CA1071528A - Treatment of biological waste material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In an activated sludge process for the treatment of liquid wastes, liquid from the main reaction vessel is circulated downwardly through one or more vertical columns.
Bubbles of oxygen are introduced at the bottom of the columns to rise upwardly relative to the liquid flow. A gas trap at the bottom of the last column collects small bubbles which are carried out of that column, and releases larger bubbles into the column. Advantageously, an air lift may be used to carry liquid from the main reaction vessel to the columns, and this serves to strip unwanted carbon dioxide from the liquid.

Description

This invention relates to ~he dissolving of gases in li.quids. It is applicable generally to the production of highly concentrated, saturated or near saturated solutions of almost any gas in any liquid where ~he resulting solution is of commercial importance.
For example, the invention is applicable to the solution of C02 in a beverage liquid, to oxygen enrichment of various liquids as required in industry or medicine, and to many other purposes.
Although the invention is widely applicable in the general sense indicated, it is primarily intended for use in obtaining concentrated solutions of oxygen in the purification treatment of sewage and industrial waste materials and the invention will be described herein mainly in terms of that particular purpose.
In the purification treatment of sewage liquids and like liquids containing biological waste materials, most methods in present use involve prolonged aeration of the liquid material. This encourages micro-organisms present ; 20 in the incoming material, or introduced with activated sludge, to break down the offensive portions of the waste into a form more fit for disposal, whilst avoiding as far as possible the production of offensive products such as sulphur compounds. The quantity of oxygen dissolved in an effluent very largely determines the aerobic digestion rate.
The conventional methods, because of the low oxygen

- 2 -concen~ration employed, have the disadvantages of being slow in operation and providing low rates of treatment.
As a result, the plant required occupies considerable areas of land.
Many proposals have been made for improving the capacity of waste treatment plant by substituting oxygen for air in the treatment. However, these plants are usually costly to install and maintain, and also require large areas of land. Moreover, the solu~ion of oxygen (as with most permanent gases in water or other aqueous solu-tion at atmospheric pressures) demand a considerable amount of power, or use of pressure vessels in which a large volume of packing is req~ired to achieve the required solution concentration.
In the field of treatment under discussion the amount of oxygen to be dissolved ln the liquid depends upon the extent to which the liquid is polluted, and the level of pollution is measured by the amount of oxygen required for the micro-organisms (bacteria) to digest ~he biological matter. The measurement scale is called the Biological Oxygen Demand ~BOD) and it is known that domestic sewage has a BOD of about 300 PPM (parts per million) unsettled. If a primary settlement stage is used, above 150 PPM of BOD is left for bacterial treatment.
With brewery and distillery effluent, dissolved sugars cause settled effluent BOD levels of in excess of 2,000 PPM. This higher level makes the aeration equipment and associated yessels as used hitherto prohibiti~ely expensive.
The use of oxygen~ instead of air, for sewage treatment has, to some extent, reduced ~he capital cost of plants to treat polluted water by increasing the level of dissolved oxygen obtainable to 45 PPM at atmos-pheric pressure, an .increase from 9 PPM which is normal solubility of oxygen in water at atmospheric pressure.
But even with this better result the problem of ~reatment of high BOD effluent still requires multi-stage oxygenation to dissolve sufficient oxygen for effective treatment.
An object of the present invention is to provide a method and apparatus for more effici0ntly dissolving gases in liquids. When applied to the oxygenation of biological waste liquids, the method and apparatus overcome, or at least ameliorate the shortcomings indicated above in a simple but highly effective manner.
Thus, the method and apparatus subject hereof effectively enable solution of up to 300 PPM of oxygen in a liquid to be treated or in water to be employed in treat-ing that liquid, and the apparatus is capable of simple construction out of readily available inexpensive materials, such as ordinary water piping.
Broadly speaking, the invention provides a method of dissolving a gas in a liquid, comprising the steps of:
~a~ causing the liquid to flow downwardly _ ~ _ through a columnar treatment duct.
(b) causing the gas to bubble upwardly through the liquid in said columnar treatment duct, and (c) adjusting the relative flow rates of said liquid and said gas so that the average velocity of the gas bubbles relative to the duct is small compared with the liquid flow rate relative to the duct.
To maximlze the time of contact between gas and liquid in the treatment duct, the average velocity of the gas bubbles relative to the duct should be substantially zero.
This inv~ntion also relates to apparatus for dissolving a gas in a llquid, camprising a vertical columnar treatment duct, means for providing a downward flaw of liquid through said duct, means for entraining bubbles of gas within said liquid in said duct, ancl a gas trap at the bottom of said cluct which, in use, receives bubbles of gas carried down frc~ said duct and releases bubbles of the gas so received to travel upwardly through said duct.
Because of variations in the sizes of individual bubbles, the rate of ascent or descent wqll ~ary frGm bubble to bubble. Furthermore, frictiQn between the lic~lid column and the walls of the duct will cause considerable turbulance generally speaking, the centre core of the column of liquid will move more rapidly downwards ~han the outer region. It is therefore ine~it-able that a greater or lesser proportion of the gas bubbles will be carried down to the bottcm of -the column. If the gas is inexpensive, e.g. air, it can simply be carried out of the system with the liquid and vented to atm~s-phere.
If relatively expensive gases such as oxygen are employed, it is preferable to provide means for trapping bu~bles of gas which reach the botb~m of the column~ and ~7~
returning them to ~he system. Accordingly, a preferred form of the invention provides:-A method of dissolving gas in liquid, comprisingthe steps of:
~a) conducting streams of the gas and o the liquid to the upper end of an upright columnar treatment duct;
(b) causing said liquid stream with gas entrained in it to descend within said duct, towards a gas trap at the bottom of said duct; and, (c) selecting the descent rate of said stream so that undissolved gas which arrives in said gas trap by way of the core portion of said stream is able to re-enter and rise within the stream by way of the slow-descending peripheral portion of that stream.
It will be appreciated that the mentioned duct may consist of an ordinary pipe, or any plurality thereof, and that the slower descent rate of the peripheral portion of the down-flowing stream relative to the core portion of that stream may be due to no other factor than that which applies to any fluid travelling through a duct; namely the frictional retardation i~posed on the peripheral portion of the stream by reason of its contact with or adjacency to the inner wall surface of the duct.
It will also be appreciated that the steps in the process of gas solution in the liquid are not abruptly dif-ferentiated. For e~ample, if the gas and liquid are brought together before they reach the top of the upright treatment duct, some degree of gas solution will take place. Some bubbles of gas during descent within -the core of the stream ln the duct wi]l become wholly dissolved, while other bubbles migrate radially and outwardly into the slower moving peripheral portion of the down-flowing stream so as to become amalgamated with up-going bubbles. Similarly up-going bubbles, may migrate into the core of the stream from the periphery of the stream.
The generally prevailing occurrence is that all bubbles of gas tend to get smaller through goi~ng into solution, and when bubbles remain large, or become large through amalgamation with others, that merely augments their upward movement so that they eventually become small enough, with buffeting and general turbulence, and break-up consequent thereof, to go into solution. In short, the larger a bubble happens to be the less will it be able to descend, and thus it may execute several descen~s (or partial descents) each followed by an ascent before it finally goes into solution.

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When the invention is applied to the aerobic activated sludge treatment of biological waste material, it may be desirable to remove C02 from the liquid particularly when low lkalinity wastes are to be treated, it is highly desirable to remove C02 to prevent the C02 formed by the bacteria from depressing the pH ~o such a level that bacterial activity is impaired.
Thus, in preferred embodiments of the invention, an oxygenation treatment process as described is preceded by the step of stripping carbon dioxide from waste material carried from the reactor/clarifier to the oxygenator by means of a gas li~t. - -The gas lift comprises a pump which supplies gas to a column of liquid to reduce the density thereof, so that it is displaced upwardly by the liquid in the reactor/clarifier. The lifting gas at low pressure, is sparged into the bottom of the column via a footpiece, usually an annular chamber around the gas lift column. The gas lift column within the footpiece has small holes in it to make small gas bubbles which travel up the column.
The gas lift serves the dual purposes of raising the waste material to the head of the oxygenator duct (or the first of said ducts in a multi stage treatment), and stripping CO2 from the liquid at the same time. To that end the gas neces-sarily is a non-acidic gas, such as air, ~2 or 2 Accordingly, a further aspect: of the invention provides a method of treating a liquid waste material comprising the steps of:
(a) treating a body of said material with biological sludge;
(b) withdrawing a stream of said material from the main body;
(c) causing said stream to flow downwardly through a columnar treatment duct;
(d) causing oxygen to bubble upwardly through the liquid in said columnar treatment duct;
(e) adjusting the respective flow rates oE said . :

liquid and said oxygen so that the average velocity of the oxygen bubbles relative to the duct is small compared with the flow rate of the liquid relative to the duct; and (f) returning said stream of liquid waste material to the main body; characterised in that said stream is withdrawn at step ~b) by means of a gas lift which simultaneously removes dissolved carbon dioxide from said material.
By way of example, various preferred forms of apparatus accord-ing to the invention are illustrated in the drawings herewith, in which:
Figures 1 and 2 are diagrammatic views of different forms of apparatus for dissolving a gas, such as oxygen, into a liquid.
Figure 3 is a diagrammatic view of a three stage dissolver which~
apart from the multiplicity of stages, is essentially the same as the single stage dissolver of Figure 1.
Figure 4 shows a system and the arrangement thereof for performing a method of treati.ng biological waste material, employing the apparatus of Figure 2.
Figure 5 is a diagrammatic cross sectional view of a reactor/clari-fier in which the digestion of oxygenated effluent may be effected.
Figure 6 is a diagrammatic view of an installation for the treat-ment of biological waste material incorporating a three stage gas dissolver as described with reference to Figure 3 and a reactor/clarifier as described with reference to Figure 5.
Figure 7 is a diagrammatic elevation of an installation for the treatment of biological waste material incorporating an alternative form of three stage gas dissolver and an alternative form of reactor/clarifier;
and also incorporating a gas lift pump.

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The dissolver of Figure 1 comprises a vertical column 10 and a supply pipe 11 whereby waste water or mixed liquor may be pumped to the top of the column 10.
Oxygen from a pressurized source is fed by way of a pressure regu-lator 12 (such as a rotameter) and a flow meter 13 into gas trap 14, from which it bubbles upwardly through the down-flowing liquid in column 10.
The gas trap 14 may comprise a tank which is partly filled with liquid and partly filled with gas. Because of the greater cross sectional area of the tank, the downward flow velocity of the liquid decreases when the liquid enters the tank, and any gas bubbles carried down are released and rise to the top of the tank/ A float-operated excess gas valve 15 main-tains the level of the liquid within tank 14 by releasing gas if the liquid head drops below a predetermined level. Such a venting arrangement is not essential to ~he practice of the invention. For example, in the embodiment shown in Figure 2, the gas trap consists merely of a widened lower portion of the column. The level of the liquid in the gas trap can be observed visually and can be controlled simply by adjustment of the gas and liquid flow rate.
When the gas dissolver is in use~ a gas pocket 16 forms at the top of column 10. At the interface 17, gas bubbles are entrained with the liquid~
The velocity of liquid down the column 10 is adjusted so that the entrained bubbles travel slowly to the bottom of the column. Large bubbles from gas trap 14 rise back up column 10. The result is a column full of bubbles in turbulent motion for very little energy expenditure. This provides an ef-fective means of contacting gas and liquid together with a large total inter-face area between the two phases.
Figure 2 shows an alternative embodiment of the gas dissolver according to the invention. This embodiment, like the embodiment of Figure 1 comprises a vertical column 20 and a supply pipe 21. Column 20 and supply pipe 21 are connected at their topmost ends by a U-bend 22. A fluid level sight glass 23 is fitted adjacent to the top of column 20 and is connected by piping to the top of U-bend 22 and to a point 24 on column 20. The gas trap 25 in this embodiment consists of a lower column portion of greater lateral cross sectional area than the fluid path of column 20. Attached by piping to both ends of gas trap 25 is a fluid level sight glass 26 from which the contents may be bled away through standard valving.
Located in column 20 at a convenient point is a sight glass 27 through which the flow of the fluid in column 20 may be observed.
In this particular embodiment, gas is fed into the system through inlet valve 28 at the lower end of supply pipe 21.
As in the embodiment of Figure 1, a gas pocket is allowed to form at the upper end of column 20. The interface 29 of the gas pocket is main-tained above point 24 on the column 20, by appropriate regulation of the liquid and gas flow rates.
This apparatus, like the apparatus of Figure 1, may be used to treat biological waste material with oxygen. The biological waste material is introduced at the bottom of the pipe 21 by known pumping apparatus, pre-ferably at a pressure of 50 to 100 p.s.i.g. Oxygen is introduced to the liquid through valve 28. The oxygen may be gaseous or in liquid form, and passes with the liquid under treatment up supply pipe 21. Some gas will dis-solve into the liquid due to the pressure. However, gas and liquid will separate in U-bend 22 to create the mentioned gas pocket at the top of the column. The level of the interface 29 can be observed through sight glass 23. Depending on the level at this point, the liquid flow and/or gas flow may be adjusted to give the desired resulting gas pocket. Liquid passing through the gas pocket in the column 20 forms bubbles of gas due to ;' " ~7~L~2~

turbulence, the gas being carried down with the liquid towards gas trap 25.
The smallest of the gas bubbles thus formed will be dissolved into the liquid and will remain in the solution while larger bubbles will float upwards through the liquid. The activity of the bubbles may be viewed via the sight glass 27.
The velocity of the liquid flowing down the centre of the column is set to be higher than the rise of bubbles through the liquid so that some oxygen bubbles will be carried down to gas trap 25.
At the periphery of the column of iiquid, the liquid velocity is less than that at the core of the column and it is arranged, by adjusting the liquid velocity, that the gas bubbles are caused to rise slowly through the liquid at the periphery.
Turbulence within the liquid column will, of course, interfere with this upward travel of the gas bubbles and in consequence, the larger bubbles are broken up and the smallest bubbles, resulting from this turbulence, are absorbed into the liquid. Some bubbles will reach the gas pocket at the top of the column to be met by the incoming liquid and carried downward, thus repeating the sequence detailed above.
The object of the bubble formation is to obtain the maximum bubble action in the column during the passage of the liquid, the greater the dis-turbance of gas bubbles, the greater is the formation of small bubbles which (at the pressure of the liquid being pumped, i.e. 50-100 p.s.i.g.) may be ab-sorbed into the liquid.
Gas bubbles carried out-of-suspension to the gas trap 25 will, because of the decrease in pressure at this point of enlarged area, separ-ate from the liquid and form into a gas space the amount of which may be ob-served in sight glass 26. Large bubbles of gas will disassociate from the gas pocket formed and will travel up the periphery of the liquid column in an endeavour to reach the top of the column.

Should the amount of gas formed in the trap 25 be too great, it may be transferred to the sight glass gas pocket at the top of the column by external piping between a valve at the top of each sight glass or may be, if desired, directed from the valve of sight glass 26 to the gas inlet at 28.
Ideally, in either of the embodiments so far described, the vel-ocity of liquid down the column 10 or 20 ~as the case may be) is set by ad-justment of the feed so that the mean velocity of the gas bubbles in the vertical direction is substantially nil. In this way~ the column 10 or 20 is filled with bubbles of gas and liquid in turbulent motion. A column in accordance with the above description 20 feet in height has effectively dis-solved a high level of oxygen in water to approximately 80% saturation.
Figure 3 illustrates a three stage gas dissolver having approxi-mately ~hree times the capacity of the dissolver of Figure 1. It comprises three columns 30, 31 and 32, associated with a single gas trap 33 of the type shown in Figure 1. The gas pockets at the top of columns 30, 31 and 32 re-spectively are interconnected. The three columns are connected in series and the waste liquid flows from point 34 through each of the columns to gas trap 33. Gas bubbles are entrained from the respective gas pockets and carried downwardly in each of the respective columns. Any gas bubbles which are carried right to the bottom of either column 31 or column 32 are simply carried over into the next downstream column (column 30 or column 31).
It is known that increasing the pressure of a gas-liquid system will increase the quantity of gas able to be dissolved in the liquid, but in preferred embodiments of the invention the only pressurization involved is that effected by the head of liquid in the dissolver column. In those em-bodiments concerned with dissolving oxygen in an aqueous medium, for example, that head may be from 15 to 30 feet of water.

Dissolved oxygen levels of 80 to 100 PPM may be reached under such conditions, provided adequate C02 stripping of the incoming liquid is pro-vided. The CO2 may be stripped with air or oxygen in a column similar to that described with respect to Figure l.
Figure ~ shows a simple system for treating liquid waste material.
The system comprises a single stage gas dissolver 40 of the type described with reference to Figure 2. It also comprises a sludge reactor/clarifier 41 consisting of a pressure vessel partly filled with bacterial sludge 42. A
fluid inlet 43 connects by internal piping or passages (not shown), in shaft 44 to pipes attached to paddle blades 45. The pipes have several outlet holes formed in them. The paddle blades 45 are attached to shaft 44 for rotation therewith. The shaft at its upper end is drive connected to an electric or like motor.
Located at convenient positions on the side of the vessel are valves through which the contents of the vessel at selected levels may be withdrawn for sampling purposes.
The method of treating biological waste material will now be des-cribed with reference to schematic diagram 4. Liquid effluent containing bi ological matter enters by pipe 46 where it is pumped to a pressure of 50-100 p.s.i.g. by the pump indicated at 47. The pressure is indicated by pressure gauge 48. If it is known that the BOD of the effluent is above 300 PPM, the effluent is pretreated by the addition to the effluent of partially treated water being discharged from the reactor by pipeline 49 (to be explained later) or a separate water supply rich in oxygen by previous treatment in a gas dis-solver, as above explained with reference to Figure 2, may be supplied to dilute the BOD of the effluent. The liquid to be treated now passes through at least one gas dissolver 40 where it is treated with oxygen to raise the dissolved oxygen content. The oxygen inlet is shown at 50.

- - -The highly oxygenated effluent mixture passes from the dissolver along pipeline 51 wherein a throttle valve 52 is located, the purpose of which is to control the pressure in the dissolver 40.
The effluent now passes into the pipes attached to the paddles 45 of the reactor 41. The paddle assembly is rotated slowly by the motor 53 usually 20-50 r.p.m. being suitable to prevent the sludge from settling and compacting without causing excessive disruption to the action of the bacteria.
The effluent jets into the sludge by way of the holes formed in the pipes and is evenly distributed throughout the sludge due to the sparging effect of the jets and the rotation of the paddle assembly.
The reactor 41 being a pressure vessel and operating under pressure assists in the oxygen remaining dissolved in the sludge bed. The effluent provides both food and oxygen for the bacteria which the bacteria consumes and in so doing, produces more bacteria and carbon dioxide which dissolves in the water resultant from the action of the bacteria.
Sa~nples taken from various levels of the reactor indicate when excess bacteria have been formed in the reactor so that the extra may be drained off through line 54.
The clear water above the sludge bed may be passed from the top of the reactor via expansion valve 55 through pipeline 56 or alternatively on large installations, may be used to drive a turbine as indicated at 57 so that some power recovery may be made before the water is discharged at 56.
As stated previously the incoming effluent may be diluted by the addition of treated water from the reactor. This water is withdrawn from the vessel through the valve indicated at 58 passing along pipe 59 to an expansion valve 60 or turbine 61 where it may be used to produce some power recovery before passing to pipeline 49 for passage into the effluent to be treated.

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For use in conjunction with embodiments of the present invention, an alternative form of reactor/clarifier is proposed. This reactor/clarifier is very simple and compact and is illustrated in Figure 5. In operation, the contents of this reactor/clarifier become somewhat stratified, at least to an extent that the processes occurring within the reactor/clarifier may be regarded as occurring within specific zones. Thus, the bottom zone 70 is used for sludge compaction. Excess sludge may be drawn off by zone 70 through drain cock 71. Such excess sludge may have 20,000 PPM suspended solids.
Above zone 70 is the main reaction zone 72. Here activated sludge of above 5,000 PPM suspended solids is gently agitated by the injection of incoming sewerage through valve 73, and oxygenated mixed liquor from the oxy-genator through valve 74. Liquor is simultaneously withdrawn from zone 72 through pipe 75 to be reoxygenated. Preferably, the rate of flow through valve 73 is from one quarter to one eighth of the recirculating flow through valve 74. The height of zone 72 may be one to two metres in a tank ~say~ 8 to 10 metres high. This zone extends about half a metre above outlet pipe 75.
Above zone 72 is zone 76, a hindered settling zone of activated sludge. Oxygen demand herè is reduced and supply is provided by dissolved oxygen in the upwardly flowing partially treated waste from zone 72.
The uppermost zone, 77, in the reactor/clarifier is a clarification zone. The treated effluent flows upward to a serrated weir 78 where it over-flows into a sterilization tank 79. This tank is located above the clarifier in order to maintain a head of water over the reaction space. Sterilization may be effected conventionally, for example, by chlorine oxygen or ozone /
Figure 6 represents a complete treatment system comprising a three stage gas dissolver 80 as described with reference to Figure 3 and a reactor clarifier 81 as described with reference to Figure 5. The system also com-prises an air stripper 82, in the form of a simple single stage gas dissolver.

Air stripper 82 removes much of the dissolved carbondioxide to enable higher oxygen concentration to be achieved. It also functions as a safety device to prevent any methane which may be produced in the reactor/clarifier by anaero-bic degradation from coming into contact with oxygen. During normal running dissolved oxygen levels are maintained at between 5 and 15 PPM in reactor 81, but should oxygen supply be lost, anaerobic condition would soon start gener-ation of methane, some of which would remain in solution. On restarting the oxygen supply, methane could be stripped out into the oxygen stream~ forming an explosive mixture. This is prevented by having the air stripping stage before the oxygenation stages. The explosion limit of methane in air is apparently narrower than that of methane in oxygen.
Capacity control for the system is achieved hydraulicly by re-cycling to a wet well 83. The wet well 83 can also be used as reaction space for the bacteria provided the dissolved oxygen levels are kept high.
The oxygen supply to the system is regulated with pressure regu-lator 12 as shown in Figure 1. In normal operation the system is largely self-regulatory, and no major adjustments are necessary even though the hour-ly sewage flow may vary considerably. Variations in oxygen demand will be met by allowing the dissolved oxygen level to vary which will cause a change in driving force in the dissolver or the oxygenator.
Due to the high oxygenation of the liquid which is achieved by the various embodiments of this invention a sludge of good settling characteris-tics ~such that bacterial and organic matter settle quickly leaving clear water for disposal) is achieved. Furthermore, because the bacteria are never short of oxygen, their activity or rate of BOD consumption is maintained at a maximum.
Although the configuration of the oxygenator of Figure 7 differs from that of the oxygenator of Figure 3, it will be understood that the oxygenator of Figure 3 can equally well be used in conjunction with a gas lift pump. Equally, the oxygenator of Figure 7 can be used with appropriate conventional pumping means in place of the gas lift pump as illustrated. Re-ferring now to Figure 7, the oxygenator, denoted generally as 101 comprises three treatment ducts 102, 103 and 104 respectively, through which liquid flows in the series. Liquid is conducted from the foot of duct 102 to the top of duct 103 and from the foot of duct 103 to the top of duct 104 by con-necting ducts 105 and 106.
Ducts 102, 103 and 104 are of equal square cross section. Con-nection ducts 105 and 106 are of rectangular cross section, and each has a cross sectional area about four times as great as that of each of ducts 102, 103 and 104.
Oxygen is entrained into the liquid flowing downwardly into col-umns 102, ~ 03 and 104 from gas pockets 107, 108 and 109 respectively. Oxygen is introduced into the system at the foot of column 104 through valve 110, and is able to pass from gas pocket 109 to gas pocket 108 and from gas pocket 108 to gas pocket 107 through lines 111 and 112 respectively. Waste oxygen leaves the system through pipe line 113. Small bubbles of oxygen which reach the foot of column 104 coalesce into larger bubbles at gas trap 114 and travel back up column 104 Small bubbles of gas which reach the bottom of either column 102 or column 103 are simply carried over into upward column 105 or 106, as the case may be, and are released into gas pocket 107 or 108. Columns 105 and 106 are provided with baffles 115 and 116, which prevent back mixing.
Newly oxygenated liquid from oxygenator 101 enters a reactor/clari-fier, denoted generally as 117, through distribution header 118. Reactor/clar-ifier 117 consists primarily of a large tank partly filled with bacterial sludge. The bacterial sludge tends to settle towards the bottom of the tank, while the liquid in the upper part of the tank tends to be relatively free of suspended solid matter. The tank may be regarded as comprising three zones, although it will be appreciated that no definite boundaries between these zones exist. The uppermost zone 119 is the zone in which the liquid is sub-stantially free from suspended solids. The middle zone 120 contains a mix-ture of liquid and suspended solids, typically at a concentration of 10,000 p.p.m. The lowest zone 121 contains compacted sludge. Dense, excess sludge from zone 121 is withdrawn at 122 for disposal. Liquid is taken from zone 119 to be cycled through the oxygenator 101.
In accordance with the present modification, an air lift denoted generally as 123 is used to raise the liquid from reactor/clarifier 117 to a higher level from which it may flow into the oxygenator.
Air lift 123 comprises a tube of generally J shape, including a foot piece 124. Air from blower 125 is fed via air line 126 into foot piece 124 and bubbles up the stem 127 of the air lift.
Untreated liquid also may be fed from a wet well 128 to the bottom of air lift 123 via pipeline 129. At the head of stem 127 the liquid and air pass into a stilling box 130 under a splash cover 131. Air reaching stilling box 130 is vented to atmosphere at 132. The air is fed to foot piece 124 at a pressure of 5 - 7 p.s.i.g. The holes communicating between foot piece 124 and stem 127 may be of about 1/32" diameter.
The submergence of the air lift ~i.e. the depth of foot piece 124 below the surface of the liquid in reactor/clarifier 117) is about 1 2/3 times the lift (i.e. the distance from the surface of the liquid in reactor/
clarifier 117 to the surface of the liquid in stilling box 130).
The turbulent contact between air and liquid in stem 127 results in the removal of dissolved carbon dioxide from the liquid. Typically~ the air vented at 132 contains 3 to 4% carbon dioxide extracted fro~ the liquid.

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From stilling box 130 the liquid flows through a vee notch flow meter 133 into a header ~ank 134 from which it flows into duct 102 to complete the cycle.
Treated li.quid which reaches the top of zone 117 of the reactor/
clarifier flows over weir 135 into chlorination tank 136. Scum which floats on the surface of the liquid in the reactor/clarifier spills over weir 137, and is returned to wet well 128. After chlorination, the purified effluent leaves tank 136 to be discharged at 138.
The efficiency of this system is demonstrated by the fact that the total cross-sectional area of the ducts of oxygenator 101 need be only about 10% of the cross-sectional area of reactor/clarifier 117. In contrast, prior art systems have required an oxygenation tank of approximately twice the size of their clarifier. The results is that the time taken for treatment of the liquid waste is approximately 30-50% of that taken by the prior art systems.

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Claims (19)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of dissolving a gas in a liquid, comprising the steps of: (a) causing the liquid to flow downwardly through a columnar treatment duct; (b) causing the gas to bubble up-wardly through the liquid in said columnar treatment duct, and (c) adjusting the respective flow rates of said liquid and said gas so that the average velocity of the gas bubbles relative to the duct is small compared with the flow rate of the liquid relative to the duct.

2. A method according to claim 1, wherein the flow rates are adjusted so that average velocity of the gas bubbles rela-tive to the duct is substantially zero.

3. A method according to claim 1, wherein bubbles of gas reaching the bottom of the duct are trapped and the trapped gas is re-used.

4. A method according to claim 1, wherein the gas comprises oxygen and the liquid comprises water.

5. A method of treating a liquid waste material comprising the steps of: (a) treating a body of said material with bio-logical sludge, (b) withdrawing a stream of said material from the main body; (c) causing said stream to flow down-wardly through a columnar treatment duct; (d) causing oxygen to bubble upwardly through the liquid in said columnar treat-ment duct; (e) adjusting the respective flow rates of said liquid and said oxygen so that the average velocity of the oxy-gen bubbles relative to the duct is small compared with the flow rate of the liquid relative to the duct; and (f) returning said stream of liquid waste material to the main body.

6. A method of treating a liquid waste material comprising the steps of: (a) treating a body of said material with bio-logical sludge; (b) withdrawing a stream of said material from the main body; (c) causing said stream to flow downwardly through a columnar treatment duct; (d) causing oxygen to bubble upwardly through the liquid in said columnar treatment duct; (e) adjusting the respective flow rates of said liquid and said oxygen so that the average velocity of the oxygen bubbles relative to the duct is small compared with the flow rate of the liquid relative to the duct, and (f) returning said stream of liquid waste material to the main body; charac-terised in that said stream is withdrawn at step (b) by means of a gas lift which simultaneously removes dissolved carbon dioxide from said material.

7. A method according to claim 6, wherein the flow rates in the columnar treatment duct are adjusted so that the average velocity of the oxygen bubbles relative to the duct is sub-stantially zero.

8. A method according to claim 6, wherein oxygen bubbles reaching the bottom of the duct are trapped and the trapped oxygen is re-used.

9. A method according to claim 6, wherein the gas of said gas lift is selected from the group consisting of oxygen, nitrogen and air.

10. Apparatus for dissolving a gas in a liquid, comprising a vertical columnar treatment duct, means for providing a downward flow of liquid through said duct, means for entraining bubbles of gas within said liquid in said duct, and a gas trap at the bottom of said duct which, in use, receives bubbles of gas carried down from said duct and releases bubbles of the gas so received to travel upwardly through said duct.

11. Apparatus for dissolving a gas in a liquid, comprising a plurality of vertical columnar treatment ducts, arranged in series, means for providing a downward flow of liquid through each of said ducts, means for entraining bubbles of gas within said liquid in each of said ducts, and a gas trap at the bottom of the last downstream duct which, in use receives bubbles of gas carried down from said downstream duct and releases bubbles of the gas so re-ceived to travel upwardly through said downstream duct.

12. Apparatus according to claim 10, wherein said gas trap consists of a widened lower extension of said vertical columnar treatment duct.

13. Apparatus according to claim 10, wherein said gas trap comprises a tank adapted to be filled partly with liquid and partly with gas and means for regulating the level of liquid within said tank.

14. Apparatus for treatment of liquid waste comprising a sludge reactor/clarifier vessel and a vertical columnar treat-ment duct, means for circulating liquid from said vessel to flow downwardly through said duct, means for entraining bubbles of oxygen within said liquid flowing downwardly through said duct, and a gas trap which, in use, receives bubbles of oxygen carried out from said duct and releases bubbles of the oxygen so received to travel upwardly through said duct.

15. Apparatus for treatment of liquid waste comprising a sludge reactor/clarifier vessel, a gas lift for removing liquid from said vessel and simultaneously stripping carbon dioxide from said liquid, a vertical columnar treatment duct adapted to receive liquid from said gas lift, means for entraining bubbles of oxygen within a downward flow of liquid through said duct, and a gas trap at the bottom of said duct which, in use, receives bubbles of oxygen carried out from said duct and releases bubbles of the oxygen so received to travel upwardly through said duct.

16. Apparatus according to claim 14, wherein said gas trap comprises a tank adapted to be filled partly with liquid and partly with oxygen, and means for regulating the level of liquid within said tank.

17. Apparatus for treatment of liquid waste comprising a sludge reactor/clarifier vessel and a plurality of vertical columnar treatment ducts, means for circulating liquid from said vessel to flow downwardly through each of said ducts in series, means for entraining bubbles of oxygen within said liquid flowing downwardly through each said duct, and a gas trap which, in use, receives bubbles of oxygen carried out from the last downstream duct and releases bubbles of the oxygen so received to travel upwardly through said downstream duct.

18. Apparatus for treatment of liquid waste comprising a sludge reactor/clarifier vessel, a gas lift for removing liquid from said vessel and simultaneously stripping carbon dioxide from said liquid, a plurality of vertical columnar treatment ducts arranged in series so that the first of said ducts is adapted to receive liquid from said gas lift, means for pro-viding a downward flow of liquid through each of said ducts, means for entraining bubbles of oxygen within said liquid in each of said ducts, and a gas trap at the bottom of the last downstream duct which, in use, receives bubbles of oxygen carried down from said downstream duct and releases bubbles of the oxygen so received to travel upwardly through said down-stream duct.

19. Apparatus according to claim 17, wherein said gas trap comprises a tank adapted to be filled partly with liquid and partly with oxygen, and means for regulating the level of liquid within said tank.