RAPID SAND FILTRATION

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RAPID SAND FILTRATION: BY NAWID QASEMY:

RAPID SAND FILTRATION: BY NAWID QASEMY

INTRODUCTION AND HISTORY::

INTRODUCTION AND HISTORY: . The   rapid sand filter   commonly used in municipal drinking water . The first modern rapid sand filtration plant was designed and built by  George Fuller  in 1920 in Little Falls, New Jersey. and its success was responsible for the change to this technology in the U.S

Operation::

Operation: . use relatively coarse sand and other granular media to remove particles and impurities of   flocculation typically salts of aluminum or iron.   . Water and flocs flows through the filter medium under gravity or under pumped pressure and the flocculated material is trapped in the sand matrix .

TYPES::

TYPES: Gravity type, e.g. Paterson's filter, and Pressure type, e.g. Candy's filter

Design criteria for Rapid Sand Filter::

Design criteria for Rapid Sand Filter: Rate of filtration:75-200/sq.m/min. Number of filter units=N=Q^(1/2)/4.69 Where Q is in m3/hr. Filter bed size: a)surface area should be between 8-40m2 b)length to breath ratio should between 1.25 to 1.33. Max Depth of sand=Qd^3h/29323B in m.

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Where Q=filtration in m3/hr per m2 of area of filter ,d=sand size in mm,h=terminal head loss in m,B=break through index which ranges from 0.4x10^-3 to 6x10^-3.(pr-treatment). Depth of base material(gravel)=30-60cm Sand size: between 0.45 to 1 mm. Depth of water over sand: 1 to 2 m.

Maintenance::

Maintenance: Rapid sand filters must be cleaned frequently, often several times a day, by backwashing, which involves reversing the direction of the water and adding compressed air. During backwashing, the bed is fluidized and care must be taken not to wash away the media

Advantages::

Advantages: Much higher flow rate than a slow sand filter; about 150 to 200 million gallons of water per acre per day Requires relatively small land area Less sensitive to changes in raw water quality, e.g. turbidity Requires less quantity of sand

Disadvantages::

Disadvantages: Requires greater maintenance than a slow sand filter. Generally ineffective against taste and odour problems. Produces large volumes of sludge for disposal. Requires ongoing investment in costly flocculation reagents. Treatment of raw water with chemicals is essential. Skilled supervision is essential. Cost of maintenance is higher. It cannot remove bacteria .

References:

References United States Environmental Protection Agency (EPA)(1990). Cincinnati, OH Environmental Engg-1 by Dr RK LAD & prof.ST MALI

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