There are many causes for water pollution but two general categories exist: direct and indirect contaminant sources.
- Direct sources include effluent outfalls from factories, refineries, waste treatment plants etc. that emits fluids of varying quality directly into urban water supplies.
- Indirect sources include contaminants that enter the water supply from soils/groundwater systems and from the atmosphere via rain water. Soils and groundwaters contain the residue of human agricultural practices (fertilizers, pesticides, etc.) and improperly disposed of industrial wastes. Atmospheric contaminants are also derived from human practices (such as gaseous emissions from automobiles, factories)
Contaminants can be broadly classified into organic, inorganic, radioactive and acid/base. Examples from each class and their potential sources are too numerous to discuss here. Some of the sources of water pollution are as follows;
- Sewage and other oxygen demanding wastes which contain decomposable organic matter and pathogens.
- Industrial wastes (metal salts to complex synthetic organic chemicals).
- Agricultural waste and agricultural inputs (fertilizer, pesticides, biocides).
- Physical pollutants (heat and radioactive substances).
Sewage and other oxygen demanding wastes
- Sewage is defined as the waterborne waste derived from home (domestic wastes) and animal or food processing plants which includes human excreta, soaps, detergents, paper and cloth. Water pollution is caused by uncontrolled dumping of wastes of villages, towns and cities into ponds, lakes, streams or rivers. The discharge of sewage into water bodies results into the following.
a.Depletion of oxygen contents
- Dissolved oxygen is the total amount of oxygen dissolved in water. Oxygen depletion is caused by aerobic bacteria in the decomposition of organic matter. The quantity of oxygen utilized by bacteria in degradation of organic substances is called biological oxygen demand. Also it can be defined as the amount of oxygen required by the bacteria to stabilize the organic matter. On an average basis, the demand for oxygen i.e.
- BOD value, is proportional to the amount of organic waste present in water. The BOD value can be a measure of waste strength and also an indicates of degree of pollution. Along with BOD, the quantity of oxygen dissolved in a body of water (DO) is indicated by the kind of biotic life, which lives there. When dissolved oxygen is reduced below 4 to 5 ppm of water, fish are scarce. BOD test should be restricted to only suitable wastes in management of treatment plants. However for other kinds of wastes chemical oxygen demand (COD) values would be more appropriate.
- COD: “It is the amount of oxygen required by organic matter in a sample of water for its oxidation by a strong chemical oxidant and is expressed as ppm of oxygen taken from a solution of potassium dichromate in two hours”.
b. Stimulation of algal growth
- A major ingredient of most detergents is phosphate. When discharged into water, the phosphate supports luxuriant growth or blooms of algae. Extensive growth of algae often withdraws great quantities of oxygen from water to the detriment of other organisms and produces bad odour when decay. In a poorly oxygenated condition, with increased CO2, fishes and other animals die and clean river is turned into a stinking drain. (Eutrophication)
c. Spreading infectious diseases
- Microorganisms, usually viruses, bacteria some protozoans and helminthes occur in water bodies as a result of sewage disposal therein spread several infectious diseases. The degree of water pollution is assessed by the presence of Escherichia coli. Their quantity in water is indicative of the degree of pollution due to animal and human excrements. On the basis of number of E. coli in water body the degree of water pollution may be distinguished as
- Drinking water - 3/ lit
- Satisfactory quality - 10/lit
- Polluted water- >100/lit
Eighty per cent of the diseases in India and other developing countries are linked with contaminated water.
2. Industrial wastewater
- Most of the rivers and fresh water streams which pass near the major cities are polluted by industrial wastes of effluents. Effluents from these contain a wide variety of both inorganic and organic pollutants such as oils, greases, plastics, methylic wastes, suspended solids, phenols, toxins and other chemical substances, many of which are not readily susceptible to degradation and cause very serious pollution problems. Arsenic, cadmium, copper, chromium, mercury, zinc and nickel are some of the most common heavy metals discharged from industries.
- Mercury: The safe level of mercury in surface water for domestic use as prescribed by Central Pollution Control Board, New Delhi is <0.002 ppm, the limit prescribed by the WHO is <0.001 ppm. Mercury poisen – minantta disease or cat tail dance syndrome
- Fluoride: Sources of fluorine compounds are nature, man’s activities and other air borne sources. According to WHO, fluoride concentration below 0.5 ppm causes dental caries and mottling of teath. But when fluoride level exceeds 0.5 ppm over a period of 5-10 years may result in fluorosis or paralysis-fluoride is not absorbed in the blood stream. It has an affinity for calcium and gets accumulated in bones resulting pain in bones and joints and outward bending of legs from the knees (knock knee syndrome)
- Lead: Lead poisoning is common in adults. Lead and processing industries constitute the major sources of serious lead pollution. Lead pollution causes gastrointestinal troubles, neuromuscular effects and affects central nervous system (CNS - Central Nervous Syndrome) also.
- Cadmium: Causes Itai – Itai (or) ouch-ouch disease to human beings
The discharge of industrial waste results into the following:
- Organic substances deplete the oxygen content.
- Inorganic substances render the water unfit for drinking and other purposes.
- Acids and alkalies adversely affect the growth of fish and other aquatic life.
- Toxic substances like cyanide, phenol and other heavy metals cause damage to flora and fauna.
- Oil and other greasy floating substances interfere with breathing system of aquatic life.
3. Agricultural waste
- Modern agricultural practices require the use of large amount of fertilizers, pesticides and other soil additives. Some of these along with waste are washed off lands through irrigation, rainfall, drainage and leaching into the rivers and streams where they can seriously disturb the aquatic ecosystem. Depletion of dissolved oxygen caused by phosphate induced algal growth leads to death of fish and other aquatic biota. In presence of phosphates, nitrates too bring about an exaggerate growth of vegetation.
a. Nitrate pollution
- Excessive and indiscriminate use of chemical fertilizers and synthetic feed for livestock often lead to accumulation of nitrates in water. When such waters are drunk by cattle or humans these nitrates taken into body are converted to toxic nitrites by intestinal bacteria. This in turn combines with the hemoglobin to form methaemoglobin, which interferes with the oxygen-carrying capacity of the blood, producing a serious disease known as methaemoglobinaemia. The various ailments that result from this disease include damage to respiratory and vascular system, blue colouration of the skin (Blue baby syndrome) and even cancer. A person in good health contains 0.8% of methaemoglobin. Symptoms of methaemoglobinaemia can be perceived as soon as the mathaemoglobin reaches a level of 10 per cent in the blood. The WHO laid down the permissible concentration of NO3 in water at 45 mg L-1. Nitrates could become toxic when it occurred in association with sulphates.
b. Eutrophication
- The non-flowing water bodies such as ponds or lakes during their early stages of formation are relatively barren and deficient in nutrients to support the aquatic life and are referred to as oligotrophic (nutrient poor). With the passage of time, organic substances from the surrounding areas accumulate and the water body becomes highly productive or Eutrophic. A comparative account of Oligotrophic and Eutrophic lakes is given below:
- Eutrophication thus denotes the enrichment of a water body by input of organic material of surface run off containing nitrates and phosphates. This may happen naturally but very slowly, often over a period of hundreds of years. Human activity generally responsible for rapid eutrophication as domestic waste, agricultural and land drainage and organic industrial waste or their decomposition products reach the water bodies and induce the productivity and composition of aquatic life. Eutrophication leads to increase in the growth of aquatic plants and often to algal blooms. The extensive algal growth have resulted in killing of fishes by interfering with recreation, excluding light intensity necessary for photosynthesis by other aquatic plants and thereby preventing the release of oxygen into the water or depleting the oxygen through decay or respiration with bloom. Some algal bloom release toxic substances that kill fishes, domestic animals and birds and water begins to stink.
- Lake Erie (USA) is an excellent example of eutrophication from man’s activities. In 1965, 87t of phosphates were dumped into the lake leading to the development of 350 t of algal slime. The various measures suggested to step eutrophication are as follows:
- Treatment of waste water in order to minimize nutrient inputs.
- Reduction in the amount of nutrient solubilized in water through stimulation of bacterial multiplication.
- Harvesting and removal of algal blooms to check recycling of nutrients into the water.
- Removal of dissolved nutrients from water physically or chemically. Phosphorus can be removed by various methods of precipipation. Nitrogen can be removed by
(a) ion exchange,
(b) electrodialysis,
(c) reverse osmosis and
(d) denitrification
4. Physical pollutants
- Chemical industries, fossil fuel and nuclear power plants use lot of water for cooling purposes and return this water to stream at a high temperature. The hot water interferes with the natural conditions in the lake and river affecting aquatic life. This is a thermal pollution as heat acts as a pollutant. The thermal pollution is thus the raising of temperature of part of the environment (water in this case) by the discharge of substances whose temperature is higher than the ambient. Some plants and animals are killed outright by the water, if it is very hot some of the adverse environmental effects of the thermal pollution are:
- Fish eggs hatch so early.
- Trout eggs commonly fail to hatch.
- BOD increases as warm water holds less oxygen.
- Change in diurnal and seasonal behaviour of organisms.
- Decrease in species diversity.
- Affects migration of some aquatic life.
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