What are the major causes of water pollution ? Explain. 

Major causes of water pollution are as below :

$(i)$ Pathogens : The most serious water pollutants are the disease causing agents called pathogens. Pathogens include bacteria and other organisms that enter water from domestic sewage and animal excreta. Human excreta contain bacteria such as Escherichia coli and Streptococcus faecalis which cause gastrointestinal diseases.

$(ii)$ Organic wastes : The other major water pollutant is organic matter such as leaves, grass, trash etc. They pollute water as a consequence of run off. Excessive phytoplankton growth within water is also a cause of water pollution. These wastes are biodegradable.

The large population of bacteria decomposes organic matter present in water. They consume oxygen dissolved in water. The amount of oxygen that water can hold in the solution is limited.

The concentration of dissolved oxygen in cold water is upto $10 \mathrm{ppm}$, whereas oxygen in air is about $200,000 \mathrm{ppm}$. Because of this a moderate amount of organic matter when decomposes in water can deplete the amount of dissolved oxvgen.

The concentration of dissolved oxygen in water is very important for aquatic life. If theconcentration of dissolved oxygen of water is below $6$ $ppm$, the growth of fish gets inhibited.

Oxygen reaches water either through atmosphere or from the process of photosynthesis carried outby many aquatic green plants during day light.

Photosynthesis stops during night but the pplants continue to respire. It resulting in reductionof dissolved oxygen.

Microorganisms also used dissloved oxygen for the oxidation of organic matter. So, too muchof organic matter is added to water, all the available oxygen is used up. Thus, oxygendependent aquatic life to die.

Anaerobic bacteria which do not require oxygen, begin to break down the organic waste andproduce chemicals which are harmful to human health.

Aerobic bacteria which require oxygen degrade the organic wastes and keep the waterdepleted in dissolved oxygen.

In a certain volume of water sample the amount of oxygen required by beacteria to breakdown the organic matter is called Biochemical Oxygen Demand ($BOD$)