FR3033164A1 - METHOD FOR DEHYDRATING SLUDGE - Google Patents

Abstract

A sludge dewatering process for achieving a high level of dryness for a limited power consumption, comprising a chemical conditioning step (S1) of the sludge to be dehydrated (10) during which a flocculation adjuvant (11b) and a Coagulation adjuvant (11a) is added to the sludge (10), and an electro-dehydration step (S4).

Description

FIELD OF THE INVENTION The present disclosure relates to a sludge dewatering process for achieving a high level of dryness for limited power consumption.

Such a process can in particular be used to dewater sludge, for example from purification plants, for incineration or burial. STATE OF THE PRIOR ART Sludges, such as those resulting from wastewater treatment plants for example, are generally subjected to dehydration followed by recovery, as organic amendment (spreading) or by combustion (incineration), or a landfill in a storage center. In the case of combustion, the dryness of the sludge reached at the end of the dehydration step, that is to say the ratio of dry matter to the total mass of sludge, must be sufficient to allow the self-maintenance of the combustion: a dryness of 30 to 45% is thus preferred. Similarly, in the case of landfilling, some national legislation requires a minimum dryness rate before sludge landfill, of the order of 40% for example. The importance of developing effective dewatering processes is therefore understood, particularly when the sludge is of a type that is difficult to dehydrate, as is the case for biological sludge in a treatment plant. Among the known methods, some are intended to mechanically dehydrate the sludge: it may be centrifugation or pressure filtration processes, including the use of a filter press or a belt filter. However, such methods are limited. For example, for sewage sludge, they allow to reach dryness of only 15 to 40%; this maximum value of 400Io being reached only in rare cases with particularly effective devices and sludge particularly easy to treat (mainly mineral). Thus, for some sludge, such as biological sludge treatment plant, it is difficult to reach a dryness of 30%, which may be insufficient depending on the fate of the sludge.

Other processes are intended to thermally dehydrate the sludge: it is then to heat the latter to dry them. These processes are very effective: depending on the heating temperature and the duration of the treatment, it is possible to reach dryness of the order of 90%. However, these processes are very energy efficient and consume at least 700 kWh per ton of water removed (kWh / tEE), or even more, 5 when a mechanical process consumes in general less than 20 kWh / tEE depending on the type of technology ( filter press, belt filter or centrifuge). Finally, electrically assisted mechanical dewatering processes, referred to as electro-dewatering processes, are intended to filter under pressure a sludge volume, for example in a filter press, by additionally applying an electric field. This, added to the hydrolysis phenomenon it creates, breaks the water-sludge bonds, facilitating the migration of water molecules in the opposite direction of the solid particles. Such electrical assistance thus makes it possible to improve the dewatering of the sludge and thus to gain some additional dryness points. However, such a process is faced with certain difficulties in the case of certain sludge. Indeed, during electro-dehydration, the sludge behaves like an electrical resistance. However, in the case of certain sludges for which the electromobility and / or the amount of ions is low, there is a rapid rise in the electrical resistance of the sludge. Therefore, by Joule effect and constant intensity, the rapid rise of the electrical resistance of the sludge results in an increase in temperature. The more dry the sludge, the lower the electromobility is and therefore the higher the electrical resistance. At the same time, this causes an increase in temperature, in particular on the side of the anode, which is the driest zone at the time of electro-dehydration. For this reason, the duration of such electro-dehydrations is necessarily limited in time so as not to damage the electro-dehydration device. In addition, this increase in electrical resistance at constant intensity and therefore the voltage causes significant energy losses. Therefore, for such types of sludge, the gain in dryness is relatively low and unattractive considering the additional electrical consumption invested.

There is therefore a real need for a sludge dewatering process which is devoid, at least in part, of the disadvantages inherent in the aforementioned known processes.

PRESENTATION OF THE INVENTION The present disclosure relates to a sludge dewatering process, comprising a step of chemically conditioning the sludge to be dehydrated during which a flocculant adjuvant and a coagulation adjuvant are added to the sludge, and a step of electro-dehydration. Individually, coagulation adjuvants and flocculation adjuvants have no noticeable effect on the function and effectiveness of electro-dehydration. On the other hand, the inventors have found that their combined use in sludge conditioning offers a synergistic effect allowing a significant gain in efficiency of the electro-dehydration stage and therefore in final dryness of the sludge. Indeed, when none or only one of these components is used, there is usually a rapid increase in the voltage across the electrodes of the electro-dehydration device, which can then stabilize and fall back or reach and ceiling at a maximum voltage value set by the operator to protect the electrodes. Therefore, this peak voltage, appearing almost immediately after the beginning of the electro-dehydration, causes a rapid rise in the temperature of the sludge on the side where it is the driest, that is to say on the side of the anode, which leads to the premature termination of the electro-dehydration to prevent the deterioration of the device. On the other hand, when a coagulation adjuvant and a flocculation adjuvant are combined, it is unexpectedly found that the usually observed peak voltage does not occur. On the contrary, the experiments carried out by the inventors have shown that the tension began to decrease during the first minutes of the electrodehydration before rising and then stabilizing or going back down. As a result, the rise in temperature in the vicinity of the anode is delayed, which makes it possible to extend the duration of the electro-dehydration step before reaching a temperature level 303 3 1 6 4 4 damage to the device: more water can be extracted, resulting in greater dryness. One hypothesis advanced by the inventors to explain this surprising phenomenon would be that, beyond improving the filterability of the sludge, the combination of a coagulation adjuvant and a flocculation adjuvant increases the amount and the electromobility of the ions contained in the mud. Such a phenomenon would contribute to reducing the electrical resistance of the sludge, in particular at the beginning of the application of the electric field, and thus to reducing the rise of the voltage and therefore the Joule heating of the electrodes. Thus, thanks to this particular process and this conditioning, the tests have shown that one gains from 10 to 60 points of dryness compared to a conventional dehydration for a reduced power consumption, the overall energy consumption of such a method remaining 15 below 300 kWh / tEE. In some embodiments, the flocculant adjuvant used is a polymer. In some embodiments, the polymer used is of the cationic flocculant type. This type of polymer indeed allows better aggregation of the sludge particles and gives good results in filtration in the context of the present disclosure. In some embodiments, the polymer used is a cationic flocculant of high density and low molecular weight. However, other anionic, cationic or nonionic flocculant additives would also be possible with different molecular weights depending on the type of sludge to be dewatered. In some embodiments, during the chemical conditioning step, the weight of flocculant adjuvant added to the slurry is between 1 and 25 kg of pure product, i.e., active material ( MA), per ton of dry matter in the sludge (kgMA / tMS), preferably between 4 and 15 kgMA / tMS. The inventors have indeed found during their tests that this concentration range gave very satisfactory results in terms of controlling the kinetics of electro-dehydration and therefore in terms of achievable dryness.

In some embodiments, the coagulation adjuvant is a mineral-type coagulant, for example FeCl3. However, other mineral or organic adjuvants would also be possible. In some embodiments, during the chemical conditioning step, the mass of pure clotting aid product added to the slurry is between 0.5 and 20% of the dry sludge mass. The inventors have indeed found during their tests that this range of concentrations gave very satisfactory results in terms of control of the kinetics of electro10 dehydration and therefore in terms of achievable dryness. In some embodiments, the electro-dehydration step is carried out within a filter press comprising at least one chamber consisting of two trays, each tray being provided with at least one electrode and at least one tray being equipped with a filter, preferably a filter cloth. Each chamber consists of a tray equipped with an anode and a tray equipped with a cathode to apply the electric field. Such a filter press allows firstly to filter and then compress a volume of sludge between two trays to extract the water through the filter cloths lining the trays. In a second step, while the sludge is still compressed, the electrodes make it possible to create an electric field between the trays to facilitate the migration and therefore the evacuation of the water through the filter cloths. In some embodiments, at least one platen of the filter press is provided with a membrane configured to be deformed by a compression fluid to compress the sludge present in the filter press. In some embodiments, the pressure exerted during compression is between 5 and 20 bar, preferably between 10 and 13 bar. In some embodiments, the electro-dehydration step is performed, at least initially, at constant intensity. This mode of regulation makes it possible to promote rapid dehydration of the sludge.

In some embodiments, the intensity used is between 5 and 200 Ainn2.

In some embodiments, during the electrodehydration step, constant current regulation is abandoned in favor of constant voltage regulation when a predetermined condition is met. Such a mode of regulation makes it possible to limit the growth of the temperature at the level of the electrodes. The constant voltage value used then is preferably that which was reached when the predetermined condition is satisfied. In some embodiments, during the electrodehydration step, constant current control is abandoned in favor of constant voltage regulation when the voltage reaches a threshold voltage. This threshold voltage is set by the operator so as to use the electrodes below their breakdown voltage. In other embodiments, constant current regulation is abandoned in favor of constant voltage regulation when the temperature in the vicinity of an electrode reaches a threshold temperature. This allows operation at constant intensity, thus with high dehydration efficiency, as long as the temperature remains low enough and switch to constant voltage once the threshold temperature has been reached in order to slow down the temperature growth and therefore to prolong the dehydration before the temperature becomes too high and dangerous for the electro-dehydration device. In some embodiments, constant current control is abandoned in favor of constant voltage regulation when the anode temperature reaches a threshold temperature of between 50 and 65 ° C. This is indeed a good compromise between the interest of benefiting as long as possible from highly effective constant intensity dehydration and that of being able to prolong the dehydration at constant tension as long as possible once this threshold temperature has been reached.

In certain embodiments, during the electro-dehydration step, the temperature of the sludge in the vicinity of the electrodes does not exceed a ceiling temperature of less than or equal to 150 ° C., preferably less than or equal to 80 ° C. C, more preferably less than or equal to 70 ° C. This reduces the risk of damage to the device. Electro-dehydration is thus preferably stopped as soon as such a ceiling temperature is reached.

In some embodiments, the electro-dehydration step lasts at least 45 minutes, preferably at least 60 minutes, more preferably at least 100 minutes. Such times, allowed by the present process, allow for greater sludge dryness levels than conventional electro-dehydration processes. In some embodiments, at the end of the electrodehydration step, the sludge dryness is greater than 30%, preferably greater than 40%, more preferably greater than 50%.

In some embodiments, the method further comprises a filtration step performed prior to the electro-dehydration step. This filtration step allows a first filtration before the application of the electric field to remove the majority of the free water contained in the sludge. This filtration step is preferably carried out after the conditioning step. In some embodiments, the method further comprises a compression step performed prior to the electro-dehydration step. This compression step alone makes it possible to mechanically extract the remainder of free water without electrical assistance. This can make it possible to extract the free water already from a sludge without consuming the electricity necessary for electro-dehydration and without starting the temperature growth of the electrodes. Preferably, this step is carried out after the filtration step and using the filter press. The compression pressure can be between 5 and 20 bar, preferably between 10 and 13 bar.

In other embodiments, the process is devoid of a compression step prior to the electro-dehydration step. The inventors have indeed found in the course of their tests that an application of the electrical treatment just after the filtration step consumes little energy and even that such a single preliminary compression stage could be unfavorable in the case of sludge difficult to dehydrate, with high electrical resistance in particular. In some embodiments, the sludge to be dewatered is a biological sludge from a treatment plant, including an aerobic biological purification process. This type of sludge specifically contains a high proportion of bound water. However, of course, this disclosure could be applied to other types of sludge.

The above-mentioned features and advantages, as well as others, will be apparent from the following detailed description of exemplary embodiments of the proposed method. This detailed description refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are schematic and are intended primarily to illustrate the principles of the invention. In these drawings, from one figure (FIG) to another, elements (or 10 parts of element) identical are identified by the same reference signs. FIG 1 is a block diagram of an exemplary method according to the invention. FIG. 2 is a block diagram of the filter press used for the needs of the process. FIG 3 is a graph comparing the evolution of the voltage during a conventional electro-dehydration and an electrodehydration according to the invention. FIG. 4 is a graph comparing the evolution of temperature during conventional electro-dehydration and electrodehydration according to the invention. FIG. 5 is a graph of time tracking of the parameters of conventional electro-dehydration at high intensity. FIG. 6 is a graph of time tracking of the parameters of conventional electro-dehydration at lower intensity. DETAILED DESCRIPTION OF THE EMBODIMENT (S) In order to make the invention more concrete, examples of dewatering processes are described in detail below with reference to the accompanying drawings. It is recalled that the invention is not limited to these examples. FIG 5 illustrates the temporal monitoring of the various parameters of an electro-dehydration performed by the inventors without the packaging according to the present invention. This first test is performed at high intensity. In this case, the electro-dehydration is carried out on a sludge difficult to dehydrate, having a high electrical resistance, with a constant intensity of 16 A, ie about 70 A / m 2, and a limited voltage at 110 V in order not to reach the breakdown voltage of the system. As can be seen in FIG. 5, because of the high electrical resistance of the sludge, the voltage 82 (strongly interrupted line) increases very rapidly to reach its maximum value of 110 V in less than one minute. The constant-intensity regulation can no longer be maintained and the intensity 81 (strong continuous line) begins to decrease before stabilizing while the voltage 82 is maintained at 110 V. There is then a rapid rise in temperature anode side 83 (continuous fine line) which reaches 70 ° C in less than fifteen minutes, then imposing the stopping of the test. On the other hand, it should be noted that the rise in temperature on the cathode side 84 (fine line interrupted) is much slower so that it is determined that the temperature on the anode side is the limiting parameter for such electro-dehydration.

Thus, in this first test not implementing the method according to the invention, it is found that the total duration of the treatment does not exceed fifteen minutes. It is therefore understandable that the level of dryness reached is not satisfactory. In a second test carried out without packaging in accordance with the present invention, the inventors sought to reduce the intensity of the current applied during electro-dehydration. In this case, the latter is made with a constant intensity of 9 A, or about 40 A / m2 As can be seen in FIG 6, the voltage 92 (strong line 25 interrupted) increases again very quickly in the first seconds of the test then slows down its growth before nevertheless reaching its safety value at 110 V after about three minutes. The constant-intensity control can no longer be maintained and the intensity 91 (strong continuous line) begins to decrease while the voltage 92 is maintained at 110 V. However, a rapid rise in the temperature on the anode side 93 ( continuous fine line) which reaches 70 ° C in about ten minutes, then imposing the stopping of the test. Here again, the rise in temperature on the cathode side 94 (thin line interrupted) is negligible compared with that of the anode 93.

Thus, in this second test which does not implement the method according to the invention, it is found that the duration of the constant intensity treatment is a little longer but does not however exceed four minutes. In addition, the total duration of treatment does not exceed ten minutes. It is therefore understandable that the level of dryness achieved is once again insufficient. To improve these dehydration conditions, the method according to the present invention will now be described. Its overall sequence is shown in FIG. 1. In the context of the present process, the sludge to be treated 10 is a biological sludge from a purification plant. This type of sludge is derived from a membrane bioreactor operating directly on untreated raw water. Sludge 10, whose initial dryness is about 0.9% in this example, that is to say 9 g of solids per liter of liquid sludge, first undergoes a chemical conditioning step Si at during which certain chemical components are added to the sludge 10 to facilitate its treatment and dehydration. Among these components, a coagulation adjuvant (11a) and a flocculation adjuvant (11b) are added. In this example, the coagulation adjuvant (11a) used is FeCl3. It is added to the sludge tank at a level of 15% by mass of dry sludge material. The flocculation adjuvant (11b) used is the Flopam EM 640 TBD polymer. It is injected in line with a target concentration of 4.6 kg of pure product per tonne of dry matter (4.6 kgMA / tMS).

The sludge 10 thus treated is injected into the filter press 20 by means of a booster pump. This filter press 20 is shown diagrammatically in FIG. 2. It comprises three trays 21a, 21b defining two chambers 22 in which the sludge 10 is introduced. Each wall of the trays 21a, 21b defining an edge 30 of a chamber 22 is provided with an electrode 23a, 23b: the lateral plates 21a are thus provided with an electrode forming the anode 23a and the central plate 21b is provided with each of its faces an electrode forming the cathode 23b. The inner wall of each side plate 21a is further provided with a filter cloth 24 disposed in front of the electrode 23a considered. Each wall of the central plate 21b is provided with a membrane 303 3 1 6 4 11 disposed behind the electrode 23b considered and a filter cloth 24 disposed in front of the electrode 23b considered. The three plates 21a, 21b of the filter press 20 are clamped against each other via a hydraulic jack 26 which seals the filtration chambers 22. Under the strong pressure of the jack 26, the plates are brought together one by one. towards the other in order to maintain the volumes of sludge present in the chambers 22. It is of course understood that it is possible to provide several tray modules 21a, 21b before the jack 26.

Each plate 21a and 21b is equipped with a first outlet for discharging the filtrates 12 and a second outlet 13a and 13b for evacuating any gas produced. During a filtration step S2, the booster pump generates a mechanical pressure of 8 bar within the sludge 10 which is then filtered using the filtering cloths 24. This step S2 stops when the flow rate filtrate 12 discharged from the trays 21 reaches a predetermined low threshold. This step S2 makes it possible to eliminate the free water present on the surface of the sludge flocs with less energy expenditure. Once the sludge 10 has been filtered in the chambers 22 of the filter press 20, a compression step S3 alone is carried out within the filter press 20. A compressed air booster makes it possible to inflate the membranes 25 of the filter press 20 so as to compress the sludge present in the chambers 22 until a pressure in the chambers 22 of about 11 to 12 bar is reached. The compression is thus maintained for 10 minutes. No electrical assistance is provided during this step S3. This compression step alone S3 makes it possible to extract the remainder of free water present in the sludge 10 in the form of filtrates 12 discharged by the outlets of the trays 21. At the end of this compression step alone S3, the compression 30 is maintained and electrical assistance is triggered to initiate an electro-dehydration step S4. An electric generator then imposes a current between the pairs of electrodes 23a, 23b of each chamber 22. Initially, regulation is carried out at constant current. In this example, an intensity of about 35 9 A is chosen, about 40 A / m2.

303 3 1 6 4 12 During this step, the water extracted from the sludge 10 is discharged in the form of filtrates 12 through the outlets of the trays 21a and 21b. In addition, the gases generated during this step S4, oxygen and hydrogen resulting from the hydrolysis of water, for example, are removed by the 5 seconds outputs trays 13a and 13b. The constant current regulation is maintained until the temperature in the vicinity of an anode 23a, measured by means of a thermocouple for example, reaches 60 ° C: when this condition is fulfilled, the regulation of the current is abandoned in favor of constant voltage regulation. The voltage value selected for this constant voltage control is the maximum safe voltage value, 110V in this example. Constant voltage electrical assistance is then maintained until the temperature near an anode 23a reaches 70 ° C: when this condition is met, the electrical treatment is stopped and the compression is released. The sludge cakes 14 thus dehydrated are then removed from the chambers 22 of the filter press 20 during a step of disintegrating the cakes S5. The dryness values obtained for these cakes 14 are on average about 38.3%. These cakes 14 can then be buried, or upgraded organic amendment or energy. Several series of tests will now be presented to illustrate the contribution of the invention compared to conventional methods.

In all these tests, the same type of sludge 10 was used: it is a biological sludge of the type that comes from purification plants. More particularly, it is a sludge of the type that is derived from a membrane bioreactor operating on untreated raw water. This sludge has an initial dryness of less than 1%, that is to say 10 g / L.

The operating conditions of the TO test are the following: FLOPAM EM 640TBD polymer at a level of 8.95 kg / tMS, ie 3.8 kg ai / tMS; no coagulant; - filtration only at 8 bar; 35 - compression alone at 11-12 bar for 10 minutes; 303 3 1 6 4 13 - compression at 11-12 bar + electro-dehydration at 9 A (ie around 40 A / m2) with constant voltage switching over anode side temperature of 60 ° C.

The operating conditions of the T1 test are as follows: FLOPAM EM 640TBD polymer at 12.8 kg / tMS, ie 5.4 kg MA / tMS; - FeCl3 coagulant at 15% of the dry sludge mass; - filtration only at 8 bar; 10 no compression step alone; - compression at 11-12 bar + electro-dehydration at 9 A (about 40 Ahri2) without tilting at constant voltage beyond anode side temperature of 60 ° C. FIGS. 3 and 4 are intended to compare the evolution of the voltage and the temperature of conventional electro-dehydration and electro-dehydration according to the invention. Thus, the voltage 31, 36 and the anode temperature 32, 37 were followed during the electro-dehydration step of these tests: the curves relating to the TO test are in broken lines while the curves 20 relating to the test T1 are in continuous lines. In the context of the test TO in which the conditioning has not used coagulant, it can be seen in FIG. 3 that the voltage 31 first increases rapidly and forms a voltage peak 31a at approximately 80 V in the first minutes of the test; the voltage then drops back to stabilize from about one-tenth minute to about 50 V. It should be noted that the voltage measured in the context of the Ti test, whose conditioning has simultaneously used a coagulant and a polymer, follows an evolution totally opposite the voltage 31 of the test TO since, after the introduction of the electric field, the voltage 36 of the test Ti decreases from its initial value instead of increasing as was the case in the test TO. The voltage 36 then reaches a minimum voltage value at about 40 V and starts to grow significantly only later, after about 30 minutes of testing, until reaching a plateau 36a at about 50 V after forty minutes. approximately, before stabilizing at a value between 30 and 40 V.

303 3 1 6 4 14 Thus, it can be seen in FIG. 4 that the anode temperature 32 of the TO test increases much more rapidly than the anode temperature 37 of the Ti test since it reaches a value of 40 ° C. as soon as the beginning of the TO test while the same threshold is reached by the anode temperature 37 of the T1 test after 15 minutes. Consequently, at the end of this transient regime, the behavior of which differs between the TO test and T1, there is a time lag of the temperature curves: the ceiling temperature of 70 ° C is thus reached after 45 minutes for the TO test and after 120 minutes for the Ti test. Thus, in total, the electro-dehydration in the test according to the invention T1 lasted about 75 minutes longer than the comparative test TO which ended after about 45 minutes. In addition, a measurement of the dryness at several points of the cakes thus obtained revealed: for the TO test, an average dryness value of 26 ± 5%; - For the T1 test, a mean dryness value of 38 ± 7% There is therefore an average gain of 12 points in dryness when the packaging combines a polymer and a coagulant compared to conditioning with a polymer alone.

In addition, the energy consumption generated by each of these electro-dehydrations was measured during the test according to the invention T1 and the reference test TO. These energy consumptions are 260 kWh / tEE for the conventional electro-dehydration of the TO test and 140 KWh / tEE 25 for electro-dehydration according to the invention of the T1 test is an energy consumption decreased by almost half. The modes or examples of embodiment described in the present description are given for illustrative and not limiting, a person skilled in the art can easily, in view of this presentation, modify these modes or embodiments, or consider others, all remaining within the scope of the invention. In addition, the various features of these modes or embodiments can be used alone or be combined with each other. When combined, these features may be as described above or differently, the invention not being limited to the specific combinations described herein. In particular, unless otherwise stated, a feature described in connection with a mode or example embodiment may be similarly applied to another embodiment or embodiment.

Claims (13)

REVENDICATIONS1. A sludge dewatering process comprising a chemical conditioning step (Si) of the sludge to be dehydrated (10) during which a flocculant adjuvant (11b) and a coagulation aid (11a) are added to the sludge (10) , and an electro-dehydration step (S4). 2. Method according to claim 1, wherein the flocculating adjuvant (11b) used is an anionic, cationic or nonionic flocculant type polymer. 3. Method according to claim 1 or 2, wherein, during the chemical conditioning step (Si), the mass of flocculation adjuvant (11b) added to the sludge is between 1 and 25 kg of pure product. per tonne of dry matter. 4. Process according to any one of claims 1 to 3, wherein the coagulation adjuvant (11a) used is of the mineral type. 5. Process according to any one of claims 1 to 3, wherein the coagulation adjuvant (11a) used is of the organic type. 6. Process according to any one of claims 1 to 5, wherein, during the chemical conditioning step (Si), the mass of pure product of the coagulation adjuvant (11a) added to the sludge is included between 0.5 and 20% of the mass of dry sludge. 7. Method according to any one of claims 1 to 6, wherein the electro-dehydration step (S4) is carried out in a filter press (20) comprising at least two trays (21a, 21b). each tray (21a, 21b) being provided with at least one electrode (23a, 23b) and at least one tray (21a) being provided with a filter (24), in which the filter (30a, 23b) press (20) compresses the sludge (10) during the electro-dehydration step (S4). 8. Process according to any one of claims 1 to 7, in which the electro-dehydration step (S4) is carried out, at least initially, at constant intensity, then the constant-intensity regulation is abandoned in favor of a constant voltage regulation when the temperature in the vicinity of an electrode (23a) reaches a threshold temperature. 10 9. A method according to any one of claims 1 to 7, wherein the electro-dehydration step (S4) is carried out, at least initially, at constant intensity then the constant intensity control is abandoned in favor of a constant voltage regulation when the voltage reaches a threshold value The method according to any one of claims 1 to 9, wherein during the electro-dehydration step (54), the temperature of the slurry in the vicinity of the electrodes (23a, 23b) does not exceed one 20 ceiling temperature lower than or equal to 150 ° C, preferably lower than or equal to 70 ° C. 11. A process according to any one of claims 1 to 9, further comprising a filtration step (S2) carried out before the electro-dehydration step (S4). 12. Method according to any one of claims 1 to 10, further comprising a compression step (53) performed before the electro-dehydration step (S4). 30 13. Method according to any one of claims 1 to 11, wherein the sludge to be dewatered (10) is a biological sludge from a purification plant.