Biological Oxygen Demand (BOD) is the amount of dissolved oxygen needed by biological organisms for the decomposition of organic compounds in a given amount of water. And a higher BOD indicates a lower level of dissolved oxygen.
Secondary treatment also known as biological treatment is a treatment that uses biological process to convert dissolved suspended and colloidal organic wastes to more stable solids. These converted stable solids are then either removed by settling or is discharged to the environment without causing any harm.
In the secondary treatment the primary effluent is passed into large aeration tanks. After passing the effluent is agitated continuously by mechanical method and then the air is pumped into it. By doing this there is vigorous growth of useful aerobic microbes into flocs. Flocs are bacterial masses coupled with fungal filaments to form mesh like structures.
These aerobic microbes consume the major part of the organic matter present in the effluent during their growth phase, thereby reducing the BOD of the effluent. The sewage water is treated till the BOD is reduced.
The effluent is next passed into settling tank after the BOD of sewage water is reduced.
In this way primary effluent is given secondary treatment thereby showing change in the level of BOD.
The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.
BOD is also a measure of the organic matter present in the water as the BOD test measures how much oxygen is taken by the micro-organisms present in the water. Hence greater the BOD of waste water more is the polluting potential of the water.
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