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Causes of pollution

Degradable wastes are organic materials that can undergo decomposition through bacterial attack. The inputs that can be included under this category are urban sewage, agricultural waste, food-processing waste, distillery waste, paper-pulp mill waste, organic discharges from chemical industry and oil spillages. In addition, inputs like leaves and grass clippings, and run-off from livestock feedlots and pastures also contribute to this. When natural bacteria and other microorganisms in the water break down to organic materials, they use up the oxygen dissolved in water. Most of the fishes and bottom-dwelling animals cannot survive when level of dissolved oxygen drops too low. When this occurs, it kills aquatic organisms in large numbers, which leads to disruptions in food chain.

Fertilizers containing nutrients such as nitrates and phosphates could have effects similar to those of organic wastes. In excess levels, nutrients over-stimulate the growth of aquatic plants and algae. Excessive growth of these clogs waterways, uses up dissolved oxygen as organisms decompose besides blocking light to deeper waters. The depletion of oxygen, in turn, proves very harmful to aquatic organisms as it affects respiration of fish and other organisms that derive oxygen from water.

Heat, acids and alkalies and some chemicals such as cyanides can be considered as dissipating wastes as they lose damaging effects soon after they enter water-body. Particulates like dredging spoil, fly-ash, China clay-waste, colliery waste and a variety of man-made materials like plastics are inert, but they may clog feeding and respiratory structures of animals, may also reduce photosynthesis by reducing light penetration or may smother benthos. Conservative wastes like heavy metals, halogenated hydrocarbons and radioactive materials are not subject to microbial attack, and therefore, exist over a long duration, and cause harm to plants and animals

Pathogens such as bacteria, viruses and protozoans, are another type of pollution. They can cause many illnesses ranging from typhoid and dysentery to minor respiratory and skin diseases. These organisms enter waterways through untreated sewage, storm drains, septic tanks, run-off from farms, etc. There is also threat from carcasses and other animal materials infecting water supplies following inadequate disposal. Though microscopic, these pollutants have tremendous effect, evidenced by their ability to cause diseases.

Pollution sources

The sources of water pollution are categorized as point and non-point. Point sources of pollution occur when polluting substance is emitted directly into waterway. A pipe spewing toxic chemicals directly into a river is an example. A non-point source occurs when there is run-off of pollutants into a waterway, for instance, when fertilizers from agricultural fields are carried into a stream by surface run-off.

The common point sources of pollution are municipal and industrial wastewater effluents; run-off and leachate from solid waste disposal sites; run-off from industrial sites; storm sewer outfalls from urban centres; run-off and drainage from industrial sites, mines and oil fields; discharge from vessels, storage tanks and piles of chemicals; run-off from construction sites; and bypasses from sewers and sanitary pipes. The non-point sources include flow from agricultural fields and orchards, run-off from logging operations, urban run-off from unsewered areas and septic tank leachates, atmospheric deposition and run-off from roads.

When toxic substances enter lakes, streams, rivers, oceans and other water-bodies, these get dissolved, lie suspended in water or get deposited on bed. This results in water pollution, whereby quality of water deteriorates, affecting aquatic ecosystems. Pollutants can also seep down and affect groundwater deposits. The most important sources of pollution are city sewage and industrial waste discharged into rivers by virtue of the quantities in which these are discharged. Presently, only about 10% of the wastewater generated is treated allowing about 90% of it to directly enter receiving water. Due to this, pollutants enter groundwater, rivers and other water-bodies, and may even harbour pathogens. Agricultural run-off or the water from fields that drains into rivers is another major source of water pollution as it could be rich in the major nutrients, viz. nitrogen, phosphorus and also pesticides.

Control and abatement

The high cost of treatment has been the major concern for wastewater management. There is considerable agreement on the need for revised technology to diminish industrial and automotive emissions, to produce degradable wastes and to dispose of all wastes in ways less damaging to environment. For example, by channeling sewage to farms as fertilizer, and by recycling glass and metal materials. Finally, improvement is required in techniques for preventing pollution by especially hazardous wastes.

(Source:Hand book of Fisheries and Aquaculture. 2006. Indian Council of Agricultural Research. New Delhi.)