On the basis of their origin, the sources of water pollutants can be categorized into:
(i) Point sources, where the effluent discharge occurs at a specific site; for example, sewage outlet of a municipal area or effluent outlet of a factory, and
(ii) Non-point sources, where inflow of pollutants occurs over a large area, for example, city storm water flow, agricultural runoff, etc. point source pollution can be effectively checked with appropriate technology. Non-point source pollution is difficult to control and needs application of control measures on a large scale. Water pollutants can be: (i) biological (pathogens, such as viruses, bacteria, protozoa, algae, helminthes), (ii) chemical (organic chemicals like biocides, polychlorinated biphenyls or PCBs; inorganic chemicals, like phosphates, nitrates, fluoride, etc; also heavy materials like As, Pb, Cd, Hg, etc.), and (iii) physical (hot water from industries, oil spills from oil carriers, etc.),
The industrial and municipal wastewaters are treated in Efficient Treatment Plant (ETP) prior to disposal in water bodies. Generally, the following treatments are given in ETP:
1. Primary treatment: This physical process involves the separation of large debris, followed by sedimentation in tasks or clarifiers.
2. Secondary treatment: This is a biological process and is carried out by microorganism. In this treatment, the wastewater is pumped in shallow stabilization or exudation ponds, where the microbes oxidize its organic matter. The process results in release of CO2 and formation of sludge or biosolid. The sludge is continuously aerated to further its oxidization. Algae grown in the upper lighted zone of the waste water provide aeration by generating O2.
3. Tertiary treatment: This physicochemical process removes turtidity in waste water caused by the presence of nutrient (nitrogen, phosphorous, etc.), Dissolved organic matter, metals or pathogens. This step involves chemical oxidation of wastewater by strong oxidizing agents, such as chlorine gas, perchlorate salts, ozone gas and UV radiation. After tertiary treatment, the wastewater can be discharged into natural waters or used for irrigation.
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