GB1584065A

GB1584065A - Two zone process for biological treatment and clarification of waste water

Info

Publication number: GB1584065A
Application number: GB41898/77A
Authority: GB
Original assignee: Canadian Liquid Air Ltd
Current assignee: Canadian Liquid Air Ltd
Priority date: 1976-10-07
Filing date: 1977-10-07
Publication date: 1981-02-04
Also published as: BE859489A; JPS5835118B2; FR2367024A1; FR2367024B1; AU2941477A; JPS5346166A; AU515052B2; IT1088855B; NL7711035A; DE2745312A1

Description

(54) TWO ZONE PROCESS FOR BIOLO GICAL TREATMENT AND CLARIFICATION OF WASTE WATER (71) We, CANADIAN LIQUID AIR LTD, a Canadian Company of 1210 Sherbrooke Street West, Montreal, Quebec, Canada H3A 1H8, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process and apparatus for the continuous treatment of waste water having a biological oxygen demand tB.O.D.) to remove the B.O.D.

The 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, namely a degritting tank, a primary clarifier, an aeration tank and a secondary clarifier.

A typical treatment plant will comprise several 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, Canada is de signed to treat waste water at a rate of 60 million gallons/day; each series of treatment tanks 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. feet.

In such conventional treatment processes the waste water is treated initially in a degritting tank in which the heavy solid particles are permitted to settle out. The water passes from the degritting tank to a primary clarifier wherein the waste water is held for a time to permit suspended solid particles to settle out and wherein floating solids and oils and grease are skimmed off, this treatment being referred to herein as "primary clarification". The liquid from the primary clarifier passes to an aeration tank which contains microorganisms for converting dissolved matter in the liquid by biological reaction into insoluble matter; air or oxygen is introduced under agitation into the tank to meet the oxygen requirement of the microorganisms. 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 liquid containing sludge from the secondary clarifier is continuously recycled to the aeration tank for further biological treatment, and the excess is wasted.

The invention provides a process and apparatus for treating waste water biologically in which the biological reaction and secondary clarification of biologically treated water are conducted in a single vessel, thus permitting economy in plant design and the successful treatment of waste water containing a higher concentration of waster matter than the conventional process employing a separate aeration tank and secondary clarifier. In accordance with the invention the primary clarification, the biological reaction and secondary clarification may, under appropriate conditions, all be conducted in a single vessel.

Apparatus according to the invention may provide significant economies in size over apparatus conventionally employed; can permit higher treatment capacity; and can permit the treatment of waste water containing relatively high concentration of waste matter.

In one aspect the invention provides a process for continuously treating waste water biologically, which process comprises continuously introducing an oxygenated mixed liquor containing dissolved oxygen into a lower part of a single reactor vessel having a lower biological reaction zone and an upper clarification zone therein, the oxygenated mixed liquor having been obtained by oxygenating a mixture of newly supplied waste water and partially treated liquor from the reaction zone, the mixed liquor in the reaction zone containing microorganisms effective in the presence of oxygen to convert organic waste in the waste water into water-insoluble solids, carbon dioxide and other products, agitating and circulating the mixed liquor in the reaction zone to maintain at least some of the solids therein in suspension while maintaining the contents of the clarification zone relatively free from agitation to allow solids contained therein to settle as liquor rises from the reaction zone into and through the clarification zone, maintaining a concentration of oxygen in the reaction zone sufficient to satisfy the oxygen demand of the microorganisms acting upon the waste in the mixed liquor in the reaction zone but less than the concentration of oxygen in the oxygenated mixed liquor to prevent release of oxygen bubbles from the mixed liquor, and withdrawing clarified liquid from an upper part of the clarification zone.

The process of the invention will be described in more detail as follows. A single reactor-clarifier vessel, in which there is a reaction zone at least in a lower part and a clarification zone in an upper part, contains a supply of mixed liquor containing microorganisms effective in the presence of oxygen to convert organic waste into waterinsoluble solids, carbon dioxide and other products. Oxygenated mixed liquor containing dissolved oxygen is continuously supplied to a lower part of the vessel. This oxygenated mixed liquor is provided by oxygenating a mixture of newly supplied waste water and partially treated substantially carbon dioxide-free mixed liquor circulated from within the vessel. Controlled agitation is provided to maintain circulation of the liquid in a part of the vessel only, that part constituting an agitated biological reaction zone in which at least some of the solids are kept in suspension and, at the same time, to maintain relatively free from agitation the liquid in a separate part of the vessel constituting a clarification zone in which the solids settle and clarified liquid rises. Partially treated mixed liquor is continuously taken from the reaction zone and added to the mixture being oxygenated. The oxygen in the reaction zone is maintained at a con centrntion to satisfy the oxygen demand of the organisms and, at the same time, at less than the concentration of oxygen in the oxygenated mixed liquor supplied to the reaction zone. Tn this way, the oxygenated mixed liquor is diluted, as it enters the reaction zone, to prevent release of oxygen from solution and thus to inhibit the formation of bubbles which would carry solids from the reaction zone into the clarified liquid. Clarified liquid is continuously overflowed from the vessel to keep pace with the waste water newly fed to the vessel. The procedure described enables separate stable clarification and biological reaction zones to be maintained in a single reactor-clarifier vessel and efficient clarification of a treated liquor on a continuous basis.

Thus, waste water continuously flows into the system and water from which waste has been progressively removed overflows as clarified liquid. This takes place as follows.

Newly added waste water mingles with the partially treated mixed liquor to form a mixture to which oxygen is added. The oxygenated mixture passes into a lower part of the reactor-clarifier vessel and into the reaction zone where it meets the mixed liquor which is being continuously circulated in contact with the treating organisms and which contain a lower concentration of oxygen than that of the incoming oxygenated mixed liquor. The incoming oxygenated mixed liquor is therefore diluted so that its oxygen remains in solution preventing the release of oxygen as bubbles which would interfere with the process by buoying solids upwards into the clarification zone at a greater rate than the net upward movement of liquid in the reaction zone. The incoming oxygenated liquor thus becomes a part of the liquor in the reaction zone which is in continuous circulation. Part of the circulating liquor is continuously taken from the reaction zone and returned to be mixed with the newly fed waste water for oxygenation as described. As this is all taking place, liquid is rising from the reaction zone at a rate determined by the rate of waste water addition into a relatively quiescent clarification zone in which the solids settle out of it and as the liquid rises it is clarified. The clarified liquid overflows at the rate new waste water is added to the system.

In brief, while there is a continuous flow of the waste water into the system and a continuous upward flow of mixed liquor through the single reactor-clarifier vessel on which oxygen Injection and agitation are superimposed, the conditions within the vessel are so controlled that the solids are subjected to resultant downward forces causing them to settle and bring about clarification.

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 carefully 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 reaction zone then the undissolved oxygen in the form of small bubbles disturbs the secondary clarification because the small bubbles 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 biological reaction zone comprising both microoganisms and waste material which has not been biologically treated and this also results in unsatisfactory secondary clarification.

According to another aspect of the invention there is provided an apparatus for treating waste water biologically which comprises a single reactor-clarifier vessel, inlet means for continuously adding waste water to said vessel, to form a content of mixed liquor therein, an oxygen dissolving device adapted to dissolve oxygen in mixed liquor contained in said device, including injecting means for injecting oxygen into the mixed liquor in said device, circulating means for impelling at least part of said mixed liquor through said oxygen dissolving device to dissolve oxygen therein and for discharging oxygenated mixed liquor into a biological reaction zone defined in said vessel, said circulating means being effective to continuously circulate the oxygenated mixed liquor up through said biological reaction zone and through said oxygen dissolving device for a time effective for completion of biological reaction in said biological reaction zone and at flow rate effective to maintain solids in the mixed liquor in suspension, means for removing separated clarified liquid from an upper part of said vessel, said circulating means being effective to produce a flow rate in said upwardly flowing mixed liquor considerably greater than flow rate of upwardly flowing clarified liquid and a flow rate of said 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, and means for controlling the supply of oxygen to said oxygen dissolving device to meet the biological oxygen demand and avoid undissolved oxygen in the biological reaction zone.

Effectively in the process and apparatus 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 clarification zone. Although physical separation of the two zones is not absolutely necessary it is found to be convenient to employ flow distributing baffles between the zones since this improves the separation of suspended solids from the upwardly flowing clarified liquid.

The two zones are effectively produced by appropriate hydraulic design by arranging a pumping or mixing system to agitate the bottom half of a tank sufficiently to keep solids in suspension to function as a biological reactor. At the same time the hydraulic design should arrange to keep the top half of the tank free from agitation so as to function as a clarifier.

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 biological reaction zone 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 biological reaction zone relative to the rate of flow in the clarification zone, is necessary both to produce the required hydraulic system permitting efficient separation of the clarified liquid, and to maintain solids precipitated from the water in the biological reaction zone in suspension; solids must remain in suspension since settling of all the solids and accumulation in the bottom of the vessel will eventually disturb the hydraulic system.

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 the time spent in the biological reaction zone. For each circulation of waste water through the oxygen dissolving device and biological reaction zone the residence time of the waste water in the oxygen dissolving device is typically from 5 to 50 seconds, and preferably from 10 to 25 more preferably about 15 seconds. The total average time that waste water spends in the biological reaction zone is about 0.5 to 5 hours, preferably about 1 to 3 and more preferably about 2 hours.

In fact it was surprising, in spite of the theory of prior workers in the art, that effective separation of the biological reaction zone and the clarification zone could be obtained and that the circulating solids did not escape into the clarified liquid. While the separation of the zones of high and low flow rate might be expected on a theoretical model of a system under ideal conditions it was highly surprising that adequate separation could be achieved in an aqueous system having a high solids content and that con tinuous separation of clarified aqueous liquid could be obtained.

Further it was not to be expected that the process and apparatus of the invention employing a single vessel would permit the effective continuous treatment of a water having a higher content of waste material than the conventional system in which aera tion and secondary clarification are con ducted in completely separate vessels.

It will be understood that in the clarifica tion zone the rate of settling of the solids must be greater than the rate of upward flow of liquid to achieve efficient clarification.

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 carefully controlled to ensure that there is no undissolved oxygen in the form of gas bubbles. For similar reasons it is appropriate to employ oxygen substantially free of other gases. Air would not be suitable as the source of oxygen in view of the high content of nitrogen; nitrogen is 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 conveying 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.

Under steady conditions the solids content in the system incleases 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 typically 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 gallons/day, the solids are suitably pumped out from the biological reaction zone at a rate of 10 gals/ min for 4 hours which represent a rate of about 4% of the influent flow rate per day.

It is also within the scope of the invention to carry out the primary clarification (as herein defined) 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 present invention is that it permits a much higher treatment capacity per unit surface area of treatment tank, than existing installations.

In the case of the Hamilton treatment plant described previously, the separate aeration tank and clarifier in each series could be converted to two single treatment tanks in parallel, according to the teachings of the present invention and in this way the treatment capacity (i.e. volume of water treated per unit time) of an existing installation could 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 could be increased to 11 to 19 million gallons/day by modifying the existing twotank series to provide two single treatment tanks in parallel.

If the tanks are modified in such a way that all three treatments (primary clarification, biological reaction 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 installation can be converted to three single treatment tanks 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 apparatus of the invention, which by careful 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.

In this specification the expression "oxygen dissolving device" or"oxygen contact device" refers to any device which can be employed to contact oxygen and waste water 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 dissolved in the waste water.

An especially preferred class of oxygen dissolving devices is the class generally illustrated in U. S. patent 3,643,403 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 in U. S. Patent 3,643,403 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 for introducing oxygen to the funnel portion in the form of a bubble disperser.

Another type of oxygen dissolving device usually located outside the treatment tank may consist of a tube sunken vertically into the ground. This tube is divided into two halves longitudinally by a vertical partition that starts at the top of the tube and ends above the bottom to provide clearance for flow.

This arrangement may also be achieved by installing two vertical tubes, side by side and connecting them at the bottom. Waste water is introduced into one side of the partition where a pump induces the flow down the tube and up the-other side. Oxygen is introduced just below the pump in the form of fine bubbles and these are swept downwards with the flow dissolving as they are carried along with the waste water.

In one embodiment, the invention provides improvements in oxygen contacting devices which employ a pump that induces a downward flow. This downward flow must not draw in air to the liquid by a vortex or any other means because the major component of air is nitrogen and the nitrogen mixes with the oxygen and dilutes it. This reduces the rate at which oxygen is dissolved into the liquid.

The pump is required to supply energy to move water and oxygen through the U tube.

It is not necessary condition to induce a spiral flow down the U tube, for there are a number of ways of inducing the required flow without the spiral effect being produced.

The invention will be described in more detail by reference to the accompanying drawings in which : FIGURE 1 is a schematic diagram of one embodiment of an apparatus of the invention for carrying out the process of the invention in which an oxygen dissolving device is located wtthin the reactor-clarifier vessel, FIGURE 2 is a schematic diagram of a different embodiment of the invention in which an oxygen dissolving device is located outside the reactor-clarifier vessel, FIGURE 3 is a schematic diagram of a carbon dioxide stripper which may be incorporated in the systems illustrated in Figures 1 and 2.

FIGURE 4 is a schematic diagram of another embodiment of the invention in which an oxygen dissolving device is located outside the reactor-clarifier vessel, and FIGURE 5 illustrates in greater detail the oxygen dissolving device employed in Figure 4.

With further reference to Figure 1, a treatment apparatus comprises a reactorclarifier tank 10 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 12, 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 provided a clarifies overflow weir 16 which communicates with an effluent line 17 for removing clarified water.

An oxygen disolving 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 recbrder controller 28 to a flow regulating valve 30 in the oxygen supply line 20.

A pump 32 is mounted above the bxygen dissolving device 18 for circulating liquids being treated through the oxygen dissolving device in the direction shown by the arrows.

The pump 32 may be, for example, an axial pump or a centrifugal pump; when frothing of the waste water is not a problem and/or when stripping of CO2 from the waste water is deemed desirable, an air-life pump can be used for the circulation; in this case a certain amount of oxygen from the air-lift ispicked up by the mixed liquor, thus reducing the overall oxygen gas requirement.

The tank 10 defines a biological reaction zone 38 and clarification zone 40 separated by a separating zone 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 between 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 oxygen supplying circuit comprising oxygen probe 24 and the related oxygen analyzer 26, recorder controller 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 microorganisms as they act upon the waste water.

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 dissolved 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 analyser 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 dissolved oxygen on a continuous basis.

The controller in the recorder controller 28 compares the input signal with a predetermined set-value and sends a signal to flow regulating valve 30 in the oxygen 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 chamber 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 which in the particular embodiment are of different diameters.

The flow directing baffle 12 is suitably located substantially centrally in an upper part of tank 10 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 the clarification zone 40; in particular an upper portion of zone 38 is defined between the inner wall of baffle 12 and the outer surface of device 18; and the zone 40 is defined between the outer wall of baffle 12 and the inside wall of tank 10. The baffle 12 suitably comprises a tubular member having an upper cylindrical tube and a lower frusto-conical housing, however, baffle 12 may also be a square sectioned member having an upper square sectioned member and a lower square section pyramid.

In operation influent is 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 turbulence.

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 detects the dissolved oxygen in the waste water and sends a signal to the analyzer 26. The analyzer 26 in turn transmits a signal to the recorder/controller 28 that modulates a flow control valve 30 on the oxygen line from the source 22. When the dissolved oxygen is below the set point of the recorder 28, the oxygen flow valve 30 opens to allow more oxygen to be dissolved, and conversely when the dissolved oxygen exceeds the set point the flow valve 30 is closed. The set point of the recorder 28 may be varied as required. The oxygen fed to the device 18 is regulated at the valve 30 under instruction from the recorder/controller 28 to ensure that adequate oxygen is provided to meet the biological oxygen demand of microorganisms in the biological reaction zone 38 while at the same tim 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 from 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 where the tank 50 is of rectangular cross-section the inlet member may comprise a tubular rectangular frame.

An outlet member 64 is spaced apart from the inlet 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 communicates 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.

There is defined in the tank 50 an upper clarification 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 biological 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 forming 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 (acrylonitrile-butadiene-styrene copolymer).

Such modules are commercially available and may be stacked side by side while being firmlv suported by clamping members.

Similarly, it is convenient to employ a flow 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.

In some cases it may be apropriate to incorporate into the system means for stripping off carbon dioxode. However, carbon dioxide dissolved in the waste water does not affect the performance of the biological treatment when present in moderate quån- tities, and for treating domestic waste water as opposed to certain industrial waste water stripping of the carbon dioxide is not necessary.

However, 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, so 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 via line 100, strips carbon dioxide from the water and exists via line 98.

With reference to Figure 4 there is shown a treatment apparatus which is similar to that of Figure 2, in as much as the oxygen dissolving device is located outside the tank.

In Figure 4 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 illus trated by reference to Figures 4 and 5 is located in the influent line 213.

The device 218 comprises a generally cylindrical tube 300 having a partition wall or baffle 302 extending between the walls of the tube 300 from the upper end 304 of tube 300 towards the lower end 306, a gap 308 being provided between wall 302 and end 306; the partition wall 302 divides the tube 300 into an upstream portion 310 and a downstream portion 312.

A recirculation impeller 314 is disposed near the top of 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. A solids outlet 214 is provided from the line 316 so that excess sludge may be removed.

The oxygen dissolving device 218 is connected by an oxygen supply line 260 to an oxygen source 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 regulating valve 276 in the oxygen supply line 260.

As shown more clearly in Figure 5 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 4 and 5 is substantially the same as that described with reference to Figures 1 and 2.

Influent is introduced into tank 210 via influent line 213 and oxygen dissolving device 218, and is recirculated through the biological reaction zone 238 and device 218 by pump 318.

Oxygen 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 zones 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).

In the biological reaction zone carbon dioxide will be generated. This will be pumped out of the vessel 210 through the line 316. This carbon dioxide preferably should be removed. This can be done by putting an air sparger in the horizontal part of the line 316. In this way carbon dioxide generated in the biological reaction zone is continuously removed.

Influent line 213 can be provided with means (not shown) to distribute flow across the entire cross-section of the tank 210 so as to avoid localised high flow conditions which might disturb the clarification zone 240.

EXAMPLE 1 A pilot plant was set up in the laboratory according to that illustrated in Figure 2 of the drawings in which the oxygen contacting device was located outside the tank. The waste water treated was synthetic and was made from a solution of glucose and added nutrients. The plant was operated under the following condition.

Waste Water Flow ... 4,800 G.P.D. (gallons/day) Qualify Total biological oxygen demand (BOD) 264 mg/l (milligrams/litre) Total chemical oxygen demand (COD) 396 mg/l Process Conditions Biological Reaction Zone Mixed liquor suspended solids (MLSS) 26,000 mg/l Temperature ... 19"C Dissolved Oxygen (D.O.) 5 mg/l Residence 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 zone 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 MLSS was 26,000 mg/l. The mixed liquor in this process was also the sludge which was wasted.

EXAMPLE 2 The following represent typical performance data obtained with municipal waste water biologically treated with the two zone process. An apparatus as illustrated in Figure 1 was employed having the oxygen contacting device in the tank, but without the flow distributor 36 and without the flow distributing baffles 34.

Waste Water Flow Min. 50.000 G:P.D.

Max. ... 110,000 G.P.D.

Average 75,000 G.P.D.

Quality Suspended Solids ... 70 mg/l Total BOD ... .. 125 mg/l Soluble BOD ... ... 60 mg/l Total COD ... ... 250 mg/l Soluble COD ... ... 175 mg/l Process Conditions Biological Reaction Zone M.L.S.S. ... ... 2500 mg/l Temperature ... ... 16"C Dissolved Oxygen ... 3 mg/l Residence Time ... 3 hours Clarifier Overflow rate ... 1000 G.P.D./sq.ft.

Sludge (Solids) Settling Velocity ... ... 7 ft./hour Effluent Quality Suspended Solids ... 20 mg/l Total BOD ... ... 25 mg/l Soluble BOD ... ... 5 mg/l Total COD ... ... 80 mg/l Soluble COD . 55 mug/1.

WHAT WE CLAIM IS:- 1. A process for continuously treating waste water biologically, which process comprises continously introducing an oxygenated mixed liquor containing dissolved oxygen into a lower part of a single reactor vessel having a lower biological reaction zone and an upper clarification zone therein, the oxygenated mixed liquor having been obtained by oxygenating a mixture of newly supplied waste water and partially treated liquor from the reaction zone, the mixed liquor in the reaction zone containing micro-organisms effective in the presence of oxygen to convert organic waste in the waste water into water-insoluble solids, carbon dioxide and other products, agitating and circulating the mixed liquor in tile reaction zone to maintain at least some of the solids therein in suspension wliile maintaining the contents of the clarification zone relatively free from agitation to allow solids contained therein to settle as liquor rises from the reaction zone into and through the clarification zone, maintaining a concentration of oxygen in the reaction zone sufficient to satisfy the oxygen demand of the microorganisms acting upon the waste in the mixed liquor in the reaction zone but less than the concentration of oxygen in the mixed liquor to prevent release of oxygen bubbles from the mixed liquor and withdrawing clarified liquid from an upper part of the clarification zone.

2. A process according to claim 1 wherein the clarified liquid is withdrawn at a rate substantially equal to the rate at which waste water is newly supplied to said vessel and the mixed liquor circulates in the reac tion zone at a flow rate 10 to 100 times the flow rate of upwardly flowing clarified liquid in the clarification zone.

3. A process according to claim 1 or 2 wherein the oxygenated mixed liquor is produced in an oxygen-dissolving device located within said vessel.

4. A process according to claim 3 wherein the oxygenated mixed liquor is produced in an oxygen-dissolving zone separated from the biological and clarification zones and the oxygen supplied in tthe dissolving zone is circulated as dissolved oxygen to the mixed liquor in the reaction zone.

5. A process according to claim 1 or 2 wherein the oxygenated mixed liquor is produced in an oxygen-dissolving device located outside said vessel.

6. A process according to any one of the preceding claims wherein a primary clarification as herein defined of the waste water is also conducted in said vessel.

7. A process according to claim 1 substantially as described with reference to any one of Figures 1, 2 and 4 of the accompanying drawings.

8. A process according to claim 1 substantially as described in Example 1 or 2.

9. Apparatus for treating waste water biologically which comprises: a single reactor-clarifier vessel, inlet means for continuously adding waste water to said vessel, to form a contact of mixed liquor therein, an oxygen dissolving device adapted to dissolve oxygen in mixed liquor contained in said device, including injecting means for injecting oxygen into the mixed liquor in said device, circulating means for impelling at least part of said mixed liquor through said oxygen dissolving device to dissolve oxygen therein and for discharging oxygenated mixed liquor into a biological reaction zone defined in said vessel, said circulating means being effective to continuously circulate the oxygenated mixed liquor through said biological reaction zone and through said oxygen dissolving device for a time effective for completion of biological reaction in said biological reaction zone and at flow rate effective to maintain solids in the mixed liquor in suspension, means for removing separated clarified liquid from an upper part of said vessel, said circulating means being effective to produce a flow rate in said mixed liquor considerably greater than the flow of upwardly flowing clarified liquid and a flow rate of said 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, and means for controlling the supply of oxygen to said oxygen dissolving device to meet the biological oxygen demand and avoid undissolved oxygen in the biological reaction zone.

10. Apparatus according to claim 9 wherein said means for controlling the sup

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Quality Suspended Solids ... 70 mg/l Total BOD ... .. 125 mg/l Soluble BOD ... ... 60 mg/l Total COD ... ... 250 mg/l Soluble COD ... ... 175 mg/l Process Conditions Biological Reaction Zone M.L.S.S. ... ... 2500 mg/l Temperature ... ... 16"C Dissolved Oxygen ... 3 mg/l Residence Time ... 3 hours Clarifier Overflow rate ... 1000 G.P.D./sq.ft. Sludge (Solids) Settling Velocity ... ... 7 ft./hour Effluent Quality Suspended Solids ... 20 mg/l Total BOD ... ... 25 mg/l Soluble BOD ... ... 5 mg/l Total COD ... ... 80 mg/l Soluble COD . 55 mug/1. WHAT WE CLAIM IS:-

1. A process for continuously treating waste water biologically, which process comprises continously introducing an oxygenated mixed liquor containing dissolved oxygen into a lower part of a single reactor vessel having a lower biological reaction zone and an upper clarification zone therein, the oxygenated mixed liquor having been obtained by oxygenating a mixture of newly supplied waste water and partially treated liquor from the reaction zone, the mixed liquor in the reaction zone containing micro-organisms effective in the presence of oxygen to convert organic waste in the waste water into water-insoluble solids, carbon dioxide and other products, agitating and circulating the mixed liquor in tile reaction zone to maintain at least some of the solids therein in suspension wliile maintaining the contents of the clarification zone relatively free from agitation to allow solids contained therein to settle as liquor rises from the reaction zone into and through the clarification zone, maintaining a concentration of oxygen in the reaction zone sufficient to satisfy the oxygen demand of the microorganisms acting upon the waste in the mixed liquor in the reaction zone but less than the concentration of oxygen in the mixed liquor to prevent release of oxygen bubbles from the mixed liquor and withdrawing clarified liquid from an upper part of the clarification zone.

8. A process according to claim 1 substantially as described in Example 1 or 2.

10. Apparatus according to claim 9 wherein said means for controlling the sup

ply of oxygen to said oxygen dissolving device comprises an oxygen probe disposed in said biological reaction zone, effective to determine dissolved oxygen in the oxygenated mixed liquor, said probe being operably connected to an oxygen analyser and control means for controlling the flow of oxygen into said oxygen dissolving device in response to a signal from said oxygen probe.

11. Apparatus according to claim 9 or 10 wherein said oxygen dissolving device is located inside said vessel.

12. Apparatus according to claim 9 or 10 wherein said oxygen dissolving device is located outside of said vessel.

13. Apparatus according to claim 9 substantially as described with reference to, and as illustrated in, any one of Figures 1, 2 and 4 of the accompanying drawings.