Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
: 10423S8
This application relates to the separation of solids
from liquids by filtration, and apparatus for effecting this
result. More particularly, this invention relates to the separa-
tion of solids from liquids, particularly for the purifi~ation of
water, by using an internal rotary filter system and an automating
mechanism therefor.
Internal rotary filter systems are known in the art.
Generally speaking, they-comprise a tub arrangement in which
there is presented a rotatable filter drum. The drum may be
made up of end walls and a connecting, open cylinder on which is
secured a filter screen.
Liquid containing solids, as for example sewage waste,
is supplied by way of a conduit to the internal portion of the
drum. As the drum rotates it separates the solids from the
liquid by causing the solids to collect on the internal surface
of the rotating screen while the "purified" li~uid drains through
the screen and into the surrounding tub. The solids, such as the
sludge obtained from waste disposal, is lifted by the screen to
a point, e.g. about the apex of rotation, at which point a fluid
~ . .
spray may be caused to impinge upon an unsubmerged portion of
the screen from an external source, thus back-flushing and dis-
lodging the solids from the interior of the screen. In order
s:::
to collect the separated solids there is provided immediately below
the dislodgement point and internally of the drum, a trough ar-
rangement which catches the dislodged solids and transmits them
through a conduit out of the drum to a collection facility.
The ~purified~ liquid in the tub is usually caused to overflo~
a spillway of preselectod height into a conduit which leads to
a disposal area such as a stream, lake, or further processing
facility.
~ , ~
- 2 -
,
~042358
Internal rotary filters, e.g. Microscreens, generally
exhibit two major operating problems. Firstly, dislodgement of
the solids from the internal portion o the filtering screen i9
often inadequate, This causes some of the solids to be carried
by the screen past the dislodgement point and back into the waste
liquid pool being treated which, in turn, tends to concentrate
the liquid and clog the screen. The efficiency of the filtration
process and the amount of liquid that can be treated is thereby
diminished.
Secondly, there is the problem of fouling of the screen
by tenaciously adherent substances such as grease, surface-active
materials and slime, including organic and/or inorganic slimes.
Such a problem i8, of course, not experienced when one is
-filtering a relatively pure ~ uid-solid mixture, e.g. one that
includes little or no bacteria, fungus, materials required for
the metabolic growth of microorganisms, and other slime-forming
ingredients. On the other hand, it has been found that when one
seeks to treat contaminated liquid-solid mixtures such as
sewage waste, the "slime" problem becomes a significant
deterrent to effective filtration due to the clogging of the
screen. Since Microscreens are being increasingly relied upon
.
for at least one step in the process of sewage waste treatment,
this slime presents a significant problem.
Several attempts have been made in the prior art to
improve solids dislodgement from the:rotating screen. Such
attempts have usually depended on a mechanical means for moving
the solids such as an internal scraper blade, vibration of the
screen, or the like, or on a fluid impacting system such as a
liquid, air or steam spray, or on chemical cleaners, including
bleaches, surfactants and germicides. Combinations of these
- 3 -
104~3S8
have also been proposed.
Mechanical means in this respect, are often unsatisfac-
tory because of the nature of the screens employed and because of
the need for exact alignment of the scraper, etc. in order to
achieve the desired result.
When employing fluid contacting or impacting techniques,
it is known that the mere use of a spray is not always the most
efficient way to clean the screen. Thus, the art has devised
various means for impacting the screen with a spray so as to
better dislodge the solids. They have, for example, designed high
pressure nozzles, used larger amounts of fluid, and either
pulsated the fluid in an on-off mode at any particular point
along the screen's length.
While these fluid flush impacting techniques have
achieved some modicum of success they are not entirely
satisfactory. Firstly, achievement of sufficient pressures
. ::
wi~thout excessive use of water in order to dislodge the solids,
.
and particularly waste sludge, from the screen by constant
pressure spray is not always possible with conventional nozzles
available to the art. On the other hand, by going to higher
pressures through the use of more liquid (air usually being un-
satisfactory) excessive liquid is injected through the screen
and into the filtering zone or solids collecting trough thus
putting an additional load on the system which receives this
~i stream. Furthermore, on-off pulsating of the liquid, either
by way of an actual timer-pump arrangement which turns the water
on and off, or by reciprocating the spray nozzles across the
face of the screen, has proved relatively unsatisfactory because
a trade-off must be made between the length of the "off" mode
and the amount of solids dislodged.
~04;~3S8
The existence of general dissatisfaction with mechanical
and fluid impact cleaning methods is evidenced by the strong
dependency on the use of chemical cleaners which has developed
in the art. Thus, for instance chlorine i9 used continuously in
some plants and intermittently in others to control slime.
However, the use of such chemicals introduces a degree of com-
plexity and additional expense into the process, and in some
instances involve shut-down of the filter. Certain of these
agents have a corrosive effect on both metallic and non-metallic
filter cloth and appurtenant structure. More importantly,
however, many chemical cleaners are themselves pollutants which
can enter rivers and other bodies of water with the sewage treat-
~` ment plant effluent.
It is, therefore, evident from the above that thereexists a dëfinite need in the art for an improved filtration
~;~ technique and/or apparatus which would increase the efficiency
~l of the separation process and solids removal and, at the same
- `1 ,
time, lessen or eliminate dependency on chemical cleaning agents.
It is a purpose of this invention to fulfill this
need as well as other needs which will become more apparent
` to the skilled artisan once given the following detailed dis-
closure of this invention.
Generaliy speaking, this invention achieves its
avowed purposes by a method which comprises operating the
above-descrlbed impacting liquid spray first at a relatively
~ - low pressure and then at a relatively high pressure, the periods
`~ of time at which the liquid spray is held at low pressure and
high pressure being, in combination, sufficient to dislodge
substantially all of the solids on the internal portion of the
screen, but in~ufficient to unduly load the system with liquid.
-- 5
104Z;~S8
Stated in another way, this invention constitutes an improve-
ment upon the process of separating a liquid from solids in
an admixture of the two by using an internal rotary filter
system comprised of a rotatable hollow drum defining there-
within a filter zone, the filter zone communicating with a liquid
collection zone external to the drum through a filter medium
located on the drum, and wherein separation is accomplished by
presenting the admixture in the filter zone, rotating the
drum and causing solids t-o adhere to the internal portion of the
filter medium while causing liquid to flow through the filter
medium to said liquid collection zone and wherein the separated
. solids are collected by contacting the filter medium with a
fluid to dislodge the solids from the medium and cause them to
çollect in a solids collection zone separate from the liquid
collection zone, the improvement of this invention comprising
. contacting the filter medium with the fluid cycled at at least
two different finite pressures such that the combination of
.~ pressures and time interval of each within the cycle removes
substantially all solids from the filter medium without over-
20- burdening the system with liquid.
While this invention contemplates the cycling of
`: the fluid through more than two preselected finite pressures,
it is preferred for the purposes of this invention that for
the majority of the time in the cleaning cycle the system is
operating at two different pressures, one pressure in a.
relatively high range the other pressure in a relatively low
range. By employing such a system in the preferred embodiments
of this invention, the combination of pressures and time interval
of each is sufficient to cause a more efficient dislod~ement
and separation of the solids than if any of the pressures were
104Z3S8
used alone and pulsated in an on-off mode or if the fluid were
applied at any of the pressures at a constant rate, as heretofore
attempted by the prior art, In certain further preferred embodi-
ments of the invention, the relative levels and time relationships
of the low and high pressure segments of the operating cycle are
preselected to reduce or eliminate the slime problem, while con-
trolling the use of wash water and power (fo~ the pump which feeds
the spray nozzles). In other preferred embodiments, the liquid
spray is taken from the "purifiedH liquid in the tub which has
been filtered through the rotary filter system. This li~uid can
nevertheless contain solids which can clog the spray nozzles. In
., ,
furtherp~eferred embodiments, so-called "self-cleaning~ nozzles
are employed to spray the liquid against the screen in combination
~`~ with control means to provide a short period of flow through the
.~ ,
nozzles at a still lower pressure to purge the nozzles of any
contaminating solids which may have collected therein.
The processes of this invention differ dramatically
from those of the prior art both in operation and effect. In
, ::
~j~ operating the prior art, when `it employed pulsation, provided
20` only limited control over wash water utilization when operated
at its maximum slime removal capability. In addition, the various
mechanisms employed to reciprocate the spray nozzles (if this is
how pulsation is achieved) are often complex, thus adding an
,. ~ :
expense and a potential maintenance item to the system as a whole.
In-contrast, the processes of this invention are simple to operate,
do not overload the system with fluid and yet effectively remove
solids from the screen -- and in particular embodiments eliminate
slime without the need for germicides.
In order to most efficiently carry out the above-
described processes of thi~ invention, thi~ invention contemplates
iO4Z358
an improvement upon the basic apparatus heretofore e~ployed in
the art as described hereinabove which improvement generally
comprises using as the means for applying the fluid to the fil-
tering medium a means which includes a pressure controlling means
for applying the fluid in a cycled fashion at pressures within
at least two different finite pressure rangeq such that the com-
bination of pressures and time interval of each within the cycle
are capable of removing substantially all solids from the filter
medium without overburdenLng the system with fluid. In certain
preferred aspects of this invention, the pressure controlling
means is also capable of applying fluid in a cycled ashion so as
to prevent any substant~al amount of slime from forming on said
fluid medium without the use of a germicide. In other preferred
forms of this invention, the pressure controlling means also includes
- ~ a pump ~eans capable of performing at at least two different speeds
and a timer means for controlling in said cycled fashion, the
speed at which the pump means operates and thus the pressure at
.~ which the fluid is applied to the filter medium.
In other preferred forms of this invention the timer
~-~ 20 means includes a control means to provide a short period of flow
,.
h~ through the nozzles at a still lower pressure, preferably between
a change-over from either high to low, or low to high pressure.
~-~ Also, the fluid applying means includes self-purging nozzles
located externally to the drum, and usually at the apex of rota-
,~. . I
tion of the drum, which are capable of purging themselves of
contaminating solids during the aforesaid short period of flow.
~ his invention will now be described by reference to
certain embodiments thereof and illustration9 in connection there-
with wherein: ¦
; 30 IN THE DRAWINGS
Figure 1 is a partially sectioned perspective ViQW
I
~0423S8
of an internal rotary filter system employed in accordancewith this invention.
Figure 2 is an end plan view of the system of
Figure 1.
Figure 3 is a partially sectioned size plan view of the
system of Figure 1.
Fiqure 4 is a sectionalized end view of the system as
taken along line 4-4 of Figure 3.
Figure 5 illustrates a preferred inlet in accordance
with the teachings of this invention.
Figure 6 is a partially sectioned side schematic view
of Figure 5 illustrating the device in operation at a relatively
~`~ low level of filtration.
Figure 7 is the same schematic view as Figure 6 except
that the liquid level is relatively high.
Figure 8 is a schematic of a circuit for controlling
~ pulsation in accordance with this invention.
;1 Referring now to Figures 1 - 7 there is illustrated
-~ what may be called a basically conventional internal rotary
filter system 1 adapted to be operated ln accordance with this
invention. Such a system generally comprises a tank or tub
.~
means 2 which forms in its internal portion a pool or reservoir
3 ~Figure 2). Located in end walls 5 of tank means 2 are drum
axle bearing means 7 which form a stationery retaining means
-:
about which the filtration drum 13 (hereinafter described) may
rotat-e. On top of tank 2 there is provided a hood or lid 9
which may extend over the entire top portion of the system so
as to prevent splattering of fluid and solids from within the
system.
Located within the front wall 5 of tank 2 is inlet
_ 9 _
~V42358
conduit 11 which communicates between a point external of the
system to the internal filtration zone 14 of drum 13. Located
within drum 13 and juxtapositioned longitudinally along its axis
and immediately below its highest point or apex of rotation,
generally illustrated as at point X (see Fig. 4), is sludge
collecting conduit 15. It consists of a collecting trough 17
and a conduit 19 which connects the trough through the rear wall
5 with a point external to the system.
Located at about the apex of rotation X and internally
of hood 9 are spray means 21. Spray means 21 may assume any con-
ventional configuration which generally would comprise fluid
conduit 23 and a plurality of axially spaced spray nozzles 25.
While nozzles 25 may be any conventional nozzle currently employed
in the art, it is particularly preferred for the purposes of this
- invention to employ nozzles of the self-purging type such as
those produced by Lodding Engineering Corporation of Auburn,
Massachusetts. Such nozzles are often known as "self-cleaning
showers" and generally comprise aspring actuated plunger which
- closes down the orifice to form a spray when water pressure is
applied behind the plunger. When water pressure is eliminated
or reduced, such as by turning off the water, the plunger retracts
and the nozzle opens thus purging it.
While back flush fluid (e.g. water) May be provided
by a source external to the system, it is preferred as illustrated
in Figures 1-3 to supply this back flush fluid by means of a
- - pump 27 which draws, for its source of fluid, from reservoir 3
and which then sends this fluid under one of two pressures ~as
described hereinafter more fully) by way of conduit 29, fluid
conduit 23 and nozzles 25 to the screen. In this respect, it is
often convenient to provide a manual throttle valve 31 for
-- 10 -- .
104Z3S~
manually adjusting the pressure to the nozzles 25.
For the purposes of this invention, pump 27 may be
any conventional type such as a centrifugal pump. However, pump
27 must be capable of delivering back flush fluid to conduit 29
at, at least, two different pressures, one relatively lower
than the other.
Filter drum 13 is rotatably driven by motor 33 which
- drives the drum 13 by way of rotating axle 35 linked to pinions
37 which are connectingly associated with gear wheels 39 on either
end of drum 13.
Drum 13 is structurally sealed against fluid flow at
its ends by walls 41 (Figure 3) such that elimination of fluid
therefrom, must be by way of microscreen filter 43. The micro-
screen filter 43 comprises a perforate supporting member, filter
cloth of interwoven strands or filaments defining apertures there-
between, the apertures being smaller than the openings in the
perforate supporting member and a locking layer for locking the
cloth in engagement with the supporting member. In addition, the
- locking layer is usually formed of solid material which, at
least prior to locking, is soft or softenable under conditions
which do not distend or impair the material of the filter cloth
and the locking layer has an outer portion fixediy secured to
the supporting member and having an inner portion which includes
integral extensions extending through and at least partially
overlapping a sufficient number of the filaments or strands of
the cloth to securely lock the cloth to the perforate supporting
member.
Such microscreens are capable of reducing the suspended
solids in the effluent of an activated sludge process in an effi-
cient manner, for example from about 20 parts/million to about
104Z3S8
10 parts/million, In addition, such microscreens are durableand exhibit high efficiency of filtration when employed in the
techniques of this invention.
As best illustrated in Figures 1, 2 and 4, one of the
side walls of tank 2 has therein a spill weir 45 for removing
the filtered "purified" liquid from the system and sending it
either to drain or to further processing. Such a spill weir
communicates with a spill tank 47 and an outlet conduit 49.
Referring now to Figures 5-7, there is illustrated a
particularly preferred embodiment of this invention when the system
is called upon to handle a large amount of liquid, such as sludge
process effluent. Such an embodiment generally comprises at
least two filtration systems 1 connected by way of a branched con-
duit arrangement generally indicated at 50. Conduit arrangement
' 50 generally comprises an inlet conduit 51 communicating with a
divider-overflow tank 53 which separates the incoming liquid to
~`~ be filtered and sends i~ to its respective systems 1.
Divider-overflow tank 53, in turn, as is best illus-
trated in Figures 6 and 7 comprises an inlet channel 55 which
20- communicates by way of spill weir 57 with overflow channel 59
and drain conduit 61. Weir 57 i~ formed so as to provide a
:
safety spill-over at a point just below the point at which liquid
inter;nal of drum 13 would begin to spill over into sludge collec-
tor 15. In this respect then Figure 6 shows a normal operation
while Figure 7 illustrates ~hat could happen if for example a
large amount of sewage suddenly rushed into the system due to rain
or the like. Rather than flooding the internal portion of the
drum, the discharge of excess influent waste water over weir
57 allow~ the condinuous operation of drum 13 in its normal mode
(with its normal efficiency).
- 12 -
10423S8
Figure 8 is a schematic of a circuit for manipulating
pump 27 in its low-high pressure mode. In this respect, elec-
trical energy is provided at source 61 to a conventional two-speed
motor starter MS. Starter coils M-l and M-2 and thelr associated
armatures (not shown) and contacts selectively energize the high
speed windings 69 or low speed windings 71 of the two speed pump
motor PM which is preferably of the variable speed, variable
torque type. Control over starter coils M-l and M-2 and selection
of the high or low speed windings is attained by a control sub-
circuit CS connected to power source 61 via conventional connec-
tions through stepdown tranSformer 63.
- In control sub'-circuit CS, switch means 67 includes
a manual selection switch 75 which may be manually adjusted to
manual setting H, open setting O or automatic setting A. When
switch 75 is in position A, it connects one side each of contacts
79A and 79B with power supply transformer 63.
Also, there is a rotating timerdr,ive; motor 65 connected
to transformer 63. This motor operates timer contacts 79A and
' 79B. This is accomplished for instance through any suitable mecha-
~; 20 nical arrangement such as a gear train and disc (not shown) driven
by the timer motor, the disc having adjustably positionable cam
means (not shown) on it and follower means (not shown) on the
timer contacts 79A and 79B whereby the respective contacts can
be held alternately open and closed for any desired portion of
one revolutlon of the disc.
When contacts 79A are closed by rotation of timer drive
motor 65, they energize high speed starter windings M-l in motor
starter circuit MS and cause pump motor PM to operate at high speed
to produce the hlgh pressure ~egment of system operation. When
contacts 79B are closed by timer motor 65,they energize low speed
.~
1042358
- starter windings M-2 and cause pump motor PM to Operate at low
speed, producing the low pressure segment of each operating cycle.
Note that the starter windings M-l and M-2 are illustrated twice
in the wiring diagram to show their relationship to both the
motor starter and control sub-circuit. Note also that indicators
73 (optional and mountable in the same control panel with switches
75 and 77 and timer motor 65) may be wired in parallel with starter
coils M-l and M-2, to show an operator which of the~ is energized.
When switch 75-is in manual operating position H, it
connects to the transformer 63 one side each of the high speed
- contacts I and low speed contacts J of manual speed selector
switch 77. When this switch is moved manually to position I,
high speed starter coil M-l is connected to the transformer
through a current path which by-passes timer contacts 79A, thus
manually operating pump motor PM at high speed. Similarly, moving
- switch 77 manually to position J by-passes contacts 79B and operates
-pump motor PM-at low speed.
Based on this disclosure, those skilled in the art
will readily substitute other equivalenttimers and controls. Such
timer mechanism is preferably, however, one which provides at
least two speeds of operation as described hereinabove for pump
27. In addition, and preferably, the timer should be capable of
providing a hiatus in the operation of the pump motor or at
least a perioa of still lower pressure operation between the prin-
clpal high and low pressure segments of the operating cycle to
enable the self-cleaning shower nozzles to purge themselves of
any contaminating solids as described hereinabovo.
In operation, the actual pressures chosen for the
low pressure and the high pressure segments of the operating
cycle will be governed by various parameters in the system such
- 14 -
10423S8
as the type of liquid being filtered, the size of the filter,
; and the like. Functionally stated, however, the low pressure
should be some definite finite pressure capable of removing
and directing to the trough a substantial portion of the sludge
presented under its spray. Then, following application of this
low pressure spray there should be provided, for a sufficient
period of time within the cycle, a spray of liquid at a pressure
above the low pressure sufficient to remove substantially all
remaining sludge from the screen and preferably to eliminate any
slime formation thereon either with a reduced amount or without
` the employment of chemical cleaners.
In general, there is a programmed operation in repe-
titive cycles. Each cycle includes a segment or period of
operation at pressures(s~ in the range of about 15 to 60 psig
and a segment or period of operation at pressure(s) in the range
of ab-out 60-200 psig, with a sufficient volumetric flow rate for
islodging solids from the interior of the screen and into the
trough beneath. The pressure or pressures in the high pressure
segment usually average at least about 1.5 times and preferably
at least about 2 times the pressure or pressures in the low
pressuro segment.
~- - In every rota~y screen filter, including those of the
prior art, there is a certain variation in the percentage of
dislodged solids which actually enter the trough as opposed to
missing the trough or running down the interior of the drum and
falling into the unfiltered sewage. At a given spray pressure,
this percentage appears to be influenced positively by larger
~, .
~- and/or more numerous screen openings and by the presence of
- protrusions which tend to break the water film on the inner sur-
face of the screen. Correspondingly, with smaller screen openings
.. . .
-- 15 --
104Z358
(and smooth inner screen surfaces) it is recommended that
pressures in excess of the above minima be employed in one or
both of the pressure ranges, but especially the low pressure
range.
The cycles vary in length from a fraction of an hour
up to about one day or more. Within each cycle, the time is
apportioned predominantly or exclusively between high and low
pressure operating time. The high pressure segment of each
cycle is at least as long as is required for at least about one,
and preferably at least about three complete revolutions of the
filter drum. The high pressure segment may represent from about
0.5% to about 33%, but more preferably 25% or less of the total
time in each cycle, generally, the balance of the time in each
cycle will comprise the low pressure segment of the cycle, which
is longer than the high pressure segment, and (optionally) a
shorter but even lower (including zero) pressu~e operating seg-
... .
ment for nozzle purging. Generally, the lower pressure nozzle-
purging segment will represent less than 5% of the time in the
cycle, and is preferably limited to only that time required to
~20 ~ fully purge the nozzles e.g. about S to 15 seconds.
The cycles are repeated substantially continuously
throughout the continuance of filtration. Thus, over an extended
period of:filtration which may last one or more hours or days,
-the total length of the low and high pressure segments of the
~1:
oycles which occur in such period may represent 100% and prefer-
ably represent at least 95% of the extended period. ThUs, the
nozzles remain "on" and spraying for extended periods under
~ ~ either high or low pressure at substantially all times during
-i~; filtration, rather than rapidly pulsating on and off or rapidly
`~ 30 pu~sating between high and low pressuro.
.-'''
., .
- 16 -
.
. 104Z358
Operation of the process invention may be illustrated
by an example wherein the system is designed to reduce suspended
solids concentration from about 20 ppm to le5s than 10 ppm and
is capable of handling a flow of about 700 gals. per minute of
effluent from an activated sludge plant properly designed and
operated in the treatment of that quantity of domestic sewage.
A screen of about 5 ft. in diameter and 10 ft. long may be
employed. This screen may be fitted with a cloth filter medium
of any weave Ibut preferably an open weave such as square weave)
of approximately 25 micron aperture and a projected open area of
20% of its total area. This screen, when operated at a peripheral
speed of 90 ft/min., may be expected to pass the aforementioned
flow with a nominal pressure drop across the media for example
6~ of water head. The shower system can provide a low pressure
of approximately 25 psi and a flow of 40 gallons/min. for approxi-
mately 27 minutes, and subsequently a period of a few seconds
duration where the pump is stopped and the pressure drops to zero.
This may be followed by a period wherein the pump is started
: :`
and comes up to speed in the high pressure mode. During the
~20~ stopping and starting phases the pressure in the spray header
at positive values of less than lS psi for a finite period
of time to accomplish flushing of the self cleaning nozzles.
Pressure in the shower pipe during the high pressure
mode may bs approximately 110 psi and flow through the showers
~ approximately 80 G.P.M. Duration of the high pressure mode may
; be about three minutes, after which period the motor is elec-
~i trically switched to low speed to commence the low pressure mods
~b of the sequence. The approximately 30 minute cycle as described
above can be and normally ~8 repoated, as described, on a con-
tinuous basis.
.
~- - 17 -
10423S8
For applications in systems differing from the pre-
ceding example, it is possible to preselect low pressures, high
pressures, ratios of high to low pressure, ratios of high pres-
sure time to low pressure time, and low and high pressure volu-
metric flows through the nozzles for varying drum diameters,
lengths and speeds, for varying media apertures and open area
ratios, and for effluents varying in the quality of their prior
treatment, influent concentration of suspended solids, and dis-
solved contaminants.
For drums of increasing diameter, generally increased
screen capacities are typically realized. However, the increased
capacity is usually less than directly proportional to the
increase in diameter. Under those circumstances somewhat higher
pressures and flow rates can ~e selected in both low and high
pressure segments, and vice versa. Where higher flows and pres-
sures are selected the ratio of high pressure to low pressure
and the ratio of time at high pressure to time at low pressure
; required may remain substantially the same.
When using screens longer in the longitudinal direc-
~- 20 tion, one normally can obtain an increase in capacity which is
typically proportional to the increase in length, when the flow
rates of shower water at both high and low pressure are corres-
pondingly increased. Thus, pressure and time requirements are
normally substantially independent of screen length.
. ~ . , .
Starting at a low peripheral speed and gradually in-
`~ creasing same, one can readily determine an optimum speed which
: ~f
produces maximum filtration capacity. As the peripheral speed
increases towards this optimum value, modest lncreases in the
- low pressure level and/or flow rate may be ound helpful in ob-
taining best results. Further increases in drum speed beyond
- 18 -
104;~3S8
this optimum value may however result in a decrease in screen
capacity.
Other things being egual, when one selects a filter
medium with a larger aperture size one typically obtains in-
creased screen capacity. However, such increase is apparently
less than proportional to the increase in aperature size,
providing the preponderance of particles in suspension are
larger than the aperature selected. Both high and low pressure
and/or flow rates may be somewhat reduced even if the ratio of
high to low pressure and high pressure time to low pressure
time remain the same.
; When one selects a medium having an increased ratio
of open area to total area, increased screen capacity may be
obtained. This increase may in some instances approach propor-
tionality with the increase in the open area ratio. Again, both
~high and low pressure and/or flow rates may be somewhat reduced
even if the ratio of high to low pressure and high pressure time
to low pressure time remain the same.
The degree of treatment of the waste stream prior j
to screening or the concentration and type of suspended and
' ~r ~ dissolved materials it contains should be taken into account.
~= A poorly treated influent to the screen may contain more dissolved
and semi-colloidal tenacious slime-forming contaminants that
tend to foul the screen and may in turn require greater flow
~'t ' or pressure or time or a combination thereof, particularly in
- .
`- the high pressure phase of the cycle. Higher concentrations of
- . :,i
solids generally indicate a need for somewhat higher low pres-
sure flow or pressure or a combination thereof
:
Operating the screen at lncreased differential head
will generally result in increAso in flow through the screen
.
,-
: -- 19 --
104Z358
which is typically less than proportional to the increase in
head. High and low pressure and flow requirements may be some-
what increased thereby. The ratio of time at high and low pres-
sure need not necessarily be altered.
The variables of time on, pressure, and volume rate
of the high pressure mode have an interrelationship within limits
by which one can produce essentially the same degree of effective-
ness of cleaning by making offsetting changes in some or all of
these variables. For instance, during the high pressure segment
illustrated in the example, the time on can be extended in connec-
tion with the use of either lower pressure or lower volume rate.
The time on can be shortened somewhat at either high pressure
or volume rate.
By way of illustrating the actual operation of a
particular ~embodiment in accordance with this invention, and
once again with reference to Figures 1-8, the process is initiated
by pumping liquid containing solids to be filtered into inlet
51 (Figure S) through divider-overflow channel S3 which connects
with conduit 11 and thus fills drum 13 to a level as indicated
. .
20~ in Figures 4 and 6.
The filtration process is then initiated by conventional
switch means which start motor 33 (Figures 1 and 2) turning
gears 37 and 39 to produce the desired peripheral speed of drum
---` 13.
- ~ At a given flow, peripheral speed, type and concentration
~ ,., ~ . . .
--~ of suspended material in the influent, substantially constant
level is inherently maintained within drum 13 and an excellent
and efficient flow of purified liquid may be filtered from the
~-- drum via weir 45, spill tank 47 and conduit 49 (Figures 1 and
4) to further processing or to disposal. On the other hand, it is
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also contemplated by this invention (although it is not express-
ly illustrated) that conventional pressure sensing means can
optionally be provided both within reservoir 3 and drum 13 so
as to measure differenkial pressure therebetween, and should
such a differential depart from a prescribed value, either slow
down or speed up the drum so as to decrease or increase the fil-
tration rate to re-establish the prescribed pressure difference.
As can be seen best by way of reference Figures 1
and 4, as filter drum 13 rotates in the direction of the arrow,
it lifts the solids 80 on filter screen 43. Water then filters
through the screen and runs down the outside of the screen into
tank 3. The solids 80 adhering to the inside of screen 43 are
caused to approach the apex of rotation X by further rotation of
screen 43. At point X, fluid pumped via pump 27 (Figure 2) from
tank 3 via conduits 29 and 23 to nozzles 25 is caused to impinge
upon the solids layer, dislodging it from the screen downwardly
into sludge collector 15.
~` The ~purified" water collected in tank 3 fills to
., .
- a level slightly above weir 45 and thus spills thereover for
outlet purposes. On the other hand, the spray flush water,
.'!1~ kept to a minimum in accordance with the teachings in this
~` invention and mixed with the solids entering collector 15, are
~``-` ` caused to flow downwardly through inclined trough 17 ~see Figure
--~i` 3) and out conduit 19 to a sludge collection tank.
'. ~
During this operation, timer means 65 has been opera-
ting in accordance with a preselected schedule so as to provide
- the desired pressures in the desired sequence at nozzle 25 by
~- varying the speed of pump 27.
As can be seen~ the filtering system~ of this invention
can be employed for many purposes, the particularly preferred
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one, i.e. filterihg water either from sewage or from partially
; treated sewage, being described hereinabove. Once given the
above disclosure many other features, modiflcations, and improve-
ments will become apparent to the s~illed artisan. Thus, such
~` other features, modications, and improvements are considered to
~; be a part of this invention and should be considered in constru-
ing the following claims:
~,
~ ~ .
~: :
,~
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