UFC 3-280-04
17 DEC 2003
filter media with the filtered water. For conventional systems, a layer of gravel is often
placed between the media and the underdrain to aid in preventing media loss. The prin-
cipal consideration in underdrain design is the uniform distribution of backwash water.
Some common underdrain systems include pipe laterals with orifices or nozzles; ce-
ramic or plastic block laterals with holes, nozzles, or porous plates; lateral T-Pees; ple-
num, precast, or monolithic concrete, with holes, nozzles or porcelain spheres
(Wheeler-type); plenum with porous plates; and porous plates in ceramic block laterals.
Table 5-5, taken from Monk and Willis (1987), compares some conventional underdrain
systems.
Table 5-5. Comparison of Underdrain Systems
TYPE
ADVANTAGES
DISADVANTAGES
Nozzles result in greater head loss
Pipe laterals with
Air-scour can be used
Cannot use concurrent air and water
nozzles
Less gravel layers needed
Shallower filter box required
Multiple gravel layers needed
Pipe lateral with
Relatively inexpensive material
Integral air-scour cannot be used
orifices
costs
Gravel layers increase depth of filter box
Simple to construct and install
Precast concrete
Very little head loss
Multiple gravel layers needed
T-Pees
Can be used to form plenum
Integral air-scour cannot be used
Casting and laying is labor intensive
Gravel layers increase depth of filter box
Integral air-scour cannot be used
Ceramic tile block
Good backwash distribution
Requires up to seven layers of gravel
Small head loss
Blocks difficult to handle
Relatively easy to install
Deeper filter box because of gravel and
depth of block
Requires up to seven layers of gravel
Plastic dual lateral
Light to handle
Deeper filter box because of gravel and
block
Small head loss
depth of block
Water and air can be used
Limited flexibility in range of air-scour rates
concurrently
Blocks require care in laying correctly
Good water-to-air distribution
Difficult to construct
With appropriate nozzles air-scour
Plenum with
Deeper box because of plenum
can be used
precast concrete
Extra care is needed to avoid nozzle
Good water-to-air distribution
block and nozzles
clogging
Gravel layer not needed
Wheeler-type
Low head loss
Multiple gravel layers required
System
Good water distribution
Integral air-scour cannot be used
Costly construction requirement
Deeper filter box because of plenum and
gravel layers
Deeper box because of plenum
Water and air can be used
Plenum with
Extra care is needed to avoid nozzle
concurrently
monolithic floor
clogging
Little or no gravel required
and nozzles
Nozzle type must be carefully specified
Nozzles available that can be
adjusted to ensure uniform air
distribution
Air-scour rates can be varied
Deeper filter box because of plenum
Water and air can be used
Plenum with
Less reliable than a monolithic floor
concurrently
precast concrete
Extra care is needed to avoid nozzle
Little or no gravel required
blocks and nozzles
clogging
5-11
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