Water Pollution
Water is a very good solvent as it dissolves many substances. This property is due to polarity of water molecules and its ability to form hydrogen bonds with other substances. For instance, a water from rain 🌧 and snow ❄️ flow over rocks and through the soil, it dissolves minerals. A fresh water that we drunk or iluse in daily life is not pure water, rather it is a dilute solution containing a number of minerals. When concentration of these minerals becomes sufficient high, the water becomes unfit for human use.
Many human activities also contribute to the pollution of surface and ground water. Human activities include household waste, agriculture waste, livestock wastes, pesticides, oil leaks, detergents, septic tanks, petroleum, natural gas production may result in water pollution.
Household wastes
Household wastewater (also known as domestic wastewater or municipal wastewater) water that contain soaps, detergents, livestock wastes, pains and oil food and vegetable waste garbage etc. Although detergents have strong cleansing action than soaps, still they remain in water for a long time and make water unfit for aquatic life. When household water containing detergents is discharged into lakes, ponds and rivers etc., it causes death of aquatic life. Chemicals and bacterial contents in household wastewater can contaminate surface and underground water. Bacterial contents may cause infectious diseases such as cholera, jaundice.
Industrial waste
Manufacturing of industrial products is always accompanied by some by-products and waste effluents. These waste products may be in the form of waste heat, smoke solid or waste water effluent. These waste may contain highly toxic compounds and heavy metals such as palladium, cadmium, mercury, chromium, antimony and arsenic etc. These toxic substances cause serious health problems such as nervous disorder, anemia, high blood pressure, kidney disease, nausea, dizziness and cancer. Industrial units generally discharge their waste either to open land or into water bodies. Water bodies lakes ,ponds, rivers and oceans. Water from leather tanneries contain large amount of Cr(VI) salts. Cr(VI) salts are highly toxic and known to cause cancer. Industrial waste water causes irreversible degeneration of environment causing health problems for the public and marine life.
Water treatment is essential for many reasons:
Through water purification, we can avoid drinking impure and contaminated water which causes many epidemic diseases and unsafe for healthy life. It removes bacteria, viruses and parasites which may cause diarrhea, dysentery, typhoid, polio, cholera and hepatitis. It also removes heavy metals like arsenic, chromium and lead which can cause long term neurological problems, kidney, nausea, dizziness and cancer. It also improves flavor and appearance of different food items.
Wastewater Treatment
The major purpose is to remove hazardous material and to reduce BOD (Biological Oxygen Demand). In many countries, waste water is passed through certain treatment stages before it is discharged into lakes, streams, rivers and oceans. This treatment involves primary, secondary and tertiary treatment. Each treatment reduces pollutants and BOD.
Primary Treatment
Primary treatment involves removal of:
>insoluble salts by filtration
>floating objects
>solid that sink into bottom due to its gravity. This process is known as sedimentation. For this purpose, water body is allowed to stand for some time.
>suspended particles are allowed to settle down using suitable chemicals. This process is known as flocculation.
During primary water treatment, Initially waste water is allowed to pass through sieves where coarse and floating particles are separated from wastewater. Then water is allowed to stand in sedimentation tank, where heavy particles settle down. After sedimentation, water is passed through flocculation tank where certain chemicals such as Al2(SO4)3 is added resulting in formation of gel like Al(OH)3. As Al(OH)3 settles down, it traps small suspended particles in the water and form the sludge. The sludge thus released is dried in air and used for secondary water treatment. Then water is treated with either chlorine or ozone to kill the pathogenic organism. If water contains sufficient pollutants, it is subjected to secondary treatment otherwise water is discharged into river or ocean.
Secondary Treatment
It involves biological treatment that is also known as activated sludge treatment. Waste water is mixed with bacterial laden sludge and oxygen which results in breakdown of organic matter by bacteria. Then waste water is passed to sedimentation tank where activated sludge is settled down. Primary waste water from activated sludge goes into aeration tank (oxygen provider) ,sedimentation ,chlorination and then tertiary treatment if impurities are present.
Tertiary Treatment
It involves removal of heavy metals such as cadmium,palladium and mercury ions.These cations can be removed by precipitation with precipitant whose solubility is low.e.g an be removed by passing gas.Cadmium sulphide is formed as product
- Phosphate can be removed by calcium and aluminium.
- Nitrates are highly soluble and can be removed by ion exchange method which contains anion exchange resins.Nitrates can also be removed by biological method that involves anaerobic organic organism that convert nitrogen in nitrates into atmospheric
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