Note: Descriptions are shown in the official language in which they were submitted.
lk 2 ~
-- 1 --
METHOD AND APPARATUS FOR PRETREATMENT OF WATER
USING A BED OF GRANULAR ACTIVAI'ED CARBON
BACKGROUND OF THE INVENrrION
FIELD OF THE INVENTION
~he present invention relates to a water
treatment method and apparatus employing granular
activated carbon beds for purification of water.
DESCRIPTION OF THE PRIOR ART
Commonly, surface water such as lake or river
water, or subterranean water, is treated in a municipal
water treatment plant for use as potable water. This
water often contains materials which can cause a bad
taste or odor, or is otherwise harmful. For example,
the water may contain oryanic substances from decaying
vegetation, or chemicals from various agricultural or
industrial applications, such as pesticides and
herbicides.
It is common for waste water and potable
water treatment plants to subject the water to a
number of pretreatment steps. Following pretreat~
ment, the pretreated water is filtered by one or
more granular coal or sand filter beds.
As one pretreatment step, the water is mixed
with chemicals to coagulate particles in the water.
This chemically ~reated water may be fed directly to
the downstream filter beds. However, especially when
the water has a heavy solids content, the filter beds
tend to clog relatively quickly. As a result, siynif-
icant amounts of backwash water is used to frequently
clean the ~ilter beds. Also while being cleaned, the
filter beds may be out of service. Thus~ e~cessive
cleaning reduces the overall efficiency of the treat-
ment plant because of the Eilter downtime.
To partially overcome these problems,
chemically treated water is sometimes subjected to
~ t~
additional pretreatment steps prior to fil-tration.
For example, a water and coagulant chemical mixture
may be passed through a flocculation ~one. In such
a zone, gentle mixing takes place so as to p~omote
flocculation of the solids without undue solids
dispersion. From the flocculation zone, the partially
pretreated water may be directed to a sedimentation
basin. Solids settle in such a basin prior to
filtration. ~ith these additional steps, the rate
of pluqginq and head loss development in downstream
filter beds is reduced. Also, cleaning water require-
ments are reduced. However, these added steps increase
the costs of a treatment plant.
Sometimes water treatment plants~ expecially
15 potable water plants, utilize water purification stages
following an initial filter bed filtration stage. For
example, filtered purified water is sometimes contacted
with granular activated carbon in a fixed bed for final
water polishing or purification. These carbon beds are
typically operated in a downflow mode to minimize the
passage of carbon fines from the bed and into the in-
ished water. As the water flows through these carbon
beds, dissolved orqanic contaminants are adsorbed from
the water. Eventually~ the carbon becomes spent~ That
is it ceases to effectively adsorb contaminants. When
this happens, spent carbon is typically removed from
the bed and thermally treated to regenerate it.
Reqenerated or previously unused fresh carbon is
introduced into the bed to replace the removed spent
carbon. These beds are relatively expensive in the
first instance. Moreover, the active life of the
granular carbon making up such beds is relatively
short. Furthermore~ prior art carbon beds known to
applicant have suffered from the drawbaclc of becoming
quickly ouled unless the water being treated is very
clear and free from suspended matter.
-- 3
~ n exemplary activated carbon filter operated
in an upflow mode, is shown in U.S. Patent No.
~202~770 of Gappa, et al. In the filter of Gappa,
activated carbon moves downwardly through the filter
in a direction countercurrent to the direction of the
flow of waste water being trea-ted. This migration
is accomplished by removing contaminant laden spent
carbon from the bottom of the filter, regenerating the
carbon, and introducing fresh or regenerated carbon at
the top of the filter bed. Like other known granulated
activated carbon filters, the Gappa filter is used to
purify previously filtered relatively pure water.
Another prior approach to water treatment,
is described in U.S. Patent No. 3,250,899 of Rice,
et al. In ~ice, fine powdered activated carbon and
chemicals is added to untreated water in a plant.
The mixture is then directed to a filter bed of sand.
The carbon adsorbs organic materials from the water.
As a result, the need for a post-filtration activated
carbon bed is frequently eliminated~ However, these
carbon fines penetrate the downstream filter bed and
contribute to its premature clogging. Also, it is
difficult to efficiently recover costly used powdered
carbon for subsequent reuse.
~5 rrherefore, a need for an apparatus and method
which expands the possible uses of activated carbon
beds in water treatment systems, which (a) takes
advantage of the adsorption characteristics of carbon;
(b) eliminates the need for the use of and problems
associated with powdered carbon; and (c) incorporates
the potential dual usage of granular carbon beds for
adsorption as well as flocculation and suspended
solids capture; (d) reduces the problem oE clogging
of granular activated filtration beds, this lat-ter
problem having led others to limit the use oE such
~3~
-- 4
beds to post-filtration or other pure water treatment
applications.
SUMMA~Y OF THE INV~NTION
In accordance with the present invention,
a water treatment plant includes a qranulated
activated carbon bed for use in pretreating water
prior to filtration oE the water. The carbon bed,
within a contactor, is designed to treat water with
high concentrations of particulate materials without
10 excessive head loss development or requiring an
excessive use of backwash water.
The contactor is operable in one mode
primarily as an adsorption granular activated carbon
bed so as to adsorb organic materials from the pre-
15 treated water. In this dual mode, the contactoraccomplishes the partial removal of particles prior
to downstream filtration. As a result, the partic-
ulate loading on downstream filter beds is reduced.
The granular activated carbon bed may be
20 operated in either an upflow or downflow mode.
However, in the illustrated preEerred embodiment,
the system is operated in an upflow manner. With
this construction, the bed is partially fluidized
by pretreated water which flows up through the bed.
25 Because of this fluidization, plugging of the bed with
solids -from the water is reduced. The forward flow
rate of water through the bed is adjusted to control
the bed expansion. This adjusts the degree which the
apparatus functions as a clarifier in addition to its
30 functioning as an adsorption device. The slower the
upward water flow rate through the bed, the less the
bed fluidizes and the greater the rate of solids
capture by the bed. Conversely, the faster the upward
water flow rate, the greater the fluidization of the
35 bed and the easier the solids passage through the
bed. Thus, by controlling the upward flow rate, the
degree to which the bed functions as a clarifier or
flocculation device is controlled. Also, the finer
the size of the particles of the carbon bed, the
greater the solids capture rate. However, -the finer
the media, the more subject the bed is to plugging.
Thus, when operated in a dual capacity as an adsorption
device and clarifier, the media particulate size is
selected to perform satisfactory adsorption and
clarification without undue clogging of the bed.
Also, the coagulant chemical dosage affects the rate
which solids are captured by the bed.
Portions of the carbon in the bed, when
heavily loaded with contaminants, are selectively
pulsed from the bottom oE the bed Eor regeneration
and subsequent reuse. Fresh carbon granules are
transmitted to the top of the contactor to rejuvinate
the bed.
Also, carbon fines which happen to leave the
bed during water processing are removed by one or more
downstream Eilter stages of the plant. However, in
comparison to the powdered carbon approaches of the
prior art, the adverse impact of these carbon ~ines
on the downstream Eiltration stages is much reduced.
Cleaning or flushing o the bed is
accomplished by periodically or selectively increasing
the rate which Eluid flows upwardly through the bed.
This expands the bed and dislodges trapped materials.
These trapped materials may be diverted away from
downstream filter stages to a separate sedimentation
basin or other treatmen-t mechanism. In applications
in which the contactor is Eunctioning primarily to
adsorbcontaminants, the dislodged solids may simply
be allowed to flow to the downstream filters where
they are captured. Also, the beds may be backwashed
to remove solids which happen to plug inlet screens
through which influent water flows to the contactor.
-- 6 --
Effluent water from another contactor may be used for
cleaning purposes. In this case, all of the water
flowing through the plants including cleaning water,
is pretreated by a carbon bed.
For the purpose of the present application,
certain terms used herein are defined as follows:
A coagulant is a material which will cause
suspended particles in water to floc or -to be altered
so that they can be removed efficiently by filtration.
The particles may be enlarged in size by the coagulant
or they may be adsorbed or enmeshed in a precipitate
formed by the coagulant. Suitable coagulants include
aluminum sulfate, ferric sulfate/ ferric chloride,
various organic polymers as well as other coagulants
known to the art.
The term pretreated water is water, such as
from a reservoir or a lake, or partially treated waste
water ~rom a municipal waste water treatment plant
or other source, which is being processed by a water
treatment plant upstream of any filtration bed of the
plant~
The term pretreating water means processing
pretreated water.
The term water treatment plant includes
plant5 for treating unfiltered water from whatever
source including partially treated waste water.
It is an object of the present invention to
provide a new and improved water processing apparatùs
and method which includes a granular activated carbon
bed designed for pretreating the water.
Another object of the invention is to provide
a method and apparatus for pretreating water utilizing
a granular activated carbon bed operable both for
adsorption and clarification or flocculation
purposes.
Still another object oE the invention is to
provide a me-thod and apparatus for pretreating water
in a granular activated carbon bed which minimizes
the plu~ging of the bed with collected solids.
Another object of the invention is to provide
a water pretreatment method and apparatus in which
solids capture by a the granular activated carbon bed
is controllable.
Another object of the invention is to provide
a method and apparatus in which the degree of clarifi-
cation performed by a water pretreatment granular
activated carbon bed is controllable by controlling
the velocity of water Elow through the bed~
A further object of the invention is to
provide a method and apparatus which includes bed
expansion and backwash cleaninq means for effectively
cleaning a qranular activated carbon bed used in
pretreatin~ water.
Still another object of the invention is
to provide a method and apparatus in which chemical
coagulants may be added to pxetreated water upstream
of a granulated activated carbon water pretreatment
bed to enhance the functioning of the bed as a
flocculation device.
A further object of the invention is to
provide a method and apparatus in which a qranular
activated carbon water pretreatment bed is replenished
in a counterflow manner without concern for the
problems normally encountered with carbon fines in
finished water, such fines being removed by one or
more downstream filtration stages of the treatment
plant.
A further objec-t of the invention is to
provide such a method and apparatus utilizing a gran-
ular activated carbon water pretreatment bed whichminimizes the need for flocculation and sedimentation
basins when the carbon water pretreatmen~ bed is used
for the dual purpose of flocculation and adsorption.
A still further object of the present
invention is to enhance overall water treatment
5 plant efficiency through factors such as adsorption
taking place before chlorination of the water and pH
adjustment, and the minimization of scale forrnation
in the plant.
It is another object of the present invention
to provide an apparatus and method by which organic
materials and solids may be removed from water in an
efficient cost effective manner.
A further object of the invention is to
provide a method and apparatus in which a granular
activated carbon bed may be successfully used to
remove solids and organic materials from pretreated
water in the presence of substantial amounts of
turbidity in the water.
Still another object of the invention is
to provide a granular activated carbon bed water
pretreatment method and apparatus which may be
readily incorporated in a presently existing water.
These and other objects, advantages and
features of the invention will become apparent with5 reference to the description and drawings which follow.
BRIEF DESCRIPTION OE THE DR~WINGS
Fig. 1 is a schematic vertical sectional
view of a water treatment plant in accordance with
the present invention, including a granular ac~ivated
carbon bed contactor for pretreating water;
Fig. 2 is an enlarged vertical sectional
view of a portion of the granular activated carbon bed
contzctor of Fig. 1, taken along lines 2-2 of Fig. l;
Fig. 3 is a bottom view of a hopper portion
of the contactor of Fig. 2;
' ~ t ~
Fig. 4 is a cross sectional view of a portion
of the hopper of Fig. 3;
Fig. 5 is an enlarged view of a screened
nozzle inlet to the hopper of Fig. 4; and
Fig. 6 is a chart plotting the percentage
expansion of a granular activated carbon bed of the
invention as a function of water flow rate through
the bed.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Figure 1 illustrates a water treatment plant
in accordance with the present invention which incor-
porates a granulated activated carbon bed containing
contactor 12. Water from a source 14 flows through
and is pretreated by contactor 12 prior to being
15 delivered to downstream treatment stages 16 of the
plant. These downstream stages include one or more
filters and may also include other treatment devices,
such as sedimentation basins. The source 14 typically
comprises a reservoir, la~e, river, or other source
of unfiltered water. Thus, the present invention
utilizes a granulated activated carbon bed as an
adsorption unit of a water pretreatment stage of the
plant~ Also, as explained below, the carbon bed is
also capable of being opeLated ~or a water floccula-
t~on and clarification purposes to reduce downstreamsolids loading of the stages l~ The use of a carbon
bed in water pretreatment overcomes the drawbacks
present in postfiltration carbon bed devices of carbon
fines being entrained in the treated water. This is
because fines in the effluent water from the contactor
12 will be filtered and removed to a desired degree
by the downstream stages 16. At the same time, the
amount of carbon fines in the water is very small in
comparison to powdered carbon treatment techniques.
Consequently, plugging of downstream filter stages 16
by carbon Eines is minimized.
3'~
- 10
In a large scale plant, plural contactors 12
are utilized. Each contactor comprises a recta~ular
upwardly opening tank 57 which is subdivided by
transverse upright interior walls 58, 60 into three
individually isolated cells 46, 48 and 50~ As a
specific example, cells 46, ~18, 50 may be oE
rectangular cross-section, fc,urteen feet wide by
twenty-eight feet long. As can be seen in ~ig. 2,
these contactors may be placed side by side with a
common wall 57 between adjacent contactors. In E'ig.
2, the cell corresponding to cell 46 in the adjacent
contactor is identified as 46'. Referring again to
Fig. 1, these cells each contain a granulated acti-
vated carbon bed 52. A range of granular activated
carbon size particles are more suitable, with the
exact size being selected for the specific application.
It is believed that a carbon particle layer of par-
ticles larger than 4 mesh (U.S. Standard Sieve) are
too large for satisfactory adsorption of contaminants
from the water. In contrast, carbon particles smaller
than 50 mesh are too small because a bed of such
particles tends to plug with solids. Thus, carbon
granules between these si~es are preferred to achieve
the absorption and flocculation and/or solids
separation objectives. In the specific example under
discussion, the contactor 12 is filled with 25,000
cubic feet of a uniform graded 12 by 30 mesh carbon.
This grade was selected for one specific application
as best the objectives of adsorption and minimum
plugging Eor that application. This carbon weighs
approximately 700,000 pounds and has upwashed and
settled density of about twenty-eight pounds per cubic
feet. These cells are suitable for treating water of
turbidity of up to 300 NTU (Nethelometric Turbidity
Units) or higher.
Since each of these cells ~6, 48 and 50 are
of identical construction, only the operation oE cell
46 will be described in detailO Water from source 14
is directed through a conduit 20, a branch 24 to a
S conduit 26, and then to a conduit 32. This latter
conduit is in turn connected, via inlet pipes 40,
42 and 44 to respective cells 46, ~8 and 50 of the
contactor 12. Water from conduit 40 enters the base
of the cell ~6 and flows upwardly, as indicated by
arrows 54, through plural openings in a bottom hopper
base 56 of the cell. The base 56 comprises respective
first and second side-by-side inverted pyramidal
shaped hoppers 62, 64. These hoppers, in the example,
are of a fourteen feet by fourteen feet dimension at
their widest point. Plural apertures are provided in
hoppers 62, 64 for example on eighteen inch centers,
as described below to permit the flow of pretreated
water into the bed. Screened nozzles are positioned
in these apertures, again as explained below, so that
water entering the bed fluidizes the bed so as to
allow the passage of solids through the bed while
at the same time enabling the carbon bed to adsorb
organic materials from the water. The tapered hopper
design rsults in an increased velocity oE water flow
through the hopper as compared to the wider dimensioned
cell above the hopper. This facilitates solids passage
through the bed. Also, the less the fluidization of
the bed, the more readily solids will be trapped in
the bed, and vice versa. Since the fluidization of
the bed increases with increasing water flow rates
(see the chart of Fig. 6), the rate of solids capture
is controllable and adjustable by controlling and
adjusting the water flow rate through the bed. Thus,
the flow rate may be established at a level which
facilitates the functioning oE the bed for both
- 12 -
adsorption and water clarification, or solids capture,
purposes.
To facilitate solids capture within the bed,
coagulating chemicals may be added frorn a conventional
coagulant supply mixing system 70 (Fig. 1) to the
conduit 20. This enhances the dual role of carbon
bed 52 in adsorption and flocculation. Treated water
from the cell 46 flows, as indicated by arrows 7~,
over notched weirs 71 (Fig. 2) and into transverse
elongated troughs 74~ These troughs empty into a
longitudinally extending effluent channel 76. As
shown in Fig. 1, channel 76 carries the partially
treated water from contactor 12 to a line 78 and hence
to the downstream stages 16. Although these downstream
stages 16 may include a sedimentation basin in addition
to filters, in certain applications desired water
quality would be achievable with filters alone. Also,
if it is desired that the cell 46 function primarily
as an adsorption device instead of a dual adsorption
and clarifier role, coagulation chemicals may be added
downstream from contactor 12 at line 78. In this
latter case, the stages 16 may also include a
flocculation zone.
As solids accumulate in the cells 46, 48
and 50, it becomes necessary to clean the beds.
To accomplish this cleaning, an upwash and backwash
cleaning system 82 is provided and described in
greater detail below. In addition, spent granular
activated carbon is periodically removed from the bed.
In the illustrated embodiment, contaminant laden
carbon is removed from the bottom of the bed by a
spent carbon removed system 84 (Fig. 1) and regenerated
carbon is introduced by a regenerated carbon delivery
system 86, at the top of the bed~ Thus, the carbon
35 migrates through the bed in a direction which is
countercurrent to the normal flow of water through
- 13 -
the bed. As a result, the most heavily contaminated
carbon is removed. Also, disruption o~ the operation
o~ the bed is reduced.
More specifically, a conventional automa-
tically operated shut-off valve 100 is provided for
closing the pretreated water influent line 20 leading
to the system. Also, a similar valve 102 in line 26
controls the flow of water to line 32 and another line
30. Line 30 leads to another contactor not shown in
Fig. 1, but which includes cell 46' in Fig. 2. Also,
a valve 104 ~Fig. 1) controls the flow of pretreated
water through line 32 to the cell inlet pipes 40, A2
and 44. A similar valve in line 30 not shown, is
provided for the same purpose. ~his latter valve,
together with the valve 104, balance the flow of water
evenly between the contactors. Also, a line 28 from
branch 24 leads to conduits 34 and 36 which are in
turn connected to additional contactorsO A valve 106
is operated in conjunction with valve 102 to balance
the flow between the branches 26 and 28. The above
described valves are used to evenly balance the flow
of pretreated water through the contactors of the plant
and also to selectively shut off portions of the system,
such as during cleaning. In the same manner, additional
contactors may be added to the plant as desired.
In addition, valves 110, 112, 114 are
provided in the respective lines 40, 42 and 44 to
control the flow of water into the associated cells
46, 48 and 50. During normal operation, the water
flow rates are adjusted by these valves to even the
flow between the different cells. During normal
operation, valves 116 and 118 oE the cleaning system
82 are closed. With valves 100, 102, 104, 110, 112
and 114 opened and valves 116 and 118 closed, water
from the reservoir 14 flows upwardly through the
respective conduits 40, 42, 44 through the granulated
~3~
caLbon beds 52 ~f cells 46, 4a, 50 and to troughs 74.
From troughs 74, the pLetreated water is directed to
effluent channel 76 and then conduit 78 to downstream
filter stages 16.
The water flow rate is adjusted by these
valves so as to expand the bed somewhat to permit a
desired amount of solids passage simul~aneously with
adsorption of contaminants by the bed. Bed expansion,
and therefore solids capture, is controlled by the
~low rate and particle size. For a given flow rate,
the smaller the particle size, the greater the becl
expansion. Typically a 5 to 15 per cent bed expansion
is preferred. In parti~ular, the desired flow rate is
established to fit the adsorption and clarification
goals o~ a particular application. Common rates would
range from two to twelve gallons per minute per square
foot oE bed. However, more typically, ~low rates oE
six to ten gallons per minute per square foot of bed
are employed. Under these conditions, the bed expands
approximately ten to twenty-five percent, there~y
reducing the overtrapping of solids in the bed which
would create an unnecessary head loss. At the same
time, the bed does collect some solids.
At predetermined time intervals, or when the
head loss as detected by conventional detectors (not
shown) reaches a predetermined level, an upwash bed
cleaning is performed. To accomplish this cleaning,
valve 104 is closed and valve 116 is opened. This
permits the cleaning system to pump cleaning water on
line 120 to conduit 32 and the respective cells 4~, 48
and 50. Cleaning system 82 includes pumps for this
purpose with cleaning water being supplied on a line
124 to the cleaning system. A valve 122 in line 120,
discussed below, is closed during such times. The
cleaning water is delivered at flow rates which cause
bed expansion and dislodgment of accumulated solids
- 15 -
from the beds. ~y applying cleanin~ water at a rate
which results in approximately twenty to forty percent
bed e~pansion, ~lushing of captured solids -Erom the
bed is accomplished. To minimize the required
capacity o~ cleaning pumps, only one of the valves
110, 112 and 114 is opened at a time during cleaning.
The solids containing cleaning water ~lows from the
beds to effluent channel 76. In applications where
contactor 12 is used primarily for adsorption and not
clarification, these solids are directed to the down-
stream treatment stages 16 where they are removed in
a conventional manner. However~ when the contactor
12 is perEorming both adsorption and clari~ication
functions, to reduce the solids load on downstream
stages 16, during cleaning, a valve in line 78 (not
shown) is closed and the solids are diverted via an
opened valve 118 to a settling basin or other treat-
ment zone 13U. It is advantageous to use carbon
pretreated effluent from other contactors of the
plant in this upwashing cleaning operation. In such
a case, line 124 may be connected to the ef~luent line
of another so that all of the water reaching the
stages 16, including cleaning water is carbon bed
pretreated.
Also, the openings or inlets through which
water flows through hoppers 62, 64 and into the cells
may at times become clogged with solids. To dislodge
these solids, a backflushing cleaning process is used.
With valves 104, 118, and the valve in line 78 closed,
30 and valves 110, 112~ 114, 116, 122 and a valve 134
opened, this backflushing operation takes place.
Water in the cells 46, 48 and 50 drains downwardly
through the inlets and carries the dislodged solids
to conduit 32. From conduit 32, the solids are
35 carried through valve 122 to a settling basin 128.
Alternately, valve 122 may lead to a line connected
~ 16 ~
to downstream stages 16 wherein solids removal is
performed. During this backwash cleaning, supple-
mental makeup water is pumped from cleaning system 82
through the valve 134 and a conduit 136 to trough 76.
5 From trough 76l the makeup water flows baGkwardly
through channel 74 and into the top of the respective
cells. As in the upwashing operation, backwashing
rnake-up water may be obtained from the effluent of
another contactor of the plant. In this case,
backwash cleaning water reaching downstream stages
16 is also carbon bed pretreated.
With reference to Figs. 1, 2 and 3r the
hoppers 62, 64 will next be described. These hoppers
have inclined upper surfaces which taper to a carbon
removal outlet 150O In the illustrated embodiment,
each of these hoppers, best seen in Fig. 3, is oE an
inverted pyramidal construction and may be precast of
concrete. Also, the angle ~ between the upper surface
of the hopper wall and horizontal, is approximately 45.
These hoppers are formed with plural frustoconical
recesses 156, in ~heix upper surfaces as
shown in Figs. 4 and 5. In the Fig~ S form, these
recesses are on eighteen inch centers~ are twelve
inches in diameter at the upper sur~ace of the hopper
and six inches in diameter at their base, and three
inches deep. An aperture 15~ is provided through the
hopper wall at each of these recesses. Conventional
nozzles 160 are provided with one such nozzle being
inserted through each recess. These nozzles each have
an upstream nipple 162 which provides an inlet to the
nozzle and thus to the cell. The nozzle fluidize the
granular activated carbon of the bed. Also a screen
163 surrounds each of the nozzles. In the Fig. 5
form, these screens comprise a four inch diameter,
two inch high, SST wedge wire screen. These screens
distribute the flow of water across the contactor bed.
- 17 -
As can be seen with reference to Fig. 3, these no~zles
are distributed in horizontal rows throughout the
surface of the hopper 56. Other configurations which
distribute the flow of water from conduit 32 generally
uniformly across the bed are also suitable.
~ ith this construction, when the carbon
removal conduit 150 is open, the combination of the
fluidized bed and the tapered hopper cause~s removal of
a cross-section or plug of the bed through the conduit
150. Thus, the most heavily contaminant laden carbon,
even from along the sides oE the cell~ is removed.
More speGifically, referring again to cell 46 of Fig.
1, spent carbon is removed as follows. Carbon purging
valves 170, 172 and 176 are opened. Thereupon, carbon
15 passes downwardly through the conduits 150 and into a
carbon transport line 178. A source of pressurized
water 180 moves the carbon along line 178 to a conven-
tional carbon regeneration plant 182. In plant 182,
the carbon is heated and cleaned to reactivate it for
reuse. Each cell 46, 48 and 50 is typically purged of
spent carbon in series. In the specific example pre-
viously mentioned, in approximately 30 to 60 minutes,
about 2500 pounds of carbon is removed from a hopper.
Although variable, approximately five to ten percent
of the carbon is typically removed from a contactor
each time that the contactor beds are cleaned in this
manner. The frequency of carbon removal depends upon
the rate of solids capture by the bed. From regener-
ated area 182 the regenerated carbon may be conveyed
to a carbon storage area 184. From storage area 184,
the carbon is transported as needed on a line 186
through respective valves 188~ 190 and 192 to the
cells 46, 48 and 50. This replenishes the carbon in
these cells. Thus, countercurrent migration of carbon
through the apparatus for purposes of cleaning the bed
- 18 -
is achieved without disrupting the operation of the
treatment plant.
If the contactor is operated in a downflow
manner, all of the carbon in the contactor is typi-
cally removed at one time and replaced with freshcarbon. This is somewhat less efficient than pulse
removal of portions of the carbon bed as previously
explained.
One specific test of a carbon contactor
used for adsorption and flocculation purposes has
been perEormed. In this test, a small carbon
contactor was constructed and operated in an upflow
manner. Water with a relatively high concentration
of coagulated particles was delivered to the test
apparatus. In this specific test, it was desired to
operate the contactor in a partially fluidized manner
to pass flocculated solids and minimize head loss
build-up in the contactor itself.
To perform the test, a non-standard 12 by 30
mesh sample of granular activated carbon was obtained.
This sample was produced by screening a commercially
available 12 by 40 mesh carbon through a 30 mesh
screen, and discarding the 30 mesh material. This
gradation of carbon was selected to improve the bed
expansion and solids passage characteristics of the
contactor, while only moderately affecting the adsorp-
tion characteristics of the bed. In particular, a 12
by 30 granular activated carbon bed was selected to
achieve the adsorption and flocculation clarification
goals. In the test, the carbon contactor bed was
operated at an initial rate at 6.1 gallons per minute
per square foot of bed. This produced approxi~ately a
ten percent initial bed expansion. Although possible,
no attempt was made to adjust the flow rate to compen-
sate for head loss developed in the carbon contactorduring the test. The initial head loss was equivalent
~1~ 3 ~
to 1.7 inches per foot of carbon depth~ After 48
hours, the flow rate had dropped a modest amount to
approximately 5.3 yallons per minute per square foot
of bed. In addition, the bed expansion had dropped to
about nine percent. Also, the head loss had decreased
slightly to 1.5 inches per foot of carbon depth. In
addition, at the end of the 48 hour test, the contactor
was operating to reduce the turbidity of the water
passage through the contactor from 7.5 NTU to a 2.9
NTU. The upflow rate was subsequently increased to
approximately a thirty percent bed expansion. This
readily flushed captured solids from the carbon bed.
Following cleaning, the flow rate and head loss in
the contactor bed returned to the initial conditions.
Although limited, this test confirmed that
a granular activated carbon bed is capable of oper-
ating in a dual mode to both collect solids and adsorb
contaminants~ Finally, utilizing a water pretreatment
granular activated carbon bed ahead of downstream
filter stages also offers other advantages. For
example, scaling of the carbon bed with CaCO3 is
not a problem as the bed may be upstream of a lime
treatment stage. Also, plant chlorine requirements
may be reduced. In addition, better carbon
utilization is possible because of lower water pH.
Having illustrated and described the
principles of my invention with reference to one
preferred embodiment, it should be apparent to those
persons skilled in the art that such invention may be
modified in arrangement and detail without departing
from such principles. I claim as my invention all
such modifications as come within the true spirit and
scope of the following claims.
