Note: Descriptions are shown in the official language in which they were submitted.
~ 3~
This invention relates to the treatment of waste water
having a biological oxygen demand (B.O.D.) to remove the B~O.D.
More especially the invention is concerned with a
process which permits the employment of a single vessel for
carrying out the biological reaction and the secondary clarifi-
cation by settling of precipitated solids from the biological
reaction, still further the invention is concerned with improve-
ments in oxygen dissolving devices which may be e~ployed in the
process and apparatus to efficiently dissolve oxygen in the
waste water.
Purification and biological treatment of waste water
from municipal and industrial sources prior to discharge into
natural water systems conventionally comprises four basic steps
carried out in four separate treatment tanks or vessels in series.
A typical treatment plant will comprise a plurality of
such series of treatment tanks disposed in parallel to treat
water from a common inlet duct and discharge it from a common
outlet duct.
By way of example a Municipal treatment plant in
Hamilton, Ontario is designed to treat waste water at a rate of
60 million gallons/day, each series of primary clarifier aera-
tion tank and clarification tank treats 7.5 million gallons/day,
and there are eight such series in parallel. In the Hamilton
plant each aeration tank is 360 ft. long, 60 ft. wide and 15.5
ft~ deep, and each clarifier is 120 ft. square and 10 ft. deep,
thus each aeration tank has an exposed surface area of 21,600 sq.
ft., and each clarifier has an exposed surface area of 14,400 sq.
ft.
In such conventional treatment processes the waste
water is treated initially in a degritting tanX in which the
heavy solid particles are permitted to settle out. The water
passes from the deyritting tank to a primary clarifiex which
:k~
comprises a tank which holds the waste water for a time to per-
mit suspended solid particles to settle out and w~erein floating
solids and oils and grease are skimmed off. The liquid from the
primary clari~ier passes to an aeration tank which contains
microorganisms for converting dissolved matter in the liquid
into insoluble matter, air or oxygen is introduced under agita-
tion into the tank to meet the oxygen requirement of the micro-
organisms. From the aeration tank liquid containing suspended
solids and dissolved matter is passed to a secondary clarifier;
clear liquid overflows from the secondary clarifier and solids
are removed from a lower portion of the clarifier. A portion
of the liquid containing sludge in the secondary clari~ier is
continuously recycled to the aeration tank for further biologi-
cal treatment, and the excess is wasted.
Various proposals have been made to modify the conven-
tional treatment apparatus to overcome different disadvantages
and to improve the efficiency, for example the modifications
described in U.S. Patent 3,476,682 Albersmeyer and U.S~ Patent
3,983,031, Kirk.
It has been recognized that as the amount of suspended
solids in the liquid entering the secondary clarifier increases,
the solids loading becomes the critical factor in desiqn criteria
governing the size of the secondary clarifier; and the size of
the secondary clarifier increases relative to the size of the
aeration tank. Consequently the capital cost of the secondary
clarifier represents a major portion of the overall cost. This
was discussed in a paper entitled Solids Thickening Limitation
and Remedy in Commercial Oxygen Activated Sludge presented by
R. E. ~Speece and Michael ~. Humenick of the University of Texas
at Austin at the 45th Annual Convention of the Water Pollution
Control Federation, October 9, 1972, in Atlanta, Georgia.
It was further suggested by Speece and Humenick in the
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87
aforementioned paper that it might be possible to meet the
problem of secondary clarifier size by reducing the amount of
solids being transferred from the aeration tank to the secondary
clarifier by employing some solids separation within the aera-
tion tank~ Speece and Humenick theorized that the secondary
clarifier could perhaps be omitted if a solids separation could
be achieved in the aeration tank which reduced the overflow
suspended solids down to a level permissible in the final
effluent. Speece and Humenick were primarily concerned with an
oxygen dissolving device which they called a Downflow Bubble
Contact Aerator (DBCA), which they developed, and in particular
were concerned with its constructional parameters.
The oxygen dissolviny device of Speece is described
in U.S. Patent 3,643,403, and Speece has also obtained U.S.
Patent 3,804,255 which describes the use of an oxygen contact
device in the treatment of waste water.
Nevertheless the disclosures of Speece and Humenick
and the U.S. patents of Speece fail to recog~ize that control
of the oxygen added is essential to successful treatment of
the waste water in a single vessel.
Gas dissolving devices are known and their function
is to introduce and dissolve a gas in a liquid. One such
device is described by Speece in the aforementioned U.S. Patent
3,6~3,403, which is especially concerned with dissolving oxygen
in water to aerate the water and increase the dissolved oxygen
concentration. The Speece device comprises an upright conical
housing through which water is passed downwardly and oxygen is
continuously injected into the downwardly flowing water through
a bubble disperser located in an upper portion of the conical
housing adjacent the water inlet.
The inlet velocity of the water entering the conical
housing is designed to exceed the upward buoyant velocity of the
~43~7
gas bubbles. The outlet velocity from the bottom of the skirt
of the housing is designed to be less than the upward buoyant
velocity of the gas bubbles so that between the inlet and outlet
a cloud of bubbles of changing size is held in suspension under
highly turbulent conditions.
In the lower portion of the conical housing an equi-
~ibriurn position is established where the down flow velocity of
the water equals the buoyancy of the oxygen bubbles and an oxygen
bubble zone is established for prolonged contact with the down-
wardly flowing water. Speece indicates that eventually thebubbles are displaced from the outlet end of the conical housing
by virtue of the continuous injection of bubbles at the bubble
disperser causing "crowding" of the bubbles at the lower outlet
end.
In U.S. Patent 3,804,255, Speece describes a modifica-
tion in his oxygen dissolving device in which he includes a
bubble harvester in the bubble zone to collect the bubbles,
including bubbles of waste gases such as nitrogen and carbon
dioxide which are continuously stripped from the water, the
collected bubbles being vented to atmosphere through a vent
tube. Speece indicates that the objective of this is to con-
fine turbulence to the interior of the conical housing of the
oxygen dissolving device.
Thus in U.S. Patent 3,804,255 Speece seeks to prevent
turbulence externally of the oxygen dissolving device, however,
Speece did not recognize that the presence of undissolved oxygen
in the biological reaction zone would disrupt the formation of
the separate clarified zone. Indeed Speece particularly indicates
that the solid separation capability in the waste treatment pro-
cess and the stability of the interface between the clarifiedsupernatant and the sludge, is preserved by confining turbulence
to the interior of the cone member.
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~3~137
Clearly Speece did not recognize the significance of
controlling the supply of oxygen introduced into the system so
as to meet the biological oxygen demand oE the microorganisms
and avoid undissolved oxygen in the biological reaction zone.
Indeed it is clear that Speece did not contemplate controlling
the oxygen supply at all since he included a vent means to avoid
build up of excess oxygen and other gases in the bubble zone.
Speece sought to eliminate turbulence in the liquid
outside the cone member which he found distrupted the formation
of the clarified supernatant layer by agitation of the liquid
outside the cone member.
It should be recognized, however, that Speece does
not eliminate the presence of gas bubbles outside the cone
member. This ~s because the pressure within the cone member is
si~nificantly higher than the pressure outside the cone member.
Consequently when oxygenated liquid emerges from the-outlet in
the cone member of Speece, the release in pressure experienced
by the liquid results in evolution of some of the oxygen
(dissolved under pressure) with the result that bubbles of
oxygen are formed.
The present inventors discovered that the released
oxygen, in the form of gas bubbles disturbed the efficiency of
~ the clarifying process by carrying suspended solid particles
into the clarification zone; and this was the case even when
the emergence of the gas bubbles from solution outside the cone
member, did not result in any significant change in the turbu-
lence characteristics of the liquid outside the cone member.
In other words, in the sense of Speece, there was substantially
no turbulence outside the cone member, but efficient clarifica-
tion was not obtained because of the bubbles emerging from
solution.
An object of this invention is to provide a process
,
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~L~4~
of treating waste water biologically in which the biological
reaction and secondary clarification of biologically treated
water are conducted in a single vessel, thereby permitting
considerable economy in plant design.
It is a further object of this invention to provide
a process of treating waste water which permits the successful
treatment of waste water containing a much higher concentration
of waste matter than the conventional process employing a
separate aeration tank and secondary clarifier.
It is a still further object of this invention to
provide a process of treating waste water in which primary
clarification, the biological reaction and secondary clarifica-
tion are all conducted in a single vessel.
Treatment of Waste Water
, The present inventors have discovered that by con-
trolling the supply of oxygen into the biological reaction zone,
not merely to avold undissolved oxygen within the reaction zone,
but more precisely so as to meet the biological oxygen demand
of the waste liquid, that emergence of oxygen from solution,
is avoided and efficient clarification of the liquid, on a
continuous basis, is obtained. In this manner stable clarifica-
tion and biological reaction zones are maintained.
In a process, according to the invention, waste water
is continuously passed through a single treating enclosure open
to the atmosphere containing waste degrading microorganisms, to
which oxygen is added to sustain the microorganisms and from
which the clarified effluent is continuously overflowed and
from which excess sludge and gases are removed.
In starting up the process, there is initially
established (a) in a lower part of the enclosure a biological
reaction zone containing mixed liquor containing saicl micro-
organisms and in which a biological reaction to degrade the
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3~
the waste is conducted, (b) in an upper part Qf the enclosure a
clarification zone in which clarified liquid rises and overflows,
and ~c) between the reaction and clarification zones a transi-
tion zone in which the li~uid of the mixed liquor rises and the
solids settle.
Then, after these conditions have been established,
there are then carried out continuously, the following steps.
A recycle stream of mixed liquor from the reaction
zone is withdrawn and conducted through an oxygen-dissolving
device dis~osed outside the reaction zone, influent waste water
added to it, oxygen dissolved in the stream, and the supplemented
stre~ injected into a lower part of the reaction zone remote
~rom the vicinity of withdrawal.
m e waste water is conducted into the recycle stream
at a variable rate within a range related to the depth and
surface area of the enclosure to provide a residence time with-
in the reaction zone effective for the biodegradation of the
waste and for the formation and settling of biological floc.
Oxygen is added to the recycle stream at a rate to
provide an oxygen concentration within a controlled range below
the saturation level of oxygen in the liquid effective to meet
the oxygen demand of the organisms and maintained in contact
with the liquid in a contact zone of the stream for a time and
under a pressure such that the oxygen is dissolved in the liquid.
The overall flow rate of the recycle stream is con-
trolled to a substantially constant rate several times that of
the incoming waste water effective to provide (d~ for dissolving
the oxygen which is added to the recycle stream, (e) an amount
of dilution of the recycle stream entering the reaction zone
effective to prevent the oxygen coming out of solution at an
upper part of the reaction zone.
The flow of the recycle stream entering the reaction
;:
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3~
zone is distributed to reach a substantial area o-f a lower part
thereof (f) to provide 2 wide spread direct flow through the
reaction zone, from the vicinity of injection to the vicinity
of withdrawal, whereby there is controlled agitation effective
to keep the solids dispersed, and good access of the organisms
to the biodegradeable waste, (g3 and to provide at an inter-
mediate level o the enclosure, an upward velocity of the mixed
liquor less than the settling rate of the solids, whereby there
- is maintained in the enclosure separate reaction and clarifica-
tion zones intervened by a transition zone.
The concentration of dissolved oxygen in th~ reactionzone is continuously monitored to determine variations thereof,
resulting from variations in the flow rate and concentration
therein of waste.
The rate of addition of the oxygen to the recycle
stream is periodically adjusted in response to variations in
the oxygen concentration in the reaction zone to maintain the
concentration within the controlled range and at a level where
there i5 substantially avoided effervescence that would lead to
gas bubbles rising into the clarification zone.
The effluent is continuously withdrawn from the
clarification zone to keep pace with the influent waste water.
And, continually, excess sludge is removed from the reaction
zone and carbon dioxide removed from the mixed liquor.
Suitably the oxygen is dissolved in the recycle stream
in an oxygen dissolving device, which comprises a housing defin-
ing a contact zone for the recycle stream and in~ected oxygen
and including means for injecting oxygen into the recycle stream
contained within the housing. Thus the oxygenated recycle
stream is circulated through the biological reaction zone and
through the device for a time effective for completion of the
biological rsaction in the biological reaction zone and at a
~3fl~d
flow rate effective to maintain solids in the mixed liquor i~
suspension, and an upwardly flowing clarified liquid is con-
tinuously separated from the oxygenated liquid in the biological
reaction æone to form the zone of clarified liquid above the
biological reaction zone.
The rate of flow of the recycle stream in the biolo-
gical reaction zone is at a flow rate several times greater than
the flow rate of the upwardly,flowing clarified liquid and the
flow rate of the upwardly flowing clarified liquid is such that
the rate of settling of suspended solids is greater than the
upward flow of li~uid to permit the clarification. The supply
of oxygen is controlled to meet the biological oxygen demand of
the microorganisms and avoid undissolved oxygen in the biological
reaction zone such that conveyance of suspended solids, by bubbles
of oxygen, into said æone of clarified liquid is avoided.
The oxygen supplied to the biological reaction zone is
controlled by careful monitoring so that the oxygen requirement
of the microorganism for efficient metabolism is met. At the
' same time, and most importantly, the supply of oxygen is care-
fully controlled to ensure that there is no undissolved oxygen
in the biological zone or the clarification zone. It is found
that if undissolved oxygen is present in the biological reac-
tion zone then the undissolved oxygen in the form of small
bubbles disturbs the secondary clarification because the small
,b,,ubbles rise through the upwardly flowing clarified liquid and
convey solid particles of waste material with them so that
satisfactory clarification is not achieved, further the oxygen
bubbles and the solid particles of waste material conveyed by
the oxygen bubbles tend to pick up active material in the bio-
logical reaction zone comprising both microorganisms and wastematerial which has not been biologically treated and this also
results in unsatisfactory secondary clarification.
87
The apparatus used in the invention comprises a single
vessel and includes one or more ox~gen dissolving devices
adapted to dissolve oxygen in the recycle stream~ The device
may be located within or externally of the vessel.
The means for controlling the recycle stream is
effective to continuously circulate the oxygenated stream
through the biological reaction zone and through the oxygen
dissolving device for a time effective for completion of the
biological reaction in the biological reaction æone and at
flow rate effective to maintain solids in the reaction zone
in suspension' further the recycle stream is controlled so as
to produce a flow rate in the recycle stream considerably
greater than the flow rate of upwardly flowing clari~ied
liquid, and a flow rate of upwardly flowing clarified liquid
such that the rate of settling of suspended solids is greater
than the upward flow,of liquid to permit the clarification
thereof. The means for adjusting the supply of oxygen to said
oxygen dissolving device responsive to the monitoring means is
effective to control the oxygen supply to meet the blological
oxygen deman~ and avoid undissolved oxygen in the biological
reaction zone.
Effectively in the process of the
invention the biological treatment of the waste water and the
clarification of the biologically treated water, known as
secondary clarification, are conducted in a single vessel
having a lower biological reaction zone and an upper clarifi- '
cation zone with an intervening transition zone. Although
physical separation of the two zones is not absolutely neces-
sary it is found to be convenient to employ flow distributing
baffles between the æones since this improves the separation
of suspended solids from the upwardly flowing clarified liquid.
The two zones are e~fectively produced by appropriate
-- 1 0 --
~3~37
hydraulic design, in any hydraulic system in which water in a
vessel is subjected to agitation there will be a zone in which
the flow rate of the water resulting from the agitation is
high and more remote zones in which the flow rate is low.
The present invention utilizes this phenomenon to advantage,
the vessel being constructed such that the biological reaction
zone is the zone in which the flow rate of water is high and
the clarification zone is the zone in which the flow rate of
water is low.
The flow rate of water in the clarification
zone is equal to the rate of flow of the influent into the
vessel, the flow rate of water in the recycle stream
being 10 to 100, preferably 25 to 50 and more preferably 35 to
45 times the rate of flow of the influent.
The significantly higher rate of flow in the
recycle stream relative to the rate of flow in the
clarification zone, is necessary both to produce the required
hydraulic system permitting efficient separation of the clari-
fied liquid, and to maintain the solids precipitated from the
water in the biological reaction zone in suspension, the solids
must remain in suspension since settling of the solids and
accumulation in the bottom of the vessel will eventually dis-
turb the hydraulic system.
As the amount of precipitated solids increases, a
portion of the solids settle and are periodically removed as
a sludge from the bottom of the vessel. By continually re-
moving settled solids and continuously removing clarified
effluent, it is found that a stable system is established for
the continuous treatment of waste water.
The flow rates and the design of the waste treatment
system is such that the time which the waste water spends in
the oxygen dissolving device is very low in comparison with
~3~
the time spent in the biologica-l reaction zone. E~or each cir-
culation of waste water through the oxygen dissolving device
and biological reaction zone the xesidence time of the waste
water in the oxygen dissolving device is typically from l to
3 minutes. The total average time that waste water spends
in the biological reaction zone is about 0.5 to 5 hours, pre-
ferably about l to 3 and more preferably about 2 hours.
The circulation of the recycle stream through the
oxygen dissolving device and the biological reaction zone is
controlled so as to provide for dissolving of the injected
oxygen, the ~eed of which may vary, and to maintain the
oxygen in solution as the recycle stream enters the biologi-
cal reaction zone.
To this end, it is found to be appropriate to con-
trol the circulation of the recycle stream such that the time
- for one complete circulation of the volume of the biological
reaction zone is 1 to 60 minutes.
.The flow rate of the influent will vary as will the
concentration and quality of the biodegradeable waste, and this
means that the oxygen requirement will vary in response to
these changes. In the present invention the parameters of the
vessel are selected so that the vessel can acco~modate changes
in the flow rate o~ the influent. Further, by monitoring the
dissolved oxy~en concentration and controlling the feed of
oxygen in response thereto in accordance with the invention,
account is taken of the variations in the oxygen requirement
of the microorganisms in response to variations in the flow
rate of the influent and the content and quality of the waste
material therein. There is thereby obtained an efficient
treatment of the waste water.
In this respect a treatment vessel within the inven-
tion can be successfully employed to treat 300 to l,500, typi-
cally 600 ~allons per sq. foot per day~
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foo-t per day of waste water on a continuous basis.
Further it was not to be expected that the proce~s
and apparatus of the invention employing a single vessel would
permit the effective continuous treatment of a water having a
higher content of was-te material than the conventional system
in which aeration and secondary clarification are conducted
in completel~ separate vessels.
It will be understood that in the clarification zone
the rate of settling of the solids must be greater than the
rate of upward flow of li~uid to achieve efficient clarifica-
tion.
Accordingly, in constructing the vessel for carrying
out the process of the invention various factors must be taken
into consideration which will depend on the conditions of the
particular case, but which are, however, well within the scope
of the competent workman in this field of technology.
As has been described previously it is essential that
the oxygen supplied to the biological reaction zone be care-
fully controlled to ~nsure that there is no undissolved
oxygen in the form of gas bubbles. For similar reasons it is
appropriate to employ commercial oxygen free of other gases.
Air would not be suitable as the source of oxygen in view of
the high content of nitrogen; nitrogen i9 much less soluble
in water than oxygen and employment of air as the source of
oxygen would result in a large number of nitrogen bubbles in
the biological reaction zone which would rise upwardly convey-
ing solids into the clarification zone. It might be possible
to employ an oxygen enriched air having a high oxygen content
as the oxygen source if this did not introduce a significant
amount of undissolved nitrogen into the system.
The o~yg~n is injected into the recy~le stream so as
to maintain the oxygen concentration in the ~iolo~ical reaction
~ , .
- - 13 -
zone in a selected range. This selected range is determined
both by the requirement of the ~icroorganisms in biodegrading
the waste solids in the reaction zone, and by the necessity
of avoiding undissolved oxygen which would disrupt the clari-
fication. The saturation value for oxygen in water is about
43 mg/l but for air is about 9.2 mg/l, at 20C. The satura-
tion values in waste water are typically 80 to 9~% of the
values in water.
Since the vessel is most conveniently operated open
to the atmosphere, the atmosphere above the clarification zone
is air. Consequently the selected range for the oxygen concen-
tration is determined on the basis of the saturation value for
air in waste water.
The saturation values of gases in water are dependent
on the temperature of the water, and the process of the inven-
tion will generally be carried out with waste water at a
temperature of from 7C to 35C, and more usual}y from 10C to
30C.
Within these operating temperature ranges, the dis-
solved oxygen concentration in the reaction zone is suitably
selected within the range of 1 to S mg/l and preferably 2 to
3 mg/l. A lower limit for effective operation would be of
the order o~ 0.1 mg/l, but concentrations of this order, while
within the scope of the invention, are less preferred. The
upper limit is the saturation value for oxygen in the waste
water, however, it is inappropriate to employ this upper limit
when the vessel is open to the at sphere. The upper limit
for operation in a vessel open to the air is more appropriately
the saturatlon value for air in the waste water, although it
is probable that the invention could be carried out while
maintaining a dissolved oxygen concentration o~ 10 to 15 mg/l
this, howev~r, is less pre~erred.
~3~
The oxygen concentration may well be higher in the
recycle stream in the oxygen dissolving device than in the
biological reaction zone, the recycle stream being diluted on
entering the biological reaction zone. ~Iowever, by controlling
the oxygen concentration in the biological reaction zone re-
lative to the saturation value for air, the oxygen concentra~
tion in the oxygen dissolving device will be below the satura-
tion value for air.
In carrying out the invention, gases such as nitrogen
are formed so that the system moves towards a nitrogen con-
taminated oxygen rather than pure oxygen. Although the
relative proportions of nitrogen and oxygen will not approach
that in air. it is safer to employ the saturation value for
air, rather than that for oxygen in selecting the oxygen
concentration so as to avoid undissolved bubbles of oxygen
and nitrogen.
Further, in a vessel open to the atmosphere, an
equilibrium is established at the interface of the air and
the exposed surface of the water. If the oxygen concentra-
; 20 tion in the water was higher than the saturation value for
air, dissolved oxygen would come out of solution at the inter-
face, as bubbles, conveying solids and forming a scum on the
clarified liquid, which would be inacceptable.
It is thus found to be preferable to maintain the
diSsolved oxygen concentration well below the saturation value
~or air in the waste water, so as to minimize the chance of
the saturation value being inadvertently exceeded such that
bubbles of oxygen would emerge from solution.
Furthermore, the rate of dissolving of the oxygen
in the waste water increases and hence the efficiency in-
creases, as the ~issolved oxygen concentration moves away
; from the saturation value, where the system is at equilibrium.
~ - 15
3~
Thus in carrying out the process of the invention the
dissolved oxygen concentration, in the biological reaction zone,
in continuously monitored, and a signal established in response
to the monitoring indicative of the dissolved oxygen concentra-
tion. The feed of oxygen to the reaction zone is then regula~
ted in response to the signal so as to rnaintain a pre-establis-
hed dissolved oxygen concentration in the reaction zone of at
least 0.1 mg/l and less than the saturation value of air in the
waste water, effective to meet the biological oxygen demand per
unit time, and avoid undissolved oxygen in the biological re-
action zone such that conveyance of suspended solids, by
bubbles of oxygen, into the clarification zone is avoided.
The oxygen probe or sensor of the oxygen monitoring
system is disposed in the biological reaction zone. A suitable
probe comprises a polargraphic cell encased in a membrane of a
chemically resistant polymer which is permeable to oxygen. A
part of the dissolved oxygen in the reaction zone proportioned
to the partial pressure, diffuses through the membrane into the
cell body and is reduced at the cathode surface. This causes a
current flow proportional to the amount of oxygen in the bio-
logical reaction zone. Such probes are sufficiently sensitive
that variations of 0.1 ppm in the oxygen concentration are
easily detected.
The oxygen is suitably introduced into the recycle
stream in finely divided form to ensure efficient dissolving of
the oxygen in the waste water. The oxygen is injected into
the recycle stream in the form of fine bubbles. The pressure
at which the oxygen is introduced into the recycle stream may
suitably vary from 1 to 60 psig.
It is also found to be highly expedient to include
in the vessel of the invention a flow distributor in a lower
part of th~ biological reaction zone but at an elevated posi-
tion with respect to the level of introduction of oxygenated
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~3~
mixed liquor to the biological reaction zone. Such a flow
distributor suitably comprises a planar element spaced apart
from the bottom of the vessel and extendin~ from wall to wall
of the vessel and having a plurality of passayes therethrough
for passage of the liquid and solids therein into the biologi-
cal reaction æone, such a flow distributor serves to direct
the flow upwardly and to discourage the setting up of side
currents in the upward flow of liquid which might disturb the
clarification zone.
Suitably the passages may be of circular cross-
section, although they may also be eliptical, rectan~ular or
square, having an area of from 0.8 to about 2a sq. ins.; pre-
ferably about 2 to about 10 sq. ins., the area of the flow
distributor occupied by the passages bein~ from ~bout 20 to
about 80% of the total area, preferably 30 to 70%. It will
be recognized that the passages must be sufficiently large to
permit passage of recirculated solids in the liquid and that
if the passages are too small in cross-sectional area that
clogging may occur and this will interrupt the continuous
treatment.
Under steady conditions the solids content in the
system increases slowly. In order to keep the concentration
of these solids constant, a small portion is pumped out at
frequent intervals. The solids may be pumped out on a daily
basis for a period of 30 minutes to 24 hours, however, typi-
cally they are pumped out for a single 4 hour period each day.
The solids are pumped out at a rate daily which is about 1.0
to 10%, preferably about 4% of the influent flow per day. In
a typical operation where the influent flow is about 60,000
~allons/day, the solids are suitably pumped out from the bio-
logical reaction zone at a rate of 10 gals/min for 4 hours which
represents a rate of about 4% of the influent flow per day.
.
- 17 -
The parameters of the vessel rnust be selected to
maintain a relationship between the volume of water treated
per unit of time and the horizontal area surfacing to the
atmosphere so as to permit the establishment of the stable
clarification and biological reaction zones with the inter-
mediate transition zone and provide a residence time within
the biological reaction zone effective for the biodegradation
of the waste and for the formation and settling of biological
floc.
A typical treatment vessel must treat 300 to 1000 U.S.
gallons per sq. ft. per day and in such a vessel the clarifi-
cation zoné typically has a depth of 5 to 15 feet and pre-
ferably about 10 feet, the transition zone depth is typi-
cally 1 to 5 ft., and the biological reaction zone typically
has a depth of 5 to 13 feet, and preferably about 10 feet.
It is also within the scope of the invention to
carry out the primary clarification of waste water in the same
vessel as the biological treatment and secondary clarification.
This can suitably be achieved by incorporating skimmers in the
vessel effective to skim floating solids and oils from the
upper surface of the biological reaction zone as well as a
conveyor device in the bottom of the vessel to collect and
~ remove heavy solids which settle rather than remaining in
suspension.
A particular advantage of the apparatus of the pre-
sent invention is that it permits a much higher treatment
capacity per unit surface area of treatment tank, than exist-
ing installations.
In the case of the Hamilton treatment plant described
previously, the separate aeration tank and clarifi~r in each
series can be converted to two single treatment tanks in
parallel, according to the teachings of the present invention
- 18 -
L3~8'~
and in this way the treatment capacity (i.e. volume of water
treated per unit time) of an existing installation can be
increased by from 50% to more than 100%. Thus in the example
of the Hamilton plant it can be shown that -the treatment
capacity of 7.5 million gallons/day can be increased to 11 to
19 million gallons/day by modifying the existing two-tank
series to provide two single treatment tanks in parallel~
If the tanks are modified to the embodiment in which
all three treatments (primary clarification, biological re-
action and secondary clarification) are carried out in the
same vessel, then the primary clarification tanks can also be
modified to provide treatment vessels of the invention. In
this case each series of three tanks in the existing installa-
tion can be converted to three single treatment tan~s in
parallel to provide a treatment capacity which is shown to be
more than three times the capacity of the single series of
three tanks.
It will thus be evident that the process and appara-
~ tus of the invention, which by caref~ll control of the added
oxygen permit treatment on a continuous basis, without
interruption, provide significant advantages especially in
that they permit a significant increase in the treatment
capacity of an existing plant and the construction of new
plants of generally smaller size for a given treatment
capacity.
Oxygen Dissolving Device
In this specification the expression "oxygen dis
solving device", or "oxygen contact device" refers to any
device which can be employed to contact oxygen and waste
water of the recycle stream and dissolve the oxygen in the
water, and which comprises a housing through which the waste
water flows, and within the confines of which, the oxygen is
-- 19 --
contacted with the recycle stream and dissolved therein.
An especially preferred class of oxygen dissolving
devices is the class generally illustrated in U.S. Patent
3,6~3,403 which comprises a flow confining chamber having an
upper inlet and a lower outlet through which waste water may
be impelled downwardly with a decreasing velocity from a
maximum at the inlet end to a minimum at the outlet end. As
described by Speece the flow confining chamber comprises a
downwardly diverging funnel or generally conical housing,
having a vertically disposed intake tube at its upper end and
an impeller mounted in the intake tube to direct the flow
downwardly. The device further includes means ~or introducing
oxygen to the funnel portion in the form of a bubble disperser.
This device can be located internally or externally of the
treatment tank.
As indicated previously, the residence time of the
waste water in the oxygen dissolving device for each circula-
tion will generally be from 5 to 100 seconds. In the afore-
mentioned device comprising a generally conical housing, the
residence time is preferably from 10 to 30, more preferably
... . .
about 15 seconds.
The pressure of the oxygen in the conical housing is
preferably about 3 to 7 psig above atmospheric pressure.
Another preferred oxygen dissolving device, parti-
cularly for location outside the treatment tank comprises a
generally vertically disposed cylindrical tube having a verti-
cally disposed partition wall separating it into an upstream
side and a downstream side, the partition wall terminating
above the bottom of the tube to provide a clearance for flow
3~ of waste water. Waste water is introduced at the upstream
side and flows downwardly in the tube, under the lower edge of
the partition wall and upwardly through the downstream side,
- 20 -
and oxygen is introduced into the waste water in the upstream
side where it is entrained in the downwardly moving water.
The parameters of the tube are such that a long path
is provided for contact between the recycle stream and the
oxygen. A typical tube may have a vertical height of 100 to
150 feet, the pressure increases as the oxygen bubbles and
recycle stream descend the downflow portion of the tube and
this increases the rate of dissolving of the oxygen. When the
recycle stream ascends the upflow portion of the tube the
pressure decreases, but as long as the oxygen content is below
the saturation value there will be substantially no tendency
for the oxygen -to come out of solution.
In the tubular oxygen dissolving device the residence
time of the waste water in the device for each circulation will
preferably be from 30 to 100, more preferably 40 to 60 seconds~
In a typical er~odiment the recycle stream may spend about 20
seconds in the downstream side and 20 seconds in the upstream
side.
The pressure of the oxygen gas introduced to the
tubular device is preferably about 3 to 60 psig above atmos-
pheric pressure.
The vertically disposed tube is suitably embedded
.
in the earth as is the treatment tank.
In an especially preferred enlbodiment it is-found
to be convenient to employ a plurality of such tubes so as
to increase the efficiency in dissolving the oxygen.
; The present invention provides improvements in such
oxygen contacting devices which improve the control of oxygen
dissolving and increase the efficiency and rate at which
oxygen is dissolved.
- 21 -
~43~7
An important characteristic of the invention is
that the oxygen dissolving device or the oxygen contact zone
be effective to efficiently dissolve oxygen in the waste water
before the waste water enters the reaction zone, undissolved
oxygen bubbles should not enter the reaction zone from the
oxygen contact zone.
In one embodiment the invention provides improve-
ments in oxygen contacting devices which employ an impeller
which produces a spiral flow in the downwardly moving liquid,
- 22 -
which spir~l flow enlarges downwardly because of the shape
of the flow confining chamber. This spiral flow produces a
vortex in the liquid in the upper intake tube which sucks
or draws in air from the atmosphere above the vessel in
which the oxygen contacting device is located. The major
component of air is nitrogen and the nitrogen mixes with
the oxygen and dilutes it and this reduces the rate at
which oxygen is dissolved into the liquid. Furthermore
when the oxygen dissolving device is employed in the two
zone waste water treatment system of the invention, the
presence of undissolved nitrogen in the biological reaction
zone disturbs the clarification and the maintenance
of the separate clarification and biological reaction zones
in the same manner as does the presence of undissolved
oxygen. This is clearly undesirable in the process and
apparatus of the invention in which the injection of oxygen
is to be carefull~ monitored and controlled to meet the
; ~ biological oxygen demand and avoid the presence of undis-
solved oxygen in the biological reaction 20ne At the
same time the uncontrolled introduction of oxygen in the
air at the intake tube may disturb the monitoring and control
of the injected oxygen and result in undissolved oxygen in
the biological reaction zone, which, as described, disturbs
the clarification and the establishment of the two zones.
Thus in one embodiment there is provided an oxygen
dissolving device of the general class described which is
to be immersed in an open body of liquid below an atmosphere
of air and which includes an impeller which directs liquid
downwardly with a spiral flo~, for example an axial pump,
wherein the improvement comprises means disposed above the
impeller effective to prevent sucking in of air from the
_ 23
upper atmosphere into the downwardly moving liquid,
According to the invention there.is provided a
device for dissolving a first gas in a body of.liquid,
adapted to be immersed in an open body of the liquid
beneath an atmosphere of a second gas, comprising a flow
confining cham~er having an inlet and an outlet end through
which a downflow of liquid is conducted wi-th a decreasing
velocity from a maximum velocity at the inlet end to a
minimum velocity at the outlet end, an intake tube at said
inlet end having at least one inlet port in the tube wall
for said liquid, an impeller mounted in said intake tu~e,
below said at least one inlet port, adapted to direct said
liquid downwardly through said flow confining chamber in an
enlarging spiral flow, means for injecting bubbles of said
first gas into the downwardly flowing liquid and means in
said intake tube disposed upstream of the impeller effec-
tive to prevent sucking in of said second gas into the
liquid.
. In an especially preferred aspect the means up-
.stream of the impeller in the intake tube comprises a
plurality of radially disposed vanes extending inwardly
from the wall of the intake tube, each vane extending along
the wall of the intake tube upstream and downstream of the
inlet ports and terminating just upstream of the impeller.
In general it is preferred to employ four
symmetrically disposed vanes, however, threa or even two
vanes can also be employed. Of course, more -than four .
vanes can be employed, and the maximum number which can
be employed will be dictated by their dimensions and the
volume of the intake.tube.
_ 24 -
~1434~ i~
The vanes are suitably disposed radially,
however, they may also be inclined towards the direction
of the spiral flow. The vanes prevent the formation of a
vortex by the spiral flow adjacent the upper atmosphere,
which vortex would suck in gas from the atmosphere. I'he
vanes should not be inclined away from the direction of
splral flow since this would promote the establishment of
the vortex.
The vane~ have been conveniently ~mployed in a
lQ gas dissolving device~in which the impeller has a
relatively low speed of about 240 rpm.
When an impeller having a higher speed, of the
order-of 17Q0 rpm, is to be employedLit is found suitable
to replace the vanes by a plate located above the inlet
ports and extending inwardly from the inner wall of the
intake tube. The plate does not prevent the establishment
of a vortex but it -does prevent the sucking in of the air
by providing only a small clearance between the plate and
the shaft of the impeller for the liquid above the plate.
The plate should also extend outwardly from the outer wall ~ --
of the intake tube since the vortex created by the high
speed impeller may extend out into the body of liquid out-
side the tube in the vicinity of the inlet ports
In a further embodmment it is found especially
advantageous to radially introduce the oxygen in a plurality
of streams from an annular ring shaped injector mounted
_ 25 -
~L341~
adjacent the inlet end of the flow confining chamber which
injector has a plurality of spaced apart orifices communi-
cating with a source of oxygen. Injection of the oxygen
in this manner increases the number and surface area of
bubbles for a given volume of injected oxygen and thus
increases the rate of dissolving of the oxygen or the rate
of mass transfer. The streams are suitably directed radially
inwardly, however, they might also be directed inwardly in a
non-radial direction or in a tangential direction.
In another embodiment it is preferred to employ
distributor means in the flow confining chamber, for
example, one or more horizontally disposed perforated
plates or sets of vertically disposed tubes. These
distributor means are effective to offset or neutralize
- the splral flow and distribute the flow in a generally
vertical direction while at the same time increasing the
turbulence in the flow confining chamber and improving the
gas~liquid contact and thus the efficiency of dissolving
the gas. It is further within the scope of the invention
to include one or more vertically disposed flow direct-
ing plates in the flow confining chamber.
The appearance of bubbles of gas in the flow con-
fining chamber appears at the point where the buoyancy of
the gas equals the velocity of downflow of the liquid,at
this point a cloud of bubbles is visible_ The turbulence
in the flow confining chamber produces a continuous shear-
ing, coalescing and reforming of the bubbles, in the cloud
and efficient dlssolving of the gas.
In a further ~mbodiment the flow confining chamber
consists of a cone made up from two parts. The upper conical
,. .. .
_ 26 _
~3fl~
part with a certain angle of divergence is connected to a lower
conical part in which the angle of divergence is greater than
that in the upper part. In this way the velocity of downflow of
the liquid in the chamber is initially maintained high in the
upper half of the chamber and then decreases rapidly in the
lower half of the chamber, this increases the turbulence and
increases the gas/liquid contact thereby increasing the rate
and efficiency of the dissolving of the oxygen. In this way
the residence time of the liquid in the o~ygen dissolving
device, for each circulation can be increased.
According to another aspect of the invention there is
provided a device for dissolving a gas in a body of liquid
comprising a flow confining chamber having an inlet encl and an
outlet end through which a downflow of liquid is conducted with
a decreasing velocity from a maximum velocity at the inlet end
to a minimum velocity at the outlet end, the walls of said
chamber diverging from said inlet end to said outlet end, said
flow confining chamber comprising an upper chamber communicating
with a lower chamber, the walls of the upper chamber diverging
less rapidly than the walls of the lower chamber.
In a further preferred embodiment of the invention,
surprising results were accomplished, by injecting the influent
stream horizontally into the bottom of the biological zone from
one vicinity of the vessel, in the form of a wide, deep, rela-
tively high-velocity high-dissolved oxygen inflow and drawing
off mixed liquor from the biological zone, at a vicinity near
the floor of the vessel remote from that of injection, in the
form of a wide outflow, to provide a low dissolved oxygen re-
cycle stream. Between the inflow and the outflow there is thus
formed along the floor of the vessel across the width of the
biological zone, an undercurrent of mixed liq~or having a
relatively high horizontal velocity. The e~tensive area of
_ 27 -
. .
interface between the undercurrent of highly oxygenated horizon-
tally moving supplemented recycle stream and the overlying mixed
liquor permits substantially maximum oxygen content with the
organism without causing undue turbulence to interfere with the
settling of the suspended solids. A mixing action takes place
in the interface zone and, undoubtedly, a certain amount of
local turbulence in the form of eddies, but this does not inter-
fere with the settling of the solids from the clarification æone.
Oxygen is continuously dissolved in the recycle stream, outside
the vessel, and combined with waste water influent, to form a
supplemented recycle stream which forms the inflow.
The applicants have found that this bxings about
extensive and intimate contact between the dissolved oxygen and
the mixed liquor afforded by the wide undercurrent running along
the bottom of the biological zone so as to result-in substan-
tially maximum consumption of dissolved oxygen by the organisms.
Maintaining the undercurrent near the floor of the vessel, it
has been found, isolates the mixed liquor in the uppe~ part of
the biological zone from undue agitation which would cause
mixing between the biological zone, and the clarification zone.
It has also been found that the undercurrent, sweeping the floor
of the vessel, keeps the solids in suspension and avoids sludge
build-up.
Most of the liquid velocity for the mixing occurs in
the horizontal direction. This increases stability, since with
low upward velocity, the solids are allowed to settle out, and
the effluent is low in suspended solids. So, compared with the
prior art, with the present process there is available a higher
capacity per unit of surface area with the same grade of effluent
or, alternatively, a better grade effluent with the same capacity.
The mixed liquor may be drawn off from the biological
zone by a pump located on the floor of the vessel. In this
87
event, the behavior characteristics-of the mixed liquor have
been observed -to be different from that of mixed liquor in a
standard aeration system, in that the`solids settle more readily,
thus aiding clarification. The explanation may be that settling
is encouraged by the vibration of the pump.
.. ~
The size of the process vessel may vary. Its hydrau-
lic depth must be effçctive to hold enough biomass for effi-
cient treatment of i~coming waste and to'provide for a clarifi-
cation zone. The depth may run from at least about 8 feet to
as much as about 100 feet~ The depth of the clarification zone
must be effective to minimi2e the carryover ofisolids from the
biomass by the effluent and should be at least a~out 2 feet.
The distance between the vicinity of the inflow and the vicinity
o~ the outflow should be long enough for the microoxganisms to
absorb the oxygen without becoming devoid of oxygen at the point
of recycle which can be from about 6 feet and 200 feet or more.
A prefer'red distance is between about 20 feet and about 100 feet.
The initial depth o~ the inflow is great enough to prevent undue
pressure drop and not great enough to allow mixed liquor to flow
into the cIarifier zone, preferably within the range from about
6 inches to about 6 feet~ The width of the inflow is really
only limited by the width of the vessel and preferably is the
entire width of the vessel. The biological zone should have a
minimum depth of at least about 2 feet and can exte'nd to about
2 feet less than the hydraulic depth of the vessel~' The calcul-
ated average linear velocity of the inflow at the vicinity of
injection should be enough to prevent undue quiescence in the
biological reaction zone without producing agitation which would
cause the oxygen to come out of solution and is preferably with-
in the range from about 1 to about 35 feet per minute, but thisis not critical. The average horizontal velocity in the re-
action zone is high enough to be effective to bring the oxygen
- 29 -
L3~7
into contact with the biomass and low enough so that the oxygen
will be substantially consumed without the organisms becoming
devoid of oxygen, preferably within the range from about 1/2 to
about 20 feet per minute. I~e recirculation rate is effective
to supply enough oxygen to carry out the process e~ficientl~ and
is preferably within the range from about 1 to about 15 times
the average waste water influent flow rate.
The invention also contemplates apparatus for carrying
out the preferred form of the process described as will be
evident from the follo,wing description.
The invention is illustrated in particular and pre-
ferred embodiments by reference to the accompanying drawings,
in which:
FIGURE 1 illustrates schematically an apparatus of
the invention for carrying out the process of the invention in
which an oxygen dissolving device is located within the vessel,
FIGURE 2 illustrates schematically a different emhodi-
ment of the inven~ion in which an oxygen dissolving device is
; located outside the ves~sel,
FIGURE 3 illustrates schematically a carbon dioxide
stripper which may be incorporated in the systems illustrated
in Figures 1 and 2,
FIGURE 4 illustrates a detail of a modi~ied intake
tube for the oxygen dissolvi,ng device of Figure 1,
FIGURE 5 is a section on a line 5-5 of the detail of
Figure 4,
FIGURE 6 illustrates a detail of another modification
of the intake tube for the oxygen dissolving device of Figure 1,
FIGURE 7 illustrates schematically a modified flow
confining chamher of an oxygen dissolving device having a
pinched ~aist,
- 30 -
~3~i~7
FIGURE 8 shows a detail of an assembly o~ flow
directing plates and a distributor plate housed in the flow
confining chamber of Figure 7,
FIGURE 9 illustrates an oxygen injector ring shown
in the device of Figure 7.
FIGURE 10 illustrates a detail of the ring of Figure.
9 showing the gas outlets,
FIGURE il illustrates schematically another embodi-
ment of the invention in which an oxygen dissolving device is
located outside the vessel,
FIGURE 12 illustrates in greater detail the oxygen
dissolving device employed in Figure 11,
FIGURE 13 illustrates schematically a variant of the
embodiment of Figure 11, employing two oxygen dissolving
devices,
FIGURE 14 is a diagrammatic illustration in vertical
cross-section of one form of apparatus suitable for carrying
out the process of the invention, and
FIGURE 15 is a similar diagrammatic illustration of
another form of apparatus.
With further reference to Figure 1, a treatment
apparatus comprises a tank lO having disposed therein a flow
directing baffle 12. An influent line 13 delivers influent
to an upper part of the tank 10 within the flow directing
baffle'l2, and an outlet 14 for solids is provided in the
lower part of the tank 10 for removing solids.
In an upper portion.of the tank 10 there is pro-
vided a clarifier overflow weir 16 which communicates with
- 31 -
~3~
an effluent l.ine 17 for removing clarified waterO
An oxygen dissolving device 18 is mounted in the
tank 10 and communicates via an oxygen supply line 20 with an
oxygen source 22. An oxygen probe 24 is suspended in the tank
10 and is connected via an oxygen analyzer 26 and a recorder
controller 28 to a flow regulating valve 30 in the oxygen
supply line 20.
A pump 32 is mounted above the oxygen dissolving
device 18 for circulating liquids being treated through the
oxygen dissolving device in the dixection shown by the
arrows. The pump 32 may be, for example, an axial pump or
a centrifugal pump; when frothing~of the waste watex is not
a problem and/or when stripping of C02 from the waste water
is deemed desirable, an air-lift pump can be used for the
- circulation; in this case a certain amount of oxygen from
the air-lift is picked up by the mixed liquor, thus reducing
the overall oxygen gas requirement.
The tank 10 defines a biological reaction zone 38
and a clarification zone 40 separated by a separating 20ne
42. The~flow directing baffle 12 assists in defining these
zones in the tank 10~
A plurality of flow distributing baffles 34 are
mounted in the separating zone 42 between the flow directing
baffle 12 and the upright walls of the tank 10~ The baffles
34 may suitably comprise a plurality of inclined tubular
members.
A flow distributor 36 which may suitably comprise
a planar member having a plurality of passages therethrough,
extends hetween the upright walls of the tank 10 and the
oxygen dissolving device 18 and is disposed in a lower portion
of the tank 10 above the outlet of the oxygen contacting
device 18~
- 32 -
The ox~gen supplying circuit comprising oxygen
probe 24 and the related oxygen analyzer 26, recorder control-
ler 28, flow regulating valve 30 and oxygen supply line 20
is of a kind known ,per se in other technologies where accurate
control of oxygen content is necessary. The oxygen supplying
circuit controls the supply of oxygen to the waste water
treatment so that it meets the demand exerted by the waste
water being treated.
In the oxygen supplying circuit the oxygen probe
24 senses the concentration of dissolved oxygen in the
biological reaction zone 38, the oxygen probe 24 may be, for
example, of the polarographic or galvanic cell type and
consists of two different metals immersed in an electrolyte
and separated from the waste water in zone 38 by a semi-
permeable membrane. Under steady state conditions the dis~
solved oxygen concentration is proportional to the current
produced between the two different metals in the cell.
An agitator forms a component part of oxygen probe
24 and continuously pumps liquid in zone 38 across the
membrane of the cell. The agitator is suitably fabricated
from a soft rubber and is disposed so as to wipe the membrane
to keep it free from oil and grease.
The current output from probe 24 as a measure of
the dissolved oxygen concentration is analysed by the oxygen
analyzer 26 and is amplified into a standard signal range
suitable for a standard controller, A recorder controller
28 comprises such a controller in conjunction with a recorder
and the recorder controller 28 indicates and records the dis-
solved oxygen on a continuous basis.
The controller in the recorder controller 28 com-
pares the input signal with a pre-determined set-value and
sends a signal to flow regulating valve 30 in the oxygen
- 33 -
~3~8~
supply line 20, If the dissolved oxygen is below the set
point the valve 30 is signalled to open and vice versa.
The set point is determined by experiment in advance by
determination of the biological oxygen demand of the waste-
water being treated,
The oxygen dissolving device 18 comprises a flow
confining ch~nber 18a having an inlet tube 18b separated
from an intake tube 18c by an inverted frusto-conical member
18d. Intake tube 18c includes inlet ports 18e in its side
walls. At ItS lower end the chamber 18a opens at an outlet
18f. The member 18d serves as a connecting piece between
the inlet tube 18b and the intake tube 18c w~ich in the
particular embodiment are of different diameters.
The flow directing baffle 12 is suitably located
substantially centrally in an upper part of tank lO so as
to circumvent an upper part of the oxygen dissolving device
18. In this way the baffle 12 assists in defining the
biological reaction zone 38 and tXe clarification zone 40,
in'particular an upper portion of zone 38 is defined between
the inner wall of baffle 12 and the outer surace of device
18, and the zone 40 is defined between the outer wall of
baffle 12 and the inside wall of tank 10. The baff:Le 12
suitably comprises a tubular member having an upper
cylindrical tube and a lower frusto-conical housing,
however, baffle L2 may also be a ~quare sectioned member
having an upper square sectioned member and a lower
square section p~ramidO
- 34 -
In operation influent i5 introduced into the tank
10 via the influent line 13 and is circulated through the
oxygen dissolving device 18 and the biological reaction zone
38 by the pump 32. The influent enters device 18 at the
inlet ports 18e, leaves at outlet 18f and passes through
zone 38 and back to the ports 18e~ The velocity of the
liquid in chamber 18a decreases as it moves downwardly from
the inlet tube 18b to the outlet 18f and the liquid is
subjected to tùrbulence.
. ~ . .
.
~ - 35 -
Oxygen is introduced to the oxygen dissolving device
18 from the oxygen source 22 via the oxygen supply line 20,
and the oxygen dissolves in the liquid passing through the
device 18.
The oxygen probe 24 in conjunction with the oxygen
analyæer 26 monitors the dissolved oxygen in the biological
reaction zone 38 and passes a signal to the recorder con-
troller 28 which interprets the signal and correlates the
information concerning the amount of dissolved oxygen of the
system, and actuates the flow regulating valve 3Q to control
the flow of oxygen from the oxygen source 22 to the device 18.
The oxygen fed to the device 18 is regulated at the valve 30
under the instruction from the recorder controller 28 to
ènsure that adequate oxygen is provided to meet the biological
oxygen demand of microorganisms in the biological reaction
zone 38 while at the same time preventing-the introduction of
excess oxygen into the biological reaction zone which would
be present as undissolved oxygen in the form of bubbles.
As the liquid circulates rapidly through the oxygen
dissolving device 18 and biological reaction zo~e 38, clari
fied liquid rises slowly upwardly in the clarification zone
40.
The liquid in the biological reaction zone is
conveniently circulated for a period of two to three hours,
the liquid being present in the oxygen dissolving de~ice 18
for only about 15 seconds in each circulation.
The apparatus illus~rated in Figure 2 differs from
that of Figure 1 in that the oxygen dissolving device is
located outside the vessel.
With further reference to Figure 2, the apparatus
represented therein comprises a tank 50 including an influent
.
- 36 -
:
3~
line 52 to supply influent to a lower part of the tank 50
and a solids outlet 54 in the lower part of the tank S0
for removal of solids.
The tank 50 includes a clarifier overflow weir 56
which communicates with an effluent line 57~
An oxygen dissolving device 58 is located in the
influent.line 52 for oxygenating the influent being intro-
duced into the tank 50.
The oxygen dissolving device 58 is connected by an
oxygen supply line 60 to an oxygen source 62
The influent line 52 terminates in the tank 50 at
an inlet member 53. Inlet member 53 may suitably comprise
a tubular member having a plurality of exit passages therein
for the influent to flow rom the inlet member 53 into the
interior of the tank 50. The inlet member 53 may be, for
example, an endless tubular frame having the same shape as the
cross-section of the tank, for example in the case where
the tank 50 is of circular cross-section the inlet member
53 may comprise a circular tubular member, and in the case
: 20 where the tank 50 is of rectangular cross-section the inlet
member may compxise a tubular rectangular frame.
An outlet member 64 is spaced apart from the in-
~' let member 53 and is suitably of similar configuration
having a plurality of holes or passages therein for entry
of liquid in the tank 50. Outlet member 64 communicates
with recirculating line 66 which communi.cates via pump 68
with the oxygen dissolving device 58.
An oxygen probe 70 is suspended in the tank 50
and is connected via an oxygen analyzer 72 and a recorder
controller 74 to a flow regulating valve 76 in the oxygen
supply line 60.
.
- 37 -
37
~here is defined in the tank 50 an upper clarifi-
cation zone 82 and a lower biological reaction zone 84
separated by a separating zone 86
The apparatus is constructed so that the oxygen
probe 70 and the outlet member 64 are located in the
biologicaI reaction zone 84.
As in the embodiment of Figure 1 it is convenient
to employ a plurality of flow distributing baffles 78 in the
separating zone 86 in order to enhance the separation. Such
baffles 78 conveniently comprise a plurality of inclined
tubular baffle members.
In one embodiment the tubes are inclined at an
angle of 60 to the base of the tank and comprise a stack
of adjacent tubes formin~ a module, each tube has a generally
rectangular, preferably square, cross-section, with a cross-
sectional area of about 4 sq. ins , suitably the tubes are
fabricated from a synthetic plastic, for example PVC or ABS.
Such modules are commercially available and may be stacked
~: ~ side by side while being firmly supported by clamping
members.
Similarly, it is convenient to employ a ~l.ow
distributor 80 at a lower part of the biological reaction
zone 84 and located vertically above the inlet member 53.
In one embodiment a flow. distributor (36 or 80)
was fabricated from plywood having a thickness of 0.75 inches
having about 30% of its total area occupied by circular holes
communicating with passages, which holes had diameters of
2 and 3 inches.
The operation of the apparatus illustrated in
Figure 2 is substantially the same as that as described with
reference to the apparatus of Figure 1.
: .
: - 3~3 -
~ 3~
In some cases it may be appropriate to incor-
porate into the system means for stripping off carbon
dioxide. However, carbon dioxide dissolved in the waste water
does not affect the performance of the biological treatment
when present in ~oderate quantities, and for treating domestic
waste water as opposed to certain industrial wa,qte water
stripping of the carbon dioxide is not necessary.
~ owever, the presence of carbon dioxide in the water
may reduce the rate and efficiency of the dissolving of
oxygen. When it is necessary to improve this efficiency
the carbon dioxide may be removed by a simple stripping
device. A suitable device functions by contact of the waste
water with air, 50 that the equilibrium conditions favour
the transfer of carbon dioxide from water to air. Thus
any of several known types of device that contact water with
air may be used, for example a surface aerator, submerged
turbine or air sparger. The operation of an air sparger
as a carbon dioxide stripper is illustrated schematically
in Figure 3.
With further reference to Figure 3 there is
illustrated schematically an air sparger 90 comprising a
wet well 92 and a vertical column 94, a line 93 is connected
to wet well 92 and lines 96 and 98 are connected to column
94; a compressed air line 100 connects column 94 to a source
of compressed air (not shown).
The air sparger 90 is disposed in the system
illustrated in Figure 1 or 2 so that a portion of the
circulating waste water being treated flows through line 93
to the wet well 92 and travels upwardly through column 94
and back to the circulating waste water in the system via
line 96. Compressed air is introduced to the waste water
in column 94 ~ia line 100, strips car~on dioxide from the
- 39
37
water and exits via line 98.
With reference to Figures 4 and 5 there is illus-
trated a modified intake tube 18c which can be employed in
the oxygen dissolving device 18 illustrated in Figure 1.
The intake tube 18c includes inl0t ports 18e in its side
walls and a pump 32 having an impeller 102 on a centrally
disposed shaft 104, the impeller being disposed just below
the inlet ports 18e, the intake tube 18c has an upper end
106 which is open to the atmosphere. Extending radially
inwardly from the inner wall of intake tube 18c are four
vanes 108 which extend vertically above and below inlet
ports 18e and terminate at their lower ends just above the
impeller 102. The inner edges of vanes 108 are spaced
apart from shaft 104 to provide a smaïl clearance,
The intake tube 18c is shown in its wor~ing
environment in an open body of liquid 110. The vanes 108
prevent the formation of a vortex in the liquid 110 in the
tube 18c above the impeller 102, which vortex would suck
in air from the atmosphere above the liquid 110.
:~20 With reference to Figure 6 there is illustrated
a further modification of the intake tube 18c of an oxygen
dissolving device 18 in which the vanes 108 of Figures ~
and 5 are replaced by a disc-shaped plate 112 having an
inner edge 112a and an outer edge 112b. The plate 112
extends inwardly of the wall of tube 18c so that inner
edge 112a is spaced apart from shaft 104 with a small
clearance; and the plate 112 extends outwardly of tube 18c
so that edge 112b is remote from tube 18c.
The plate 112 does not prevent the formation of
a vortex in liquid llb above impeller 102, however, it does
prevent air being drawn from the atmosphere into the liquid
- 40 -
~ 3~
by the vortex. The edge 112b should be sufficiently re~ote
from the tube 18c to prevent air being sucked in to a vortex
extending out of tube 18c through inlet ports 18e.
; In otherwords the parameters of the plate 112 are
determined by the vortex which will be produced.
With further reference to Figure 7 th~re is shown
a modified flow confining chamber 11~ adapted -to form part
of an oxygen dissolving device. The chamber 118 includes
an upper conical chamber 120 and a lower frusto-conical
chamber 122 mounted on legs 138, the wall of chamber 122
diverging more rapidly than the wall of chamber 120. The
chamber 120 is connected to an intake tube 124 via an inlet
tube 126 and an inverted frusto-conical connecting member
128. An oxygen injector ring 130 is mounted in the inlet
tube 126. Perfoxated, vertically disposed, flow directing
plates 132 and 134 extend betwean the walls of chamber 120;
plate 132 being substantially perpendicular to plate 134 and
a disc-shaped perforated flow distributor plate 136 extends
horizontally through and is.welded-to the vertical plates.
The upper conical chamber 120 may 3uitably define
about 30 to 7~/OI typically about 50/0 of the total height
of chamber 118. The walls of chamber 120 may suitably
include an angle of about 10 to about 35, typically about
25 and the wall~ of chamber 122 include an angle of about
40 to about 60, typically about 50.
- 41 -
With further reference to Figure 8 there is shown a
detail of the plate assembly 132, 134, 136 of Figure 7. Each
of the vertical plates 132 and 134 and the horizontal plate
136 are perforated with holes 138 over their whole surface;
the vertical plates include brackets 140 by means of which
they can be mounted inside chamber 120.
The vertically disposed plates 132 and 134 direct
the flow of liquid generally downwardly and offset the spiral
flow of liquid formed by the impeller. The perforations 138
in the vertical plates 132 and 134 ensure pressure equali-
zation between the quadrants of the chamber 120 formed by
the plates 132 and~134 and at the same time the passage of
the liquid through the perforations increases the shbaring
of the liquid and gas thereby increasing the g~s/liquid
contact. The perforated horizontal plate 136 functions to
ofEset the spiral flow of the ~iquid and distributes the
liquid in a downward direction, while producing a shearing
action similar to that of the vertical plates 132 and 134.
The perforations 138 in plates 132, 134 and 136
are suitably circular having a diameter of about 1 to 3
inches typically about 2 inches and may suitably occupy
about 30 to 70%, typically about 50% oE the plate area.
- 42 ~
An assembly similar to that of Figure 8 can be
employed in the oxygen dissolving device 18 of Figure 1,
further there can be employed solely the vertical flow
directing plates 132 and 134 or solely the horizontal
flow distributing plate 136 or a plurality of plates 136
spaced vertically apart.
In one especially preferred embodiment employing
an oxygen dissolving device 18 of Figure 1, there was
emp1oyed two horizontal, perforated flow distributing
plates 136, a lower plate being located at half the verti-
cal height of flow confining chamber 18a and an upper plate
located at one-thlrd the vertical height of chamber 18a
measured from the upper end.
With further reference to Figures 9 and 10 there
is illustrated an oxygen injector ring 142 having an in-
wardly facing surface 144 and an oxygen inlet pipe 146.
The inwardly facing surface 144 has a plurality of holes
148 therein, as shown in Figure 10. In one particular
embodiment there were 40 holes 148 in surface 144, located in
four groups of 10, each hole 148 being located on a common
circumferential line. The holes 148 which suitably have a
diameter of 1/32 inches provide an e-fficient injection of
oxygen and increase the rate of dissolving of the oxygen.
_ 43 _
41~7
With reference to ~igure 11 there is shown a treat-
ment apparatus which is similar to that of Figure 2, inasmuch
as the oxygen dissolving device is located outside the tank.
In Figure 11 the treatment apparatus comprises a
tank 210, an oxygen dissolving device 218 located outside the
tank 210 and a controlled oxygen supply system 211.
The tank 210 includes an influent line 213, an
effluent line 217 and a solids outlet 214.
An overflow weir 216 is located in an upper portion
of tank 210 and is in communication with effluent line 217 for
removing clarified water; and a rotatable sludge rake 215 is
disposed in a lower portion of tank 210.
The tank 210 provides for a lower biological reaction
zone 238 and an upper clarification zone 240.
The oxygen dissolving device 218 illustrated by
reference to ~igures 11 and 12 is located in the influent
line 213.
The device 218 comprises a generally cylindrical
tube 300 havingl~a partition wall or baffle 302 extencling
between the walls of the tube 300 from an upper end 304 of
tube 300 towards a lower end 306, a gap 308 bei.ng provided
between wall 302 and end 306, the partition wall 302 divid-
ing the tube 300 into an upstream portion 310 and a down-
stream porti.on 312.
A recirculation impeller 314 is disposed near the
top o~ the upstream portion 310.
A recirculation line 316 in which is disposed a
pump 318 communicates the biological reaction zone 238 in
tank 210 with influent line 213 upstream of tube 300,
The oxygen dissolving device 218 is connected by
an oxygen supply line 260 to an oxygen sourcé 262.
- 44 -
An oxygen probe 270 is suspended in the biological
reaction zone 238 in tank 210 and is connected via an oxygen
analyzer 272 and a recorder controller 274 to a flow regulat-
ing valve 276 in the o~ygen supply line 260.
As shown more clearly in Figure 12, the oxygen
supply line 260 terminates in upstream portion 310 in an
oxygen injector 261 comprising an injector ring 263 having
an array of holes therein.
The operation of the apparatus illustrated in
Figures 11 and 12 is substantially the same as that des-
cribed with reference to Figures 1 and 2.
Influent is introduced into tank 210 via influent
llne 213 and oxygen dissolving device 218, and is recirculated
through the biological reaction zone 238 and device 218 by
pump 318.
O~ygen is introduced to upstream portion 310 of
device 218 and is entrained in the liquid passing to the
downstream portion 312 and from there to biological reaction
zone 238.
The oxygen content is monitored and controlled in
the same manner as described with reference to Figure 1.
As the liquid circulates rapidly through biological
reaction zone 238 and device 218, clarified liquid rises
slowly upwardly in the clarification zone 240.
The zone~ 238 and 240 may optionally be separated
by a separating zone and flow distributing baffles such as
are described with reference to Figure 1 (42 and 34).
With further reference to Figure 13 there is shown
a treatment apparatus 400~ This comprises a tank 210 with
an open top so the surface is accessible to the atmosphere,
oxygen dissolving devices, in this case U-tubes 218, located
outsi.de the tank and a controlled oxygen supply system 211.
- - 45 _
The tank 210 includes recycle lines 213, effluent
lines 217 and a solids outlet 214.
An overflow weir 216 is located at the upper part
of the tank 210 and leads to the effluent lines 217 for re-
moving clarified water. A rotating sludge rake 215 is dis-
posed in a lower part of the-tank 210. One function of the
rake 215 is to prevent solids from stagnating at the bottom
part of the tank 210.
The tank 210 provides for a lower biological re-
action zone 238 and an upper clarification zone 240 wlth an
intervening transition zone 239 which are maintained as will
be described.
The ~-tubes 218 are different from the device 218
in Figures 11 and 12 and are connected to the recycle lines
213,
Each U-tube 218 is made up of a vertical elongated
tube or shaft 300, lined with a cylindrical tube 300a, having
an inner concentric tube 302 éxtending from the upper end 304
of the shaft 300 and terminating near its lower end 306. A
space 308 is provided between the bottom end of the tube 302
and the end 306. The tubes 300 and 302 thus provide a down-
flow channel 312 and a concentric upflow channel 310.
The flow to the downflow channels 312 is provided
through a line 322 leading from a head tank 320. The tank
320 is supplied with incoming waste water (influent) from a
pump 321. The downward flow in the channel 312 may be
induced by elevating the t~k 320 or other means as will be
described.
The recycle line 213, ln which is disposed a pump
318, leads from a well 328 in the bottom of the hiological
reaction zone 238 to the head tank 320. A solids outlet 214
leads from a solids collection~well 214a, in the foot of the
.
.. - 46 -
vessel 210, to facilitate the removal of excess sludge.
Each oxygen dissolving device 218 is connected by
an oxygen supply line 260 to the oxygen source 211 through
flow regulating valves 275.
A dissolved oxygen probe 270 is suspended in the
biological reaction zone 238 in the tank 210 and is connected
via an oxygen analyzer 272 and a recorder-controller 274 to
the flow regulating valves 276 in the oxygen supply line 260.
The oxygen supply line 260 is connected to an
o~ygen injector 261 located in the upper part of the downflow
channel 312. The oxygen injector 261 in the embodiment shown
is in the form of a ring having an array of holes in it so
that the oxygen is injected in the form of small bubbles to
facilitate its dissolving in the liqùid (see Figure 5).
The operation of the process is as follows.
Influent is introduced into the head tank 320 via
the line 321 where it is combined with recycled mixed liquor
in the line 213 coming from the reaction zone 238. A mixture
of incoming waste liquor and recycled partly treated mixed
liquor is passed from the tank 320 through the line 322 into
the downflow channel 312 of the U-tubes 218. The resulting
mixture of oxygen and liquid passes through the downflow
channel 312 and then through the upflow channel 310 so that
the oxygen is dissolved in the liquid.
The dissolved oxygen concentration in the biological
reaction zone 238 is monitored continually by the device 270.
The oxygen feed is adjusted according to the variations in the
oxygen concentration (oxygen demand) through the instruments
272, 274 and the valve 276 to maintain the oxygen concentration
in the biological reaction zone within predetermined desixed
limits n
As the liquid circulates in the biological reaction
- 47 -
zone 238 and through the U-tubes 218, clarified liquid rises
quiescently in the clarification zone 240 and overflows the
weirs 216 and is carried away through the pipes 217.
Between the zones 23~ and 240 is the tra~sition zone
239 in which solids separate from the liquid and settle into
the biological reaction ~one 238.
In the biological reaction zone 238 carbon dioxide
will be generated. This may conveniently be re~oved from the
feed tank 320 by a conventional surface aerator 329 or other
device. Alternatively, the carbon dioxide may be removed at
other places in the system.
In the embodiment of the invention shown in Figure
13, the sludge rake 215 is mounted on the lower end of a
hollow shaft 323 which is journalled in upper and lower bear-
ings 325 and 326, respectively, suitably mounted on the tank.
Surroundin~ the shaft 323 above the tank 210 is a collection
reservoir 327 which communicates with the inside of the shaft
323 through openings 328. The rake 215 includes outwardly
extending pipes 324, communicating with the inside of the
shaft 323. The pipes 324 have outlet openings or nozzles 338.
The shaft 323 is rotated by an electric motor 330 through a
reduction gear system 331.
In accordance with the invention, for the effective
treatment of the waste water, a number of interdependent
factors are controlled, for example
Waste water will be received by the system at a
variable rate. The flow rate of the influent to the system
is related to the depth and surface area of the treatment
enclosure to provide a residence time within the reaction zone
effective for the biodegradation of the waste and for the
biological floc to settle. This is built into the desi~n of
the vessel 210.
- 48 -
~34~7
~ he recycle stream of mixed liquor is controlled to
a constant rate effective to provide for dissolving the oxygen
added to the recycle stream at a variable rate, and for an
amount of dilution of the recycle strec~m entering the reaction
zone effective to prevent the oxygen coming out of solution at
the top of the reaction zone.
The rate,direction and type of fiow of the incomin~
recycle stream to the biological reaction zone is controlled
to provide controlled agitation effective to keep the solids
dispersed and to provide, at an intermediate level of the
enclosure, an upward velocity of the mixed li~uor less than
the sett~ing rate of the solids so that there is maintained
in the enclosure separate reaction and clarification zones,
intervened by a transition zone.
The concentration of dissolved oxygen in the re-
action zone is monitored constantly to determine variations
thereof. The rate of flow of the oxygen to the recycle stream
is adjusted, in response to the variations in the concentra-
tion of dissolved oxygen in the reaction zone, so as to
restore~the concentration of oxygen in the reaction zone to
within a selected range effective to biodegrade the waste
solids and to maintain the oxygen in solution so as to avoid
effervescence that would lead to gas bubbles rising to the
surface and entraining solids.
The invention has been explained by reference to
the preferred apparatus shown in Figure 13. It will be under-
stood that this apparatus may be varied considerably and still
perform the functions described and provide for effective
control of the interdependent factors necessary to operate
under practical conditions. A head tank 330 is shown in
Figure 13 to which influent waste and partially oxygenated
mi~ed liquor is pumped using an airlift, centrifugal, positive
: .
- 49 -
3~
displacement, or axial flow pump. A centrifugal, axial flow,
or positive displacement pump can be employed to pump down
the U-tube 218. A centrifugal, axial flow, positive displace-
ment or airlift pump can be employed to draw flow from the U-
tube up flow channel 310.
A centrifugal, axial flow, positive displacement or
airlift pump may be employed to draw from a sump in the bottom
of the tank 210. The returning flow from the biological re-
actor zone 238 to the U-tube 218 may be induced by using a
centrifugal, axial flow, positive displacement or airlift pump
to draw from a sump in the bottom of the tank or through
nozzles attached to the sludge rake 215 or drawing through
nozzles attached to a piping header laid on the bottom of the
tank 210.
Flow distribution in the tank 210 can be achieved
by sludge rake 215 which comprises a rotating rake and scraper
with flow nozzles 325 installed close to the top of the rake
215, as shown, or by introducing flow at the periphery of
the tank 210.
The total surface area of the flow nozzles 325 is
suitably at least equal to the cross-sectional area of the
inside of shaft 323. Conveniently the apparatus may include
a second rake 215 which may conveniently be angularly offset
90 to the first rake 215, while lying in the s~ne horizontal
plane.
Effluent overflow may be achieved by collection of
flow around the periphery or from the center or a mid-point
of the tank 210.
Excess sludge may be removed from the tank 210 by
an external batch operated decantation tank, an external con-
tinuously operated decantation tank, or by a decantation
basin in the bottom of the tank 210.
- 50 -
~L3~
Addition of oxygen to the U-tubes 218 may be by the
use of a single tube or a multiplicity of tubes, by a porous
diffuser, or by an orifice plate or venturi injector.
Carbon dioxide stripping may be accomplished by a
submerged aerator in the head tank, by sparging-in air at the
head tank 320 or l~-tube 218, or by a second U-tube~
The tank 210 may be of various configurations, for
example, cylindrical, square or rectangular.
A further preferred embodiment of the invention, which
has considerdble advantage over the other embodiments, is
illustrated in Figures 14 and 15.
There is shown a treatment tank A having a rectangular
floor 515 and upwardly extending walls 517, 517a, 517b, 517c
terminating în a top 519 open to the atmosphere. The tank con-
tains a charge made up of a dispersion of a biological mass of
biodegradeable material in oxygenated water, in a biological
reaction zone S, and supernatant liquid in a clarification zone
C. mere is also a transltion zone.
A false wall or baffle 521 extends from the wall 517a
to the wall 517c and is spaced from the wall 517 to form a
vertical passage 524 for the influent defining the start of the
inflow. The wall 521 extends from near the top of the tank to
a point spaced from the floor 515 to provide for a narrow
elongated inflow slot 525.
Towards the wall 517b there is a mixed liquor recycle
pump P, resting on the floor 515. The pump P has an intake
from a pair of orificed intake pipes 530 and 531 near the floor
515 and extending substantially the entire width of the tank to
provide an extensive intake across the width of the tank. The
intake pipes 530 and 531 lead to a recycle conduit 533 which
extends upward from the biological reaction zone S through and
out of the tank A and is connected to the downcomex 535 of an
.
-- 51 --
3~8~
oxygen-dissolving de~ice R.
The device R has an outer tube 537, forming with the
downcomer 535, an annulus 534. The upper part of the tube 537
is connected to an inflow conduit 539 which terminates in a
nozzle 541 in a widened upper part of a passage 524. ~ waste
water influent conduit 543 also enters the top of the passage
524 to deliver influent waste water from a source of supply.
The tube 533 is provided with a sludge wasting outlet conduit
534 controlled by a valve 536. The tube 533 includes a flow
control valve 538 and a flow meter S~0 in series.
An o~ygen injector 545 is operatively connected to
the downcomer tube 535 near the top and to an oxygen supply
conduit 547 leading from a suitable source of oxygen. The
conduit 547 is controlled by a valve 549 having operating
mechanism 551 controlled from a dissolved oxygen recorder-
controller 553 connected to a dissolved oxygen analyzer 555.
The oxygen analyzer 555 is, in turn, connected to a dissolved
oxygen probe 557 suspended in the biological reaction zone
within the sludge blanket S~
A sludge rake T is provided in the bottom of the tank
to be used, if necessary, to prevent the accumulation of solids.
However, the high velocity sweeping the floor 515 will~normally
inhibit the accumulation of solids.
General Operation
Generally speaking, the operation of the device is
as follows.
In starting up, the tank A is filled with a charge
consisting of waste water and sludge containing the biodegrad- ,
ing organisms which eventually settle to the bottom of the tank
and a flow of waste water is induced through the ~onduit 543
until circulation throughout the system is possible as will be
described.
- 52 -
Once the systemis operating,-waste water influent
continuously enters the conduit 543 and flows into the channel
524l where it is mixed with continuously recirculated mixed
liquor from the biological reaction zone S, in which oxygen is
continuously dissolved in the oxygen-dissolving device R. The
mixture of recycled and oxygenated mixed liquor and newly added
effluent flows downward between the false wall or baffle 521
and wall 517 to the bottom of the.tank A at the slot 525
(bottom on the tank) and the mixture is directed from the slot
10 525, horizontally, as a wide shallow inflow. The pump P with-
draws the mixed liquor suspended solids uniformly, as a wide
~ shallow outflow, through the orificed pipes 530 and 531, extend-
: ing from one side of the tank to the other near the bottom of
the tank and circulates this fluid through the tube 533 to the
.. oxygen dissolving device R. The oxygenated fluid flows through
the device R to the process tank A via tube 539. '.
By the combined push-pull effect of the head of liquid
in the device R and the pump P, the inflow enters at a relatively
high velocity through~the slot 525 and an undercurrent of a
dispersion of undissolved solids is caused to flow continuously
across t.he floor 515 from the inlet 525 to the outlets 530 and
531, with a minimum of turbulence. While the velocity of the
~~ undercurrent is effective in preventing the settling o~ solids
it may be assisted, if necessary, by the action of the rake T,
to ensure that the solids will not settle to the bottom 515.
O~ygen is ingested by the microorganisms from the
undercurrent stream in its transit from the inlet 525 to the
outlets 530 and 531. At the same time, the concentration of
dissolved oxygen in the layer S is continuously monitored by
the dissolved oxygen probe 557. The measurement of the oxygen
count registers in the dissolved oxygen analyzer 555 to which
the probe 557 is connected and is recorded and controlled by
- 53 -
the dissolved oxygen recorder-controller 553, which, in turn,
controls the oxygen admitting valve 549.
In this way, the amount of dissolved oxygen, in the
biological reaction zone S, may be kept within predetermined
limits to support vigorous aerobic activity, regardless of
variations in the quantity and quality of waste water influent.
The incoming influent is diluted, by the mixed liquor in the
zone S, as it emerges through the slot 525 into the reaction
zone S. Thi5 allows a substantial maximum concentration of
oxygen to be contained in the incoming stream since, immediately
it leaves the slot 525, it is diluted by entering the larger
volume of mixed liquor S, which Xeeps the dilution above the
point where oxygen would come out of solution. The extensive
area of interface between the undercurrent of highly oxygenated
horizontally moving supplemented recycle stream and the over-
lying mixed liquor permits substantially maximum oxygen contact
with the organisms without causing undue turbulence to inter-
fere with the settling of the suspended solids from the
clarification zone.
By proceeding a5 described, it is possible to build
up a biomass, which may be of any desired depth in which the
solids are maintained in suspension, due to the horizontal
velocity of the liquid introduced at 525, and the vertical
component of this velocity, which essentially averages out to
the velocity equivalent to the overflow rate, i.e., to the
rate of the influent flow. In other words, the vertical
velocity of the M.L.S.S. i8 less than the settling rate of the
solids in the M.L.S.S., which makes it possible to maintain a
clarified zone above the mixed liquor in which the solids
settle rather than being carried upward into the influent.
There is a high degree of stability of the interface
between the mixed liquor zone and the clarified zone, even when
- 54 -
~39L~
there are wide fluctuations in the influent flow rate. For
example, the stability of the interface is ~aintained, even at
overflow rates which vary between 300 and 1500 gals/ft2/day.
The biological reaction zone and the clarified zcne do not
actually mer~e immediately, one into the other, but there is a
transition zone between the two containing settling solids.
And, according to the invention, the upper part of the bio-
logical reaction zone (above the undercurrent) acts as a buffer
zone insofar as turbulence is concerned, between the under-
current and the transition zone.
It will be evident that, when oxygenated mixed li~uoris returned to the biological zone S from the oxygenator R,
there is a time lapse before this mixed liquor arrives at the
withdrawal points 530 and 531, to be recycled to the U-tube.
For example, when the recycle rate is 1300 USG/~, and where the
distance between the point of re-entry to the biological zone
and the point of withdrawal is 20 ft. it takes about 2.8
minutes for the mixed liquor to travel this 20 ft. distance.
Thus, the average holrizontal velocity is about 7 feet per
minu'te. With a tank 21 ft. wide and a recycle flow rate of
1300 USG/Min., an undercurrent of mixed liquor with an average
thickness of 1.25 feet thick moves across the bottom at an
average velocity of 7 ft/min. This velocity varies depending
Oll the variation of the undercurrent and, thus, can vary 2 to
12 ft/min., even under otherwise constant process conditions.
An alternative form of the invention is shown in
Figure 15 in which similar reference numerals to Figure 14
have been employed for similar feature~ butraised by 100 and
the subscript 1 has been given to the reference letters
identifying similar parts. In this case, the false wall 621
is replaced by a directional baffle 621 which extends diago-
nally downwards from the wall 617 to a point spaced from the
- 55 -
/7
floor 615 so as to leave between the bottom edge of the baffle
621 and the floor 615 an opening or slot 625 for the inElow of
liquid to tank Al. The baffle 621, which extends all the way
from the wall 617a to the wall 617c, forrns with the walls 617,
617a and 617c, a pocket or compartment 618.
A conduit 639 leads from the oxygenator Rl and is
connected to a waste water influent line 643. The conduit 639
enters the tank just above the floor 615 and is connected to a
distributor 641, which extends across the tank/ for distribut-
ing a mixture of oxygenated mixed liquor and waste water
influent coming in through the line 639. In this way, a rela-
tively high velocity inflow of waste water influent and recycled
oxygenated mixed liquor passes through the slot 625 between the
baffle 621 and the floor 615. Figure 15 is similar to that of
Figure 14. The inflow through the slot 625 forms an under-
current which passes along the floor 615, in the lower part of
the biological reaction zone Sl, to form an outflow at the dis- -
charge pipes 630 and 631 under the suction of the pump Pl.
The directional baffle 621 provides a slot 625 of a
size effective to impart a relatively high velocity to the
liquid along the floor 615, as described in connection with
the version of Figure 14. The pocket 618 separates and traps
undissolved gases from the oxygenated mixed liquor which will
collect at the apex 618a from which there is a discharge to
the atmosphere.
The effect of the horizontal motion of the Eluid is
to maintain the solids in suspension without, however, creat-
ing excessive turbulence which would hinder clarification by
increasing the upward movement of the solids. The upward
velocity of the fluid is less than the settling velocity of
the solids in the M.L~S.S. which makes it possible to maintain
a clarified zone ~ove the mixed liquor. There is a high degree
.
-- 56 --
of stability of the interface between the mixed liquor zone and
the clarified zone despite the flow occasioned by the recycle
rate to prevent the settling out of the sludge and despite the
wide fluctuations of the influent flow rate~
As one example, to illustrate these statements, the
stability of the interface is maintained even at overflow rates
which vary between 300 and 1500 gals/ft2/day. In spite of the
high circulation rate of the mixed liquor, it is possible to
maintain a sta~le biological zone which is necessary for good
clarification.
The ~iological zojne contains sufficient oxygen to
support vigorous aerobic activity. The dissolved oxygen de-
creases for example-from approximately 30 mg. per litre at
the slot 625 to approximately 2 mg. per litre at the pump
inIets 630 and 631.
The ahility of the biological reactor/clarifier to
perform effectively is dependent on the combination of the
system described abovel the use of an external oxygen dis~
solving device, and the use of pure oxygen.
A vessel with a single pump has been illustrated.
The process may be carried out in a wide vessel elonyated in
the direction of the length of the walls 617 and 617b in which
there may be a common,inlet slot 625 and a plurality of pumps
Pl at the other end of the vessel for withdrawing the common
undercurrent created.
The invention has also been illustrated with a
rectangular vessel. It should be understood that the same
principle may be employed in a circular or other shaped tank -
~n which the recycle stream is injected at one vicinity near
the bottom of the reaction zone and withdrawn at a remote
vicinity near the bottom so as to create a wide shallow hori-
zontal undercurrent functioning as described.
- 57 -
1~L3~L87
,
EXAMPLE 1
A pilot plant was set up in the laboratory accord-
ing to that illustrated in Figure 2 of the drawings in w~ich
the oxygen contacting device was located outside the tank.
The waste water treated was synthetic and was made from a
solution o glucose and added nutrients, The plant was
operated under the following condition,
Waste Water
~ Flow 4,800 G.P.D. (gallons/day)
` 10 Quality
Total biological oxygen demand (BOD) 264 mg~l
(milligrams/litre)
Total chemical oxygen demand (COD) 396 mg/l
Process Condltions
Biological Reaction Zone
j Mixed liquor suspended solids (MLSS) 26,000 mg/l
Temperature 1~ C
Dissolved Oxygen (D.O.) 5 mg/l ~,
Resldence time 1.5 hours
Clarification Zone
Overflow rate 383 g./d./sq.ft,
(equivalent to
4,800 G.P.D.)
Effluent Quality
Suspended solids 85 mg/l
Total BOD 95 mg/l
Total COD 200 mg/l
-
- 58 -
Although, in this example, the effluent quality
was not too good, the principle of the two ~one process was
found to be practical. The wasting of sludge, was determined
by the level of the mixed liquor in the biological reactor.
In this example, the M~SS was 26,000 mg/l. The mixed liquor
in this process was also the sludge which was wasted,
EXAMPLE 2
The following represent typical performance data
obtained with municipal waste water biologically treated with
. 10 the two zone process. An apparatus as illustr~ted in Figure
1 was employed having the oxygen contacting device :in the
tank, but without the flow di~tributor 36 and without the
flow distributing baffles 34.
: Waste Water
Flow Min, S0,000 G.P.D.
Max 110,000 G,P D.
Average 75,000 G.P~D.
Quality
: Suspended Solids 70 mg/l
Total BOD 12-5 mg/l
Soluble BOD 60 mg/l
Total COD 250 mg/l
Soluble COD. 175 m~l
- 59
~3~
Process Conditions
Bioloaical Reaction Zone
~,L.S.S. 2500 mg/l
Temperature 16C
Dissolved Oxygen 3 mg/l
Residence Time 3 - 4 hours
Clarifier
Overflow rate 1000 G.P.D./sq.ft.
Sludge (Solids) Settling
; 10 Velocity 7 ft,/hour
Effluent Quality
Suspended Solids 20 mg/l
Total BOD 25 mg/1
Soluble BOD 5 mg/l
Total COD 80 mg/l
Soluble COD 55 mg/l.
- 60 -
. . .
EXAMPLE 3
The following represent typical performance data and
parameters obtained with municipal waste water biologically
treated with the apparatus of Figure 13:
a) Characteristics of waste water to be treated:
The flow of waste water and its quality as defined by
B.O.D., C.O.D., suspended solids, pH, were determined
in a preliminary study. The results of these were as
follows:
(i) B.O.D. ~ 300 mg/l for 95% of time
(ii) C.O.D. < 600 mg/l for 95% of time
(iii) Suspended solids ~ 300 mg/l for 95% of time
(iv) Flow rate < 700 g.p.m. for 90% of time
i~e. 1,000,000 g.p. day.
b) Effluent quality required from process:
(i) B.O.D. 20 mg/l
(ii) C.O.D. 100 mg/l
(iii) Suspended solids 20 mg/l
c) In a trial study in the laboratory it was detennined
that to reduce the B.O.D. from 300 mg/l to 20 mg/l, the
residence time required in the biological reaction zone
238 was 3 hours.
d) Volume of biological reaction zone 238 = 1'2' X 3
= 125,000 Gallons
= 20,032 ft.3
e) ~n a trial study in the laboratory it was determined that
the overflow rata required to obtain an effluent quality
with suspended solids of 20 mg/l was 500 g.p.d./ft.2.
f) Hence the overflow area re~uired
l,001 = 2,000 ft.
Ass~uning a circular tank 210 is chosen the diameter =
d feet
7~4 = 2,0()0
d ~ 50 feet
For a residence time in the clarification zone 240 of
4 hours, the depth of the zone 240 is determined:
Depth of zone 240 = 214624 x 2000 = 13-36 ft-
me volume of the biological reaction zone 238 is
20,032 ft.3 and so the depth of the biological reaction
zone is 10 ft.
g) Size of U-tube 218:
Oxygen demand of the waste water is:
1,000,000 x 10 x 300 = 3,000 lbs~/day.
For a U~tube 100 feet deep, the oxygen added per
circulation = 40 mg/l, i.e. ~DO - 40 mg/l.
Consequently the total recirculated flow to dissolve
3,000 lbsu/day is:
34 x 100,000 = 7.5 m.g.d.
_ 5208 g.p.m.
Considering a velocity in the downflow channel 312 of
5 feet/sec.; the appropriate diameter of tube 302 ~o
give this velocity at a flow of 5208 g.p~m. is about
24"
The diameter of concentric tube 300 or of shaft 300
to provide about the same annular area is 36"~
h) Size of head tank 320:
Assuming a total residence time in the head tank 320
of 10 minutes:
Influent flow = 700 g.p.m.
Recirculation flow = 5,208 g.p~m.
Total flow = 5,908 g.p.m~
-- 62 -
3~8~7
There the volume of head tank 320 = 59,080 gals.
- 9,468 ~t.
i.e. a tank 320 having, for example the diameter
20' x 20' x 24'
Thus in summary the tank 210 in Figure 13 suitribly
has a diameter of 50 feèt and a depth of 23.4 feet.
10 feet depth for biological reaction zone 238
13.4 feet depth for clarification zone 240
The volumetric .size of U-tube 218:
2~ in. diameter for internal pipe 302
36 in. dir~meter for external pipe 300a
Depth of pipes 302 and 300a is about 150 feet.
Recycled flow rate = 5,208 g.p.m.
Size o~ head tank 20 ft. wide x 20 ft. l~ng x 24 ft. deep.
,
..
.
~ ~3 _
L3~37
EXAMPLF 4
A typical reactor/clarifier tank was employed having
dimensions of 20.5 feet wide by 24.5 feet long by 14 feet
working depth (effective volume equals 52,600 gallons). ~le
system is designed to process an average of 260,000 gallons per
day.
The influent flow over a period surveyed varied widely
and for the first 25 days of a given month, the average daily
flow ranged from 142,000 to 427,000 gallons per day (average
equals 248,000 GPD). The peak diurnal flows often exceeded
550,000 GPD (limit of the flow meter).
The average process performance for the first 21 days
of the month are summarized as follows:
Primary Effluent Two Zone
Feed to Two Zone Effluent
Total B.O.D.5 (mg/l) 109 22
Soluble B.O.D.5 (mg/l) S3 2
Suspended Solids (mg/l~ 58 21
The M.L.S.S. concentrations were allowed to rise
during the month reaching 5,200 to 5,900 mg/l (as measured in
the sludge recycle stream) by the 4th week. Based on the above
concentration the system SRT is stabilized in the range of 9
to 12 days.
During the first 25 days of the month the sludge
blanket occupied 8.4 feet of the total 14 feet liquid depth,
or 60% of the liquid volume. The daily variation in blanket
depth was from 5.8 feet to 11.2 feet. The hourly variations
in the depth of the biological zone were minimal and affected
mainly by the influent flow. Daily variations throughout the
month occurred gradually and appeared to respond more to the
mixed liquor wasting patterns than to diurnal changes in
influent flow. The large circulation flow from the oxygen
dissolving device apparently so dwarfed the net influent flow
- 64 -
that influent diurnal variations (even 2/1 peak/average) react
as minor disturbances with minimal effect on the total dynamic
flow regime within the tank.
The oxygen dissolving device employed was a U-tube
having a 10 inch diameter downcomer and a 20 inch outer shaft,
and a 146 foot shaft depth. The U-tube consisted solely of a
straight downcomer and a straight riser with no deliberate head
loss (constrictions or mixers) built in to promote turbulence.
. - 65 -
