Note: Descriptions are shown in the official language in which they were submitted.
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DBS J ~ NY9 180
DIG~STER INCL~DING S DG~ R~CIRC~LATOR
NIT~ GAS_SI~PLE~ENT
BACKGROUND OF THE INVENTION
Field of the Invention
'rhe invention re~ates to anaerobic sludge dige~ters, and more
particularly to ~ludye dige3ters including 8y~tem8 for mixing the
sludge within the digester tank to improve dige~tion and to prevent
the unwanted accumulation of solid material in the digester.
Reference to Prior Art
Anaerobic digestion systems are commonly used to
bacteriologically treat waste such as sewage sludge before the
byproducts of the trea~ment are discharged into the environment or
otherwise disposed of. A typical digestion sy~tem includes a large
digester tank for containing the sludge during digestion and piping
for adding raw sludge to the tank and for removing partially
digested sludge from the tank.
It is well known that to improve dige~tion, the digester
contents must be periodically mixed or blended to reduce the
settling and accumulation of ~olids in normally quiescPnt area~
such as on the bottom of the digester tank. Mixing the ~ludge al~s
distributes the bacteria to insure active digestion vf the sludge,
maintains uniform chemical and thermal cond~ t~ on~ within the
digester, and disperses toxic material entering the system to
minimize its effect on the digestion process. A known digester
mixing system for hydraulically mixing the contents of the digester
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D~ DU/E)lY918û
~ank i8 provided in V~S. Patent No. 4/092,338 is~ued May 30, 1978
to Tossey. This mixing system includes a network of pipe~ spread
out over the base of the tank, each pipe leading to a ~eparate
di6charge point in a different region of the digester tank. Sludge
undergoing digestion is withdrawn from the digester tank and pumped
through a slide val~e which sequentially delivers the recirculated
sludge to sel~cted ones of the various pipe~ to mix ~ludge from
less active regions of the digester tank into other region~.
It is ~lso well known to introduce a gas into the digester
tank to cause turbulence at the surface of the sludge to reduce the
accumulation of scum on the surace. A known gas mixing system is
produced by Envirex, Inc. of Waukesha, Wisconsin, and is known as
a Pearth ~as mixing system. In this system, methane ga~ collected
from a gas dome on the cover of the digester tank i5 compressed and
returned to the tank through a series of discharge lances. The
discharge lances are spaced apart and extend downwardly from the
cover into the sludge to a desired depth. A multi-port rotary
valve between the compres~or and the lances causes the gas to be
sequentially discharged through the lance~.
5UMMARY OF T~E_INVENTION
The present inYent~on provides a digester apparatus
incorporating an improved mixing system in which sludge and gas
streams are combined into a single stream before being di~charged
into the digester tank through a common distribution network. The
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DU/IDI~/~NV910~
distribution network includes a ~ingle ~equencing mech~nism through
which the mixture of sludge and gas i6 sequen~ially delivered to
only one d.ischarge point ak a time.
The prior art hydraulic mixing systams employed to mix sludge
in anaerobic dige~ters are inadequate to provide sufficient
~urbulence at the surface of the ~ludge to avoid ~cum build-up~
Redirecting some of the di~charge points of the hydraulic mixing
system to regions ad~acent the sludge surface would not provide
adequate coverage of the ~urface without reducing mixing bene~its
to other parts of the digester to unacceptable level$. Similarly,
systems for agitating the surface of the sludge to reduce scum
accumulation are inadequate to accompli~h sufficient mixing
throu~hout the digester ank. For these reasons, ~ome prior art
digesters have required the u~e o~ separate hydraulic and gas
mi~ing sy~tems, each mixing system including it~ own recirculation
ox distribution network and ~equencirlg mechanism~ The mixin~
system of the present invention eliminates the need for separate
systems by achiev.ing in a single, integr~ted syst~m the benefits of
both hydraulic mixing and gas mixing. Thi~ reduce~ thQ size,
compiexity and number of parts needed to accomplish successful
mixing of the entire contents of the di~ester tank.
More specifically, the invention provide~ a sewage ~ludge
digestion apparatus includinq a tank adapted to contain sewage
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sludge and wherein the sewage sludge i~ digested by bacteria, and
a plurality of pipes within the tank. The pipes have respectiv2
effluent discharge ends which are ~paced apart within the tank.
The digestion apparatus al50 includes means for pumping partially
digested sludge derived from the tank back into the tank to provide
fer mixing and agitation of the sludge contained in the tank to
promote biologi~al activity in the tanX by the bactPria ~ and means
for supplying partially digested ludge from the tan~ to the means
for pumping. To di~tribute the partially d~gested sludg2
sequentially to at least one selected pipe at a time, valve means
in communication with the means for pumping is provided. Mean~ for
supplying gas under pressure to the partially digested ~ludge being
supplied to the valve means is also provided so that the partially
digested sludge and gas are delivered together sequentially to the
selected ones of the pipes.
The invention also provide~ ~ ~ewage Yludge dige~tion
apparatus including a tank adapted to contain sewage sludge and
wherein the sewag~ sludqe Ls digested by bacteria, a manifold
including a plurality of efluent ports, and a plurality of pipes
within ~he tank, each of ~he pipe~ communicating with the manifold
through one of the effluent ports. ~ach of the pipes has an
effluent discharge end, the effluen~ discharge end~ o the pipes
being fipaced apart within the tank. Means are provided for
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D~SI~i~/E)lV91aO
supplying Gludge to the manifold to di~tribute the sludge to the
pipes for introduction in~o the tank. Means for ~upplying gas
under pressure to the sludge being supplied to ~he manifold is also
provided ~o that the gas under pressure i8 mixed with th~ sludge
and travels together with the sludge through the pipe~ for
introduction into the tank.
The invention further provides a sewage sludge d~ge~tion
apparatus including a tank adapted to contain sewage ~ludge and
wherein the sewage sludge i~ digested by bacteria, and a cover on
the tank and including a gas collection reservoir. The digestion
apparatus al80 includes a ~ludge ~upply assembly including means
for pumping partially digested sludge derived from the tank back
into the tank to provide for mixing and agitation of the ~ludge
contained in the tank to promote biological acti~ity in the tank by
the bacteria. The sludge supply asnembly also includes means for
supplying partially digested 81udge from the tank to the means for
pumping, the means for supplying including a ~ludge recirculation
conduit extending from the tank. To redistribute the sludge
supplied by the sludge 6upply assembly, the diges~ion apparatus
includes a distribution network including a plural~ty of pipes
within the tanX, Each of the pipes has a discharge end, the
discharge end~ of the pipes being spaced apart within the tank.
The distribution network also includes valve means in colmnunica~ion
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D~SIID111D1~9~80
with the means for pumping for distributing the partially digested
~ludge sequentially to a~ leaR~ one selected pipe at a time. The
val~re means incl udes a llousing having an inf luent end portion and
a manifold portion, the manifold portion including a plurality of
effluent ports. ~ach of the plpes communicates with the manifold
portion through one of the effluent ports. The valve means also
includes a valve member housed in the housing for movement relative
thereto to open an~ close selected ones of the effluent ports to
facilitate the sequen ial distribution of the partially digested
slud~e to the selected ones of the pipe~ at a time. The
distribution network also operates to deliver gas to the pipes to
agitate the surface of the sludge ln the tank. Thus, means are
provided for ~upplying ~as under pres~ure to the partially digested
~ludge bein~ supplied to the valve means ~uch that par~ially
digested sludge and gas are deliver~d together sequentially to the
selected ones of the pipes. The means for supplying gas under
pressure includes a gas conduit communicating between the valve
means and the gas collection reservoir and a compressor disposed
within the gas conduit.
Other features and advantages of the invention are ~et foxth
in the following description, claims and drawinqs.
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D~S/I~ 7V9180
BRIEF DESCRIPTIQN OF THE DRA~INGS
FIG. 1 is a partially diagrammatic side view, partially in
seetion, of an anaerobic sludge digestion apparatus embodying
various features of the invention.
FIG. 2 is a view taken along line 2-2 in FIG. 1, and ~hows in
top elevation the distribution network of the dige~tion By~tem~
FIG. 3 i~ a ViPW taken along linP 3-3 in FIG. 1, and ~hows,
partially in section, the mechanism for sequentially delivering a
sludge and ga6 mixture to the various pip~s of the distribution
network.
Before one embodiment of the invention i5 explained in detail,
it i8 to be understood that ~he invention i~ not limited to the
details of construction and the arrangement of component~ set forth
in the following de~cription or illustrated in the drawings. The
invention is capable of other embodiments and of being practiced or
carried out in various ways. Also, it .i~ to b~ understood that the
phxaseology and terminology u~ed herein is for the purpo~e of
description and should not be regarded as limiting.
GENERAL DESCRIPl'ION
Illustrated in the drawingY i~ an anaerobic sludge digestion
apparatus 10 embodying various features of the invention. The
dige~tion apparatus 10 forms part of a wastewater treatment
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D~S /I~DU/ ENV9 ~ ~0
facility and function~ to contain organic ~olids or ~ewage sludge
until the sludge i~ adequately digested by bacteria. Raw sludge is
received in the digestion apparatus 10 after being separated from
sewage water in a~other part of the treatment facility. The water
is processed in other biological treatment steps and returned to
the environment while the sludge is stabilized in the digestion
apparatu~ 10 before also being returned to the environment or
oth~rwise di~posed of.
As shown in FIG. 1 t the dige~tion apparatus 10 includes a
digester tank 12 for containing the sludge while it i~ digested by
the bacteria. The d;gester tank 12 includes a cylindrical wall 14
and a floor 16 which i8 generally conically ~haped and which slopes
downwardly toward a central sludqe collection point or trough 18.
The size of the digester tank 12 can be varied to suit the
requirements of the installation, bu~ typically on the order of
100 feet in diameter and 30 feet high. The digester tank 12 is
closed by a floating cover 20 including a centrally located ga~
dome 22 defining a ~as collection reservoir 24 in which gases that
evolve as a b~product of the biological digestion proce~s are
collected. Such gases include methane and carbon dioxide, amonQ
others. Excess gas in the collection reservoir 24 can, if desired,
be vented to the atmosphere or otherwise disposed of.
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~BS/~ /~91~0
The digest~r apparatus 10 al~o incll~des a sludge distribution
or mixing sys~em for mixing or blending the ~ludge within the
digester tank 12. As more fully e~plained below, the mixing system
incorporates both a hydraulic mixing component for mixing the
sludge in the dlgester tank 12 and a ga mixing component for
agitating the surface of the sludge~
M~re specifically, the sludge mixing system 26 includes a
sludge supply system or assembly 28 for supplying ~ludge to be
introduced into the digester tank 12. In the illustrated
arrangement, the sludge supply assembly 28 is operable to supply a
mixture of raw sludge and recirculated partially digested sludge
for introduction into the digester tank 12. However, it should be
understood that in other constructions the ~ludge supply a~sembly
28 could be arranged to supply only recixculated sludge ~r only raw
sludge for introduction into the digester tank 12.
As shown in FIGS. 1 and 2, the sludge aupply a~sembly 28
includes a pump 30 for pumping partially digested 51udge derived
from the digester tank 12 back into the tank 12 to positively
displace ~ludge in o~herwise quiescent regions of the tank 12. The
pump 30 is preferably a high volume, low head pump of conventional
design, however, any suitable pumplng means can be employed. A
tank outlet pipe or suc~ion conduit 32 extends between the bottom
of the digest~r tank 12 and the pump 30. As shown in FIG. 1, the
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D~SIl~Dtt/l!lN91130
suction conduit 32 communicate~ with the collection trough 18 so
that sludge which ~e~tles to the bottom of the dige~ter tank 12 and
slides down the floor 16 to the trough 18 i6 rccirculat2d. A
second sludge conduit 34 extend~ from the pump 30 for dQlivering
the sludge pumped therefrom to ~ distrlbution system for delivering
the sludge back i2ltO the digester tank 12, as will be further
explained below.
The sludge supply assembly 28 also includes a discharge pipe
or conduit 36 communicating with the suction conduit 32 through a
valve 38. The Yalve 38 regulates the flow of partially digested
sludge entering the discharge conduit 36 from the suction conduit
32 for ultimate di~posal at a remote location. While in the
illustrated arrangement partially digested sludge can be withdrawn
from the sludge supply assembly 28 for di~posal, in other
arrangements this ~ludge can be withdrawn from the dige~ter
apparatu~ lO at other location~ or using other suitable means.
The sludge supply assem~ly 28 is al~o provided with a ~ludge
inflow pipe or conduit 40 for supplying raw ~ludge to the diges~er
apparatus lO. A second val~e 42 regulates the flow of raw slud~e
into the sludge conduit 34. While in the illustrated arrangement
raw sludge is introduced into the partially digested sludge bein~
pumped from the digester tank 12, in other arrangementR raw sludge
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DBSI~)U/I~NV9180
can be lntroduced at other location~ in the systQm u~ing other
suitable meansO
To ~aintain de6ired thermal condition~ within the digester
tank 12, a heat exchanger 44 i~ provided to h~at the raw ~lud~e
flowing through the inflow conduit 40 before its introduction into
the sludge conduit 34. It should be understood that one or more
heat exchangers could be employed at othex locations ~o heat either
or both the raw sludge and the recirculated sludge, if de~ired.
The sludge mixing system 26 also include8 a gas supply ~ystem
or assembly 48 for supplying gas to be introduced into ~he diges~er
tank 12 ~o that the gas bubbles to the surface of the sludge to
create surface turbulence. This surface agitation is desired to
prevent the accumulation of scum on the urface of the sludge.
The gas supply assembly ~a i~cludes means for supplying ga~
under pressure to the partially digested ~ludge (and raw sludge if
present) 60 that the sludge and the gas form a mixture before be~ng
introduced into the digester tank 12. While Yarivus pres~urized
gas supply means can be used, in the illustrated arrangement such
means recycles gas collecting in the gas dome 22 and include8 a
compressor 50 communicating with the ga~ collection xeservoir 24
through a gas recirculation pipe or conduit ~2. While gas from the
gas dome 22 is preferably recycled, any ga~ ~ource capable of
supplying suitable ga~ that does not contain amounts of oxygen
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DBS11~ 9180
sufficient to interfer~ with anaerobic digestion could be u~ed.
The compressor 50 i8 located on the floating co~er 20, but could
also be located elsewhere such as in a nearby building (not shown)
or other suitable remote location. A second yaæ pipe or conduit 54
extends from the compressor 50 for deli~ering the pre~surized
recycled qa~ to the aforementioned di~tribution system for its
ultimat~ reintroduction ~nto the dige~ter tank 12, a~ explained
below.
The mixing system 26 also includes a distribution system or
netwoxk 56 which receive~ the recirculated ~ludge and gas from the
sludge and gas supply assemblies 28 and 48 and distributes this
mixtuxe back into the digester tank 12. As shown in FIG. 3, the
distribution network 56 include~ a hou~ing 58 having a tubular
influent end portion 60 defining an entry chamber 62. The influent
end portion 60 i8 provided with a pair of elbows 64 and 66
respectively connected to the sludge conduit 34 and the ga~ conduit
54 so that recirculated sludge and ga~ are introduced into the
entry chamber 60 where they are mixed. The housing 58 al~o
includes a tubular manifold portion 68 connected ~o the influent
end portivn 60 and extending into the digester tank 12. The
manifold portion 68 include~ a plurality of effluent portæ 70
arranged in spaced apart, staggered relation along the length of
the manifold portion 68.
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DôS/l~DU/EUY9180
To deliver the rec~rculated ~ludge and gas mixture from the
hou~ing 58 to the dig~ster tank 12, the distribution network 56
includes a plurality of distribution pipes 72 each connected to one
of the effluent ports 70. The pipes 72 are positione~ above and
closely adjacent the floor 16 of the digester tank 12 and each
includes a di charge end 74. It is preferred that the dl~charge
ends 74 of the pipes 72 be oriented to sweep sludge from different
regions of the floor area in which sedimentation would otherwise be
expected to occur. Thus, the pipes 72 are of various lengthfi and
some are appropriately bent or jointed to qenerally follow the
contour of the cylindrical wall 14 of the dige~ter tank 12 80 that
the discharge ends 74 of the pipes 72 are spaced around the tank 12
and along the cylindrical wall 14. The number and arrangement
of the pipe~ 72 in the distribution network 56 can be varied from
what is shown in the drawings. For example, the pipes 72 could be
arranged in a fan-shaped configuration ~uch as i6 disclosed in
Patent No. 4,092,338 which iB hersin incorporated by reference.
Due to the high power requirements that would be needed to
forcefully deliver sludge and gas to all of the pipes 72 at the
same time, it is preferred tha~ delivery occur to only selected
ones of the pipe~ 72 at a time. To sequence the delivery of the
sludge and gas mixture ~o the variou~ di~charge point~ ~erYed by
the pipes 72, the di~tribution network 56 includes a valve means
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DIIS / l~D~1/ EN1~9 l ôO
for selectively distributing the mixture to on~ of tha pipe8 72 at
a time. While variou~ valve means c~n be employed, ln the
illustrated arrangement the valve means is of the type di~closed ln
Patent No. 4,092,~38. Thus, the valve means incorporate~ the
tubular housing 58 and include~ a sliding ~alve apparatus 78
reciprocally movable within the hou3ing 58.
The valve apparatus 78 includes a vane member 80 attached to
the proximal end of a hollow tube 82, the vane member includin~
openings 83 communicating with the interior of the tube 82. The
valve apparatus 78 al~o includes a valve member 84 which is
attached to the distal end of the tube 82 and which travels within
the manifold portion 68. The valve member 84 include~ a circular
~eal member 86 which slideably ~eal~ to the interior wall of the
manifold portion 68. The valve member 84 al~o includes a ring 88
which has holes or opening~ 89 communicatinq with the interior of
the kube 82, and a central web 90 interconnecting the ring 88 and
the seal mamber 86. When a selected one of the effluent ports 70
is centered between the seal member 86 and the ring 88, the sludge
and gas mixture flow from the ~ntry chamber 62 through the
openings 83 in the vane member 80, the tube 82, and the holes 89 in
~he ring 88 to the distribution pipe 72 corresponding to the
selected port 70.
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D~S / ~DU/ E~rr~ 180
To advance the valve apparatus ~8, the valve means includes an
operating rod 92 connected to the vane member 80 and extending
outside the housing 58 to a rack 94. The rack 94 ls drivingly
engaged by a pinion 96 which is driven by ~ motor 98. Indexing
circuitry or a m~nual ~witch (not shown) can be used in con~unction
with the motor 98 to advance the valve apparatus 78 ~o as to
sequentially direct the recirculated material from the entry
chamber 62 to the distribution pipes 72.
The distribution network 56 can be employed a~ a retrofit
device in digester apparatus which were originally equipped with
other ~ludge and/or gas mixing systems to replace or ~upplement
these systems. Where the distribution apparatus 56 i8 built into
a newly constructed digester apparatus, the pipes 72 can be
partially embedded in the concrete which forms the tank 12.
Operation of the mixing system 26 to circulate the slu~ge
within the digester tank 12 and to prevent the build-up of scum on
the surface of the sludge i~ a~ follows. Partially digested sludge
and gas from the digester tank 12 are simultaneously pumped through
the sludge supply assembly 28 and gas supply assembly 48,
respectively, to the entry chamber 62 of the d~stribution network
56. The sludge and ga~ mixture traveling through the distribution
network 58 is sequentially delivered, via operation of the valve
member 84, to the pipes 72. As the mixture is forcefully emitted
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D~ 91aO
from the discharge end ~4 of a ~elected one of the pipes 72, the
61udge in the r~gion of the digester tank 12 served by that plpe 72
i8 hydraulically mixed to provide mixing turnover in that region.
This prevents settleable solids from accumulating in significant
amounts on the portion of the floor 16 in the region and insures
adequate digestion in the region. By sequentially di~charging the
sludge and gas mixture through all of the pipes 72 the entire
contents of the digester tank 12 can be hydraulically mixed. While
the gas emitted through the pipes 72 may provide some hydraulic
mixing benefits, the primary mission of the gas i~ to ri~e to the
sludge surface to agitate the surface, thereby preventing ~he
accumulation of 8CUm thereon. The positioning of the di~charge
ends 74 of the pipes 7~ around the tank 12 facilitate3
substantially complete coverage of ~he ~urface.
Advantageously, the mixing system 26 provide~ both hydraulic
mixing for circulating the 51udge within the tank to insure active
digestion throughout substantially the entire digester tank volume,
and gas mixing for agitating the surface of the sludge to avoid
scum build-up. By combining previously separate hydraulic and gas
mixing systems into a ingle system, a single distribution network
can be used to introduce both the recirculated gas and slud~e into
the digester tank 12. Since only a 6ingle distxibufion network 56
is employed, only one ~equencing mechanism~ i.e. the valve
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DBSII~DU/2NV9180
apparatus 78, is required to simultaneously ~equence tha delivery
of sludge and ga~ to the various dlscharge points. Accordln~ly,
the digester spparatus 10, and particularly the mixing 8y8te~ 26,
includes fewer parts and is les~ expensive to operate and maintain
than are prior art arxangements employing ~eparate hydraulic and
gas mixing system~.
Various features of the invention are set forth in the
following claims.
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