Biochemical Oxygen Demand in Water Quality

Biochemical Oxygen Demand or alternatively termed as Biological Oxygen Demand (BOD) is the amount of oxygen needed or demanded by aerobic microorganisms to break down the organic matter present in a certain sample of water at a specific temperature and over a given time period.

Water bodies have a certain amount of oxygen dissolved in it on which the aquatic life is dependent for its respiratory needs. When there is organic matter present in the water body, aerobic microbes use the dissolved oxygen in the water to break down the organic substance thus reducing the oxygen available for aquatic life. The molecular oxygen present in water is either a byproduct of photosynthesis occurring in aquatic plants or is atmospheric oxygen in the dissolved state. The more the BOD of a water body or water sample, the more it is polluted. The increase in organic matter can be due to environmental factors but are mostly due to anthropogenic causes like pollution. BOD is used as an index for measuring water quality. Determining organic matter present in a water body and its effect on the ecosystem and aesthetics of the water body is an integral part of water quality management.

Measuring Biochemical Oxygen Demand

Biochemical Oxygen Demand of a water sample is measured by a Bioassay procedure which measures the oxygen consumed by the bacteria from the decomposition of the organic matter over a period of five days at an incubation temperature of 20°C. BOD is expressed in milligrams per litre of sample water. Although this is not a precise quantitative test, it is widely used as an indication of the polluting potential of water. This test was given by Sawyer and McCarty in 1978.

Typical Values of BOD and its Indication:

• Below 1 mg/L- Pristine water quality.

• 2-8 mg/L- Moderately polluted water.

• Above 8mg/L- Severely polluted water.

Factors Affecting BOD

BOD of a Water Body is Affected by the Following Factors -

• Temperature

• pH value of the water

• The presence of certain kinds of microorganisms affects the growth of aerobic bacterias.

• Type of inorganic substances in water.

• Amount and type of organic substance in water.

Significance of BOD

Biochemical Oxygen Demand finds its importance in various fields. These are -

• BOD finds its primary importance in sewage treatment plants. It gives the respiration rate of sewage, sludge, soil, and garbage.

• It determines the rate of respiration in living beings.

• Measuring BOD gives the COD or Chemical Oxygen Demand of inorganic substances.

• It indicates the polluting potential of water.

• BOD is used in the medical and pharmaceutical industries to measure the oxygen consumption of cell cultures. 

Sources of BOD

Sources that increase the Biological Oxygen Demand of water are both natural and man-made. Pollution is a major contributor to increasing the BOD of water bodies. A good lifestyle is associated with an ample usage of water on a regular basis which results in a lot of wastewater with organic content in it. With increasing industrialization, pollution is increasing manifold. Factories have enormous wastewater being generated. Few industries that have huge quantities of wastewater are paper mills, food processing plants, jute mills, etc. The environmental factors contributing to increasing BOD include surface runoff, floating debris, dead animals and plants, soil erosion, etc. There are few chemicals that affect the BOD of drinking water. One of these is phosphate, which when present in high amounts increases the BOD of water.

Usage of BOD in Sewage Treatment Plants

Biochemical Oxygen Demand is used in secondary sewage treatment or biological sewage treatment. After the primary treatment in which the floating debris is removed by sequential filtration and sedimentation, the primary effluent is passed to aeration tanks where it is constantly agitated and the air is pumped into it. In aeration tanks, there are vigorous growth heterotrophic microbes into flocs. Flocs are masses of bacteria associated with fungal filaments. 

These microbes consume the organic matter present in the primary effluent. The water is treated until the BOD of the water is reduced. This is now called the activated sludge. This effluent from aeration tanks goes into settling tanks for bacterial flocs to settle down and after which they are treated with anaerobic microbes and physicochemical processes before discharge in water bodies.

Effect of High BOD on the Aquatic Ecosystem

Increasing BOD has the same effect as the effects of dissolved depleting oxygen. When the BOD of a water body increases significantly, aquatic life is adversely affected. The oxygen used by aquatic organisms for respiration and metabolism is significantly reduced by the microbes for breaking down of organic waste. This results in the death of fishes and aquatic plants and complete disruption in the aquatic ecosystem. Oxygen concentration below 5ppm (parts per million) risks even low oxygen organisms like catfish and carps. The freshwater fishes like Catla and rohu do not survive at these concentrations. The overall aesthetic and beauty of the water body are damaged.

Pollution and Its Effects on Biochemical Oxygen Demand

With increasing pollution and urbanization, the water quality of the water bodies is significantly reducing. Water quality management is essential for the correct ecological functions. Urbanization leads to the production of much larger quantities of sewage. The number of sewage treatment plants was not enough for treating these large quantities of sewage.  Untreated sewage was often discharged directly into water bodies which caused massive pollution and an increase in BOD of the water bodies. This also led to an increase in water-borne diseases like cholera, dysentery, jaundice, etc.

This increasing amount of BOD and pollution led to extreme pollution of the two main river bodies in India, Ganga, and Yamuna. The Ministry of Environment and Forests initiated the Ganga Action Plan in 1985 and the Yamuna Action Plan in 1993, to save these major rivers of the country. These plans initiated the building of a large number of sewage treatment plants to facilitate the discharge of only treated sewage in rivers.

Methods to Reduce BOD in Water

The biological oxygen demand of the water can be reduced by the following methods 

• Advanced Oxidation Processes (AOP) by using H2O2/ UV, O3/UV, Fenton's reagent ( H2O2+FeSO4), etc.

• Coagulation using alum or cationic polymers

•  Flocculation (e.g. chitosan, isinglass, polyelectrolyte) and sedimentation.

• Adsorption using activated charcoal.

• Electro flocculation.

• Using the up-flow anaerobic sludge blanket reactor(UASB).

• Reverse osmosis.

• Dissolved air floatation technique.

Water is a basic need for all life forms to exist on earth. Therefore it is extremely important for us to conserve water and not pollute it. Polluting water directly harms every living creature be it mankind or the other forms of life. Rising biochemical oxygen demand levels in water are harming the ecosystem of the water bodies which indirectly is harming the whole ecology of the whole biosphere as a whole. It is our duty to keep these ecosystems alive. The other forms of life have the same amount of rights on environmental resources as human beings. We must keep our greed away and strive towards conservation of water bodies and reduction of BOD levels in them.

The measurement of the amount of dissolved oxygen consumed by aerobic microorganisms while decomposing organic matter in stream water is known as Biochemical Oxygen Demand.

Affecting Factors

Mentioned below are the affecting factors of biological chemical demand:-

• Biological oxygen demand affects the amount of dissolved oxygen in streams and rivers. The rate of oxygen consumption is affected by: pH, temperature,  various kinds of microorganisms, and organic and inorganic materials.

• Less oxygen is consumed by higher forms of aquatic life. The consequences of the high BOD are the same as those for low dissolved oxygen.

Uses

Various uses of biological  oxygen demand have been mentioned as follows :

• Biological oxygen demand is used in studies for measuring self purification capacity of the streams.

• It is an important method in sanitary analysis in determining industrial waste, the strength of sewage, and polluted water.

• It also serves as a source through which the quality of effluents discharged into the stream water can be checked.

Importance

Let's discuss about the environmental significance  of biological oxygen demand:

• Biological oxygen demand is important for water quality because it provides an index to assess the effect discharged wastewater will have on our environment.

• BOD is used extensively for treating wastewater, as decomposition of organic waste by microorganisms is commonly used for treatment.

• Data obtained from the BOD test is used for designing the wastewater treatment plants.