Pathogenesis: The most serious water pollutants are the diseases causing agents called pathogens. Pathogens include bacteria and other organisms that enter water from domestic sewage and animal excretra. Human excretra contain bacteria such as Escherichia coli and Streptococcus faecolis which causes gastrointestinal diseases.
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.
In cold water, dissolved oxygen (DO) can reach a concentration up to 10 ppm (parts per million), whereas oxygen in air is about 200,000 ppm. That is why even s moderate amount of organic matter when decompose in water can deplete the water of decompose in water can deplete the water of its dissolved oxygen. The conertration of dissolved oxygen in water is very important for aquatic life.
If the concentration 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 out by many aquatic green plants durning day light.
However, during night , photosynthesis stops but the plants continue to respire, resulting in reduction of dissolved oxygen. The dissolved oxygen is also used by microorganisms to oxidise organic matter.
If too much of organic matter is added to water, all the available oxygen is used up. This causes oxygen dependent aquatic life to die. Thus, anaerobic bacteria ( which do not require oxygen) begin to break down the organic waste and produce chemicals that have a foul smell and are harmful to human health. Aerobic ( oxygen requiring) bacteria degrade these organic wastes and keep the water depleted in dissolved oxygen.
Thus, the amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water, is called Biochemical Oxygen Demand (BDO). The amount of BDO in the water is a measure of the amount of organic material in the water, in terms of how much oxygen will be required to break it down biologically.
Clean water would have BDO value of less than 5 ppm whereas highly polluted water could have a BDO value of 17 ppm or more.
Chemical pollutions: As we know that water is an excellent solvent, water soluble inorganic chemical that include heavy metals such as cadmium, mercury, nickel etc constitute an important class of pollutants. All these metals are dangerous to humans because our body cannot excrete them. Over the time, it crosses the tolerance limit. These metals then can damage kindneys, central nervous system, liver damage etc. Acid ( like sulphuric acid) from mine drainage and salts from many different sources including raw salt used to melt snow and ice in the colder climates ( sodium and calcium chloride) are water soluble chemical pollutants.
