| (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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