UFC 3-280-04
17 DEC 2003
trough. Air may be introduced either above the gravel layer or through the orifices of the
underdrain. For design, it should be assumed that there is no reduction in a backwash
rate when air scour is used.
5-1.3.5
Backwash Control
5-1.3.5.1 Rates/Times. Backwash must be carried out at a rate sufficient to fluidize the
entire bed and for a time sufficient to wash the dislodged particles out of the bed and
into the wash water gutter. For combined waterair backwash, fluidization is not neces-
sary (although the bed will expand a bit). The backwash rate should be adjusted at the
end of the cleaning cycle to ensure reclassification of the filter media. Quick shutdown
may result in increased packing and consequent smaller porosity and less pore space
for filtration. For operation on hazardous and toxic waste sites, the designer must keep
in mind that the turbulence during backwash may release volatile emissions. Potential
design solutions for this problem may include enclosing the vessel and scrubbing off-
gases or using an upstream unit operation to remove volatile constituents prior to the
filtration process. Maximum backwash rates are typically between 600 and 1000
(L/min.)/m2 (15 and 25 gpm/ft2) and the filter is backwashed for a period of at least 90
seconds or possibly for as long as 8 to 10 minutes, depending on the manufacturer's
requirements. The required backwash rate for a given filter depends on the filter media
particle size and density, and the backwash water temperature. More detailed informa-
tion regarding the calculation of the required backwash rate is presented below. Appro-
priate filter backwash rates at a water temperature of 20 degrees C are shown in Figure
5-3.
5-1.3.5.2 Source/Storage. The source of the backwash water can either be filtered wa-
ter (effluent stream) or water from an off-site source (i.e., potable water supply). The
backwash water generally should be stored in a wash water tank to provide adequate
capacity for backwashing. The volume of the tank will be determined by filter size, rate
and time of backwash, and frequency of backwash. The water may be supplied for
backwashing by using an elevated tank or a wash water pump. Typical required storage
capacity is 6 m3/m2 of filter area (150 gal/ft2). The daily backwash volume is normally in
the range of 1 to 4% of the daily treatment rate, but during peak conditions, 10% or
more may be necessary. On multiple unit pressure filter systems, backwash may also
come from other filter units operating in parallel, thus obviating the need for a backwash
reservoir.
5-19
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