There are different degrees of the water impurity. It can be due to the different organisms which differ in the tolerance level of temperature, ph, oxygen, calcium and phosphorous content. It also involves the organic matter, poisons and some pollutants. There are certain fishes known as daphnia which are sensitive to poison and metals. The study of aquatic life gives information regarding the water pollution. The bio chemical oxygen demand also helps in the measurement of organic wastes in the polluted water. It is denoted by BOD. It is defined as the requirement of oxygen by small organisms so that the organic matter which is easy to be decomposed can be stabilized under the oxygen rich environment.
The oxygen is measured in the milligrams for few days. It is measured in I liter of water at a temperature around twenty degree Celsius. More is the organic waste more is the BOD value. The water which is not rich in the organic matter has a value of BOD equal to 1500 milligram per liter. The water which is rich in the organic matter has a value of BOD more than 1500 milligram per liter. It relies only on the organic matter measurements and other factors are not taken into account. So, it is not a reliable method for the water pollution. Another method is there which can substitute this method and is known as the COD which refers to chemical oxygen demand. It takes into account all the oxygen consuming pollutants which are present in the water.   _{Your Comment}