Some of the important sources of water pollution are: (i) Domestic effluents and sewage, (ii) Industrial effluents, (iii) Agricultural effluents, (iv) Radioactive wastes, (v) Thermal pollution, and (vi) Oil pollution.
(i) Domestic Effluents and Sewage:
Man, for his various domestic purposes such as drinking, cooking, bathing, cleaning, cooling, etc., uses on an average 135 litres of water per day. About 70 to 80 per cent of this is discharged and drained out, which through municipal drains poured into, in many cases, a river, tank or lake.
This water is known as domestic waste water and, when other waste material such as paper, plastic, detergents, cloth, etc., is mixed in it; it becomes municipal waste or sewage.
The domestic waste water and sewage is the main source of the water pollution. This is the inevitable and unfortunate fallout of urbanisation. This organic waste depletes the oxygen from water and upsets the natural balance of the aquatic ecosystem.
Municipal sewage is considered to be the main pollutant of water. Most of the sewage receives no treatment before discharge, especially in developing countries like India. In Delhi alone, 120 crore litres of water is consumed per day, out of which 96 crore litres is drained into the Yamuna river through 17 big drains. In the same manner, all the 47 towns located on the bank of river Ganga drain their sewage into it.
With the growth of population, the quantity of waste water is also increasing in addition to the production of large quantities of sewage. Sewage contains decomposable organic matter and exert an oxygen demand on the receiving waters.
The common organic materials found in sewage are soaps, synthetic detergents, fatty acids, and proteinaceous matters such as amines, amino acids, amides and amino sugars.
Besides, it also contains numerous micro-organisms in the form of pathogenic bacteria and viruses derived from human faces. Untreated waste water is often the carrier of viruses and bacteria and, with poor household sanitation practices, has been linked to high infant mortality rates in developing countries.
Even where most sewage is treated, as in the developed world, recent measurements of fecal coli forms in some countries indicate increasing pollution. Sewage supports the growth of other forms of life that consume oxygen; it is measured in terms of Biochemical Oxygen Demand (BOD). It is the lack of oxygen that kills fish and other aquatic life.
In recent years, there has been considerable growth in the use of detergents, which causes severe water pollution. Many modern detergents contain phosphates, which are an essential component of agricultural fertilisers. When phosphate detergents are discharged into waterways, they supply a needed nutrient and promote rapid growth of algae.
This enrichment process is known as ‘eutrophication’. In many areas of the world, aquatic weeds have multiplied explosively. They have interfered with fishing, navigation, irrigation and even production of hydroelectricity. In developing countries, human population and settlements are growing fast, often faster than waste water treatment facilities can be provided.
Thus, much of the untreated waste water and sewage is discharged into rivers and other water bodies, making the water unsuitable for drinking.
(ii) Industrial Effluents:
Industrial activities generate a wide variety of waste products, which are normally discharged into water courses. Major contributors are the pulp and paper, chemicals, petrochemicals and refining, metal working, food processing, textile, distillery, etc. The wastes, broadly categorised as heavy metals or synthetic organic compounds, reach bodies of water either through direct discharge or by leaching from waste dumps.
In developed countries of the world, many industrial discharges are strictly controlled. Yet, water pollution continues from accumulations of wastes discharged over the past 100 years. But, in developing countries, industrial discharges are largely uncontrolled and thus, a major cause of water pollution.
All the Indian rivers have been polluted by industrial effluents. The ‘holy’ river Ganga has become a highly polluted river today due to various types of industrial discharges. Along the Ganga, several chemical, textile, tanning, pulp and paper, petrochemical, rubber, fertiliser and other industries are located and all of them discharge their waste water and other effluents, directly or indirectly, into the river, resulting in the pollution to such an extent that even the Ganga Action Plan, to control water pollution, has failed miserably.
From Delhi industrial area alone, more than 8 lakh tonnes of industrial waste is discharged into the river Yamuna.
Damodar river of Bihar is a highly polluted river due to industrial wastes discharged from Bokaro, Rourkela, Indian Iron and Steel Company (IISCO), Bengal Paper Mills, Sindhri Fertiliser Factory, etc.
A study reveals that from Durgapur Steel Plant 1,800 cu/m washed coal has been discharged into the river. Similarly, from IISCO 15,000 cu/m and from Bengal Paper Mills 12,000 cu/m industrial waste is discharged per day into river water.
Likewise, about 10 to 15 tonnes of sulfuric acid from Sindri and 5 to 10 tonnes toxic chemicals are discharged into the river water. The story of the Hooghly river in West Bengal is more or less same. Its water has been polluted to such an extent that even fish fertilisation has become difficult.
The paper mills located along the river discharge about 11.4 tonnes liquid wastes into the river. Almost all other rivers of India have the same fate. Chambal, Narmada, Kaveri, Godavari, Mahanadi and all other small rivers have been polluted and if this is not stopped, it will result in greater water pollution.
The nature and effects of the pollutants from the effluents of paper and pulp, textile, food processing, chemicals, metal and petroleum industries are as follows:
1. Effluents from paper and pulp industries include wood chips, bits of bark, cellulose fibres and dissolved lignin, in addition to a mixture of chemicals. All these produce a sludge which blankets fish spawning grounds and destroys certain types of aquatic life.
The effluents contain chlorine, sulfur dioxide, methyl mercaptan, etc., which are considered to be highly poisonous to fish.
2. Effluents from textile industries are alkaline in nature and have a higher demand for oxygen.
3. Food processing industries include dairies, breweries, distilleries, meat-packing, etc., where the waste products include fats, proteins and organic wastes.
These industries discharge wastes containing nitrogen, sugar, proteins, etc. This waste contains a higher BOD and is therefore responsible for water pollution.
4. Chemical industries include acid manufacturing, alkali manufacturing, fertiliser, pesticides and several other industries. The effluents from these industries contain acids which have corrosive effects. The effluents from fertiliser industries contain phosphorus, fluorine, silica, and a large amount of suspended solids.
5. Metal industries usually discharge effluents containing copper, lead, chromium, cadmium, zinc, etc., which are toxic to man as well to aquatic life. These wastes also contain acids, oils, greases and cleansing agents.
6. Petroleum industries include oil refineries and petrochemical plants. The effluents include hydrocarbons, phenolic compounds and other organic and inorganic sulphur compounds.
7. Other industries, which pollute water, are tanneries, soaps and detergent industries, glass, electroplating, bleaching, atomic plants, explosive factories, etc.
8. Mining operations can result in metals leaching into the acidic effluents, thus adding to the metal load in rivers, lakes and groundwater. Discharge of mercury from gold mining activities has polluted some streams in Brazil and Ecuador and created serious health problems.
With reference to water pollution through mercury, mention of Minimata Gulf incident must be made. In 1950, near the Japanese coast, in Minimata Gulf, fishermen suffered from blindness, weakness, mental illness, paralysis, etc.
It was found that effluents discharged from a plastic factory contained mercury, which entered the fish and by eating those fish, all the fishermen suffered from effects of mercury poisoning. Thus water pollution through industrial effluents has become a major environmental problem today and needs measures to control it.
(iii) Agricultural Effluents:
Agricultural water pollution is caused by fertilisers, insecticides and pesticides, farm animal wastes and sediments. In recent years, use of chemical fertilisers has increased manifold. The green revolution in India is a reflection of the increased use of fertilisers. The chemicals used in fertilisers enter the groundwater by leaching and the surface waters by run-off.
The nitrates, when mixed with water, may cause methemoglobinemia in infants. Incidences of nitrate poisoning are also seen in livestock. The plant nutrients, nitrogen and phosphorus are reported to stimulate the growth of algae and other aquatic plants.
The use of various types of pesticides and insecticides in agriculture is also one of the causes of water pollution. Their presence in water is highly toxic to both man and animals, because these entire have a high persistence capacity, i.e., their residues remain for long periods.
The farm animal wastes often pose serious problems of odour and water pollution. These wastes also contain pathogenic organisms which get transmitted to humans. Sediments of soil and mineral particles washed out from fields also cause water pollution. They fill stream channels and reservoirs and reduce the sunlight available to aquatic plants.
(iv) Radioactive Wastes:
Radioactive elements, such as uranium and radium, possess highly unstable atomic nuclei. This disintegration results in radiation emission which may be highly injurious. During nuclear tests, radioactive dust may encircle the globe at altitudes of 3,000 metres or more, which often comes down to the earth as rain.
Eventually, some of the radioactive material, such as Strontium 90 (which can cause bone cancer), percolates down through the soil into groundwater reservoirs or is carried out into streams and rivers.
In both cases, public water supplies may be contaminated. The construction of more nuclear reactors and the increasing use of radioactive materials in medical research represent other potential contamination sources.
(v) Thermal Pollution:
Most of the thermal and electric power plants also discharge considerable quantities (about 66%) of hot effluent/water into nearby streams or rivers. This has resulted in thermal pollution of our water courses. Thermal pollution is undesirable for several reasons. Warm water does not have the same oxygen holding capacity as cold water.
Therefore, fishes like black bass, trout and walleyes, etc., which require a minimal oxygen concentration of about 4 ppm, would either have to emigrate from the polluted area or die in large numbers. When the temperature of the receiving water is raised, the dissolved oxygen level decreases and the demand for oxygen increases, hence anaerobic conditions will set in resulting in the release of foul gases.
Thermal pollution is considered hazardous for the whole aquatic ecosystem. Several industries have installed cooling towers, where the heated water is cooled. But even so, thermal pollution has become a serious problem for water bodies located near thermal plants.
(vi) Oil Pollution:
The spread of oil in the sea has become a common feature nowadays. Oil is transported across oceans through tankers and either due to some accident or leakage oil spills onto the water and causes the degradation of aquatic and marine environment. Between 1968 and 1983, there were more than 500 tanker accidents that involved oil spills. Altogether, more than one million tonnes of oil was released. A dramatic incident was that of the tanker Torrey Canyon, when it struck off the southern tip of the British
Isles in March 1967. The Torry Canyon was the largest oil spill up to that time. The pollution caused widespread destruction of many- forms of marine life despite strenuous efforts to clean up the spill. Similarly, on March 16, 1978, the oil tanker Amoco Cadiz lost its steering off the coast of Brittany in France and the total spilling of oil was 1.6 million barrels.
Such accidents have become very common due to technical problems or heavy marine traffic. During the 1991 Gulf War, there was heavy bombing on oil tanks, which resulted in the spilling of oil.
The impact of this oil spill on the marine ecosystem in this area has not yet been remedied. Offshore drilling operations also contribute their share of oil to the sea. The total quantity of oil that finds its way into sea each year is very large. It has been estimated that about one million tonnes of oil spills into the ocean each year from tankers and oil drilling operations.