GB1560732A - Electrolytic waste water purification method and apparatus - Google Patents

Description

(54) ELECTROLYTIC WASTE WATER PURIFICATION METHOD AND APPARATUS (71) I, RONALD VAN BERLYN of 23, Centre Heights, London, NW3, a British Subject do hereby declare the invention, which was communicated from Westinghouse Electric Corporation of Westinghouse Building, Gateway Center, Pittsburgh, Pennsylvania, United States of America, a company organised and existing under the laws of the Commonwealth of Pennsylvania, United States of America, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to an electrolytic method and an apparatus for purifying waste water by continuously agglomerating solids of colloidal size or larger, suspended therein.

The prior art contains a variety of different methods wherein direct current, or direct current having alternating currents superimposed thereon, is utilised to agglomerate solids suspended in aqueous liquid. U.S. Patent No. 3,767,046 is exemplary thereof.

The prior methods incorporate many disadvantages which tend to render them either economically unfeasible or very expensive to operate. For example, prior direct current agglomeration methods are characterised by rapid deterioration of the anode and/or scaling or fouling thereof with organic materials from the solution, impurities from the metal itself, or impurities resulting from grain reactions at the anode surface. Therefore, batch operation is required due to the necessity for frequent shut down to clean or replace the anode, and continuous operation, or operation for extended periods of time is impossible.

A variety of cell additive materials are known in the art for reducing the tendency of the electrodes to scale, to improve current efficiency, and to prolong electrode life.

These additives are effective in varying degrees. However, the expense involved in utilising additives is substantial and is a definite disadvantage in direct current operation.

Furthermore, as will be obvious to those skilled in the art, continuous operation or at least semi-continuous operation over a prolonged period of time without the need to shut down to replace or clean the anode is preferable from an economic standpoint to batch operation even with additives.

We have found that suspended solids in an aqueous solution can be agglomerated by the efficient use of alternating current, so as to provide continuous operation without the necessity to frequently replace or descale the anode. Alternating current has been found to exert an electrostrictive effect on the suspended materials. In addition, alternating current continuously produces hydrogen peroxide at both electrodes which tends to maintain said electrodes in clean condition during operation. Finally, the hydrogen produced on the electrode has the capability of reacting with the electrode itself to produce, ultimately, minimal quantities of metal hydroxides which act as flocculation seeds to promote flocculation or agglomeration of the solid impurities. The electrostrictive effect and the production of hydrogen peroxide at each electrode are not possible in a direct current cell without the presence of sulphate radicals or other additives. Accordingly, said process is characterised by much more efficient operation in a continuous mode, without the necessity of using costly additives.

It has now been discovered that if the waste water containing solids to be agglomerated is passed between electrodes on a continuous basis and turbulence is created in the water as it flows past facing surfaces of the electrodes, vastly improved agglomeration or flocculation of said solids results from the multiplicity of electrolytic cells presented in a collapsing field of alternating current. The efficiency of the procedure is improved, if a particulate conductive material is added to, or is present in, the water flowing past and between the electrodes. In addition, by utilising conductive particules which are substantially resistent to attrition, a mechanical scrubbing effect will be exerted upon the electrodes which will tend to keep said electrodes clean during continuous operation and thereby avoiding frequent shut downs for descaling. Furthermore, it has been discovered that by utilising the method of this invention, electrode erosion may be reduced to approximately 0.2 percent of that experienced in direct current procedures.

We have found that by utilising a turbulent fluidised bed of conductive particles, the advantage of continuous operation with high current efficiency can be achieved.

It should be noted that the fluidised bed may be established in the apparatus of this invention with the process liquid alone, or with the process liquid augmented by an injected gas such as air. Once a fluidised state has been established, the likelihood of bridg- ing or short circuits between the electrodes is eliminated. Increased turbulence then increases the efficiency of the method of this invention with the upper limit thereof to be governed only by the economic factors.

In an embodiment of this invention, gas assisted turbulence is aided by disposing the side walls of a cell vessel of the apparatus at an angle to the vertical axis of the vessel. The angled side walls then increase the ease with which a true fluidised bed may be established within the vessel and increase the scrubbing action against the electrode faces by the bed particles.

Under an alternating current electric field, the conductive particles become bipolar electrodes so that the cell contains a multiplicity of such electrodes with each electrode providing its own boundary layer. The particle movement inherent in the fluidised bed not only ensures a continuous scrubbing or cleaning of the electrode faces, but of the indi vidual particles and their boundary layers.

Accordingly, the maximum turbulence consistent with a given apparatus will provide the maximum efficiency achievable according to the method of this invention.

We utilise alternating currents having a frequency between 0.2 and 800 Hz (preferably between 10Hz and 400 Hz) across the elctrodes generating an electric current having a magnitude between 0.08 and 6.0 amperes per square inch through said bed. In addition, however, it is also preferred to maintain a residence time in said bed of between nine and twenty-five seconds and a spacing of the facing surfaces of said electrodes of between one-quarter and two inches and most preferably one-half inch.

The spacing between the electrode faces, however, may be greater than two inches, if desired. However, it is preferred to utilise a spacing of one-half inch, and multiple cells in parallel to accommodate the volume of process liquid desired.

Accordingly, it is an object of this invention to provide an improved method for agglomerating solids of colloidal size or larger suspended in waste water by passing the waste water to be processed through an alternating current electric field while maintaining a turbulent condition thereof for the generation of hydrogen peroxide at each electrode.

The invention consists in a continuous method for purifying waste water by agglomerating solids of colloidal size or larger suspended therein, for subsequent removal comprising the steps of providing at least a pair of metal electrodes having facing surface areas spaced with respect to one another, causing the water to continuously flow between and past the surfaces of the electrodes, applying an alternating (ac) voltage at a frequency between 0.2 and 800 H3 across the electrodes, controlling the ac voltage in relation to the spacing between the elctrode surface areas and the conductivity of the water so as to provide an alternating electric current flowing between the elctrode surfaces with the value of the alternating current density being between 0.08 and 6.0 amperes per square inch of electrode surface, and causing agitation of the water so as to create turbulence in the water as the same flows between and past the spaced facing surfaces of the electrodes.

The invention also consists in an apparatus when used in performing the above defined method comprising a vessel having front, rear, and side walls, and having an inlet therein for admitting water to be purified and an outlet for purified water and agglomerated solids, the side walls of the vessel lying in a plane disposed at an angle to this vertical axis of the vessel, at least one pair of upstanding metal electrodes being disposed within the vessel between the inlet and the outlet, the electrodes having opposed, spaced, metal faces defining with the side walls of the vessel a water treatment chamber which is in communication with the vessel inlet and outlet, means coupled to the elctrodes being provided for applying an alternating current thereto of a frequency between 0.2 and 800 Hz with a current density between 0.08 and 6.0 amperes per square inch of electrode surface means being provided for directing water to be purified through the inlet into the vessel, through the treatment chamber and subsequently for directing treated water from the vessel through the outlet, means being provided for agitating the water to be purified, within the treatment chamber, to create turbulence in the water while the water is disposed between the electrodes, conductive means being disposed within the treatment chamber and between the electrodes for enhancing the alternating electrical field across the electrodes when water to be purified is disposed within the chamber.

In order to make the invention clearly understood, reference will now be made to the accompanying drawings which are given by way of example and in which: Fig 1 is a plan view of an apparatus of this invention; Fig. 2 is a vertical sectional view of the vessel portion of the apparatus of Fig. 1, having the centre portion removed; Fig. 3 is a sectional view taken along lines 3-3 of Fig. 1; Fig. 4 is a sectional view taken along lines 4-4 of Fig. 2; Fig. 5 is a sectional view taken along lines 5-5 of Fig. 2; Fig. 6 is a sectional view taken along lines 6-6 of Fig. 2; Fig. 7 is a sectional view taken along lines 7-7 of Fig. 2 Fig 8 is a sectional view taken along lines 8-8 of Fig. 2; Fig. 9 is a graph representing the efficiency of the method of this invention as measured by percent transmittance versus residence time in a vessel of an apparatus of this invention; Fig. 10 is a graph representing current density versus residence time, current consumed, and flow rate; Fig. 11 is a graphical representation of the relationship between the amount of process water treated and current density versus the velocity and flow rate; Fig. 12 is a graphical representation of the cell resistance as graphite particles are added to water for both alternating current and direct current; Fig. 13 is a graph representing the temperature in the cell with relation to the cell resistance in a situation wherein there is no fluid bed, and a situation wherein there is a fluidised bed within said cell; Fig. 14 is a graphical representation of the linear flow rate versus bed height for establishing fluidisation with different particle bed constituents.

With reference to the drawings, Fig. 1 represents a preferred version of an apparatus of this invention for purifying waste water by agglomerating solids of colloidal size or larger, suspended therein. The apparatus includes a vessel 10 which may contain any desired number of cells such as two as illustrated herein (see Fig. 3), and as will be subsequently described. Each cell 12 in vessel 10 is controlled through a console 14. Console 14 is utilised to monitor and regulate the turbulence in the cell and to apply the desired current and voltage to the electrodes. The console 14 may be coupled to a source of electrical energy by, for example, a cable 16, and supplies the desired voltage and current to the cell electrodes through cables 18. It will be obvious to those skilled in the art that console 14 is not an essential feature of this invention, and individual control may be utilised.

Although the range within which the current density should be maintained, according to the invention, is 0.08 to 6.0 amperes per square inch of electrode surface, we have found it advantageous for the current density to be maintained between 0.08 and 3.0 amperes per square inch of electrode surface, or between 0.08 and 0.5 amperes per square inch. Preferably, the current density of the alternating current passing between the electrode surfaces is maintained at one of the following values in amperes per square inch: 0.19; 1.0; 1.5; 2,0; 2.5; 3.0; 3.5; 4.0; 5.0; 6.0. Turbulence, as noted previously, may be established by the process water admitted to the cell, or may be augmented with a pressurized gas such as air, preferably to such an extent that the turbulence is of Reynolds number of at least 10,000. In the case of a multiplicity of cells, it is preferred to provide a separate water inlet and air inlet for each cell. Accordingly, as shown in Fig. 1, process water is pumped through a conduit 20 to cell inlets 22 and 22' in the lower portion of the vessel 10. The flow of process water may be monitored, for example, by flow meters 24.

The gas for augmenting the fluidised bed condition within each cell, which in the preferred embodiment is air, is pumped to the lower portion of each cell through conduit 26 and admitted through inlets 28 and 28'. Console 14 may contain a variety of meters and gauges for regulating the flow and monitoring the flow into cells 12.

The vessel 10 also includes an outlet 29 in the upper portion thereof for the water with agglomerated solids which has been processed according to the method of this invention. The particle size of the agglomerated solids may be at least 5000 Angstrom units.

Water with solids leaving outlet 29 may be conveyed by conventional means to any desired separating device such as a settling tank, centrifuge, skimmer or floatation cell.

As will be seen in Fig. 3, the vessel 10 comprises in this embodiment twin cells 12 housing opposed electrodes 30. Electrodes 30 are separated by an insulating spacer 32, and are supported by side walls 34 and face walls 36. The inner electrodes are also separated by an insulating spacer 38.

The electrodes in each cell 12 may preferably be of any metal other than noble metals and valve metals. Noble metals are metals which are substantially inert to attack by oxygen. Examples of noble metals are gold and platinum. Valve metals are metals which exhibit or may function in a combination which exhibits unidirectional electrical properties. Examples are tantalum, niobium and titanium. The electrodes may be of aluminium, aluminium alloy, iron, magnesium or steel, for example. For most applications, however, the electrodes will be made of or contain aluminium. Preferably, at least the facing surfaces of the electrodes will be of aluminium. The opposed electrodes are spaced a distance from one-quarter to at least two inches, but preferably one-half inch apart, at the commencement of operation of the method, with their opposed faces being in planes parallel to the vertical axis of the vessel. Any number of cells 12 may be utilised in vessel 10 as preferred from one or two as shown in Fig 3 up to ten or more. In each instance, the electrodes will preferably be disposed in parallel to each other.

While the upstanding side walls 34 of vessel 10 may be vertical, it is preferred to have the walls disposed at an angle of up to 30 to the vertical axis of the vessel. Preferably the angle will be between five and thirty degrees to the vertical and most preferably ten degrees. The angle of side walls 34 is intended to facilitate the establishment of a turbulent fluidised bed within each cell 12.

Electrodes 30 extend substantially through the entire central portion of vessel 12 nearly coextensive with the angled side walls 34.

As will be seen in Figs. 2,4,5 and 7 an inlet chamber 37 is disposed in the base 39 of vessel 10 for mixing the process water admitted through inlets 22 and 22' and the air may be admitted through inlets 28 and 28' into mixing chamber 42 to establish the desired turbulence. A screen 40 is disposed above the inlet chamber 37 and below the mixing chamber 42. A screen 44 is also disposed above mixing chamber 42 to, as will be subsequently described, support the fluidised bed and maintain it confined to the central portion of vessel 10 between the angled side walls 34. The side walls 46 of the lower portion 39 of vessel 10 may be vertical as shown.

Vessel 10 may be supported on a base 48, as desired.

As will be seen in Figs. 2, 6 and 8, the upper portion 50 of vessel 10 houses an upper disengagement chamber 52 disposed above electrodes 30. A screen 54 is disposed above chamber 52 to maintain the fluidised bed within vessel 10. Electrode leads 56 extend from electrodes 30, and are adapted to be coupled to cables 18 connecting the electrode 30 with console 14 and the source of electrical energy transmitted therethrough.

As process water passes upwardly through screen 54, a weir 58 is provided which separates the interior of vessel 10 from an outlet collection chamber 60 wherein the processed water 62 is collected for removal through outlet 29. The processed water from outlet 29 is then, as noted above, transported by any conventional means to a solid separation device such as a skimmer, settling tank or centrifuge (not shown). In the alternative the processed water may be recycled through inlet 20 in vessel 10 with the agglomerated solids serving as flocculation seeds to achieve greater purity.

The side walls 64 and front and rear face walls 36 may be integral with the corresponding side and front and rear walls of the central portion of vessel 10, or may be welded thereto. Said walls may be of any desired configuration adapted to form chamber 52, and outlet chamber 60 as will be obvious to those skilled in the art.

The exact particle size of the constituents of the fluidised bed is not critical, and may range from three millimetres to up to six millimetres, for example. On the basis of tests conducted, to achieve bed stability at a residence time desired, the ideal particle would be a sphere of approximately oneeighth inch diameter and a specific gravity of approximately two. The material utilised, however, could be any conducting material which has excellent mechanical and chemical stability. It is preferred, however, that the bed of particles be capable of true fluidisation, and that the particles be attrition resistant.. In the absence of a fluidised bed condition, as noted above, bridging and short circuits can result which may diminish current efficiency. In preferred version of this invention, a graphite pellet is utilised which is cylindrical and nominally 0.125 inches in diameter by 0.19 inches long, with a specific gravity of 2 to 2.1. The cylindrical pellets were found to enhance the turbulence of the fluidised bed, and to increase the electrical effect thereof.

Fig. 14 illustrates the results of tests directed to establishing a fluidised bed in a tube having an inside diameter of 5/8 inches, filled with possible bed constituents. Each was fluidised with water at the various velocities shown, and the bed height relative to velocity is illustrated in Fig. 14.

A series of tests was also conducted utilising a cell having parallel side walls and an electrode plate spacing of 0.403 inches to evaluate the conductivity of the graphite particles selected relative to temperature in a fluidised bed, and alternatively in a condition wherein fluidisation is not established. It was found that, for example, at a water flow rate of 1.5 gallons per minute and an air flow rate of 0.2 SCFM, (standard cubic feet per minute) the bed was expanded from a "no flow" height of 15 3/4 inches to a completely fluidised height of 32 inches. Dramatic flocculation occurred with a current density of 0.19 amps per square inch and a power consumption equivalent to 3.11 kwhrs per 1000 gallons. Significantly lower power consumption was experienced with said fluidised bed compared with the power consumption experienced with no bed as shown in Fig. 13.

Fig 12 also illustrates cell resistance as graphite pellets are added to 225 ccs of water to illustrate the greatly increased conductivity found with alternating current as compared to direct current in a fluidised bed environment.

Figs. 9 to 11 illustrate the results achieved in tests conducted in the device of Figs. 1 to 8 wherein the plate spacing was one-half inch between electrodes, and the bed volume was about two-thirds of the actual volume between the electrodes. In addition, the side walls were disposed at a ten degree angle to the vertical.

The graph of Fig 9 depicts efficiency of the apparatus of this invention as compared to residence time for the liquid being processed.

Efficiency is measured as the percent of light transmittance as measured with a spectrophotometer at A = 5500 A with distilled water as a standard equalling 100 percent.

Thus, the efficiency, or percent transmittance, measures the clarity of the water processed as compared to distilled water. While the water processed in this example, the results of which are depicted in Fig 9, was white water paper mill effluent, the results are representative for other waste waters processed. As shown therein, the optimum residence time should fall in the linear portion of the graph between about nine and twentyfive seconds. A longer residence time achieved little increase in transmittance as compared to the current required. Accordingly, residence time in the apparatus of this invention is preferred between nine and twenty-five seconds total.

The graph of Fig. 10 represents current density versus residence time, current consumed and flow rate and the graph of Fig 11 represents the relationship between the amount of process water treated and current density versus the velocity and flow rate.

The following are exemplary results achieved utilising the apparatus of this invention: The apparatus of this invention has been used to treat a variety of waste waters including coke oven flushing liquor, well water containing colloidal iron, and process water containing colloidal graphite, paper mill kraft and soda mill waters, machine shop oil in water emulsions, cheese whey, copper drawing soap effluent, domestic sewage, paper mill lagoon sludge, and various other commercial and domestic waste streams. In all cases suspended solids were significantly reduced within the preferred residence time as above noted.

The following are specific examples utilising the method of this invention. In each the fluidised bed was established with graphite pellets having a specific gravity of about 2.1.

EXAMPLE 1 A flow rate, total, of 3.5 to 5 g/m of colloidal graphite contaminated water and an air flow rate of one to 1.5 SCFM in the two cell vessel of Figs 1 to 8 was used to establish good fluidisation with cylindrical graphite pellets as described above having a specific gravity of about 2.1. A total current of 30 amps, an ac voltage of 50 volts and 60 Hz was applied across the electrodes and the colloidal suspension was "broken". Suspended solids were reduced from 1285 ppm in the untreated water to 38 ppm after one pass through the unit followed by approximately two hours settling time in a holding tank, not shown. The influent water was at a temperature of 34 to 38 degrees F. Higher influent temperatures would obviously result in a higher conductivity, e.g., as a general rule water at 70 degrees F has a conductivity of 2 to 3.75 times that of water at 32 degrees F depending on the amount and type of impurities.

EXAMPLE 2 Paper mill lagoon sludge was processed in an apparatus of this invention as described in Figs. 1 to 8. The sludge had originally been treated with aluminium sulphate but contained suspended solids which under normal conditions would remain suspended for years. The sludge had a very high viscosity and zero percent transmittance. The suspended solids amounted to 5.4 percent by weight, and the aluminium hydroxide equalled 1.3 percent of the solids.

The sludge was diluted 3 : 1 with city water. The flow rate of the device of this invention was three gallons per minute, an ac voltage of 60 volts was applied across the electrodes, at a total current of 60 amps.

The processed sludge contained solids which precipitated within ten minutes and the suspension forces were totally broken.

The transmittance of the supernatant liquid of the processed sludge after one hour was measured at 86.5 percent at ssooA. Suspended solids remaining were measured at 11 ppm.

In summary then it has been discovered that alternating current applied through a turbulent region of an aqueous liquid, and preferably through a turbulent fluidised bed of conductive particles in said region, will efficiently and economically reduce solids suspended in the liquid on a continuous basis. The turbulence established increases current efficiency, and contributes to an electrostrictive effect whereby a multitude of bipolar electrodes are established within the cell to break down the suspending forces of the solids. The solids then clump together whereby they may be easily removed by settling or skimming.

In contrast to prior art procedures, by utilising alternating current the electrode surfaces remain relatively clean for efficient, continuous operation due primarily to the generation of hydrogen peroxide, and a mechanical scrubbing action by the bed particles. The hydrogen peroxide as will be obvious to those skilled m the art also contributes to decolorisation and deodorisation of the water processed. The hydrogen peroxide produced at the electrodes causes the formation of metal hydroxides at the electrodes.

When the waste water contains alkalimetal-halide salts, the metal hydroxides are alkali-metal hydroxides.

Alternating current allows continuous operation without the need to shut down and replace a sacrificial anode, or to descale the anode, and does not require costly additives for efficient operation.

We have found it to be advantageous, if a pH range of between 2 and 9 is maintained during the flow of the waste water past and between the electrodes. We have also found it advantageous if the waste water contains or has added thereto conductive material in the form of at least one salt soluble in the waste water. Such salts may be alkali-metal-halide salts WHAT I CLAIM IS: 1. A continuous method for purifying waste water by agglomerating solids of colloidal size of larger suspended therein for subsequent removal, comprising the steps of providing at least a pair of metal electrodes having facing surface areas spaced with respect to one another, causing the water to continuously flow between and past the facing surfaces of the electrodes, applying an alternating (ac) voltage at a frequency between 0.2 and 800 Hz across the electrodes, controlling the ac voltage in relation to the spacing between the elctrode surface areas and the conductivity of the water so as to provide an alternating electric current flowing between the electrode surfaces with the value of the alternating current density being between 0.08 and 6.0 amperes per square inch of electrode surface, and causing agitation of the water so as to create turbulence in the water as the same flows between and past the spaced facing surfaces of the electrodes.

2. A method as claimed in claim 1, wherein the ac voltage is at a frequency between 10 Hz and 400 Hz.

3. A method as claimed in claim 1 or 2, wherein the agitation is fluid-induced and is such as to create turbulence of a Reynolds number of at least 10,000 in the water.

4. A method as claimed in claim 1,2 or 3, wherein the agitation is by means of compressed gas.

5. A method as claimed in any one of claims 1 to 4, wherein the electrodes are of a metal other than the noble metals (hereinbefore defined) and valve metals (hereinbefore defined).

6. A method as claimed in claim 5, wherein the metal of the electrodes is aluminium, an aluminium alloy, iron, magnesium, or steel.

7. A method as claimed in claim 6, wherein the electrodes contain aluminium, and at least the facing surfaces of the electrodes are of aluminium.

8. A method as claimed in any one of claims 1 to 7, wherein the facing surfaces of the electrodes are substantially parallel to each other.

9. A method as claimed in any one of claims 1 to 8, wherein the conditions stated in any of claims 1 to 8 are maintained so that as the waste water passes between the elctrodes, metal hydroxides are caused to be formed at the electrode surfaces during the passing of current therebetween, the rate of flow of the waste water being controlled so that the same passes between the electrode surfaces with a residence time sufficiently long to cause the particles in colloidal suspension to be agglomerated into solid particles of such a size that they are no longer in colloidal suspension in the waste water.

10. A method as claimed in claim 9, wherein the flow rate of the waste water is such that the residence time is between 9 and 25 seconds.

11. A method as claimed in claim 9 or 10, wherein the metal hydroxides are caused to be

Claims (36)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   surfaces remain relatively clean for efficient, continuous operation due primarily to the generation of hydrogen peroxide, and a mechanical scrubbing action by the bed particles. The hydrogen peroxide as will be obvious to those skilled m the art also contributes to decolorisation and deodorisation of the water processed. The hydrogen peroxide produced at the electrodes causes the formation of metal hydroxides at the electrodes.

   When the waste water contains alkalimetal-halide salts, the metal hydroxides are alkali-metal hydroxides.

   Alternating current allows continuous operation without the need to shut down and replace a sacrificial anode, or to descale the anode, and does not require costly additives for efficient operation.

   We have found it to be advantageous, if a pH range of between 2 and 9 is maintained during the flow of the waste water past and between the electrodes. We have also found it advantageous if the waste water contains or has added thereto conductive material in the form of at least one salt soluble in the waste water. Such salts may be alkali-metal-halide salts WHAT I CLAIM IS: 1. A continuous method for purifying waste water by agglomerating solids of colloidal size of larger suspended therein for subsequent removal, comprising the steps of providing at least a pair of metal electrodes having facing surface areas spaced with respect to one another, causing the water to continuously flow between and past the facing surfaces of the electrodes, applying an alternating (ac) voltage at a frequency between 0.2 and 800 Hz across the electrodes, controlling the ac voltage in relation to the spacing between the elctrode surface areas and the conductivity of the water so as to provide an alternating electric current flowing between the electrode surfaces with the value of the alternating current density being between 0.08 and 6.0 amperes per square inch of electrode surface, and causing agitation of the water so as to create turbulence in the water as the same flows between and past the spaced facing surfaces of the electrodes.
2. 2. A method as claimed in claim 1, wherein the ac voltage is at a frequency between 10 Hz and 400 Hz.
3. 3. A method as claimed in claim 1 or 2, wherein the agitation is fluid-induced and is such as to create turbulence of a Reynolds number of at least 10,000 in the water.
4. 4. A method as claimed in claim 1,2 or 3, wherein the agitation is by means of compressed gas.
5. 5. A method as claimed in any one of claims 1 to 4, wherein the electrodes are of a metal other than the noble metals (hereinbefore defined) and valve metals (hereinbefore defined).
6. 6. A method as claimed in claim 5, wherein the metal of the electrodes is aluminium, an aluminium alloy, iron, magnesium, or steel.
7. 7. A method as claimed in claim 6, wherein the electrodes contain aluminium, and at least the facing surfaces of the electrodes are of aluminium.
8. 8. A method as claimed in any one of claims 1 to 7, wherein the facing surfaces of the electrodes are substantially parallel to each other.
9. 9. A method as claimed in any one of claims 1 to 8, wherein the conditions stated in any of claims 1 to 8 are maintained so that as the waste water passes between the elctrodes, metal hydroxides are caused to be formed at the electrode surfaces during the passing of current therebetween, the rate of flow of the waste water being controlled so that the same passes between the electrode surfaces with a residence time sufficiently long to cause the particles in colloidal suspension to be agglomerated into solid particles of such a size that they are no longer in colloidal suspension in the waste water.
10. 10. A method as claimed in claim 9, wherein the flow rate of the waste water is such that the residence time is between 9 and 25 seconds.
11. 11. A method as claimed in claim 9 or 10, wherein the metal hydroxides are caused to be formed at the electrode surfaces without the presence of alkali-metal-halide salt additives in the waste water.
12. 12. A method as claimed in claim 9 or 10, wherein alkali-metal-halide salts are present in the waste water as it flows past and between the surfaces of the electrodes.
13. 13. A method as claimed in claim 9 or 10, wherein the alkali metal hydroxides are caused to be formed at the electrode surfaces without external addition of hydroxides to the waste water passing between said electrodes.
14. 14. A method as claimed in claim 9 or 10, wherein the stated conditions are maintained so that the alternating current passing between the electrodes produces hydrogen peroxide at the electrodes, thus causing the metal hydroxides to be formed at the electrodes.
15. 15. A method as claimed in any one of claims 1 to 14, wherein the facing surfaces of the electrodes are spaced from each other by one-half inch at the commencement of operation of the method.
16. 16. A method as claimed in any one of claims 1 to 15, wherein a pH range of between 2 and 9 is maintained during the flow of the waste water past and between the electrodes.
17. 17. A method as claimed in any one of claims 1 to 16, wherein there is added to, or is present in, the waste water flowing past and between the electrodes a conductive material

    which enhances establishing and maintaining an alternating current passing between the electrode surfaces with a current density within the range stated in claim 1.
18. 18. A method as claimed in claim 17, wherein the conductive material is provided as a fluidised bed of conductive attrition resistant particles between the facing surfaces of the electrodes, the said agitation also causing agitation of the bed, the waste water being caused to continuously flow through the bed while flowing between and past the facing surfaces of the electrodes.
19. 19. A method as claimed in claim 18, wherein the conductive particles are cylindrical in form.
20. 20. A method as claimed in claim 17, 18 or 19, wherein the conductive material is graphite.
21. 21. A method as claimed in claim 17, 18 or 19, wherein the conductive material has a specific gravity of 2 to 2.1.
22. 22. A method as claimed in claim 17, wherein the conductive material is at least one salt soluble in the waste water.
23. 23. A method as claimed in any one of claims 1 to 22, wherein the current density of the alternating current passing between the electrode surfaces is maintained between 0.08 and 3.0 amperes per square inch.
24. 24. A method as claimed in claim 23, wherein the current density is maintained between 0.08 and 0.5 amperes per square inch.
25. 25. A method as claimed in any one of claims 1 to 24, wherein the current density is maintained at one of the following values in amperes per square inch: 0.19; 1.0; 1.5; 2.0; 2.5; 3.0; 3.5; 4.0; 5.0; 6.0.
26. 26. An apparatus when used in performing the method of claim 1, comprising a vessel having front, rear, and side walls, and having an inlet therein for admitting water to be purified and an outlet for purified water and agglomerated solids, the side walls of the vessel lying in a plane disposed at an angle to the vertical axis of the vessel, at least one pair of upstanding metal electrodes being disposed within the vessel between the inlet and the outlet, the electrodes having opposed, spaced, metal faces defining between them a water treatment chamber which is in communication with the vessel inlet and outlet, means coupled to the electrodes being provided for applying an alternating current thereto of a frequency of between 0.2 and 800 Hz, the current applying means being such that, having regard to the spacing between the elctrodes and the conductivity of the water being treated, it is capable of supporting a current density of between 0.08 and 6.0 amperes per square inch of electrode surface, means being provided for directing water to be purified through the inlet into the vessel, through the treatment chamber and subsequently for directing treated water from the vessel through the outlet, means being provided for agitating the water to be purified, within the treatment chamber, to create turbulence in the water while the water is disposed between the electrodes, conductive means being disposed within the treatment chamber and between the elctrodes for enhancing the alternating electrical field across the electrodes when'water to be purified is disposed within the chamber.
27. 27. An apparatus as claimed in claim 26, wherein the side walls are disposed at an angle to the vertical axis of the vessel of up to 30 degrees.
28. 28. An apparatus as claimed in claim 27, wherein said angle is between 5 and 30 degrees.
29. 29. An apparatus as claimed in claim 28, wherein the angle is 10 degrees.
30. 30. An apparatus as claimed in any one of claims 26 to 29,wherein the opposed faces of the electrodes are in planes parallel to the vertical axis of the vessel.
31. 31. An apparatus as claimed in any one of claims 26 to 30, wherein the means for agitating includes a source of gas under pressure, in communication with the chamber, for injection of gas thereinto.
32. 32. An apparatus as claimed in claim 31, wherein the gas is air.
33. 33. An apparatus as claimed in any one of claims 26 to 32. wherein the conductive means comprises graphite pellets.
34. 34. An apparatus as claimed in any one of claims 26 to 33, wherein the faces of the electrodes are of aluminium, iron, steel or magnesium.
35. 35. Methods for purifying water by agglomerating solids suspended therein for subsequent removal, as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying draw ings.
36. 36. Apparatus as claimed in claim 26 and substantially as hereinbefore described with reference to the accompanying drawings.