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    Rapid Sand Filtration Detailed

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    Abhijith SP
    Slow sand filters and rapid sand filters are two common methods for purifying large quantities of water. Slow sand filters rely on a biological layer that forms to remove bacteria through sedimentation, adsorption, and bacterial action. They are simple to operate but require more space. Rapid sand filters use coagulation, flocculation, and sedimentation prior to filtration to remove impurities mechanically. They have higher filtration rates and require less space but are more complex to operate. Both methods can significantly reduce bacterial counts to provide clean, safe drinking water.

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    Rapid Sand Filtration Detailed

    Uploaded by

    Abhijith SP
    300 views13 pages
    Slow sand filters and rapid sand filters are two common methods for purifying large quantities of water. Slow sand filters rely on a biological layer that forms to remove bacteria through sedimentation, adsorption, and bacterial action. They are simple to operate but require more space. Rapid sand filters use coagulation, flocculation, and sedimentation prior to filtration to remove impurities mechanically. They have higher filtration rates and require less space but are more complex to operate. Both methods can significantly reduce bacterial counts to provide clean, safe drinking water.

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    Rapid sand filtration detailed

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    Slow sand filters and rapid sand filters are two common methods for purifying large quantities of water. Slow sand filters rely on a biological layer that forms to remove bacteria through sedimentation, adsorption, and bacterial action. They are simple to operate but require more space. Rapid sand filters use coagulation, flocculation, and sedimentation prior to filtration to remove impurities mechanically. They have higher filtration rates and require less space but are more complex to operate. Both methods can significantly reduce bacterial counts to provide clean, safe drinking water.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    300 views13 pages

    Rapid Sand Filtration Detailed

    Uploaded by

    Abhijith SP
    Slow sand filters and rapid sand filters are two common methods for purifying large quantities of water. Slow sand filters rely on a biological layer that forms to remove bacteria through sedimentation, adsorption, and bacterial action. They are simple to operate but require more space. Rapid sand filters use coagulation, flocculation, and sedimentation prior to filtration to remove impurities mechanically. They have higher filtration rates and require less space but are more complex to operate. Both methods can significantly reduce bacterial counts to provide clean, safe drinking water.

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    © All Rights Reserved
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    of 13

    Water Purification

    1.Purification of Water on Large Scale


    2.Purification of Water on Small Scale
    1.Purification of Water on Large Scale
    Components
    1.Storage 2. Filtration 3. Disinfection
    1.Storage
    a) Physical: About 90 % of suspended impurities settle down by 24 hrs. by gravity.
    b) Chemical: The aerobic bacteria oxidize the organic matter present in the water with the aid of dissolved
    oxygen. As a result, the content of free ammonia is reduced and rise in nitrate occurs.
    c)Biological: The pathogenic organism gradually dies out. It is found that when river water is stored the total
    bacterial count drops by as much as 90% in the first 5-7 days. This is one of the best advantages of storage.

    ABHIJITH SP
    The optimum period of storage of river is considered to be about 10-14 days. If the water is stored for long
    periods there is likelihood of development of vegetation growth such as algae which impart bad smell and

    odor to water .
    2.Filtration
    Filtration is the second stage in purification of water, quite an important stage because 98-99% of the bacteria

    are removed by filtration, apart from other impurities.


    Two types of filters are:
    i)Slow Sand Filter /Biological Filter
    ii)Rapid Sand Filter/ Mechanical Filter
    1.SLOW SAND FILTER/BIOLOGICAL FILTER
    These were first used in Scotland in 1804and then in London
    During the 19th century, their use spread throughout the world.
    Even today, they are generally accepted as standard, method for water purification.

    ABHIJITH SP
    Elements of slow sand filter
    1.Supernatant Water (Raw)
    i. Supernatant water above the sand bed whose depth varies from 1-1.5m.
    ABHIJITH SP
    ii. Constant head of water
    iii. Provides waiting period
    2.A bed of graded Sand
    0.The thickness of the sand bed is about 1 metre. The sand grains have an "effective diameter"
    between 0.2 and 0.3 mm.
    0.The sand bed is supported by a layer of graded gravel 30-40 cm deep which also prevents the fine grains
    being carried into the drainage pipes. This provides 15,000 sq. metres of surface area.
    0.Water is subjected to a number of purification processes - mechanical straining,
    sedimentation, adsorption, oxidation and bacterial action, all playing their part.
    0.The designed rate of filtration of water normally lies between 0.1 and 0.4 m3/hour/per square metre of sand
    bed surface

    Vital layer
    Vital layer is not truely "biological“.
    The surface of the sand bed gets covered with a slimy growth known as "Schmutzdecke", vital layer,
    zoogleal 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 formation of vital layer is known as "ripening“ of the filter.The vital
    layer is the"heart" of the slow sand filter.It removes organic matter, holds back bacteria and oxidizes
    ammoniacal nitrogen into nitrates and helps in yielding a bacteria-free water..

    ABHIJITH SP
    3.An Under-drainage System and
    It consists of porous or perforated pipes.
    Once the filter bed has been laid, the under-drainage system cannot be seen.
    Filter box : The first 3 elements (e.g. supernatant water- 1 to 1.5 metre, sand bed- 1.2 metre, and
    under-drainage system) are contained in the filter box.
    The filter box is an open box, usually rectangular in shape, from 2.5 to 4 metres deep and is built
    wholly or partly below ground.

    Supernatant water-1 to 1.5 m


    Sand bed-1.2 m
    Gravel support-0.30 m
    Filter bottom -0.16 m

    ABHIJITH SP
    4.A system of filter control valves.
    The filter is equipped with certain valves and devices which are incorporated in the outlet-pipe system.
    An important component of the regulation system is the "Venturi meter “which measures the bed resistance
    or "loss of head".
    When the "loss of head" exceeds 1.3 metre it is uneconomical to run the filter.
    ABHIJITH SP
    The advantages of a slow sand filter
    Simple to construct and operate.
    The cost of construction is cheaper than that of rapid sand filters.
    The physical, chemical and bacteriological quality of filtered water is very high.
    When working ideally, slow sand filters have been shown to reduce total bacterial counts by 99.9 to
    99.99 per cent and E. coli by 99 to 99. 9 per cent.

    ABHIJITH SP
    2.RAPID SAND OR MECHANICAL FILTERS
    In 1885, the first rapid sand filters were installed in the USA. Since that time, they
    have gained considerable popularity especially in highly industrialized countries.
    Rapid sand filters are of two types, the gravity type (e.g. Paterson's filter) and the
    pressure type (e.g. Candy's filter).
    The following steps are involved.
    1.Coagulation
    2. Rapid Mixing
    3. Flocculation
    4. Sedimentation
    5. Filtration

    ABHIJITH SP
    RAPID SAND FILTERS

    :
    1.Coagulation The raw water is first treated with a chemical coagulant such as alum, the dose of which
    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 treated water is then subjected to violent agitation in a "mixing chamber" for a few minutes.
    This allows a quick and thorough dissemination of alum throughout the bulk of the water, which is very
    necessary.

    ABHIJITH SP
    3.Flocculation : This phase involves a slow and gentle stirring of the treated water in a "flocculation chamber"
    for about 30 minutes.It consists of a number of paddles which rotate at 2 to 4 rpm.The paddles rotate with the
    help of motors.This slow and gentle stirring results in the formation of a thick, copious, white flocculant
    precipitate of aluminium hydroxide.

    4.Sedimentation
    The coagulated water is now led info sedimentation tanks where it is detained for periods varying from 2-6
    hours, bacteria settle down in the tank.At least 95 per cent of the flocculant precipitate needs to be removed
    before the water is admitted into the rapid sand filters..For proper maintenance, the tanks should be cleaned.
    regularly from time to time, otherwise they may become a breeding ground for molluscs and sponges.
    5.Filtration
    The partly clarified water is now subjected to rapid sand filtration.

    Filter beds : Each unit of Filter bed surface :80 to 90 m2 (about 900 sq. feet).

    The "effective size" of the sand particles is between 0.4-0. 7 mm.

    The depth of the sand bed is usually about 1 metre (2 1/ to 3 feet).

    Below the sand bed is a layer of graded gravel, 36 to 40 cm. (l-l1/2 feet)

    The depth of the water on the top of the sand bed is 1.0 to 1.5 m (5-6 feet).

    The under-drains at the bottom of the filter beds collect the filtered water.

    The rate of filtration is 5-15 m3/m2/hour

    ABHIJITH SP
    Filtration
    As filtration proceeds, the "alum-floe" not removed by sedimentation is held back on the sand bed.
    It forms a slimy layer comparable to the zoogleal layer in the· slow sand filters.
    It adsorbs bacteria from the water and effects purification.
    When the "loss of head" approaches 7-8 feet, filtration is stopped and the filters are subjected to a washing
    process known as "backwashing".
    Advantages
    Rapid sand filter can deal with raw water directly. No preliminary storage is needed
    The filter beds occupy less space
    filtRation is rapid, 40-50 times that of a slow sand filter
    The washing of the filter is easy
    There is more flexibility in operation.

    ABHIJITH SP
    COMPARISON OF RAPID AND SLOW SAND FILTERS

    ABHIJITH SP
    ABHIJITH SP

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