Biochemical Oxygen Demand (BOD)

Introduction- What is Biochemical Oxygen Demand (BOD)?

Biochemical oxygen demand (BOD) or biological oxygen demand is the amount of oxygen demanded by the bacteria present in a water body to decompose the organic material present in the body at a given temperature over a specific period of time. The decomposition or breaking down of the organic master often takes place by aerobic methods in water bodies. So BOD leads to a shortage of oxygen in the water body.

Water bodies usually have a definite amount of oxygen dissolved in them. The amount of dissolved oxygen (DO) indicates the liveability of aquatic organisms in the water body. However, when there is organic matter present in the water body, they are decomposed by bacteria. In this process, oxygen is absorbed. Therefore, the amount of BOD increases with increased biomass of organic matter.

The amount of oxygen present in water bodies are either a byproduct of the photosynthesis of the aquatic plants or it is a state of dissolved oxygen from the atmosphere in the water body. The more the amount of BOD, the more polluted is the water body.

Image 1: Anthropogenic effects on the marine nitrogen cycle

An increase in BOD usually occurs due to anthropogenic reasons enhanced by human activities. For example, BOD increases in the case of anthropogenic eutrophication where dead algae biomass accumulates in the bed of water bodies which prevents any other life form to sustain in the water bed.

BOD is used to determine the water quality levels of water bodies. Measuring the BOD levels of a water body and therefore the amount of organic matter present in the water body in order to test the aesthetics of the water bodies is an integral part of water quality management.

How Biological Oxygen Demand is measured

Biological oxygen demand is measured by a bioassay procedure. In this process, the water of a water body is tested for how much oxygen is required for bacteria to decompose the organic matter over a period of five days and at a temperature of 20 degrees Celsius. BOD is expressed in a unit of milligrams per liter of water. This process of determining BOD is not a precise test for determining BOD but it is widely used to test the polluting potential of water. This test was first given by Sawyer and McCarty in 1978.

Typical values of BOD and their indication

• For pristine quality water 1 mg/L.

• For moderately polluted water 2 to 8 mg/L.

• For severely polluted water 8 mg/L or above.

Factors Affecting BOD

• The temperature of the environment of the water body.

• The pH level of the water body.

• Presence of certain microorganisms in the water body help in the growth of aerobic bacteria.

• Inorganic substances in the water body.

• Types and amounts of organic substances in the water body.

Importance of BOD

BOD is an important measure for various fields. Some of the uses and importance of BOD include the following:

• BOD is an important measure in sewage treatment plants. It provides the respiration rate of sewage, soil, sludge, and garbage bacteria.

• BOD determines the rate of respiration in living organisms.

• BOD measurement gives the value of Chemical Oxygen Demand (COD) of inorganic substances.

• It is important to measure the polluting potential of water.

• To measure oxygen consumption in cell cultures in the pharmaceutical and medical industries, BOD is an important factor.

Reasons for Increase in BOD Amount

• The reasons for the increase in the amount of BOD can be both natural and man-made. For example, pollution is a major contributor to the increase of BOD in water bodies. Humans require ample amounts of water for a good lifestyle. The usage of water often results in wastewater with an enormous amount of organic matter in them. Increasing industrialization is also a contributing factor. Factories generate a very high amount of wastewater. Some industries that generate huge wastewater quantities are paper mills, jute mills, and food processing plants.

• Untreated sewage water also contains a lot of inorganic material in them. Environmental factors that contribute to increasing BOD include surface runoffs, floods, landslides, and soil erosion, etc. which erode soil rich in nutrients and take them to nearby water bodies to make the water bodies rich in nutrients which are fed by bacteria to increase the BOD. The BOD of drinking water is increased by certain chemicals, such as phosphates which increase the BOD when mixed with water.

Use of BOD in Sewage Treatment Plants

• The biochemical oxygen demand process is used to treat sewage in the secondary stage. In the preliminary stage, the sewage is treated with sequential filtration and sedimentation which remove the floating debris from the sewage water.

• The water is then processed vigorously in aeration tanks where the air is pumped in to provide oxygen to the water so that heterotrophic microbes can grow into flocs.

• Flocs are masses of bacteria with fungal filaments.

• The microbes in the aeration tanks consume the organic matter present in primary effluents.

• The treatment is continued until the level of BOD is reduced to a certain extent. The effluent from aeration tanks is taken to settling tanks where flocs settle down. The water is then treated with anaerobic microbes and physiochemical processes before discharge into water bodies.

Effect of Higher BOD on Aquatic Ecosystems

Image 2: Scheme eutrophication on aquatic life

The effects of high BOD can be seen in the process of Eutrophication of water bodies where various life forms find it impossible to sustain in water bodies. When the BOD goes above a certain level, the dissolved oxygen in water bodies gets reduced until it reaches a significant level where no other life form can live as the BOD is consumed by bacteria to decompose organic materials present in the water body. As bacteria use dissolved oxygen to break down dead algae biomass in water bodies, only a little amount of oxygen is left for use by other living organisms.

When the amount of BOD results in an oxygen concentration of 5 ppm (parts per million) or below even low oxygen-consuming organisms such as catfish and carp find it tough to sustain in the water bodies. Freshwater species such as Catla and Rohu do not survive in these situations. Moreover, the aesthetics of water also turns unpleasant due to the high amount of BOD.

Conclusion

It is important to learn about BOD because it indicates the health of water body ecosystems. Moreover, as high BOD represents high amounts of pollution we must be aware of it to reduce pollution. It is also important to preserve aquatic diversity by maintaining the levels of BOD within limits.

Frequently Asked Questions

Qns 1. What is the level of BOD for severely polluted water?

Ans. The level of BOD for severely polluted water is 8 mg/L or above.

Qns 2. What is the concentration of oxygen below which low oxygen-consuming organisms die out?

Ans. Low oxygen-consuming species such as catfish and carp fie out below 5 ppm of oxygen.

Qns 3. Name some environmental causes that lead to increased BOD in water bodies.

Ans. Environmental factors that contribute to increasing BOD include surface runoffs, floods, landslides, and soil erosion, etc.