What are the major causes of water pollution? Explain.

(N/A) Major causes of water pollution are as follows:
$(i)$ Pathogens: The most serious water pollutants are 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: Another major water pollutant is organic matter such as leaves,grass,and trash. They pollute water as a consequence of runoff. 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 and consumes oxygen dissolved in water. The amount of oxygen that water can hold in solution is limited.
The concentration of dissolved oxygen in cold water is up to $10 \ ppm$,whereas oxygen in air is about $200,000 \ ppm$. Because of this,a moderate amount of organic matter,when decomposed in water,can deplete the amount of dissolved oxygen.
The concentration of dissolved oxygen in water is very important for aquatic life. If the concentration of dissolved oxygen in water is below $6 \ ppm$,the growth of fish is inhibited.
Oxygen reaches water either through the atmosphere or from the process of photosynthesis carried out by aquatic green plants during daylight.
Photosynthesis stops during the night,but plants continue to respire,resulting in a reduction of dissolved oxygen.
Microorganisms also use dissolved oxygen for the oxidation of organic matter. If too much organic matter is added to water,all available oxygen is used up,causing oxygen-dependent aquatic life to die.
Anaerobic bacteria,which do not require oxygen,begin to break down organic waste and produce chemicals harmful to human health.
Aerobic bacteria,which require oxygen,degrade organic wastes and keep the water depleted of dissolved oxygen.
In a certain volume of water sample,the amount of oxygen required by bacteria to break down organic matter is called Biochemical Oxygen Demand $(BOD)$.