Note: Descriptions are shown in the official language in which they were submitted.
~1~9~3~5
SPECIFICATION
This invention relates to a sewage or water treatment
system. The invention is directed particularly to improvements
in rapid sludge dewatering or sludge reduction beds associated ;
with such systems, and progressive removal of sludge cake
as a continuing operation by mechanical means.
BACKGROUMD OF THE ART
In the past, technological advances in the treatment
of sewage have been directed for the most part to the various
processes utilized in the reduction of raw sewage, such
as aeration, settling coagulation, chemical precipitation
of metallic ions, etc. The end products of all such sewage
from water treatment facilities, however, are a clear effluent
and waste sludge. This invention is directed particularly
to novel and innovative means fPr the handling, processing,
dewatering and disposal of such sludge in an efficient and
economical manner.
The first requirement in sludge handling is to reduce
moisture, and to increase handling ability by reducing volume.
The process is usually referred to as concentration, thickening,
; dewatering or drying, according to the amount of moisture
being removed. Different sewage and water treatment processes
yield sludges of different solids concentrations ranging
approximately from 2 to 20 percent. Further moisture reduction
may be desirable to 25~ solids for some types of mechanical
handling. Higher solids content may be desirable for specific
uses or disposal methods.
Existing sludge dewatering equipment falls primarily
into two categories (1) simple and inexpensive, but slow,
sand drying beds; and (2) fast, but expensive, highly mechanized
~k
9975
devices such as presses, vacuum filters, centrifuges, heat dryers and
incinerators.
Because of the increase in the separation of both free and bound
water from the sludge solids effected by the instant invention, a 200%
savings is realized in the area of land requircd for the dewatering pro-
cess, and further savings in the operating and maintenance costs of
sludge processing and utilization are experienced. No other sludge
dewatering system is more cost effective and efficient in continued
operation.
Sand drying beds require up to seven weeks to achieve cake
concentrations of 25 to 40 percent. The slowness of this method requires
bed areas ranging from 1 to 3 square feet per capita of population
served, depending on sludge quality and climate conditions. The extensive
land requirements of such systems have commonly forced plants serving more
than 30,000 persons into mechanica~ equipment. Sand beds are, however,
used by 38 percent of cities serving populations over 100,000.
The disadvantages of mechanical equipment stem from its high
initial cost, high maintenance and operational costs, frequent breakdown
problems and large energy consumption.
SUMMARY OF T~lE INVENTION
According to one aspect of the invention; there is provided
a process for dewatering sludge on a filter plate wherein the improvement
comprises: (a) where required, pretreating the sludge with a chemical
conditioner to form soft masses of various sizes including larger and
smaller sludge masses; (b) passing the sludge onto a filter plate
characterized by a high volume percolation rate and having an upper
surface comprised of an aluminum oxide aggregate which is
.~
~9~75
in turn comprised o aluminum oxide particles having sharp, llpwardly
extending points, which particles are bonded together with a binder;
(c) permitting the larger sludge masses to settle by force of gravity so
that they are pierced by the sharp, upwardly ex~ending points of the
aggregate particles on the upper surface of the filter plates and form
a layer over the top of the filter plate; (d) applying a vacuum of
sufficient strength to draw filtra~e from the sludge through the layer
of larger sludge masses on the surface of the filter plate and through
the filter plate, but not so strong as to break down the larger sludge
masses, such that the smaller sludge masses are trapped by the already
settled larger sludge masses, until the sludge is dewatered to an extent
making it removable by mobile disposal means; and (e) removing the
dewatered sludge from the surface of the filter plate by mobile disposal
means; wherein the process is characterized by rapid dewatering and sub-
stantial self-filtering action.
According to another aspect of the invention there is provided
a multi-layered filter plate for sludge dewatering comprising: (a) a
thin top layer having a smooth, hard upper surface and a lower surface and
consisting essentially of individually sharp pointed fine aluminum oxide
particles, rigidified and ~hen bonded together with a binding agent
which does r.ot impede high volume percolation; (b) a middle levelling layer
having upper and lower surfaces and composed of fine aggregate particles,
rigidified and then bonded together and to the lower surface of the top
layer with a binding agent which does not impede high volume percolation;
and (c) a bottom support layer composed of relatively coarse aggregate
particles, rigidified and then bonded together and to the lower surface of
the middle layer with a binding agent which does not impede high volume
percolation, which filter plate is monolithic and s~ructurally rugged enough
~?
~1~9~75
to support mobile sludge remoyal means on the upper surface of the top
layer of the filter plate without damage to the filter plate.
It should be understood that this disclosure emphasizes
certain specific embodiments and that all modifications or alternatives
-- equivalent thereto are within the spirit or scope of the invention as
set forth in the appended claims.
DESCRIPTION OF THE FIGURES
Figure 1 is a perspective view of a sludge reduction or
; dewatering bed embodying the invention;
Figure 2 is a top plan view;
Figure 3 is a longitudinal cross-sectional view, taken along
the line 3-3 of Figure 2 in the direction of the arrows;
Figure 4 is a transverse cross-sectional view taken along
the line 4-4 of Figure 2 in the direction of the arrows;
Figure 4A is a view in detail of the emersion heater steam
supply assembly;
Figure 5 is a perspective view similar to that of Pigure 1 but
further illustrating the use of a transparent canopy for keeping out rain, "
high humidity, snow etc., and for capturing solar heat and retaining it;
Figure 6 is a longitudinal cross-sectional view similar to
that of Figure 3 but illustrating an alternative support and drainage
structure for the modular Ullit filter plates;
Pigure 7 is a schematic diagram of the sludge reduction system
illustrating its interrelation with a typical sewage treatment plant;
3~75
FIGURE 8 is a side perspective view of a representative
pair of sludge reduction beds e~uipped for electroosmosis;
FIGURE 9 is an elevational view of the control panel;
FIGURE 10 iS a cross-sectional view of one of the beds
of FIGURE 8;
FIGURE 11 is an enlarged sectional view on the circular
line 11 of FIGURE 10.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Described below is an embodiment of the inventive apparatus
chosen for purposes of illustration as the environment for
the practice of the inventive method.
There is a typical 2-unit sludge reduction bed 10.
A first unit 11 is shown in its entirety and a second unit
is only partially illustrated. As illustrated in FIGURES
1 through 4, each bed unit 11 comprises a shallow, open-
topped, box-like sludge container for receiving sludge to
be chemically pretreated and processed. The sludge container
is integrally formed of monolithically poured, reinforced
concrete, or alternatively, it could be fabricated of other
structural materials such as fiberglass, concrete blocks,
steel, etc. The container comprises a bottom slab portion
12, upstanding sidewall portions 13 and 14, and upstanding
front and back wall portions 15 and 16, respectively. The
internal dimensions of each sludge container unit 11 may,
for example, be 20 x 40 feet to provide in one form of the
invention for the close fitting assembly therein of a plurality
of modular filter plates 17 which may conveniently be 2
ft. x 4 ft. in size, for example. The sludge containers
are made impervious by float finishing or by the use of
concrete additives, or a suitable sealer, thereby preventing
pollution of the subsoil by leakage of the filtrate.
~1~9~ 5
The plates 17 or monolithic poured slab serves as the
filtering media in the sludge dewatering or reduction process,
and is sufficiently strong to withstand pressures imposed
by mechanized dried sludge handling equipment which will
be driven over the bed from time to time for the removal
of the sludge cake at the end of the rapid sludge dewatering
process.
The above described rigid filter plate is reinforced
with inert glass fibers providing the required strength
or durability during daily mechanical unloading. The filter
plate is preferably installed over a mass 60 of large size
clean aggregate (3/4" to 1 1/2") which is keyed together
by a vibratory compactor during construction. After keying,
the aggrega~e is given a light coating of sprayed-on epoxy
to prevent movement of the aggregate during daily unloading
of the dewatered and dried sludge cake by mechanical means.
Above the aggregate 60 is a leveling layer 60' of 6-10 sand,
alqo keyed and sprayed with epoxy. The aggregate and leveling
layer thu~ installed to a depth of 12" to 24" inside the
outer impervious shell provides a void area of approximately
40% where the vacuum is uniformly applied over the whole
poured filter media underdrive. This void area also provides
a reservoir for the water filtrate during the filling-up
operation. Should a power failure occur, the same area
serves to collect all the filtrate.
The bottom slab portion 12 of the sludge container
unit 11 in the alternative may be integrally formed with
a plurality of equidistantly-spaced, parallel longitudinally-
extending ribs 18, said ribs being approximately square
in cross-section and having upper surface portions lying
in the same horizontal plane. The ribs 18 are so spaced
~ 1~9~75
as to support marginal longitudinal underside portions of
adjacent rows of the filter plates 17.
As illustrated in FIGURES 3 and 4 the upstanding sidewall
portions 15 and 16 are each formed with inwardly directed
ledges 19 defining shoulders at the same height as that
of the ribs 18 for the support of outer marginal edge portions,
at the underside of the peripherally positioned filter plates
17. Thus, as illustrated in FIGURE 4, each of the longitudinally-
extending fluid and gas flow chambers serve as a through
passage for sludge filtrate, for backwash, or for vacuum
voiding, as is hereinafter more particularly described.
As further illustrated in FIGURES 3 and 4, the upper
surface of the bottom slab portion 12 of the sludge container
11 is sloped downwardly from back to front so as to provide
-; for gravity drainage of sludge filtrate or processing fluids
into a transversely-extending ~anifold 22 formed within
the ~ludge container.
Drainage of the sludge container 11 is provided for
by a manifold fluid conduit 23 communicating with the bottom
of the manifold 22 and extending outwardly of the upstanding
sidewall 13. There is a gas conduit 24 also extending through
the sidewall 13 at the top of the manifold. The manifold
and conduits 23 and 24 extend through individual electric
control valves 25 and 26 to join at a common inlet conduit
27 leading to a sump pump 28. The output of the sump pump
28 i8 fed through conduits 29 and 30 to sludge filtrate
return line 31 and chemical solution return line 32, respectively,
as controlled by respective control valves 33 and 34, as
shown in FIGURES 1 and 2. As many as four or more rapid
sludge dewatering units, if desired, may be connected in
a basic design to one central vacuum reservoir and effluent
~ 9~'75
tank by means of cross-connecting lines and control valves,
thereby conserving material and capital investment.
Means is provided for filling one or the other or both
of the side-by-side sludge container units 11, lla, comprising
the two-unit sludge reduction bed 10, with fluid waste sludge.
To this end, adjacent zones of the backwall portion 16 of
the side-by-side sludge containers 11, lla are joined by
a rearwardly-extending sludge-receiving filling and mixing
box 39. Slide gates 40 and 40a in respective backwall portions
16 and 16a permit selective passage of liquid sludge fed
into the filling and mixing box 39 into one or the other
or both of the sludge containers 11 and lla selectively,
depending upon the volume of sludge being fed into the beds
for treatment. A comparatively large diameter conduit 41
discharges through a manually controlled valve 42 into the
filling and mixing box 39, said sludge being pumped from
i a typical sewage or water treatment facility for disposal.
Although not shown in the drawings, an important factor
in the practice of the method herein disclosed is a conventional
mobile mechanized vehicle capable of entering on the filter
plate, there to be loaded with sludge cake for removal.
Such mobile units are commercially available and if not
already equipped with a scraper and lifting mechanism, accessories
of such description can be installed.
Mobile mechanized units most advantageous for use in
the system here disclosed are four wheel vehicles of sufficient
length and breadth to carry the operator and an appreciable
load of sludge cake. With wheels providing traction, the
operator can mount and drive to and from a loading location
on the filter plate. It is of consequence, therefore, that
the length and breadth of each sludge container 11, lla
be as large as described to allow the mobile unit to move
375
about while the sludge cake is being scraped clear from
the top surface of the filter plate, and ultimately loaded
on the mobile unit for transportation elsewhere.
The nature of the mechanized units is clearly such
that care must be taken in the construction of the ~ilter
plate and its support to carry the load of the mobile mechanized
unit and facilitate repeated cleaning and scraping cycles
for removal of sludge cake when dried, while avoiding damage
to the filter plate's upper surface.
Clearly, because of the semifluid character of the
sludge, side, front and back wall portions 13, 14, 15 and
16 must provide a liquid-tight container while the dewatering
of the sludge is taking place. To accommodate entrance
and exit of the mobile mechanized units, a slide gate, such
as gate 43, is provided in the back wall portion 16 of the
sludge container unit 11. A similar gate (not shown) is
provided for the back wall 16a of the sludge container unit
lla and for each of any additional sludge container units
which may make up an installation. Accordingly the gate
must have a liquid tight fit when closed, and provide an
opening wide enough to accommodate a loaded mobile mechanized
unit such, for example, as a front end loader.
Optional means is provided for simultaneously heating
and moving sludge in the sludge container units 11 and lla
during the dewatering process. This system will provide
for a 7% solid sludge cake content thereby facilitating
prolongation of the drying process to achieve a drier condition.
In other words, by closing the cracks in the sludge as it
starts to crack at 7~, the sludge heating and moving means
serve to extend the dewatering process, thus producing drier
sludge cake in a shorter time.
S
More specifically, there is a transverse support bar
44 which straddles the upstanding sidewall portions 13 and
14 of each of the sludge container units 11 and lla. The
ends of the support bar 44 carry roller bearings 45 which
ride along the upper surfaces of the sludge container sidewall
portions 13 and 14 to minimize frictional resistance in
the movement of said support bar reciprocatively between
the front and backwall portions 15 and 16, respectively,
of the associated sludge container unit.
A continuous drive cable 46 is looped between an idler
pully 47 upstanding from and fixed with respect to the center
of the backwall portion 16 and the drive pulley 48 of a
reversible electric motor 49. The electric motor is fixed
with respect to and extends upwardly of a central portion
of the container front wall portion 15.
The lower loop portion or run of the drive cable 46
extends through and is attached to a central portion of
the transverse support bar 44, as indicated at 50 in FIGURE
3. The transverse support bar 44 carries a plurality, three
in the embodiment illustrated, of transversely-extending
emersion heater conduits 51a, 51b and 51c. The conduits
are po~itioned at various heights to extend at various levels
into the semifluid mass of sludge fed into a sludge container
11 for treatment.
As illustrated schematically in FIGURE 4a, a steam
generator 52 supplies superheated steam through the emersion
heater conduits 51a, 51b, and 51c, connected in series,
through flexible conduits 53 and 54. An electric energization
circuit (not shown) for the reversible drive motor 49 serves
to reciprocatively move the transverse support bar 44 and
its associated emersion heater conduits back and forth within
the sludge container units 11 and lla during the sludge
--10--
dewatering or drying process. ~lternatively, elongated
electric heating units could be used.
The sludge moving and heating means, comprising the
reciprocative transverse support bar 44 and its associated
emersion heater conduits 51a, 51b and 51c, serves primarily
as a device for uniformly moving the sludge to minimize
- any tendency to the formation of shrinkage cracks therein,
particularly during the vacuum drying process. The sludge,
being a gelatinous mass, is much more easily cut for mixing
by the heated conduits as compared with unheated conduits,
to which the sludge would have tendency to stick. The heat
imparted to the sludge being mixed also renders it more
fluid, particularly in cold weather, thereby further enhancing
the time efficiency of the vacuum drying process.
An important feature of the invention resides in the
composition of the filter plates 17. They must not only
exhibit the requisite porosity for rapid and efficient filtering,
be inert with respect to reaction with any of the various
caustic and corrosive chemicals commonly found in sludge
and sludge treatment, but must also withstand loads imposed
! by the mobile mechanical units employed to remove the dried
sludge cake. It has been established that a 3" thick filter
plate having the required strength, resistance to chemicals
and filtering or percolation rate is obtained with a mixture
of angular silica filter sand having a sieve size of between
6 and 10 millimeters used as the aggregate, and a bonding
agent of chatahoochie epoxy in the ratio of one pound of
the epoxy to 20 pounds of the aggregate.
An effective filter plate is shown in FIGURE 11 of
the drawings. The plate 17, whether individual plates or
a poured slab, is 1 1/2 to 2 inches thick and has a lower
permeable layer 17a of 3/4 inch to 1 1/2 inches of aggregate.
--11--
~9~7S
This is rigidified, meaning that the particles are worked
into a mass such that they are substantially all interlocked
with each other, to inhibit any further shifting or movement.
-~ The choice of agqregate is such, however, that the layer
-~ remains permeable and permits free flow of water withdrawn
during the dewatering cycle.
Above the lower permeable layer is a leveling layer
17b of about 1/4" thickness for which 6-20 sand is suitable.
This leveling layer 17b is also rigidified by working or
10 tamping similar to, and for the same purpose as, the lower
permeable layer, leaving it firm, and at the same time,
permeable.
Above the leveling layer is a layer 17c of sharp, 16
~; grit anthracite and/or aluminum oxide aggregate also rigidified
though permeable. Individual sharp points of the 16 grit
anthracite and/or aluminum oxide present a "sand paper"
;~ like character for the upper surface of the plate 17. Included
in all the layers is an appropriate binder such as epoxy,
f which, after hardening, renders the multi-layered filter
20 plate monolithic and physically strong to be heavy equipment
traffic rated as described.
The columnar structure 60, 60', whether gravel as shown
in FIGURE 11, or ribs, resting on the impermeable bottom
slab portion 12, substantially as has been already described,
leveled off by the layer 60' of 6-10 sand, supports the
filter plate 17 at a location within wall portions 13, 14,
15 and 16.
In a second form of the invention shown in FIGURE 5,
a transparent canopy 55 extends over the sludge container
30 11, fully enclosing the same, thereby serving not only as
a rain water shed, but also providing for capturing solar
heat to accelerate the sludge drying process. The canopy
-12-
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may also serve to contain heat generated by fuel-fired heaters.
Preferably, the canopy may be of a dark colored vinyl sheet
of the type commonly used in hothouses to create the so-
called greenhouse effect. The canopy also serves to contain
gaseous pollutants, often malodorous, which might otherwise
contaminate the surrounding atmosphere. These gaseous pollutants
can be recycled to the primary treatment facility from which
the sludge is obtained for sludge reduction, to be reabsorbed
in the fluid state. Solar heat collector panels provide
warm dry air to be pulled through while the vacuum below
is applying an accelerating dewatering assist.
By heating air in the space above the sludge bed, whether
by solar energy in the event a canopy is employed, or by
some other appropriate heater, multiple benefits result.
Warm air when drawn through the filter plate with the filtrate
appreciably accelerates drying of the sludge cake.
The drawing of warm air warms the semi-liquid sludge
and accelerates the coagulating of the mass which, at the
same time, accelerates the dewatering process, and, consequently,
the drying of residue.
Occasionally reversing air flow through the filter
plate has added benefits, namely loosening up the accumulation
of partially dewatered sludge, as well as enhancing the
dewatering cycle.
FIGURE 6 is a longitudinal cross-sectional view, similar
to that of FIGURE 3 but with portions broken away, illustrating
a simplified form of sludge container embodying the invention.
The structure there shown is one particularly well-suited
for installation on land area having high bearing strength
such as coral rock or naturally dense sand or sand and rock
mixtures. Under such conditions, as illustrated in FIGURE
6, the impervious concrete bottom slab can be eliminated
7S
and the upstanding sidewalls 13a and front and backwall
portions 15a and 16a can be integrally formed of reinforced
poured concrete with a peripheral footing 56.
As further illustrated in FIGURE 6 and as indicated
at 57, the interior bottom surface will be sloped for drainage
from back to front into a recessed, transversely-extending
manifold 58 corresponding with the manifold 22 of the embodiment
illustrated in FIGURES 1 through 4. A moisture impervious
:
layer 59 of 12 gauge Neoprene sheeting may cover the interior
of the sludge container thus formed. The two to four inch
thick layer of coarse clean gravel 60 on the bottom area
of the Neoprene sheeting 59 presents a horizontal upper
; bearing surface upon which the filter blocks 17 are placed,
as in the embodiment of the invention illustrated in FIGURES
1 through 4. Preferably, the gravel will be of a sieve
8i ze from 3/4 to 1 inch.
The spaces within the mass of gravel form a continuous
flow passage for filtrate which passes through the filter
plate. Accordingly the gravel serves a double purpose,
- 20 namely, that of establishing flow passages and that of acting
as a columnar support for the filter plates throughout the
area of the sludge container unit.
Instead of using modular plates 17, the filtering medium
can be a single monolithic slab (not shown) of the silica
sand and epoxy binding agent mixture. Gravel is effective
for use as a combined support and structure forming flow
passages. The monolithic slab may be supported by 3/4"
to 1 1/2" aggregate, providing for the approximately 40
void area, or by ribs like the ribs 18 of FIGURES 1 and
4. The use and operation of the invention illustrated in
FIGURE 6 is otherwise the same as the principal embodiment
of FIGURES 1 through 4.
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g7S
FIGURE 7 illustrates, schematically, a typical sewage
treatment plant and associated two-unit sludge reduction
system embodying the invention. The sewage treatment system
may comprise, for example, the usual raw sewage input surge
- tank 61 feeding successive aeration, settling and chlorine
contact tanks 62, 63 and 64, respectively, which serve to
reduce the solid matter to a substantially homogeneous sludge
discharged to a sludge holding tank 65.
Reagent for ion removal is supplied to the aeration
lQ tank 62 from a supply tan~ 66 through a pump 67. Effluent
is passed from the chlorine contact tank 64 to a tertiary
filter 68 for further removal of suspended solids, and the
filtered effluent is discharged through conduit 69 to a
drain field or other suitable means of disposal. The filtered
solids are returned through conduit 70 to the surge tank
61 for reprocessing and eventual disposal in the sludge.
Supply water is fed through conduit 71 and control
valve 72 into a chlorinator 73 discharging chlorine in solution
into the chlorine contact tank 64 for treatment of the effluent
therein. The water supply line 71 also serves to feed chemicals
in solution to the sludge container units 11, lla in the
sludge treatment process.
In operation of the sludge dewatering or reduction
system in the embodiment illustrated in FIGURES 1 through
4 and 7, fluid sludge to be treated is pumped through a
sludge pump 74, conduit 41 and control valve 42 into the
; common sludge filling and mixing box 39, wherein it will
mix to a more or less homogeneous mass, then allowed to
flow into one or the other or both of the sludge container
units 11 and lla by appropriately opening the sludge gates
40 and 40a respectively. The sludge container units 11
and lla will ~e filled to a depth of approximately 12 inches,
--15--
s
whereupon the liquid fraction of sludge will begin to percolate
through the filter plates 17 and drain into the manifold
22 at the front of the unit. This separated filtrate will
ordinarily be returned by the sump pumps 28 to the surge
tank 61 through appropriately valved conduits 75 for re-
treatment (See FIGURB 7).
Various forms of treatment of the sludge in the sludge
reduction units is provided for, depending upon the quality
of the sludge to be treated. For exmple, washing with clean
water or elutriation may be required of sludges high in
soluble inhibitors of coagulation. To this end, supply
water fed through conduit 71 and passing through chemical
feed tank 76 is pumped into backwash chemical feed line
35 by backwash pump 77. Sludges rich in greases may require
lime conditioning. Industrial waste sludges may require
other chemical conditionings which can be supplied as a
backwash solution by adding appropriate chemicals to the
chemical feed tank 76. Alternatively, chemical treatment
solutions can be added directly to the sludge by passing
supply water solvent through a chemical feed tank 78 and
pump 79 to conduit 80 and 80a discharging into the sludge
container units 11 and lla respectively.
It is also to be understood that cracking of the sludge
cake during vacuum drying can also be inhibited by the use
of chemical coagulants added to the sludge at the filling
pump.
In following a cyclical pattern for adjacent sludge
beds the process includes pretreating raw sewage with a
polymer coagulant to break up solids and flocculate the
sludge particles. Numerous tests dictate that the sludge
be treated to create large soft particles and soft particles
of various molecular and sieve sizes. The larger particles
-16-
gain weight and then being heavy are settled first. Smaller
finer particles are settled thereafter and are trapped by
the already settled larger particles. Settling of the particles
is assisted by application of a vacuum beneath the filter
plate.
The added coagulant, by flocculating the sludge, causes
formation of soft particles or masses of various sizes.
As stated, the larger particles tend to settle first by
gravity action and spread a gel-like slime layer over the
top of the filter plate. Free water drains rapidly through
the relatively large porous spaces of the initially formed
layer, and through the filter plate. This initially formed
layer at the same time traps the smaller particles which
continue to settle out.
When a vacuum is applied as a next step, a fast liquid
filtrate flow is induced through the porous sludge cake
and the filter plate, rapidly dewatering the sludge.
The choice of anthracite and/or aluminum oxide for
the top layer of the monolithic filter plate is of significance.
Anthracite and/or aluminum oxide provides sharp, upwardly
extending points and these tend to pierce the material of
the larger particles in the slime layer adding to the speed
and completeness of the dewatering.
A circumstance of some note is that a vacuum applied
quickly after formation of the flocculated slime expedites
formation of the cake. This is directly opposite to the
cake formation in a standard vacuum filter, in which the
smaller particles are picked up first in the cake or, at
best, a homogeneous sludge cake is achieved thus forming
a sludge cake which is more permeable and harder to dewater.
Wide latitude is possible in strength of the vacuum applied
which may vary with circumstances between 1 and 27 inches.
r~
97S
The sludge cake will continue to dry while water is pulled
from the floc. This differential type of cake formation
- produces a thicker cake and delays formation of cracks,
permitting the maintenance of the vacuum for a longer period.
In many cases, this is as far as the dewatering need go ~;
for quick and economical disposal since 75 to 80% of the
-- water will have been removed. A final optional stage can
~ be the further dewatering of the sludge cake by during the
adherent water by hot air pulled by a vacuum with or without
~ 10 mechanical raking.
; Pneumatic lines 36 extend from the sludge container
sumps to a reversible pneumatic system 81 which, when operated
in its vacuum mode, provides a substantially reduced pressure
beneath the filter plates 17, thereby greatly accelerating
the dewatering process. An added benefit of applied vacuum
will be the suctioning for in-plant disposal of any undesirable
gases and odors. This vacuum drawn filtrate will normally
be returned to the sewage treatment facility surge tank
61 for reprocessing along with the gravity filtered or percolated
filtrate.
In FIGURE 8 is shown a pair of sludge containers 11'
connected in tandem utilizing common sidewall 13'. In other
respects the containers are identical each comprising, as
shown, a second side wall 14' with front wall and back wall
portions 15' and 16', respectively, surrounding an impervious
bottom slab 12'. The customary monolithic filter plate
17 is here shown extending over the entire upper surface
of a level, rigidified supporting course 60 of aggregate.
Of special consequence in this form of the invention
is the presence of an anode 85 and cathode 86. Where the
side walls of the container 11' are to be about 12~ high
above the filter plate the anode is fastened to the wall
-18-
structure about 4" above the top level of the filter plate.
The anode is preferably copper and located so as to be continuously
immersed in the semi-liquid sludge.
This anode extends around all four side walls.
The cathode 86, likewise extending around all four
side walls, is located at the bottom of the body of semi-
liquid sludge, preferably at and touching the monolithic
filter plate itself. The material of the cathode is non-
ferrous and a material other than copper. Carbon is found
to be satisfactory. An appropriate non-ferrous metal may
be, for example, zinc or aluminum.
The anode and cathode are not connected to any source
of electric power but serve, instead, as anode and cathode
in a bath of electrolite, as for example, the semi-liquid
sludge. In this arrangement the electrodes establish an
electroosmosis of the sludge causing the solid sludge particles
to flow to the cathode and the water molecules to flow to
the anode. The flow of water thus induced accelerates the
speed of dewatering and, in fact, significantly contributes
to the increased speed and completeness of dewatering and
formation of sludge cake. Details of construction of the
electrodes have not been included in the disclosure inasmuch
as such details may vary considerably without departing
from the scope of the invention.
For convenience there is shown a control panel with
appropriate instrumentation 87 which hàs controls to motivate
the sundry mechanical means such as the coagulant metering
pump 88, blower control 89, valves for the various lines
and the sump pump and vacuum chambers 91.
Following dewatering, isotope irradiation or addition
of a chlorine solution may be employed to disinfect and
~ ~9!375
deodorize the reduced sludge. This can be e~fected by supplying
chlorine to the chemical feed tanks 76, 78 for backwash
or forwardwash. An appropriately valved chlorine solution
return line 82 provides for recycling the chlorine solution
through the sludge as pumped by sump pumps 28 for efficient
use of the chlorine. It will be understood that the various
~ above described filtration, backwash and chemical treatment
- steps, both liquid and gaseous, in the sludge reduction
process can be programmed for automatic operation, with
selective variations depending upon the quality of the sludge
to be dewatered or reduced.
i Among the advantages of the sludge reduction dewatering
system embodying the invention are the following:
1. Sludge having a dry solids content of 2~ can be
dewatered in approximately five hours sufficiently
to be removed from the sludge containers mechanically
and transferred by open truck to a disposal site
without difficulty or loss.
2. Sludge can be dewatered to 15% dry solids in approximately
eight hours.
3. Recapture of approximately 99.5% of the solids
in the sludge reduction process.
4. Chemical disinfecting of the dry sludge is easily
effected.
5. Disinfection by isotope irradiation.
6. At least 40 times less land area is required as
compared with sludge dewatering systems heretofore
employed.
7. Substantial reduction in odor pollution of the
surrounding atmosphere.
,~ ,
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