Slow sand filtration water treatment plants
Practical Action
Design criteria
PARAMETER
Number of pre-filters (n)
VF Filtration rate (flow/area) (m/h)
FL
Sedimentator or pre-filter + FL
Sedimentator + pre-filter + FL
Area of a filter (A)
Minimum cost coefficient (K)
Length of filter (m)
Total height of the gravel bed (m)
Layers
First layer (gravel)
Second layer (gravel)
Third layer (gravel)
Height of the sand bed (m)
Effective diameter of the sand (D10) (mm)
Uniformity coefficient (D60/D10)
Height of free border (m)
Height of supernatant layer (m)
thickness (m)
≤ 0.05
≤ 0.05
≤ 0.15
VALUES
≤2
0.1 – 0.2
0.15 – 0.3
0.3 – 0.5
A = Q / (n x VF)
K = 2n / (n+1)
L = √(area x K)
0.1 – 0.3
size (mm)
1.5 – 4.0
4 - 15
10 – 40
0.3 – 0.4
0.15 – 0.4
1.8 – 3
0.2
1 – 1.5
The smaller the effective diameter of the sand, the more efficiently will bacteria be
removed in the filter; however, the cleaning frequency will also increase. For clear
water with a high bacteriological content, small effective diameters will be selected,
whereas large effective diameters will be chosen for cloudy water.
For water with a high bacteriological content, low filtration velocities are
recommended, and a 0.4m-thick bed.
Drainage pipes consist of a main drain and porous side drains with 12.7mm diameter
holes at intervals of 50mm on the bottom, through which the treated water will enter.
The space between the side drains should be equivalent to 1/16 of the total length of
the side drains and 1/32 of the total length of the wall.
The sizing of the pipes should be based on the criteria that the speed limit at any
point in the pipes should not exceed 0.30 m/s. The list of velocities between the
main drain (VD) and the secondary drains (Vd) should be VD/ Vd < 0.462 to obtain an
even distribution.
A maximum height of 3m from the outlet of the pre-filters to the base of the filter
should be considered, so that the water flowing from the pre-filters can be used for
flow or counter-flow filter cleaning. This height prevents the loss of head generated
by silting in the filters, allowing the water to flow normally during the cleaning
process.
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