FIELD The present invention relates to methods and apparatus for electrocoagulation treatment of aqueous dispersions, in particular for electrocoagulation treatment of aqueous dispersions or slurries in order to facilitate removal of particles therefrom by flocculation. The invention is concerned with improvements to the maintenance and cleaning of electrocoagulation electrodes in use.

BACKGROUND

The stabilisation and aggregation of colloidal dispersions or emulsions of particles in water or in aqueous solutions, has been explained in terms of DLVO theory (an acronym for the workers Derjaguin, Landau, Verwey and Overbeek who developed the theory) which combines the effects of van der Waals attraction with electrical double layer repulsion between dispersed, charged colloidal particles.

Commonly charged colloidal particles (i.e. colloidal particles having the same sign of charge) are stabilised in colloidal dispersions by mutual electrostatic repulsion forces exceeding the attractive van der Waals attraction. Particles in a colloidal state typically have a particle diameter from about 1 to 10,000 nm.

The charged particles may attract counterions, of opposite charge, to their charged surfaces, from their aqueous surroundings, resulting in the formation of an electrical double layer (EDL) at the particle surface. This EDL screens the electrical repulsion between particles, and so by formation of a suitable EDL, the electrostatic repulsion between the commonly charged colloidal particles may be sufficiently screened in order to allow van der Waals forces to drive coalescence of the particles into larger, bulk agglomerates or floes.

Typically, for water purification, or for winning of desired materials, such as heavy metals, from an aqueous dispersion or slurry, in order to remove colloidal particles from water by flocculation, modification of the EDL may be achieved by addition of electrolyte to the colloidal dispersion to be flocculated. However, for water purification, this has the disadvantage that high levels of dissolved electrolyte may remain in the water remaining after flocculated particles of material have been removed.

Electrocoagulation is based upon the use of electrochemical dissolution of an electrode by electrolytic oxidation with OH ^" to form counterions of high charge, at the anodes, which can aid flocculation (typically cations such as Fe or Al for flocculation of fatty particles) without the need for addition of corresponding salt-derived anions into the aqueous dispersion to be treated (typically OH ^" will be the counterions formed in the electrocoagulation process). In parallel with the formation of the cations formed at the anode, gas bubbles (hydrogen) are also formed at the cathode.

For a typical electrocoagulation system, opposed electrodes may be used to provide a voltage difference across one or more sacrificial electrodes positioned between the opposed electrodes, usually with the sacrificial electrodes not electrically connected to each other or to the opposed electrodes other than through the aqueous dispersion. This results in an electrical field being set up across the sacrificial electrodes, causing them to have cathodic and anodic surfaces and causing a current to flow between them and the opposed electrodes, typically with the material of the sacrificial electrodes oxidising and dissolving at the anodic surfaces and hydrogen bubbles being generated at the cathodic surfaces. For instance with sacrificial electrodes of aluminium, aluminium hydroxide is formed at the cathode and can lead to flocculation or co-precipitation of colloidal particles within the aqueous dispersion to be treated.

For removal of dispersed particulate matter from water, particularly fatty particles, the presence of gas bubbles from the cathode, subsequently entrained within the resulting floe of the particulate matter, may assist in removal of the flocculated particulate matter by flotation and bulk separation, as when the matter is fatty in nature it is typically of lower density than water. The additional presence of entrained gas bubbles generated in the electrocoagulation (EC) process may further reduce the density of the floe formed, assisting in speeding separation by flotation of the floe to form a separate layer for subsequent removal to leave purified water.

A problem with electrocoagulation systems is the need to replace or clean the electrodes at intervals as the electrodes become coated with impervious oxide layers or deposits of fatty or other flocculated or co-flocculated material during use (referred to hereinafter as oxide/debris for the sake of brevity). Whilst electrodes are being replaced, the aqueous dispersion to be treated, such as waste water to be purified, may continue to accumulate and so there is a need to provide electrocoagulation apparatus and methods which reduce the need for cleaning of electrodes or which allow for the electrodes to perform adequately for longer periods before cleaning is required, or which do not require cleaning prior to the need to replace the electrodes as a result of their eventual dissolution in use.

SUMMARY

It is one aim of the present invention, amongst others, to provide electrocoagulation methods and apparatus which allow for maintenance of electrodes in a sufficiently clean state for continued operation in use. It is also an aim of the invention to provide electrocoagulation methods and apparatus which address problems known from prior art electrocoagulation systems or which address other problems, such as those mentioned herinafter or otherwise present for electrocoagulation systems. For instance, one aim of the invention is to provide an electrocoagulation system suitable for treatment of waste water streams for which accumulation of waste water cannot be easily halted whilst maintenance is carried out on the electrocoagulation apparatus, making less frequent maintenance stoppages highly desirable. In particular, it is an aim of the invention to provide electrocoagulation apparatus and methods suitable for purification of water by flotation separation of fatty matter from a waste water stream. Another aim of the invention is to provide electrocoagulation apparatus and methods suitable for use in separating particulate matter from an aqueous slurry or dispersion as part of a process for winning and extracting desired materials, such as heavy metals. It is also an aim of the invention to provide an alternative to prior art method and apparatuses. According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

Throughout this specification, the term "comprising" or "comprises" means including the component(s) specified but not to the exclusion of the presence of other components. The term "consisting essentially of or "consists essentially of" means including the components specified but excluding other components except for components added for a purpose other than achieving the technical effect of the invention. The term "consisting of or "consists of means including the components specified but excluding other components.

Whenever appropriate, depending upon the context, the use of the term "comprises" or "comprising" may also be taken to include the meaning "consists essentially of" or "consisting essentially of, and also may also be taken to include the meaning "consists of or "consisting of.

By the term "substantially constant", as used herein, is meant varying by less than +/-3%, preferably less than more preferably less than +/-0.5%, from a predetermined value.

The optional features set out herein may be used either individually or in combination with each other where appropriate, and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention, as set out herein, are also applicable to any other aspects or exemplary embodiments of the invention where appropriate. In other words, the skilled person reading this specification should consider the optional features for each aspect or embodiment of the invention as interchangeable and combinable between different aspects or exemplary embodiments of the invention.

A first aspect of the invention provides a method for applying electrocoagulation treatment to an aqueous dispersion, the method comprising:

a) flowing the aqueous dispersion through a region comprising sacrificial electrodes and located between opposed electrodes,

b) applying a voltage across the opposed electrodes whereby a current is passed between the opposed electrodes through the sacrificial electrodes whereby the sacrificial electrodes donate cations to the aqueous dispersion, and

c) periodically reversing the polarity of the voltage applied across the opposed electrodes with an interval T between the current having zero amplitude at each reversal, wherein following each reversal of polarity, the aqueous dispersion flow rate through the region between the opposed electrodes is arranged to have a value of F _R or more , for a period T _R of 0.05T or more, wherein F _R is 1 .1 F _M or more , and wherein F _M is the mean flow rate between each reversal.

The term electrocoagulation as used herein is also meant to encompass electroprecipitation. The sacrificial electrodes may be of any suitable material for electrochemical dissolution, depending upon the nature of the aqueous dispersion to be treated. Typically, the sacrificial electrodes may be of metal, and may comprise or consist essentially of aluminium or iron (e.g. steel). Aluminium-based electrodes may be particularly useful for the treatment of waste water in order to provide coagulation and coalescence of fatty materials dispersed therein whereby purification by bulk separation of fatty material and purified water may be facilitated. The opposed electrodes may suitably be of a material having a higher resistance to electrochemical dissolution then the sacrificial electrodes. For instance, if the sacrificial electrodes are of aluminium, the opposed electrodes may be of steel. If the sacrificial electrodes are of one grade of steel, the opposed electrodes may be of a different grade of steel, more resistant to electrolytic dissolution than the steel of the sacrificial electrodes.

The term aqueous dispersion as used herein refers to any liquid suitable for application of electrocoagulation treatment, and includes flowable dispersions or slurries of particulate solids or liquids present in a continuous phase of solvent or solution typically including water as a component, for instance with at least 50% water by weight of the solvent or solution. Typically the solvent or solution may be aqueous solvent or solution. The term particle merely means "small portion" and particles may be of liquid or solid, so for instance the oil droplets in an oil- in-water emulsion used as liquid are referred to herein as oil particles dispersed in a continuous aqueous phase. Typically the particles may have a diameter, for instance as measured by light scattering techniques, from 1 to 10,000nm. Typically, for the method of the first aspect of the invention, the sacrificial electrodes are not electrically connected to each other or to the opposed electrodes other than through the aqueous dispersion. For instance, the sacrificial electrodes may be supported between the opposed electrodes by being held in an electrically insulating carrier.

The region comprising the opposed electrodes and sacrificial electrodes may be within a flow- through assembly comprising: sacrificial electrodes retained within a chamber, for instance held within an insulating frame; an inlet port and an outlet port arranged for flow of the aqueous dispersion through the chamber, into the inlet port, over the sacrificial electrodes, and out of the outlet port; a first electrode on an inner face of the chamber and a second electrode positioned opposite to the first electrode, such that the sacrificial electrodes are located between the opposed (first and second) electrodes. In one suitable arrangement, an insulating frame for holding the sacrificial electrodes between the opposed electrodes in use may comprise a pair of opposed jambs or pillars of electrically insulating material having one or more sheets forming the sacrificial electrodes each having opposed edges retained in a respective slot in each opposed jamb. The sheets may typically be rectangular in shape, though this is not essential to the invention. The insulating frame may act as a replaceable cartridge to facilitate rapid replacement of the sacrificial electrodes when they are spent or damaged.

It will be understood that any suitable arrangement may be used for the opposed electrodes, for instance with an inner electrode located within a surrounding outer electrode to provide the opposed electrodes. For instance the inner electrode may be a rod with an outer electrode as a coaxial cylinder surrounding it and the sacrificial electrodes may be cylinders of various diameters coaxially positioned between the opposed electrodes.

The method of the invention involves applying a voltage across the opposed electrodes whereby a current is passed between the opposed electrodes through the sacrificial electrodes whereby the sacrificial electrodes donate cations to the aqueous dispersion. This current passes through the aqueous dispersion and will lead to the sacrificial electrodes having anodic and cathodic surfaces as a result of the applied electrical field. The voltage may be applied, for instance, by means of an electrical power supply arranged across the opposed electrodes. Typically, a voltage of up to 600V, say 1 to 550V may be applied, with a direct current in the range from up to 60 Amperes (A), say 1 to 55A, passing between the opposed electrodes. Usually, a voltage of 200 to 550V may be applied, with a direct current from 5 to 50 A, such as 10 to 25A, passing between the opposed electrodes. In order to prevent excessive build-up of oxide/debris on the sacrificial electrodes, the method of the invention involves periodically reversing the polarity of the voltage applied across the opposed electrodes with an interval T between the current having zero amplitude at each reversal. It will be understood that this switches the cathodic surfaces to become anodic surfaces and vice versa for the opposed electrodes and for the sacrificial electrodes.

Following each reversal of polarity, the aqueous dispersion flow rate through the region between the opposed electrodes is arranged to have a value of F _R or more, for a period T _R of 0.05T or more, wherein F _R is 1 .1 F _M or more. F _M is the mean flow rate between each reversal. In other words, for the period T _R , the flow rate of the aqueous dispersion is increased to a level which is at least 1 .1 times the mean flow rate between reversals, and which may be even more, say up to 6 times the mean flow rate between reversals. The mean flow rate between reversals is simply the time integral of the flow rate as a function of time over the period T, divided by T. Without wishing to be found by any theory, it is thought that the high level of flow rate for the period T _R results in the oxide/debris layer, formed on the opposed and sacrificial electrodes during the previous period T, being removed in a synergistic manner when combined with the reversal of current leading to electrostatic repulsion of the oxide/debris particles on the electrodes.

The flow rate of the aqueous dispersion may, for instance, be controlled by means of a pumping arrangement, such as a pump in a feed line running from a storage tank for the aqueous dispersion to an electrocoagulation chamber holding the electrodes. It will be understood that there may be a time lag between the pumping arrangement increasing the flow rate of the aqueous dispersion and the flow rate actually increasing within the electrocoagulation chamber, for instance determined by the speed of sound through the aqueous dispersion, and for the sake of any doubt, the flow rates referred to herein are the flow rates as measured at the entrance to a chamber holding the electrodes for flow of aqueous dispersion between the electrodes.

The pumping arrangement may be controlled by a controller which is synchronised to the voltage and current reversals between the opposed electrodes. In a preferred arrangement, F _R may be 1 .2 F _M or more such as 1 .3 F _M or more, for instance 1 .5 F _M or more in order to provide better synergistic removal of oxide/debris layers at polarity reversal. However, in order to avoid excessive stress on any chamber holding the electrodes, and to avoid wear to the pumping arrangement, the value of F _R is preferable 5F _M or less, with the aqueous dispersion flow rate preferably not exceeding a value of 6F _M during the period T _R . The time period T _R may by 0.1 T or more, or may be 0.2T or more. However, T _R should be less than 0.5T, preferably less than 0.4T and more preferably less than 0.3T. It will be understood that when the flow rate of liquid is higher, the level of dissolved sacrificial electrode material entering the liquid, for a particular current value, will be lower than it would be when the liquid is flowing at a lower flow rate. This may lead to the undesired consequence of reduced flocculation at constant current.

It will also be understood that the mean flow rate, determined by integration as explained above, includes the flow rate during the time period T _R . The aqueous dispersion flow rate may be varied over the period T _R or may be maintained at a substantially constant value over the period T _R .

Following each period T _R the aqueous dispersion flow rate may be reduced to a substantially constant value F _c over a flow drop period of less than 0.05T and maintained at F _c until an increase in aqueous dispersion flow rate to a value of F _R or more, over a flow rise period of less than 0.05T, to provide an aqueous dispersion flow rate of F _R or more for a next, subsequent period T _R . The period T _R may commence at a time from 0.1 T before, to 0.1 T after, each respective polarity reversal. For instance, the period T _R may commence at reversal (i.e. when the current is zero) or the flow rise period may commence at reversal. Preferably, the period T _R takes place immediately after reversal of the polarity. The interval T is suitably from 1 to 60 minutes, such as from 2 to 30 minutes. Shorter intervals than 1 minute may not allow sufficient time for removal of oxide/debris layers from the electrodes following reversal, whereas intervals longer than 1 hour can lead to excessive consolidation of oxide/debris layers whereby removal is more difficult. For the first aspect of the invention, following each polarity reversal, the amplitude of the current may be controlled to have an amplitude of C _R or more, for a period T _P of 0.05T or more, wherein C _R is 1 .1 C _M or more, and wherein C _M is a mean current amplitude between each reversal. It should be understood that the amplitude of the current is its absolute value or modulus, and so is always positive or zero, irrespective of the direction of flow of current between the opposed electrodes and through the sacrificial electrodes. C _R may be 1 .2 C _M or more, such as 1 .3 C _M or 1 .5 C _M or more. However, C _R is suitably 5 C _M or less, such as 4 C _M or less or 3 C _M or less. It will be understood that the amount of dissolved sacrificial electrode material will depend upon the value of current, so excessively high currents may lead to excessively rapid degradation of the sacrificial electrodes.

Without wishing to be bound by any theory, it is thought that the increase in the current following reversal may assist in repelling oxide/debris from the relevant surfaces of the sacrificial electrodes, and this may act synergistically with the increased flow rate of the invention in order to facilitate removal of oxide/debris following each reversal of current.

The period T _P may be 0.1 T or more, for instance 0.2T or more. T _P may be less than 0.5 T, preferably less than 0.4 T and more preferably less than 0.3T. It will be understood that when the current amplitude is higher, the level of dissolved sacrificial electrode material entering the aqueous dispersion, for a particular current value, will be higher than it would be when the current amplitude is at a lower value.

Preferably, the periods T _R and T _P may run substantially concurrently in order to provide a synergistic effect for removal of oxide/debris and also to assist in maintaining a relatively even level of dissolved material within the aqueous dispersion, with the higher current contributing greater amounts of dissolved material as required for the greater flow rate of aqueous dispersion at the time.

The current amplitude may be maintained at a substantially constant value over the period T _P , or may vary provided it remains in excess of C _R .

Following each period T _P the current amplitude may be reduced to a substantially constant value C _c over a current drop period of 0.05T or less, and maintained at C _c until a subsequent reversal. The current amplitude may be controlled to increase monotonically from zero at reversal to a value of C _R or more within a current rise period of 0.05T or less.

The mean current amplitude between each reversal may suitably be from 5 to 50 Amperes. The power supply providing the voltage and current between the opposed electrodes may be a constant current power supply, configured to provide a desired current as a function of time and arranged to vary the voltage to achieve this depending upon the circumstances, such as the conductivity of the aqueous dispersion between the electrodes. However, it will be understood that such a constant current supply may have to operate within certain voltage limits, say with the voltage being variable from 200 to 600 Volts (V). Should higher or lower voltages be required in order to maintain a constant current, then the power supply may be arranged to maintain the voltage at its boundary value whilst permitting the current to change, rather than attempting to hold the current at a constant value. Additionally, the method may comprise applying a voltage V across the opposed electrodes whereby a current C is passed between the opposed electrodes through the sacrificial electrodes, whereby the sacrificial electrodes donate cations to the aqueous dispersion, wherein the voltage is maintained at or below a value V _max (for instance within 10% V _max such as within 5%) of when the conductivity of the aqueous dispersion is S _min or less and wherein the voltage is allowed to decrease to values less than V _max as the conductivity of the aqueous dispersion increases above S _min .

The voltage between the electrodes may be maintained at a value V _max when the conductivity of the aqueous dispersion is S _min or less and the voltage may be allowed to decrease to values less than V _max when the conductivity of the aqueous dispersion is greater than S _min . In this way, the current passing between the electrodes may be determined by the conductivity of the aqueous dispersion when the conductivity of the aqueous dispersion is S _min or less: as the voltage remains at V _max for these low conductivities, the current will decrease in accordance with Ohms law as the conductivity decreases below the value S _min . For treatment of aqueous dispersions where the electrolyte concentration, and hence conductivity, of the aqueous dispersion, increases or decreases along with the concentration of particulate matter in the aqueous dispersion. When the conductivity of the aqueous dispersion falls below the level S _min , it follows that there will only be low levels of particulate matter required for flocculation and so lower levels of dissolved coagulant are required from the sacrificial electrodes. As the amount of coagulant increases or decreases with the amplitude of the current, at conductivity levels below S _min , the current may be allowed to decrease as the conductivity decreases (i.e. as resistance between the first and second electrodes increases). As the electrical power consumption is (current) ² x resistance, or (voltage) ² / resistance, by not allowing the voltage to exceed V _max , as the conductivity drops below S _min , the power consumption of the electrocoagulation process may be reduced as the current is allowed to decrease when lower levels of coagulant are acceptable.

Suitably, V _max may be from 240 to 520 V. It will be understood that for any particular aqueous dispersion, the skilled person will easily be ably to establish a value for S _min , by simple measurement of the particulate levels following flocculation and separation, in order to ensure that a required level of particulate removal is achieves, for instance so that purified water separated from the aqueous dispersion may meet local requirements for disposal or re-use. A current from C _min to C _max may be passed between the first and second electrodes when the conductivity of the aqueous dispersion in the flow-through cell has a value in excess of S _min , and the current may be allowed to fall below C _min when the conductivity of the aqueous dispersion in the flow-through cell has a value of S _min or less.

In this way, it may be ensured that when the conductivity is above a certain level, and so the level of particulates in the aqueous dispersion is also correspondingly high, the current is maintained at a sufficient level to ensure that an adequate level of coagulant is present for particulate flocculation and separation to subsequently take place, following passage of the aqueous dispersion through the electrocoagulation apparatus.

S _min may be such that the current passed between the first and second electrodes, when the voltage applied is V _max , is from 5 to 20 A. The conductivity of the aqueous dispersion may be measured by a conductivity monitor. Alternatively or additionally, the conductivity of the aqueous dispersion may be derived from measurements of the voltage and current across the first and second electrodes.

The current may maintained at a substantially constant value C _min when the conductivity of the aqueous dispersion is in excess of S _min and the voltage is in excess of a value V _min , and the current may be controlled to increase up to a value C _max to maintain a substantially constant voltage V _min across the first and second electrodes when the conductivity of the aqueous dispersion is in excess of a value (S _max ) such that the current C _min corresponds to the voltage V _min at that conductivity.

The current may be maintained at a constant value C _min over the conductivity range from S _min to S _max , or it may be desirable to control the current and voltage such that the current increases from C _min at a conductivity of S _min to a current up to C _max corresponding to the conductivity of S _max . In one suitable arrangement according to the invention, the electrical power consumption may be maintained substantially constant over the range S _min to S _max , with the current increasing as the electrical resistance between the first and second electrodes decreases as the conductivity increases.

In one exemplary embodiment according to the invention, an electrolyte may added to the aqueous dispersion when the conductivity of the aqueous dispersion prior to electrolyte addition is S _crit or less, but greater than S _min , where S _crit is greater than S _min , whereby the conductivity of the aqueous dispersion in the flow-through cell after electrolyte addition is S _crit or more. This arrangement means that in a situation where the aqueous dispersion still contains high levels of particulates which need to be flocculated, yet has a low conductivity associated with the aqueous portion of the aqueous dispersion such that an excessive electrical power consumption would occur at the desired current for dissolution of adequate flocculant from the sacrificial electrodes, then by the addition of further electrolyte to the aqueous dispersion, it can be arranged that the conductivity of the aqueous dispersion is increased so that an adequate level of current may still be passed through the aqueous dispersion in order to generate sufficient dissolved sacrificial electrode material to provide adequate flocculation, without excessive electrical power being required. The added electrolyte reduces the conductivity of the aqueous dispersion so that a higher current may be passed through the aqueous dispersion without excessive electrical power consumption that would otherwise be associated with such current if the conductivity of the aqueous dispersion had not been decreased by addition of electrolyte. For this exemplary embodiment of the invention, it may be arranged that no electrolyte is added to the aqueous dispersion when the conductivity of the aqueous dispersion prior to electrolyte addition is S _min or less.

Once again, it will be understood that the value chosen for S _crit will depend upon the nature of the particular aqueous dispersion being treated, and S _crit will be easily determinable, for instance by setting an upper limit on electrical power consumption that may be tolerated alongside the maximum particulate levels that are acceptable following flocculation and separation of the purified water from the aqueous dispersion. The electrolyte may be added as a sodium chloride solution having a greater conductivity than

Scrit-

The conductivity of the aqueous dispersion prior to any electrolyte addition may be derived from measurements of the voltage and current across the opposed electrodes and the quantity of any electrolyte added to the aqueous dispersion.

In other words, the voltage may be maintained at a value V _max when the conductivity of the aqueous dispersion is S _min or less and the voltage may be allowed to decrease to values less than V _max as the conductivity of the aqueous dispersion increases. Electrolyte may be added to the aqueous dispersion at low conductivities to further reduce power consumption. This optional feature allows the electrocoagulation process to operate automatically, without operator intervention, over a wide range of particulate levels with reduced electrical power consumption. A second aspect of the invention provides an apparatus for applying electrocoagulation treatment to an aqueous dispersion comprising:

a flow-through chamber comprising opposed electrodes and sacrificial electrodes positioned therebetween;

a power supply arranged to apply a voltage across the electrodes and to control a current therebetween;

a flow control unit arranged to control the flow rate of said aqueous dispersion through the flow-through chamber; and

a controller arranged to control the power supply and the flow control unit to vary the voltage, the current and the flow rate according to the method of the first aspect of the invention.

The optional and preferred features as set out in relation to the first aspect of the invention are also applicable, where appropriate, to this second, apparatus aspect of the invention.

Also disclosed herein is a controller arranged to control the power supply and the flow control unit to vary the voltage, the current and the flow rate for an electrocoagulation apparatus according to the method of the first aspect of the invention. It will be evident that the invention may be put into effect by means of pumps used as flow control units arranged to vary the flow of the aqueous dispersion. The controller may be a programmable computer apparatus programmed to put the method of the invention into effect on a suitable electrocoagulation apparatus. Aspects of the invention may be implemented in any convenient form. For example computer programs may be provided to carry out the methods described herein. Such computer programs may be carried on appropriate computer readable media which term includes appropriate tangible storage devices (e.g. discs). Aspects of the invention can also be implemented by way of appropriately programmed computers, for instance as the controller for use in aspect of the invention.

Also disclosed herein is a method for applying electrocoagulation treatment to an aqueous dispersion, in which the current is varied between each reversal, but the flow rate does not have to be varied. This disclosed method comprises:

a) flowing the aqueous dispersion through a region comprising sacrificial electrodes and located between first and second opposed electrodes

b) applying a voltage across the opposed electrodes whereby a current is passed between the opposed electrodes through the sacrificial electrodes whereby the sacrificial electrodes donate cations to the aqueous dispersion, and c) periodically reversing the polarity of the voltage applied across the opposed electrodes with an interval T between the current having zero amplitude at each reversal, wherein following each polarity reversal, the amplitude of the current is controlled to have an amplitude of C _R or more, for a period T _P of 0.05T or more, wherein C _R is 1 .1 C _M or more, and wherein C _M is a mean current amplitude between each reversal.

C _R may be 1 .2 C _M or more, such as 1 .3 C _M or more, for instance 1 .5 C _M or more. However, C _R is suitably 5 C _M or less, such as 4 C _M or less or 3 C _M or less. It will be understood that the amount of dissolved sacrificial electrode material will depend upon the value of current, so excessively high currents may lead to excessively rapid degradation of the sacrificial electrodes.

Without wishing to be bound by any theory, it is thought that the increase in the current following reversal may assist in repelling oxide/debris from the relevant surfaces of the sacrificial electrodes.

The period T _P may be 0.1 T or more. T _P should be less than 0.5 T, preferably less than 0.4 T and more preferably less than 0.3T. It will be understood that when the current amplitude is higher, the level of dissolved sacrificial electrode material entering the aqueous dispersion, for a particular current value, will be higher than it would be when the current amplitude is at a lower value.

The current amplitude may be maintained at a substantially constant value over the period T _P , or may vary provided it remains in excess of C _R .

Following each period T _P the current amplitude may be reduced to a substantially constant value C _c over a current drop period of 0.05T or less, and maintained at C _c until a subsequent reversal. The current amplitude may be controlled to increase monotonically from zero at reversal to a value of C _R or more within a current rise period of 0.05T or less.

This disclosure may be employed in its own right or may be combined with the method of the first aspect of the invention, as already set out hereinbefore, and this may act synergistically with the increased flow rate of the invention in order to facilitate removal of oxide/debris following each reversal of current.

The aqueous dispersion to be used with the invention may typically be an aqueous flowable liquid (meaning having at least 70% by weight of water present, such as 80% or more, or 90% or more), where the liquid may include contaminants in particulate form, the removal of which is desired, either to purify the water for later use or to extract the contaminant for separation, purification and subsequent re-use. For instance, where a contaminant includes a heavy metal, it may be desirable to both remove the metal from the water for the sake of water re-use and also desirable to extract and separate and purify the metal for recycling purposes. This may also be true where the contaminant is a fatty material, for instance in colloidal form, so that the purified water may be re-used and the separated fat may be recycled as fuel or fodder for animals.

Also disclosed herein is the use of the aspects of the invention when the aqueous dispersion is waste water, contaminated with a fatty materials dispersed therein, so that the electrocoagulation process carried out in the apparatus of the invention may be used to facilitate coagulation and coalescence of the dispersed fatty materials in order to facilitate subsequent bulk separation of the fatty materials from consequently purified water.

Also disclosed herein is the use of the aspects of the invention when the aqueous dispersion is water containing a material to be extracted dispersed therein, so that the electrocoagulation process carried out in the apparatus of the invention may be used to facilitate coagulation and coalescence of the materials to be extracted in order to facilitate subsequent bulk separation of the material for subsequent isolation of the material from consequently purified water. DETAILED DESCRIPTION

For a better understanding of the invention, and to show how exemplary embodiments of the same may be carried into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

Figure 1 schematically depicts a cross-sectional view through an embodiment of an apparatus according to the second aspect of the invention;

Figure 2 shows a graph of flow rate Q as a function of time t for aqueous dispersion flowing through an electrocoagulation apparatus, with the flow controlled in accordance with an embodiment of a method according to the first aspect of the invention;

Figure 3 shows a graph of current amplitude C as a function of time t controlled to pass through an aqueous dispersion flowing through an electrocoagulation apparatus, with the current amplitude controlled to be in accordance with an embodiment of a method according to the first aspect of the invention.

Turning to the embodiment of the first aspect of the invention shown schematically in Figure 1 , a flow-through chamber 1 encloses a pair of opposed electrodes 3 with sacrificial electrodes 4 positioned between the opposed electrodes and not electrically connected to the opposed electrodes 3 other than through the aqueous dispersion 7.

A power supply 2 is arranged to provide a voltage difference between the two opposed electrodes in order to generate an electric field between the electrodes so the current may pass from one electrode to the other opposed electrode through the aqueous dispersion and the sacrificial electrodes 4. A voltage of 200 to 600V may be applied, with a direct current in the range from 15 to 20 A passing between the electrodes controlled in the manner set out below in accordance with the invention.

A flow control unit 5 in the form of an in-line pump draws aqueous dispersion 7 to be treated, such as waste water containing oil or fat particles dispersed therein, particularly colloidal oil or fat particles in the form of an emulsion or Pickering emulsion, from a pre-treatment holding tank 8 and forces it upwards through the flow-through chamber and past and between the opposed and sacrificial electrodes for collection in a post-EC (post-electrocoagulation) holding tank 9.

In this example, the aqueous dispersion 7 may include colloidal particles of fatty matter dispersed in water, which are fed into the pre-EC (pre-electrocoagulation) holding tank 8 from a downstream process in the direction of arrow I. Following electrocoagulation treatment in the flow-through chamber 1 , the colloidal fat particles will flocculate as a result of the presence of the highly charged dissolved material (aluminium hydroxide) from the sacrificial electrodes 4. Following transient collection of the post-EC treated aqueous dispersion 7 in the post-EC holding tank 9 the aqueous dispersion 7 flows in the direction of arrow O to a further processing stage, not shown, in which the flocculated fatty material is separated from the water, leading to purification of the water. This may be achieved, for instance, in a separate settling tank.

The holding tanks 8, 9 allow for the changes in flow rate of aqueous dispersion 7 required for putting the method of the invention into effect to be handled with in a relatively efficient manner with reduced risk of air locks or overflows of aqueous dispersion.

For this embodiment, the opposed electrodes 3 are of steel while the sacrificial electrodes 4 are of aluminium. With such an arrangement, the opposed steel electrodes 3 may endure through many replacement, or refurbished, sets of aluminium sacrificial electrodes 4.

A controller 6 is operably connected to the flow control unit 5 and to the power supply to in order to control these pieces of apparatus to vary the flow rate of the aqueous dispersion 7 through the flow-through chamber and to vary the current amplitude passing between the opposed electrodes 3 through the aqueous dispersion 7 in accordance with the method of the first aspect of the invention. Details are set out below in relation to the graphs shown in Figures 2 and 3. Figure 2 shows the flow rate of 2 of the aqueous dispersion 7 as a function of time t for a method according to the first aspect of the invention. Reversal of the polarity between the opposed electrodes 3 occurs regularly at intervals T as shown in Figure 2 and in Figure 3.

Figure 3 shows the current amplitude C as a function of time t, once again with the current shown as reversing direction at intervals T. Because the graph of Figure 3 shows only the current amplitude, which is always positive, the current amplitude falls to the value C=0 at each of the reversals.

Turning to Figure 2, F _M and F _R are indicated by lines drawn parallel to the time axis, with the value of F _M corresponding to the average or mean flow rate obtained by integrating the area under the flow rate/time curve over period T with F _M corresponding to the value of the area divided by T.

The value of F _R in this example is set as 1 .1 F _M , and, as required by the method of the first aspect of the invention, the flow rate Q is in excess of the value F _R are for a period T _R following each current reversal with T _R being greater than or equal to 0.05 T. After each period T _R , the flow rate Q is controlled by the flow control unit 5 to drop back to a constant value F _c until the next reversal of current. On reversal, the controller again increases the flow rate Q to a value in excess of F _R for a further period T _R .

The actual flow rate will depend upon the circumstances, and suitable flow rates can be used to provide a suitable level of dissolved sacrificial electrode material in order to achieve flocculation. A plurality of electrocoagulation chambers may be used together, for instance connected in parallel.

Although it is not necessary for the current to be varied along with the flow rate in order to meet the requirements of the method of the first aspect of the invention, for this particular embodiment, as shown in Figure 3, the amplitude of the current is also varied along with the flow rate Q in order to provide a synergistic cleaning benefit at each reversal of polarity.

As is shown in Figure 3, the current amplitude C is increased, after each reversal of current (at which the current amplitude falls to 0) to a value in excess of C _R , where C _R equals 1 .1 C _M . Lines corresponding to C _R and C _M have been shown in Figure 3 lying parallel to the time axis. The current amplitude see is maintained at a value in excess of C _R for a period T _P as shown in the Figure. After this period, the current amplitude falls back to a constant value C _c with the mean current amplitude C _M being calculated by integrating the area under the current amplitude/time curve over each period T, and dividing the resulting area by time T in order to arrive at the value for C _M .

For this particular example, the value T _P is a little smaller than T _R in order to take into account the time taken for the current to be ramped up from zero to a value in excess of C _R following each reversal. However, other arrangements where T _P is longer than T _R also fall within the scope of the invention

In a waste-water treatment system for purification of waste-water including colloidal fatty particles, the waste-water, after passing though the apparatus and being subjected to electrocoagulation treatment, may be transferred to a separation chamber, in which the particles of fatty material, now less mutually repulsive as a result of the presence of highly charged cations, may flocculate together to form a floating mass over the remaining purified water in the separation chamber, with the flocculated mass also including entrapped gas generated in the electrocoagulation process. The purified water may be flowed out of the separation chamber for storage, further treatment or disposal, with the floating mass removed be a suitable means such as surface scraping or overflow to a fat collection chamber.

In summary, the invention provides methods and apparatus for applying electrocoagulation treatment to an aqueous dispersion includes flowing the aqueous dispersion through a region comprising sacrificial electrodes between opposed electrodes and applying a voltage across the opposed electrodes to pass a current through the sacrificial electrodes causing donation of cations to the aqueous dispersion. The voltage polarity is alternated at intervals T periodically. Following each reversal of polarity, the aqueous dispersion flow rate through the region between the electrodes is arranged to be in excess of a value F _R , for a period T _R of at least 0.05T wherein F _R is greater than 1 .1 F _M , wherein F _M is the mean flow rate between each reversal. This arrangement reduces oxide build-up on the electrodes and sacrificial electrodes and reduces maintenance requirements.

Although a few exemplary embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention, as defined in the appended claims.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.