Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
PROC~SS AN~ APPA~TUS FOR HIGF~ KATE UPFLOW ~ATER
Li'IL'rRATIO~ WIT~I BUOYANT FILI'ER ~EDIA
BACKGROU~D OE' T~l~ INVENTION
The present inven~ion relates to methods and
apparatus for purification of water by filtration. More
specifically, it concerns upElow filters which contain a
buoyant filter mediaO
It has been known for some time that water can be
filtered by passing it upwardly through a bed of filter
meclia comprising grains or small pellets of a buoyant
material such as polyethylene. Such a filtration method
is shown in United Kingdom Patent Specification No~
833,327 to Smith. A related upflow filtration process is
described in Example 4 of United States Patent No.
3,343,680 (Rice et al.).
While such filters may have shown promise in
their time, they have never been successful since they
heretofore have been contained within substantially closecl
tanks or in tanks internally divided by screens to prevent
escape of the buoyant media. The filtration zones and
media bed of such filters are inaccessible during
filtration which limits control over the filtration
process.
Furthermore, no effective methods have been found
for cleaning the media in such filters. Backwashing of
such filters has proved to be wasteful of energy, finished
water and time since a very large downward flow of liquid
is required before the media particles will separate.
~echanical agitation of the media, as described in the
Smith specification, enhances cleaning, but requires a
wasteful consumption of energy in order to sufficiently
agitate a packed media bed that impurities are released.
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SU~RY OF THE INVENTION
It has now been found that it i6 possible to
provide trough-type liquid collectors to remove filtered
water from the effluent side of an upflow filter. The use
o~ such a collector makes it possible to improve the
accessability to the filter apparatus by eliminating any
top covering or screen when used in an appropriately
shaped filter vessel. This arrangement allows ~or the
incorporation of movable mechanisms which can extend from
the exterior to the interior of the tank for free motion
therethrough without the use o~ complicated seals or
tortured screening arrangements.
The nature of buoyant media particles used in a
filter bed has a substantial impact on the flow rate,
solids capture rate and cleanability of the bed.
Particularly significant are the shape and size of the
media particles. Irregular, angu~arly shaped particles
are found to have significant advantages.
It is also discovered that a relatively gentle
cleaning procedure can be used if the madia is
sufficiently close in specific gravity to the specific
gravity of water. ~ir bubbles dispersed throughout water
flowing into the bed cause the overall specific gravity of
the water and air mixture to fall to a level below that of
the media in the bed so that the bed uniformly expands
downwardly, thereby releasing trapped impurities without
mechanical agitation.
Free standing filters according to this invention
are particularly advantageous because they can be
constructed in any desired extexior configuration or
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shape. The size of such a filter depends only on the
amount of liquid to be filtered in a given amount of
time.
In accordance with one aspect, the invention is
a filter system comprising: an upflow filter vessel
defining a vertical passageway for water and having an
upflow filter inlet; an amount of buoyant particulate
media in the vessel sufficient to form a buoyant
filtration bed extending above the inlet in the
passageway; a downflow filter vessel located laterally of
the upflow filter vessel and having a downflow filter
outlet; and an amount of nonbuoyant particulate filter
media in the downflow filter vessel sufficient to form a
nonbuoyant filtration bed therein extending above the
outlet, the vessels being in communication in such a
manner that, during operation, water flows through the
inlet, then upwardly through the buoyant filtration bed,
then downwardly through the nonbuoyant filtration bed,
then through the outlet.
In accordance with a second aspect, the
invention is a filter system comprising: an upflow filter
vessel defining a vertical passageway for water and
having an upflow filter inlet and an upflow filter outlet
located above the upflow filter inlet; an amount of
buoyant particulate media in the vessel sufficient to
form a buoyant filtration bed in the passageway between
the upflow inlet and outlet, the bed having particles of
a density less than the density of water and greater than
the density of an admixture of water and gas bubbles; gas
injection means including stationary vented lateral gas
delivery tubes located upstream of the bed; gas supply
means for supplying gas to said gas delivery tubes, the
gas supply means being adapted periodically to supply gas
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to said gas delivery tubes so as to uniformly dispense
minute gas bubbles throughout water flowing upwardly into
the bed in such a manner that at least some of the
particles will descend to expand the bed and, due to such
bed expansion, release solids trapped therein during
filtration, a downflow filter vessel having a downflow
filter inlet which is connected to the up10w filter
outlet so that water can flow from the upflow vessel into
the downflow vessel and a downflow filter outlet located
below the downflow filter inlet, and an amount of
nonbuoyant particulate filter media in the downflow
filter vessel sufficient to form a filtration bed therein
between the downflow inlet and outlet.
In accordance with a third aspect, the
invention is a filter system comprising: an upflow
filter vessel defining a vertical passageway for water
and having an upflow fiiter inlet, an upflow filter
outlet located above the upflow filter inlet, and a drain
outlet located below the inlet; an amount of buoyant
particulate media in the vessel sufficient to form a
floating filtration bed in the passageway between the
upflow inlet and outlet when the vessel is filled with
water to the level of the upflow filter outlet; cleaning
means including bed maintenance means for periodically
withdrawing water from the vessel through the drain
outlet, the rate of flow through the drain outlet being
automatically adjusted in response to a sensing means so
that, during operation of the bed maintenance means,
water is withdrawn through the drain outlet at
substantially the same rate as water is admitted through
the inlet and so that flow through the upflow filter
outlet stops and the elevation of the bed is maintained
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to allow cleaning of the bed in situ without interruption
of flow through the inlet; a downflow filter vessel
having a downflow filter inlet which is connected to the
upflow filter outlet and a downflow filter outlet located
below the downflow filter inlet; and an amount of
nonbuoyant particulate filter media in the downflow
filter vessel sufficient to form a filtration bed therein
between the downflow filter inlet and outlet.
The invention may also comprise the inclusion
of at least one jet of water used to break up any mat of
fiberous material which may form at the bottom of the
bed.
In accordance with another aspect, the
invention is a filter system comprising: an upflow filter
vessel defining a vertical passageway for water and
having an upflow filter inlet and an upflow filter outlet
located above the upflow filter inlet; an amount of
buoyant particulate media in the vessel sufficient to
form a buoyant filtration bed in the passageway between
the upflow inlet and oulet; a manifold means with at
least its lowest portion positioned below the level of
water within the vessel, said manifold means having a
plurality of horizontally spaced collection orifices
positioned below the level of water within the vessel and
above the level of the lowest portion of the manifold
means, said manifold means being connected to the outlet
to deliver such water from the orifices to the outlet; a
plurality of screen cages, one of which is located in the
flow path of water from the passageway to each orifice,
to prevent particles of the media from clogging the
orifices and from being carried into the manifold; a
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down:Elow filter vessel having a downflow filter inlet
which is connected to the upflow filter outlet and a
downflow filter outlet located below the down10w filter
inlet; and an amount of nonbuoyant particulate filter
media in the downflow filter vessel sufficient to form a
filtration bed therein between the downflow inlet and
outlet.
It is therefore an object of this invention to
provide a filter which is easily accessible for
inspection, treatment by mechanical apparatus inserted
from the exterior and for viewing and maintenance
purposes.
Also an object is to provide a filter which can
operate at a high flow rate and which rapidly can be
cleaned and returned to service after cleaning.
Another object is to provide an apparatus which
can be cleaned easily and automatically.
A related object is to provide a plurality of
such filters which operate in tandem so that filtering
processes can proceed uninterrupted while one filter unit
is being cleaned.
A further object is to provide a filter for use
in virtually any location, the filter being a free
standing unit which will operate with or independentl~ of
other associated equipment and which can be built in any
si~e or shape.
These and other features, objects and
advantages of the present invention will be apparent from
the following detailed description thereof and from the
a-ttached drawings.
In the drawings:
Figure 1 is a perspective view of a filter
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apparatus according to the present invention shown in
vertical section;
Figure 2 i5 a seotional side elevation of a
collector trough shown in Figure l;
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Fig. 3 is a sectional side elevation of a first
alternate embodiment of the trough shown in E'ig. 2;
Fig. 3a is a sectional side elevation of a second
alternate embodiment of the trough shown in Fig. 2;
Fig. 4 is a sectional side elevation of a third
alternate embodiment of the trough shown in Fig. 2;
Fig. 5 is a sectional side elevation of a fourth
alternate embodiment of the trough shown in Fig. 2;
E'ig. ~ is a schematic sectional side elevation of
a filter according to the present invention with its bed
in the operating position;
Fig. 7 is a schematic sectional side elevation of
the apparatus shown in E'ig. ~ with the bed expanded during
cleaning;
Fig. 8 is a perspective view of the filter
apparatus of Fig. 1 incorporating another effluent
collector and flow controller;
Fig. 9 is a perspective view of a horizontally
oriented filter according to the present invention with a
portion of the outer casing broken away to show interior
detail;
Fig. lO is a perspective view of a filter
according to the present invention incorporating suction
sludge removal apparatus;
Fig. 11 is a perspective view of a
multi-compartment filter apparatus according to the
present invention shown in vertical section;
Fig. 12 is a schematic diagram showing a water
purification system according to the present invention
including a clarifier tank, and both upflow and downflow
filtration units;
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E'ig. 13 is a seckional side elevation of a filter
according to the present invention especially suited for
filtering water containing cellulosic fibers or other
suspended solids which tend to form a mat on the influent
side of a filter bed;
Eig. 14 is a graph showing a wash waste profile
of a filter according to the present invention; and
Fig. 1~ is a yraph showing wash waste profiles of
another filter according to the present invention.
DETAILED DESCRIP~ION
As shown in Fig. 1, a filter vessel 20 provides
an internal passageway 22 for water moving therethrough.
An inlet 24 is provided near the base of the vessel 20 for
supplying influent water into the passageway 22 and an
outlet 26 is located near the top of the vessel for
removing filtered water from the passageway 22.
Located between the inlet 24 and outlet 26 is a
bed of buoyant media particles 30 on which impurities
collect as water to be filtered moves upwardly through the
bed. l'his media must have a specific gravity less than
that of water and, for the reasons described below, should
have a specific gravity no less than 0.80. Most
preferably, the media particles should have a specific
gravity no less than 0.96.
To achieve effective filtration, a media of
proper characteristics will be chosen depending upon the
nature of the water to be filtered. For example, the
media should have an effective size between two and twenty
millimeters; but optimal filtration of storm water or
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effluent from a biological treatment clarifier, requires a
media llaving an effective size between about 2.0 and 10.0
millimeters in diameter. The particles will have a
uniformity coefficient no greater than 2.0 and sphericity
of less than 0.7.
Water is filt~ered by passing it upwardly through
the bed of such particles 30 and is collected in a
horizontal collector apparatus 34 which delivers the
filtered water to the outlet 26. To allow for access and
ease of cleaning, the collector 34 should extend
horizontally into the filter vessel at a location near the
top thereof without impeding flow through the entire
cross-section of the vessel. ~owever, the collector
should be capable of receiving liquid at multiple
locations in the cross section of the passageway 22
wi-thout loss of media particles. And yet the collector
should be such that the filter can be established in an
uncovered tank for ease of access.
Collectors can be constructed in a number of
advantageous configurations, Figs. 1~5 and 8 illustrating
most preferred arrangements. In E'iy. 2, a solid channel
38 is covered by a flat screen 40. While in Fig. 3, a
similar channel 42 is covered by an arched screen 44. The
arched screen is preferred since it is entirely self
supporting and will not collapse even if, for some reason,
a substantial amount of the filter media particles 30
climbs over the trough 42.
A screen 44a, which flares upwardly from a trough
42a is shown in Fig. 3a. ~liS arrangement is desirable
because media and other solids tend to fall outwardly and
downwardly from the sides the screen 44a~ It i5 almost
impossible for media to climb over the screen; and the
screen is less subject to plugging than in mos~ other
collector types.
Fig. 4 shows another alternate trough
embodiment. Irhis trough is substantially tubular in c~oss
section, and includes a lower hemicylinder made of a solid
material 46 and an upper hemicylinder or arch made of
screening 48. In Fig. 5, the trou~h constitutes a tubular
screen. Another variation (not shown), is a perforated
tube with screen covered openings.
Any of the troughs can include means for
automatically cleaniny the screen should it become clogged
either with media or debris. As illustrated in Fig. 4, a
tube 52 can extend through the interior of the trough and
include spray orifices at periodical intervals. Should
the screens become clogged, pressurized water is delivered
to the interior of the tube 52 to form spray jets which
backwash the screen as illustrated.
As an alternativa, one or more external tubes S3
can supply water for jets which spray onto the exterior of
the screen to wash off adhering solids.
A somewhat different collector, as shown in Fig.
8, comprises a pipe manifold 54 having a plurality of
collector heads 55 located in a common hori~.ontal plane.
The heads 55 are made of screen and surround the inlets to
~he manifold 54 so that filter effluent can flow into the
manifold inlets while buoyant media particles 30 are held
back by the heads.
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All of ~he illustrated collec~or trough
arrangemerlts avoid horizontal, cross-sectional,
media-retainirlg screens and thus allow easy access,
throuyh the top of the filter vessel, to bed and region
t'herebelow. E'urt'hermore, the collector arrangement is
compatible with effective methods and apparatus for
cleaning upflow filters of the type illustrated.
As previously mentioned, cleaning the media
particles can be a substantial problem in this type of
filter. And, the availability of a practical cleaning
method is essential to filter operability.
In the present invention, cleaning is
accomplished by periodically dispersing gas bubbles
throughout liquid in the filter bed. The amount of gas
provided is selected to be just sufficient to reduce the
density of the fluid in the passageway 22 to below the
density of the particles 30 which make up the bed so that
at least some of the media particles descend by gravity
and the filter bed expands in volume. As the bed expands,
media particles separate from one another, allowing
trapped impurities to move outwardly from the bed to be
; collected and discarded.
To effectively accomplish the dispersion of gas
into liquid in the bed, the apparatus of Fig. 1 includes a
gas injection mechanism 56 located inside the vessel 20
beneath the bed of particles 30. The injection mechanism
is a manifold having a feeder pipe 58 connected to lateral
delivery tubes 60. ~hen air is introduced into the
manifold through an air intake line 64, air bubbles are
produced at the numerous orifices located in the tubes 60
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throughout ~he cross-sec-tion of the passageway 22 so that,
during cleaning, the fluid within the passageway 22 is an
air-water mixture. ~y dispersing bubbles of air
throughout the fluid in the passageway 22, bed expansion
is substantially uniform at each horizontal cross-section
of the passageway ~2.
A different gas injection mechanism 56a is shown
in Figs. 6 and 7. This mechanism includes a vertical
feeder pipe 58a on which are mounted per~orated delivery
tubes 60a. This mechanism is constructed so that the
tubes 60a revolve horizontally to dispense air bubbles
throughout the passageway 22 when pressurized air is
supplied through the pipe 5~a.
Regardless of the vessel or collector used,
region R is provided between the bottom of the bed of
particles 30 and the air distribution manifold. This
region R must be unobstructed by screens or other
structures which would limit downward movement of the
media particles. It must also be of sufficient volume to
allow the bed to expand downwardly during cleaning to the
extent that trapped impurities in the bed are released.
A comparison of a bed before and during cleaning
is found in Figs. 6 and 7. Fig. 6 shows a bed in the
normal unexpanded condition which exists durins normal
filtration. During cleaning the bed is expanded as
illustrated by Fig. 7, which shows a partially expanded
bed.
~ hen air is introduced through the manifold, the
media particles 30 descend, usually to the extent that the
lowest particles of the bed are located immediately above
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the manifold. Particles will typicalLy fill the region R
when the bed is expanded, but will not descend below the
air manifold since liquid below that level remains at a
greater density than the media.
~ 'o achieve effective cleaning with a minimum of
procedural steps, it is advantageous to prevent all flow
of liquid through the outlet 26 during cleaning of the
bed. 'rhis is accomplished by closing valves 66, 67
provided in lines extending from the outlet 26. In most:
open vessels, however, the valves 66, G7 cannot be closed
without simultaneously diverting the influent to prevent
the open filter vessel from overflowing. Even in a closed
vessèl, merely closing the outlet 26 would not be
acceptable because it would cause water to back up at some
location upstream of the filter bed.
To divert the filter influent, a drain outlet 68,
; connected with a drain line 70 is provided to remove water
from the passageway 22. During expansion of the bed, the
impurities trapped in the bed are separated, descend by
gravity and are carried out of the vessel through the open
drain outlet 68.
Valves controlling the drain line 70 can be
opened to match continued flow through the inlet 24.
Removal of liquid through a drain can, however, create
problems since it is not only essential to keep liquid
from flowing over the top of the vessel 22, but also
important to prevent media particles 30 from being carried
outwardly through the drain.
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Screens can be placed over the outlet 68; but
these are prone to rapid clogging during draining. In the
filters according to the present invention, therefore, the
drain outlet 68 is loca~ed at a position at least one foot
below the air delivery tubes 60 of the manifold 56. This
positioning prevents media from descending to the level of
the outlet 68 even when the bed is at its maximum
expansion. Also, a mechanism is provided to automatically
match the flow entering through the inlet 24 to ~he flow
of liquid exiting through the drain line 70 so tha~ the
media bed remains at the surface.
In the apparatus of Fig. 1, such automated
flow-matching is accomplished by means of a flow meter 74
to measure the rate at which influent is entering the
filter vessel 22. A combined flow meter and automatically
actuated valve 76 are incorporated in the drain line 70.
A processor 78 is connected between the entrance flow
valve 74 and exit flow valve 76 to continuously monitor
and compare the rate of inflow and outflow. If the rate
of inflow becomes ou~ of balance wi~h the rate of outflow,
the processor 78 signals the automatically actuated valve
76 to open or close an appropriate amount to balance the
flow. In this manner, the level of water in the vessel 22
i5 maintained at a substantially constant leveli even
during cleaning while the bed is expanded.
Fig. 8 shows another automatic control system for
maintaining a substantially constant level of liquid in
the vessel during cleaning with the effluent line closed.
In this embodiment, a float valve chamber 82 is connected
by a pipe 84 to the interior of the vessel 20 so that
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liquid in the chamber 82 is always at the same level as
liquid inside the passageway 22. A float 88 inside the
vessel 82 connscts to an automatic valve controller 90.
If the level of liquid inside the chamber 82
falls below a predetermined level during cleaning of the
filter, the controller 90 automatically closes a valve 92
in the drain line 70 which causes the level of liquid to
rise in both the vessel 20 and chamber 82. The control
may also be constructed so that if the float 88 exceeds a
predetermined maximum level in the chamber 82, th~
controller 90 signals the valve 92 to open, thereby
draining liquid from the vessel until an acceptable
reduced level is reached.
A fil~er according to the present invention can
be constructed in a variety of configurations other than
the open-topped, vertical, circular cylinder shown in
Figs. 1 and 8. For instance, Fig. 9 shows a vessel 100
which is a substantially horizontally extending circular
cylinder closed at both ends. A passageway 102 is defined
20 between the level of the inlet 104 and outlet 106 with
media particles 110 forming a bed through which liquid
must travel from the inlet 104 to the outlet 106. A
collector trough 114 is a foraminous tube which extends
longitudinally along the top of the container and connects
to the outlet 106. A gas injection manifold 116 including
a plurality o~ lateral tubes 120 connects to an intake
line 124 and is located below the level of the particles
110 which form the bed. Liquid is removed through a drain
ou~let ~not visible) which connects to a drain line 130.
30 ~he operating controls and mode of operation for the
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apparatus shown in E'ig. 9 are similar t~o those discussed
above in relation to E'igs. 1 and 8.
Yet another embodiment is shown in Fig. 10
wherein a filter apparatus 136 includes a vessel 138
having a wall 140 in the shape of a rectangular cylinder.
Adjacent to the vessel 138 is a headwater compartment 142
which receives water to build up a head sufficient for
moving the water upwardly through the filter bed. The
bottoms of the headwater chamber 142 and the vessel 138
are interconnected to provide an inlet 1~4 for water to
enter the vessel. An outlet 146 is provided near the top
of the vessel 140 with a bed of buoyant media particles
150 being located beween the inlet 144 and outlet 146. A
trough-type collector 154 lies parallel to a wall of the
vessel 138. Air injection apparatus 156 are provided
below the bed along with a drain outlet 158 and drain line
160.
Fig. 10 illustrates some of the specific
advantages of the present invention. Water delivered to
the filtering system shown collects in the headwater
chamber 142 and is free to rise or fall independently of
the source of the water. The chamber 142 is constructed
so that water cannot flow back from the chamber 142 into
upstream apparatus to cause backups and overflows.
r~rhe open top of the apparatus shown in Fig. 10
accommodates a movable bridge mechanism 16~ which can
travel across the entire! surface of the filter and which
supports apparatus that depends into the water inside the
vessel 140. In the illustrated embodiment, a siphon-type
sludge removal device of a type described in
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U.~. Patent Mo. ~,094,785 to Booty is carried by the
bridge 164. 'rhe use of such a device would be awkward or
impossible i~ the filter vessel were closed at the top or
if the bed of media particles 150 was retained above or
below a horizontal screen.
Fig. 11 shows an apparatus similar to the one
illustrated in Fig. 10 and corresponding components are
marked with similar reference numerals. The apparatus of
Fig. 11, however, includes multiple filter vessels
138a-138d which share common walls. These multiple cells
are connected in parallel and can operate together.
Advantageously, flow through any individual filter can be
halted during cleaning or maintenance of a particular cell
while uninterrupted flow continues through the others.
I'he present invention further comprises other
multi-unit systems for the purification of water
containing suspended impurities. Such systems are
illustrated by Fig. 12. A clarifier tank 160 has a wall
162 which defines a settling zone 16~. Inlet means 166
delivers a stream of water with suspended solids into the
settling ~one 164 wherein the water is partially ~larified
by settling. Partially clarified water leaves the
clarifier -tank 160 by means of an outlet, illustrated in
Fig. 12 as a weir 168.
Water passing over the weir 168 is delivered to a
headwater container such as a pipe 170 wherein its level
can, to some extent, vary independently of the level of
the liquid in the clarifier. The pipe 170 delivers
partially clarified water to the inlet 174 of an upflow
filter vessel 176 which defines a vertical passageway for
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water between the inlet 174 and an ou~let 178 located
above the inlet. Buoyant media of the type previousl~
described is located in the vessel in an amount sufficient
to form a floating filtration bed. Gravity urges water to
flow from the pipe 170 into the vessel 176 and through the
bed so long as the level of water in the pipe 170 is
greater than the level of water in the filter vessel 176.
Another advantageous system illustrated by Fig.
12 has side-by-side upflow and downflow filter vessels
176, 180. These may or may not share a common wall.
~ater which leaves the filter vessel 176 through the
outlet 178 flows into the top of the downflow filter where
the water is finished by flowiny downwardly through sand
or some other non-buoyant media. The arrangement is ideal
to minimize energy consumption because the driving force
which raises liquid through the filter vessel 176 is all
that is necessary to supply influent to the downflow
filter 180, even if the filters 176, 180 are built to the
same elevation and located alongside each other on level
ground. Chemical compounds to aid filtration,
particularly in the downflow filter 180, can be added
either upstream or downstream of the upflow filter. Thus,
in ~`ig. 12, coagulants, filter conditioners, pH adjustment
chemicals and the like can be added through either an
upstream line 182 or a downstream line 183.
It can fur~her be seen from Fig. 12 that a
comprehensive water purification system comprises a
clarifier, followed by an upflow and then a downflow
filter, to make maximum use of the available head and
minimize or avoid the cost of incorporating and operating
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pumping mechanisms. A very fine quality water is produced
by such a three stage system. And, the system is very
compact since only a small difference in height is
required between the units to accomplish operation
entirely by gravity.
It is also a discovery that certain upflow
filters can be used to solve specific, difficult
filtration problems. Fig. 13 shows an apparatus according
to the invention specifically adapted for use in filtering
liquids containing cellulosic fibers or other material
which forms a mat as the suspending liquid flows through
bedded filter media. The apparatus of Fig. 13 is very
similar to that shown in Fig. 1 so that both figures
contain a number of correponding reference numerals. The
apparatus of Fig~ 13 can employ filter media having an
effective size of up to twenty millimeters. Media of
larger effective size clogs less rapidly so the use of
media of an effective size between ten and twenty
millimeters is advantageous if the liquid to be filtered
contains fibers or other solids which form a mat. A water
injection means 184 is located about six to twelve inches
below the bed of media particles 30 for directing jets of
water into the bed to break up the mats of fiber which
form on the bottom of the bed. Pressurized water is
supplied to a distribution pipe 186 having a number of
nozzles 188, preferably directed horizontally or upwardly.
~ he pipe 188 can be fixed or, as an alternative,
may comprise a rotor having an interior cavity which
connects to the nozzles. In the illustrated embodiment,
water is supplied to the cavity through a central vertical
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shaft and means are provided to turn the rotor so that the
jets move along a horizontal path to break up any mat wich
forms. The procedure for operation using this embodiment
is more fully explained below.
OPERATION
While it is believed the general operation of the
invention will be understood from the previous description
of the apparatus, various aspects of the operation,
particularly the filter cleaning procedures, are explained
as follows.
In each embodiment, the bed is established by
providing the desired amount of particulate filter media
inside a vessel and then filling the vessel with water so
that a floating bed is formed. Next, water to be filtered
is introduced into the inlet of the vessel and flowed
upwardly through the bed to the outlet where filtered
water is collected.
Periodically, when flow through the bed drops off
or suspenaed solids break through, it is necessary to
clean trapped impurities from the bed. To accomplish the
cleaning, a stream of gas bubbles is distributed uni~ormly
into water at a location upstream of the bed. As the air
bubbles move into the bed, the average density of fluid in
the bed is reduced to the point where particles in the bed
descend. As particles descend, the bed expands and
impurities trapped during filtration are released so that
they can be washed out of the filter vessel.
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rl'here are ~wo ways ~o discharge separated
impurities from the fil~er vessel. One way, described in
re~erence to the apparatus of Fig. 12, is to close the
valve 66 which connects to the finished water line and to
open the valve 67 which connects to a drain line. While
air expands the bed, water is allowed to continue moving
upwardly therethrough, carrying with it separated
impurities. The valve 76 remain~ closed so that water
laden with separa~ed solids flows into the collector 34
and through the valve 67 to the drain. Normal filtration
is resumed by stopping the air flow and allowing any
residual solids to flow through the drain valve 67. As
soon as water of sufficient clarity is being collected,
the drain valve 67 is closed and valve 66 is reopened.
Another method for discharging separated
impurities from the vessel is to drain water from the
bottom of the vessel at a rate substantially equal to the
rate of flow through the inlet while gas bubbles are being
introduced into the water. This halts the flow of water
through the outlet, without closing a valve on the outlet
or substantially lowering the level of liquid in the
vessel. Preferably, the draining rate and influent ra~e
are automatically sensed and matched so that the height of
liquid inside the vessel is maintained within a
predetermined range.
If the liquid being filtered contains fiber or
other materials which tend to form a mat on the filter
media, the cleaning process will further include the step
of directing jets of water into the mat sufficiently to
disrupt the mat, preferably while draining water from
- 18 -
,~
~ t~4
below the bed so that the mat is moved downwardly and
disintegrated as i~ passes through the level of the water
jets. Such draining can be accomplished by opening the
valve 76 in the case of the apparatus of Fig. 12 so that
the bed of media particles 30 push the mat downwardly past
the distribution pipe 186. The disintegrated mat pieces
fall to the bottom of the vessel and are carried out
through the drain line 70.
The operation of the present invention will
further be understood from the following examples.
Example 1
Tests were conducted to determine the suitability
of apparatus and process of the present invention or use
for the filtration of effluent from activated sludge
sewage treatment plants. The apparatus tested was
generally as shown in Fig. 1. Certain of the tests were
conducted in a filter column three inches in diameter and
ten feet high. Other vessels having a square
cross-section sixteen inches on a side and eight feet high
were also used. During the tests, filtered influent was
fed to the bottom of each column and effluent collected at
the top. 'rhe effluent collector was a trough covered with
No. 16 wire screen to prevent the loss of media. A drain
line was provided at the base of the columns and air for
cleaning was supplied through 1/4 inch tubing located at a
distance below the bed.
Separate tests were conducted using polyethylene
pellets and polypropylene pellets for filter media
particles:
-- 19 --
99L
.
a~ ~he polyethylene pellets were the more
spherical in shape and had a very smooth surface. Sieve
analysis showed this test media to have an effective size
of 2.9 ~illimeters and a uniformity coe~ficient of abou~
1.2. Specific gravity of the beads was 0.96.
b. The polypropylene media had a rough surface
and were angularly shaped. Specific gravity of the
polypropylene was 0.90. Pellets had an effective si~e of
about 3.5 millimeters and a uniformity coefficient of
about 1.8.
In comparing the two types of media, it was found
that the polyethylene media was more sensitive to flow
rate. Specifically, it was inefficient to operate a
polyethylene media bed at a rate of higher than 10 gpm per
square foot. At that rate, no more than about 30% of
suspended solids could be removed. A 50% removal of
solids requred reducing the flow rate to 6 gpm per square
foot or less. The results were much better when the
polypropylene media was used, most likely due to the
irregular shape of the particles which increased the
interstitial volume of the bed. Fifty percent suspended
solids removal could be achieved even at flow rates of
twenty gpm per foot square.
~ eds ranging in depth from three to seven feet
were tested. Suspended solid removal was significantly
lower when a bed of three feet was used. A substantial
increase in efficiency was used when the bed depth was
increased to five feet. But, operaion at a depth of seven
feet produced little or no improvement over filters
containing five feet of media.
- 20 -
1'7~
Tile above test results and efficiency percentages
were determined when the influent contained less t'han lO
mg/l of suspended solids, typical for the sewage treatment
plants where testing occurred. ~owever, when the
suspended solids content was increased to 56 mg/l for a
short period, the effluent from a filter containing the
polypropylene media was essentially unchanged, displaying
a removal efficiency of about 94%. It thus appears that
removal efficiency will increase wit'h increasing influent
solids content so the above figures concerning removal
efficiency are for comparison purposes only.
Tes~s further demonstrated that the media beds
could be cleaned effectively with only a small addition of
air properly applied at the bottom of the filter. Air
was injected into the liquid below the bed at a sufficient
distance that when the upwardly travelling mixture of air
and water entered the bed, particles in the lower part of
the bed would descend, apparently due ~o a reduction in
fluid density, to about the level of the air inletO l`he
result was an expansion of the bed and release of trapped
impurities due to the increase in the distance between
media particles and enhanced fluid flow therethrough.
To achieve such bed expansion, it is necessary to
reduce the density of the liquid by an amount sufficient
to overcome the buoyancy of the media. The volume ratio
of air to water needed to match the specific gravity of
the liquid to the media is about O.l to specific gravity
of the media. For example, if the media has a sepcific
gravity of O.g, the ratio of air to water needed to
counteract the force of buoyancy is about .1 to .9 or
1/9. This is calculated on the following basis:
- 21 -
^~
7~
~m = (Pw/P)~) + (Pa/P) ~a
~m, ~, and ~a are specific gravities of
mixture, water and air respectively,
P Pa Pw '
Pa is the air fraction in the pore, and
Pw is the water fraction in the pore.
Since Pa is much less than p, the above equation may be
approximated to:
~m = irpw/(pw ~ Pa)]~
For a buoyant media having a specific gravity of O.9
~ m < O.g, for bed expanslon.
Therefore, a limiting value of PW/(Pw + Pa) ~ 90,
for ~= 1, or
~ Pw = ~ 90 (Pw + Pa)
(1 - 0-90~PW = -g Pa
Pw/Pa = gO/G. 10 = 9
Thus, the maximum water-air ration for bed expansion is
about 9 to 1.
Tests showed that for a seven foot column of
water containing a five foot buoyant media bed, expansion
would occur in about one minute when air was added at the
base of the column at a rate of about 1 cubic foot per
minute per square foot of filter area regardless of media
size. 1~us, the energy expenditure for operating air
pumps to clean buoyant media by this method is minimal.
This is in sharp contrast to energy expenditure
requirements to clean a conventional rapid sand or like
heavy media filter by air scouring during bacXwashing.
For example, to obtain equivalent cleaning of a heavy
media bed using air scouring, it would be necessary to
- 22 -
'q~
supply about three cubic feet of air per minu-te per s~uare
foot of fil-ter media while backwashing with water at
between about fifteen and forty-five gallons per minute
depending on the sir~e and type of meclia.
While the solids removal efficiency of tested
buoyant media filters would vary depending UpOIl bed depth,
flow rate and influent characteristics, it was found that
the average solids retaining capacity for polypropylene
media was about 0.11 pounds per square foot per inch of
headloss increase, which was about ten fold higher than
for a conventional heavy filter media. Using the
polypropylene media, an exceptional 5~ solids removal
could be achieved at a rate of 20 gallons per minute per
square foot with a five foot bed.
Example 2
In another set of experiments, filter columns 1
inches in diameter and ten feet high were arranged
generally as the apparatus shown in Fig. 1. Submersible
pumps installed in the final clarifier of the sewage
treatment plant at Philomath, Oregon were used to provide
secondary clarifier effluent for filtration.
Comparison tests were conducted to determine the
size of screen suitable for use on the effluent collector
of the filter. The tests showed that No. 8 or No. 10
screen is the optimum choice for use with a filter in this
situation. If the filter is upwashed for cleaning so that
washing waste is discharged by way of the effluent
collector, the screens would become clogged. But,
cloyginy is cleared by spraying the collector screen with
- 23 -
7~
~, ~
water as shown in Fig. 4. If cleaning was accomplished
using balanced influent and drain flows, no cleaning of
the collector screen was required except for the
occasional removal of slime.
A single quarter inch air orifice supplied 2.4
cfm of air (1.36 cfm per square foot) during cleaning,
which was sufficient to cause bed expansion in the
eighteen inch circular column. 'rhe minimum air
requirement was about 1 cfm per square foot. Using the
single air injector, the whole four foot high bed could be
expanded within a minute. 'rhe air distribution in a
larger filter will be less ideal; but 2 cfm per square
foot would be adequate for most larger filters.
Using the filters, tests were conducted to
compare methods for cleaning buoyant media beds:
a. Bottom draining through screen.
A wash cycle was started by closing the influent
valve 74 and lowering the water level in the passageway 22
~o about six inches below the effluent collector 34. 'rhen
air was introduced at a rate of about 1 cfm per square
foot for one to one and a half minutes. After that, the
drain valve 76 was opened to allow approximately four feet
of water to drain from the column through a No. 4 screen
placed over the drain outlet 68 to prevent media loss.
Then the column was refilled with influent water and air
applied followed by a repetition of the draining.
As shown in 'rable I, the total amount of solids
in the wash out of one filter drain was relatively low.
I.e. only 0.06 pounds per square foot were drained whereas
0.~8 pounds per square foot were collected during
- 24 -
9~
filtration. Even after a second draining, significant
amounts oE solids remained in the filter medium. l'he
inadequacy of this cleaning method was further evidenced
by the rela-~ively rapid increase of headloss in filter
runs conducted after cleaning according to this method.
,'
- 25 -
'7~'~
.
TABLE I
Wash Waste Solids
~ottom discharge through drain screen at 7.9 gpm/sq. ft.
with influent and air shut off.
Time (min.) Suspended Solids (mg/L)
-
0-1 268
1-2 114
2-3 102
3-4 91
4-5 76
5-6 58
6-7 50
7-8 49
8-9 49
Total 857
Total solids drained = 0.06 lb/sq. ft.
Total solids filtered = 0.48 lb/sq. ft.
` 30
- 26 -
'799L
. Discharge through effluent collector.
In this me~hod, the washing cycle was started by
closing the influent valve 74 and lowering the water level
to about six inches below the effluent collector 34. Air
was introduced for about a minute and a half or until the
bed was fully expanded. Then, influent valve 74 was
opened while the flow o~ air remained on so that washing
waste was collected through the effluent collector 34 and
passed to a waste line via the valve 67.
When the filter bed was washed according to this
process, more solids were washed out within th~ same time
interval as compared to a washing with process "a". Table
II shows the wash out solids profile for filter cleaning
according to this method.
- 27 -
94
TAB~E II
Wash Waste Solids
Time (min.)Suspended Solids (m
0-1 1,035
: 1-2 364
2-3 255
3_4 205
4-5 144
Total 2,003
Total solids discharged = 0.19
Total solids filtered = 0.47 lb/sq. ft.
~'
: 20
,:
- 28 -
~.'h~:~'7~3~
Although good solids removal was achieved by this
method, significant clogging of the effluent screen was
experienced due to the presence of fibrous material in the
wash water. q`he clogging was alleviated by spray washing
as illustrated in Fig. 4.
It is a possible drawback of this cleaning method
that some portion of media near the top oE khe bed might
be restrained by the effluent collector 34 from free
expansion when the collector 34 is used for collecting and
washing waste. But, such a problem is likely to occur
only when the influent flow rate is high.
c. Bottom draining without screen.
The same procedure was followed as mentioned in
paragraph "a" above. ~lowever, no screen was present over
the drain and the drain rate was increased substantially.
It took about one half minute to drain out three feet of
water which was equivalent to an average rate of 25
gallons per minute per square foot. For each draining,
the maximum amount of water to be drained out was limited
by the depth of the tank and the media.
With the screen absent, the drain valve 76 was
closed before any media reached the drain line 70. After
draining, it took about two and one half minutes to refill
the tank with influent at a rate of about eight and one
half gallons per minute per square foot so that each
drain-refill cycle took about three minutes. About four
cycles were required for complete washing. The typical
wash out profile shown in Fig. 14 illustrates that removal
of the drain screen produced some improvement in media
- 29 -
~'~
3gL
cleaniny. This drain-refill method, howeYer, would
require complicated control apparatus or very diligent
operators.
d. Drain the wash waste with both influent and
air on without drain screen.
In this procedure, waste was effectively drained
through the drain line 70 while influent continued to flow
through the inlet 24 and air was applied continuously.
For successful opera$ion, the piping was arranged
substantially as illustrated in Fig. 1 with the air
distributor at an elevation at least twelve inches, and
preferably fifteen inches, above the invert elevation of
the drain pipe. In other words, the dimension v in Fig.
13 is at least twelve inches. rrhe region R should be
large enough to provide for at least 50% expansion of the
bed to achieve optimum cleaning. In other words, the
dimension of w shown in Fig. 13 is at least 50% of
dimension x for a vertically cylindrical filter vessel.
Satisfactory results could be obtained if the space
provided for expansion was at least 50~ of the bed volume,
i.e. w = 40% x in Fig. 13. r~O achieve any meaningful bed
expansion, the unobstructed expansion space must be at
least 10% of the volume of the bed, i.e. w = 10% x in Fig.
13. Most preferably, the influent should be applied to
the filter at a distance y about two feet below the bottom
of the media with the influent inlet 24 located at a
vertical distance z above the air distributory 56,
preferably of at least one foot.
- 30 -
'
7~3~L
The wash procedure was started by adjusting the
drain valve 76 to maintain a draining rate equal to the
influent rate through inlet 24 as previously described.
Automatic controls would maintain a constant water level
in the filter during washing. Once balance flow is
established, ~he air valve is opened to allow about one
cubic ~oot per square foot of air to enter the bed to
cause expansion. Operation is continued in this mode
until water passing through the drain line 70 is
substantially clear. Since the inlet and drain flows are
balanced, operation is very simple; and no screens need be
positioned over the inlet 24 or drain outlet 56~ Thus,
clogging problems are entirely eliminated.
In some instances, a very small amount of media
could escape to the drain at the start of the wash cycle
if some aglomerates of solids and media happened to fall
below the level of the air distributor 56 before breaking
up. To prevent such loss of media, the wash cycle may be
started with the influent valve 74 and drain valve 76
closed. I~e air is applied for about one minute to expand
the bed and then the valves 74 and 76 opened.
Specific cleaning tests were conducted with
filters that had previously been operated at 8.5 gpm per
square foot for about 20 hours. Total solids filtered
were estimated to be about 1.5 pounds per square foot.
As shown by the draining waste profiles of Fig.
15, such a filter was cleaned adequately within 10 minutes
at the highest washing rate, although the wash out solids
had not fully reached the turbidity level of the
influent. A small amount of residual solids left in the
~'
7~
sys~em was tolerable and sometimes could be beneficial for
ripening of the filter bed. The required wash time was
shortest at the highest wash rate of 22.6 gallons per
minute per square foot. The required time for washing
also varied as a function of the total amount of solids
removed during the filtration.
In addi~ion to the washing tests, a brief test of
the filtration capability was made at a filter rate of 36
gallons per minute per square foot. At this rate, 50~
removal efficiency was achieved for suspended solids of
good filterability. Thus, a filter rate higher than 20
gpm per square foot is feasible under certain
circumstances.
However, for polishing secondary clarifier
effluent, the filtration rate should not exceed 20 gpm per
square foot at peak flow. At that rate, a bed comprising
five feet of polypropylene media appears to be adequate;
but, a shallower bed could be used if the filtration rate
were lower.
It was generally observed that, when the influent
had a solids concentration higher than 200 or turbidity
higher than 100, the filter could not be cleaned
adequately by using influent as wash water. The filter
effluent or other water sources should be used instead.
Exam~le 3
'L'o determine the suitability of filters according
to this invention for the filtration of water containing
cellulose fiber or other materials which tend to mat and
clog a filter bed, tests were made using upflow filters
- 32 -
.~ ~
~3L7!~9~
with buo~ant media on paper mill effluent, brown
suspension from pulp plants, suspensions of recycled shor~
fibers and final effluent from aeration lagoons. The test
apparatus comprised filter columns three inches in
diameter and ~en feet high generally arranged as shown in
Fig. 13. The media depth varied from eighteen inches to
84 inches among the test runs. Influent was provided to
the filter vessel by a submersible pump.
a. A substantial amount of testing was done on
paper mill effluent since this liquid has the greatest
potential for reuse. A major portion of the paper mill
effluent comes from the recirculated water of paper
machines. The main constituent of the waste solids is
fibrous materials, particularly cellulose fibers. The
particular paper mill effluent filtered in the tests was
at a temperature of about 105F and contained fibers of
various lengths which were able to pass through 3/16 inch
holes. Suspended solids concentration of the influent
varied from 108 to 1,617 milligrams per liter, the average
being about 690 milligrams per literO Settleable solids
varied from 32 to 3~0 mg/l and averaged at around 140 mg/l
after thirty minutes settling.
Using polypropylene media of the type previously
described, the removal efficiency of filters tested was
over 98~ owever, the fibers accumulated very rapidly at
the lower surface of the media and formed a mat about one
half inch thick every fifteen to thirty minutes.
Operating at constant pressure of 3.2 psi, the flow rate
would initially be as high as twenty gpm per square foot,
but would decline to about two gpm per square foot within
,~
- 33 -
'7~
. . .
thirty minutes. During that thirty minute period, the
total volume of water filtered was equivalen~ to a run at
a constant rate of about six gpm per square foot. l~e
headloss would reach about seven feet in thirty minutes.
In order to provide a practical filter, various
methods were attempted to extend the length of the run by
periodically breaking up the mat which formed in the
bottom of the media bed. Mechanical raking, water jetting
and air bumping all proved to be less than satisfactory.
Because the media particles were tightly pac~ed,
mechanical raking was difficult to accomplish. Water
jetting would disrupt the mat but could not release it
from the media surface; and air bumping could expand or
agitate the media but had little effect on the mat.
It was discovered, however, that cleaning could
be effectively accomplished by partially draining the
vessel 20 through the drain outlet 68 every fifteen to
thirty minutes and simultaneously directing water jets
produced by the nozzles 188 into the mat.
For this purpose, the surface wash system 184
should be located at about six to twelve inches below the
bottom of the unexpanded bed. During the simultaneous
drainings and jettings, preferably regulated by automatic
controls, the water level is lowered by about twelve
inches, or until the lower bed boundary has passed to
below the level of the nozzles.
The water jetting commences while the mat and
media bed are still located about s~x inches above the
nozzles 188. With continued draining, the mat passes
downwardly through the level of the w,ater distributor 134
- 34 -
)4
where the water jets break it up. Still fur~her draining
would lower media particles 30 into the ~ets which would
cause agitation and facili~ate the release of solids from
lower portions of -the bed. This would make the eventual
filter washing at the end of a run easier to accomplish.
Using this mat disintegrating technique, the
filter can run for approximately three hours. At the end
of that time, the filter is washed by following one of the
procedures described in Example 2.
b. Brown suspension from a pulp plant was also
tested. This water source contained about 1700 mg/l of
usable fibrous material. Operating at six gpm per square
foot, the test filter was able to reduce the suspended
solids to 50 mg/l. The settleable solids in the influent
water were 120 mg/l after thirty minutes of settling.
; Effective cleaning required the same procedure previously
described for operation with papermill effluent.
c. The tested effluent containing short fibers
had a relatively low solids concentration of about 300
my/l. The fibers in the test samples were shorter than
those which were present in the paper mill effluent and
readily penetrated the filter.
Operating at five gpm per square foot, about
47-57~ removal of suspended solids was achieved without
chemical addition. When 30 mg/l of alum and 0.3 mg/l of
985N were added, the removal efficiency was increased to
60-86~ for a filter emp~oying a five foot bed.
d. Tests were conducted on aerated lagoon
effluent which comprised effluent from the paper and pulp
plant after settling in a clarifier and biological
treatment in aerated lagoons.
~:,
- 35 -
~L~9:17'`~3~
The lagoon effluent had a temperature of around
80E' and suspended solids of about 2040 mg/l. About 40%
removal was achieved at a flow of 15 gpm per square foot.
This was comparable to the removal experienced with
domestic waste water and described in previous examples.
While I have shown and described the preferred
embodiments of my invention, it will be apparent to those
skilled in the art than changes and modifications may be
made without departing from my invention in its broader
aspects. I therefore intend the appended claims to cover
all such changes and modifications as follow the true
spirit and scope of my invention.
- 36 -
