MXPA98004058A - Method for cleaning a filtration installation of type that has sumergi membranes - Google Patents

Abstract

The present invention relates to a method for cleaning a filtration installation of the type constituted by a plurality of membranes submerged in a tank at least, containing an effluent to be filtered, said method being characterized because it includes the steps consisting of: - draining at least partially the effluent contained in said tank so that said membranes are exposed to the air, - passing at least one cleaning solution through said membranes along a flow path in the opposite direction to the flow of effluent filtering, feeding said cleaning solution from the permeate side of said membranes

Description

METHOD FOR CLEANING A FILTRATION INSTALLATION OF THE TYPE THAT HAS SUBMERGED MEMBRANES The invention relates to the field of installations for filtering effluents, specifically water, for the purpose of purifying and making it potable. More precisely, the invention relates to the field of such facilities that include filtering membranes that are directly immersed in the effluent to be treated. More precisely, the invention relates to a method for cleaning the membranes of such installations.

Installations with submerged membranes are characterized in that they use microfiltration or ultrafiltration membranes that can be flat, tubular or with hollow fibers, which are generally grouped together in modules and do not include any housing. These modules are immersed directly in the treatment tank containing the effluent to be filtered, and the permeate is extracted by suction. Such filtering installations are described in detail in US-A-5248424, in the name of Cote and others, in European Patent Application EP-A-510328 and in the article entitled "Direct separation of solids and liquids using a membrane of hollow fiber, in an activated sludge aeration tank "in the name of Yamamoto et al., which appeared in 1989 in the journal Water Sciencie Technology, vol. pp.43-54. Submerged membranes that are used in such installations are generally used under conditions that result in little obstruction with low pressure across the membranes, which generally does not exceed 0.5 bar, so that operations can be spaced as much as possible. cleaning of these membranes. However, cleaning operations are still necessary and are usually carried out with the help of chemical substances that are generally corrosive. In installations with traditional membranes, in which the filtering modules are not immersed directly in the effluent to be filtered, but which have a housing and are equipped with a filter circuit, the cleaning of the membranes can be carried out easily without having to remove the membranes from the installation. This type of cleaning, called cleaning in itself, is simply to circulate a cleaning solution through the recycling circuit. Such method is effective, since it allows to control well the concentration of the chemical in the cleaning solution, the temperature of the solution and the contact time of the same with the membranes. In addition, such a cleaning procedure can be done completely automatically. Finally, the volume of discharge is low and is proportional to the shrinkage of the recycling circuit. However, filtering installations of the type having submerged membranes do not incorporate housing or a recycling circuit. Consequently, one of the disadvantages related to the use of such facilities lies in the fact that the cleaning operations are made much more difficult by the absence of a housing that surrounds the filtering modules and also by the lack of a circuit such recirculation.

Current technology has provided several methods for cleaning such filtering installations with submerged membranes. One of these methods, called ex-si tu, consists simply of removing the filtering modules from the tank one by one and cleaning them in an apparatus specially provided for this purpose. Such method allows to carry out an effective cleaning of the membranes, but has several disadvantages. On the one hand, it has to interrupt the treatment or reduce its efficiency during the time, which can be relatively long, which is needed to transfer the modules to the cleaning apparatus and carry out the cleaning operation. In addition, such a method also has the disadvantage that it can not be easily automated, which increases the cost thereof. It has also been suggested in current technology to clean filtration facilities that have submerged membranes, replacing the effluent that is present in the treatment tank with a cleaning solution and operating the installation normally to allow the cleaning solution to pass through the pores of the membranes. That technique also has many disadvantages. Although this method is effective and can be automated, in fact it requires a large volume of cleaning solutions to be used. Apart from the fact that the cost of the reagents is increased, it is much more difficult and more expensive to heat such a large volume of cleaning solutions. Finally, the volume of the discharge (the dirty cleaning solution) is also increased. It should also be noted that in current technology a method has been proposed whose objective is to allow in situ cleaning of the membranes of an installation that includes such submerged membranes. Such a method, which is described in detail in US-A-5403479, in the name of Smith et al., Consists of circulating a cleaning solution through the membranes along a flow path in the opposite direction to the filter flow, and all without emptying the tank inside which the membranes are installed. The excess cleaning solution that does not pass through the membranes is recycled in such a way that the volume of the solution transferred to said tank is minimized. The effectiveness of this method is limited, since the cleaning solution used is inevitably diluted by the effluent present in the tank as soon as it passes through the membranes, which considerably reduces its effectiveness. At the same time, the temperature of this cleaning solution is also abruptly reduced as it passes, which also reduces its effectiveness. In addition, the scheduled time for injecting the cleaning solution must be limited, and as a result, the treatment being applied is not evenly distributed, since when it is a biological treatment, the biomass present in the tank can be rapidly decimated if the Cleaning solution is injected for too long. Finally, such a method can not be applied when the installation of the submerged membranes in question is used precisely to make the water drinkable, since the chemical reagents used in the cleaning solutions are not compatible with such treatment. The object of the present invention is to provide a cleaning method for a filtration installation of the type having submerged membranes, which does not have the disadvantages of current technology. More specifically, one of the objects of the invention is to provide such a method that can be applied at the same time that the membranes are kept in place in the installation. Yet another object of the invention is to describe such a cleaning method in situ with a small volume of cleaning solutions and which does not result in them being diluted. Another object of the invention is to offer a method that can be automated easily. Still another objective of the invention is to provide a filtering installation that allows the implementation of such a method. These different objectives, as well as others that will be obvious below, are achieved by the invention which relates to a method for cleaning a filtration installation of the type including a plurality of submerged membranes in a tank, at least, containing a effluent to be filtered, such method being characterized because it includes the following steps: - to evacuate at least partially the effluent contained in said tank, in order to expose said membranes to the air; - passing at least one cleaning solution through the pores of said membranes along a flow path in the opposite direction to the filtrate flow of the effluent, passing said cleaning solution from the permeate side of said membranes. Preferably, said method includes a step that consists of recovering or neutralizing said cleaning solution that has been passed through said membranes actively employed in said tank. Accordingly, the invention offers an original way to clean in situ the membranes of the installation by emptying, at least partially, the tank in which the filtration membranes are installed, in such a way that the cleaning solution is allowed to pass through. used from the permeate side of the membranes to the outside thereof, and then a flow of this solution on the outer surface of the membranes towards the bottom of the tank. In this way, the cleaning solution is used to the maximum of its capacity, since it does not dissolve when leaving the membranes and, on the contrary, it can run along after having passed through its membranes. pores In addition, such method allows a uniform distribution of the cleaning solution in the membranes, since no pressure is exerted against it. Taking into account the fact that the cleaning solution is not diluted after passing through the pores of the membranes, it is also possible to use less volumes of cleaning solutions than those needed to achieve an effective cleaning in the context of the method described above according to US-A-5403479. This is another advantage of the method since it can be applied with less cost. Although such a method can be applied in any filtration installation having submerged membranes, this method can be used more in installations whose membranes are placed vertically inside said tank. Indeed, such a position favors the flow of the cleaning solution on the outer surface of the membranes after it passes through the pores thereof. In this case, according to a preferred and especially interesting aspect, said step of the method according to the invention which consists of having at least one cleaning solution passed through the pores of said membranes along a path flow in the opposite direction to the flow for filtering the effluent, is carried out by feeding alternatively or simultaneously said cleaning solution by the upper part and by the lower part of said membranes. Such a unique feature allows the membranes to be well moistened and, as a consequence, to use smaller volumes of cleaning solutions. On this question, it should be noted that in the technique described by the aforementioned US patent, the cleaning solution is always fed from the bottom of the filtering modules. Advantageously, said method consists in performing a cleaning sequence that includes at least one step which consists of having at least one basic cleaning solution pass through said membranes, and also includes at least one step consisting of in passing through at least one acid cleaning solution through the pores of said membranes. The use of such a basic cleaning solution and such an acid cleaning solution allows the effectiveness of the cleaning operation carried out by applying the method according to the invention to be further increased. Preferably, said method also includes at least one step which consists of causing at least one cleaning solution containing an oxidizing agent to pass through the pores of said membranes. For example, such a solution could be constituted by sodium hypochlorite or hydrogen peroxide. Still more preferably, said method includes at least one step which consists of causing at least one solution containing a base and a chlorine compound to pass through the pores of said membranes, and includes a step of making that at least one acid cleaning solution passes through the pores of said membranes. In fact, it has been observed that the application of such a sequence results in a truly effective cleaning of the membranes, as will be explained in more detail below. It is important to say that such a sequence allows the use of cleaning solutions at room temperature, thus eliminating the almost inescapable need of current technology to heat the cleaning solutions that are normally used. A further advantage consists in that said steps of said sequence are interspersed with one or more rinsing steps, or are preceded or followed by them, consisting in causing water to pass inside said membranes. Yet another advantage is that said method can include at least one impregnation phase during which the supply of the cleaning solution is cut off to allow it to impregnate the membranes and thereby increase its effectiveness. As a further advantage, the solution or cleaning solutions are applied at a rate of a total volume ranging from 2 to 20 liters per square meter of membrane. These volumes are much smaller than those used in current technology, which are traditionally of the order of 50 liters per square meter. Preferably, the total duration of said cleaning sequence fluctuates between 30 minutes and 4 hours.

The invention also relates to an installation for carrying out such a method, such installation including at least one treatment tank within which are installed vertically and jointly several filter membranes, with means for feeding the effluent to be filtered in said tank, means for draining said tank, means for discharging the permeate from said membranes, at least one tank for storing a solution for cleaning said membranes, means for feeding said cleaning solution from the permeate side of said membranes, and characterized in that said means for feeding said cleaning solution include means that allow said cleaning solution to be supplied alternatively or simultaneously by the upper part or by the lower part of said membranes. According to an interesting variant of the invention, this installation includes, as an advantage, at least two treatment tanks mounted in parallel and each having in its interior a plurality of filter membranes installed vertically, and including means to allow the cleaning of the membranes of the first tank and means that allow the contents of this first tank to be stored in the second tank during the cleaning operation. Preferably, such installation includes means for connecting said drainage means to said supply means.

The invention, together with the various advantages it offers, will be understood more easily thanks to the following description, in a non-limiting manner, of putting into practice the invention to which the drawings refer, in which: Figure 1 represents a diagram of a first embodiment of an installation according to this invention; - Figure 2 represents a diagram of a second embodiment of an installation according to this invention; With reference to Figure 1, the filtering installation depicted includes a treatment tank 1. As is normal, that tank has means 2 for feeding the effluent to be filtered including a bypass valve 2a, and means for draining the tank 5 including a through valve 5a. The installation is continuously fed with the water that is going to be filtered through the valve 2a controlled by the water level of the tank. Organized in a module 4 there is a plurality of membranes 3 installed vertically in said tank. In the described embodiment, these membranes are constituted by ultrafiltration membranes made of hollow fibers, the filtering being carried out from the outside towards the interior, and they are mounted in a module of 12m2. The installation also includes a suction pump 6 that allows the extraction of the treated effluent constituted by the permeate of the membranes, through a network of channels 7, 8, 9, 10, 15 and 19. The installation also includes three tanks 11, 12 and 13 for storing cleaning solutions, each having a through valve, 12a and 13a, and also means including a set of channels 14 and 15 and a set of valves 16, 17 and 18 that allow these Cleaning solutions are fed at the foot of module 4; that is, by the bottom of said membranes 3. According to the invention, the installation also includes means that contain a channel 19, a valve 20, plus channels 8, 9 and 14 and valves 16, 17 and 18 that allow these cleaning solutions are fed through the head of module 4; that is, by the upper part of said membranes 3. Finally, it should be noted that the content of the tanks is connected to the water of the main pipe 21, which has a passage valve 22. In the filtering mode, the bypass valve 2a for the medium 2 which supplies the unfiltered effluent to the tank 1, is opened, and the bypass valve 5a of the means 5 for draining the tank is closed, and consequently the effluent fills tank 1 in a manner such that the filtering membranes are immersed 3. In addition, valve 20 is activated so that channel 7 communicates with channel 8, and valve 18 is activated to communicate channel 15 with channel 19. Finally, valve 16 is activated so that channels 8 and 19 communicate with channel 9, and valve 17 is activated to communicate channel 9 with channel 10 to discharge the permeate. In this way, the permeate is extracted at the same time, by the head and by the foot of the module 4. When the membranes 4 are clogged and they have to be cleaned, the installation is put into operation according to the cleaning method of the invention. The cleaning solutions contained in the reservoirs 11, 12 and 13 can be used at this time to unblock these membranes and can be easily injected, alternatively, through the upper part or through the lower part of the membranes. It should also be noted that the cleaning solutions can also be injected simultaneously through the upper part and the lower part of the membranes. According to the first step of this cleaning method, the tank 1 is drained by closing the passage valve 2a that is in the medium supplying the effluent to the tank 1, and opening the passage valve 5a of the drainage means 5 , so as to allow the effluent discharge from that tank and allow the membranes to be exposed to the air. According to the second step, which consists of carrying out the cleaning cycle itself, the three cleaning solutions are used one after the other. For example, if it is chosen first to employ the solution contained in the tank 13, as shown in Figure 1, valve 13a of this deposit is opened (The valves lia and 12a of the other tanks remain closed). Simultaneously, valve 20 is closed, valve 16 is activated so as to communicate to reservoir 13 with channel 9, valve 17 is activated so that channel 9 is communicated with channel 14, and the valve is activated 18 so that channel 14 communicates with channel 15. In this way, the cleaning solution reaches through the bottom of the membranes and propagates upwards through the entire height thereof. The flow rate of this solution is obviously calculated to allow the membranes to get wet well. The cleaning solution passes easily through the membranes, since there is no liquid present in the tank that could exert counterflow pressure. Then, the solution flows along the membranes. After having passed through the membranes, the cleaning solution, which is already dirty due to the impurities present in the membranes, is discharged through the medium 5 in order to drain the tank 1. It should be noted that in other embodiments to implement the method of the present invention, it is also possible to not discharge the cleaning solution or tank cleaning solutions but simply neutralize them. After a certain time to supply the cleaning solution through the lower part of the membranes, this same solution can be fed subsequently through the upper part of the membranes in order to increase the effectiveness of the cleaning and also to carry out the humidification of the membranes. To this end, the valve 18 is activated so that the channel 14 is communicated with the channel 19, and the valve 20 is activated so as to communicate the channel 8 with the channel 7. Afterwards, the cleaning solution is fed by the upper part of the membranes. After passing through the membranes it flows along them and is discharged by the drainage means 5. After having used the cleaning solution contained in the tank 13, the cleaning sequence can be continued using the solutions of cleaning contained in tanks 12 and 11, also feeding them alternatively by the upper part and by the lower part of the membranes 4. Each time that you change the cleaning solution, you can use the water of the main line or the permeate to wash the channels through which this solution passes. In addition, it should be noted that for each step of the cleaning sequence, the supply of the cleaning solution can be stopped (closing the valve lia, 12a, or 13a, as the case may be, and deactivating the pump 6), to so give time to the membranes to be impregnated with the cleaning solution. The installation according to Figure 1 has been operated according to several cleaning sequences, No. 1, No. 2, No. 3 and No. 4, the details of which are shown in Table 1 which is set forth then, after the membranes have become clogged with water from the Seine (30 NTU) or sewage from urban sewage. Regarding Seine water, the first three sequences (1,2,3) have been carried out with an empty tank, and the last (4) has been tested with an empty tank and with a full tank. Regarding urban wastewater, sequence 1 has been tested with an empty tank and a full tank.

All these cleaning sequences consist of a basic step, a step with acids and a step with chlorine, with the exception of the last sequence with respect to which only two steps have been carried out (with sodium hypochlorite and acid). To put it more clearly, in sequences No. 1, No, 2 and No. 3, three cleaning solutions were successively applied: the first solution contained a base, the second solution contained 0.5% citric acid and a third solution of 0.03% sodium hopochlorite. In sequence No. 4, only two cleaning solutions were used: the first solution was constituted by mixing in an aqueous solution a base and 0.03% of a sodium hypochlorite solution and a second 0.5% citric acid solution. Between each cleaning solution, the membranes were washed with water from the main piping, which was fed to the tank using the channel 21 and the bypass valve 22.

In addition, in the sequences No. 1, No. 3 and No. 4, the supply was carried out through the upper part and after the lower part of the membranes, with respect to each cleaning solution, and for each washing HE • Flow rates of 100 1 / h were applied, and the delivery time fluctuated between 2.5 and 30 minutes. Regarding sequence No. 2, only a supply was carried out at the bottom with a flow rate of 250 l / h and a delivery time of 30 minutes with respect to the basic cleaning solution, 15 minutes for the another two cleaning solutions and 5 minutes for washing with water from the main pipe.

Finally, in sequences No. 2 and No. 3, the impregnation time fluctuated between 15 and 40 minutes, which was applied after injecting the cleaning solutions.

Table 1: Cleaning sequences with proven chemical solutions * (Supply for 20 minutes at the top, then at the bottom, supplying for 10 minutes at the top and then at the bottom Table 1 (continued): Cleaning sequences with proven chemical solutions • (Supply for 20 minutes at the top, then at the bottom, supply for 10 minutes at the top, then at the bottom) **** (Supply from the top for 5 minutes, impregnated 10 minutes, Supply for the lower part 5 minutes, impregnated 10 minutes) was carried out 2 times. ***** (Supply by the top for 5 minutes, impregnated 10 minutes, Supply by the bottom 5 minutes, impregnated 10 minutes). The 3 * sequence consists in lengthening the recirculation time and reducing the impregnation time. The sequence 4 * consists of reducing the recirculation time and lengthening the impregnation time.

These different cleaning solutions have been implemented in the course of different cleaning operations after the membranes were clogged with water from the Seine as described in Table 2. Cleaning sequence No. 1 was implemented with respect to a cleaning operation, in which 1% sodium hydroxide at 25 ° C (pH = 11.7) was tested as a basic cleaning solution (cleaning operation No. 7). Cleaning sequence No. 2 was implemented with respect to a single cleaning operation, in which the base used as a basic cleaning solution consisted of 1% sodium hydroxide at 25 ° C (pH = 11.9) (cleaning operation No. 2). Cleaning sequence No. 3 was implemented in two cleaning operations (cleaning operation No. 8 and No. 9) in which the base used as a basic cleaning solution consisted of 1% sodium hydroxide at 25 ° C ( pH = 11.9). Cleaning sequence No. 4 was implemented with respect to three cleaning operations, in which the base used as a cleaning solution, which contained both a base and sodium hypochlorite, was constituted by 0.5% sodium hydroxide (cleaning operations No 12, 14 and 15). According to the invention, all these cleaning operations were carried out while tank No. 1 was empty. But for comparison purposes, only the operation 18 Cleaning No. 15 was carried out with the tank full of effluent. During these different cleaning operations, different volumes of cleaning solutions were used; from 4.6 to 23.3 1 / m2. Two cleaning operations were also carried out after the membranes had become clogged with urban sewage and activated sludge, as described in Table 3. During these two cleaning operations (cleaning operations No. 16 and No. 17), cleaning sequence No. 1 was used, with Ultrasil at 60%, as the base. Cleaning operation No. 16 was carried out according to the invention (with the tank empty), while cleaning operation No. 17 was carried out, for comparison purposes., with the tank full. The quality of the cleaning operations carried out was evaluated, on the one hand, by calculating the percentage of permeability that the membranes had with respect to the water coming from the main pipeline after cleaning in relation to the permeability of new membranes and, on the other hand, part, evaluating the increase in permeability in these membranes. Table 2 provides the results obtained with water from the Seine. Table 3 provides the results obtained with urban wastewater and activated sludge. Picture These results demonstrate that the cleaning method according to the invention, in comparison with the cleaning operation consisting of filling the tank with solutions for washing, allows to reduce the volumes of chemical reagents while maintaining an excellent effectiveness. In fact, within the context of the invention it can be seen that the permeability of the membrane, measured after the cleaning operations, is almost equal to or equal to that of a new membrane. Furthermore, the results also show that the injection of reagents carried out alternately by the upper part and by the lower part (cleaning operation No. 7) is more effective than when the reagent is simply injected from the upper part to the lower part (operation of cleaning No. 2). The prudent combination of different reagents during the various cleaning sequences allows the reagent volumes used to be reduced and also the time spent on cleaning. In particular, the cleaning operations carried out according to washing sequence No. 4, in which first a cleaning solution containing both sodium hydroxide and chlorine is used, are shown to be especially effective (cleaning operations Nos. 12 and 14). These results also indicate that there is a considerably greater cleaning effectiveness within the context of the invention, when it is performed having the tank empty than when it is done with the tank full (cleaning operations Nos. 15 and 17). With reference to Figure 2, there is shown a second embodiment of an installation that allows cleaning the membranes according to the method of the invention without interrupting the use thereof in the filtering mode. In addition to having a distribution system for cleaning solutions similar to that of the installation described above, this solution to the problem has four treatment tanks la, ib, le and Id, with a plurality of filter membranes (hollow fibers) organized in modules 3a, 3b, 3c and 3d, installed inside each one. In turn, these modules can be supplied with cleaning solutions thanks to the valves 18a, 18b, 18c and 18d (to supply them through the lower part of the membranes), and thanks to the valves 20a, 20b, 20c and 20d ( to supply them on the upper part of the membranes). The means 2 for supplying the tanks with the effluent to be filtered includes an intermediate tank 23, a general flow valve J and a through valve A, B, C and D for each treatment tank. As regards the means for draining the tanks, there is a general passage valve I and a bypass valve for each tank E, F, G, H. Finally, the drainage medium and the supply means of the tanks are connected one to the other by channel 21 having a valve of 21 Table 2: Effectiveness of cleaning operations carried out with chemical solutions Obstruction caused by Seine water Cleaning operations 1 to 6: Permeate flow = 500 1 / h Cleaning operations 7 to 13 Permeate flow = 400 1 / h Reference permeability of the new membrane = 220 1 / h.m2. barias Table 3: Effectiveness of cleaning operations carried out with chemical solutions Desobstruction of urban waste water - activated sludge Reference permeability of the new membrane = 350 1 / h.m2. arias Sequence 1 * = Sequence 1 with wash times in steps 2 and 4, were 20 minutes Step K, which allows, as desired, to transfer the drained volume of one tank to the other tanks and vice versa. When the facility is operating at full capacity, the four tanks la, Ib, le and Id are supplied with the effluent to be filtered. For this purpose, the valves J and A, B, C and D open at the same time as the drain valves E, F, G, H and I and valve K are closed. When it is desired to clean the membranes of one of the tanks, for example those of the tank Id, the general supply valve J and the supply valve D of the tank are closed, and valves H and K are opened, the other valves remaining in the same condition as they were previously. Once the general supply valve J is closed, the effluent to be filtered reaching the installation is stored in the intermediate tank 23. In the embodiments in which the contents of the tank are active (for example when it contains activated carbon or activated sludge) , the volume of effluent present in tank Id is drained and transferred to the other three tanks la, Ib and le, while filtering continues its course. Once the contents of the tank Id have been completely transferred to the other tanks, the drain valve H and the valve K are closed, and valve J is opened. Then the cleaning solutions can be fed to the empty tank. Id from deposits 11, 12 and 22 13, alternatively, by the bottom and the upper part of the membranes. At the end of the cleaning sequence, valve H and valve I are opened to discharge the dirty cleaning solutions present at the bottom of the tank. Id. Then, the tank can be washed simply by opening the valve D that supplies the tank. So, to refill the tank Id with the effluent that is going to be treated with the excess effluent present in the tanks the, Ib and le, the supply valves A, B and C are closed and the valves are opened. drain E, F and G. The drain valve H of the tank Id is closed, as is the general supply valve J and the general drain valve L. In order to be able to transfer the surplus of the tanks, Ib and To tank Id, valve K is opened. Finally, in order to allow the installation to return to its normal filtering mode, valves A, B, C, D and J are opened, and the other valves are closed. Accordingly, such an installation allows to implement the method for cleaning membranes according to the invention, while preserving the useful content of the treatment tanks. Therefore, the invention provides a cleaning method that can be easily automated and in which undiluted cleaning solutions are used in small volumes, as well as an installation for practicing the method.

The embodiments of the invention described herein are not intended to reduce the scope of this patent application. Consequently, it will be possible to make numerous modifications to the invention without departing from its scope. Regarding the method, it can be contemplated the use of other cleaning solutions apart from those indicated, and also other types of membranes. Regarding the installation, a different number of tanks and pumping circuits can be supplied that allow the filtering to continue during the cleaning of the membranes of a tank. 24

Claims (14)

1. Method for cleaning a filtration installation of the type including a plurality of submerged membranes in a tank, at least, containing an effluent to be filtered, said method being characterized because it includes several steps that consist of: - draining at least partially the effluent contained in said tank so that said membranes are exposed to the air; - passing at least one cleaning solution through said membranes along a flow path in the opposite direction to the filtrate flow of the effluent, providing said cleaning solution from the permeate side of said membranes.

2. A method according to claim 1, characterized in that it also includes a step that consists of recovering or neutralizing said cleaning solution that has passed through said membranes at the foot of said tank.

3. A method according to claim 1 or 2, characterized in that said membranes are placed in vertical position inside said tank.

4. A method according to claim 3, characterized in that said step consisting of passing at least one cleaning solution through said membranes along a flow path in the opposite direction to the filtrate flow of the effluent, it is carried 25 end supplying said cleaning solution, alternatively or simultaneously, by the upper part and by the lower part of said membranes.

A method according to one of Claims 1 to 4, characterized in that it consists in carrying out a cleaning sequence including at least one step which consists of passing at least one basic cleaning solution through of said membranes, and in one step, at least, which consists in passing at least one acid cleaning solution through said membranes.

6. A method according to claim 5, characterized in that said cleaning sequence includes at least one step consisting of passing at least one cleaning solution containing at least one oxidizing agent through said membranes. .

A method according to Claims 5 and 6, characterized in that said cleaning sequence includes at least one step which consists in passing at least one cleaning solution containing a base and an oxidizing agent through the said membranes, and at least one step consisting of passing at least one acid cleaning solution through said membranes.

A method according to one of Claims 5 to 7, characterized in that said steps of said cleaning sequence are interposed between one or more washing steps consisting of passing water through 26 said membranes, or are followed or preceded by them.

A method according to one of Claims 5 to 7, characterized in that said steps of said cleaning sequence are interspersed between one or more impregnation steps, or are followed or preceded by them.

A method according to one of Claims 1 to 9, characterized in that said cleaning solution or said cleaning solutions are used with a total volume flow ranging from 2 to 20 liters per square meter of membrane.

11. A method according to one of Claims 5 to 10, characterized in that the total duration of said cleaning sequence fluctuates between 30 minutes and 4 hours.

12. An installation for carrying out the method according to one of Claims 3 to 11, including at least one treatment tank (1) within which a plurality of filter membranes (3) are installed vertically together with means (2) for feeding an effluent to be filtered in said tank, means (5) for draining said tank (1), means (7, 8, 9, and 10) to discharge the permeate from said membranes, at least one storage tank (11, 12 and 13) of a solution for cleaning said membranes and means for feeding said cleaning solution from the permeate side of said membranes, characterized in that said means for feeding 27 said cleaning solution includes means for allowing said cleaning solution to be supplied alternatively or simultaneously by the upper part and by the lower part of said membranes.

13. An installation according to claim 12, characterized in that it includes at least two treatment tanks (la, Ib, le, Id) mounted in parallel, each having a plurality of filter membranes (3a, 3b, 3c, 3d) installed vertically in the interior, and because it includes means for cleaning the membranes of the first tank and means for storing the contents of this first tank in the second tank during the cleaning operation.

14. An installation according to Claim 13, characterized in that it includes means (21, K) for communicating said drainage means (5) with said supply means (2). 28