WO1998022813A1

WO1998022813A1 - pH ALTERING DEVICE AND METHOD

Info

Publication number: WO1998022813A1
Application number: PCT/GB1997/003195
Authority: WO
Inventors: Douglas William Meredith; Trevor Kim Gibbs; Nigel John Cade; John Anthony Barney
Original assignee: Enviros Monitors Limited
Priority date: 1996-11-21
Filing date: 1997-11-21
Publication date: 1998-05-28
Also published as: AU5061498A

Abstract

A pH altering device comprises a fluid receptacle, a microporous membrane disposed within the fluid receptacle and dividing fluid within the interior of the receptacle into first and second fluid chambers, and an electrode disposed within each fluid chamber. The microporous membrane restricts or substantially prevents the flow of the fluid from one fluid chamber to the other, but does not prevent the flow of any anions or cations necessary for a flow of an electric current between the electrodes. This makes it possible to produce two separate fluid streams of different pH from a single fluid. The device has applications to detecting impurities in fluids, producing cleaning fluids, and decontaminating fluids.

Description

pH Altering Device and Method

This invention relates to a device for, and a method of, altering the pH of a fluid. The invention also relates to a device for, and a method of, detecting the concentration of a material in a fluid. For the avoidance of doubt, in this specification a "relatively high pH" means a pH that is numerically greater than a "relatively low pH".

In recent years there has been an increasing demand to use sensors to monitor products in, for example, environmental, mining, agricultural, food, medical and process industries. New sensor technologies have been developed to meet this demand, but most require some form of reagent addition to optimise performance. For portable instruments, whose sensor probe is dipped into the sample stream such as a river, reagent addition is not a practical proposition. With in-line instruments, reagent addition can be a feature of the design, but the maintenance associated with the frequent failure of the reagent manipulation system and the high reagent usage, result in a high "cost of ownership". The addition of reagents for in-line instruments in, for example, the food industry, may not be permissible because of the contamination of the process stream. Consequently there is a genuine need for instruments requiring no, or minimal, reagent addition. An example of a material that often needs to be measured is ammonia, which dissolves and dissociates in water according to the following reaction:

NH₃ + H₂O - NH/ + OH^" (I)

At pH values around 7 the equilibrium is well over to the right.

However, at higher values of pH (eg pH around 11), the equilibrium moves over to the left. In order to detect the ammonium ion concentration, it is possible to use a detector for NH₄ ⁺ ions. However, such detectors can be unreliable, because they may also detect, for example, K⁺ and Na" ions. Another possibility is to generate ammonia gas, by adding a suitable reagent containing hydroxyl ions (eg sodium hydroxide) to the liquid. The ammonia gas can then be detected using an ammonia gas detector. Another material that it is often desirable to measure is chlorine, which dissolves and dissociates in water according to the following reactions:

Cl₂ + H₂0 ^ HOCI + HCl (II)

HOCL H⁺ + OCr (IIIA)

HCl H⁺ + Cr (IIIB)

The concentration of hypochlorite ion can be measured using a three- electrode polarographic system, but in order to do this the pH must be higher than 11.

In EP-B-0637381 there is disclosed an ammonia gas sensor which is housed within a container partially immersed in a solution containing ammonium ions. An electrochemical generator is provided to generate hydroxyl ions in a region of the solution adjacent the container. This converts ammonium ions to ammonia gas, which is sensed by the sensor after having diffused into a gas permeable membrane. This sensing provides an indication of the ammonium ions in the solution. This sensor is useful in a number of applications including testing for contamination in water.

The addition of hydroxyl ions is achieved by means of the electrochemical generator, which generates hydroxyl groups according to the following reaction:

2H₂O + 2e^' ^ 2OH^" + H₂ (IV)

WO-A-9625662 discloses a similar system. It also discloses apparatus for detecting chlorine levels, by generating hydrogen ions with an electrochemical generator. The addition of hydrogen ions can be achieved by means of the electrochemical generator, according to the following reaction: 2H₂O - 4e^" ** O₂ + 4H⁺ (V)

EP-B-0637381 and WO-A-9625662 disclose a method of detecting a material by increasing or decreasing the pH of the solution by electrochemical means. Thus, it is no longer necessary to add a reagent to the solution to increase or decrease the pH.

The detection methods described in EP-B-0637381 and WO-A- 9625662 provided an improvement over the earlier methods that involved the addition of reagents. However, there are problems with these methods. In particular, the liquid being analysed may be subjected to stirring, or, when there is a liquid flow, to flow variations caused by turbulence. This has the effect that the OH^" ions generated at the cathode can be neutralised by H⁺ ions generated at the anode. Another problem is that in certain liquids the different concentrations of ions cause solids to precipitate upon the electrodes. A further problem is that, in certain liquids, stagnation encourages bacterial growth, or biological contamination, of the sensor by the accumulation of biofilms.

Further techniques for detecting the concentrations of impurities in a liquid are disclosed in US-A-4822456, US-A-4900422, US-A-4961163, US-A- 5016201, US-A-5046028, US-A-5098547 and US-A-5162077. These specifications disclose a device having a membrane through which a species to be measured can diffuse into a chamber in which its concentration can be measured. A first electrode is disposed within the chamber, and a second electrode is disposed outside of the chamber, but near to the first electrode. A sensor for measuring the species is also disposed in the chamber. There is no fluid flow to and from the chamber other than by diffusion. This device has the disadvantage that the buffering capacity of some ions in the process stream can prevent a sufficient alteration of the pH, which can prevent, or limit, accurate measurement of the species by the sensor. This buffering capacity causes ions in the sample to soak up hydrogen and hydroxyl ions, preventing or reducing any changes in pH. In order to solve these problems we have now found a way of measuring the concentration of a material in a fluid, wherein reagent addition is completely avoided or minimised, and wherein the problems of neutralisation experienced in EP-B-0637381 and WO-A-9625662 are reduced or eliminated. Furthermore, we have found a way to deal with the buffering problem which can occur in the devices described in the specifications of the US patents mentioned above. We have also found a way to deal with the problems of precipitation and biological fouling.

Furthermore, we have also found that our solution to the problems in the prior art has far wider applications than just liquid analysis. Broadly, we have found a way to produce fluids of raised and lowered pH from a single fluid.

According to one aspect of the invention there is provided a pH altering device comprising a fluid receptacle, a microporous membrane disposed within the fluid receptacle and dividing fluid within the interior of the receptacle into first and second fluid chambers, and an electrode disposed within each fluid chamber, wherein the microporous membrane restricts or substantially prevents the flow of the fluid from one fluid chamber to the other, but does not prevent the flow of any anions or cations necessary for a flow of an electric current between the electrodes.

It is possible to use the device in a batch process in which an amount of the fluid is placed in each fluid chamber, the device is operated for a period of time, then the fluid is removed from each chamber. However, it is preferred that the device is used in a continuous process, wherein fluid is continuously fed to, and continuously withdrawn from, both fluid chambers.

In continuous processes, the characteristics of the flow of the fluid through the device can help to prevent mixing of the fluid across the microporous membrane. It is preferable to have a chamber configuration and fluid flowrates that promote laminar flow both sides of the membrane, rather than turbulent flow as this minimises the risk of any mixing across the membrane.

The purpose of the microporous membrane is to permit an electrical current to pass between the electrodes, while restricting or substantially preventing intermixing of the fluid in the fluid chambers. Thus, the microporous membrane can maintain the separation and flow of the fluid sample in two separate fluid streams.ln order to achieve this, the pores of the microporous membrane must be small enough to restrict or substantially prevent the flow of the fluid therethrough, and must be large enough to permit the flow therethrough of the anions and cations necessary for current flow between the electrodes. The migration of charge particles across the microporous membrane causes the pH of the fluid flowing in one chamber to decrease relative to the pH of the fluid flowing in the other chamber. In this way it is possible to generate two fluid streams of different pH from a fluid source of a single pH. The streams of different pH may both be acidic, or may both be alkaline, or one may be alkaline and the other may be acidic. In the present invention the pH is changed by alterations in the hydrogen and hydroxyl ion concentrations, and there is no requirement to alter the concentration of other ions.

In the present invention only a single membrane is needed, in which case the membrane should not be ion-selective.

Although it is preferred that the flow of the fluid across the microporous membrane is substantially prevented, it is usually acceptable for small amounts of the fluid to cross the membrane, so long as this does not have any significant effect on the pH of the fluid on each side of the membrane. The microporous membrane may have an open area in the range 10% to 70%, preferably 30% to 60%. More preferably the open area is in the range 40% to 50%, most preferably about 45%.

The pore size of the microporous membrane may be from 0.001 to 100 μ², preferably from 0.005 to 10 (1 μ = ft? m ). More preferably the pore size is from 0.01 to 1 μ², most preferably from 0.01 to 0.1 μ . We have obtained very good results with a pore size of about 0.02 μ².

The thickness of the microporous membrane may be from 1 to 1000 μ. In general the membrane will have a thickness not greater than 100 μ. Moreover, the minimum thickness of the membrane will typically be no less than 10 μ. We have found good results with thicknesses in the range 10 to 50 μ, especially about 25 μ.

The microporous membrane may be rigid or flexible. It is highly desirable that the membrane is resistant to high and low pH fluids. It is preferred that the microporous membrane has hydrophilic properties; these properties can be imparted, for example, by surface treatment. Preferably the microporous membrane is a dielectric material such as a polymer, a glass, a cellulosic, a ceramic or a fibrous material. The microporous membrane may be a composite material comprising or consisting of a suitable combination of the aforementioned materials to achieve a de sired set of properties. More specifically, the microporous membrane may be PTFE, PVC, polyethylene, polypropylene, nylon, paper, or silicone rubber.

An example of one suitable microporous membrane is available from Hoechst under the trade name Celgard 3500. This is a non-woven polypropylene material having a 45% open area, a pore size of 0.075 μ by 0.25 μ, and a thickness of 25 μ.

In most practical embodiments the receptacle is provided with means to enable fluid to be supplied to and withdrawn from the fluid chambers. Typically, the fluid is supplied and withdrawn through an aperture in the receptacle. In an advantageous embodiment, the receptacle is provided a fluid inlet and a fluid outlet for the first fluid chamber, and a separate fluid inlet and fluid outlet for the second fluid chamber. This arrangement is especially suitable for a continuous process, where fluid can be continuously fed to the fluid chambers through the fluid inlets and can be continuously withdrawn from the fluid chambers through the fluid outlets. It is desirable to provide a fluid flow splitter upstream of the fluid inlets, whereby a single fluid stream can be split into two streams, one for each fluid inlet.

The electrodes are desirably connected to an electrical power source, which may be a battery, or any other suitable source of electrical power. It is preferred that means is provided to adjust the electrical power delivered to the electrodes by the power source. It is also preferred that polarity reversing means is provided for reversing the polarity of the electrodes; by means of this arrangement it is possible from time to time to reverse the polarities of the electrodes, which helps to clean the electrodes, and the rest of the device, of deposits that accumulate during the use of the device. The flowrates of the fluid through the device, and the electrical power delivered to the electrodes, may be varied to optimise the cleaning time, depending on the buffering capacity of the fluid flow.

There is a wide variety of electrodes that are suitable for use with the present invention. The electrodes may be simple planar units of conductive material. They may comprise a metal, carbon, a semiconductor (such as a metal oxide), a conducting polymer, or a composite of two or more of these materials. We have found that electrodes comprising vitreous carbon, platinum, gold, stainless steel, or ruthenium oxide are ideal; however, other metals, such as silver may be used instead. Composites of two metals, such as platinum coated on titanium may be used. The electrodes may comprise solid, woven, porous or film electrodes. Electrodes comprising a piece of metal gauze are very effective. The potential difference to be applied across the electrodes can vary considerably between different applications. The optimum potential difference depends on factors such as the ionic species present in the fluid, the fluid flow rate, and the size of the chambers. It is possible to produce useful results in some systems with very low voltages of 1 mV or more. The potential difference would usually be at least 100 mV, and typically would be at least 1 V. For large scale applications the potential difference may be very high, possibly up to 10,000 V, or even up to 1 MV. For small scale sensing operations using fluid flows of the order of 1 to 100 ml per min, a potential difference in the range 1 to 100 V would be typical, and a potential difference in the range 15 to 30 V would generally be preferred.

In one embodiment, the device includes a pH sensor for sensing the pH of the fluid. The pH sensor may be disposed in one or both of the fluid chambers, and/or may be disposed upstream of the fluid chambers, and/or may be disposed downstream of the fluid chambers. Several separate pH sensors may be provided. In a batch process the or each pH sensor would be arranged to sense the fluid pH in the fluid chambers, while in a continuous process the or each pH sensor would be arranged to sense the fluid pH downstream of the fluid chambers. In a preferred construction a separate pH sensor is provided downstream of each fluid chamber.

A flow regulator, such as a pump or a valve may be provided for regulating the flowrates of fluid supplied to each fluid chamber. A separate flow regulator may be provided for each fluid chamber. The flowrates of the fluid to each fluid chamber may be the same or different.

A controller may be provided for controlling the pH of the fluid discharged from the fluid chambers. Preferably, the controller is operatively connected to the or each pH sensor whereby the controller receives a signal indicative of the pH of the fluid discharged from at least one of the fluid chambers. The controller is preferably also operatively connected to the means for adjusting the electrical power, whereby the controller can adjust the electrical power delivered to the electrodes in response to signals received from the or each pH detector. The controller is preferably also operatively connected to the flow regulator, whereby the controller can adjust the flowrate of fluid supplied to the fluid chamber in response to signals received from the or each pH detector. The controller can also be provided with some form of manual input means, such as a keyboard, whereby an operator can set a desired level of pH for the fluid discharged from the fluid chambers. This arrangement makes possible a very precise control of the fluids produced by the device. In a preferred embodiment a recycle is provided, whereby at least part of the fluid discharged from one of the fluid chambers is recycled to the inlet for that chamber. It is possible for a separate recycle to be provided for the fluid discharged from each fluid chamber. The presence of a recycle makes it possible to achieve better control of the pH, partly by overcoming the influence of buffering ions. A pH sensor may be provided in the recycle, and the pH sensor in the recycle may be operatively connected to the controller described above. The recycle may be connected to a downstream valve means disposed downstream of the fluid chamber from which fluid is to be recycled; this enables the proportion of the recycled fluid to be controlled. The downstream valve means is preferably operatively connected to the controller. The recycle may also be connected to an upstream valve means disposed upstream of the fluid chamber to which fluid is to be recycled; this enables the proportion of fresh fluid fed to the chamber to be controlled. The upstream valve means is preferably operatively connected to the controller. When there is a flow splitter, the upstream valve means would normally be disposed downstream of the flow splitter, but upstream of the inlet for the fluid chamber.

The use of the upstream and downstream valve means makes it possible to recycle 100% of the fluid from one or more of the fluid chambers, and to prevent the flow of any fresh fluid to one or more of the chambers. This makes it possible to generate very large and controlled alterations in pH, especially in large buffering capacity fluids.

Another technique for generating very large and controlled alterations in pH is to arrange two or more of the devices according to the invention in series. For example, the fluid discharged from each fluid chamber can be fed to another device. This arrangement can be used in conjunction with the or each recycle described above.

In another embodiment, the device includes an additional chamber downstream of one of the first and second chambers. The additional chamber may be provided in the receptacle, or may be provided in an additional receptacle linked to the first receptacle via a flow pipe. The additional chamber is provided with a fluid inlet in fluid communication with the outlet of one of the first and second chambers, and with a fluid outlet. The additional chamber is preferably provided with two electrodes arranged so that fluid flowing through the additional chamber can flow between said electrodes. However, the additional chamber is not provided with any membrane. This embodiment is useful in order to avoid problems with precipitation of solid deposits caused by the pH changes in the fluid. It is important to maintain laminar flow conditions in the second chamber, and this can be achieved by means of the flow regulator. In this embodiment, it is particularly advantageous to provide the polarity reversing means described above for reversing the polarities of the electrodes in the first and second chambers. It is preferred that means to sense chemical changes in the fluid is provided in the second chamber. This embodiment is useful when the device is being used for chemical analysis, as described in greater detail below, because it is possible to clean the device without interrupting the analysis.

The device can be used to produce fluids for chemical analysis, cleansing, sterilisation or other sample treatment. One particularly advantageous application of the device is for destroying bacteria in a fluid. A flow of fluid can be diverted through the device so that part of the fluid is at a high pH and the other part is at a low pH, whereby bacteria in the fluid are destroyed by the acidic and/or alkaline conditions. The two streams can subsequently be recombined into a single stream containing the destroyed bacteria. It is envisaged that this form of the device may be used in the treatment of drinking water and sewerage; however, it will be appreciated that there are many other applications.

Another especially important application of the pH altering device and method described above relates to fluid analysis. Broadly, this involves electrochemically modifying a region of the fluid to convert a material to be measured into a form in which its concentration can be measured (such as a gas), and sensing the amount of said form that is generated in order to provide a measure of the concentration of the material in the fluid. In this application the microporous membrane restricts or prevents any neutralisation caused by turbulence or mixing, so that an accurate measurement of the concentration can be obtained.

In order to use the device in chemical analysis, it is preferred to provide a sensing means in or downstream of the first and/or second chambers. In order to perform chemical analysis it is important for one or both electrodes to be capable of electrochemically modifying the fluid to convert a material of interest into a form in which it can be measured by the sensing means.

The sensing means may be adapted to sense ions in the solution (e.g. hypochlorite ions formed by equation IIIA), or it may be adapted to sense a gas formed by the electrochemical modification (eg the ammonia gas formed by equation (I)).

When the sensor is a gas sensor it may include a membrane permeable to said gas, and detection means for detecting the amount of said gas that diffuses into the membrane. The gas-permeable membrane of the sensor may be any membrane that will permit the diffusion therethrough of the gaseous form of said material, but will prevent the passage of the fluid. The membrane may contain a dye sensitive to said gas, so that the concentration of the material can be measured by measuring the magnitude of the colour change of the dye. Means can be provided to measure the magnitude of the colour change.

In one embodiment, the gas sensor may be adapted to measure carbon dioxide gas. This provides a means to measure the total inorganic carbon level of an aqueous sample: the acidification of the sample liberates carbon dioxide from any bicarbonates and carbonates present in the sample.

In another embodiment, the gas sensor may instead be adapted to measure sulphur dioxide gas. Acidification of the sample (to a pH below 0.7) enables the "free" sulphur dioxide levels to be monitored; this is especially useful in the food and drink industries. Samples containing "bound" sulphur dioxide could be analysed by increasing the pH to a value above 12, when sulphur dioxide will be liberated and can be measured. These two techniques could be combined to provide a measure of the total sulphur dioxide levels; both measurements could be made simultaneously with the present invention.

The sensor may be an ion selective electrode adapted to sense the presence of a particular ion in solution. These electrodes are well known and include an active membrane material, which selects the ion to be detected. Ion selective electrodes are sensitive to the pH of the solution, as shown by some examples in the following table.

Ion Detected Membrane Material Detection limit/M pH Ranqe

Fluoride Lanthanum Fluoride 10^"7 5-8

Chloride Mercurous Chloride 4 x 10^"7 0-5

Bromide Mercurous Chloride 10^"7 1-6

Thiocyanide Mercurous Thiocyanide 5 x 10^"7 1-7

Cyanide Silver Iodide 10^"6 11-13

Sulphide Silver Sulphide <10^"7 13-14

The above table relates to anion sensitive electrodes. With cation sensitive electrodes, such as those for lead, cadmium, copper and mercury, the pH range generally needs to be between 3 and 7 (in fact, for mercury, the pH must be in the range 4 to 5).

Other sensors can be used instead. For example, other optical or electrical sensors may be used, and they may be invasive or non-invasive.

In EP-B-0637381 and in WO-A-9625662, and in the US patents referred to above, the sensor had to be placed near to the electrode at which the species to be sensed was generated. Typically the sensor would be within a fraction of a millimetre, or at most a few millimetres, of the electrode. The device according to the invention has the advantage that the sensor does not need to be placed near one of the electrodes. For example, the electrode could be a centimetre or more, 0.1 m or more, or even 1 m or more, away from the sensor. This makes it possible to use several different sensors, remote from the electrodes, for analysing the presence of different materials. For example, several sensors can be arranged downstream of the first and/or second fluid chambers, at, near or remote from each of the fluid outlets. Sensing does not need to be carried out while the fluid streams are being electrochemically modified: sensing can be carried out on the pH altered fluid at any time after the electrochemical modification. The device according to the invention can be part of a fixed installation or it can be removable. The device can be made small enough to be readily portable. The device can be easily fitted to existing installations.

According to another aspect of the invention there is provided a method of altering the pH of a fluid, comprising flowing a first fluid through a first fluid chamber and in contact with a first electrode disposed in the first fluid chamber, flowing a second fluid through a second fluid chamber and in contact with a second electrode disposed in the second fluid chamber, the first and second fluids being separated by a microporous membrane which restricts or substantially prevents the flow of the first fluid to the second chamber or the flow of the second fluid to the first chamber, but does not prevent the flow of any anions or cations necessary for a flow of an electric current between the electrodes; passing an electrical current between the first and second electrodes, through the first and second fluids, to generate hydroxyl ions at one electrode and hydrogen ions at the other, thereby generating an increase in pH in one of the fluids and a decrease in pH in the other of the fluids, and discharging fluid from the first and second chambers to recover two separate fluids of different pH.

The two fluids may have substantially identical compositions, and may be derived from the same source, or they may be different.

From time to time the polarities of the electrodes may be reversed. This helps to clean the electrodes, and the rest of the device, of depositions. The flow rate of the fluid through the device, and the electrical power delivered to the electrodes, may be varied to reduce the cleaning time.

In a preferred embodiment, the fluid exiting one of the chambers is fed to an additional chamber containing two electrodes, and a potential difference is applied to the fluid flowing through the additional chamber. In this embodiment, the fluid in the additional chamber does not contain any microporous membrane between the electrodes. Preferably the flow of the fluid within the additional chamber is controlled in order to maintain laminar flow conditions. The periodic reversal of the polarity is particulariy advantageous with this embodiment, because it enables any precipitated solids in the first and second chambers to be dissolved. Furthermore, we have found that it is possible to measure continuous chemical changes in the fluid in the additional chamber, despite the periodic reversals of polarity, because the pH near to the electrodes in the additional chamber does not change greatly. Thus, this embodiment is particularly useful for measuring the concentration of a material in the fluid.

This method can be used in conjunction with a pH altering device having any combination of the features of the pH altering device described above.

According to a further aspect of the invention there is provided a method of detecting the concentration of a material in a fluid, comprising flowing the fluid through a device as described above to produce two fluids of different pH, sensing the concentration of said material, or of a species related to said material, downstream of said first and second chambers, generating a signal indicative of said sensed concentration, and calculating the concentration of the material from said signal. It will be appreciated that the result of the concentration can be displayed or printed by any suitable means.

According to a further aspect of the invention there is provided a method of destroying bacteria in a fluid comprising flowing the fluid through a device as described above to produce two fluids of sufficient acidity or alkalinity to destroy at least part of said bacteria.

According to a further aspect of the invention there is provided a method of producing high and/or low pH cleaning fluids from a single substantially neutral fluid, comprising flowing the fluid through a device as described above to produce two fluids of sufficient acidity or alkalinity to act as a cleaning fluid.

It will be appreciated that invention is for use with fluids that are electrically conductive. In practice the fluids used with the present invention are almost invariably liquids, and are predominantly aqueous solutions. The fluid may be a single liquid, or may be a mixture of two or more liquids. When the fluid is a liquid, then the references to "fluid" above should be read as "liquid", and when the fluid is an aqueous solution, then the reference to "fluid" above should be read as "aqueous solution". In general, the liquid comprises a mixture of water with at least one other electrolyte dissolved therein. The fluid may also contain other liquids apart from water. The liquid may contain entrained or dissolved gases.

The liquid may be, for example, river water or potable water. In this case, the polarity reversal for cleaning purposes would ideally be carried out about once per week. The liquid may instead be brackish water, estuary water, sea water, or effluent from a wide variety of industrial processes, in which case the polarity reversal would probably be necessary more than once per week.

The device and method can be used to detect a wide variety of contaminants in water including, but not limited to, dissolved ammonia, ammonium salts, salts of alkali metals and salts of alkaline earth metals. Examples of salts that can be detected include fluorides, chlorides, bromides, iodides, sulphates, sulfides, nitrates, carbonates and phosphates. Particular salts that can be detected include sodium chloride, calcium and magnesium carbonates, magnesium phosphate, silver sulfide and so on.

When the device is used to sense the concentration of a material in the fluid, a wide range of different materials may be detected, including liquid components, solids entrained or dissolved in the fluid, and gases entrained in or dissolved in the fluid. In the present invention, the ability of the fluid streams to flow through the fluid chambers containing the electrodes overcomes the buffering effects of the ions present in the fluid, i.e., their ability to soak up the hydroxyl and hydrogen ions generated during the application of the potential difference.

Reference is now made to the accompanying drawings, in which: Fig. 1 is a schematic view of an embodiment of a pH altering device according to the invention;

Fig. 2 is a schematic view of a modification of the invention;

Fig. 3 is a schematic view of another modification of the invention;

Fig. 4 is a schematic view of a further modification of the invention; Fig. 5 is a schematic view showing the application of the device of Fig.

1 to chemical analysis;

Fig. 6 is a schematic view of a modification of the device shown in Fig. 5; and

Fig. 7 is a schematic view of another modification of the device shown in Fig. 5.

Fig. 1 shows the most basic unit of the invention, which is a pH altering device generally designated 10. The device 10 comprises a receptacle 12 having liquid inlets 14 and 16 and liquid outlets 18 and 20. An interior 22 of the receptacle is divided into first and second chambers 22a and 22b by a microporous membrane 24 which extends across the interior 22. The microporous membrane 24 is arranged such that the chamber 22a is in fluid communication with the inlet 14 and the outlet 18, but is not in fluid communication with the inlet 16 and the outlet 20, and such that the chamber 22b is in fluid communication with the inlet 16 and the outlet 20, but is not in fluid communication with the inlet 14 and the outlet 18.

Electrodes 26 and 28 are disposed in the chambers 22a and 22b respectively. The electrodes 26 and 28 are shown secured to opposite walls of the interior 22 of the receptacle 12, which is the preferred location, but they could be secured at any position within their respective chambers 22a, 22b. The electrodes 26 and 28 are electrically connected to an electrical power source 30 by wires 32 and 34. The electrical power source 30 may be any D.C. source, and is preferably a battery. The power source 30 incorporates means to reverse the polarity of its output and means to vary the electrical power of its output.

A feed pipe 36, which can be connected to a liquid source S, is connected to a flow splitter 38 which splits liquid from the pipe 36 into two streams, each of which is fed to a respective one of the inlets 14 and 16.

The operation of the device 10 will now be described. In the following description it will be assumed that the electrode 26 acts initially as a cathode and the electrode 28 acts initially as an anode. However, it will be appreciated that this initial position would be reversed if the polarity of the power source 30 were reversed.

The liquid to be analysed is flowed from the pipe 36 to the flow splitter

38, where it is split into two separate streams, each of which is fed to a respective one of the inlets 14 and 16. The feed pipe 36 may be in communication with any suitable source of liquid, such as, for example, a river. It will be appreciated that only part of the flow from the river would be diverted to the device 10. When a potential difference is applied across the electrodes 26 and 28 an electrical current flows through the liquid by virtue of the ability of anions and cations in the liquid to pass through the microporous membrane 24.

The liquid stream fed to the chamber 22a comes into contact with the cathode 26. Upon the application of a potential difference from the source 30, hydroxyl ions are generated at the cathode 26, in accordance with Equation (IV) above. In this example, the generation of the hydroxyl ions increases the pH of the liquid in the chamber 22a.

The liquid stream fed to the chamber 22b comes into contact with the anode 28. Upon the application of a potential difference from the source 30, hydrogen ions are generated at the anode 28, in accordance with Equation (V) above. In this example, the generation of these hydrogen ions reduces the pH of the liquid in the chamber 22b.

The liquid stream in chamber 22a is discharged from the receptacle 12 via the outlet 18, and the liquid stream in the chamber 22b is discharged from the receptacle 12 via the outlet 20. The liquid stream discharged through the outlet 18 will be relatively alkaline compared with the liquid stream discharged through the outlet 20. If the liquid supplied to the device 10 from the pipe 36 were approximately neutral, then the pH of the liquid stream discharged via the outlet 18 would be greater than 7, and the pH of the liquid stream discharged via the outlet 20 would be less than 7.

After the liquid has been discharged from the receptacle 12, the two liquid streams may be immediately recombined to form a combined liquid having a pH approximately the same as the liquid in the pipe 36. However, it is possible to further delay recombination of the liquid streams. This may be done for a number of reasons. For example the acidic and alkaline liquids may be used for cleaning purposes. In addition, or instead, recombination of the liquid streams may be delayed for a time sufficient to kill bacteria therein. In some embodiments, recombination of the liquid streams need not be carried out at all, and the pH values of the portions remain stable.

The polarities of the electrodes 28 and 30 may be reversed from time to time in order to help to remove any deposits which form on the electrodes 26 and 28, on the membrane 24, or in other parts of the interior 22 of the receptacle 12.

In Fig. 2 there is shown a modification of the invention in which two liquids of different pH are produced and stored. The embodiment of Fig. 2 involves the use of the device 10. In the embodiment of Fig. 2 the outlet 18 communicates with a pipe 52, which delivers its contents to a receptacle 54, and the outlet 20 communicates with a pipe 56, which delivers its contents to a receptacle 58. The liquids in the receptacles 54 and 58 can be stored and/or transported for subsequent analysis. The altered pH of the liquid can be maintained almost indefinitely.

Fig. 3 shows a modification which makes it possible to produce a liquid stream of a predetermined pH. In Fig. 3 the device 10 is provided with a pH sensor 60 disposed in the liquid stream discharged through the outlet 20. The pH sensor 60 measures the pH of the liquid discharged through the outlet 20, and generates a signal indicative of the magnitude of the pH. This signal from the pH sensor 60 is fed to an amplifier 62 via a wire 64, and the amplifier 62 amplifies the signal. The amplified signal is fed to a controller 66 via a wire 68. The controller 66 compares the desired pH with the actual pH and generates an appropriate signal to control the power source 30 via a wire 70. The controller 66 causes the size of the electrical power delivered to the electrodes 26 and 28 by the source 30 to be varied, so that the pH of the liquid in the outlet 20 is kept at a desired level. Means can be provided to vary the value of the predetermined pH. Instead, or in addition, the controller 66 could be arranged to control the rate of liquid flow to the chambers 22a and 22b.

The pH sensor 60 could instead be disposed in the liquid stream discharged through the outlet 18. It is possible to use two of the pH sensors 60, one of which is disposed in the liquid stream discharged through the outlet 18, and the other of which is disposed in the liquid stream discharged through the outlet 20, so that both pH sensors can be used to control the process.

Fig. 4 shows another embodiment of the invention. This embodiment can be used to provide very good control of the pH and also to provide very large alterations in pH. Some of the parts in Fig. 4 are identical to the parts in Fig. 3, and like parts have been designated with like reference numerals.

In Fig. 4, the device 10 is provided with a recycle 80 arranged so that liquid discharged from the chamber 22b can be recycled back into the chamber 22b. A downstream valve means in the form of a valve 82 is disposed downstream of the chamber 22b, and one end of recycle 80 is connected to the valve 82. An upstream valve means in the form of a valve 84 is disposed upstream of the chamber 22b, and the other end of the recycle 80 is connected to the valve 84. The pH sensor 60 is disposed in the recycle 80 and provides the controller 66 with a measure of the pH in therein. The controller is additional operatively connected to the valves 82 and 84, and a pump P, via wires 72, 74 and 76 respectively. The pump P is arranged to meter the flow through the inlets 16 and 18; although the pump P has been illustrated as a single pump, it is possible to provide a separate pump for each inlet 16 and 18. The pump P is arranged downstream of the valve 84.

With the arrangement shown in Fig. 4 it is possible to adjust the valves 82 and 84 so that a predetermined percentage of the liquid stream discharged from the chamber 22b is recycled. It is possible for the valves 82 and 84 to be completely closed, so that 100% of the liquid stream in the outlet 20 is recycled, and no fresh liquid is added; this makes it possible to achieve very large and controlled changes in pH.

It is also possible to use the valves 82 and 84 to balance the volumes on both sides of the membrane 24.

In Fig.5 there is shown an embodiment of the invention for use in sensing the concentration of selected materials.

The device 10 includes a gas sensor 46, for example for detecting ammonia gas, which is disposed downstream of the chamber 22a. The sensor 46 is connected to a concentration calculator 48 by a wire 50. The sensor generates a signal indicative of the measured concentration, which is fed to the calculator 48 via the wire 50. The calculator 48 converts the signal into a measure of the concentration of the gas dissolved in the liquid. The calculator 48 can be calibrated, by conventional techniques, to perform this calculation.

The device further includes an ion sensor 40, for example for detecting hypochlorite ions, which is disposed downstream of the chamber 22b. The sensor 40 is connected to a concentration calculator 42 by a wire 44. The sensor 40 generates a signal indicative of the measured concentration, which is fed to the calculator 42 via the wire 44. The calculator 44 converts the signal into a measure of the concentration of species present in the liquid. The calculator 42 can be calibrated, by conventional techniques, to perform this calculation.

Additional sensors (not shown) for detecting other material in the liquid could be provided downstream of the chambers 22a and/or 22b. The operation of the device shown in Fig. 5 will now be described. In the following description it will be assumed that the electrode 26 acts initially as a cathode and the electrode 28 acts initially as an anode. However, it will be appreciated that this initial position would be reversed if the polarity of the power source 30 were reversed. The following description relates to the detection of the concentration of ammonia and chlorine in water; it will also be appreciated that the device 10 could be used for the detection of other materials.

The liquid to be analysed is flowed through the pipe 36 to the flow splitter 38, where it is split into two separate streams, each of which is fed to a respective one of the inlets 14 and 16. The liquid fed to the device 10 may comprise the entire flow of the liquid, or may be just part of the flow of the liquid. For example, if the liquid being analysed were part of a river, then liquid fed to the device 10 would only comprise part of the total liquid flow. The liquid may contain ammonia and chlorine species in dissolved, dissociated or combined forms, and the device 10 can measure the concentrations of the ammonia gas and hypochlorite ions generated in the liquid, from which the concentrations of the ammonia and chlorine species can be calculated .

The liquid stream fed to the chamber 22a comes into contact with the cathode 26. Upon the application of a potential difference from the source 30, hydroxyl ions are generated at the cathode 26, in accordance with Equation (IV) above. In this example, the generation of the hydroxyl ions increases the pH of the liquid stream in the chamber 22a above 7, and these hydroxyl ions react with ammonium ions, in accordance with equation (I), to generate ammonia gas. The ammonia gas is sensed by the sensor 46, and the calculator 48 converts the measurements from the sensor 46 into a measure of ammonia concentration in the liquid.

The liquid stream fed to the chamber 22b comes into contact with the anode 28. Upon the application of a potential difference from the source 30, hydrogen ions are generated at the anode 28, in accordance with Equation (V) above. In this example, the generation of these hydrogen ions reduces the pH of the liquid stream in the chamber 22b to a level below 7, at which the concentration of the hypochlorite ions can be measured with the sensor 40. The liquid stream in the chamber 22a is discharged via the outlet 18, and the liquid stream in the chamber 22b is discharged via the outlet 20. The liquid stream discharged from the chamber 22a will be relatively alkaline compared with the liquid stream discharged from the chamber 22b.

After the liquid analysis has been performed, the two liquid streams may be immediately recombined to form a combined liquid having a pH approximately the same as the liquid in the pipe 36. However, it is possible to further delay recombination of the liquid streams. This may be done for a number of reasons. For example the acidic and alkaline liquids may be used for cleaning purposes. In addition, or instead, recombination of the liquid streams may be delayed for a time sufficient to kill bacteria therein. In some embodiments, recombination of the liquid streams need not be carried out at all, and the pH values of the portions remain stable.

Another embodiment of the invention is shown in Fig. 6. The embodiment of Fig. 6 is also used for sensing the concentration of materials in the liquid. The embodiment of Fig. 6 utilises two of the devices 10 arranged in series. Many of the parts of the embodiment of Fig. 6 are identical to the parts of the embodiment of Fig. 5, and like parts have been designated with like reference numerals.

In Fig. 6 the liquid stream discharged through the outlet 20 of the first device 10 is fed to the feed pipe 36 of the second device 10. The first device 10 has not been provided with the sensor 40, the calculator 42 or the wire 44 near its outlet 20; however, it will be appreciated that these components could have been provided, if desired.

In a modification of the embodiment shown in Fig. 6, the pipe 36 of the second device 10 can be fed with the liquid stream discharged through the outlet 18 of the first device 10, instead of the liquid stream discharged through the outlet 20 of the first device 10.

In a further modification of the embodiment shown in Fig. 6, the liquid stream discharged through the outlet 18 of the first device 10 could have been fed to the pipe 36 of a third of said devices 10, with the liquid stream discharged through the outlet 20 still being fed to the pipe 36 of the second device 10.

Fig. 7 shows a modification of the embodiments shown in Figs. 5 and

6. In Fig. 7 the liquid stream discharged through the outlet 20 of the first device 10 is fed to an inlet pipe of an additional unit 502. The device 10 has not been provided with the sensor 40, the calculator 42 or the wire 44 near its outlet 20; however, it will be appreciated that these components could have been provided, if desired.

The unit 502 comprises a receptacle 512 having a liquid inlet 514 and a liquid outlet 520. The receptacle 512 has an interior defining a chamber 522; unlike the receptacle 12, the interior of the receptacle 512 is not divided into two chambers.

Electrodes 526 and 528 are disposed in the chamber 522. The electrodes 526 and 528 are shown secured to opposite walls of the receptacle 512, which is the preferred location. The electrodes 526 and 528 are electrically connected to an electrical power source 530 by wires 532 and 534. The power source 530 may be any D.C. source, and is preferably a battery.

A gas sensor 546, for example for detecting ammonia gas, is disposed close to the electrode 528. The sensor 546 is connected to a concentration calculator 548 by a wire 550. The sensor 546, the calculator 548 and the wire 550 operate in a similar manner to the senor 46, the calculator 48 and the wire 50. A liquid stream is discharged from the chamber 522 via an outlet 520.

The liquid discharged through the outlet 520 may be combined with the liquid from the outlet 18.

During the course of operation of the device 10 and unit 502 there may be solids precipitated from the liquid; this may occur, for example, if the liquid is hard water. These solids can be dissolved by periodically reversing the polarities of the electrodes 26 and 28. During a reversal of polarity in the device 10, the sensing of the concentrations in the outlets 18 and 20 would normally have to be stopped. However, the provision of the unit 502 enables the sensing to be carried out continuously with the sensor 546. It has been found that, provided that the flow in the chamber 522 is laminar, the pH of the liquid near to the electrode 528 can be maintained at the correct level for measurement even when the polarities of the electrodes 526 and 528 have been reversed.

Another one of the units 502 may be provided for the liquid stream discharged through the outlet 18.

The liquid flow can be carefully controlled via a pump P', in order to ensure laminar flow in the chamber 522.

Example

A device in accordance with the embodiment of Fig. 4 was used to generate pH changes in water containing ammonium ions and buffering. The liquid was at a temperature of 15°C. The liquid was pumped through the feed pipe 36 at a flowrate of 16 ml/min, so that the flow rate in each chamber was 8 ml/min. A potential difference of 30V was applied across the electrodes 26 and 28. After a continuous flow had been achieved in the chamber 22b, the valves 82 and 84 were completely closed, so that all the liquid in the chamber 22b was recycled via the recycle 80. A steady flow of the liquid was maintained by the pump P. The liquid stream discharged from the outlet 18 was fed to waste.

After 12-15 minutes, the pH measured by the sensor 60 had reached pH 11. The ammonia concentration was then measured by the sensor 40 in the recycle 80. The sensor was capable of measuring reliably ammonia concentrations in the range 100 ppb to 10 ppm.

After the concentration had been measured, the polarity of the electrodes was reversed to solubilise electrode deposits, or to degas them.

Finally, the valves 82 and 84 were opened, the potential difference was removed, and the chambers 22a and 22b were flushed to neutrality in readiness for another sample. The total cycle time was about 24 minutes.

By repeating this experiment on several samples of the same liquid, it was possible to establish that the device 10 was capable of measuring ammonia concentration accurately and reproducibly.

It will be appreciated that modifications may be made to the embodiments described above. In each of the above examples it is possible to provide pH sensors at any selected position. Furthermore, sensors for sensing materials in the liquid can be provided in each of the embodiments. In particular pH sensors and the controller 66 can also be used in the embodiments of Figs. 5 to 7. Several of the devices 10 can be arranged in series and/or in parallel, and the embodiments of Figs. 1 to 7 can be combined as necessary.

Furthermore, although the liquid supplied to each chamber 22a and 22b in the embodiments described above is derived from a common source, it will be appreciated that it could have been derived from separate sources.

Claims

1. A pH altering device comprising a fluid receptacle, a microporous membrane disposed within the fluid receptacle and dividing fluid within the interior of the receptacle into first and second fluid chambers, and an electrode disposed within each fluid chamber, wherein the receptacle is provided with an inlet and an outlet for the first fluid chamber, and with a separate inlet and outlet for the second fluid chamber, and wherein the microporous membrane restricts or substantially prevents the flow of the fluid from one fluid chamber to the other, but does not prevent the flow of any anions or cations necessary for a flow of an electric current between the electrodes, whereby a stream of fluid can flow through the first fluid chamber from the first fluid inlet to the first fluid outlet, and a separate stream of fluid can flow through the second fluid chamber from the second fluid inlet to the second fluid outlet.

2. A device according to claim 1, wherein the microporous membrane has an open area of 30% to 60%.

3. A device according to claim 1 or 2, wherein the microporous membrane has a pore size of 0.01 to 1 μ².

4. A device according to claim 1 , 2 or 3, wherein the microporous membrane has a thickness of 10 to 100 μ.

5. A device according to claim 4, further comprising a fluid flow splitter upstream of said fluid inlets whereby a single fluid stream can be divided into two separate fluid streams, each of said two fluid streams being fed to a respective one of the fluid inlets.

6. A device according to any preceding claim, wherein said electrodes are connected to an electrical power source.

7. A device according to claim 6, further comprising means to reverse the polarity of the electrical current supplied to said electrodes from said power source.

8. A device according to claim 6 or 7, further comprising means to adjust the electrical power delivered to the electrodes by the electrical power source.

9. A device according to any preceding claim, further comprising at least one pH sensor for sensing the pH of the fluid, the or each pH sensor being disposed in the fluid chambers, upstream of the fluid chambers and/or downstream of the fluid chambers.

10. A device according to any preceding claim, further comprising a flow regulator for regulating the flowrate of fluid supplied to each fluid chamber.

11. A device according to claim 10, wherein there is a separate flow regulator for regulating the flow of fluid to each fluid chamber.

12. A device according to any preceding claim, further comprising a controller for controlling the pH of the fluid discharged from at least one of the fluid chambers.

13. A device according to claim 12, when dependent upon claim 9, wherein the controller is operatively connected to the or each pH sensor, whereby the controller receives a signal indicative of the pH of the fluid discharged from at least one of the fluid chambers.

14. A device according to claim 13, when dependent upon claim 8, wherein the controller is also operatively connected to the means for adjusting the electrical power, whereby the controller can adjust the electrical power delivered to the electrodes in response to the signal received from the or each pH detector.

15. A device according to claim 13 or 14, when dependent upon claim 10 or 11 , wherein the controller is operatively connected to the or each flow regulator, whereby the controller can adjust the flowrate of fluid supplied to the fluid chambers in response to the signal received from the or each pH detector.

16. A device according to any preceding claim, further comprising a recycle, whereby at least part of the fluid discharged from one of the fluid chambers is recycled to the inlet for that chamber.

17. A device according to any preceding claim, further comprising a pH sensor disposed in the recycle.

18. A device according to claim 17, when dependent upon any of claims 12 to 15, wherein the pH sensor disposed in the recycle is operatively connected to the controller.

19. A device according to claim 16, 17 or 18, wherein the recycle is connected at one end to a downstream valve means disposed downstream of the fluid chamber from which fluid is to be recycled, and is connected at the other end to an upstream valve means disposed upstream of the fluid chamber to which fluid is to be recycled.

20. A device according to claim 19, when dependent upon any of claims 12 to 15, wherein the downstream and upstream valve means are operatively connected to the controller.

21. A device according to any preceding claim, further comprising an additional chamber disposed downstream of one or both of the first and second chambers, the or each additional chamber being provided with two electrodes disposed therein.

22. A device according to any preceding claim, further comprising at least one sensor for sensing the presence of a material in the fluid, the or each sensor being disposed in one or both of the fluid chambers and/or being disposed downstream of one or both of the fluid chambers. O 98/22813

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23. A device according to any one of claims 1 to 21, further comprising at least one sensor for sensing the presence of a material in the fluid, the or each sensor being disposed downstream of one or both of the fluid chambers.

24. A sensing device for sensing the concentration of a material in a fluid comprising: a fluid receptacle; a microporous membrane disposed within the fluid receptacle and dividing fluid within the interior of the receptacle into first and second fluid chambers; an electrode disposed within each fluid chamber; sensing means disposed downstream of said fluid chambers for sensing said material, or a species related to said material, and generating an electrical signal representative of the concentration of said material or species; and means for converting said signal into a measure of the concentration of said material in the fluid; wherein the microporous membrane restricts or substantially prevents the flow of the fluid from one fluid chamber to the other, without preventing the flow of any anions or cations necessary for a flow of an electric current between the electrodes, whereby the pH of the fluid in one of said fluid chambers is increased and the pH of the fluid in the other of said fluid chambers is decreased, and the material in the fluid is thereby modified to a form in which it can be sensed by the sensing means.

25. A device according to claim 24, wherein the receptacle is provided with an inlet and an outlet for the first fluid chamber, and with a separate inlet and outlet for the second fluid chamber, whereby a stream of fluid can flow through the first fluid chamber from the first fluid inlet to the first fluid outlet, and a separate stream of fluid can flow through the second fluid chamber from the second fluid inlet to the second fluid outlet.

26. A device according to claim 24 or 25, wherein the sensing means is adapted to sense ions in the fluid, or is adapted to sense a gas formed by the alteration in the pH of the fluid.

27. A method of altering the pH of a fluid, comprising flowing a first fluid through a first fluid chamber and in contact with a first electrode, flowing a second fluid O 98/22813

-28- through a second fluid chamber and in contact with a second electrode, the first and second fluids being separated by a microporous membrane which restricts or substantially prevents the flow of the first fluid to the second chamber or the flow of the second fluid to the first chamber, but does not prevent the flow of any anions or cations necessary for a flow of an electric current between the electrodes; passing an electrical current between the first and second electrodes, through the first and second fluids, to generate hydroxyl ions at one electrode and hydrogen ions at the other, thereby generating an increase in pH in one of the fluids and a decrease in pH in the other of the fluids, and discharging fluid from the first and second chambers to recover two separate fluids of different pH.

28. A method according to claim 27, further comprising intermittently reversing the polarity of the electrodes.

29. A method according to claim 27, further comprising flowing the fluid discharged from one of the fluid chambers to an additional chamber containing two electrodes, passing an electrical current between the electrodes in the additional chamber, through the fluid in the second chamber, activating a sensor in the additional chamber to sense the concentration of at least one material in the fluid, and intermittently reversing the polarity of the electrodes in the first and second fluid chambers.

30. A method according to claim 27, 28 or 29, further comprising recycling at least part of the fluid discharged from at least one of the fluid chambers back into the chamber from which the fluid was discharged.

31. A method according to claim 27, 28, 29 or 30, wherein the first and second fluids are derived from a common fluid source.

32. A method of destroying bacteria in a device comprising a fluid receptacle, a microporous membrane disposed within the fluid receptacle and dividing fluid within the interior of the receptacle into first and second fluid chambers, and an electrode O 98/22813

-29- disposed within each fluid chamber, wherein the microporous membrane restricts or substantially prevents the flow of the fluid from one fluid chamber to the other, but does not prevent the flow of any anions or cations necessary for a flow of an electric current between the electrodes, said method comprising flowing a separate stream of fluid through each of said fluid chambers, and applying a potential difference across said electrodes to produce a fluid of sufficient acidity in the first chamber to kill bacteria in the fluid and/or to produce a fluid of sufficient alkalinity in the second chamber to kill bacteria in the fluid.

33. A method of detecting the concentration of a material contained in a fluid, by means of a device comprising a fluid receptacle, a microporous membrane disposed within the fluid receptacle and dividing fluid within the interior of the receptacle into first and second fluid chambers, and an electrode disposed within each fluid chamber, wherein the microporous membrane restricts or substantially prevents the flow of the fluid from one fluid chamber to the other, but does not prevent the flow of any anions or cations necessary for a flow of an electric current between the electrodes, said method comprising flowing a separate stream of fluid through each of the first and second chambers, applying a potential difference across the electrodes to cause the pH of the fluid in each chamber to be altered, thereby modifying the material to a form in which it can be sensed; discharging a separate stream of fluid from each of the fluid chambers, and sensing the concentration of the material downstream of at least one of the first and second chambers.

34. A method of producing a high and/or low pH cleaning fluid from a single substantially neutral pH fluid, comprising: splitting said single substantially neutral fluid into two fluid streams, flowing a first of said fluid streams through a first fluid chamber and in contact with a first electrode, flowing a second of said fluid streams through a second fluid chamber and in contact with a second electrode, the first and second fluids being separated by a microporous membrane which restricts or substantially prevents the flow of the first fluid to the second chamber or the flow of the second fluid to the first chamber, but does not prevent the flow of any anions or cations necessary for a flow of an electric current between the electrodes; passing O 98/22813

-30- an electrical current between the first and second electrodes, through the first and second fluids, to generate hydroxyl ions at one electrode and hydrogen ions at the other, thereby generating an increase in pH in one of the fluids and a decrease in pH in the other of the fluids, discharging the fluid streams from the first and second fluid chambers, and recovering a cleaning fluid of high pH from one fluid stream and/or recovering a cleaning fluid of low pH from the other fluid stream.