerobic microbes and this deprives other aquatic organisms including fish of the oxygen they need to thrive.       

Further reading

Jee C and Shagufta (2007). Environmental Biotechnology. APH Publishing Corporation, Darya Ganj, New Delhi, India.

Latha C.D.S and Rao D.B (2007). Microbial Biotechnology. First edition. Discovery Publishing House (DPH), Darya Ganj, New Delhi, India.

Maier R.M, Pepper I.L. and Gerba C.P (2000). Environmental Microbiology. Academic Press, San Diego.

Mishra B.B, Nanda D.R and Dave S.R (2009). Environmental Microbiology. First edition. APH Publishing Corporation, Ansari Road, Darya Ganj, New Delhi, India.

Paul E.A (2007). Soil Microbiology, ecology and biochemistry. 3rd edition. Oxford: Elsevier Publications, New York.

Pelczar M.J., Chan E.C.S. and Krieg N.R. (2003). Microbiology of Soil.  Microbiology, 5th Edition. Tata McGraw-Hill Publishing Company Limited, New Delhi, India.

Pepper I.L and Gerba C.P (2005). Environmental Microbiology: A Laboratory Manual. Second Edition. Elsevier Academic Press, New York, USA. 

Roberto P. Anitori (2012). Extremophiles: Microbiology and Biotechnology. First edition. Caister Academic Press, Norfolk, England.

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