The agents which causes pollution is called pollutants. Pollutants are classified on different basis like nature, decomposition etc.
On the Basis of Nature
Depending upon the nature of the pollutants and their interaction with environment process, the pollution caused by different agents can be classified into the following categories:
1) Pollution Caused by Solid Wastes: The solid wastes includes the pollutants like garbage, rubbish, ashes, large wastes formed due to demolition and construction processes, dead animals wastes, agricultural wastes, etc.
2) Pollution Caused by Liquid Wastes: Oxygen cycle is nicely operated in aquatic system maintaining ecological balance. That is, the dissolved oxygen is used by aquatic living organisms for their respiration and in return, these liberate carbon dioxide. Carbon dioxide molecules are again used by green plants and algae in the process of photosynthesis. During photosynthesis, oxygen is again liberated to water which remains in dissolved state. However, if some organic matter (food for bacteria) enters the water course, then bacteria oxidize these materials consuming oxygen from water. At such a condition, if the process of re-oxygenation is slower than the process of deoxygenation, then the river will be devoid of life sustaining dissolved oxygen and aquatic living organisms will die.
The most important source of organic pollutants is sewage which contains faecal matter, urine, kitchen washing and oil washings. Sewage also contains a large number of pathogenic and harmless bacteria. The strength of organic waste material of sewage is measured in terms of Bio-chemical Oxygen Demand (B.O.D). The value is expressed in terms of mg of oxygen per litre of waste for 5 days at 20C. If the volume of B.O.D. is below 1500mg per litre, the sewage is termed as weak waste, if it is 4000mg per litre, it is medium and above this value it is termed as strong waste. However, if liquid industrial wastes containing acids, alkalis and poisonous substances enter the river, the aquatic life is affected and self-purification system of water is impaired. Pesticides and herbicides which enter water may kill some organisms or accumulate in fishes which, when consumed by man, pass on the chemicals giving rise to cumulative poisoning.
3) Pollution Caused by Gaseous Wastes: The gaseous wastes include Carbon monoxide (CO), Sulphur dioxide (SO2), Nitrogen dioxide (NO2), Ozone (O3) and smog gases (composed of a complex mixture of photochemical oxidation products of hydrocarbons. These gases are more abundant in the atmosphere of industrial cities.
4) Pollution from Waste without Weight: This type of pollution is also known as pollution by energy waste; Wastes without weight may be of the following types:
i) Radio-active Substance: Despite of all possible precautions in the functioning and maintenance of nuclear reactors, it is seen that minute quantity of radio-active waste escapes out into the environment. From the mining operation of the uranium to the use and final disposal of wastes from the reactor, radio-active materials continuously escape out into the environment. Besides, a lot of radio-active wastes enter into environment during the nuclear tests.
ii) Heat: A large quantity of waste heat energy is dissipated into environment by the way of hot liquid streams or hot gases released by industries and automobiles.
iii) Noise: The unwanted sound is known as noise. This sources of noise for the general public are the machines in the industry, traffic noise, indiscriminate use of transistor, radios, public address systems, etc.
On the Basis of Decomposition
1) Non-Degradable Pollutants: These are not broken down by the natural processes like action of microbes. Most of these pollutants get accumulated in the environment and also get biologically magnified as these moves along the food chains in an under-composed state. These may also react with other compounds in the environment to produce toxins. These can be further sub-divided into two more classes:
i) Waste: e.g., glass, plastic, phenolics, aluminium cans, etc.
ii) Poisons: e.g., radio-active substances, pesticides, smog gases, heavy metals like mercury, lead and their salts.
2) Degradable Pollutants or Bio-degradable Pollutants: These are natural organic substances which can be decomposed, removed or consumed and thus, reduced to acceptable levels either by natural processes like biological or microbial action or by some engineered systems, like sewage treatment plants. The degradable pollutants can be further sub-divided into two categories:
i) Rapidly Degradable or Non-Persistent Pollutant: The degradation of these pollutants is very faster process. For example, the decomposition of sewage and wastes of animals and plants is a faster process. The domestic sewage can be rapidly decomposed by natural processes. However, the problems become complicated when the input into environment get exceeded of the decomposition or dispersal capacity.
ii) Slowly Degradable or Persistent Pollutant: The degradation of these pollutants, is a very slower process. It seems as if the amount of pollutant remains unchanged with time. For example, degradation of synthetic compounds and radio-active elements like Iodine 137, Strontium 90 or Plutonium 239 takes a longer period of time.