CA1143487A - Process for biological treatment of waste water - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A biological waste water treatment carried out in a single tank. The conditions in the tank are controlled to provide a biological reaction zone containing waste water and biodegrading organisms and an overlying clarification zone from which an effluent of treated waste water flows. To maintain these conditions, a recycle stream is continuously withdrawn from the biological reaction zone, passed through an oxygen-dissolving device, and supplemented with influent raw waste water and the supplemented stream returned to the reaction zone.
The oxygen in the biological reaction zone is monitored and supplied to the oxygen-dissolving device in an amount to satisfy the demands of the organisms and, at the same time, to keep the oxygen in solution. For preferred results, the supplemented recycle stream is continuously injected, at one vicinity, along the bottom of the biological reaction zone in a horizontal shallow inflow having a width substantially greater than its depth and at a flow rate considerably greater than that of the raw influent. The recycle stream is withdrawn from near the bottom of the reaction zone, at a vicinity remote from that of the inflow, in an outflow having a substantially greater width than its depth. In this way, there is created, between the inflow and the outflow, a horizontally relatively fast flowing undercurrent of initially high dissolved-oxygen content, and having an extensive uninterrupted interface with an overlying relatively quiescent body of mixed liquor flowing upwardly relatively slowly.

Description

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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 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.

~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 , ~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 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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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.

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 ~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 ~ , .

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.

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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.

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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 ~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.
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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 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 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, 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.

~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, 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 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 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, ~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 ~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~

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 ~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

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 .

:

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.

.

~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 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 ~ 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.

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.

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 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.

~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 : .

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.

~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 .

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.

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 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 ~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 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 .

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.

1~L3~L87 , 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 -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, 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 ~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.

. . .

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~

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 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 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 -

Claims (22)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A process of treating waste water containing bio-degradeable waste to provide a clarified liquid effluent and a disposable sludge, in which 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, comprising, initially establishing:
(a) in a lower part of the enclosure a biological reaction zone containing mixed liquor containing said microorganisms and in which a biological reaction to degrade the waste is conducted, (b) in an upper part of the enclosure a clarification zone in which clarified liquid rises and over-flows, and (c) between the reaction and clarification zones a transition zone in which the liquid of the mixed liquor rises and the solids settle, and continuously, withdrawing a recycle stream of mixed liquor from the reaction zone and conducting the stream through an oxygen dissolving device disposed outside the reaction zone, adding influent waste water to it, dissolving oxygen in the stream and injecting the supplemented stream into a lower part of the reaction zone remote from the vicinity of withdrawal, conducting the waste water 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 within the reaction zone effective for the biodegradation of the waste and for the formation and settling of biological floc, adding oxygen to said 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 maintaining it in contact with the liquid in a contact zone of said stream for a time and under a pressure such that the oxygen is dis-solved in the liquid, controlling the overall flow rate of said recycle stream 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, distributing the flow of said recycle stream enter-ing the reaction zone to reach a substantial area of a lower part thereof:
(f) to provide a 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, (g) and to provide at an intermediate level of the enclosure, and upward velocity of the mixed liquor less than the settling rate of the solids, whereby there is maintained in the enclosure said separate reaction and clarification zones inter-vened by said transition zone, continuously monitoring the concentration of dis-solved oxygen in the reaction zone to determine variations thereof resulting from variations in the flow rate and con-centration therein of waste, periodically adjusting the rate of addition of the oxygen to the recycle stream in response to variations in the oxygen concentration in the reaction zone to maintain said concentration within said controlled range and at a level where there is substantially avoided effervescence that would lead to gas bubbles rising into the clarification zone, continuously withdrawing said effluent from the clarification zone to keep pace with the influent waste water, and continually removing excess sludge from the re-action zone and carbon dioxide from the mixed liquor.

2. A process, according to claim 1, wherein the con-centration of oxygen in said reaction zone is maintained in the range of 1 to 5 mg/l.

I

3. A process, according to claim 2, wherein said re-cycle stream is flowed at a rate such that the time for one complete circulation of the volume of the reaction zone is 1 to 60 minutes.

4. A process, according to claim 3, wherein the water in the clarification zone has a flow rate to keep pace with the flow rate of waste water fed into said enclosure, and the flow rate of water in said recycle stream is 25 to 50 times the rate of upward flow in the clarification zone; and wherein the recycle stream has a residence time in said con-tact zone of 30 to 100 seconds for each circulation of said stream through said reaction zone and said contact zone.

5. A process, according to claim 4, wherein said clarification zone has a depth of 5 to 15 feet, said transi-tion zone has a depth of 1 to 5 feet, and said reaction zone has a depth of 5 to 13 feet, when said waste water is fed into said enclosure at a rate of 300 to 600 gallons per sq.
ft. per day of the exposed area of the liquid.

6. A process, according to claim 5, wherein said con-tact zone is defined by an elongated downflow channel and an elongated upflow channel.

7. A process of treating waste water containing bio-degradeable waste to provide a clarified liquid effluent and a disposable sludge, in which waste water is continuously passed through a single treating enclosure open to the atmos-phere 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 which there is initially established a charge including (a) in a lower part of the enclosure a biological reaction zone containing mixed liquor containing said microorganisms and in which a biological reaction to degrade the waste is conducted, (b) in an upper part of the enclosure a clarification zone in which clarified liquid rises and overflows, and (c) between the reaction and clarification zones a transition zone in which the liquid of the mixed liquor rises and the solids settle; and continuously there is withdrawn from the biological reaction zone a recycle stream of mixed liquor from the reaction zone and the stream conducted through an oxygen-dissolving device disposed outside the reaction zone and influent waste water and oxygen are added to it; the thus supplemented stream is injected into a lower part of the reaction zone remote from the vicinity of with-drawal, the 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 within the reaction zone effective for the biodegradation of the waste and for the formation and settling of biological floc, oxygen is added to said 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 maintaining it in contact with the liquid in a contact zone of said stream for a time and under a pressure such that the oxygen is dis-solved in the liquid; the overall flow rate of said recycle stream is controlled 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 enter-ing the reaction zone effective to prevent the oxygen coming out of solution at an upper part of the reaction zone, the flow of said supplemented recycle stream entering the reaction zone is distributed to reach a substantial area of a lower part thereof, (f) to provide a 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, (g) and to provide, at an inter-mediate level of the enclosure, an upward velocity of the mixed liquor less than the settling rate of the solids, where-by there is maintained in the enclosure said separate reaction and clarification zones intervened by said transition zone;
the concentration of dissolved oxygen in the reaction zone is continuously monitored to determine variations thereof result-ing 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 said concentration within said controlled range and at a level where there is 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 the excess sludge is continuously removed from the reaction zone and carbon dioxide from the mixed liquor, comprising the steps of, continuously injecting along the bottom of the bio-logical reaction zone said supplemented recycle stream in a horizontal shallow inflow having a width substantially greater than its depth, and withdrawing. mixed liquor from near the bottom of the reaction zone at a vicinity remote from the inflow in an outflow having a substantially greater width than its depth, thereby to provide between the inflow and the out-flow a horizontally flowing undercurrent having an extensive uninterrupted interface with an overlying relatively quiescent upwardly flowing body of mixed liquor, and in which, the depth of the charge is from about 8 feet to about 100 feet, the depth of the clarification zone is at least about 2 feet, the distance between the inflow and the outflow is from about 6 feet to about 200 feet, the initial depth of the inflow is within the range from about 6 inches to about 6 feet, the depth of the biological zone is at least about 2 feet, the calculated average linear velocity of the inflow at the vicinity of the injection is within the range from about 1 to about 35 feet per minute, the average horizontal velocity in the reaction zone is within the range from about 1/2 to about 20 feet per minute, and the recirculation rate is within the range from about 1 to about 15 times the average waste water influent flow rate.

8. A process, as defined in claim 7, in which the tank is rectangular and the width of the inflow is substantially the entire width of the enclosure.

9. A process, as defined in claim 7, in which the ratio of length to width of the inflow is l/2 to 8 times.

10. A process, as defined in claim 7, in which the depth of the biological reaction zone is from about 2 feet to about 2 feet below the surface of the charge.

11. A process, as defined in claim 7, in which the depth of the biological reaction zone is from about 4 feet to about 2 feet less than the depth of the charge.

12. A process, as defined in claim 7, in which the effluent is overflowed at a rate of up to about 1,500 gals.per sq. ft./day.

13. A process, as defined in claim 7, in which the rate of injection in the supplemented stream is between about 2 and about 15 times the average waste water influent flow rate.

14. An apparatus for treating waste water containing biodegradeable waste to provide a clarified liquid effluent and a disposable sludge including a single treating enclosure open to the atmosphere for containing waste-degrading micro-organisms and through which waste water is continuously passed, and 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 which a lower part of the enclosure defines a biological reaction zone for containing mixed liquor containing said microorganisms and in which a biological reaction to degrade the waste is conducted, an upper part of the enclosure defines a clarifica-tion zone in which clarified liquid rises and overflows, and there is between the reaction and clarification zones a transi-tion zone to effect rising of the liquid of the mixed liquor and settling of the solids, an oxygen-dissolving device, means for continuously withdrawing a recycle stream of mixed liquor from the reaction zone and conducting the stream through said oxygen-dissolving device, means for continuously adding influent waste water to said stream, means including a source of oxygen for continuously adding oxygen to the oxygen-dissolving device to dissolve oxygen in the stream and means for passing the thusly supple-mented recycled stream into a lower part of the reaction zone of the enclosure remote from the vicinity of withdrawal, means for continuously conducting the waste water 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 within the reaction zone effective for the biodegradation of the waste and for the formation and settling of biological floc, means for continuously adding oxygen to said 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 to maintain it in contact with the liquid in a contact zone of said stream for a time and under a pressure such that the oxygen is dissolved in the liquid, means for continuously controlling the overall flow rate of said recycle stream to a substantially constant rate several times that of the influent waste water effective to provide for dissolving the oxygen which is added to the recycle stream, and 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, means for continuously distributing the flow of said recycle stream entering the reaction zone to reach a substan-tial area of a lower part thereof to provide a 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 and to provide at an intermediate level of the enclosure, an upward velocity of the mixed liquor less than the settling rate of the solids, whereby there is maintained in the enclo-sure separate reaction and clarification zones intervened by said transition zone, means for continuously monitoring the concentration of dissolved oxygen in the reaction zone to determine varia-tions thereof resulting from variations in the flow rate and concentration therein of waste including a probe located within said reaction zone, means including a dissolved oxygen analyzer and controller responsive to the probe, for periodically adjust-ing the rate of addition of the oxygen to the recycle stream in response to variations in the oxygen concentration in the reaction zone to maintain said concentration within said controlled range and at a level where there is substantially avoided effervescence that would lead to gas bubbles rising into the clarification zone, means for continuously withdrawing said effluent from the clarification zone to keep pace with the influent waste water, and means for continually removing excess sludge from the reaction zone and carbon dioxide from the mixed liquor.

15. An apparatus, as defined in claim 14, in which the agitation is provided by means subdividing the stream into jets arranged across the container.

16. An apparatus, as defined in claim 14, in which the oxygen-dissolving device is a chamber of tapering cross-section within the enclosure below the liquid level and having an entrance at its narrow end towards the top of the enclosure and an outlet at its wide end towards the bottom of the enclosure.

17. An apparatus, as defined in claim 14, in which the oxygen-dissolving device includes an elongated cooperating downcomer for receiving the supplemented recycle stream and an upcomer leading back to the enclosure.

18. An apparatus, as defined in claim 14, wherein the means for distributing the flow of the influent recycle stream includes, a rotatable hollow shaft mounted centrally for rota-tion in said enclosure and providing a vertical conduit, a distributing conduit extending outwardly from a lower part of the shaft and having an operable connection with the vertical conduit, said distributing conduit being provided with a plurality of outlets for directing said influent recycle stream in predetermined direction to provide said controlled agitation.

19. An apparatus, as defined in claim 18, in which the distributing conduit is part of a foot including raking means for removing solids from the bottom of the enclosure.

20. An apparatus, as defined in claim 16, in which the means for distributing the flow of the influent recycle stream includes means for conducting the stream to a lower part of the container and means connected therewith for dis-tributing the stream so conducted in jets at positions across the container.

21. An apparatus, as defined in claim 19, in which the distributing conduit and the raking means are part of a foot in which the distributing conduit extends outwardly from the central shaft at a downward angle and forms a top beam, the raking member extends outwardly from the central shaft at an upward angle and is spaced from the outwardly extending conduit, and there are connecting members extending between the conduit and the raking member so that the entire unit is substantially in the form of a truss.

22. An apparatus, according to claim 16, wherein said oxygen-dissolving device is defined by at least one vertical column having an elongated downflow channel and an elongated upflow channel, said means for introducing oxygen being adapted to inject oxygen into said downflow channel; said means for conducting influent waste water to said oxygen-dissolving device comprising first conduit means communicating said reaction zone with an upper end of said downflow channel for passage of liquid from said reaction zone to said downflow channel, and second conduit means communicating said reaction zone with an upper end of said upflow channel for passage of liquid from said upper end to said reaction zone.