There are large numbers of water pollutants. For our convenience, they have been classified into five different categories: (i) Organic pollutants (ii) Inorganic pollutants (iii) Suspended solids and sediments (iv) Radioactive pollutants and (v) Thermal pollutants Organic pollutants: The organic pollutants present in water may be classified into five different categories such as: (i) oxygen demanding waste (ii) disease causing waste (iii) synthetic organic compounds (iv) oil (v) sewage and agricultural runoff.
Oxygen demanding wastes: One of the most important measures of water quality is the amount of dissolved oxygen (DO) present in it. The saturated value of DO in water is in the order of 8 to 15 mg /l. Optimum DO required for healthy fish and other aquatic life in natural water is 5 to 8 mg/1.
If DO drops, fish and other aquatic life is threatened and in extreme cases, killed. The other affect of reduced DO is undesirable taste, odor and color, prohibiting its use for domestic and recreational purposes.
The oxygen demanding wastes are generally biodegradable organic matter contained in sewage, industrial wastes from food processing, paper mills and tanneries, agriculture return, etc.
These organic wastes provide a good substrate for the luxuriant growth of aerobic bacteria which decompose the waste and deplete the oxygen.
The two most important measures of oxygen demand are (1) chemical oxygen demand (COD) (2) biological oxygen demand (BOD).
Chemical oxygen demand (COD) is the amount of oxygen required to oxidize the wastes chemically. BOD is the amount of oxygen required by microorganism to decompose the wastes biologically.