| (B) Flash mixing equipment shall have sufficient flexibility
to ensure adequate dispersion and mixing of coagulants and other chemicals
under varying raw water characteristics and raw water flow rates.
(9) Flocculation equipment shall be provided.
(A) Plants with a design capacity greater than 3.0
MGD must provide at least two sets of flocculation equipment which
are designed to operate in parallel. Public water systems with other
surface water treatment plants, interconnections with other systems,
or wells that can meet the system's average daily demand are exempt
from the requirement for redundant flocculation equipment.
(B) Flocculation facilities shall be designed to provide
adequate time and mixing intensity to produce a settleable floc under
varying raw water characteristics and raw water flow rates.
(i) Flocculation facilities for straight-flow and up-flow
sedimentation basins shall provide a minimum theoretical detention
time of at least 20 minutes when operated at their design capacity.
Flocculation facilities constructed prior to October 1, 2000, are
exempt from this requirement if the settled water turbidity of each
sedimentation basin remains below 10.0 nephelometric turbidity units
and the treatment plant meets with turbidity requirements of §290.111
of this title.
(ii) The mixing intensity in multiple-stage flocculators
shall decrease as the coagulated water passes from one stage to the
next.
(C) Coagulated water or water from flocculators shall
flow to sedimentation basins in such a manner as to prevent destruction
of floc. Piping, flumes, and troughs shall be designed to provide
a flow velocity of 0.5 to 1.5 feet per second. Gates, ports, and valves
shall be designed at a maximum flow velocity of 4.0 feet per second
in the transfer of water between units.
(10) Clarification facilities shall be provided.
(A) Plants with a design capacity greater than 3.0
MGD must provide at least two sedimentation basins or clarification
units which are designed to operate in parallel. Public water systems
with other surface water treatment plants, interconnections with other
systems, or wells that can meet the system's average daily demand
are exempt from the requirement for redundant sedimentation basins
or clarification units.
(B) The inlet and outlet of clarification facilities
shall be designed to prevent short-circuiting of flow or the destruction
of floc.
(C) Clarification facilities shall be designed to remove
flocculated particles effectively.
(i) When operated at their design capacity, basins
for straight-flow or up-flow sedimentation of coagulated waters shall
provide either a theoretical detention time of at least six hours
in the flocculation and sedimentation chambers or a maximum surface
overflow rate of 0.6 gpm/sq ft of surface area in the sedimentation
chamber.
(ii) When operated at their design capacity, basins
for straight-flow or up-flow sedimentation of softened waters shall
provide either a theoretical detention time of at least 4.5 hours
in the flocculation and sedimentation chambers or a maximum surface
overflow rate of 1.0 gpm/sq ft of surface area in the sedimentation
chamber.
(iii) When operated at their design capacity, sludge-blanket
and solids-recirculation clarifiers shall provide either a theoretical
detention time of at least two hours in the flocculation and sedimentation
chambers or a maximum surface overflow rate of 1.0 gpm/sq ft in the
settling chamber.
(iv) A side wall water depth of at least 12 feet shall
be provided in clarification basins that are not equipped with mechanical
sludge removal facilities.
(v) The effective length of a straight-flow sedimentation
basin shall be at least twice its effective width.
(D) Clarification facilities shall be designed to prevent
the accumulation of settled solids.
(i) At treatment plants with a single clarification
basin, facilities shall be provided to drain the basin within six
hours. In the event that the plant site topography is such that gravity
draining cannot be realized, a permanently installed electric-powered
pump station shall be provided to dewater the basin. Public water
systems with other potable water sources that can meet the system's
average daily demand are exempt from this requirement.
(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.
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