TREATMENT OF WATER BY CAVITATION

Abstract: A method and apparatus for treating water by a cavitation process in which microscopic bubbles are formed and collapsed by pressure pulses, the energy generated in the collapse destroying any living organisms associated with the bubble. The invention is directed to an improved method in which the water passing through the cavitation region of bubble formation and collapse is subjected to a pressure reduction which greatly facilitates the generation of bubbles. An inverted U-tube (2) is a convenient water duct, providing a reduced pressure at the upper end (4) by a siphonic effect. The cavitation devices (5, 9) are positioned near this upper end and thus subjected to reduced pressure.

Description: Treatment of water by cavitation This invention relates to a method of, and apparatus for, treating water to destroy any harmful living organisms which may be present, such as bacteria, protozoa and viruses, and has special application to the treatment of waste water. It is known to treat waste water by a cavitation process in which very small bubbles, down to microscopic dimensions, are produced and then very rapidly collapsed. Such alternate rarefaction and compression effects can be produced in various ways, for example by pumps, propellers etc, in which cavitation effects are well known and generally undesirable. It has been found that, by making the alternate low and high pressure waves sufficiently intense the succession of rarefaction and compression fronts have a destructive effect on living micro-organisms within the water. Thus a considerable number of bubbles are formed down to quite small sizes, and it is believed that bacteria, viruses or other small living organisms, which may be present within the water, act as nucleating points for the microbubble formation in the rarefaction fronts. The subsequent compression fronts then cause the rapid collapse of the bubbles, creating forces and temperatures sufficient to destroy the associated organisms. An object of the present invention is to improve the efficiency of the treatment process and in particular to increase the "kill rate" of the organisms to, or at least towards, 100%. According to one aspect of the invention, in a method of treating water by cavitation in which a series of alternate low pressure and high pressure conditions are produced in the water to give rise to the formation and subsequent collapse of bubbles, a relatively steady pressure reduction is produced throughout the region of the bubble formation and collapse, to promote bubble formation. The pressure reduction preferably lies in the range 400 to 700, and is preferably 500, millimetres of mercury below atmospheric pressure. The water may be passed through a duct having an upflow inlet portion, a downflow exit portion and an intermediate chamber to produce a siphon effect. Water maybe induced to flow up the upflow portion into the intermediate chamber and down the downflow portion by the application of a vacuum source to the duct. The vacuum source is preferably connected to the intermediate chamber. A pump applied to the inlet of the duct maybe controlled to drive water up the upflow portion through the intermediate chamber and into the fownflow portion and then controlled to produce the above pressure reduction in the intermediate chamber. According to another aspect of the invention, apparatus for treating water comprises a duct for the passage of water, cavitation means operable to impart low pressure and high pressure shock conditions to the water to give rise to the formation and subsequent collapse of bubbles, and means for producing a relatively steady reduction of pressure of the water to promote bubble formation. The apparatus may further comprise a duct having an upflow inlet portion, a downflow outlet portion and an intermediate chamber and means for inducing water to rise in the upflow portion to such a leveli that it overflows through the intermediate chamber into the downflow portion so producing the above pressure reduction at the upper water levels. The means for inducing water to rise into the intermediate chamber may comprises a pump applied to the inlet of the duct and means for evacuating the intermediate chamber. Alternatively, the means for inducing water to rise into the intermediate chamber may comprise a pump having a controllable pressure head for establishing a siphon condition and subsequently reducing the pump head to establish the pressure reduction. The cavitation means are preferably located in the upper part of the downflow portion and may comprise a rotatable impeller, means for introducing a liquid at high velocity into the water, an ultrasonic or sonic transducer, means for producing a series of underwater explosions, a venturi device a jet or an orifice providing rapid pressure changes, or a combination of two or more of these means. The cavitation means operable to impart positive and negative shock waves may include an impeller directed across the water flow path confronting the opening of a branch duct to cause water to recirculate through the region of bubble formation and collapse. A method of treating water in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram of water duct equipment for use in a continuous water treatment process; Figure 2(a) is a schematic diagram of a similar but closed loop system and also employing an alternative cavitation means; and Figure 2(b) is a modification of the equipment of Figure 2(a). In Figure 1 of the drawings there is shown a pipe 1 for the passage of effluent water as indicated by the arrows. The effluent water is driven through the pipe by a pump 12, the flow velocity being controlled by a control valve 14. The pipe incorporates a region 2 in the form of an inverted 'IT. To the top of the U- region 2 of the pipe there is connected a vacuum pump 3 to enable the setting up of normal flow conditions and provide control of the water height as will be explained. The arrangement of Figure 1 is designed to reduce the pressure in the upper surface levels of the water to a vacuum of about 500 mms of mercury, i.e. 500 mms below atmospheric pressure. In the situation shown in Figure 1 , the pressure at the upper water level is: (working from the inlet end) P 0 (atmospheric), plus the pump head P p , minus the column head H. This pressure is less than atomospheric by the excess of the head H over the pump head P p . Thus the height H must be selected in relation to the pump head so that this excess is the pressure drop required in the cavitation region (assuming the cavitation region is in or near the upper water levels) i.e. 500 mm H g in the example. This situation obtains whether there is a space 4 having a sub-atmospheric pressure (as shown) equalling the top water level pressure or the U-tube is completely filled (assume the valve 16 isolates the vacuum pump 3). In setting up the apparatus it is not possible to establish a completely full U-tube, with a pump head P p less than the height H unless the pump is assisted by a vacuum until such time as the siphon effect is achieved and atmospheric pressure on the upper water surface is relieved. It would be possible to run the pump 12 initially at high power with a head greater than H until the syphon effect is established, and then reduce the pump head to a value less than H for normal operation. The use of a vacuum to establish the siphon condition provides flexibility and control. Merely to establish a full U-tube condition the vacuum may be applied anywhere downstream of the upflow pipe 13. Thus a vacuum connection may be fitted at the top of the U-tube as shown or even at the outlet of the downflow pipe. An advantage of the arrangement shown in Figure 1 is that close control of the local sub-atmospheric pressure can be obtained. In a modification of the U-tube the intermediate chamber formed by the bend of the U-tube is a long horizontal chamber providing space for the fitment of various cavitation means. In the downflow arm 15 of the 'U' there is located an impeller 5 coupled to a shaft 6 passing through a water-tight seal 7 in the wall of the tube to a drive motor 8 arranged to rotate the impeller at a high speed, resulting in cavitation and the production of microbubbles within the water, followed by positive pressures which cause the bubbles to collapse, the succession of high pressure and low pressure shock conditions resulting in the formation and subsequent collapse of the microbubbles being effective to destroy living organisms. The effect can be enhanced by the introduction into the water, for example through one or more bored passageways (not shown) in the shaft 6, of a gas or gases, for example one or more of air, O 2 , O 3 C1 2 , and H 2 and O 2 mixtures. Such gases, which are oxidative and damaging to the target organisms in the conditions achieved, may be introduced through a very small orifice or orifices, and may form many more bubbles to which the organisms may attach themselves. Expansion and collapse of these bubbles improves the "kill rate". The impeller may be disposed within a surrounding housing and arranged so that it not only produces alternate low and high pressure conditions, but also causes water to circulate through the impeller a number of times during its passage through the plant. One such method of achieving this recirculation of water in the cavitation region is illustrated by the parallel branch pipe 17 shown in broken lines. This is open to the main tube 15 at its ends. If the impeller 5 drives water in the direction indicated, the upper end of the pipe 17 is arranged opposite supplementary cavitation means 9 so that there is partial recirculation of water descending the pipe 15. Cavitation means, as indicated at 9, may be provided downstream of the impeller to give more positive pressure pulses to collapse the microbubbles; this may comprise a source of sound or ultrasound as shown, or an explosive device in which explosive mixtures of gases are fed in small quantities into an inverted bell and ignited by spark periodically. The cavitation impeller 5 may be replaced or supplemented by other cavitation means providing rarefaction/compression, e.g., a ventuπ tube through which the main water, or a branch section of the flow, passes. This venturi flow may rely on the basic flow pressure or may be aided by, for example, the impeller 5. Again, the impeller 5 may be replaced by an orifice or jet, for example as shown in Figure 2. It will be appreciated that the drawing is not to scale and that the cavitation region will be subject to the sub-atmospheric pressure that obtains at its particular location in the downflow arm 15. Clearly therefore the height of the apparatus can be kept to a minimum by positioning the cavitation means near to the top of the arm. It may also be noted that the cavitation means are located in the downflow arm so that the increase of static pressure downstream of the cavitation means assists in the collapse of the bubbles. For similar reasons it is preferable that the bubble-collapsing sonic device 9 is located downstream of the impeller 5. The reduction in pressure with which the invention is concerned is determined partially by the vacuum pump 3 in that the vacuum pressure controls the water height. A number of devices such as that shown in Figure 1 may be connected in series to improve the kill rate, or in parallel to increase the throughput. The apparatus illustrated in Figure 2(a) is primarily a closed loop version of the system of Figure 1 , for treating a "batch" of water. A reservoir 19 of waste water has an outlet in the form of a venturi device 21 which in turn opens in to a chamber 23. This chamber has a return pipe 29 to the reservoir 19, the pipe including a circulating pump 25 and a control valve 27. An outlet 31 on the chamber 23 enables the treated water to be removed, and a vacuum-tight cap 33 on the reservoir 19 permits re-filling. Water in the reservoir 19 is sucked through the venturi 21 by the pump 25. The venturi device 21 provides the required cavitating action of rarefaction and compression by means of the rapid succession of high and low pressure conditions. A vacuum pump 3 produces a pressure reduction on the surface 11 of the water in the reservoir 19 and facilitates the generation of microbubbles by the venturi device. The closed loop system of Figure 2(a) can, if required, be operated as a continuous system by breaking the return path and providing connections to flow and return waste water paths as shown by the broken line modification. The reservoir is in this case completely filled. An alternative to the venturi device 21 of Figure 2(a) is a jet device as shown diagrammatically in Figure 2(b). This provides sudden changes of pressure conditions and consequent cavitation in the turbulent adhres at the jet boundary.

Claims: CLAIMS 1. A method of treating water by cavitation in which a series of alternate low pressure and high pressure conditions are produced in the water to give rise to the formation and subsequent collapse of bubbles, and in which a relatively steady pressure reduction is produced throughout the region of the bubble formation and collapse to promote bubble formation. 2. A method according to Claim 1 , wherein said pressure reduction lies in the range 400 to 700 millimetres of mercury below atmospheric pressure. 3. A method according to Claim 2, wherein said pressure reduction is approximately 500 millimetres below atmospheric pressure. 4. A method according to any of Claims 1 to 3, wherein said water is passed through a duct having an upflow inlet portion, a downflow exit portion and an intermediate chamber to produce a siphon effect. 5. A method according to Claim 4, wherein water is induced to flow up the upflow portion into the intermediate chamber and down the downflow portion by the application of a vacuum source to said duct. 6. A method according to Claim 5, wherein said vacuum source is connected to the intermediate chamber. 7. A method according to Claim 4, wherein a pump applied to the inlet of said duct is controlled to drive water up the upflow portion through the intermediate chamber and into the downflow portion and then controlled to produce said pressure reduction in the intermediate chamber. 8. A method according to any of Claims 4 to 7, wherein said duct is in the form of an inverted 'U', the intermediate chamber at the bend of the 'U' having access means for evacuating the bend region. 9. A method according to Claim 6 or Claim 7, wherein the water is subjected to negative and positive pressure shock conditions during its passage through the downflow portion of said duct. 10. A method according to any preceding claim, wherein the water passes through the treatment apparatus continuously. 11. A method according to any of Claims 1 to 3, wherein the water circulates through the treatment apparatus in a closed loop. 12. Apparatus for treating water, comprising a duct for the passage of water, cavitation means operable to impart low pressure and high pressure shock conditions to the water to give rise to the formation and subsequent collapse of bubbles, and means for producing a relatively steady reduction of pressure of the water to promote bubble formation. 13. Apparatus according to Claim 12, comprising a duct having an upflow inlet portion, a downflow outlet portion and an intermediate chamber, means for inducing water to rise in the upflow portion to such a level that it overflows through the intermediate chamber and into the downflow portion so producing said pressure reduction at the upper water levels. 14. Apparatus according to Claim 13, wherein said means for inducing water to rise into said intermediate chamber comprises a pump applied to the inlet of the duct and means for evacuating the intermediate chamber. 15. Apparatus according to Claim 13, wherein said means for inducing water to rise into said intermediate chamber comprises a pump having a controllable pressure head for establishing a siphon condition and subsequently reducing the pump head to establish said pressure reduction. 16. Apparatus according to any of Claims 13 to 15, wherein said cavitation means are located in the upper part of said downflow portion. 17. Apparatus according to any of Claims 12 to 16, wherein said cavitation means comprises a rotatable impeller, means for introducing a liquid at high velocity into the water, an ultrasonic or sonic transducer, means for producing a series of underwater explosions, a venturi device a jet or orifice providing rapid pressure changes, or a combination of two or more of these means. 18. Apparatus according to any of Claims 12 to 16, wherein said cavitation means includes an impeller directed across the water flow path confronting the opening of a branch duct to cause water to recirculate through said region of bubble formation and collapse. 19. Apparatus according to any of Claims 12 to 18, adapted to run continuously with an untreated water input and treated water output. 20. Apparatus according to Claim 12, and including a return path for the water to provide a closed loop system, the apparatus including a reservoir having an outlet through cavitation means, and means for filling and emptying the reservoir after treatment of the water contained in it. 21. Apparatus according to Claim 20, wherein said cavitation means comprises a venturi device or a jet. 22. A water treatment system including two or more apparatuses, each according to any of Claims 12 to 19, connected in series or parallel. 23. Apparatus or a system according to any of Claims 12 to 22 for treating water by the destruction of harmful living organisms. 24. A method of treating water substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings. 25. Apparatus for treating water substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

Inventors: REECE MICHAEL PETER, WHEELER DAVID JOHN, REECE, MICHAEL, PETER, WHEELER, DAVID, JOHN
Applicants: GEC ALSTHOM LTD, REECE MICHAEL PETER, WHEELER DAVID JOHN, GEC ALSTHOM LIMITED, REECE, MICHAEL, PETER, WHEELER, DAVID, JOHN
Application year: 1993
Main classification: C02F1
IPC codes: C02F1