UFC 3-280-04
17 DEC 2003
TYPE
ADVANTAGES
DISADVANTAGES
Plenum with porous
Excellent water distribution
Integral air-scour cannot be used
plates
No gravel layers needed
Filter box is deeper because of plenum
History of damaged plates
Little, if any, competitive market
Usually not recommended for wastewater
filtration
Source: Monk and Willis (1987)
Considerations in selecting an underdrain system include the size of the
underdrain, depth of the gravel layer, head loss during backwash, and material of con-
struction. The size of the underdrain will affect the depth of the filter box. The gravel
layer depth depends on the orifice size and spacing. Orifice size will greatly affect head
loss during backwash. The underdrain should be constructed of a material that will be
resistant to any contaminants in the water to be filtered.
Concrete filters are not generally used for flows less than 12 L/s (200 gpm).
Package systems used for low-flow applications will generally have standard under-
drains designed for the system. The manufacturers will provide guidance on whether
the particular application requires a different underdrain system.
5-1.2.10 Number of Cells. The number of filtration cells must be sufficient to assure
that the backwash flow rates do not become excessively large and that when one filter
cell is taken out of service for backwashing, routine maintenance, or repairs, the loading
on the remaining cells is within acceptable design criteria. The number of cells should
be kept at a minimum to reduce the cost of piping and construction. After peak filtration
and plant flow rates are established, the number of cells should be determined based
on total required surface area and space, and cost. Where multiple cells are specified,
the number of cells should be based on one cell being out of service at all times. Usu-
ally, the minimum number of cells is two, with four often recommended. For package
pressure filter systems, it is common to size the system with three cells, anticipating that
the design load through two cells will be exceeded for short periods of time while the fil-
trate from those two cells is used to backwash the cell requiring cleaning. For some low-
flow HTRW applications and continuous backwash systems, one cell may be sufficient if
it is acceptable to interrupt filtration (e.g., shut off recovery wells or increase equaliza-
tion storage) for backwash or maintenance.
5-1.2.11 Filter Size. Generally, the surface area required is based on the peak filtra-
tion and peak flow rate. Bed depth, filtration rate, head loss, and filter run length also
help determine the required filter size. Capital and operating costs must also be consid-
ered in designing the filter. The filter box must be large enough to house the media, un-
derdrain, any control mechanism, and troughs. Additionally, the filter box size will be, in
part, determined by the backwash requirements (the bed expands during backwash)
and control system.
5-1.2.12 Valves and Piping. The necessary valves and piping are for influent flow
control, effluent flow control, and the backwash cycle. Additionally, washwater troughs
must be designed.
5-12
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