The polluted water may contain large amounts of inorganic and organic compounds. Some of these can be oxidised by dissolved oxygen in the presence of microorganisms.

BOD, is a measure of the dissolved oxygen that would be needed by the microorganisms to oxidise these compounds. BOD, therefore is a measure of the total contamination caused by compounds which can be oxidised in the presence of microorganisms. The BOD is taken as a realistic measure of water quality – clean water would have a BOD value of less than 5 ppm whereas highly polluted river water could have a BOD value of 17 ppm or more.

BOD Determination

In order to find out BOD, the water sample is first saturated with oxygen. It is then incubated at constant temperature. This allows time for microorganisms in water sample to oxidise pollutants. The remaining amount of dissolved oxygen is determined and BOD is obtained by subtraction BOD measurements takes a few days, so another parameter called the chemical oxygen demand (COD) is sometimes measured.

COD determination

In COD determination the water sample is treated with a known quantity of an oxidising agent, usually   in acidic medium. This reagent oxidises most of the polluting substances, including those which are resistant to microbial oxidation. The remaining (24) is determined by back titration with a suitable reducing agent. From the concentration of  consumed, the amount of oxygen used in oxidation may be calculated using the following chemical equation:

(25)

The results are expressed in terms of amounts of oxygen in ppm, that would be required to oxidise the contaminants. This is called COD.

Control of water pollution

Some of the steps which are helpful for the control of water pollution are being described as follows:

(i) Waste water treatment techniques can be applied before the polluted water enters a river, lake or pool.

(ii) Septic tanks should be used for each house.

(iii) Rivers, lakes etc. should not be used for bathing and washing purposes.

(iv) Too much use of pesticides which are not biodexampleradable should be avoided. These are highly toxic substances.

(v) Efforts should be made to increase the use of low grade or polluted water.

Illustration 1. Differentiate between biodexampleradable and non biodexampleradable substances with examples.

Solution: Organic wastes which can be readily decomposed by microorganism are called biodexampleradable example. Plant wastes.

Certain wastes can not be decomposed or take a very large time for their decomposition are called non biodexampleradable. They are harmful for healthy environment as they persist forever or for very long span of time example. Polythene.

Illustration 2. What are the main sources of thermal pollution?

Solution: Thermal power plants and nuclear plants

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