Note: Descriptions are shown in the official language in which they were submitted.
9523-1
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BACKGROUND OF THE INVENTION
Thls invention relates to an activated sludge
sewage treatment process ~or treating raw or treated
phosphate-containing sewage to obtain a substantially
phosphorous-~ree e~fluent which is returned to natural
water resources.
~ n the conventional actlvated sludge system
in use today, sewage is sub~ected to the usual screening
and prelimlnary sedimentation procedures, then mixed with
activated sludge recycled ~rom a settling tank to form a
mixed liquor which is sub~ected to aeration. During
aeratlon o~ the mixed liquorJ the organisms present cause
the aerobic decomposition o~ sollds and a high degree of
BOD removal is achieved.
Phosphates, which are present in organic wastes
and detergents, escape conventional sewage treatment
processes and are released with the e~fluent into natural
water resources, e.g., lakes, rivers and streams. These
phosphates result ln over ~ertilization or eutrophication
of waters causing unsightly algal blooms and serious
pollution problems.
It is known that aeration of the mixed liquor
in an activated sludge sewage treatment process lnitially
causes the microorganisms present to take up phosphate.
U.S, Patent ~o. 3,236,766 discloses a process which util-
izes this phenomenon for removing phosphates ~rom sewage.
According to the process disclosed in that patent, the
pH o~ raw sewage is ad~usted, if necessary, to maintain
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a range of from about 6.2 to about 8.5, the sewage is
mixed with activated sludge to form a mixed liquor, the
mixed liquor is aerated to maintain a dissolved oxygen
content of at least 0.3 mg. per liter in the mixed liquor
and a phosphate-enriched sludge is separated from the`
mixed liquor to provide a sùbstantially phosphate-free
effluent. The phosphate-enriched sludge is treated to
reduce the phosphate content thereof prior to recycling
for mixing with the influent sewage. This is accomplished
by maintaining the phosphate-enriched sludge in an an-
aerobic condition for several hours.
Several other processes have been proposed for
reducing the phosphate content of phosphate-enriched
sludge following the aeration step in an activated sludge
sewage treatment process. Thus~ for example, U.S. Patent
No. 3,38S,785 discloses adjusting the pH of phosphate-
enriched sludge to between about 3.5 and 6 and agitating
the sludge in contact with a low phosphate-containing
aqueous medium in a tank for a time sufficient to effect
transfer of water-soluble phosphate material from the
sludge to the aqueous phase. The low phosphate-containing
aqueous medium used in this tank is water added from an
external source to leach out the phosphate from the sludge.
After the soluble phosphate has been leached out of the
sludge into the aqueous medium, the mixture is passed to
a settling tank wherein the phosphate-enriched aqueous
medium is separated from the phosphate-depleted sludge.
Since this process requires the addition of considerable
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quantlties of water from an external source and requires
two separate tanks -- i.e., a phosphate leaching tank
and a settllng tank, to separate the soluble phosphate
rrom the sludge, a large capital lnvestment is required
in such a process.
Accordingly, it is an ob~ect o~ thi5 invention
to provlde an improved process for reducing the phosphate
content of phosphate-contalning sewage in an activated
sludge sewage treatment prooess.
It ls a further ob~ect o~ thls invention to
provlde such a process whlch includes an improved phos-
phate strlpping operation of phosphorous-enriched sludge
and which results in lncreased efficiency of the phosphate
removal from the sewage.
Other ob~ects and advantages of this invention
will be apparent from the ensuing disclosure and claims.
SUMMARY
This invention relates to an activated sludge
process for treating raw or treated phosphate-containlng
sewage to obtaln a substantially phosphorous-free efflu-
ent.
The process of this invention comprises aeratlng
a mlxed liquor comprising phosphate-containing in M uent
sewage material and activated sludge in an aeration zone
to reduce the BOD content o~ the sewage material and to
cause the m~croorganlsms present to take up phosphate.
Phosphate-enriched sludge is separated from the mixed
llquor to provide a substantially phosphate-free effluent.
106326~
This phosphate-enriched sludge is passed to a phosphate
stripping zone and settled to form supernatant liquor
in the stripping zone upper section, and settled sludge
comprising a supernatant sludge layer and a subnatant
sludge layer. The subnatant sludge layer is maintained
under anaerobic conditions for a time sufficient to re-
lease phosphate to the liquid phase of the subnatant
sludge layer. Subnatant anaerobic sludge is contacted
with a lower soluble phosphate content medium to trans-
fer the soluble phosphate in the subnatant anaerobicsludge liquid phase to said lower soluble phosphate
content medium, for phosphate enrichment of supernatant
liquor in the stripping zone upper section. At least a
portion of the subnatant anaerobic sludge is recycled
from the phosphate stripping zone to the aeration zone
as the aforementioned activated sludge therefor.
As used herein, the term "sludge" refers to a
solids-liquid mixture characterized by a sludge solids
phase and an associated liquid phase. The term "lower
soluble phosphate content medium" refers to an aqueous
or water-containing medium which contains a lower con-
centration of soluble phosphate than the subnatnat an-
aerobic sludge with which it is contacted.
In accordance with the present invention,
phosphate-enriched sludge, in which the phosphate is
present in the cells of the sludge microorganisms, i.e.,
biological solids, is maintained under anaerobic condi-
tions in a settled subnatant sludge layer in the phosphate
9~23-
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strlpping ~one for a tlme sufflclent to cause the micro-
organisms to release phosphate to the liquid phase Or the
sludge. The resultant subnatant anaerobic sludge ls con-
tacted with a lower soluble phosphate content medlum. The
purpose of such contacting is to transfer soluble phos-
phate out of the subnatant anaerobic sludge and ultimately
to the supernatant li~uor in the stripping zoneJ so as to
e~ect a phosphate enrlchment Or the supernatant llquor
thereln. In accordance wlth the lnvention, the soluble
pho~phate ln the subnatant anaerobic sludge may be trans-
rerred either directly by contacting the anaerobic sludge
with the supernatant liquor or indirectly, as for example
by contacting the anaerobic sludge with the phosphate-
enriched sludge passed to the stripping zone.
Without the aforementloned contactlng/transfer
steps, a conslderable time would be required for the sol-
uble phosphate in the subnatant anaerobic sludge to migrate
through the supernatant portion of the sludge and into the
supernatant liquor in the stripping zone. Under such condi-
tions, ir the sludge were withdrawn from the stripping zone
and recycled to the aeration zone be~ore a sufficient arnount
o~ the soluble phosphate is transferred to the supernatant
liquor, an excess amount of soluble phosphate will be re-
cycled to the aeration zone and the phosphate removal
er~lciency of the overall process will be lowered. The
pre~ent invention overcomessuch difficulty and has been
demonstrated in actual operation to achieve a high level
of pho~phate removal efficiency.
9~23-
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BRIEF DESCRIPTION OF ~HE DRAWINGS
Figure 1 is a schematlc flowsheet Or an actlvated
sludge process accordlng to one embodiment of the inven-
tion, wherein subnatant anaerobic ~ludge is contacted
wlth the phosphate-enrlched sludge passed to the stripping
zone.
Figure 2 is a schematic ~lowsheet Or an activated
sludge process accordlng to another embodiment of the in-
vention, wherein a portion o~ the supernatant liquor in
the phosphate stripping zone iR wlthdrawn therefrom and
is reintroduced into the stripping zone underneath the
layer Or subnatant anaerobic sludge for contacting there-
with.
DESCRIPTION OF THE PREFERRED
EMBODIMENTS
~ererring now to the drawings, Figure 1 shows
an illustrative process system according to the present
inventlon. A raw phosphate-containing sewage influent
stream 1 is passed through conventional screenlng and
grit-removing units and is optionally sub~ected to primary
settling in a primary settling tank 2 ~rom which primary
sludge is removed in line 3. The primary ~ettled sewage
1~ mlxed with recycled, activated sludge hereinafter
described to form a mixed liquor and is passed by line 4
to the aeration tank 5.
In the aeration tank, the mixed liquor ls
aerated at a rate sufficient to maintain it ~erobic --
i.e., so that there is a measurable amount of dissolved
9923-1
~063264
oxygen present in the mlxed liquor in at least a part of
the aeration tank, for a period o~ 1 to 8 hours. During
aeratlon, the microorganlsms present take up phosphate
and consume organic matter present in the sewage. A hl~h
degree o~ BOD removal ls obtained durlng aeration.
A~ter aeratlon, the mixed liquor is fed lnto a
secondary settling tank 6. In the secondary settllng
tank 6) phosphate-enrlched sludge settles and thereby
3eparates ~rom the llquor. The sludge contains a substan-
tial portlon of the phosphate present in the sewage The
substantlally phosphate-free e~luent ls discharged for
dlsposal ln a conventlonal manner by line 7.
The phosphate-enrlched sludge ls removed from
the settllng tank 6 by llne 8. A portlon of the sludge
may be dellvered to waste and the remalnder contacts and
mixes wlth recycled, subnatant, anaeroblc sludge from the
phosphate strlpper whlch contains a high concentration o~
soluble phosphate and the mlxture ls passed to the phos-
phate stripper 9. In the phosphate stripper 9, the
phosphate-enrlched sludge is settled to ~orm supernatant
liquor ln the upper sectlon o~ the strlpplng zone, and
settled sludge comprlsing a supernatant sludge layer and
a subnatant sludge layer. The subnatant sludge layer ls
maintalned under anaeroblc conditions ~or a tlme suf~lclent
to cause the microorganlsms in the subnatant anaeroblc
sludge to release phosphate to the llquld pha~e of the
subnatant sludge layer. The phosphate leaks out of the
sludge sollds lnto the llquld phase. ~he mechanlsm o~
thls treatment will be descrlbed ln more detail hereinbelow.
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A phosphate-enrlched supernatant ll~uor is
produced in the phosphate strlpper 9 and is passed by
llne 10 to the phosphate preclpitator 11. A phosphate
precipitant, such as alumlnum or lron salts or lime, is
mixed with the phosphate-enriched supernatant liquor in
the phosp~late precipitator 11 to precipitate phosphate.
The precipitated phosphate is passed by line 12 for
mixing with raw sewage in line 1.
In the phosphate stripper 9, sludge solids,
contalnlng a high amount of phosphate ln the cells of
the organisms in the sludge, separate from the aqueous
phase of the sludge and settle toward the bottom of the
phosphate stripper. Thus, the solld particles in the
sludge contain intracellular phosphate and these partlcles
settle into the subnatant anaeroblc layer of sludge in
the phosphate stripper 9. A density gradient exists in
the layer of settled sludge, the density of the sludge
being greater at the bottom of the sludge layer than at
the top. As the sollds in the sludge containing intra- -
cellular phosphate mlgrate to the bottom of the sludge
layer, because of the anaerobic conditions existing in
this portion of the sludge layer, the organisms release
phosphate to the liquid phase of the sludge in the form
of water-~oluble phosphate ions. Preferably, the residence
time of the sludge in the phosphate stripper is from 2
to 30 hours. The concentration of soluble phosphate is
initially greatest in the lower portion of the sludge
layer. As discussed earlier herein, a considerable time
-` 9923-
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ls requlred rOr the soluble phosphate to mlgrate through
the less dense and lower solids containing supernatant
portion of the sludge and into the supernatant llquor.
If the sludge ls withdrawn and recycled to the aeration
zone be~ore a su~icient amount o~ the soluble phosphate
is trans~erred to the supernatant llquor, an excess amount
o~ ~oluble phosphate wlll be recycled to the aeration tank
and the phosphate removal e~riciency o~ the overall pro-
ce~s wlll be lowered. It ls pre~erred that no more than
75% o~ the soluble phosphate which is relea~qed in the phos-
phate stripper be recycled to the aeration tank with the
recycle activated sludge.
Accordlng to one embodiment o~ the invention,
subnatant, anaerobic sludge containing a substantlal
portion o~ the ~oluble phosphate which ls released in the
phosphate 3trlpper 9 is withdrawn from the phosphate
stripper and split lnto two portions. One portlon Or this
high soluble phosphate-containing sludge ls recycled
through line 13 for mixing with the raw sewage which is
being red to the aeration tank in line 4, and the other
portion of anaerobic, high soluble phosphate-containing
sludge ls recycled through llne 14 for contacting and mix-
ing wlth aeroblc sludge from the secondary ~ettling tank
6 as it is being passed to the phosphate stripper 9. Since
the sludge withdrawn from the secondary settling tank 6
contains lntracellular phosphate in the solids phase
thereor and since the liquid phase, which may comprise
98 to 99~ o~ the sludge, contains little or no soluble
9923-
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phosphate, the llquld portion of thls aerobic sludge
leaches or elutriates the soluble phosphate from the
high soluble phosphate-contalnlng anaeroblc sludge por-
tion from line 14. It is preferred that each Or the high
soluble phosphate-contalnlng anaeroblc sludge portlonQ
wlthdrawn from the strlpper and passed through llnes 13
and 14 comprlse from about 25 to 75~ o~ the total amount
o~ high soluble phosphate-containlng sludge wlthdrawn
~rom the stripper and that at least 25% of the soluble
phosphate released in the strlpper be removed from the
stripper along wlth the supernatant`llquor ln line 10.
The portion of the hlgh soluble phosphate-
contalning sludge ln llne 14 may also be introduced dir-
ectly into the phosphate strlpper 9 above the sludge layer
contalned therein so that the soluble phosphate will come
in contact with the supernatant ln the phosphate sbrlpper
and be dlsso'ved by the supernatant.
In the embodlment of the invention shown in
Fig. 2, phosphate-containing influent sewage material is
introduced to the sewage treatment system by line 15 and
mixed wlth the phosphate-depleted supernatant liquor from
line 34, to be described more fully hereina~ter. The in-
~luent sewage and recycle sludge in line 24 are pas~ed
into aeration zone 16 wherein the mixed llquor formed ~rom
the sewage material and the recycled activated sludge are
aerated to reduce the BOD content of the sewage and to
cause the microorganisms present to take up phosphate.
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1063264
In practlce, the aeration zone may be Or a
conventional type wherein atmospheric air is used as
the oxidant in open aeration chambers. Alternatively,
the aeration may be conducted in a manner as taught by
U.S. Patent No~.3,547,813 - 3,547,815 to J.~. McWhlrter
et al ~n ~hlch at least one enclosed covered aeration
chamber i~ employed wherein the llquid undergoing treat-
ment is lntimately contacted in the presence of activ-
ated sludge with oxygen enrlched gas ~rom an overlying
gas space to dissolve the oxygen necessary for aeroblc
biological activity. Such oxygenation systems are able
to operate at biological suspended æolids levels se~reral
times ~reater and aeration detentlon periods several times
less than those Or conventional alr aeration systems while
maintaining comparable or higher overall levels o~ treat-
ment, and have been found to be hlghly e~fective in the
practice of the present invention.
The aerated mixed liquor is conducted from the
aeration zone in line 17 and passed to the secondary
settling zone 18. In the settling zone, phosphate-
enriched sludge is separated from the mixed liquor to
provide a substantially phosphate-free effluent which is
discharged Irom the system in line 19. The separated
phosphate-enriched sludge is passed from the secondary
settllng zone by line 20 to the phosphate strlpping zone
21. In the stripping zone, the phosphate-enriched sludge
is settled to rorm supernatant liquor in the upper section
22 Or the si~ripping zone, and settled sludge comprising a
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supernatant sludge layer and a ~ubnatant æludge layer.
The subnatant sludge layer ln the stripplng zone is main-
tained under anaerobic conditions in the same manner pre-
viously descrlbed for a time sufrlcient to release phos-
phate to the liquid phase of the subnatant ~ludge layer.
The subnatant anaerobic sludge contacting step
is conducted in this system by withdrawing supernatant
liquor from the stripping zone upper section in line 25
and dlverting a portlon thereof into line 26, having pump
means 27 disposed therein, for reclrculation to the
stripping zone lower section, The dlverted supernatant
liquor ln line 26 is reintroduced lnto the phosphate
stripplng zone by the sparging means 28, whlch may for
example co~prise multiple stationary nozzles. In thls
manner a countercurrent elutriation of the ~oluble phos-
phate in the subnatant anaeroblc sludge ls established,
as the soluble phosphate is trans~erred to the upflowing
recirculated supernatant liquor and subsequently to the
bulk liquid volume of the supernatant liquor in the
strlpplng zone upper section.
The portion of the supernatant liquor withdrawn
from the stripping zone ln line 25 whlch is not diverted
as recycle contacting medium in line 26 is flowed to
quick mix tank 29. In this tank, the undiverted super-
natant liquor is rapldly mlxed (by means not shown) with
a phosphate precipitant, e.g., lime, lntroduced to the
tank by me~ns of line 30. The supernatant liquor-phos-
phate preclpitant mixture is then passed by line 31 to
13
1063Z64
flocculator tank 32 in which the precipitated phosphate
is settled and removed from the system as waste chemlcal
sludge in line 33. The overflow phosphate-depleted super-
natant from the flocculator tank 32 is recycled in line 34
for ~oining with the sewage influent entering the process
in line 15.
By means of the above-describéd subnatant sludge
contacting scheme, a significant amount of the phosphate
which is released in the subnatant anaerobic sludge layer
is transferred to the supernatant liquor in the stripping
zone upper section, whereby the subnatant sludge withdrawn
from the bottom of the stripping zone in line 23 and re-
circulated to the aeration zone in line 24 has a suffic-
iently lowered phosphate content to permit high removal
of phosphate from the sewage to be achieved in the aeration
zone.
Means other than those illustratively described
hereinabove may be used in the broad practice of the pres-
ent invention to maximize the contact between the subnatant
anaerobic sludge portion containing a high soluble phos-
phate content and the supernatant liquor in the phosphate
stripper so that the soluble phosphate will be eluted into
the supernatant liquor. For example, two or more phosphate
stripping tanks may be used. While one tank is maintained
in a relatively unagitated state and the sludge is main-
tained under anaerobic conditions so that the microorgan-
isms will release their phosphate content, another tank,
in which the microorganisms have already released their
phosphate content is agitated to cause the subnatant,
anaerobic layer of sludge containing the soluble phosphate
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1063264
to mix wi~h the supernatant liquor. Arter the supernatant
liquor has eluted substantlally all of the soluble phos-
phate from the sludge sollds, the vigorous agitation is
stopped and the solids are permltted to settle. After the
solids have settled, the supernatant llquor, whlch then
contains substantlally all Or the soluble phosphate in
the tank, ls removed and passed to a phosphate preciplta~
tor and the phosphate-depleted sludge is recycled for
mixing with raw sewage which 1~ being ~ed to the aeration
tank. The tank is then ~illed with sludge from the
secondary settling tank, the sludge is permltted to settle
and become anaerobic and is maintained under anaerobic
conditions while the previously descrlbed process of vigor-
ously stirrlng the subnatant anaerobic sludge and super-
natant liquor, settllngJ and removing a phosphate-enriched
supernatant is repeated with the first tank. In other
words, the two tanks are maintained 180 out of phase
with each other.
Another means for effecting transfer o~ the
soluble phosphate from the subnatant anaerobic sludge is
to introduce rresh sludge from the secondary settling tank
into the bottom of the stripping zone so that it diffuses
up through the anaerobic layer and elutes the soluble
phosphate contained in this layer.
Another means for maximizing the contact
between the supernatant liquor and the subnatant anaerobic
sludge layer containing a high concentration of soluble
phosphate i~ the phosphate stripper is to intermittently
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and vigorously stir the contents o~ the phosphate stripper
so that the anaerobic layer of sludge will be dispersed
into the supernatant. The supernatant will thereby elute
the phosphate from the slud~e solids The contents of
the phosphate stripper are then permitted to settle. Dur-
ing agitation and settling, no recycle sludge would be re-
moved rrom the bottom of the phosphate stripper. Agitation
may be accomplished by introduction Or a non-oxygen con-
taining gas at the bottom of the stripper 9. Different
portions of the anaerobic sludge layer may also be raised
above the sludge layer and then permitted to settle again.
During settling, the soluble phosphate is eluted by the
supernatant. While this i9 being done, another portion
of the anaerobic sludge layer may be withdrawn from the
phosphate stripping for recycle to the aeration tank.
Any o~ these processes may be conducted either
continuously or intermittently. Thus, sufficient elutria-
tion of phosphatemay be obtained in the Fig. 1 system if,
~or example, the anaerobic sludge withdrawn ~rom the
phosphate stripper is recycled through line 14 one hour
out Or every five hours of operation.
The following examples illustrate the specific
advantages of the present invention in acheiving high
overall removals of phosphate from phosphate-containing
sewage relative to the prior art process systems lacking
the subnatant sludge contacting feature of the instant
process.
16
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1063264
EXAMRLE I
In this test, a process system of a type simllar
to that shown in Figure 2 herein was operated inltially ln
the manner of the prior art wlthout the subnatant sludge
contacting ~eature Or the present invention and subse-
quently was operated ln accordance with the present ln-
vention, using supernatant llquor wlthdrawn ~rom the
strlpplng zone upper sectlon a~ the lower soluble phos-
phate content contacting medlum, ln a manner substan-
tially simllar to that described herelnabove in connec-
tlon with Figure 2.
In both phases the comparative evaluatlon test,
inrluent sewage was mixed with recycle activated sludge
to form a mixed liquor which was then aerated in the aera-
tion zone to cause the mlcroorganisms present to take up
phosphate. Phosphate-enriched sludge was then separated
from the mixed liquor in the secondary clarifier to pro-
vide a substantlally phosphate-rree e~fluent. The sep-
arated phosphate-enriched sludge was passed to the phos-
phate stripping zone and settled therein to form a super-
natant liquor in the stripping zone upper section, and
settled sludge comprislng a supernatant sludge layer and
a subnatant sludge layer. The subnatant sludge layer wa~
maintalned under anaerobic conditions rOr a time sufric-
lent to release phosphate to the liquid phase o~ the sub-
natant sludge and to provide a phosphate-enrlched super-
natant llquor. This phosphate-enriched supernatant liq-
uor was withdrawn rr~m the phosphate stripping zone,
17
9923-1
1063264
mixed with phosphate precipitant (lime) in a qulc~ mix
tank; the resulting precipitated phosphate was removed
as waste chemical sludge in a flocculation tank and
phosphate-depleted supernatant liquor was recirculated
to the influent sewage llne. Settled sludge was withdrawn
frorn the phosphate stripping zone and recirculated to the
influent sewage line as the aforementioned activated
sludge.
Tn the first phase of the comparative evalua-
tion test, operated in accordance with the prior art
teachings, none of the supernatant liquor withdrawn from
the stripping zone upper section was recirculated or re-
introduced into the stripplng zone. All of the withdrawn
supernatant liquor was treated with phosphate precipitant
and recirculated to the influent sewage line. As des-
cribed in terms of the Figure 2 system,the recycle pump
27, schematically shown as disposed in the recycle line
26 which is connected to the phosphate stripping zone
supernatant liquor discharge line 25 and which terminates
in the lower portion of the stripping zone, was not ac-
tuated and no flow was conducted through the recycle line.
In the second phase of the comparative evalua-
tion test, operated in accordance with the present inven-
tion, the process system was operated as above, except
that the aforedescribed recycle pump was actuated so as
to d~vert a portion of the lower soluble phosphate con-
tent supernatant liquor from the stripping zone super-
natant llq~or discharge line and to introduce same into
18
9~23-
1063264
the phosphate strirping zone beneath the subnatant anaer-
obic sludge. In this manner a countercurrent elutriation
of the soluble phosphate ~n the subnatant anaerobic sludge
was established whereby the phosphate was transferred to
the elutrlant surernatant liquor and subsequently into
the supernatant liquor in the stripping zone upper section,
~or ~hosphate enrichment thereof. ~he duration o~ the
first phase test, without subnatant sludge contacting,
was 8 days o~ contlnuous operation and the second ~hase
test, wherein subnatant sludge contacting was employed,
was contlnuously o~erated ~or 10 days.
The data which were taken during the comparatlve
evaluation test of the above described systems is set
rorth in the Table below. These data demonstrate the
substantial improvement in phosphate removal efficiency
which is achieved by the proGess of this invention (data
shown in column A) over the system whlch is taught by the
prior art (data shown in column B). As shown by the data,
the ~rocess ~arameters in the respective systems, includ-
lng influent sewage ~low rate, phosphate-enriched sludge
recycle rlow rate, strip~ing zone underflow rate, stripping
zone over~low rate, mlxed liquor suspended solid~ under
aeration, mixed liquor volatile suspended solids under
aeration, influent blochemical oxygen demand (BOD5), and
effluent blochemical oxygen demand (BODs) all had closely
corresponding measured numerical value~. Acco~dingly, the
entries in tAe Table relating to measured phosphate con-
centrations in selected process streams in the two systems,
19
~ 9923-1
~063264
.. . ...
TABLE
''A~ B''
Process ~eaturing Process w~thout
subnatant slud~e subnatant sludge
Proces~ Par~meter contacting contact~ng
INFLUENT FLOW RATE,GPM12.1 12.0
PHOSPHATE-ENRICHED SLUDGE
RECYCLE F~OW RATE (SLUDGE
FROM SECONDARY SETTLING ZONE
PASSED TO STRIPPER), GPM 3.0 3.0
STRIPPING ZONE UNDE~FLOW
RA~E, aPM 1.~ 1.4
STRIPPING ZONE OVERFLOW
RATE, GPM 1.6 1.6
MIXED LIQUOR SUSPENDED SOLIDS
IN AERATION ZONE, MG-/L-2929 3448
MIXED LIQUOR VOLATILE SUS-
PENDED SOLIDS IN AERATION
ZONE, MG./L. 2324 2454
INFLUENT BOD5, MG./L. 7 51
EFFLUENT BODs, MG-/L- 10 16
INFLUENT PHOSPHATE* MG./L. 5.3 4.2
LUENT PHospHATE* MG-/L- -7
PERCENTAGE OVERALL
PHOSPHATE*REMOVAL 87% 16.7%
PHOSPHATE*IN STRIPPER
UNDERFLOW, MG./~. 468 685
PHOSPHATE*IN STRIPPER
SUPERNATANT. M~.~L- 35 9
* Measured as total
phosphorous content
9923-1
~063Z64
vis.~ phos~hate in the in~luent sewage, phosphate in the
efrluent sewage, phosphate overall percentage removal,
~hosphate in the stripper under~low, and phosphate in the
stripper supernatant llquor, clearly demonstrate that the
proceQs Or this invention, wherein the subnatant anaeroblc
sludge i9 contacted with a lower soluble phosphate content
medium to effect an ultimate transfer to and enrlchment of
the supernatant liquor ln the stripping zonè, provldes a
sub~tantlally enhanced overall removal of phosphate (87%
v. 16.7~) from the sewage~being treated relative to the
prior art process, which dld not employ such 9ubnatant
anaerobic sludge contactlng tep.
The rea~on for such striking difference in phos-
phate removal levels between the respective systems is
readlly apparent based on a comparison Or the phosphate
concentrations in the phosphate strlpper underflow and
supernatant in these systems. In the process conducted
in accordance with the present (data tabulated in column
A), the phosphate stripper underflow phosphate concentra-
tion was 468 milligrams/liter and the phosphate stripper
supernatant liquor concentration was 35 milligrams/liter,
whereas in the prior art process corresponding phosphate
concentrations were 685 milligrams/liter in the stripper
underflow and only 4.9 milligrams/liter in the stripping
zone supernatant liquor. These data lndicate that in the
prior art process, the phosphate released by the subnatant
anaerobic ~ludge was retained in the settled sludge layer
and was not significantly transferred to the supernatant
21
1063Z64 9923~1
liquor in the stripping zone, whereas in the process
according to the present invention a substantial transfer
of the phosphate, concommitant with significantly higher
overall phosphate removals than the prior art process,
were achieved.
EXAMPLE II
Raw sewage (about 1,000,000 gallons per day --
g.p.d.) containing about 100 parts per million (p.p.m.)
of solids and about 10 p.p.m. of total phosphate is passed
through conventional screening and grit-removing units
and is mixed with recycle activated sludge (about 100,000
g.p.d.) containing about 50 p.p.m. of soluble phosphate.
The mixed liquor is fed to an aeration zone and is aerated
at a rate of 2 cubic feet of air per gallon of sewage
for 6 hours. The effluent mixed liquor from the aeration
zone is fed to a secondary settling tank. Clarified waste
liquor which is substantially free of phosphate is dis-
charged to the effluent outflow after chlorination at a
rate of about 1,000,000 g.p.d. The settled mixture of
phosphate-enriched sludge is withdrawn from the secondary
settling tank at a rate of about 210,000 g.p.d. A por-
tion of this sludge (about 10,000 g.p.d.) is passed to
waste sludge, and the remainder is passed to an anaerobic
phosphate stripper wherein it is held under anaerobic
conditions for about 10 hours. The conditions existing
in the stripper induce considerable quantities of intra-
cellular phosphate to be released by the microorganisms.
The sludge is permitted to thicken and settle under slow
9~23-
1063Z64
mechanical stirrlng, which stlrring ls not su~flcient to
infuse the phosphate secreted by the sludge layer into
the supernatant liquor at a fast enough rate to render the
process satisfactorily e~icient. Thus, the soluble phos-
phate which is released by the microorgani9ms tends to
remain ln the anaerobic, subnatant sludge. The anaerobic
subnatant ~iquor is removed ~rom the bottom o~ the phos-
phate stripper at a rate of 200,000 g.p.d. A portion of
this anaerobic sludge (100,000 g.p.d.) is recycled for
mlxing with lncoming raw sewage and the remainder (100,000
g.p.d ) is recycled for mixlng with aerobic sludge wlth-
drawn from the secondary settling tank as lt is being
passed to the phosphate stripper. The soluble phosphate
contained in the anaerobic sludge portion is thus dis-
solved by the liquor portion o~ the aerobic sludge, and
the soluble phosphate is thereby distributed into the
supernatant liquor in the stripper tank. Phosphate-
enriched supernatant liquor containing about 50 p.p.m.
Or soluble phosphate (100,000 g.p.d.) is wlthdrawn ~rom
the stripper tan~ and red to a chemical precipitation
tank where llme ls added and mlxed to form a phosphate
precipitate. The precipitated phosphorous is recycled
and mlxed wlth influent raw sewage. In the aeration zone,
the soluble phosphate introduced along with the recycled
sludge from the phosphate stripper is taken up by the
microorganisms present in the sludge along with the phos-
phate contained in the influent sewage.
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1063264
Although illustratlve embodiments of the lnven-
tlon have been set forth above which variously employ
supernatant liquor and phosphate-enriched sludge as a
lower soluble phosphate content medium, it is apparent
that other such media, as for example a portion of the
substantially phosphate free ef~luent from the process,
may be used in the contacting of the subnatant anaerobic
step in accordance wlth the process of this invention.
Thus, although preferred embodiments of this invention
have been descrlbed in detail it will be appreciated
that other embodiments are contemplated only with modi-
fication of the disclosed ~eatures, as being within the
scope of the invention.
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