Water is a key necessity for the existence of life on planet earth (plants and animals). It serves as a chemical medium in which all biological and biochemical processes occur. However, sewage leads to observable disturbances in the normal properties and functions of water.
The dissolved oxygen (DO) present in water is essential to support fish and other aquatic life. Sewage results in the decrease of the total dissolved oxygen present in water thus affecting the life in water.
Sewage contains a wide number of microorganisms such as viruses, bacteria, protozoa which maybe pathogenic and cause diseases. These are referred to as water born diseases. Example: Infectious Hepatitis (Jaundice) caused by Hepatitis A virus
Sewage contains radioactive pollutants such as radioactive isotopes of Uranium, Radium which are carcinogenic (cancer causing).
Sewage contains chemicals which may remain in the environment for a long time and lead to bioaccumulation which ultimately enters the food chain via the aquatic life.