2.9 Design of Rapid Sand Gravity Filters

The processes of coagulation, flocculation and subsequent settlement have considerably reduced the turbidity of water, but it still has a turbidity of 40 - 50 NTU and is undrinkable. According to the WHO Guidelines for the Drinking water, the turbidity of drinkable water should be less than 25 NTU. The situation demands that water should be subjected to further treatment in order to make it drinkable. The next treatment unit is Rapid Sand Filter in which water passes through a bed of sand for further cleaning. The process is called Rapid Sand Filtration.


Overall the plant will have the capacity to treat 18 MGD of water. But initially it has been planned to construct plant that can cater to 12 MGD demand of water. Therefore, Rapid Sand Gravity Filters shall be developed to fulfill the water need of 12 MGD leaving space for extension by 6 MGD in future.








Design Criteria

Design Parameter
Ref 1
Ref 2
Ref 3
Ref 4
Adopted
Filtration rate
5.0 m/h
120 m/d
5 - 15 m/h
120 – 235 m3/m2/d
5.4 m/h
Media Diameter


0.5 - 1.2 mm
0.35 - 0.55
0.8 mm
Bed Depth


0.6 - 1.8 m

1.4 m
Uniformity Coefficient



1.3 – 1.7
1.4
Required Head


1.8 - 3.0 m

3.0 m

1Water Supply & Sewerage by E.W Steel & Terence J.McGhee (P -233)
2Hand Book of Environmental Engineering Calculations by C. C. Lee & Shun Dar Lin (P - 1.363)
3. Water Treatment Principles & Design by MWH (P -743)

Design

Flow = 12.0 MGD
Add 5 % water losses in backwashing = 0.6 MGD
Total Flow requirement = 12.6 MGD
= 2019230.8 / D
= 23.37 Cusecs
Time Required for Filter Cleaning = 0.5 Hrs

Filtration Rate = 17.7 Cft / Sft / Hr

Area of Filter Required = 2019230.8 / 17.70 x23.5
= 4854 Sft
No of Filter Beds = 8
Area of each Filter Bed = 4854/8
= 606.81 Sft.
Length of each Filter Bed = 31.0 Ft
Width of each Filter Bed = 19.57 Ft

A channel shall pass through the center of each filter bed to serve as a feeding channel as well as backwashing channel. This channel shall divide each filter bed in to two parts, the width of each part shall become 9’ - 10”.