APPARATUS FOR THE DISINFECTION OF LIQUIDS

Abstract: Microbes can be destroyed or inactivated by subjecting them to a high potential gradient. This fact does not lead to a practical process for disinfection unless the apparatus for its implementation incorporates certain features and, in addition thereto, the electrical, mechanical and chemical parameters of the process are appropriate to the particular desired disinfection. This application discloses practical apparatus which accomplishes these objectives and constitutes an innovation.

Description: APPARATUS FOR THE DISINFECTION OF LIQUIDS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the destruction or inactivation of microbes by subjecting them to a high potential gradient. 2. Description of the Related Art As shown by publications in the open technical literature (see Technical Bibliography, below) it has been known for at least fifty years that microbes can be destroyed or deactivated by high potential gradients. In the earliest publications (1949-1965) practical application of this phenomenon for the purpose of disinfecting liquids - and liquid foods in particular - was emphasized. Somewhat later (1970-1985) , carefully controlled application of high potential gradients to the manipulation of biological cells was studied and reported. Publications described, inter alia, the use of high potential gradients to render the membranes of biological cells permeable, to organize a number of cells into a group and to accomplish the fusion of two or more cells. Practical devices to accomplish these things were developed and offered for sale as commercial products for use in biological laboratories. The microbiological studies associated with this work provided valuable fundamental information on the effect of high potential gradients on cells. Among other things, it became evident that high potential gradients produce porosity and, in some cases, permanent damage to cell membranes by exerting electromechanical force (electrophoretic force) on the membrane. In other words, the observed effects were due to these forces and not due to electric currents or ohmic heating. While some microbes - such as spores, in particular, - can be found in gaseous media (in the air, for example) or in solid materials or on the surfaces of solid materials, the vital processes of most microbes require a liquid medium - usually an aqueous medium. Such liquid media are usually weak electrolytes (water being an example) and consequently these media have comparatively high electrical conductivity. Conductivity of 0.05 Siemens (S) per meter is a typical value; but values differing from this by a factor of IO 6 would still be classified as "semi-conducting", or "non-insulating" . Consequently, it was recognized from the earliest work (1949- 1960) that high potential gradients could be applied successfully to media containing microbes only under certain special conditions. A potential gradient of IO 6 volts/meter is of the order of the lowest gradient which will have a permanent effect on a microbe. Such a gradient, applied as a steady (DC) gradient to a medium having a conductivity of 0.05 S/m, would result in a current density of 5 x IO 4 amp/m 2 , with consequent power dissipation of 50,000 Megawatts/m 3 ; and the temperature of the medium would rise at an initial rate of roughly 12,000 °C/sec. Of course, these considerations were well-known to all who have worked on this subject. Some of the earliest work applied high-frequency AC voltages (Burton - 1949; Doevenspeck - 1961) but by 1965, pulsed voltages had become accepted as the preferred means of creating high potential gradients in the various media which were studied. (E.g., Hamilton & Sale - 1967; Sale & Hamilton - 1967,1968) . Pulse lengths in the range .1 microsec to several milliseconds were employed. It was established that the effect of potential gradients on cells - as measured by the induced porosity of the membrane or by the lethal effect on the cells - increased rapidly as the magnitude of the gradient increased. Sale and Hamilton (1968) presented a formula for the potential difference across a microbe in an electric field which has been widely used ever since. They assumed a spherical cell of radius a 0 in an electrolytic medium (specific resistance p and dielectric constant K ) . The cell membrane is assumed to have a very high resistance - high enough that it can be approximated as a perfectly-insulating membrane - while the interior of the cell is assumed to be conducting; that is, its specific resistance is less than p . A uniform potential gradient E 0 is impressed on the medium containing the cell. Although the cited paper does not give the derivation, it can be shown that the maximum potential difference occurs between the poles of the spherical cell, in the direction of E 0 . This potential difference, V,- ^ is Presumably, this potential would be divided equally between the membranes at either pole. If the membrane has a thickness T, then the potential gradient E p at the poles (where this gradient has its maximum value) would be E p = (3/2) (a 0 /τ) E 0 Thus, the gradient in the membrane increases with an increase in the overall size (represented by the radius a 0 ) of the cell and decreases with an increase in the membrane thickness. Below a certain critical gradient - which depends upon the type of cell and is of the order of 10 kV/cm - the porosity induced in the cell membrane is reversible. That is, when the gradient is removed, the membrane regenerates its properties. Whereas, for values of gradient above the critical value, there is an increasing probability that the cell will be destroyed. There is also evidence that - for a given value of potential gradient - the effect increaseε with increasing time of application. Various systems for applying high potential gradients to a medium containing microbes are disclosed in U.S. Patent No. 5,048,404 to Bushnell et al. and in U.S. Patent No. 5,235,905 to Bushnell et al. Apparatus for inactivation of viruses using pulsed high electric field is disclosed in an article by Mizuno et al. entitled "Inactivation of Viruses using Pulsed High Electric Field" at Conference Record, Annual Meeting, IEEE Industry Applications Society, page 674, 1990. Technical Bibliography 1949 H. Burton, National Institute for Research in Dairying, Paper #1041, Reading, England 1961 Doevenspeck, Fleischwirstschaft 13., 986 1967 A.J.H. Sale & W.A. Hamilton, Biochimica & Biophysica Acta 148, 781 1967 W.A. Hamilton & A.J.H. Sale, Biochimica & Biophysica 1968 A.J.H. Sale & W.A. Hamilton, Biochimica & Biophysica Acta 163. 37 1971 Roland Benz & K. Janko, Biochimica & Biophysics Acta 455. 721 1973 J.M. Crowley, Biophysics Journal 13 . , 711 1974 Ulrich Zimmermann, Gunther Pilwat -i F. Riemann, Biophysics Journal 2A, 881 1974 S.H. White, Biophysics Journal 14, 155 1974 Ulrich Zimmermann, Gunther Pilwat & F.Riemann, Dielectric Breakdown in Cell Membranes , in: Membrane Transport in Plants, p. 146, Springer, Berlin 1975 H.G.L. Coster & Ulrich Zimmermann, Biochimica & Biophysica Acta 382. 410 1975 F. Riemann, Ulrich Zimmermann & Gunther Pilwat, Biochimica & Biophysica Acta 394. 449 1975 Gunther Pilwat, Ulrich Zimmermann & F. Riemann, Biochimica & Biophysica Acta 406. 424 1975 J. Requena & D.A. Haydon, Biophysics Journal 15., 77 1976 Ulrich Zimmermann, Gunther Pilwat, G. Beckers & F. Riemann, Bioelectrochemistry & Bioenergetics 3., 58 1976 Roland Benz & P. Lauger, Journal of Membrane Biology 22, 171 1977 Ulrich Zimmermann, F. Beckers & H.G.L. Coster, Biochimica & Biophysica Acta 464. 399 1978 G. Boheim & Roland Benz, Biochimica & Biophysica Acta 507. 262 1978 J. Vienken, E. Jeltsch & Ulrich Zimmermann, Cytobiology 17 . , 182 1979 Roland Benz & Ulrich Zimmermann, Journal of Membrane Bilogy, 48., 181 1980 Roland Benz & Ulrich Zimmermann, Biochimica & Biophysica Acta 597, 637 1980 Ulrich Zimmermann, J. Vienken & Gunther Pilwat, Bioelectrochemistry & Bioenergetics 1_, 553 1980 H. Hulsheger & Eberhard Neumann, Radiation & Environmental Biophysics 2 ., 281 1980 Ulrich Zimmermann, Gunther Pilwat, A Pequeux & R. Giles, Journal of Membrane Biology 5_4, 103 1981 Ulrich Zimmermann, Peter Scheurich, Gunther Pilwat & Roland Benz, Angewandte Chemie £3., 332 1983 H.Hulsheger, J Potel S- Eberhard Neumann, Radiation & Environmental Biophysics 20., 53 1986 Akihira Mizuno & Yuji Hori, IEEE Trans , on Indust . Applicationε 24, 387 1989 Eberhard Niemann, A.E. Sowers & CA. Jordan, Electroporation and Electrofusion in Cell Biology, Plenum Press, N.Y. 1990 Akihira Mizuno, et al. , Conference Record, Ann. Mtg. Industrial Applications Soc, IEEE, p. 713 1991 S. Jayaram, G.S.P. Castle & A. Margaritis, Proc. Annual Mtg. Industry Applications Soc . , IEEE, p. 674 1991 Yoichi Matsumoto, Norio Shioji, Tokuki Satake & Akihiro Sakuma, Ibid., p. 652 1991 J. Wilschut & D. Hoekstra, Membrane Fusion, Marcel Dekker N.Y. SUMMARY OF THE INVENTION The fact that microbes can be destroyed or inactivated by subjecting them to a high potential gradient does not lead to a practical process for disinfection unless the apparatus for its implementation incorporates certain features and, in addition thereto, the electrical, mechanical and chemical parameters of the process are appropriate to the particular desired disinfection. The practical apparatus of the invention accomplishes these objectives and constitutes an innovation. The advantages of the invention include the following: (1) The lethal effect of electric fields on microbes increases very rapidly as the strength of the field (i.e., the magnitude of the potential gradient) increases. An example of this is shown in Figure 1. The subject invention makes it possible to apply very high potential gradients to the electrolytic liquids and partially-conducting liquids in which microbes are found. (2) A practical apparatus must be capable of running reliably for long periods of time without being removed from service for repairs and maintenance work. Typical causes of operating problems are weakening of electrical insulation, which results in electrical discharges, and occasional breakdowns; contamination of electrodes and other internal parts by accumulated debris, and the effect of electrolysis. The subject invention overcomes these reliability problems. (3) The pulsed, high-gradient disinfection technique is not practical if the apparatus and methods employed in its application result in high power consumption. A high power requirement in itself - together with the necessary cooling equipment - would increase the cost of the process and could ultimately make it less advantageous than other alternative processes. The invention herein described iε a pulsed, high- gradient apparatus of high efficiency. As used throughout this specification and claims, the term "liquid" meanε and includes any conformable substance which can be made to flow through the processing chamber. Thus, the term "liquid" includes, for example, thixatropic fluids and non- newtonian fluids. BRIEF DESCRIPTION OF THE DRAWINGS The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawings, in which: Fig. 1 is a graph showing percentage of bacteria surviving as a function of peak electric field. The solid line applies to fecal coliform bacteria and the broken line to total coliform bacteria counts. (For each experimental point ten pulses were applied.) Fig. 2 is an example of the application of an insulating barrier within the processing cell, in which a cylindrical barrier is placed concentrically between cylindrical electrodes. Fig. 3 is another example of the application of an insulating barrier within the processing cell, in which two parallel, insulating barriers are placed perpendicular to the liquid flow. In this embodiment, pairs of grids, of conducting material, could be placed on either side of the insulating barriers, to provide greater control over potential distribution within the processing chamber. In either Fig. 2 or Fig. 3, E, is a grounded electrode, E 2 is a high voltage electrode and V l7 V 2 and V 3 are fluid volumes at successive places in the flow. Thus, the portions of the boundary of the processing cell through which the liquid enters and leaves are at ground potential; this is an important feature of the invention. Fig. 4 is a graph showing breakdown potential gradient E M in water as a function of the length of the applied voltage pulse. In these experiments a sharp-edged electrode faces a plane counter-electrode. The sharp-edged electrode is negative in the case of the upper curve, and it is positive in the case of the lower curve. (From J. Pace VanDevender and T.H. Martin, Untriαcrered Water Switching. Sandia Laboratories Report, 1968.) Fig. 5 is an example of a possible arrangement of processing chamber and high-voltage supply which provides low inductance in the voltage supply circuit. In this example, the processing chamber is the inner element in a concentric- cylindrical aεεembly. Fig. 6 iε a view, εimilar to that of Fig. 3, and showing the structure in greater detail. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, and first to Figs. 2, 3 and 6 thereof, therein are shown two embodiments of apparatus in accordance with thiε invention. The featureε of the invention may be εummarized aε follows: 1. Use of an insulating barrier within the interelectrode gap. 2. Pulse length. 3. Control of the pulse length and pulεe εhape by varying the physical and geometrical parameters of the proceεsing chamber. 4. A pulεe conεiεting of a εucceεεion of very narrow pulεeε. 5. Pulses alternating in polarity. 6. Low inductance structure in the assembly of voltage source and processing chamber. 7. A variety of electrode geometrieε in the proceεεing chamber to meet the requirementε of a variety of different microbeε in the liquid. 8. Meanε for excluding gaε voidε from the processing chamber. 9. Operation of the processing chamber at elevated pressure. Referring now to each of these features in sequence, 1. Use of an insulating barrier within the interelectrode gap: A solid dielectric is placed within the chamber wherein the flowing liquid is exposed to the pulsed potential gradient. The dielectric, which is pierced with a number of narrow passageways, is so placed within the chamber that it constituteε a barrier to the flow of the liquid. That is, any liquid passing through the chamber must pasε through the paεεagewayε in the dielectric. Two examples of this use of the dielectric barrier are shown in Figureε 2 and 3. The path of the liquid being processed is therefore aε follows: Through an inlet pipe to a volume V j , through the small pasεagewayε in the dielectric barrier into volume V 2 , and εo on, until it reacheε the outlet pipe. The passageways have a length equal to or greater than the thicknesε of the dielectric barrier. Theεe paεεageways are preferably slotε, but may have other geometrieε, εuch aε that of pin-holeε. The transverse section of the slots may have any shape, as long aε each paεεageway haε a minor transverse dimension small compared to the thicknesε of the barrier. The electrical conductivity of the liquid medium, containing the microbeε, is very much higher (for example, one million-fold higher) than the conductivity of the dielectric barrier; and the dielectric constant of the liquid is much higher (typically at least 20 times higher) than the dielectric constant of the barrier. Therefore, the gradient which resultε from a potential difference between E t and E 2 is much higher in the liquid in the passsageways than it is in the liquid in the volumeε V t , V 2 and V 3 . The velocity of the fluid εhould be maintained conεtant in the paεεageway . The advantages of this arrangement are: (a) There is a high degree of control over the liquid flow with reεpect to the regionε of the processing chamber which experience the highest potential gradient. In other wordε, no liquid can pass through the chamber without experiencing the highest potential gradient. (b) The gradient at the electrodes is quite low; and this has at least two advantages: First, this iε a situation which is highly favorable to the electrical performance of the apparatus, since electrical breakdown must involve the electrodes, and theεe are in thiε case exposed to reduced potential gradients. Second, this meanε that the current density at the electrodes will be comparatively low and possible accumulation of electrolytic products at the electrode surfaces will be greatly reduced. This idea can be incorporated into a large variety of electrode geometries, of which Figures 2 and 3 show two examples. 2. Pulse length: The maximum gradient which can be supported in a liquid depends on the duration of the gradient, in time. With the maximum, continuous (DC) gradient aε a basis, it is found that reducing the time of application permits a higher gradient to be εustained. This increaεe in gradient with reduction in the time of application is gradual until the time of application becomes very short (of the order of a microsecond) . But then for times leεε than one microεecond the insulating strength begins to rise very rapidly (see Figure 4) . Since the disinfecting effect increaεeε with increaεing gradient, the proceεε becomes more effective when very short pulses and the highest possible gradients are employed. This is particularly important when more resistant microbes are to be treated. 3. Control of the pulse length and pulse shape by varying the physical and geometrical parameters of the processing chamber: The arrangement described in (1) ("Use of an insulating barrier within the interelectrode gap") provideε a meanε for controlling the electrical parameterε - reεistance and capacitance - of the processing cell. This could be accompliεhed by providing a meanε for closing and opening - or partly closing and partly opening - the pasεagewayε through the dielectric barrier. The pulse shape and pulse length depend - amongst other things - on the resistance and capacitance of the electrical load preεented to the source of pulsed voltage. So in this way the pulse characteristicε can be modified, aε required, without the use of external high-voltage circuit elements connected in parallel with the processing cell, or by modifications to the pulse power supply. Such modifications in pulεe shape and pulse length contribute to attaining optimum procesεing conditions. 4. A pulse consisting of a succession of very narrow pulses: One individual pulse can be formed of a succession (a "train") of very narrow pulses. The train of very narrow pulseε may be of decreaεing amplitude,- and succesεive pulεeε may alternate in polarity, aε might be the caεe - for example - in a damped, sinuεoidal oεcillation. The advantage of thiε method of operation iε that it permits - because of the very narrow individual component pulses - the attainment of a higher potential gradient in the processing chamber. 5. Pulses alternating in polarity: A succession of pulses which alternate in polarity should have the εame anti-microbial effectiveness as a succeεsion of pulses, all of the same polarity. However, pulses alternating in polarity would have various advantages, including: (1) Electrolytic effects, in the liquid being processed, would be eliminated or greatly reduced. (2) the circuitry required to generate sharp pulses from an AC input -- of the desired frequency -- would be simpler than that required for conventional pulεe-generating circuitε. (The conventional circuitε produce mono-polar pulses from a DC input.) 6. Low inductance structure in the assembly of voltage source and processing chamber: Very short, high-voltage pulseε imply a high dl/dt (rate-of-change of current with reεpect to time) . Thiε, in turn, puts an upper limit on the permissible inductance in the circuit which connectε the voltage source to the processing chamber. Certain arrangements - of which coaxial cylindrical arrangements are an example - permit low inductance connections. An example of a low-inductance asεembly of the voltage εource and the processing chamber is shown in Figure 5. 7. A variety of electrode geometries in the processing chamber to meet the requirements of a variety of dif erent microbes in the liquid: Process requirements may differ widely for different microbes. For example, a liquid may be contaminated with cystε which require very high potential gradientε for disinfection, but disinfection requirements for these cysts may be met by a surviving fraction of IO" 3 . Meanwhile, the same liquid may contain a virus, which iε inactivated by a significantly lower value of gradient, but disinfection requirements may be for a surviving fraction of only IO" 6 . The very small surviving fraction is achievable only if every bit of the liquid experiences the high gradient. This, in turn, means a uniform-field geometry - such as concentric cylinders, or an arrangement such as that described in (1) hereinabove ("Use of an insulating barrier within the interelectrode gap") . On the other hand, the very high gradients necessary to deal with the cysts are more easily achieved in a non-uniform geometry, εuch aε would be provided by a small-diameter rod surrounded by a concentric-cylindrical opposing electrode or a rod or cylinder εurrounded by one or more circular diεkε, or by any one of - or a combination of - the many configuration which yield non-uniform fields. These electrode arrangements do not subject every bit of the flowing liquid to the same high potential gradient. However, this diεadvantage can be overcome to a considerable extent by providing several εuccessive non¬ uniform field gapε, and also by providing - up-stream from the high-voltage gaps - devices which introduce turbulence into the liquid flow. Examples of such devices to excite turbulence are: (a) A sharp lip or edge protruding into the flow, (b) A group of vanes protruding into the flow, (c) A sudden increaεe in croεs-sectional area of the flow. The combination, within the proceεεing chamber, of various electrode geometries providing both uniform and nonuniform fieldε makeε it possible to deal with a variety of disinfection requirements. 8. Means for excluding gas voids from the processing chamber: Included gas in the liquid can lead to corona diεchargeε and breakdown in the liquid. Thiε will reεult in unreliable operation and - poεεibly - in undeεirable changeε in the liquid under treatment. In addition, the dielectric barrier and the aεεociated electrodeε may be εhaped to minimize the retention of bubbles at any point in the flowing liquid stream. The posεibility of included air in the high-gradient regions is reduced if the procesεing cell iε oriented εo that it "fills from the bottom". That is, the inlet occurs at the loweεt point in the processing chamber and the outlet is at the highest point. 9. Operation of the processing chamber at elevated pressure: Operation of the procesεing chamber at an elevated preεεure may be uεeful under certain conditions: Except at very low Reynolds' Number, there may be some degree of turbulence in a flowing liquid. For example, around bendε, at diεcontinuitieε, and so forth. If disεolved gases are present in the liquid, they may form bubbles, under these circumstances, and this could lead to electrical discharges. Operation of the system under a positive pressure will reduce the formation of bubbles under these and other conditions, and lead to a more reliable process. Furthermore, in some cases the microbicidal action of the process may be enhanced by an elevated presεure during processing. Having thus described the principles of the invention, together with several illustrative embodiments thereof, it is to be understood that, although specific terms are employed, they are used in a generic and descriptive sense, and not for purposes of limitation, the scope of the invention being set forth in the following claimε.

Claims: CLAIMS 1. An apparatus for the deεtruction of microbeε, or the deactivation of microbeε, preεent in a liquid, compriεing in combination: a processing chamber containing electrodes spaced apart by a certain distance, at least one solid insulating barrier dividing the space within said chamber into at least two volumes, said barrier having a number of passageways such that any passageway has a minor transverεe dimension which is small compared to the thickness of the barrier, the construction of the foregoing being such that no fluid can pass through the apparatus without flowing through said small passageways, a voltage source adapted to provide a pulse of voltage such that a gradient of at least 30 kV/cm is produced in at least some portion of the space between said electrodes. 2. Apparatus according to claim 1, in which the geometry of the insulating barrier and/or of the pasεageways through the insulating barrier can be changed so as to alter the electrical characteristicε of the processing cell. 3. Apparatus according to claim 1, wherein each individual voltage pulse has the form of a "train" or succession of very narrow pulses. 4. Apparatus according to claim 3, wherein said succeεεive narrow pulεeε diminish in amplitude. 5. Apparatus according to claim 3, wherein said εucceεεive narrow pulεes are all of the same polarity. 6. Apparatus according to claim 3, wherein succesεive pulεeε of εaid εucceεsive narrow pulses alternate in polarity. 7. Apparatuε according to claim 3, wherein εuccessive pulεeε have the form of a damped sinusoidal oscillation. 8. Apparatus according to claim 1, wherein the pulseε applied are a continuouε succesεion of pulses, uniformly εpaced in time, and identical in εhape but alternating in polarity. 9. Apparatus according to claim 8, wherein said continuous succesεion of alternating pulεeε iε generated from an AC input εignal having a frequency equal to the desired repetition frequency of the pulses. 10. Apparatuε according to claim 1, wherein the proceεεing chamber and the voltage source are connected in a circuit whose electrical inductance is lesε than 500 nanohenries. 11. Apparatus for the destruction of microbes, or the deactivation of microbes, present in a liquid, comprising in combination at leaεt one proceεεing chamber having electrodeε forming two or more interelectrode εpaceε, meanε for transporting liquid being procesεed εucceεεively through εaid spaces, means for subjecting said liquid to pulseε of high potential gradient cauεed by pulεeε of high voltage applied between the electrodeε, one of εaid interelectrode εpaceε providing a uniform (or approximately uniform) potential gradient to all of the interelectrode εpace through which the liquid flows, another of said interelectrode εpaceε providing a region of highly-nonuniform gradient (or succesεive regions of highly non-uniform gradient) , which therefore varieε within the gap (or gaps) from a value substantially higher than the average potential gradient to a value less than the average potential gradient. 12. Apparatuε according to claim ll, including meanε for inducing turbulence in said flowing liquid. 13. Apparatus according to claim 1 in which the processing chamber is oriented so that the inlet for the flowing liquid iε at the loweεt point in the chamber and the outlet for the flowing liquid is at the highest point in the chamber. 14. Apparatus according to claim 1 including means for controlling or suppressing the formation of bubbles in the liquid. 15. Apparatus according to claim 1, including means for maintaining elevated pressure during the processing of the liquid. 16. Apparatus according to claim 11 in which the processing chamber is oriented so that the inlet for the flowing liquid is at the lowest point in the chamber and the outlet for the flowing liquid is at the highest point in the chamber. 17. Apparatuε according to claim 11 including means for controlling or suppreεεing the formation of bubbles in the liquid. 18. Apparatus according to claim 11, including means for maintaining elevated presεure during the processing of the liquid. 19. Apparatus according to claim 1, wherein said pasεageways are slots. 20. Apparatus according to claim 1, wherein the electric field near said electrodes iε low compared to the average field. 21. Apparatuε according to claim l, wherein the portions of the boundary of said chamber through which said liquid enters and leaves are at ground potential. 22. Apparatus according to claim l, wherein at least one of said interelectrode spaces includes a conductive grid or other electrode structure transverεely arranged to render the electric field more uniform.

Inventors: MILDE HELMUT I, PHILP SANBORN F, MILDE, HELMUT, I., PHILP, SANBORN, F.
Applicants: MILDE HELMUT I, PHILP SANBORN F, MILDE, HELMUT, I., PHILP, SANBORN, F.
Application year: 1996
Main classification: B01D61
IPC codes: A61L2, C02F1