TECH N I CAL FI ELD

The invention relates to a m ethod for removing contam inants from wastewater by electroflotation or electrocoagulation, in which the wastewater to be cleaned is passed through an asym m etrical electrolytic cell, resulting in a cell reaction in which both m etal hydroxide and hydrogen gas are produced. For metal hydroxides with low solubility, im purities coagulate with the m etal hydroxides into flakes.

PRI OR ART

Electrocoagu lation is the coagu lation or flocculation of dissolved or suspended solids using electricity. At the cathode, the electrolytic cell produces a gas, usually hydrogen gas. At the anode, the electrolytic cell produces m etal ions. These ions act as coagulants for the im purities in the wastewater. The gas provides a buoyant effect for the resulting flakes, which are then m echanically separated from the water.

Electrocoagulation is known from US patent publications US 5,888,359 and US 6,086,732.

A problem with electrocoagulation is the contam ination of the anode. Contam ination coagulates at the anode, where m etal ions are form ed. Contam ination of the anode lowers the efficiency of the electrolytic cell. A m ethod for cleaning the electrolytic cell is known from WO 2003/062152. Therein, the lam inar flow pattern in the electrolytic cell is broken at each flush, thereby removing the contam inating film on the anode.

SU M MARY OF TH E I NVEN TI ON

I n the first aspect, the invention com prises a m ethod for cleaning the electrode cells used for removing contam inants from wastewater by electrocoagulation, the m ethod com prising the following steps:

a) passing the wastewater to be purified through an electrolytic cell which is provided with two m etal electrodes with different electronegativities, consisting of coaxial pipes with the inner pipe com prising the more electronegative electrode, b) perform ing electrolysis between the two electrodes, such that the more electronegative electrode, which does not wear in a cleaning process, is used to produce hydrogen gas and hydroxyl ions from water, and that the less electronegative electrode, which is an active, wearing electrode in a cleaning process, is used to produce m etal ions in a solution to be cleaned,

c) producing an electric field in the electrolytic cell, whereby desired redox reactions take place to isolate one or more contam inants from the wastewater in the form of flakes,

d) passing the wastewater with said flakes from the electrolytic cell to a separation device for flakes and purified water, and

e) producing axial waves in the wastewater along the inner surface of the outer electrode interm ittently.

Axial waves help the accum ulation of pollution along the surface of the anode. These are advantageous over radial and tangential waves or turbulent flow patterns as the eddies as a result of the above lead to the break-up of the coagulated flakes.

This m akes the process more efficient and easier to m anage. With rapid accum ulation of contam ination along the anode, the capacity of the cell m ust be continually increased to guarantee the sam e water purification. The process according to the first aspect counteracts this contam ination, as a result of which electricity consum ption is reduced, and the process has to be adjusted less regularly.

I n the second aspect, the invention relates to a device for removing contam inants from wastewater by m eans of electrocoagulation, com prising two coaxial tubular m etal electrodes, an inner and an outer pipe, the inner pipe being coupled to a negative pole of a power source and the outer pipe being coupled to a positive pole of the power source, with an electrolysis space between the electrodes, the inner pipe being m ade at least in its su rface layer of a m ore electronegative m aterial than the outer pipe, with a jet cap along one end of the coaxial pipes, provided with at least one radial opening suitable for supplying or discharging the wastewater to the electrolysis space and at least one axial opening with a control valve, su itable for producing a pressure wave that propagates through said electrolysis space with a rinsing liquid.

The device is suitable for producing an axial pressure wave without stopping the electrolytic cell or substantially changing the flow pattern in the electrolytic cell. This promotes coagu lation of im purities in the electrolytic cell, since these are easily broken by whirlwinds and lateral m ixing.

On the other hand, the rinsing liquid does not have to contain chem ical cleaning agents such as detergents. The rinsing fluid can be water without the addition of chem icals. I n this way, they do not need to be removed from the wastewater afterwards, nor does the process have to be stopped regularly for removing the accumulated contamination along the anode.

Finally, the construction of both the cell and the jet cap is very simple. The pressure wave can be produced by connecting the rinsing liquid to high pressure. When the control valve is opened for a short period, it propagates through the electrolytic cell in the form of a pressure wave. In view of the design, this gives an axial pressure wave within the electrolytic cell with only small eddies. Preventing lateral mixing and eddies improves the coagulation of the contaminants.

In a following aspect, the invention relates to an assembly for purifying wastewater comprising: (i) a device for separating contaminants from wastewater by means of electrocoagulation according to the second aspect and (ii) a separating device suitable for separating purified water and contaminants coagulated into flakes.

BRI EF DESCRI PTI ON OF THE DRAWI NGS

Figure 1 A: A schematic overview of an embodiment of the electrolytic cell according to the present invention.

Figure 2A: A schematic overview of an embodiment of ultrasonic cleaning according to the present invention.

Figure 2B: A detail view of an embodiment of ultrasonic cleaning according to the present invention.

Figure 3A: A schematic overview of an embodiment of cleaning with vane- shaped brushes according to the present invention.

Figure 3B: A detail view of an embodiment of cleaning with vane-shaped brushes according to the present invention.

Figure 4A: A schematic overview of an embodiment of cleaning with a pressure wave or jet according to the present invention.

Figure 4B: A detail view of a preferred embodiment of a jet cap according to the present invention.

DETAI LED DESCRI PTI ON

The invention relates to a method for removing contaminants by means of electrocoagulation. The invention also relates to a device and assembly for purifying wastewater. Unless otherwise defined, all terms used in the description of the invention, including technical and scientific term s, have the m eaning as com monly understood by a person skilled in the art to which the invention pertains. For a better understanding of the description of the invention, the following terms are explained explicitly.

Unless otherwise defined, all terms used in the description of the invention, including technical and scientific terms, have the meaning as com monly u nderstood by the skilled person in the technical field of the invention. For a better assessm ent of the description of the invention, the following terms are explained explicitly.

I n this docum ent,‘a’ and‘the’ refer to both the singular and the plural, unless the context clearly im plies otherwise. For exam ple,‘a segm ent’ m eans one or more than segm ents.

The terms‘com prise’, ‘com prising’, ‘consist of’, ‘consisting of’, ‘provided with’, ‘include’, ‘including’, ‘contain’, ‘containing’, ‘encompass’, ‘encom passing’ are synonyms and are inclusive or open terms indicating the presence of what follows and which do not exclude or prevent the presence of other com ponents, features, elem ents, m em bers, steps known from or described in the prior art.

‘Electrocoagu lation’ is the coagu lation (flocculation) of dissolved or suspended solids using electricity. ‘Electroflotation’ com prises electrocoagulation, with the additional step of releasing gas bubbles that bring the coagulated flakes to the su rface.

Quoting nu m erical intervals by endpoints includes all integers, fractions and/or real num bers between the endpoints, these endpoints included.

I n a first aspect, the invention relates to a m ethod for removing contam inants from wastewater by m eans of electrocoagulation, the m ethod com prising the following steps: a) passing the wastewater to be purified through an electrolytic cell which is provided with two m etal electrodes with different electronegativities, consisting of coaxial pipes with the inner pipe com prising the more electronegative electrode, b) perform ing electrolysis between the two electrodes, such that the more electronegative electrode, which does not wear in a cleaning process, is used to produce hydrogen gas and hydroxyl ions from water, and that the less electronegative electrode, which is an active, wearing electrode in a cleaning process, is used to produce m etal ions in a solution to be cleaned, c) producing an electric field in the electrolytic cell, whereby desired redox reactions take place to isolate one or more contam inants from the wastewater in the form of flakes,

d) passing the wastewater with said flakes from the electrolytic cell to a separation device for flakes and purified water, and

e) producing axial waves in the wastewater along the inner surface of the outer electrode interm ittently.

This way, wastewater can be stripped of organic waste load, while the heavy m etals are also cleaned.

A‘wave’ as discussed herein is a pressure wave, being a deviating pressure that [passes] through the electrolytic cell along the inner su rface of the outer electrode. The effluent always flows axially through the electrolytic cell, and the pressure wave or pressure waves propagate according to the invention in the axial flow direction. The pressure waves can propagate with the flow, or against the flow. Preferably, the pressure waves propagate with the flow of the wastewater through the electrolytic cell. The pressure waves preferably propagate considerably faster than the flow of the wastewater through the electrolytic cell. The flow pattern of the effluent, which is preferably mainly lam inar flow, is thus broken as little as possible. As a result, the water purification is stopped less by the cleaning of the electrode su rface.

The coaxial pipes can be supplied in diam eters and lengths that vary depending on a specific application. As the size of a processing plant becom es larger and the flow rate increases, it is advantageous that a sufficient num ber of cells are connected in parallel.

I n a preferred em bodim ent, the axial wave propagates in the flow direction of the wastewater. This helps prevent water ham m er at the feeding line of the wastewater to be cleaned and gives better results with electrocoagulation. I t also ensures that we get proper pu rification at a constant flow rate.

The anode is the outer pipe. This is the electrode with a less electronegative surface m aterial in which m etal ions are released to the wastewater. Preferably, the anode, at least in the surface, is m ade of alum inium or iron. The choice between alum iniu m and iron depends on the pollution of the wastewater. The anode is an active, wearing electrode, and m ust be replaced over tim e. I t is advantageous to use this as outer electrode. This m akes it easier to replace the anode. This is also the electrode with the largest surface area, which prevents the accum ulation of contam ination at the anode and promotes the dissolution of m etal ions in the wastewater.

The cathode is the inner pipe. This is the electrode with a less electronegative surface m aterial. Hydrogen gas is produced from water at the cathode, so this is not an active, wearing electrode. Preferably, the cathode is m ade of steel. Even more preferably, the cathode is m ade of stainless steel.

Preferably, the length is considerably higher than the diam eter. Preferably, the ratio of the length over the inner diam eter, m easured from the inner surface, of the outer pipe is higher than 5, even more preferably higher than 7, most preferably higher than 1 0.

The use of elongated, concentrically nested electrode pipes using axial waves provides a sim ple way to keep the electrode surface clean. The elongated concentric pipes prom ote a lam inar flow pattern in the electrolysis space. This is advantageous for coagulating flakes, and therefore the efficiency of the cleaning process. The axial waves result in a sufficient pressure difference along the surface to keep the su rface clean, without breaking this lam inar flow pattern with turbulent effects such as eddies or lateral m ixing, which counteracts the coagulation.

At the cathode, the electrolytic cell dissociates water into H ⁺ ions and OH ions. The H ⁺ ions absorb electrons and escape from the m ixture as hydrogen. Since these H ⁺ ions escape faster than the OH ions, a m ildly alkaline solution is formed along the cathode. Metal ions dissolve in the wastewater at the anode. These m etal ions then form m etal hydroxides, which are poorly soluble in water for both iron and alum iniu m . Organic substances and heavy m etals coprecipitate with the form ed m etal hydroxides. The precipitate rises together with H gas as a flake to the su rface of clean water.

The oxidation of iron into Fe ²⁺ or Fe ³⁺ ions and the purification of the wastewater take place in a cell at a certain point of resonance energy. I n other words, the electrical energy that is introduced into a cell must be dim ensioned according to the dim ensioning and flow of the cell, i.e. the retention time of wastewater in the cell space. The search for a good point in resonance energy m ust be carried out experim entally and then the cell flow regu lated by autom ation with respect to the wastewater flow. This is considerably more difficult when the surface of the anode rapidly becom es contam inated, since this contam ination strongly drives up the resonance energy. The use of the axial waves keeps the surface clean for longer periods, m aking the process easier to control. The through-flow of wastewater does not have to be interrupted during an axial pressure wave or jet, because the rinsing with a jet is carried out at a considerably higher pressure and with a sm aller liquid volum e than the pressure and liquid volu me of through-flowing wastewater.

According to an em bodim ent, the axial waves are ultrasonic. Ultrasonic waves are well su ited for cleaning a su rface. Ultrasonic cleaning can be done interm ittently or continuously and gives a long-lasting clean surface along the anode.

According to a further preferred em bodim ent, the ultrasonic waves are produced in the wastewater to be purified before it is passed through the electrolytic cell.

Ultrasonic devices for liquids are available on the m arket. These com prise, among others, Y-shaped tubes, one leg containing the ultrasonic device and the other legs allowing the liqu id to flow through. Producing ultrasonic waves in the wastewater m eans no further addition of liquids or chem icals for proper cleaning. Although these devices are not sim ple, they are available on the m arket and easy to install. The device can nam ely be placed in series upstream from the electrolytic cell.

I n a preferred em bodim ent, the u ltrasonic waves have a frequency between 20 and 200 kHz, preferably between 20 and 1 00 kHz, even more preferably between 20 and 60 kHz.

These frequencies result in a clean surface, with only m inor adverse effects on the coagulation of contam inants into flakes. Preferably, the ultrasonic waves are used only interm ittently, where necessary, to keep the surface clean.

I n another em bodim ent, the invention com prises rinsing the electrolytic cell with an axial jet or pressure wave.

This jet or pressure wave can propagate in the wastewater or other liquid m edium . Preferably, the jet or pressu re wave is produced by bringing the cell into contact with a pressurised rinsing liquid in an axial direction. Preferably, this is done during a very short period. Preferably, the pressure of the rinsing liquid is between 0.5 and 3 bar, even more preferably between 0.6 and 2.5 bar, even more preferably between 0.6 and 2.0 bar, even more preferably between 0.6 and 1 .5, even more preferably between 0.8 and 3 1 .2 bar and most preferably between 0.9 and 1 .1 bar. This method produces a pressure wave that propagates axially through the wastewater. Hereby the eddies and lateral (in this case in the radial and tangential direction with respect to the coaxial electrodes) m ixing are prevented. Thus, coagulated flakes are not pu lled apart and only slightly affected. However, the surface does rem ain clean. The j et can be used at relatively long intervals, for exam ple every m inute to every 2 hours. Preferably, the intervals are between 5 m inutes and 2 hours. The length of the intervals m ainly depends on the amou nt of contam ination, which depends on the amount and type of im purities in the water that can be removed by electrocoagulation.

The rinsing liqu id can be any liquid, preferably water. Even more preferably, recycled ‘purified’ water from the separation device, which is located downstream from the electrolytic cell.

I n another em bodim ent, the invention com prises producing axial waves by m eans of vanes. Preferably, the vanes consist essentially of brushes. These brushes give less cause for eddies and break coagulating flakes less. The liquid passes more or less through the brushes, and the flakes stick to the brushes. Coagulation with vane-shaped brushes is thus not strongly prevented, but a clean surface is nevertheless obtained.

These brushes should not touch the surface of the outer pipe. Preferably, the brushes do not abut the surface of the outer pipe. The clean scrubbing of the surface of the outer pipe is accom panied by a faster wear of the active electrode. The surface is not cleaned by the scrubbing of the brushes against the surface, but by the flow brought about by said brushes. I n an em bodim ent, the brushes can move axially so as to cause this flow. An axial pressure wave results from the axial movem ent of the brush.

I n another em bodiment, the brushes are vane-shaped and can rotate, thereby creating axial flow. I n a preferred em bodim ent, the brushes can both move axially and rotate, and are still vane shaped.

Preferably, the brush is used only interm ittently. Although the present em bodim ents attem pt to prevent the breaking up of flakes, the cleaning of the anode surface is detrim ental to purifying the wastewater. Optim ising the time between cleaning the surface can be done by trial and error and is trivial for a person skilled in the art.

Preferably, the brushes consist of m aterials that are resistant to both slightly acidic and slightly basic conditions. Preferably, the brushes consist of a non-electrically conductive m aterial. Even more preferably, the brushes consist of polypropylene or polyam ide. I n an embodim ent, the cell is cleaned interm ittently. The cell can thus be used as m uch as possible with a strict lam inar flow pattern, which is beneficial to coagu lation. The length of the intervals can be determ ined with the help of trial and error. I n a preferred em bodim ent, the cell is operated at a constant current intensity, and the cell is cleaned when the voltage necessary to m aintain this constant current with respect to a clean cell increases by more than 30% , preferably the increase is more than 25% , even more preferably the increase is more than 20% , even more preferably the increase is more than 15% , most preferably the increase is m ore than 1 0% . Preferably, the increase of voltage at a constant current is at least 1 % before the electrolytic cell is cleaned. Even more preferably, the increase is at least 3% , m ost preferably at least 5% . Allowing a high voltage before cleaning leads to a very high power consum ption and/or low consistency in water treatm ent. Cleaning at very low increases in voltage leads to frequent cleaning, which disrupts the flow pattern in the electrolytic cell. Selected values lead to an optim um , in order to achieve a constant water treatm ent with a relatively low power consum ption.

I n a second aspect, the invention com prises a device for removing contam inants from wastewater by m eans of electrocoagulation, com prising two coaxial tubular m etal electrodes, an inner and an outer pipe, the inner pipe being coupled to a negative pole of a power source and the outer pipe being coupled to a positive pole of the power source, with an electrolysis space between the electrodes, the inner pipe being m ade at least in its su rface layer of a more electronegative m aterial than the outer pipe, characterised by a jet cap at the bottom of the electric cell along one end of the coaxial pipes, provided with at least one radial opening suitable for supplying or discharging the wastewater to the electrolysis space and at least one axial opening with a control valve, suitable for producing a pressu re wave that propagates through said electrolysis space with a rinsing liquid.

This is a sim ple set-up that is easily adjustable and m aintainable. The continuous cleaning of the surface m akes the control of the electrolytic cell easier. The pressure wave can be produced by briefly opening and closing the axial valve. Furthermore, the sam e electrolytic cell is stored. Without contam ination along the outer pipe, electrocoagulation provides better water pu rification and more consistent water pu rification when the electrolytic cells are operational for a long tim e.

I n a preferred em bodim ent, the j et cap com prises an opening suitable for em ptying the electrolysis space of wastewater and flakes. This is advantageous for the m aintenance of the electrolytic cell, for, among other things, replacing the active, wearing electrode. Furthermore, the cell m ust still be cleaned after a while.

Device for removing contam inants from wastewater by m eans of electrocoagulation according to claim 1 0, wherein said partitions are arranged obliquely, preferably at an angle of 0-40° , even more preferably an angle of 1 0-25° .

A swirling jet can be produced with the aid of skewed partitions. This introduces a lim ited amount of turbulence into the system . This benefits the purification of the surface. Since the eddy propagates through the m edium only once, the influence on coagu lation in the electrolytic cell is sm all. Nevertheless, a sm all angle is preferably used so that flakes are carried along by the eddy rather than pu lled apart.

I n a further, preferred em bodim ent, the invention com prises a jet cap which com prises two radial openings and one axial opening, the radial openings being located at a different height with respect to the axis.

The highest located radial opening is suitable for the supply of water. The lowest located radial opening is suitable for em ptying the cell for m aintenance. The axial opening is su itable for producing the axial jet. This very sim ple construction makes it possible to operate the electrolytic cell efficiently.

I n a fu rther, preferred em bodim ent, the jet cap com prises four partitions, which define four com partm ents, the radial openings opening into opposite com partm ents, and the axial opening opening into interm ediate com partm ents.

This partially prevents water ham m er in the radial openings, m ainly the feed pipe for the wastewater to be cleaned. Furthermore, the partitions help to direct the pressure wave evenly and axially. This way the flakes are less disturbed by the jet.

Preferably, the inner electrode is m ade of steel, at least in the surface layer. Steel is advantageous since the alloy can be controlled for electronegativity. Thus, with a good choice of steel, the difference in electronegativity can be controlled. Preferably, the outer electrode consists of iron or alum inium . Both are inexpensive, easy to process and both iron hydroxide and alum iniu m hydroxide are poorly soluble in water. Furthermore, iron hydroxide and alum inium hydroxide coagulate well with contam inants such as heavy m etals. In the third aspect, the invention comprises an assembly for purifying wastewater comprising: (i) a device for separating contaminants from wastewater by means of electrocoagulation according to the second aspect and (ii) a separating device suitable for separating purified water and contaminants coagulated into flakes.

Thanks to the cleaning of the surface of the electrolytic cell according to the second aspect, a more consistent water quality and more consistent flake properties are obtained. This also makes the efficient separation of purified water and flakes easier.

EXAMPLES

Example 1

An embodiment of the electrolytic cell 1 is shown in Figure 1. This electrolytic cell 1 consists of two concentric pipes, the inner 2 consisting of steel and the outer 3 consisting of iron. Here the outer electrode 3 is more electronegative. The inner electrode 2 has a radius of 4 cm and a length of 100 cm. The outer electrode 3 has a radius of 7 cm and a length of 100 cm. The concentric pipes 2, 3 are attached at the bottom to a base 4, and at the top to a top 5.

This base holds the electrodes in their concentric position and is radially equipped with two valves 7, 9. A first valve 7 is suitable for supplying wastewater to the electrolytic cell. A second, lower located valve 9 is suitable for emptying the electrolytic cell 1 for maintenance. This is desirable for the complete cleaning of the cell or replacement of an affected outer electrode 3.

The top 5 has two concentric fastening rings at the bottom. These fit closely with the electrodes and hold them in a fixed position. The top is provided centrally at the top with a pipe connection 10, 11. This is intended for the discharge of water containing flakes of coagulated impurities to a separating device, preferably a separation tower.

The effluent arrives at the bottom and is passed through cell 1 between the concentric electrodes 12. In the cell under the influence of redox reactions, contaminants coagulate into flakes. I n the top, the water is conducted from between the two electrodes to a central position 10 and is discharged along this central position 10 to a separation tower. The cell can process flow rates between 100 and 1000 l/h. The current intensity in the cell is between 5 and 250 amperes. The electrical voltage in the cell is between 1 and 60 volts. The rate of coagulation in the cell is proportional to the electrical power in the cell. The average consum ption of the electrolytic cell is 1 .5 kWh/m ³ . The m axim um consum ption of the electrolytic cell is 5 kWh/ m ³ .

The cleansing capacity of the cell was tested. Heavy m etals were almost com pletely removed from the wastewater. Many organic com pounds were also not found in the purified water. The chem ical oxygen dem and of the water (COD) decreased considerably.

I n the case of salt water, the water was only partially desalted. Alkali m etal ions were virtually not removed. Alkaline earth m etal ions were partially removed, usually between 30 and 60% . I ons of other m etals, m ainly heavy m etals, including Ni, Co, Cu, Zn, Ag and Sn are almost com pletely removed. More than 95% of the ions of these m etals are rem oved from the wastewater. The cleaning capacity depends on the electrical power acting on the cell.

I f the cell 1 is not continuously cleaned, flakes also coagulated along the outer, active electrode 3. There, these flakes form a film . Once this contam inating layer forms along the su rface, it grows rapidly due to coagulation. These layer of im pu rities along the electrode lead to a considerably higher cu rrent consum ption for the sam e cleaning capacity of cell 1 . As the layer grows rapidly, this current consumption also increases rapidly. I n the case of a heavily contam inated active electrode, an increase in the electrical power is not sufficient to guarantee the cleaning capacity of the cell. Cell 1 is operated at a constant current intensity. When the voltage rises by more than 1 0% com pared to the clean electric cell, said cell was cleaned.

Exam ple 2

The sam e electrolytic cell 1 as in the preceding exam ple was equipped with an u ltrasonic wave generator 20. This wave generator 20 was placed centrally over the entire circumference of the reactor 1 as shown in Figure 2A. The wave generator 20 produces ultrasonic waves, which break up the contam inating layer and detach it from the electrode. The flakes are carried along with the effluent and separated in the separation tower. The wave generator was activated every 15 m inutes for 30 seconds. The ultrasonic waves have a frequency of 40 kHz. Exam ple 3

The same electrolytic cell 1 as in the first exam ple was equipped with an ultrasonic wave generator 21 . This wave generator was provided with a Y-shaped tube 21 in one leg. The other leg 22 of the Y-shaped tube allows the influent through. This Y-shaped tube was placed in front of the electrolytic cell. The device produces ultrasonic waves in the influent, which causes som e degree of turbulence. This significantly delayed the form ation of a contam inating layer and can remove the contam ination itself. I f, after a considerable period of tim e, a contam inating layer is still form ed, the ultrasonic device is installed for a longer period of tim e, e.g. 2 m in. I n certain places it was broken by the turbulence, and the resu lting flakes carried by the water to the separation tower.

Com pared to the preceding exam ples, this ensured that for heavily organically loaded wastewater the contam inating film along the inside su rface of the outer electrode 3 is not form ed after 8 hours of operation and that a j et with water is no longer required. Fu rthermore, this arrangem ent also proved to im prove the cleaning capacity of cell 1 .

Exam ple 4

The sam e electrolytic cell 1 as in the first exam ple, wherein the base is replaced by a jet cap 40 as shown in Figure 4A. The jet cap 40 is shown in detail in Figure 4B. Like the base in Exam ple 1 , this j et cap 40 has two radial connections, one for supplying wastewater 41 and one for em ptying the tank 42. However, the nozzle has a third axial connection 43 on the sam e axis as the concentric pipes. The jet cap is provided with fou r radial partitions 44a, 44b, 44c, 44d, resulting in four different sem i-open com partm ents 45a, 45b, 45c, 45d.

The first com partm ent is connected to the influent supply 45a. The third com partm ent 45c is connected to the valve for em ptying the tank. The second 45b and fou rth com partm ent 45d are connected to the axial connection. The radial partitions 44a, 44b, 44c, 44d do not extend all the way to the bottom of the j et cap 40. All the com partm ents are connected to the axial connection 43 com pletely at the bottom of the jet hat 40. This results in a jet in each of the four com partm ents 45a, 45b, 45c, 45d. This also causes the influent water that enters the cell to be sucked along, which increases the acceleration and the flow. The partitions 44a, 44b, 44c, 44d do, however, extend through the connection for the influent 41 . This creates an axial wave and partially counteracts water ham m er.

I n use, a rinsing liqu id was pushed through cell 1 at a pressure of 1 bar along this axial connection 43. The rinsing liquid contains purified water. Thanks to the high pressure, the rinsing liquid is pushed through the cell, creating a jet that moves through the cell. Behind this j et, a negative pressu re develops, which pu lls along the effluent. This creates som e turbulence.

The cell was rinsed for 1 m inute every 30 m inutes. This significantly prevented the form ation of the contam inating layer.

Exam ple 5

The sam e electrolytic cell 1 as in the first exam ple, further provided with a rotatable brush 30, is shown in Figu re 3A. The brush is shown in detail in Figure 3B. The brush can rotate around the inner pipe 2, as well as move axially along this pipe by m eans of a fastening ring 31 . The brush bristles 32 consist of a plastic such as: polypropylene or polyam ide, and do not come into contact with the inner surface of the outer electrode 3. The brush 30 is tilted, at an angle of 45° .

The surface of the outer electrode 3 was not cleaned by the brushing of a brush 30 against this surface, since the brush 30 does not touch this surface. The turbulence and flow, which the brush 30 form ed as a vane causes, provides an axial flow along the inner su rface of the outer electrode 3. This flow reduces the coagulation of the flakes against this su rface. The flakes are m ainly carried by the flow to the separation tower. Thanks to the angle and the existence of space between each row of brush bristles 32, only a sm all part of the flakes is carried along by the brush 30. Nevertheless, som e of the coagulated flakes rem ain hanging along the brush 30. I f too m any flakes coagulate on the brush 30, they com e loose due to the flow and are carried along with the water.

Exam ple 6

The sam e electrolytic cell 1 as in the first exam ple, wherein the cell is purified by rinsing with hot water. To this end, the cell 1 is first em ptied, with the aid of the outlet 8 which is lower than the inlet for wastewater 6. The wastewater is collected and later recycled to the inlet 6.

The em pty electrolytic cell 1 is rinsed with hot water. Detergents, su rfactants, pH regu lators, silica and other substances can also be added to the hot water for good cleaning. I n this exam ple, water containing citric acid and a sm all amount of detergent is used. The result is good cleaning which almost completely removes the contaminating film. The cell 1 is rinsed every 2 hours.

Example 7

The same electrolytic cell 1 as in the fourth example, wherein the jet cap is adapted to a rotating jet cap. A rotating jet cap has a structure like the jet cap in Example 4, but the partitions have been partially replaced by a rotating nozzle near the axis of the electrolytic cell. This rotating nozzle produces a jet wave with cleaning liquid, with which a surprisingly good cleaning of the electrodes was obtained.

The jet cap allows, on the one hand, to admit influent into the electrocoagulation reactor and, on the other hand, to supply a jet with cleaning liquid under pressure. This jet is supplied at a pressure of 2 to 16 bar to the connection of the cleaning liquid of the nozzle of the jet cap. The cleaning liquid may comprise water, wastewater, hot water, organic solvents, detergents and surfactants, depending on the application.

Just like the base in examples 1 and 4, the jet cap has two radial connections, one for supplying wastewater and one for emptying the tank. The jet cap also has a third axial connection on the same axis as the concentric pipes. The jet cap is equipped with a rotating nozzle for producing a jet. This nozzle is arranged so that the jet propagates upwards from the bottom to the top through the jet.

The nozzle consists of a housing, provided with a connection for the supply line of the cleaning fluid and an outlet for the cleaning fluid and a nozzle body through which the cleaning fluid flows. The third axial connection of the jet cap is connected to the nozzle entrance so that the cleaning fluid can be supplied through it.

The nozzle body has a spherical end. The nozzle body is provided in the housing and is mounted at the spherical end on a pan-shaped bearing which is provided around the outlet of the housing. The nozzle body is made to rotate through the flow of cleaning fluid through the housing. The longitudinal axis of the nozzle body revolves around a generated cone. The bearing supporting the nozzle body is formed by a depression arranged in the inner wall of the housing, concentric with respect to the outlet of the housing.

With the help of this improved cleaning technique, the flow of wastewater that is cleaned by the coagulation cell could be increased to 2-5 m ³ of wastewater per hour. The cell remained sufficiently clean for long-term use.