WATER PURIFICATION ON LARGE SCALE - ENGLISH

                                                

                      WATER PURIFICATION ON LARGE SCALE - ENGLISH

                          watch my youtube video to understand this topic in easy way-

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WATER PURIFICATION-

}  Water Purification is the process of making water clean and safe to drink.  Normally it is discussed under two headings:

}  1. Purification of water on a large scale.

}  2. Purification of water on a small scale.

LARGE SCAL WATER PURIFICATION-

}  Large scale purification of water  is needed to provide safe drinking water to public. Normally this is responsibility of government to provide continuous safe water supply. Large scale water purification needs water treatment plant.

}  Main steps for Large scale purification of water  are as follows-

}  1. Storage

}  2. Filtration

}  3. Disinfection

 

STORAGE-

}  Storage provides a reserve of water from a source. Water source may be a river or canal which provide continuous water supply to the reservoir. Water is stored in the reservoir till it is used for next steps of purification

}  As a result of storage, a very considerable amount of purification takes place. This is natural purification. About 90 per cent of the suspended impurities settle down in 24 hours by gravity.

}  Some chemical changes also take place during storage. The aerobic bacteria oxidize the organic matter present in the water with the help of dissolved oxygen. By this process the content of free ammonia is reduced and a rise in nitrates occurs.

}  On storage the pathogenic organisms gradually die out. It is found by the research that when river water is stored the total bacterial count drops by as much as 90 per cent in the first 5-7 days. This is one of the greatest benefits of storage. The optimum period of storage of river water is  about 10-14 days.

FILTRATION-

}  Filtration is the second stage in the purification of water. It is very important stage where 98-99 per cent of the bacteria are removed apart from other impurities. Two types of filters are used for this process-

}  1. Slow sand filters and

}  2.Rapid sand filters.

SLOW SAND FILTERS-

}  Slow sand filters are also known as biological filters. the various elements of a slow sand filter are as follows-

}  (1) supernatant water

}  (2) a bed of graded sand

}  (3) an under-drainage system; and

}  (4) a system of filter control valves.

}  (1) supernatant water-  The supernatant water above the sand bed, whose depth varies from 1 to 1.5 meter, it provides a constant pressure of water so as to overcome the resistance of the filter bed and thereby promote the downward flow of water through the sand bed.

}  The supernatant water provides waiting period of some hours (3 to 12 hours, depending upon the filtration velocity) for the raw water to undergo partial purification by sedimentation as well as filtration.

}  (2) Sand bed- The most important part of the filter is the sand bed. it is the main filtration medium.  The thickness of the sand bed is about 1 metre. The sand grains should be preferably rounded and have an "effective diameter" between 0.2 and 0.3 mm.

}  The sand should be clean and free from clay and organic matter. The sand bed is supported by a layer of graded gravel of 30-40 cm deep which prevents the fine sand grains being carried into the drainage pipes.

}  Water percolates through the sand bed very slowly. it is subjected to a number of purification processes - mechanical straining, sedimentation, adsorption, oxidation and bacterial action . The designed rate of filtration of water normally lies between 0.1 and 0.4 m3/hour/per square metre of sand bed surface.

}  When the filter is newly constructed , it acts only as a mechanical strainer but  soon, the surface of the sand bed gets covered with a slimy growth known as vital layer or biological layer. This layer is slimy and gelatinous and consists of threadlike algae and numerous forms of life including plankton, diatoms and bacteria.

}  The vital layer is very important  It removes organic matter, holds back bacteria and oxidizes ammonia into nitrates and helps in producing a bacteria-free water. Until the vital layer is fully formed, the first few days filtrate is usually considered waste water

}  (3) Under-drainage system-  At the bottom of the filter bed there is an under-drainage system. It consists of porous or perforated pipes which acts as outlet system for the filter. Once the filter bed has been fully formed, the under-drainage system cannot be seen.

}  (4) Filter control-  The filter is equipped with certain valves and devices which are attached to  the outlet-pipe system. The purpose of these devices is to maintain a constant rate of filtration.

RAPID SAND FILTER-

}  Rapid sand filters are also known as mechanical filters.  It involves following process-

}  (1) Coagulation- The raw water is first treated with a chemical coagulant such as alum to clean the water. The dose of alum varies from 5 to 40 mg or more per litre, depending upon the turbidity and colour, temperature and the pH value of the water.

}  (2) Rapid mixing- The alum water is than sent to mixing chamber.  Mixing process uses mechanical rapid mixing blades to mix raw water with alum completely and evenly.

}  (3) Flocculation- This phase involves a slow and gentle stirring of alum mixed water in flocculation chamber. The mechanical type of flocculator is the most widely used. It consists of a number of paddles which rotate at 2 to 4 rpm.

}  (4) Sedimentation- The coagulated water is now passed into sedimentation tanks where it is allowed to stay for periods varying from 2-6 hours.  The flocculent precipitate together with impurities and bacteria and settle down in the tank.

}  The precipitate or sludge which settles at the bottom is removed mechanically from time to time without disturbing the operation of the tank. The water now becomes partially clear.

}  (5) Filtration - The partly clear water is now sent to send bed of rapid sand filter. The depth of the sand bed is usually about 1 meter supported by graded gravel. The under-drains at the bottom of the filter beds collect the filtered water

DISINFECTION ( CHLORINATION)-

}  Disinfection of clean water is carried out by adding chlorine to it. Chlorine kills pathogenic bacteria. Apart from its germicidal effect, chlorine has several other important functions as it oxidizes iron, manganese and hydrogen sulphide and it destroys some taste and odour-producing constituents.

}  In this way large scale purification of water  is achieved. This purified water is ready to consume. It is sent through water supply system of water distribution department to the houses of end users as tap water.