EP3976235A1

EP3976235A1 - Membrane-based liquid filtration installation and method for producing drinking water therewith without post-mineralization

Info

Publication number: EP3976235A1
Application number: EP20726498.7A
Authority: EP
Inventors: Claire Ventresque
Original assignee: Veolia Water Solutions and Technologies Support SAS
Current assignee: Veolia Water Solutions and Technologies Support SAS
Priority date: 2019-05-24
Filing date: 2020-05-25
Publication date: 2022-04-06
Also published as: US20220234915A1; AU2020281687A1; FR3096279B1; CN113874103A; FR3096279A1; WO2020239707A1

Abstract

Installation for the pressurized filtration of liquid with a view to producing drinking water, comprising at least one membrane-based drinking-water production unit (MPU), each MPU comprising: a plurality of filtration blocks each containing a bundle of pressure tubes mounted in parallel, each pressure tube accommodating at least two membrane-based filtration modules with spiral membranes or hollow-fibre membranes mounted in series, means (20) for feeding the liquid that is to be filtered, means for removing the filtered liquid, and means (30) for removing the concentrate, characterized in that the membranes of the filtration modules are of at least two different types selected from the group consisting of reverse-osmosis membranes and low-pressure reverse-osmosis membranes (4-6), on the one hand, and nanofiltration membranes (1-3) on the other hand, and in that at least one MPU comprises means (21-26) making it possible to alter the order in which the blocks of pressure tubes that it groups together are supplied with fluid. The method consists in supplying the filtration blocks of at least one MPU in a first order of supply in which the tubes containing nanofiltration membranes are at the head of the MPU and then in supplying the pressure tubes in a second order of supply in which the pressure tubes containing reverse-osmosis membranes or low-pressure reverse-osmosis membranes are at the head of the MPU.

Description

DESCRIPTION

TITLE: MEMBRANAR LIQUID FILTRATION INSTALLATION AND PROCESS FOR

PRODUCTION OF DRINKING WATER WITH THIS WITHOUT POST-MINERALIZATION

FIELD

The field of the invention is that of the design and production of installations used to filter liquids under pressure using filtration membranes with a view to removing pollutants, in particular micropollutants, and of reduce the hardness to produce drinking water. Such liquids can in particular be sea water, borehole water or surface water.

PRIOR ART

Membrane filtration processes are widely used in the production of drinking water. The membranes they use have a porous structure which allows them to retain pollutants, in particular micropollutants such as herbicides, pesticides or drug residues and to limit the transfer of one or more solutes with respect to the 'water.

Thus, microfiltration membranes have pores of 0.1 μm to 1 μm, those for ultrafiltration have pores from 5 nm to 0.1 μm, and those for nanofiltration (NF) have pores of a few nanometers. Finally, reverse osmosis membranes have an even more dense structure. Among the reverse osmosis membranes, a distinction is made between low pressure reverse osmosis membranes (OIBP) which have a higher permeability than that of reverse osmosis membranes used in seawater desalination (Ol). These reverse osmosis membranes thus make it possible to retain almost all of the solutes. Nanofiltration membranes differ from low pressure reverse osmosis membranes in that they only partially retain monovalent ions.

In drinking water production facilities, reverse osmosis, low pressure reverse osmosis and nanofiltration membrane filtration modules are generally spiral membrane modules or hollow fiber membrane modules. These modules are arranged in series within pressure tubes which commonly have a size allowing them to accommodate up to eight modules positioned in series. The water to be filtered is introduced at one end of the pressure tube and passes through the filtration membranes. The filtered liquid (permeate) is collected by a perforated recovery tube arranged along the longitudinal axis at the center of modules. Inter-connector devices make it possible to connect the permeate recovery tubes of the various membrane filtration modules arranged in series inside the pressure tube. The permeate is recovered at the end of the pressure tubes using permeate collectors, each connected to permeate discharge means. The concentrate, consisting of water concentrated in solutes, is collected at the opposite end of the pressure tube from the inlet water.

In practice, these pressure tubes are associated in blocks (also designated by the English term “skids”) in which they are mounted in parallel. Collectors are placed at the outlet of the pressure tubes to collect the permeate and the concentrate. These different collectors are each connected to a common collector.

The installations can be organized into several filtration stages: the concentrate leaving the first stage feeds and is treated by the membranes of a second stage, the concentrate coming from the second stage feeds and is treated by the membranes of a third stage. The permeates of each stage are brought together.

Many installations using this technology include a significant number of filter blocks. This is particularly the case for installations for the production of drinking water from seawater, which often include more than ten blocks, each of which can group together up to two hundred pressure tubes each accommodating up to eight filtration modules. membrane mounted in series.

To minimize the energy consumed in membrane systems in water desalination, reverse osmosis membranes can be used in the same pressure tube, the salt passages of which are of the same order of magnitude but with different permeabilities, in order to distribute the membrane production flows in the pressure tubes.

For example, the patent application DOW (WO200582497A1) relates to a method and an apparatus for treating water of high osmotic pressure, especially sea water, by passing the water through pressure tubes containing at least three elements of pressure. Spiral nanofiltration or reverse osmosis membranes having different permeabilities, the most permeable membrane being placed at the tail (concentrate side). This invention consists in better distributing the flows between the spiral membranes of the same pressure tube and in reducing the operating pressure.

Membrane installations for the production of drinking water, including the membranes of filtration are only reverse osmosis or low pressure reverse osmosis membranes have the drawback of producing completely demineralized water. In fact, all of the dissolved salts, in particular the dissolved calcium, in the liquids to be filtered are retained by the reverse osmosis membranes or the low pressure reverse osmosis membranes. It follows that these waters must be remineralized to be made drinkable.

Installations for the production of drinking water whose filtration membranes are only nanofiltration membranes have the drawback of producing water which, depending on the resource, may contain levels of monovalent ions or of low molecular weight micropollutants. too large or, due to the lower rejection of divalent ions, may not be sufficiently demineralized.

In fact, there are no membranes on the market capable of both retaining certain low molecular weight micropollutants and allowing some of the calcium ions to pass. The post-mineralization step involves the use of chemicals which can lead to an increase in the turbidity of the water produced. This is the case, for example, with lime remineralization. This step therefore requires the inclusion of additional equipment in the installations. This post-mineralization step therefore significantly increases the overall cost of the drinking water operation and the risks of contamination of the water produced.

To avoid or minimize such post-mineralization, it is known installations integrating two dies mounted in parallel, one with only reverse osmosis membranes and the other without membrane, one then speaks of bypass, or only with membranes whose salt passage is higher, typically nanofiltration membranes, in which part of the liquids is treated in one process and another part in the other. This gives treated water consisting of the mixture of water from the two channels which does not need to be re-mineralized. Such double-dies have the drawback of being more expensive to use than conventional dies. In addition, they do not make it possible to obtain potable water having a given fixed hardness, in the case where the temperature or the hardness of the water to be treated varies.

It will also be noted that the hydraulic yields of the membranes of the installations of the prior art are generally limited, which implies relatively large water losses. To reduce this phenomenon, treatment with chemical reagents of water at filter can be set up. However, this leads to an increase in the overall costs.

OBJECTIVES OF THE INVENTION

An objective of the present invention is to provide a membrane filtration technique for the production of drinking water which makes it possible to dispense with any post-mineralization stage of the water produced by the membranes, whether by addition of reagent or by supply of water. hard water, while allowing the effective reduction of the micropollutants contained in the liquids to be treated.

An objective of the invention is also to provide such a technique which makes it possible to obtain filtered water having a given target hardness even when the temperature of the liquids to be treated or their calcium concentration varies.

Another objective of the present invention is to provide a method implementing such an installation making it possible to operate the membranes with higher hydraulic yields than those obtainable with the methods of the prior art.

DISCLOSURE OF THE INVENTION

These objectives, as well as others which will appear subsequently, are achieved thanks to the invention which relates to any installation for the filtration under pressure of liquid for the production of drinking water comprising at least one membrane production unit of drinking water (UPM), each UPM comprising:

a plurality of filtration units each containing a bundle of pressure tubes mounted in parallel, each pressure tube accommodating at least two membrane filtration modules with spiral membranes or hollow fibers mounted in series,

means for supplying liquid to be filtered, means for discharging filtered liquid, and means for discharging concentrate, characterized in that said membranes of said filtration modules of said installation are at least of two different types chosen from the group consisting of reverse osmosis membranes and low pressure reverse osmosis membranes on the one hand, and nanofiltration membranes on the other hand, and in that said at least one UPM comprises means for modifying the order of supply of said blocks of pressure tubes which it groups together.

Thus, according to the invention, the installation comprises two types of membranes, nanofiltration membranes on the one hand and reverse osmosis membranes and / or low pressure reverse osmosis membranes on the other hand. According to a first embodiment, said membranes of at least two different types are provided in different pressure tubes.

According to another embodiment, said membranes of at least two different types are provided in the same pressure tubes.

Advantageously, said nanofiltration membranes used allow a rejection rate of less than or equal to 70% of the calcium during a standard CaC test. This standard test is performed on synthetic water consisting of demineralized water containing 500 mg / l of CaCh, the modulus being subjected to a pressure of 75 psi (0.52 MPa) and producing an efficiency of 15% (efficiency defined as flow rate of permeate produced during the test divided by the flow rate of feed water to the membrane module). The rejection of calcium during the standard test is measured at a temperature of 25 ° C and with a flow of 31 L / h / m ² / bar. This test is called the “standard CaCh test”. The rejection rate is defined as the rate of solute removal (released into the concentrate), expressed as: concentration in the permeate / concentration in the feed.

Also advantageously, said reverse osmosis membranes and / or said low pressure reverse osmosis membranes allow a rejection rate greater than 90% of the calcium during a standard CaC test.

Advantageously, the installation has a ratio of the number of nanofiltration membranes to the total number of membranes of between 5% and 95%.

Preferably, said nanofiltration membranes have a standard specific flux, or permeability, greater than 3 L / h / m ² / bar and allow a rejection rate of monovalent salts of less than 82%, said rejection being that observed during a test standard on synthetic water consisting of demineralized water containing 2 g / l of NaCl, the modulus being subjected to a pressure of 70 psi (4.8 bars) and producing a yield of 15%. The sodium rejection during the standard test is measured at a temperature of 25 ° C. This test is called the “standard NaCl test”.

Also preferably, said reverse osmosis or low pressure reverse osmosis membranes have a standard specific flow greater than 3 L / h / m ² / bar and allow a rejection rate of monovalent salts greater than or equal to 82%, said rejection being that observed during a standard test with NaCl. The invention also relates to any liquid filtration process for the production of drinking water using such an installation, characterized in that it comprises the steps of supplying the filtration units with at least one UPM according to a first order. supply in which the tubes containing nanofiltration membranes are at the head of UPM, then, depending on the variation of a parameter, to supply the pressure tubes according to a second supply order in which the pressure tubes containing reverse osmosis membranes or low pressure reverse osmosis membranes are at the top of UPM. Advantageously, each UPM is organized in filtration stages connected in series, each filtration stage comprising, according to the possible configurations thanks to the use of said means making it possible to modify the order of supply of said blocks of pressure tubes, that is to say a single filtration unit, i.e. 2 to 6 filtration units mounted in parallel.

Thus, according to the invention one or more blocks of pressure tubes operating in parallel in the first configuration can operate in series in the second configuration and one or more blocks of pressure tubes operating in parallel series in the first configuration can operate in parallel in the second configuration. the second configuration.

The invention relates in particular to such a method implemented with an installation comprising a plurality N of UPMs, with N> 1 integer, characterized in that it comprises a step consisting in supplying the blocks of x / N UPM, with x integer varying from 0 to N, according to said first order, and the blocks of the remaining UPMs according to said second order, and to vary x so that the filtered water obtained at the outlet of the installation meets a predetermined quality factor .

Preferably, said parameter is chosen from the group consisting of the temperature of the water to be treated.

Also preferably, said quality factor is the hardness of said filtered water obtained at the outlet of the installation.

LIST OF FIGURES

The invention, as well as the various advantages that it presents, will be more easily understood thanks to the following description of embodiments thereof, given by way of illustration and without limitation, with reference to the drawings in which: [Fig 1] schematically shows a first embodiment of an installation according to the present invention, the direction of the liquids passing through it being indicated by arrows corresponds to a first operating configuration called “high temperature configuration”.

[Fig 2] schematically represents the same installation operating according to a second configuration called “low temperature water configuration”.

[Fig 3] schematically shows a second embodiment of an installation according to the present invention, the direction of the liquids passing through it being indicated by arrows corresponds to a first operating configuration called “high temperature configuration”.

[Fig 4] schematically represents the same installation as that shown in Figure 3 operating according to a second configuration called "low temperature water configuration".

DESCRIPTION OF EMBODIMENTS

A first embodiment of an installation according to the present invention consists of six identical UPM (N = 6), each being dimensioned to produce 695 m ³ / h of drinking water with a yield of 85% from water. having an osmotic pressure of less than 20 bars. One of these UPMs is shown in Figures 1 and 2. It comprises, in addition to supply means 20 for liquid to be filtered and discharge means 30 for rejected liquid or concentrate, six filtration units 1 to 6. Each block filter contains thirty pressure tubes. Each pressure tube contains six membrane filtration modules mounted in series. This UPM therefore contains 1080 membrane filtration modules. So the plant, which contains six UPMs, each with 1080 membranes, has a total of 6480 membranes.

More precisely, three of these blocks contain NF modules and the other three OIBP modules.

In the context of this embodiment, the NF membranes are spiral membranes marketed under the trade reference DOW FILMTEC NF270-400 and the OIBP membranes are spiral membranes marketed under the trade reference DOW FILMTEC ECO-400.

The NF membranes used exhibit a flow rate of 55.6 m ³ / day under the standard CaC test and a calcium rejection rate of 40 to 60% under the standard CaC test.

The OIBP membranes used have a flow rate of 44 m ³ / day under the standard NaCl test and make it possible to reject 99.7% of the monovalent ions. In FIGS. 1 and 2 we therefore distinguish blocks 1, 2 and 3 with NC modules and blocks 4, 5 and 6 with OIBP modules.

According to the invention, each UM P of the installation is provided with means making it possible to modify the order of supply of the blocks of pressure tubes which it groups together. These means consist of valves, or any other isolation device, 21 to 26.

According to the invention, the order of supplying the modules of each UMP can vary as a function of a parameter.

Thus, with reference to FIG. 1, according to a first configuration, the valves or isolation device 21, 23 and 26 are open while the valves or isolation devices 22, 24 and 25 are closed. Are thus delimited, a first filtration stage made up of NF blocks 1, 2 and 3 fed in parallel, a second filtration stage made up of OIBP 4,5 blocks fed in parallel, and a third filtration stage made up of the OIBP module block 6. According to such a configuration, the OIBP takes place at the tail of the UPM on two stages, which allows a good rejection of the micropollutants and of the calcium without degrading the operating pressure. According to the invention, the supply order of the blocks 1 to 6 can be modified by closing the valves or isolation devices 21, 23 and 26 and by opening the valves or isolation devices 22, 24 and 25. According to the invention this second configuration, the first filtration stage consists of the blocks of OIBP 4, 5, 6 modules mounted in parallel, the second filtration stage consists of the blocks of modules of NF 2 and 3 and the third filtration stage consists of the NC module block 1.

According to such a second configuration, the nanofiltration takes place at the tail of the UPM on two stages, thus promoting a drop in pressure while offering good rejection of the micropollutants.

Thus, block 1 operating in parallel in the first configuration operates in series in the second configuration and block 6 operating in series in the first configuration operates in parallel in the second configuration.

Simulations have been carried out to show that, thanks to the invention, it is possible to maintain the hardness of the water produced by the installation within a fixed range, for example between 8 and 9 ° F, while maintaining good rejection of the micropollutants, even if the temperature or the quality of the water to be filtered varies.

In a first battery of simulations, the installation was calculated to filter water of constant hardness and variable temperature according to the seasons, having the following characteristics: Calcium content = 110 mg / l,

Magnesium content = 8 mg / l,

Sodium content = 22 mg / l,

Bicarbonate content = 230 mg / l, nitrates = 20 mg / l,

Chloride content = 30 mg / l,

Sulphate content = 120 mg / l,

SiC content = 10 mg / l,

pH = 7.1

and whose temperature has varied over a year from 5 ° C to 25 ° C.

The flow rate of treated water produced is 100,000 m ³ / d - The objective for the water produced is set at a minimum of 30 mg / l of calcium (corresponds to a hardness of 8 ° F) whatever the temperature of the water to be filtered. The maximum desired concentration is 35 mg / L of calcium (corresponds to a hardness of 9 ° F). As part of this first battery of simulations, the proportion (x / 6) of UPM of the installation operating in the first configuration and of UPM operating in the second configuration was varied as a function of the temperature of the water to be filtered. . Table 1 below indicates for each temperature interval, and for each proportion (0/6; 1/6; 2/6; 3/6; 4/6; 5/6; 6/6), the concentration of calcium (Ca, major component of the hardness) of the permeate, the nitrate (NO3) concentration of the permeate as well as the supply pressure (P) of the UPMs.

[TABLE 1]

By way of comparison, the same water was treated with a double-line type installation of the prior art using eight UPMs each containing 135 pressure tubes arranged in three stages. Each pressure tube containing six membrane filtration modules, this installation therefore contains the same number of membrane filtration modules as that according to the invention, ie 6,480 modules. The same NF and OIBP modules as in the installation according to the invention were used.

In order to allow obtaining in winter, at a temperature between 5 ° C and 10 ° C, a minimum hardness of the water produced corresponding to a content of at least 30 mg / l of calcium, the installation in double stream contains a proportion of 25% of NF modules and 75% of OIPB modules.

This double channel was used to filter the same water with the same temperature variations. Table 2 below indicates, for each temperature window, the concentrations of calcium (the major component of hardness), of nitrates and of the supply pressure.

[TABLE 2]

As can be seen in Table 1, it was possible to maintain the calcium concentration of the water produced by the installation between 30 mg / l and 35 mg / l by varying the proportion (UPM operating in the first configuration / Total UPM) therefore by modifying the supply order of the UMP pressure tube blocks. The invention thus makes it possible to maintain the calcium content of the treated water at approximately 30 mg / L. Such a result could not be obtained with the double route of the prior art, as shown by the results of Table 2 which show an increase in the calcium content well beyond the desired limit of 35 mg / l in practical in summer up to 46 mg / l.

These simulations also make it possible to demonstrate that the invention makes it possible to obtain better nitrate reduction rates.

It is demonstrated by tests on 5 membranes of different types that the rejection of nitrates is a good indicator of the rejection of micropollutants. The results of these tests are shown in Table 3 below. Thus, the configurations allowing the good rejection of nitrates, including including at the highest temperatures, are preferred when better rejection of micropollutants is desired.

[TABLE 3]

In a second battery of simulations, the installation was calculated to filter water of constant temperature and variable hardness with the following characteristics:

Magnesium content = 8 mg / l,

Sodium content = 22 mg / l,

Bicarbonate content = 230 mg / l, nitrates = 20 mg / l,

Chloride content = 30 mg / l,

Sulphate content = 120 mg / l,

SiC content = 10 mg / l,

Temperature = 15 ° C

pH = 7.1

and whose calcium content varied from 90 to 140 mg / l.

The flow of treated water produced is 100,000 m3 / d. The objective for produced water is set at a minimum of 30 mg / l of calcium regardless of the calcium concentration of the inlet water to be filtered. The maximum desired concentration is 35 mg / l of calcium.

As part of this second battery of simulations, the proportion (x / 6) of UPM of the installation operating in the first configuration and of UPM operating in the second configuration was varied as a function of the calcium content of the water to be filtered.

Table 4 below indicates for each calcium content in the inlet water to be filtered, and for each proportion (0/6; 1/6; 2/6; 3/6; 4/6; 5/6 ; 6/6), the calcium concentration (major component of the hardness) of the permeate, the nitrate concentration of the permeate and the supply pressure. [TABLE 4]

In order to ensure a minimum hardness of the water produced corresponding to a content of at least 30 mg / l of calcium, for feed water containing 90 mg / l of calcium, the double-channel installation contains a proportion of 30% of NF modules and 70% of OIPB modules.

This double channel was used to filter the same water with the same variations in calcium concentrations. Table 5 below indicates, for each calcium content in the inlet water, the concentrations of calcium (the major component of hardness) and of nitrates in the permeate and the supply pressure.

[TABLE 5]

This second battery of simulations makes it possible to demonstrate that the prior art leads to a calcium content of the water produced ranging from 30 to 47 mg / l while the invention makes it possible to maintain this concentration around 30 - 35 mg / l. .

The reduction of nitrates, and therefore of micropollutants, is moreover better thanks to the invention, in the majority of cases. A second embodiment of an installation according to the present invention comprises six UPMs such as that shown in Figures 3 and 4 similar to that shown with reference to Figures 1 and 2 except in that it has valves or other device for 'additional insulation (21a, 22a, 22b, 22c, 23a, 23b) and in that it includes four filter blocks instead of six. More precisely, one of these blocks (block 1) contains thirty pressure tubes, each pressure tube containing six NF modules, another block (block 2a) contains sixty pressure tubes, each pressure tube containing six NF modules, another block (block 4) contains thirty pressure tubes, each pressure tube containing six OIBP modules and another block (block 6a) contains sixty pressure tubes, each pressure tube containing six OIBP modules. The capacity of the UPM shown in these Figures 3 and 4 is therefore the same as that shown in Figures 1 and 2.

Referring to Figure 3, according to a first configuration, the valves or isolation device 21, 21a, 23, 23a, 23b and 26 are open while the valves or isolation devices 22, 22a, 22b, 22c, 24 and 25 are closed. A first filtration stage made up of two NF blocks 1 and 2a supplied in parallel, a second filtration stage made up of OIBP block 6a, and a third filtration stage made up of OIBP block 4 are thus defined.

According to such a configuration, the OIBP takes place at the tail of the UPM on two stages, which allows good rejection of micropollutants and calcium without degrading the operating pressure. According to the invention, the order of supply of the blocks can be modified by closing the valves or isolation devices 21, 21 a, 23, 23a, 23b and 26 and by opening the valves or isolation devices 22, 22a , 22b, 22c, 24 and 25.

According to this second configuration, the first filtration stage consists of blocks of OIBP 4 and 6a modules mounted in parallel, the second filtration stage consists of the NF 2a block and the third filtration stage consists of the NF 1 module block.

Thus, the block 1 operating in parallel in the first configuration operates in series in the second configuration and the blocks 6a operating in series in the first configuration operate in parallel in the second configuration.

Claims

1. Installation for the pressurized filtration of liquid for the production of drinking water comprising at least one membrane drinking water production unit (UPM), each UPM comprising:

a plurality of filtration units each containing a bundle of pressure tubes mounted in parallel, each pressure tube accommodating at least two membrane filtration modules with spiral membranes or hollow fibers mounted in series, supply means (20) of liquid to be filtered,

means for removing filtered liquid, and

concentrate evacuation means (30),

characterized in that said membranes of said filtration modules of said installation are at least of two different types chosen from the group consisting of reverse osmosis membranes and low pressure reverse osmosis membranes (4-6) on the one hand , and nanofiltration membranes (1-3) on the other hand,

and in that said at least one UPM comprises means (21-26) making it possible to modify the order of supply of said blocks of pressure tubes which it groups together.

2. Installation according to claim 1 characterized in that said membranes of at least two different types are provided in different pressure tubes.

3. Installation according to claim 1 characterized in that said membranes of at least two different types are provided in the same pressure tubes.

4. Installation according to one of claims 1 to 3 characterized in that said nanofiltration membranes allow a rejection rate less than or equal to 70% of the calcium during a standard test with CaCh.

5. Installation according to one of claims 1 to 4 characterized in that said reverse osmosis membranes and / or said low pressure reverse osmosis membranes allow a rejection rate greater than 90% of calcium during a standard test. at the CaC.

6. Installation according to any one of claims 1 to 5 characterized in that it has a ratio of the number of nanofiltration membranes to the total number of membranes of between 5% and 95%.

7. Installation according to any one of claims 1 to 6 characterized in that said nanofiltration membranes have a permeability greater than 3 L / h / m ² / bar. and allow a rejection rate of monovalent salts of less than 82%, according to the “standard NaCl test”.

8. Installation according to any one of claims 1 to 6 characterized in that said low pressure reverse osmosis membranes have a permeability greater than 3 L / h / m ² / bar and allow a higher monovalent salt rejection rate or equal to 82 %%, according to the “standard NaCl test”.

9. Installation according to any one of claims 1 to 6 characterized in that said reverse osmosis membranes have a permeability greater than 3 L / h / m ² / bar and allow a rejection rate of monovalent salts greater than or equal to 82%, according to the “standard NaCl test”.

10. A method of filtering liquid for the production of drinking water using an installation according to any one of claims 1 to 9 characterized in that it comprises the steps of supplying the filtration units with at least one UPM according to a first supply order in which the tubes containing nanofiltration membranes are at the head of UPM, then, depending on the variation of a parameter, to supply the pressure tubes according to a second supply order in which pressure tubes containing reverse osmosis membranes or low pressure reverse osmosis membranes are at the top of UPM.

11. The method of claim 10 characterized in that each UPM is organized in filtration stages connected in series, each filtration stage comprising, according to the possible configurations thanks to the implementation of said means making it possible to modify the order of supply. of said blocks of pressure tubes, either a single filtration unit, or 2 or more filtration units mounted in parallel.

12. Method according to claim 11 implemented with an installation according to one of claims 1 to 9 comprising a plurality N of UPMs, with N> 1 integer, characterized in that it comprises a step consisting in supplying the blocks. of x / N UPM, with x integer varying from 0 to N, according to said first order, and the blocks of the remaining UPM according to said second order, and in varying x so that the filtered water obtained at the outlet of the installation meets a quality factor.

13. Method according to one of claims 10 to 12 characterized in that said parameter is chosen from the group consisting of the temperature of the water to be treated and the calcium concentration of the liquid to be treated.

14. Method according to one of claims 10 to 13 characterized in that said quality factor is the hardness of said filtered water obtained at the outlet of the installation.