| (B) Design filtration rates shall not exceed three
gpm/square foot for single media filters, four gpm/square foot for
dual media filters, and five gpm/square foot for mixed media filters.
The filter area required shall be calculated utilizing the previously
listed specified rates at the design flow of the facility. A minimum
of two filter units shall be provided with the required filter area
calculated with one unit out of service.
(C) Facilities to provide periodic treatment utilizing
chlorine or other suitable agents, introduced to the influent stream
of the filter units, shall be provided as an operational technique
to control slime growth on the filter surface and the backwash storage
basin.
(D) A graded gravel layer of a minimum of 15 inches
or variable thickness of other filter media support material shall
be provided over the filter underdrain system. Filter media support
material other than gravel will be reviewed on a case-by-case basis.
Normal media depths for the various filter types are as specified
below. Media depths significantly different than these must be justified
to the commission. The justification must include an analysis of the
backwash rates. The uniformity coefficient shall be 1.7 or less. The
particle size distribution for dual and mixed media filters shall
result in a hydraulic grading of material during backwash which will
result in a filter bed with a pore space graded progressively coarse
to fine from the top of the media to the supporting layer.
Attached Graphic
(E) The unit piping for the filter units shall be designed
to return backwash waste to upstream treatment units. In order to
minimize a hydraulic surge, a backwash tank must be included into
the design for those plants that do not have some means of flow equalization
or surge control. A backwash tank shall be designed to provide storage
for filter backwash based upon the number of design daily backwash
cycles and the volume required for each backwash. Calculations must
be provided to the commission demonstrating that the performance of
the plant will not diminish with the discharging of the backwash water
into the treatment process. Enclosed backwash tanks shall be vented
to maintain atmospheric pressure. Surge control shall be provided
to the backwash system to limit flow rate variations to no more than
15% of the design flow of the treatment units that will receive the
backwash water. For these calculations, an influent lift station is
not considered as a treatment unit and, therefore, is not bound by
the 15% design flow requirement.
(F) Pumps for backwashing filter units shall be designed
to deliver the required rate with the largest pump out of service.
The backup pump unit may be uninstalled provided that the commission
is satisfied that the spare unit can be quickly installed and placed
into operation. Valve arrangement for isolating a filter unit for
backwashing shall provide ready access for the operator. Provision
for manual override shall be provided for any backwash system employing
automatic control.
(G) Head loss indicators shall be provided for all
filter units.
(H) Backwash for dual or mixed media filters shall
provide a minimum bed expansion of 20%. A surface scour shall be provided
prior to or during the backwash cycle. Backwash flow rates at 15 to
20 gpm/square foot and at a cycle time of 10 to 15 minutes should
be provided. The backwash cycle shall provide media fluidization at
the end of the cycle to restratify the media. Backwash for single
media filters should be provided by a surface air scour or combination
air-water scour and washwater at recommended rates as follows.
Attached Graphic
(I) The filter underdrain system shall be of a design
adaptable to wastewater treatment, providing a uniform distribution
of filter backwash and freedom from excessive orifice plugging. Wash
water collection trough bottoms shall be located a minimum of six
inches above the maximum elevations of the expanded media. A minimum
freeboard of three inches shall be provided in addition to the design
upstream depth of the wash water media. A minimum freeboard of three
inches shall be provided in addition to the design upstream depth
of the wash water trough to prevent submerged trough conditions during
filter backwashing.
(3) Multi-compartmented low head filters with continuous
operation (automatic backwash). This paragraph contains the design
criteria for multi-compartmented low head filters where the applicable
criteria are different than those contained in paragraphs (1) and
(2) of this subsection. All other criteria included in paragraphs
(1) and (2) of this subsection will apply to multi-compartmented low
head filters with continuous operation.
(A) Filtration rates. Filtration rates shall not exceed
three gpm/square foot for single media filters and four gpm/square
foot for dual media filters based on the design flow rate applied
to the filters. The total filter area should be provided in two or
more units and the filtration rate shall be calculated on the total
available filter area with one cell of each unit out of service. Manufacturer's
recommended rates should be utilized if substantiated by test data.
(B) Backwash. The backwash rate shall be adequate to
fluidize and expand each media layer a minimum of 20%. Provision should
be made for an approximate rate of 10 gpm/square foot over a 30 to
60 second interval. Manufacturer's recommended rates should be utilized
if substantiated by test data. Pumps for backwashing filter units
shall be adequate to provide the required rate with the largest pump
out of service. It is permissible for the backup unit to be an uninstalled
unit, provided that the installed unit can be easily removed and replaced.
Waste filter backwash water shall be returned to upstream units, preferably
the final clarifiers, for treatment.
(C) Backwash surge control. The rate of return of waste
filter backwash water to treatment units shall be controlled such
that the rate does not exceed 15% of the design flow of the treatment
units. The hydraulic and organic load from waste backwash water shall
be considered in the overall design of the treatment plant. Where
waste backwash water is returned for treatment by pumping, adequate
pumping capacity shall be provided with the largest unit out of service.
It is permissible for the backup unit to be an uninstalled unit, provided
that the installed unit can be easily removed and replaced.
(4) Alternative design for effluent polishing. Where
filters are proposed to remove remaining visible particles, other
criteria will be considered on a case-by-case basis.
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