Civil Engineering Reference
In-Depth Information
The underdrain design must be capable of uniformly distributing backwash water
and air scour across the filter bed and uniformly collecting filtered water over the
entire area of the filter. There are three categories of filter underdrain systems available:
False filter bottom
•
Floor-mounted filter bottom
•
Pipe lateral
•
False Filter Bottoms
The false filter bottom underdrain system utilizes a false floor
within the filter chamber to mount the underdrain system. A plenum is provided under
the false bottom to transport water and air to and from the underdrain. The false bottom
is normally a precast or cast-in-place concrete system supported by pedestals off the
filter chamber floor. Less commonly used are false bottoms constructed of metal plate.
The underdrain associated with the false bottoms can take a variety of forms:
Strainer type
•
Inverted pyramidal depression with balls (Wheeler bottoms)
•
Porous plate
•
The strainer-type system employs a number of different strainers, often referred to
as
nozzles,
constructed of plastic or stainless steel. The strainers contain orifices to
equalize backwash flow over the filter bed and drop pipes to control the inlet of air
through the nozzle. The porous plate-type underdrains are made of fused silicas or
aluminum oxide particles. They are formed into plates mounted on piers and allow
flow to pass through the entire plate structure. The Wheeler bottom is built with in-
terconnected modules, each module containing four inverted pyramidal spaces. Por-
celain balls are positioned in the pyramid space to diffuse flow into the gravel. A
porcelain thimble at the apex of each pyramid admits water to the plenum beneath the
modules. The strainer type and porous plate type of underdrain does not require an
overlying gravel layer, whereas the Wheeler bottom requires the gravel to maintain
the filter media in place. The strainer-type and porous plate-type underdrains utilize
fine openings to prevent the migration of media into the plenum. Normally, strainer-
type systems are used for combined air / water backwash systems, while the porous
plate and Wheeler bottom underdrains are used for backwash systems not utilizing air
scour.
Floor-mounted Filter Bottoms
This type of underdrain system can take the form
of one of several configurations. The most popular is filter blocks composed of vitrified
clay or plastic blocks, mounted directly on the floor of the filter chamber. Originally
developed using vitrified clay, the plastic blocks have become more popular in recent
years, relegating the clay blocks to retrofit applications. The top surface of the blocks
contains closely spaced orifices, with internal orifices to regulate flow. A channel in
the floor below the underdrains directs flow to and from the rows of blocks, and
connects to the filter gallery piping. The water backwash blocks are designed with
dual manifold configuration, with water entering the lower conduit and flowing into
the upper conduit along the entire length of the block manifold. The combination
air / water backwash blocks utilize triangular passages. As an alternative, filter blocks
are available that utilize strainer nozzles mounted in the top of the block or a porous
plate, which eliminates the need for support gravel.
