WO2015079205A1

WO2015079205A1 - Method and plant for treatment of dispersion

Info

Publication number: WO2015079205A1
Application number: PCT/GB2014/053463
Authority: WO
Inventors: Daniel Thomas Exley RITCHIE; Roger Nicholas HENSBY
Original assignee: Surewaters Consultancy Limited
Priority date: 2013-11-29
Filing date: 2014-11-25
Publication date: 2015-06-04
Also published as: GB201321104D0; GB2520743A

Abstract

A method and plant for separation of an aqueous dispersion of particles into separated particles and aqueous solution involves using electrocoagulation treatment followed by removing the particles as a flocculated component. The remaining solution is filtered and each filter is cleaned by halting filtration, and sending a reverse flow of cleaning solution, for a predetermined time, through the filter, to dislodge sediment deposited on the filters, when the pressure monitored at the inlet to the filter reaches a predetermined value. The filtered solution may be recirculated as cleaning solution and the solution containing dislodged sediment may be recirculated to be trapped in the flocculated component, allowing for a self-contained self-cleaning system.

Description

Method and Plant for Treatment of Dispersion

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 particularly concerned with improvements to electrocoagulation treatment when applied to purification of effluent streams of varying concentrations or to extraction of materials from aqueous dispersions by flotation.

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.

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.

A problem with plants based on electrocoagulation systems, accompanied by floc-separation apparatus, is that although floc-separation is an effective way to separate flocculated particles to leave a remaining, clarified aqueous solution, some fine particulate material may remain dispersed within the remaining clarified aqueous solution.

Although such material could be removed from the clarified aqueous solution by filtration, there may be a problem that the filters used may rapidly become blocked, so that the filters may subsequently no longer pass the clarified aqueous solution at a sufficiently rapid rate to match the desired flow of aqueous dispersion through the plant.

When the floe is a waste that needs to be separated from an effluent stream to generate purified water for disposal, particles of waste remaining in the water may lead to contamination levels that may prevent the water being disposed of in the environment or local sewage systems or lead to damage if the water is allowed to enter the environment or local sewage system. Alternatively, re-use of the water for low-grade functions (such as toilet flushing, garden watering) may be prevented if the contamination levels are too high. When the floe is used for extraction of desired materials from an aqueous slurry, such as in the ore processing industry, then failure to capture all of the particles may lead to unnecessary waste and poor extraction efficiency. Hence, there is a need for improvement in methods and apparatus the removal of fine particulate materials from aqueous dispersions.

SUMMARY

It is one aim of the present invention, amongst others, to provide electrocoagulation-based methods and plant which allow for efficient flocculation of particulates from aqueous dispersions or slurries and efficient and rapid separation of flocculated material from remaining aqueous solution. 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 both dilute and concentrated waste water streams from plant operations, such as food plant operations, abattoirs, polymer recycling plants and the like. 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 electrocoagulation method and apparatus which may be used continuously to treat effluent streams of varying concentrations and which reduce or eliminate risk of polluting waste entering the public sewage system or environment. It is an aim of the invention to provide methods and plant which are capable of treating aqueous dispersions of particles dispersed in aqueous solution in order to separate them into a component comprising the particles and a component comprising the aqueous solution in a continuous fashion, without frequent stoppage for cleaning or maintenance, whilst also providing low levels of particulate material, even fine particulate material present in the resulting aqueous solution component. It is also an aim of the invention to provide an alternative to prior art apparatuses and methods.

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.

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 separation of an aqueous dispersion of particles into a first component comprising the particles and a second component comprising a filtered, clarified aqueous solution, the method comprising sequentially: a) subjecting the aqueous dispersion to electrocoagulation treatment by flowing the aqueous dispersion through a region comprising sacrificial electrodes and located between opposed electrodes, and applying a voltage across the opposed electrodes to pass a current through the sacrificial electrodes whereby the sacrificial electrodes donate cations to the aqueous dispersion to promote formation of a flocculate comprising the particles; b) collecting the flocculate comprising the particles as a flocculated layer floating on a clarified aqueous solution and separating the flocculate from the remaining clarified aqueous solution; and c) filtering the remaining clarified solution by filtration through a filtration apparatus comprising a first filter; the method further comprising: d) causing the clarified solution to flow into the filtration apparatus from a first location, through a first valve, and then through the first filter, to a second location, in a first direction of flow, whilst monitoring a pressure of the clarified solution at a first pressure monitor located prior to passage through the filter, to provide filtered clarified aqueous solution at the second location when the pressure is less than a first value, e) when the pressure is greater than or equal to a first value, halting the flow of the clarified solution through the first filter and causing a cleaning solution to flow through the first filter, and then through the first valve, from a third location, in a second direction of flow, opposite to the first direction of flow, and wherein the first valve is arranged to divert the cleaning solution to a fourth location when flowing in the second direction, and f) after a predetermined period of time, halting the flow of the cleaning solution and causing the clarified aqueous solution to flow again in the first direction of flow, from the first location to the second location, through the first valve and the first filter, whilst monitoring the pressure of the clarified aqueous solution at the first pressure monitor. The sacrificial electrodes used for electrocoagulation 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 (i.e. of an alloy comprising aluminium as a major component) 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 (first and second) 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. 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. Such particles may be referred to as colloidal particles. The invention is of use for particles removed by flotation following flocculation, wherein the particles may have a density less than that of water or less than that of the aqueous solution in which they are originally dispersed. However, the presence of entrained gas from the electrocoagulation process may allow the invention to be applicable to particles of higher density, with the gas bubbles entrained with the particles to provide flotation. The invention is particularly useful when the particles are oils or fats such as natural fat from a renewable resource (such as an animal or vegetable fat or oil) or petrochemical oil and/or fat and/or wax. The term fat is used herein to describe such materials and typically, fats will have a density which is less than that of water.

For the method of the first aspect of the invention, the sacrificial electrodes are typically 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 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. In order to prevent excessive build-up of oxide/debris on the sacrificial electrodes, the method of the invention may involve 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. 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. 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 55 A, 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.

The filtration is arranged to take place downstream of floc-separation and is arranged to remove fine particulate solids from the clarified aqueous solution.

The particulate filters may suitably be configured to remove particulate material having a diameter of 2 μηι or more such as 5 μηι or more, say 10 μηι or more. For instance, the particulate filters may have an absolute pore diameter (the diameter of the smallest sphere capable of passing the filter), of from 2 to 500 μηι, such as from 5 to 200 μηι. Further filters may additionally be present downstream of the particulate filters, such as for ultrafiltration or reverse osmosis filtration.

Steps (e) and (f) of the first aspect of the invention, in which the flow direction through the filter of the filtration apparatus is reversed in order to clean the filter by removing sediment deposited on the filter, may be repeated each time the pressure of the clarified solution, prior to passage through the first filter, attains or exceeds the first value. It will be understood that the first value may be chosen, based upon experience of the particular type and nature of filter and aqueous dispersion, to be a value at which the flow rate of the clarified aqueous solution through the filter has been reduced, by the filter clogging, to an unacceptably low rate, indicating the need for the filter to be cleaned by reversal of flow through the filter. The flow rate used when the filter is being cleaned may be higher than the flow rate when the filter is used for filtration, for instance 2x higher or more, such as 5x higher or more, in order to better dislodge sediment. The second location may be a holding tank for filtered clarified aqueous solution, and may be the same as the third location, such that the cleaning solution is filtered clarified aqueous solution which has already passed at least once through the filtration apparatus in the first direction. In other words, the outlet for filtered clarified aqueous solution from a plant operating according to the method of the invention may include a holding tank which can be used to recirculate a portion of the filtered clarified aqueous solution back for use as a cleaning solution to be passed back through the first filter to unblock it. This gives the benefit that the plant using the method of the invention may be self-contained and not require an additional water supply for filter cleaning. The first location may suitably be an outlet for clarified aqueous solution from a floc-separation apparatus arranged for separating the flocculate comprising the particles from the clarified aqueous solution and the fourth location may be an inlet to the floc-separation apparatus. By an outlet from a floc-separation apparatus is meant either a direct outlet, or an intermediate location, such as a temporary storage tank fed from an outlet of the floc-separation apparatus. In other words, the clarified aqueous solution from the first location may pass through intermediate stages on its way to the first filter. This arrangement provides the advantage that sediment which is cleaned from the filter when flow is reversed in step (d) is returned to the floc-separation tank, where at least a proportion, if not all, of the dislodged sediment may be captured in the separated flocculated particles, and so prevented from returning back through the filtration apparatus to potentially cause further clogging of the filter. This arrangement means that the plant is capable of self-cleaning in an efficient manner, with the dislodged sediment generated during cleaning of the filter by reverse flow is returned to the component comprising the separated particles. This avoids the need to dispose of a separate waste stream which would contain the dislodged sediment and the cleaning solution. Instead, this waste stream is incorporated into the separated particles and the clarified aqueous solution from the floc-separation apparatus.

The floc-separation apparatus may, for instance, be a settling tank arranged for collecting the flocculate comprising the particles as a flocculated layer floating on a clarified aqueous solution and for separating the flocculate from the remaining clarified aqueous solution and the fourth location may be an inlet to the settling tank. In order to better ensure capture of the dislodged sediment generated by the reverse-flow cleaning of the filter, the fourth location, as an inlet of the settling tank, may be positioned to encourage trapping of the dislodged sediment in the floating flocculated layer, for instance by locating the inlet above the floating flocculated layer whereby the sediment is filtered out by the flocculated floating layer, or by positioning the fourth location as inlet below the floating layer, but above the inlet for the aqueous dispersion entering for separation, whereby the rising, flocculating particles from the aqueous dispersion, after electrocoagulation treatment, are likely to entrap the dislodged sediment particles from the filter-cleaning as they enter the settling tank at the fourth location as inlet.

In one preferred embodiment, the cleaning solution containing dislodged sediment may be mixed or blended with the aqueous dispersion prior to the entry of the aqueous dispersion into the settling tank.

The filtration apparatus may comprises a plurality of filters, each having a respective valve and a respective pressure monitor, and wherein the steps (d), (e) and (f) are applied, mutatis mutandis, with respect to each filter. In other words, in addition to the first filter as discussed above, the filtration apparatus may have one or more further filters, each arranged for reverse- flow cleaning in the same manner as the first filter, when the pressure at their respective inlet reaches or exceeds a predetermined value. This multiplicity of filters means that the separation process may continue uninterrupted whilst one or more of the filters is being subject to reverse-flow cleaning, by the flow continuing through one or more of the other additional filters.

When the flow of the clarified solution in the first direction through a filter is halted, the flow may be diverted to pass though the other filters for which flow is not halted.

The method of the first aspect of the invention may further comprises blending an aqueous solution of a gas with the aqueous dispersion of particles, prior to separating the flocculate from the remaining clarified aqueous solution; wherein the aqueous solution of gas is arranged to be supersaturated with the gas on blending with the aqueous dispersion.

The aqueous solution of gas may be formed by subjecting an aqueous solution, or water, to the gas at a pressure that is higher than the pressure of the aqueous dispersion upon blending with the solution of gas. By the term "supersaturated" it is meant that the gas is present at a concentration in excess of its solubility under the conditions after blending with the aqueous dispersion, with the consequence that gas bubbles may be nucleated as the gas comes out of solution following blending with the aqueous dispersion. Typically this may be achieved by the gas bubble generation apparatus comprising a chamber arranged for dissolving a gas into aqueous solution under pressure (i.e. with the gas at a pressure in excess of atmospheric pressure, such as at 2 Bar or more, for instance 5 Bar or more, to form the aqueous solution of gas solution in a supersaturated state). Any suitable gas which is soluble in water may be employed as the gas, for instance carbon dioxide, nitrogen or air. Air is a preferred gas as compressed air is readily available commercially or may be generated in situ, by means of an air compressor.

The aqueous solution of gas may be formed from a recirculated portion of the clarified aqueous solution resulting from the separation of the flocculated layer of particles from the aqueous dispersion, i.e. by using the recirculated portion as solvent for the gas.

It will be understood that at start-up of a method according to the invention, there may be no clarified aqueous solution available from the process of the method of the invention, in which circumstance, either water may be used to provide the aqueous solution of gas for start-up, or the formation of aqueous solution of gas may be delayed until clarified water is available from the process operating initially without blending of the aqueous solution of gas and the aqueous dispersion. The use of the aqueous gas solution may be particularly helpful in ensuring that dislodged sediment generated by the reverse cleaning of the filters can be entrained into the separated flocculated particulate component from floc-separation. This may be achieved by blending the aqueous gas solution with the aqueous dispersion near the fourth location, for instance at an inlet to the floc-separation apparatus, such as a settling tank, whereby gas bubbles generated from the supersaturated aqueous gas solution may attach to the dislodged sediment and encourage its flotation and incorporation into a layer of floating, flocculated particles.

A second aspect of the invention provides a plant for separation of dispersed particles from an aqueous dispersion of particles into a first component comprising the particles and a second component comprising a filtered, clarified aqueous solution, the apparatus comprising: i) a flow-through assembly for electrocoagulation treatment of the aqueous dispersion, the assembly comprising:

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

a power supply arranged to apply a voltage across the electrodes and to cause a current to flow therebetween through said aqueous dispersion in use; ii) a floc-separation apparatus arranged for separation and removal from a remaining clarified aqueous solution of a flocculated layer formed by flocculation of said particles after electrocoagulation treatment of said aqueous dispersion; iii) a filtration apparatus, arranged downstream of the floc-separation apparatus, and comprising a filter arranged to remove fine particulate solids from the clarified aqueous solution, and iv) a controller arranged to control flow through the filtration apparatus; wherein the controller is arranged to cause the clarified solution received from the floc- separation apparatus to flow, through a first valve, and then through the first filter, to a second location, in a first direction of flow, whilst monitoring a pressure of the clarified solution at a first pressure monitor located upstream of the filter, to provide filtered clarified aqueous solution at the second location, when the pressure is less than a first value; and the controller is arranged to cause a cleaning solution to flow through the first filter, and then through the first valve, from a third location, in a second direction of flow, opposite to the first direction of flow, when the pressure attains or exceeds the first value and for a predetermined period of time thereafter, whilst halting the flow of the clarified solution through the first filter, and arranging the first valve to divert the cleaning solution to a fourth location when flowing in the second direction; and when the predetermined time period has elapsed, the controller is arranged to halt the flow of the cleaning solution and causing the clarified aqueous solution to flow again in the first direction of flow, from the floc-separation apparatus to the second location, through the first valve and the first filter, whilst monitoring the pressure of the clarified solution at the first pressure monitor.

The second location may be a holding tank adapted for retaining filtered clarified aqueous solution, and may be the same as the third location, such that the cleaning solution is filtered clarified aqueous solution which has already passed at least once through the filtration apparatus. As already explained above, the fourth location may advantageously be an inlet to the floc-separation apparatus whereby at least some the sediment dislodged from the filter may be entrapped within the separated particulate flocculate.

The filtration apparatus may comprises one or more further filters in addition to the first filter, with each further filter having a respective valve and a respective pressure monitor, and wherein the controller is arranged to control flow through the further filters in the same manner that it is arranged to control flow through the first filter, mutatis mutandis. When the controller is arranged to halt the flow of the clarified solution in the first direction through one or more of the filters, the controller may also be arranged to divert the flow of the clarified aqueous solution to pass though other filters for which flow is not halted The plant of the second aspect of the invention may include a plurality of flow-through assemblies for electrocoagulation treatment arranged in parallel, with the controller is arranged to direct the waste water flow through each flow-through assembly according to demand and/or a maintenance schedule. Hence, if the passage through an electrocoagulation (EC) flow-through assembly is a rate determining step at certain times, the controller may switch the plant to utilise two or more flow-through electrocoagulation assemblies. When one of the flow- through electrocoagulation assemblies has become inefficient over a usage period, for instance due to need for cleaning or for replacement of sacrificial electrodes, then the flow may be switched from the inefficient EC flow-through assembly to a different EC flow-through assembly so that the clogged EC flow-through assembly may be cleaned without halting the continued operation of the waste stream treatment plant.

As explained in relation to the method of the first aspect of the invention, the plant may also comprise a gas solution generation apparatus arranged to generate an aqueous solution of a gas for blending with the aqueous dispersion at, or prior to, entry of the aqueous dispersion into the floc-separation apparatus.

The gas solution generation apparatus may comprise a chamber arranged for dissolving the gas into an aqueous solution or water, with the gas at a pressure in excess of atmospheric pressure, to form the aqueous solution of gas in use.

The gas solution generation apparatus may be arranged to form the aqueous solution of gas from a portion of the clarified aqueous solution output from the floc-separation apparatus in use.

The apparatus of the second aspect of the invention may further comprise a pump arranged for transferring said aqueous solution of gas for blending with said aqueous dispersion through a valve arranged to maintain the aqueous solution of gas at a pressure higher than the pressure of said aqueous dispersion on blending.

The valve may be positioned to blend the aqueous solution of gas with the aqueous dispersion after the aqueous dispersion has passed through the flow-through assembly for electrocoagulation but before, or at, entry into the floc-separation apparatus. Suitably, the valve may be arranged to blend the aqueous solution of gas with the aqueous dispersion at or near the fourth location so that aqueous solution comprising dislodged sediment from the filter is also blended therewith. The controller may be arranged to control the power supply to vary the voltage and the current for electrocoagulation treatment and may also be arranged to control the flow of aqueous dispersion through the flow-through cell(s) of the electrocoagulation assembly, for instance by controlling pumps arranged to pump the aqueous dispersion. The controller may also be arranged to control the blending of any aqueous gas solution with the aqueous dispersion, for instance by controlling the pumps and valves involved in effecting such blending.

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 may also be implemented by way of appropriately programmed computers, for instance as the control unit for use in aspect of the invention. 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 schematic representation of an embodiment of a waste stream treatment plant according to the second aspect of the invention and employing the method of the first aspect of the invention; Figure 2 schematically depicts a more detailed cross-sectional side view of the flow- through electrocoagulation (EC) assembly for use in the apparatus of the first embodiment as shown in Figure 1 .

Common reference numerals have been used throughout the Figures, and in the description, as set out below, reference is made to the same embodiment of the invention with the various features of the embodiment illustrated in the Figures. For the sake of clarity, connections between the controller 10 and the various pumps P, meter 7, pressure monitors 52, 53, power supply 27 and flow control valves 12, 22, 41 , 42, 44, 45, 46 have not been shown in the Figures. It should be understood that such connections will be present in the embodiment as set out in the Figures, even though not indicated directly, and the connections may be implemented as hard-wired connections, wireless connections or a mixture of these.

Turning to Figure 1 , which shows a schematic depiction of an embodiment of a waste treatment plant operating according to the invention, a waste water stream enters the plant of the embodiment at an inlet I from an adjacent manufacturing site. In this embodiment, the aqueous dispersion may include comprises fatty particles in addition to other solid particles and electrolyte in aqueous solution. The waste water stream is first passed through a pre- treatment apparatus which consists of a skimming tank 1 for removal of oil from the waste water stream by skimming a floating oil layer 2 from the waste water stream. This is indicated in Figure 1 by an oil collection gutter 3 which removes the surface layer of floating oil 2 and any other matter trapped in the floating layer of oil by collecting the oil from the surface of the skimming tank l and draining it to waste W. The remaining aqueous dispersion, of particles dispersed in an aqueous solution, is transferred from the skimming tank 1 by a pump P1 to a nozzle 37 through which it is sprayed onto a screening apparatus 6 having a cylindrical sieve in the form of a drum 4 arranged to rotate about an axle 5.

Coarse particulate matter is captured on the outer surface of the rotating sieve drum 4 and jets of water 32 are emitted from the axle 5 in order to dislodge the coarse particulate matter collected on the outer face of the sieve drum 4 so that the particles can be washed to waste W.

From the screening apparatus 6, the aqueous dispersion, now containing predominantly colloidal particles, is transferred by a further pump P2 through a pH and conductivity meter 7 into a balancing tank 1 1 . The balancing tank 1 1 is provided with a recirculation pump P3 and reagent dosing tanks 8, 9 are positioned to pump reagent (i.e. solutions of chemicals in this case) into the balancing tank 1 1 through pumps P8 and P9. The dosing of the reagents from dosing tanks 8, 9 via the pumps P8, P9 is controlled by the controller 10 in response to the values of pH and conductivity measured by the meter 7 and transmitted from the meter 7 to the controller 10. As explained below, the controller 10 may also dose the reagents in response to the measured fat content of the output clarified aqueous solution from the plant, for instance to increase conductivity so that more current can be supplied to the electrocoagulation treatment in order to increase flocculation without excessive power drain. A baffle, 33 is positioned at the outlet to the balancing tank 1 1 in order that the circulation pump P3 mixes the reagents with the aqueous dispersion prior to exit from the balancing tank 1 1 . The baffle is positioned to present direct flow of reagent from the dosing tanks 8, 9 through the outlet of the balancing tank 1 1 . The aqueous dispersion is transferred from the balancing tank 1 1 by pump P1 1 through a flow control valve 12 which determines, under the control of controller 10, through which of two flow-through electrocoagulation chambers 13, 14, the aqueous dispersion will flow. A power supply 27 is provided to supply a voltage across the opposed electrodes 28 of the flow-through electrocoagulation chambers 13, 14 and this is shown in more detail in Figure 2 for the flow- through electrocoagulation chamber 13. For this embodiment, the opposed electrodes 28 are of steel while the sacrificial electrodes 29 are of aluminium. With such an arrangement, the steel electrodes may endure through many replacement, or refurbished, sets of aluminium sacrificial electrodes.

A voltage is applied, by power supply 27, across the opposed electrodes 28 and the resulting electric field causes the sacrificial electrodes 29 to have cathodic and anodic surfaces, with the material of the sacrificial electrodes oxidising and dissolving at the anodic surfaces and hydrogen bubbles being generated at the cathodic surfaces. Typically, a voltage of 50 to 600V may be applied, with a direct current in the range from 5 to 20 A passing between the opposed electrodes 29 and through the sacrificial electrodes. In order to prevent excessive build-up of oxide on the sacrificial electrodes, the direct current may be reversed at intervals in order to switch the cathodic surfaces to become anodic surfaces and vice versa. The controller 10 controls the aqueous dispersion to flow through either one of, or both of, the flow-through electrocoagulation chambers 13, 14 depending upon circumstances, such as total volume of waste stream entering at the inlet I, the condition of the electrodes (e.g. whether cleaning is required or whether the sacrificial electrodes 29 are nearly spent), or optionally the fat content of the outlet aqueous solution from the plant.

From the flow-through chambers for electrocoagulation treatment 13, 14, the aqueous dispersion, now also comprising hydrogen bubbles and dissolved cations from the sacrificial electrodes 29, passes to a floc-separation apparatus 15 in the form of a settling tank 15 through which the aqueous dispersion, following the electrocoagulation treatment, gently flows from an inlet 38 to an outlet 35 in order to allow time for the fat particles dispersed within the aqueous dispersion to flocculate and so to form a flocculated fat-containing layer 16 of lower density than the remaining clarified aqueous solution 17.

The settling tank is provided with a blade 18 arranged to move to-and-fro while positioned over the surface of the settling tank 15 in order to scrape the flocculated fat-containing layer 16 over a rim of the settling tank into a sump 30 for collection and disposal to waste W.

In addition to the outlet 35 arranged for the exit of the bulk of clarified aqueous solution 17 from the settling tank 15, a further outlet 34 is provided in the base of the settling tank 15, positioned to collect sediment, so that any sediment which is denser than the clarified aqueous solution will collect at this outlet 34 and can be removed at intervals using pump P12 to go to waste W. From the outlet 35, a pump P 13 transfers the clarified aqueous solution 17 to a filtration tank 36 and from the filtration tank 36 the clarified aqueous solution 17 is pumped through one or more of the particulate filters 40, 43, by pumps P36, P37, in accordance with flow control valves 41 , 44, which are controlled by the controller 10 to divert the clarified aqueous solution 17 to the particulate filters 41 , 43. The pressure monitors 52, 53 measure the pressure at the inlets to filters 41 , 43 respectively and provide the pressure measurement value to controller 10.

Referring first to filter 40, when the pressure measurement value from pressure monitor 52 is less than a preset value, indicating that the filter is not blocked by sediment, then the flow control valve is set by controller 10 for the clarified aqueous solution 17 to be pumped by pump P36 through the filter 40 and out through flow control valve 42, with pump P40 set by the controller to allow passage of the resulting filtered clarified aqueous solution 51 to pass therethrough. Flow control valve 42 is set to direct the flow of the filtered clarified aqueous solution 51 to a holding tank 50, where a portion is retained and a portion allowed to flow to the outlet of the plant S under control of flow control valve 47 under control of the controller 10. The holding tank 50 includes a level monitor 48 which transmits level information to the controller 10 so that it can open flow valve 47 when the holding tank 50 is full so that excess solution flows out through the plant outlet S.

As the filter 40 gradually becomes blocked as a result of build-up of sediment, the pressure value measured at pressure monitor 52 increases until a predetermined value is attained. When the pressure monitor 52 transmits this to the controller 10, the controller 10 switches flow control valves 41 and 42 and pumps P36 and P40 in order to to halt flow of the clarified aqueous solution 17 from the tank 36 through filter 41 , and instead pump P40 draws filtered, clarified aqueous solution from holding tank 50 and directs it through flow control valve 42 to flow in the opposite direction through the filter, in order that the deposited sediment may be dislodged by the reverse-flow cleaning solution to clean and unblock the filter 40. Flow control valve 41 is set by the controller to divert the solution containing dislodged sediment to a flow control valve 46 at the inlet to the settling tank 15 where the solution containing dislodged sediment is blended with the aqueous dispersion on entering the settling tank 15. By this arrangement, the dislodged sediment may become entrapped in the floating flocculated particulate layer 16 which is separated and sent to waste. After a predetermined time, the controller 10 returns the pumps P36, P40 and flow control valves 41 , 42 to their previous configurations so that once again the clarified aqueous solution 17 is pumped by pump P36 through the filter 40 and out through flow control valve 42 as filtered clarified aqueous solution 51 to pass to the holding tank 50. Referring now to the second filter, 43, this is operated in the same manner as the first filter 40, but using pressure monitor 53, flow control valves 44 and 45 and pumps P43, P47 in an analogous manner to the use of their equivalents 52, 41 , 42, P36 and P40 in relation to the first filter 40.

From the outlet of the plant S, the filtered clarified aqueous solution may be directed to one or more of: i) re-use within the factory or manufacturing site from which it came (e.g. for low grade use such as toilet flushing), or ii) into the local sewage system, or iii) into the environment.

A portion of the filtered clarified aqueous solution 51 is directed from the holding tank 50 to a pressure tank 23 through the pump P22. A gas source 24 provides pressurised gas 25 over clarified aqueous solution 26 held in the pressure tank 23, and in this embodiment a gas pressure of 5 bar is used in order to dissolve the gas (in this case air) into the filtered clarified aqueous solution 26 to form a supersaturated aqueous gas solution 26. Pump P26 is arranged to pump the resulting supersaturated aqueous gas solution through pressure control valve 31 under the control from the controller 10 in order to blend the supersaturated aqueous gas solution with the aqueous dispersion at the pressure control valve 31 before the aqueous dispersion enters the floc-separation tank 15.

As explained hereinbefore, the blending of the supersaturated aqueous gas solution with the aqueous dispersion 17, which already includes hydrogen bubbles generated during the electrocoagulation process, results in improved flotation and separation of the fat-containing layer 16 following flocculation, and it is thought that this may be due to improved nucleation of gas bubbles of a suitable size and their incorporation within the flocculated fat containing layer 16, improving its buoyancy and so improving rate of separation from the remaining clarified aqueous solution 17. In particular, when there is also dislodged sediment present at the inlet to the settling tank, arising from the reverse-flow filter cleaning as set out hereinbefore, the presence of the additional gas from the aqueous solution of gas may assist in flotation f the dislodged sediment leading to its entrapment within the floating layer 16.

The plant according to the invention may also include a meter (not shown) to monitor the remaining fat content of the clarified aqueous solution, for instance a clarity meter, such as a nephelometer, arranged to measure the turbidity of the solution at the exit S, and the controller 10 may adjust the electrocoagulation conditions (current/voltage) and may adjust the reagent dosing at the balancing tank 11 in order to control the fat content to meet a specific requirement whilst minimising the electrical power input used in the electrocoagulation assembly.

In summary, the method and plant of the invention allow separation of an aqueous dispersion of particles into separated particles and aqueous solution using electrocoagulation treatment followed by removing the particles as a flocculated component. The remaining solution is filtered and each filter is cleaned by halting filtration, and sending a reverse flow of cleaning solution, for a predetermined time, through the filter, to dislodge sediment deposited on the filters, when the pressure monitored at the inlet to the filter reaches a predetermined value. The filtered solution may be recirculated as cleaning solution and the solution containing dislodged sediment may be recirculated to be trapped in the flocculated component, allowing for a self-contained self-cleaning system.

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. For instance, although in this embodiment the invention has been described with reference to a waste treatment plant, where the aqueous dispersion to be treated contains fatty particles, it will be understood that the invention would also be applicable to a plant intended for extraction of particulate compounds or elements from an aqueous ore dispersion by flotation.

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.

Claims

1 . A method for separation of an aqueous dispersion of particles into a first component comprising the particles and a second component comprising a filtered, clarified aqueous solution, the method comprising sequentially: a) subjecting the aqueous dispersion to electrocoagulation treatment by flowing the aqueous dispersion through a region comprising sacrificial electrodes and located between opposed electrodes, and applying a voltage across the opposed electrodes to pass a current through the sacrificial electrodes whereby the sacrificial electrodes donate cations to the aqueous dispersion to promote formation of a flocculate comprising the particles; b) collecting the flocculate comprising the particles as a flocculated layer floating on a clarified aqueous solution and separating the flocculate from the remaining clarified aqueous solution; and c) filtering the remaining clarified solution by filtration through a filtration apparatus comprising a first filter; the method further comprising: d) causing the clarified solution to flow into the filtration apparatus from a first location, through a first valve, and then through the first filter, to a second location, in a first direction of flow, whilst monitoring a pressure of the clarified solution at a first pressure monitor located prior to passage through the filter, to provide filtered clarified aqueous solution at the second location when the pressure is less than a first value, e) when the pressure is greater than or equal to a first value, halting the flow of the clarified solution through the first filter and causing a cleaning solution to flow through the first filter, and then through the first valve, from a third location, in a second direction of flow, opposite to the first direction of flow, and wherein the first valve is arranged to divert the cleaning solution to a fourth location when flowing in the second direction, and f) after a predetermined period of time, halting the flow of the cleaning solution and causing the clarified aqueous solution to flow again in the first direction of flow, from the first location to the second location, through the first valve and the first filter, whilst monitoring the pressure of the clarified aqueous solution at the first pressure monitor.

2. A method according to claim 1 wherein steps (d) and (e) are repeated each time the pressure of the clarified solution, prior to passage through the first filter, is equal to or greater than the first value.

3. A method according to claim 1 or claim 2 wherein the second location is a holding tank for filtered clarified aqueous solution, and is the same as the third location, such that the cleaning solution is filtered clarified aqueous solution which has already passed at least once through the filtration apparatus.

4. A method according to any preceding claim wherein the first location is an outlet for clarified aqueous solution from a floc-separation apparatus arranged for separating the flocculate comprising the particles from the clarified aqueous solution and the fourth location is an inlet to the floc-separation apparatus.

5. A method according to claim 4 wherein the floc-separation apparatus is a settling tank arranged for collecting the flocculate comprising the particles as a flocculated layer floating on a clarified aqueous solution and for separating the flocculate from the remaining clarified aqueous solution and the fourth location is an inlet to the settling tank.

6. A method according to any preceding claim wherein the filtration apparatus comprises a plurality of filters, each having a respective valve and a respective pressure monitor, and wherein the steps (c), (d) and (e) are applied, mutatis mutandis, with respect to each filter.

7. A method according to claim 6 wherein, when the flow of the clarified solution in the first direction through a filter is halted, the flow is diverted to pass though the other filters for which flow is not halted.

8. A plant for separation of dispersed particles from an aqueous dispersion of particles into a first component comprising the particles and a second component comprising a filtered, clarified aqueous solution, the apparatus comprising: i) a flow-through assembly for electrocoagulation treatment of the aqueous dispersion, the assembly comprising:

9. A plant according claim 8 wherein the second location is a holding tank adapted for retaining filtered clarified aqueous solution, and is the same as the third location, such that the cleaning solution is filtered clarified aqueous solution which has already passed at least once through the filtration apparatus.

10. A plant according to claim 8 or claim 9 wherein the fourth location is an inlet to the floc- separation apparatus.

1 1 . A plant according to any one of claims 8 to 10 wherein the filtration apparatus comprises one or more further filters in addition to the first filter, each further filter having a respective valve and a respective pressure monitor, and wherein the controller is arranged to control flow through the further filters in the same manner that it is arranged to control flow through the first filter, mutatis mutandis.

12. A plant according to claim 1 1 wherein, when the controller is arranged to halt the flow of the clarified solution in the first direction through one or more of the filters, the controller is also arranged to divert the flow of the clarified aqueous solution to pass though other filters for which flow is not halted.

13. A method or plant substantially as hereinbefore described with reference to and as shown in the accompanying figures.