Table of Contents
- Introduction to Water Pollution
- Sources of Water Pollution
- Domestic Sewage
- Industrial Wastes
- Water Quality Parameters and International Standards
- Importance of Dissolved Oxygen In Water
- Biochemical Oxygen Demand (BOD)
- Chemical Oxygen Demand (COD)
Water is most plentiful of all the natural sources. About three-fourth of the earth’s surface is covered by water in the form of snow over mountains, as a liquid in the rivers, lakes, springs and oceans. Water is very essential for life. Without water there would be no life. It is very essential for the growth of human body and has significant role in industry, agriculture and civilization.
The quality of water especially for drinking purposes has been a vital factor for the human welfare. The pollution of drinking water has frequently caused many water borne diseases and epidemics such as cholera, typhoid fever, etc. Therefore, water pollution is a matter of serious concern for every citizen. The main culprits for water pollutions are domestic waste from urban and rural areas and discharging of industrial wastes into natural water supplies.
Water pollution may be defined as any change in its physical, chemical or biological properties or contamination with foreign materials that can adversely affect human beings or reduce its utility for the intended use.
The degree of purity required for water depends upon its use. For Example, the water polluted for drinking purposes may be satisfactorily used for irrigation or producing electricity at hydroelectric power plant for cooling purposes.
Most of the water pollution is because of human activities. Ground water also gets polluted because of various activities. The easily identified source or place of pollution is called Point Source e.g., Municipal and industrial discharge pipes where the pollutants enter the water source. Those where a source of pollution cannot be easily identified are called Non-point Sources. For Example, Acid rain, Storm water drainage, agricultural runs off (from farm, animals and crop lands etc.). The major sources of water pollutants are domestic sewage and industrial wastage.
The major water pollutants and their common sources are given in Table below.
Let us briefly study the methods to control the pollution from these sources.
Major Water Pollutants and their Sources
Plant nutrients and Pesticides
Chemical fertilizers, Chemicals used for killing insects, fungi and weeds
Insoluble particles of soil and rock also inorganic and organic compounds which wash into water from mining, agricultural activities, construction activities and soil erosion
Domestic sewage, animal manure, food processing wastage, paper discards.
Mining of uranium containing minerals and accidental release from nuclear plants
Water used by industrial plants for cooling which is discharged as hot water
Toxic Heavy Metals
Industries and Chemical factories
The domestic sewage contains oils, human excreta, dirt, paper, rags, sand grains, dissolved material such as detergents and inorganic compounds such as sodium chloride, ammonium sulphate and ammonium phosphate, decomposed kitchen waste. The sewage also contains many disease causing bacteria called pathogens which are most serious water pollutants. Because of dumping of domestic sewage and animal excreta into water various pathogens like bacteria and other organisms enters into water. Human excreta contains bacteria such as escherichia coli and streptococcus faecalis which cause gastrointestinal diseases. These bacteria can cause great epidemic diseases. If this sewage is directly disposed into the surface water sources, they may pollute the whole water and make it unsafe for human use.
Industrial Wastes are also the major source of water pollution. The industrial wastes polluting water are mainly from industries such as coal or ore mines, textile industries, paper industries, food processing industries, dairies, chemical industries, pharmaceuticals, sugar and distilleries, oil refineries, tanneries, vegetable oil and soap industries etc. These wastes contains inorganic and organic suspended particles and inorganic and organic soluble matter which are considered harmful for the human body.
Some common types of industrial wastes polluting water are:
These are metals such as cadmium, lead and mercury which are present in industrial or mining wastes. These metals are poisonous and can be dangerous to humans. For Example, cadmium and mercury can cause kidney damage whereas lead poisoning can cause damage to the kidneys, liver, intestines, brain and central nervous system. Mercury poisoning causes a disease called Minamata in human beings which weakens the muscles and results in weakness in hearing and vision power, mental retardation and paralysis. All of these metals are cumulative poisons because the body does not excrete them and their concentration increases in the human body causing harm to it.
Various detergents contain phosphates as an additives in them. It may be noted that these do not pose any threat to the aquatic life but serve as nutrients for plants leading to their excessive growth in ponds, lakes and rivers. Dissolved oxygen concentration of water gets reduced because of formation of algae which are encouraged by these detergents and fertilizers.This process of over nutrition is known as Eutrophication. This impedes the development of higher life forms such as fish.
Petroleum products also pollute many sources of water. e.g., major oil spills in ocean. This is because of wreckage of oil tankers in open sea or accidents of ships carrying oil in the sea. The spreading of oil into sea is called oil spill and the thick layer of oil on the surface of sea water is called oil slick. In India an oil spill occurred in Bombay on March 17, 1993 due to rupture of pipeline which damaged the ecosystem and marine life.
If the pH of water is less than 3, then the water is said to be acid polluted water and is considered harmful to aquatic life. The water coming out of the mine or the water which is used for mining process contaminates the ground water because of microbial oxidation of discarded waste materials at the mine site. The acid mine water mainly contains sulphuric acid produced by the oxidation of iron pyrites (FeS2). Acid rain also contribute to the acidic nature of natural water. Natural water becomes acidic by industrial wastes and acid rain
As water is a good solvent it dissolves various water soluble inorganic chemicals which include heavy metals such as cadmium, mercury, nickel, etc. These metals cannot be digested by the human body and there are considered dangerous. By the time these cross the tolerance limit, these metals can cause harm to central nervous system kidneys, liver, etc.The raw salt (sodium and calcium chloride) used to melt snow and ice in colder climates also causes water soluble chemical pollution.
Polychlorinated Biphenyls have very high stabilities and therefore, are finding many applications these days. For Example, fluids inside transformers and capacitors is made up of PCB’s. They as cleansing solvents, detergents and fertilizers. PCBs are resistant to oxidation and their release into the environment has become a serious pollutant. These are carcinogenic and cause skin disorders in humans. Nowadays most of the detergents available are biodegradable. But using them create other problems. The bacteria responsible for their degradation feed on degradable detergent and grow very rapidly. During their growth, the bacteria may take all the oxygen dissolved in water. As a result other forms of aquatic life such as fish and plants may die because of dearth of oxygen in water. Fertilizers, as we know contain phosphates as additives. The addition of phosphorus in water promotes algae growth. Such a large growth of algae may cover the water surface. These algae growth often release toxins in water. This bloom which occurs in water affects the living organisms in the water body by inhibiting their growth. The process of nutrient enrichment of water bodies, which results in loss of biodiversity is called eutrophication.
Until recently, it was a common practice to dump the industrial wastes into nearby stream. These days, this practice has been stopped to prevent stream pollution and to recover some expensive chemicals from these wastes. However, it may be noted that the treatment of industrial wastes is very tedious problem because of the large variety of chemicals present in black liquor coming from the industries. The treatment of industrial wastes depend upon the nature of the pollutants present.
For mankind, the quality of water is an important concern because it is directly linked with human welfare. The main requirements for drinking water are:
The drinking water should be fit for human consumption and it should obey the following essential parameters:
It should be colorless and odorless.
It should be pleasant in taste.
It should be clear and turbidity should be less than 10 ppm.
Its pH should be between 5.5 to 9.5.
The total dissolved solids should not be more than 500 ppm.
It should be free from disease causing micro’ organisms.
It should be reasonably soft.
There should be no objectionable chemicals present.
Water must always obey certain international standards if water is to be used for drinking purposes. The parameters and standards for water quality and harmful effects of pollutants beyond permissible limits are given in Table 2 for some common chemicals. Some of these are also briefly discussed below:
Fluoride: Soluble fluoride is normally added to drinking water and its concentration should be upto 1-1.5 ppm or 1-1.5 mg dm-3. It protects teeth against decay if limit is within agreed safety limits. If this compound is not present in sufficient quantity (witihin limits) in the drinking water it may be harmful to man, causing diseases such as tooth decay, etc. The F- ions make the enamel on teeth much harder, by converting hydroxyapatite [3Ca3 (PO4)2. Ca (OH) 2] the enamel on the surface of the teeth into harder Fluorapatite [3Ca3 (PO4)2. CaF2]
However, F- ion concentration above 2ppm causes brown mottling of teeth. Higher concentrations of fluoride are poisonous and are harmful to teeth and bones at levels above 10 ppm (mg dm-3). This problem has been reported from some parts of Rajasthan.
pH: The pH of the drinking water should be between 5.5 to 9.5. Solubility of the heavy metal decreases if pH of the water increases the ions.
Sulphate: Sulphate is harmless at moderate concentrations but excessive concentration of Sulphate above 500 pm produces laxative effects and hypertension.
Nitrate: Methemoglobinemia (blue baby syndrome) is caused by the presence of excessive nitrate in drinking water. It is linked to stomach cancer. 50 ppm is the maximum limit of nitrate ion in drinking water.
Lead: Lead pipes are very commonly used for water transport and is liable to get contaminated with lead, particularly when water is acidic. The permissible limit for lead ions in drinking water is 50 ppb ( g dm-3). Excessive lead cause anemia, kidney dysfunction, nervous disorder, brain damage etc.
Other Metals: The maximum permissible limit recommended of common metals in drinking water and their harmul effects are given below:
Maximum Concentration (ppm or mg dm-3)
Table 3. Maximum prescribed concentration of some metals in drinking water.
The concentration of dissolved oxygen in water is very important to support the aquatic life. The water is said to be polluted if the concentration of dissolved oxygen becomes low. The water sample is considered more polluted if the concentration of dissolved oxygen is less. The fish growth is inhibited if the dissolved concentration of oxygen in water is below 6 ppm. The atmosphere and from photosynthesis forms two main sources of dissolved oxygen at the surface of the water from. It may be noted that still water takes up oxygen slowly whereas turbulent water takes it up more rapidly because bubbles are commonly submerged. Moreover, at the places where there are many aquatic green plants, the water becomes supersaturated with oxygen during the hours of daylight due to photosynthesis. However, after dark, photosynthesis stops but the plants continue to respire and therefore, the amount of dissolved oxygen decreases. Consequently, during a period of 24 hours, some water samples have considerable amount of dissolved oxygen.
The deoxygenation of water is carried out by various processes. For Example, micro-organisms consume large amount of dissolved oxygen in water to oxidize organic matter of sewage.
[CH2O] (aq) + O2 Microorganisms CO2 (aq) + H2O (l)
(From organic matter)
Water will be unable to support life of many organisms if the concentration of the dissolved oxygen is not restored either by the turbulent flow of shallow water or by a reaction, The dissolved oxygen in water is also consumed by the bio oxidation of the nitrogenous material.
NH4+ (aq) + 2O2 (aq) → 2H+ (aq) + NO3- (aq) + H2O)
and by the chemical or biochemical oxidation of chemical reducing agents.
4Fe2+ (aq) + O2 (aq) + 10 H2O (l) → 4Fe (OH) 3 (s) + 8H+ (aq)
2SO32- (aq) + O2 (aq) → 2SO42- (aq)
By using both aerobic and an aerobic bacteria which act under different conditions the organic waste can be broken down by both. Aerobic bacteria are oxygen consuming decomposers which bring about the oxidation of degradable organic matter into carbon dioxide, nitrates, sulphates, phosphates, etc. Thus dissolved oxygen in the water decreases because of aerobic oxidation. Due to short supply of oxygen, the aquatic organisms especially fish and shell fish die from suffocation. Complete oxygen removal kills many other forms of aquatic life except anaerobic bacteria which do not require oxygen.
While anaerobic bacteria do not require oxygen for the decomposition of organic wastes. Toxic and foul smelling substances such as hydrogen sulphide, ammonia, methane, phosphine, ammonium sulphide, etc. are produced because of decomposition of organic waste by anaerobic bacteria. This kind of oxidation of sewage is called anaerobic oxidation.
Dissolved oxygen is an essential requirement of aquatic life that is, animal and plant population in water. The quantity of oxygen consuming wastes in water is usually determined by measuring the biological oxygen demand (BOD). Amount of dissolved oxygen needed by aerobic bacteria to break down the organic matter is called Biochemical Oxygen Demand (BOD). The carbon of organic matter is converted into CO and since 1 mol of C requires 1 mol of oxygen, BOD is directly related to the concentration of organic matter. Therefore, that water is considered polluted the value of BOD is high. Therefore, the BOD is taken as a realistic measure of water quality. If BOD value of water is less than 5 ppm then the water is considered clean whereas that having BOD of 17 ppm or more is considered highly polluted water. In order to measure BOD, first step is to saturate the water sample with oxygen. It is then incubated at constant temperature (usually 20°C) for 5 days. Thus in this period micro-organisms in water sample oxidizes the pollutants.The remaining oxygen is determined and the difference gives BOD. There is another parameter called Chemical Oxygen Demand (COD).
Certain chemicals other than organic wastes also react with dissolved oxygen in water. This is referred as Chemical Oxygen Demand (COD). Thus, BOD measures the oxygen consumed by living organisms assimilating organic matter present in waste while COD is a measure of biological oxidizable as well as biological inert organic matter such as cellulose.
Thus, if BOD or COD values of water are large, it indicates that water is heavily polluted. In order to measure COD, the first step is the treatment of given water sample with a known quantity of an oxidizing agent (potassium dichromate, K2Cr2O7) in acidic medium. Thus those pollutants which are resistant to microbial oxidation gets oxidizes because of K2Cr2O7. Then by back titration with a suitable reducing agent such as Mohr’s salt, excess of K2Cr2O7 is determined. From the concentration of K2Cr2O7 consumed, the amount of oxygen used in oxidation may be calculated using the following chemical equation:
K2Cr2O7 (aq) + 4H2SO K2Cr2O4 (aq) → K2SO4 (aq) + Cr2 (SO4) 3 (aq) + 4H2O (l) + 3O (aq)
The results are expressed in terms of amount of oxygen in ppm that would be required to oxidize the contaminants. This is called Chemical Oxygen Demand (COD).
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