WO1999050185A1

WO1999050185A1 - Method and apparatus for sterilising liquids

Info

Publication number: WO1999050185A1
Application number: PCT/AU1999/000227
Authority: WO
Inventors: Donald Edward Tame; Paul Maitland Storer
Original assignee: Greathall Overseas Ltd.
Priority date: 1998-03-30
Filing date: 1999-03-30
Publication date: 1999-10-07
Also published as: AUPP263898A0

Abstract

Contaminants in an aqueous medium are killed or neutralised by application of a pulsed voltage to electrodes (11) in contact with the aqueous medium. The applied voltage has a rectangular waveform consisting of periodic DC pulses. The width of the pulses is automatically varied to achieve a desired average current between the electrodes. At regular intervals, the polarity of the DC pulses is reversed to minimise erosion and deposition of the electrodes. The pulsed voltage can be considered as having a DC component and AC components. It is believed that contaminant molecules are charged by the DC component to effectively form dipoles, which are agitated by the sharp transitions in the applied voltage, leading to cell disruption in the contaminant microorganisms. Electrode coagulation of metallic ions in the aqueous media has also been observed.

Description

"METHOD .AND APPARATUS FOR STERILISING LIQUIDS"

THIS INVENTION relates to sterilisation of, or reduction of contaminants in, aqueous systems. In particular, the invention is directed to a method and apparatus for killing or increasing morbidity, or reducing viability or growth rate, of microorganisms in aqueous media.

Throughout the specification, where the context permits, the term "aqueous media" is intended to include not only water-based liquids but also sludges, gels, colloidal suspensions and other compositions having a substantial water component. The term "microorganisms" includes bacteria, viruses, algae, fungi and parasites found in biological systems.

The disclosure of Australian patent application no. PP2638, from which priority is claimed, is incorporated herein by reference .

This invention has particular but not exclusive application to sterilising or disinfecting sewage and general waste waters, bore water and waste water contaminated by microorganisms, contaminated water in airconditioning cooling towers, reservoirs, foundry water, medical liquid waste, parasite contaminated waters; algae elimination in water storage areas, ponds, aquaculture; sterilising water for drinking, incorporation in foods or beverages or for cleaning; treatment of swimming pool and spa water; reclaiming water for recycling; and for illustrative purposes, reference will be made to such applications. However, it is to be understood that this invention could be used in other applications, such as for disinfecting or depopulating waters containing higher living organisms, such as guardia, Cryptosporidium, Staphylococcus aureus, Legionnaire's bacteria, Cholera, Typhoid viruses, flocculation of suspended solids in a viscous matter and cleaning of oil-rig tanks and other tanks by electro-oxidation. BACKGROUND ART Methods of treating waste water prior to discharge into the environment, or drinking water for consumption, usually include chlorination. Chlorination may be effected by adding a hypochlorite solution, electrolysis of a chloride salt solution, or by dissolving chlorine gas directly into the water. Ammonia may be used in conjunction with chlorine to produce chloramine which has a longer residual than chlorine, but requires a higher concentration to have the same effective kill rate.

Other sanitisation agents can also be used, such as formaldehyde and the like but these are expensive, difficult to handle, or may break down to produce nutrients for bacterial growth.

Although chlorination is commonly used, it suffers from problems of incomplete disinfection and production of organochlorides, and a declining residual. Where chlorine gas is used, there is a safety risk in handling the poisonous chlorine gas. Chlorination also affects the taste and smell of water detrimentally, and an appropriate dosing rate is often difficult to achieve.

As an alternative to the use of sanitising agents such as chlorine, cations of heavy metals which produce a toxic effect on living organisms have also been used.

The dosing of cations to an appropriate concentration has been a problem in prior art systems and accordingly such systems have not been successfully used on a large scale. Additionally, metal cations added to water have caused contamination due to ineffective removal of the cations.

It is also known to sterilise or treat waste water or sewerage by electrolysis. Methods of electrolytic treatment of waste water can be found in U.S. patents

3,625,884; 3,923,629; 4,963,268; 5,304,289; 5,326,446;

5,376,242 and 5,575,974.

In such methods of electrolytic treatment of waste water, electrodes are inserted in the water, and connected to a source of alternating current in the form of a sine wave or saw tooth wave. Although alternating current (AC) is considered to be effective in killing microorganisms, it is believed that the use of such waveforms does not achieve optimal results. Similarly, the use of direct current (DC) alone is considered to be unsatisfactory, and also results in electrolytic deposition. It is an object of the present invention to provide an improved method and apparatus for sterilising treatment of waste water and similar liquids.

SUMMARY OF THE INVENTION In one form, this invention provides apparatus for sterilising or otherwise treating aqueous media, the apparatus having at least a pair of spaced electrodes adapted to be placed in an aqueous medium in use, and means for applying a pulsed voltage between the electrodes .

In another form, the invention provides a method of sterilising or otherwise treating aqueous media, the method including the steps of placing at least a pair of spaced electrodes in electrical contact with an aqueous medium, and applying a pulsed voltage between the electrodes .

Preferably, the voltage is in the form of a rectangular waveform having spaced DC pulses, the waveform being offset from zero so that it has a DC component. The amplitude of the voltage pulses is typically between 0.5 volts and 100 volts, depending on the application.

The pulses may be of variable width in order to maintain a desired average current between the electrodes for any particular application. The width of each pulse may suitably be varied between 1% and 99% of the total period between pulses, but preferably the pulse width is around 5% of the period. The variation of the pulse width is referred to in this specification as pulse width modulation. Either leading edge modulation or trailing edge modulation or a combination of both, may be used. The pulse frequency may be varied to suit the particular liquid being treated or the particular purpose of the treatment. Typically, the frequency of the pulses is between 100 per second and 20000 per second, but preferably around 1000 per second. To avoid undue erosion or coating of the electrodes, the polarity of the voltage applied between the electrodes is reversed periodically. Typically, the polarity is reversed at intervals ranging between 1 millisecond and 15 minutes. The electrodes may suitably comprise graphite rods encased in stainless steel mesh.

In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic drawing of a waste water treatment installation, Fig. 2 is an elevation of the electrodes used in the installation of Fig. 1,

Fig. 3 is a block diagram of the electrical circuit of the installation of Fig. 1,

Fig. 4 is a circuit diagram of part of the electrical circuit of Fig. 3, and

Fig. 5 illustrates the waveform of the voltage between the electrodes over time.

DESCRIPTION OF PREFERRED EMBODIMENT Fig. 1 depicts apparatus 10 for treating aqueous media, which in this example is waste water. The apparatus includes a feed stock vessel 15, a treated effluent vessel

16, a conduit 18 extending therebetween, and a pump 17 for pumping feed stock 22 from vessel 15 through the conduit 18. .An electrode assembly 11 is provided in the conduit 18. A recycle conduit 19 extends from the supply conduit 18 downstream from the electrode assembly 11 to the feed stock vessel 15.

.An electrode controller 12 is electrically connected to a pair of electrodes in the electrode assembly 11 by respective conductors 13, 14. An ammeter 26 may suitably be provided on the controller 12 for indicating the average current flowing between the pair of energised electrodes.

A supply valve 23 is provided in the supply conduit 18 downstream of its junction with the recycling conduit 19. A recycle valve 24 is provided in the recycling conduit.

A conductivity sensor 25 may be provided in the supply conduit 28 downstream from the supply valve 23, for sensing the conductivity of the liquid passing therethrough. In use, the supply valve 23 is closed and the recycle valve 24 is opened, and the pump is activated to recycle feed stock from vessel 15 through conduits 18, 19. As the feed stock water is pumped between the electrodes of the electrode assembly 11, the molecules therein are charged or "energised" as a result of the current passing between the electrodes. After sufficient treatment of the waste water, the recycle valve 24 is closed and the supply valve 23 is opened to feed the treated water into vessel 16. As the treated water is pumped into vessel 16, it passes the conductivity sensor 25 which monitors the conductivity of the treated water.

The electrode assembly 11 is shown in more detail in Fig. 2. The electrode assembly includes substantially identical electrodes 27, 28, each comprising a stainless steel threaded rod 31 screwed into a threaded socket 32 of a graphite electrode 30, and secured in place with a locking nut 33. The lower ends of the graphite electrodes 31 are maintained at the desired, but adjustable, spacing by an insulating spacer 26 extending between threaded stainless steel studs 31A screwed into the respective graphite electrodes 30. The threaded rods 31 are each encased in a respective PVC plastic tube 34 extending between the associated locking nut 33 and a spacer nut 35 located under the top plate 29 of the electrode assembly 11. The threaded rods 31 extend through respective spaced apertures in top plate 29. The passage of the threaded rods 31 through the top plate 29 is sealed by 0-ring seals 40 sandwiched between bearing nut 36 and spacer nut 35.

The threaded rods 31 are connected to the electrode controller by respective conductors 13, 14 which are connected to the threaded rods 31 and secured in place by fixing nuts 38, as can be seen in Fig. 2.

The graphite electrodes 30 may each be suitably encased in stainless steel mesh 41 to prolong the life of the electrodes. In use, a voltage is applied across the electrodes by the electrode controller 12. Depending on the polarity of the voltage applied to the electrodes, one functions as an anode, while the other functions as a cathode . A block diagram of the electrode controller is illustrated in Fig. 3. The electrode controller 12 comprises an electronic control circuit, typically a microprocessor controller 41, connected to an input module 42 having a keypad and switch or dial settings which enable the user to set, for example, the desired current level between the electrodes, voltage amplitude, pulse frequency and other operating parameters. A display 43 connected to the microprocessor controller 41 displays selected and actual operating parameters. The display 43 may include the ammeter 26.

The microprocessor controller 41 generates a rectangular waveform comprising a series of periodic DC voltage pulses, and feeds this pulsed waveform to a switching circuit 45. The switching circuit amplifies the pulsed waveform and applies it to the conductors 13, 14 connected to respective electrodes 27, 28 in the electrode assembly 11. A current sensor 46 associated with the switching circuit 45 measures the average current passing between the electrodes, and feeds this value back to the microprocessor controller 41. The microprocessor controller 41, in turn, varies the duration or width of the voltage pulses in the rectangular waveform so that the resultant average current matches the current as set at the input module 42.

The microprocessor controller, switching circuit and current sensor are shown in more detail in Fig. 4. The controller 41 is typically a microprocessor having a pulsed output 44 in the shape of a rectangular waveform of 5 volt pulses with sharp transitions. The pulses may occur from 100 per second to 20000 per second depending upon the particular application, but typically around 1000 per second.

The microprocessor also provides two polarity control outputs, namely a forward control output 47 and a reverse control output 48. The two polarity control outputs are of opposite senses, i.e. when one is high, the other is low.

The forward control output 47 controls two switchable gates 50, 52, while the reverse control output 48 controls another pair of switchable gates 51, 53. The outputs of gates 50, 51 are connected to p-channel MOSFETs 54, 55 respectively, while the outputs of gates 52, 53 are connected to the inputs of n-channel MOSFETs 56, 57 respectively. The MOSFETs are connected in a bridge circuit to which the electrode conductors 13, 14 are connected. The bridge circuit is connected between a rail voltage V_cc and a resistor Rl connected to ground.

In use, when the forward control output is high and the reverse control output is low, gate 50 is closed, and gate 51 is open. Accordingly, MOSFET 54 is switched on, thereby raising electrode conductor 13 to approximately the rail voltage V_cc while MOSFET 55 is switched off. Similarly, gate 52 is closed and gate 53 is open. MOSFET 56 is switched on and off by the rectangular waveform at output 44 so that the voltage at the electrodes switches between approximately 0 volts and approximately V_cc. The current passing between the electrodes passes through resistor Rl .

At regular intervals, which may be from one millisecond to 15 minutes, the forward and reverse control outputs 47, 47 are reversed in order to reverse the polarity of the voltage/current at the electrodes. With the forward control output low and the reverse control output high, gates 51 and 53 are open, while gates 50, 52 are closed. MOSFET 55 is switched on, and MOSFET 57 is switched on and off by the pulsed output 44 from the microprocessor 41. A pulsed voltage now appears between the electrodes as described above, but in the opposite sense, i.e. between approximately 0 volts and approximately —V_cc. Current now flows through the electrodes in the opposite direction, i.e. from the voltage rail through MOSFET 55 and conductor 14, between the electrodes and back through conductor 13, MOSFET 57, resistor Rl and eventually to earth. An illustrative waveform of the voltage at the electrode terminals is shown in Fig. 5. The voltage is pulsed periodically at periods T, the pulses being of a duration t. At intervals T_p/ the polarity of the pulsed voltage at the terminals is reversed. Such reversal of polarity minimises erosion and deposition of the electrodes .

It is to be noted that the voltage/current waveform at the electrodes, when resolved into its DC and AC components by, say, a Fourier transform, comprises both a DC component and an AC component. The DC component (disregarding the polarity reversal) can be expressed as Vt/T. This DC component is shown in broken line in Fig. 5. The current passing through resistor Rl is sensed and averaged (e.g. by rectification or integration) to provide an indication of the average current passing through the electrodes. The voltage across resistor Rl (which is proportional to that current) is amplified, converted from analogue to digital, and fed back to the microprocessor 41 as a control input representing average current. The microprocessor 41 varies the width or duration "t" of the pulses so that the measured average current matches the current set by the user via input module 42. Thus, the controller automatically varies or modulates the width of the pulses of the pulsed voltage/current at the electrodes to maintain the average current at a desired level despite changes in the properties of the liquid being treated. The ratio t/T known as the duty cycle, may be varied between 1% and 99%, but typically is maintained around 5% in steady state conditions .

The display 43 can be used to display various information including output current, whether the control outputs are active or not, and if an overcurrent or undercurrent situation exists.

Various parameters can be programmed into the microprocessor controller 41 via the input module 42, including the frequency of the pulsed waveform, the period of the polarity change signal, under current and over current trip limits, and the desired average output current . A serial EEPROM is provided in the input module 42 so that these parameters can be held in non-volatile storage and used later if there is loss of power to the apparatus .

Although the precise mechanism by which the microorganisms are destroyed is not known with certainty, it is believed that the combination of (i) the sudden changes in voltage and current at the electrodes due to the pulsed voltage, and (ii) the use of a pulse waveform having both DC and 10

AC components, results in more efficient sterilisation of the liquid being treated.

In particular, it is believed that the DC component causes the molecules of the contaminant organisms to become polarised, i.e. to act as miniature dipoles . The sudden changes in voltage at the electrodes at the leading and trailing edges of the pulses causes rapid electromechanical agitation of the molecules of the microorganisms at a cellular level. The molecular "dipoles" are rapidly oscillated by the AC component, i.e. the pulsing voltage, to cause heating and cell rupture. Higher harmonic frequencies arising from the pulsed electromagnetic field contribute to the destabilisation of the cell structure, possibly as a result of resonant interaction.

Preliminary investigation of the operation of this invention suggests that the physical or electromechanical agitation of the microorganisms resulting from the application of a suddenly changing electromagnetic field to the cells or molecules of the microorganisms which have been polarised by a DC component, leads to destruction of the microorganisms.

The operation of this invention is different from other electrochemical systems in that it does not rely for its effective operation on electrolysis or electrochemical action at the electrodes or any changes in surface tension or other properties of non-cellular water. Rather, it appears that the process of this invention involves direct electromechanical agitation of the microorganisms .

It has also been found that when the invention is applied to treatment of industrial waste liquids, such as inorganic dyes and tanning wastes, there is an electrocoagulation of metal ions, causing precipitation of contaminants which may then be easily removed. That is, the process of this invention may cause coagulation of 11

complex ions . These metal ions may come from the electrodes themselves or may be contained in the industrial wastes being treated. These metal ions coagulate and are neutralised. Once precipitated, they can be easily removed, e.g. by filtering.

It has been found that in some aqueous media, such as swimming pool water, the production of inorganic ions using the process of this invention can result in a residual effect that provides sustained morbidity of microorganisms. That is, even after the voltage is removed from the electrodes, the sterilising effect is maintained. This is believed to be due to the formation of complex ion groups in the water which have a continuing toxic effect on microorganisms . The use of different electrode materials, or mixtures of electrode materials, such as copper/silver, brass, titanium/ceramic and graphite, can give rise to different overpotentials, as well as a range of redox potentials. These can result in the production of highly reactive nascent gases such as hydrogen, oxygen (and, if present) chlorine) , hydroxide ions and inorganic ions which contribute to the sustained and/or increased morbidity of microorganisms.

The foregoing describes only one embodiment of the invention, and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention. For example, although the illustrated embodiment uses graphite electrodes, any other suitable material may be used for the electrodes. Similarly, the switching circuit of Fig. 4 may be replaced by any other suitable circuit for providing the pulsed output at the electrodes .

Claims

12CLAIMS :

1. A method of sterilsing or otherwise treating aqueous media, including the steps of placing at least a pair of spaced electrodes in electrical contact with an aqueous medium, and applying a pulsed voltage between the electrodes, the voltage pulses having sharp transitions.

2. A method as claimed in claim 1, wherein the voltage is in the form of a rectangular waveform having spaced DC pulses.

3. A method as claimed in claim 1, wherein the polarity of the voltage applied between the electrodes is reversed periodically.

4. A method as claimed in claim 1, wherein the width of the pulses is varied automatically to maintain a desired average current between the electrodes .

5. A method as claimed in claim 2, wherein the amplitude of the voltage pulses is between 0.5 volts and 100 volts.

6. A method as claimed in claim 2, wherein the frequency of the pulses is between 100 per second and 20000 per second.

7. Apparatus for sterilising or otherwise treating aqueous media, the apparatus having at least a pair of spaced electrodes to be placed in an aqueous medium in use, and means for applying a pulsed voltage between the electrodes, the voltage pulses having sharp transitions.

8. Apparatus as claimed in claim 7, wherein the voltage is in the form of a rectangular waveform having spaced DC pulses.

9. Apparatus as claimed in claim 8, wherein the amplitude of the voltage pulses is between 0.5 volts and 100 volts.

10. Apparatus as claimed in claim 7, further comprising sensing means for sensing the average current passing between the electrodes, and pulse width modulation means for automatically varying the width of the pulses so 13

that the resultant average current approximates a preset value .

11. Apparatus as claimed in claim 7, further comprising control means for varying the frequency of the voltage pulses.

12. Apparatus as claimed in claim 7, further comprising polarity reversal means for periodically reversing the polarity of the voltage pulses.

13. Apparatus as claimed in claim 7, wherein the electrodes each comprise a graphite core encased in stainless steel mesh.

14. A method of sterilising aqueous media, including the steps of placing at least a pair of electrodes in electrical contact with an aqueous medium, applying a pulsed voltage to the electrodes comprising period pulses of DC voltage, sensing the average current passing between the electrodes, and varying the duration of the pulses so that the sensed average current approximates a preset value .