2.9.1 Composition of Filter Bed

Filter bed shall comprise a Balancing Chamber (5’ depth) with perforated slab fitted with nozzles placed on it. A 4.5 Ft deep filtration media i.e sand shall be placed on the perforated slab. Filter Media shall have the following characteristics;

Depth of Filter Media Sand = 4’ - 6”
Water Depth over Filter Bed = 3’ - 3”

Silica Sand Effective Size (95 % Silica) = 0.85 mm
Uniformity Coefficient = 1.3 -1.7
Density = 2.65 mg /l
Porosity = 40 - 43 %

The source of the filter media will be selected after investigating various sites to suite the requirement.

Depth, size and other characteristics of the filter media has been selected based on the experience of Water Treatment Plant, Rawalpindi, where sand has been used as filter media having bed depth of 4.5 Ft and effective media size of 0.85 without any gravel base. Kawamura (2000), P- 231 has given the following relationship between bed depth and the size of the filter media:



l/de ≥
1000
For ordinary mono sand & dual media filters
1250
For regular trimedia (coal, sand & garnet) beds
1250 - 1500
For most coarse deep monomedium beds
(de is 1.2 to 1.4 mm)
1500 - 2000
For very coarse monomedium beds)
(de is 1.5 to 2.0 mm)
Note: l is a depth of filter bed in mm & de is an effective size of the media in mm


L / de = 4.5 x 1000 / 3.28 x 0.85
= 1614


Water after passing through the sand bed and nozzles shall be collected in the Balancing Tank and shall come out of the Filter Bed through a Filtered Water Outlet Pipe which discharges in Filtered Water Collection Channel flowing in the Pipe gallery below the ground. The Filtered Water Channel shall discharge in to Backwash Water Sump. The capacity of the Backwash Water Sump will be slightly more than the requirement of cleaning of one Filter bed. The excess water shall move in to the Clear Water Tank after flowing over a Weir through a pipe.