| (a) Application Rates. With one unit out of service,
the peak application rate to any deep bed, intermittently backwashed,
granular media (deep bed) filtration unit must not exceed twice the
design application rate.
(1) Single Media (Sand).
(A) The design filtration rate for single media (sand)
filters must not exceed three gallons per minute per square foot (gpm/sf)
of media surface.
(B) The maximum filtration run time between backwash
periods is 6.0 hours.
(2) Dual Media (Stratified Anthracite and Sand). The
design filtration rate for a dual media (anthracite and sand) filter
must not exceed 4.0 gpm/sf of media surface.
(3) Mixed Media (Non-stratified Anthracite, Sand, Garnet,
or Other Materials). The design filtration rate for mixed media (non-stratified
anthracite, sand, garnet, or other materials) must not exceed 5.0
gpm/sf of media surface.
(b) Media Design.
(1) A deep bed filter underdrain system must include
a graded gravel layer with a minimum depth of 15 inches. Filter support
media other than gravel may be used if justified in the engineering
report.
(2) The uniformity coefficient of media used in a deep
bed filter must be 1.7 or less.
(3) A dual or mixed deep bed filter must be designed
to maintain stratification of media during backwash, based on the
particle size distribution.
(4) Media material depths for the various deep bed
filter types must conform to the values in the following table, unless
other media depths are justified in the engineering report with an
analysis of the backwash rates:
Attached Graphic
(c) Backwash Systems.
(1) Flowrate and Media Expansion.
(A) A backwash system must allow a fluidized media
expansion of at least 20% by volume.
(B) A single media deep bed filter must provide a minimum
backwash flowrate of 6.0 gpm/sf of media area.
(C) Dual and mixed media deep bed filters must provide
a minimum backwash rate of 15 gpm/sf of media area.
(D) Backwash times must be at least 10 minutes but
not more than 15 minutes, unless the engineering report justifies
a different time.
(2) Surge Control.
(A) A wastewater treatment facility that does not have
flow equalization or other means of surge control must have a backwash
tank.
(B) A surge control device must prevent increases in
flow greater than 15% of the design flow of the upstream treatment
units if backwash is taken directly to the headworks of the wastewater
treatment facility or the influent lift station.
(C) The design of a surge control device must be based
on calculations that demonstrate the slug effects of backwash water
and that demonstrate treatment capabilities are not diminished with
the return of backwash water to the headworks of the wastewater treatment
facility or the influent lift station.
(D) An enclosed backwash tank must be vented.
(3) Pumps.
(A) Pumps for backwashing deep bed filters units must
deliver the required flow rate for backwash operations according to
paragraph (1) of this subsection with the largest pump out of service.
(B) A backup pump must be available on-site.
(C) A valve arrangement for isolating a deep bed filter
unit for backwashing must be accessible for maintenance.
(D) A backwash system employing automatic controls
must include a manual override system.
(4) Supplemental Systems.
(A) A single deep bed media filter system must include
either an air scour system or a combination air and water scour system
in addition to an up-flow backwash water system.
(B) A dual or mixed deep bed media filter system must
include either a surface air or water scour system that meets one
of the following requirements for air or water scouring.
(i) Air scour system flowrates must be at least 3.0
standard cubic feet per minute per square foot (scfm/sf) of media
surface area but not more than 5.0 scfm/sf of media surface area.
(ii) Water scour system flowrates must be at least
0.5 gpm/sf of media area but not more than 2.0 gpm/sf of media area.
(d) Underdrain System. An underdrain system must provide
a uniform distribution for filter backwash without plugging or exceeding
the manufacturer's recommendation for maximum head loss. The manufacturer's
recommendation for maximum head loss must be included in the engineering
report.
(e) Tank Design.
(1) The bottom of a wash water collection trough must
be a minimum of 6.0 inches above the maximum elevation of the expanded
media during backwash.
(2) A wash water collection trough must have a minimum
freeboard of 3.0 inches during the maximum backwash flowrate.
(f) Controls.
(1) The deep bed filter operation controls may be manual
or automatic.
(2) Control indicators must be visible to a wastewater
treatment facility operator while adjusting the controls.
(3) An automatically controlled system must include
a manual override system.
(4) Each deep bed filter unit must have a head loss
indicator.
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