The contamination of water by foreign substances which would cause health hazards and make it unfit for all purposes (domestic, industrial or agriculture etc) is known as water pollution. The polluted water may have foul odour, bad taste, unpleasant colour etc.
Maximum prescribed concentration of some metals in drinking water is as:
Sources of Water Pollution:
(i) Domestic sewage: Discharge from kitchens, baths, etc.
(ii) Industrial water: Wastes from manufacturing processes which includes acids, alkalies, pesticides, insecticides, metals. fungicides etc.
(iii) Oil: From oil spills or washings of automobiles.
(iv) Atomic explosion: Processing of radioactive materials.
(v) Suspended particles (organic or inorganic): Viruses, bacteria, algae, protozoa etc.
(vi) Wastes from fertilizers: Industries such as phosphates, nitrates, ammonia etc.
(vii) Clay: Ores, minerals, fine particles of soil.
Effects of Impurities in Water:
(a) Fluorides: Mottling of teeth enamel, above 1 mg/L fluoride causes fluorosis.
(b) Sulphates: Sulphates of Na, K, Mg cause diarrhoea.
(c) Lead: It damages kidney, liver, brain and central nervous system.
(d) Cadmium and mercury: They cause kidney damage.
(e) Zn: It causes dizziness and diarrhoea. .
(f) Arsenic: It can cause cramps and paralysis.
(g) Phosphates from fertilizers: They promote algae growth and reduce dissolved oxygen concentration of water. This process is known as eutrophication.
Aerobic and Anaerobic Oxidation:
The oxidation of organic compounds present in sewage in the presence of good amount of dissolved or free oxygen (approx, 8.5 mlJL) by aerobic bacteria is called aerobic oxidation. When dissolved or free oxygen is below a certain value, the sewage is called stale.
Anaerobic bacteria bring out putrefaction by producing H2S, NH3, CH4, (NH4)2S etc. This type of oxidation is called anaerobic oxidation.
The optimum value of dissolved oxygen for good quality of water is 4·6 ppm (4-6 mg/L). The lower the concentration of dissolved oxygen, the more polluted is the water.
Biological oxygen demand (BOD): It is defined as the amount of free oxygen required for biological oxidation of the organic matter under aerobic conditions at 20°C for a period of five days. Its unit is mg/L or ppm.
An average sewage has BOD of 100 to 150 mg/L.
Chemical oxygen demand (COD): It is the measure of all types of oxidisable impurities (biologically oxidisable and biologically inert organic matter such as cellulose) present in the sewage. COD values are higher than BOD values.
Control of Water Pollution:
(i) Recycling of waste water
(ii) Use of chemicals: Lead poisoning can be cured by giving the patient an aqueous solution of calcium complex of EDTA. Lead ions displace calcium in the EDTA complex to form chelated lead and Ca2+. The soluble lead chelate is excreted with the urine.
Ca – EDTA + Pb2+ → Pb – EDTA + Ca2+
(iii) Special techniques such as adsorption, ion exchangers, reverse osmosis, electrodialysis etc.
(iv) Waste water reclamation
It involves the following steps-
(i) Preliminary process Passing sewage through screens to remove large suspended matter and then through mesh screens to remove solids, gravels, silt etc.
(ii) Settling process (sedimentation) The residual water when allowed to stand in tanks, the oils and grease, float on the surface and skimmed off and solids settle down. The colloidal material is removed by adding alum, ferrous sulphate etc. Primary sludge can be separated.
(iii) Secondary treatment or biological treatment It is aerobic chemical oxidation or aeration which converts carbon of the organic matter to CO2, nitrogen into NO and finally into nitrite and nitrates. Dissolved bases form salts such as NH4O2, NH4NO3 and Ca(NO3)2etc., and secondary sludge is obtained.
(iv) Tertiary treatment: It is treatment of waste water with time for removal of phosphate which is then coagulated by adding alum and ferric chloride and removed by filtration.
Water is disinfected by adding chlorine.
Secondary sludge forms a good fertilizer for soil as it contains nitrogen and phosphorus compounds.
Soil or Land Pollution:
The addition of substances in an indefinite proportion changes the productivity of the soil. This is known as soil or land pollution.
Sources of Soil Pollution:
(i) Agricultural pollutants e.g., chemicals like pesticides, fertilizers, bactericides, fumigants. insecticides, herbicides, fungicides.
(ii) Domestic refuge and industrial wastes.
(iii) Radioactive wastes from research centres, and hospitals.
(iv) Soil conditioners containing toxic metals like Hg, Pb, As. Cd etc.
(v) Farm wastes from poultry, dairies and piggery farms.
Control of Soil Pollution:
(i) Use of manures- Manures prepared from animal dung is much better than the commonly used fertilizers.
(ii) Use of bio- fertilizers- These are the organisms which are inoculated in order to bring about nutrient enrichment of the soil. e.g., nitrogen fixing bacteria and blue-green algae.
(iii) Proper sewerage system- A proper sewerage system must be employed and sewage recycling plants must be installed.
(iv) Salvage and recycling- Rag pickers remove a large number of waste articles such as paper, polythene, card board, rags, empty bottles and metallic articles. These are subjected to recycling and this helps in checking soil pollution.
Cosmic rays that reach the earth from outer space and terrestrial radiation from radioactive elements are natural radiations. This natural or background radiation is not a health hazard due to its low concentration.
Man-made sources of radiations include mining; and refining of plutonium and thorium, atomic reactors and nuclear fuel. These are produced during preparation of radio-isotopes. These are of two types: electromagnetic (radio waves UV, IR, α-rays) and particulate.
Other Sources of Radioactive Pollution:
(i) Atomic explosions: Atomic explosions produce radioactive particles which are thrown high up into the air as huge clouds.
The process releases large amount of energy as heat due to atomic -explosion nuclear fallout. These radioactive elements may reach the human beings through food chain.
(ii) Radioactive wastes Wastes from atomic power plants come in the form of spent fuels of uranium, and plutonium. People working in such power plants, nuclear reactors, fuel processors etc., are vulnerable to their exposure.
(iii) Radio isotopes Many radioactive isotopes like C14, I125, P32 and their compounds are used in scientific researches. The waste water of these research centres contain the radioactive elements which may reach the human beings through water and food chains.
Effects of Radiations:
1. Strontium-90 accumulates in the bones to cause bone cancer and tissue degeneration in number of organs.
2. 1-131 damages WBCs, bone marrow, lymph nodes and causes skin cancer, sterility and defective eye sight.
3. These may cause ionisation of various body fluids, chromosomal aberrations and gene mutations.
4. Radioactive iodine may also cause cancer of thyroid glands.
5. Cesium-137 brings about nervous, muscular and genetic change.
6. Uranium causes skin cancers and tumours in the miners.
7. Radon-222 causes leukemia, brain tumours and kidney cancers.
Bhopal Gas Tragedy
On Dec. 2, 1984 a dense cloud of methyl isocyanate gas (Mlq leaked from a storage tank of the Union Carbide ltd plant in Bhopal. It caused a great loss of life to people and animals. Methyl Isocyanate was prepared by the reaction of methyl amine with phosgene and stored in abundance.
Green Chemistry - An Alternative Tool for Reducing Pollution:
Green chemistry may be called chemistry involved in the design, development, and implementation of chemical products and processes to reduce or eliminate the use and generation of substances hazardous to human health and the environment.
Thus, the goal of green chemistry is ‘to promote the development of products and processes that reduce or eliminate the use or generation of toxic substances associated with the design, manufacture, and use of hazardous chemicals. Some important principles and method of green chemistry are-
1. It is better to prevent waste than to treat or clean up waste after it is formed.
2. Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
3. Whenever possible, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
4. Chemical products should be designed to preserve efficiency of function while reducing toxicity.
5. The use of auxiliary substance (e.g., solvents, separation agents etc.) should be avoided as far as possible.
6. Energy requirements should be recognized for their environmental and economic impacts and should be minimized.
7. Synthetic methods should be conducted at ambient temperature and pressure.