Civil Engineering Reference
In-Depth Information
Floor-Mounted Type
Advantages
Underdrains are simple to install in filter chamber.
•
Underdrain is not susceptible to rupture problems.
•
Disadvantages
Requires use of gravel layer above the blocks in some instances.
•
Disruption of the gravel layer with air is possible.
•
Underdrain interior is not accessible for cleaning.
•
Pipe Lateral Type
Advantages
Low installation cost.
•
Not susceptible to rupture problems.
•
Disadvantages
Experiences relatively high headlosses.
•
Flow distribution is not as effective.
•
Vertical space required for gravel bed
•
Filter Gravel
A graded gravel layer, usually 12 to 18 inches (0.31 to 0.46 m) deep, is placed over
the pipe underdrain system to prevent the filter media from entering the underdrain
orifices and to aid in distribution of the backwash flow. Table 12-1 shows typical
gravel bed design criteria for a pipe lateral underdrain.
A weakness of the gravel support system has been the tendency for the gravel
eventually to intermix with the filter media. Such gravel ''upsets'' are caused by lo-
calized high velocity during backwash, introduction of air into the backwash system,
TABLE 12-1. Typical Gravel Bed for Pipe Underdrain System
Number of Layers
Description
Bottom
2
3
Top*
Depth of layer, inches (mm)
1
3 (76)
3 (76)
4 (122)
Square mesh screen opening, inches
Passing
3
⁄
4
1
⁄
2
1
⁄
4
3
⁄
4
1
⁄
2
1
⁄
4
1
⁄
8
Source:
Reference 46. (Reprinted from
Water Treatment Plant Design,
3d ed., by permission. Copyright
Retained
1998, American Water Works Association.)
*Plus coarse garnet.
†Bottom layer should extend to a point 4 inches (100 mm) above the highest outlet of wash water.
