| (ii) Facilities for sludge removal shall be provided
by mechanical means or by hopper-bottomed basins with valves capable
of complete draining of the units.
(11) Gravity or pressure type filters shall be provided.
(A) The use of pressure filters shall be limited to
installations with a treatment capacity of less than 0.50 MGD.
(B) Filtration facilities shall be designed to operate
at filtration rates which assure effective filtration at all times.
(i) The design capacity of gravity rapid sand filters
shall not exceed a maximum filtration rate of 2.0 gpm/sq ft. At the
beginning of filter runs for declining rate filters, a maximum filtration
rate of 3.0 gpm/sq ft is allowed.
(ii) Where high-rate gravity filters are used, the
design capacity shall not exceed a maximum filtration rate of 5.0
gpm/sq ft. At the beginning of filter runs for declining rate filters,
a maximum filtration rate of 6.5 gpm/sq ft is allowed.
(iii) The design capacity of pressure filters shall
not exceed a maximum filtration rate of 2.0 gpm/sq ft with the largest
filter off-line.
(iv) Except as provided in clause (vi) of this subparagraph,
any surface water treatment plant that provides, or is being designed
to provide, less than 7.5 MGD must be able to meet either the maximum
daily demand or the minimum required 0.6 gpm per connection, whichever
is larger, with all filters on-line.
(v) Any surface water treatment plant that provides,
or is being designed to provide, 7.5 MGD or more must be able to meet
either the maximum daily demand or the minimum required 0.6 gpm per
connection, whichever is larger, with the largest filter off-line.
(vi) Any surface water treatment plant that uses pressure
filters must be able to meet either the maximum daily demand or the
minimum required 0.6 gpm per connection, whichever is larger, with
the largest filter off-line.
(C) The depth and condition of the media and support
material shall be sufficient to provide effective filtration.
(i) The filtering material shall conform to American
Water Works Association (AWWA) standards and be free from clay, dirt,
organic matter, and other impurities.
(ii) The grain size distribution of the filtering material
shall be as prescribed by AWWA standards.
(iii) The depth of filter sand, anthracite, granular
activated carbon, or other filtering materials shall be 24 inches
or greater and provide an L/d ratio, as defined in §290.38 of
this title , of at least 1,000.
(I) Rapid sand filters typically contain a minimum
of eight inches of fine sand with an effective size of 0.35 to 0.45
millimeter (mm), eight inches of medium sand with an effective size
of 0.45 to 0.55 mm, and eight inches of coarse sand with an effective
size of 0.55 to 0.65 mm. The uniformity coefficient of each size range
should not exceed 1.6.
(II) High-rate dual media filters typically contain
a minimum of 12 inches of sand with an effective size of 0.45 to 0.55
mm and 24 inches of anthracite with an effective size of 0.9 to 1.1
mm. The uniformity coefficient of each material should not exceed
1.6.
(III) High-rate multi-media filters typically contain
a minimum of three inches of garnet media with an effective size of
0.2 to 0.3 mm, nine inches of sand with an effective size of 0.5 to
0.6 mm, and 24 inches of anthracite with an effective size of 0.9
to 1.1 mm. The uniformity coefficient of each size range should not
exceed 1.6.
(IV) High-rate mono-media anthracite or granular activated
carbon filters typically contain a minimum of 48 inches of anthracite
or granular activated carbon with an effective size of 1.0 to 1.2
mm. The uniformity coefficient of each size range should not exceed
1.6.
(iv) Under the filtering material, at least 12 inches
of support gravel shall be placed varying in size from 1/16 inch to
2.5 inches. The gravel may be arranged in three to five layers such
that each layer contains material about twice the size of the material
above it. Other support material may be approved on an individual
basis.
(D) The filter shall be provided with facilities to
regulate the filtration rate.
(i) With the exception of declining rate filters, each
filter unit shall be equipped with a manually adjustable rate-of-flow
controller with rate-of-flow indication or flow control valves with
indicators.
(ii) Each declining rate filter shall be equipped with
a rate-of-flow limiting device or an adjustable flow control valve
with a rate-of-flow indicator.
(iii) The effluent line of each filter installed after
January 1, 1996, must be equipped with a slow opening valve or another
means of automatically preventing flow surges when the filter begins
operation.
(E) The filters shall be provided with facilities to
monitor the performance of the filter. Monitoring devices shall be
designed to provide the ability to measure and record turbidity as
required by §290.111 of this title.
(i) Each filter shall be equipped with a sampling tap
so that the effluent turbidity of the filter can be individually monitored.
(ii) Each filter operated by a public water system
that serves fewer than 10,000 people shall be equipped with an on-line
turbidimeter and recorder which will allow the operator to measure
and record the turbidity at 15-minute intervals. The executive director
may allow combined filter effluent monitoring in lieu of individual
filter effluent monitoring under the following conditions:
(I) The public water system has only two filters that
were installed prior to October 1, 2000, and were never equipped with
individual on-line turbidimeters and recorders; and
(II) The plant is equipped with an on-line turbidimeter
and recorder which will allow the operator to measure and record the
turbidity level of the combined filter effluent at a location prior
to clearwell storage at 15-minute intervals.
(iii) Each filter operated by a public water system
that serves at least 10,000 people shall be equipped with an on-line
turbidimeter and recorder which will allow the operator to measure
and record the turbidity at 15-minute intervals.
(iv) Each filter installed after October 1, 2000, shall
be equipped with an on-line turbidimeter and recorder which will allow
the operator to determine the turbidity at 15-minute intervals.
(v) Each filter unit that is not equipped with an on-line
turbidimeter and recorder shall be equipped with a device to indicate
loss of head through the filter. In lieu of loss-of-head indicators,
declining rate filter units may be equipped with rate-of-flow indicators.
(F) Filters shall be designed to ensure adequate cleaning
during the backwash cycle.
(i) Only filtered water shall be used to backwash the
filters. This water may be supplied by elevated wash water tanks,
by the effluent of other filters, or by pumps which take suction from
the clearwell and are provided for backwashing filters only. For installations
having a treatment capacity no greater than 150,000 gallons per day,
water for backwashing may be secured directly from the distribution
system if proper controls and rate-of-flow limiters are provided.
(ii) The rate of filter backwashing shall be regulated
by a rate-of-flow controller or flow control valve.
(iii) The rate of flow of backwash water shall not
be less than 20 inches vertical rise per minute (12.5 gpm/sq ft) and
usually not more than 35 inches vertical rise per minute (21.8 gpm/sq
ft).
(iv) The backwash facilities shall be capable of expanding
the filtering bed during the backwash cycle.
(I) For facilities equipped with air scour, the backwash
facilities shall be capable of expanding the filtering bed at least
15% during the backwash cycle.
(II) For mixed-media filters without air scour, the
backwash facilities shall be capable of expanding the filtering bed
at least 25% during the backwash cycle.
(III) For mono-media sand filters without air scour,
the backwash facilities shall be capable of expanding the filtering
bed at least 40% during the backwash cycle.
(v) The filter freeboard in inches shall exceed the
wash rate in inches of vertical rise per minute.
(vi) When used, surface filter wash systems shall be
installed with an atmospheric vacuum breaker or a reduced pressure
principle backflow assembly in the supply line. If an atmospheric
vacuum breaker is used, it shall be installed in a section of the
supply line through which all the water passes and which is located
above the overflow level of the filter.
Cont'd... |
