Unit for treating raw water with membrane filtration devices and associated treatment method

Abstract

The present invention relates to a unit (10) for treating raw water (12), comprising: - a supply line (14), - a high-pressure pump (16), - a high-pressure reverse osmosis filtration device (18) forming a high-pressure filtration stage, - a low-pressure membrane filtration unit (20) forming a low-pressure stage comprising at least one low-pressure membrane filtration device (34), filtering the high-pressure concentrate stream (26) and providing at least one permeate stream (38), - a recirculation loop (21) for recirculating a portion of at least one of the permeate streams (38) upstream of the high-pressure pump (16) so as to provide the inlet (22) of the high-pressure pump (16) with a mixed stream (46).

Description

Raw water treatment unit with membrane filtration devices and associated treatment process

[0001] The present invention relates to a raw water treatment unit.

[0002] The present invention relates more particularly to the treatment of seawater to desalinate it and produce drinking water using membrane filtration devices.

[0003] To desalinate seawater, it is known to use a reverse osmosis device.

[0004] The conventional reverse osmosis setup involves feeding several pressure tubes in parallel, each tube consisting of multiple membranes arranged in series. Seawater is fed under pressure into the top of each pressure tube. It then flows through the membranes in series and exits at the other end of the pressure tube. Under pressure, only water molecules pass through the reverse osmosis membranes while the salts are retained. The water that passes through the membranes, called permeate, is desalinated. The flow that does not pass through the membranes is called concentrate. The concentrate from the first membrane feeds the second membrane, and so on down the pressure tube.

[0005] The net driving force of this process corresponds to the difference between the average feed pressure to the membrane and the osmotic pressure difference between the seawater to be desalinated and the permeate. According to Van't Hoff's law, since osmotic pressure is proportional to salt concentration, the more concentrated the seawater, the greater the osmotic pressure. To allow desalination throughout the pressure tube, the feed pressure must therefore be greater than the osmotic pressure of the concentrate. At the outlet of the pressure tube, the permeate is recovered and the concentrate is discharged. Reverse osmosis desalination is a relatively energy-intensive drinking water production technology because it requires pressurizing the seawater to pressures exceeding 50 bar to allow the water to pass through the membrane.

[0006] The efficiency, or recovery rate, of reverse osmosis is the ratio between the volume of permeate and the volume of feed water. Typical reverse osmosis efficiency for seawater desalination ranges from 40% to 50%. This efficiency is limited by factors such as the energy required to further concentrate the seawater, the number of membranes arranged in series within the pressure tube, and / or the tangential flow rate necessary for proper membrane operation.

[0007] Typically, between five and seven diaphragms are arranged within a pressure tube. Beyond that, maintaining optimal operating conditions becomes more difficult.

[0008] One way to increase the efficiency of reverse osmosis is to implement a second filtration stage fed with the concentrate from the previous stage. By doing so, it is possible to increase the recovery rate while maintaining an optimal number of membranes in series.

[0009] Alternative configurations with a recirculation loop ("semi-batch reverse osmosis" or "batch reverse osmosis") also exist, which aim to improve the efficiency of the conventional configuration by recirculating the concentrate several times through the first filtration stage. This operation improves efficiency while maintaining a good balance in terms of energy consumption. However, these configurations do not provide permeate of consistent quality and require additional equipment to enable continuous production despite cyclical operation.

[0010] One aim of the invention is to provide a raw water treatment unit with optimized operation that increases the recovery rate while limiting energy consumption.

[0011] To this end, the invention relates to a raw water treatment unit, said treatment unit comprising:

[0012] - a raw water supply pipe,

[0013] - a high-pressure pump comprising an inlet fluidly connected to the raw water supply line, said high-pressure pump being intended to provide a flow of pressurized water,

[0014] - a high-pressure reverse osmosis filtration device forming a high-pressure filtration stage, designed to filter the pressurized water flow to provide a permeate flow from the high-pressure stage and a concentrate flow from the high-pressure stage,

[0015] - a low-pressure membrane filtration unit forming a low-pressure stage comprising at least one low-pressure membrane filtration device, said filtration unit being intended to filter the high-pressure concentrate stream to provide at least one permeate stream from the low-pressure stage and at least one concentrate stream from the low-pressure stage, said permeate stream from the low-pressure stage having a salinity less than or equal to the salinity of the raw water,

[0016] - a recirculation loop intended to recirculate at least part of at least one of the permeate streams from the low-pressure stage, upstream of the high-pressure pump so as to provide at the inlet of the high-pressure pump a mixed stream formed by a mixture between the raw water and at least part of at least one of the permeate streams from the low-pressure stage,

[0017] - a control unit comprising a first control system configured to regulate the permeate flow from the low pressure stage recirculated upstream of the high pressure pump, and a second control system configured to circulate part of the pressurized water flow directly to the inlet of the low pressure membrane filtration unit, so as to maintain the salinity of the pressurized water flow substantially constant over time and equal to a setpoint value.

[0018] Thus, the permeate stream from the first filtration stage is recovered for drinking water production, and the high-pressure concentrate stream is sent to the low-pressure membrane filtration unit, which includes at least one low-pressure membrane filtration device forming a second filtration stage. The use of such a low-pressure membrane device makes it possible to desalinate water with a higher salt concentration without having to increase the pressure using an interstage pump, thereby increasing the efficiency of the process without increasing pressure. However, the membranes of the low-pressure membrane filtration device have a lower salt retention capacity, resulting in a lower-quality permeate that cannot be used for drinking water production.The permeate stream, with a salinity equal to or lower than the feed seawater, is returned upstream of the first reverse osmosis stage to be mixed with the feed seawater before filtration. This allows for an increase in overall efficiency without additional energy costs. Furthermore, the increased efficiency can even lead to reduced energy costs when the lower salinity permeate stream is mixed with the seawater, as this lowers the osmotic pressure and therefore the required feed pressure at the inlet of the high-pressure reverse osmosis filtration unit.

[0019] Furthermore, the presence of the regulation unit makes it possible to maintain a constant salinity at the inlet of the high pressure stage despite variations in the salinity of the raw water and thus to limit the variations in operating pressure of the high pressure pump over time to operate at the best efficiency by reducing energy consumption.

[0020] The processing unit according to the invention may comprise one or more of the following features, taken individually or in any technically feasible combination:

[0021] - the control unit includes at least one sensor of a parameter representative of the salinity of the mixed flow configured to acquire at least one measurement of a parameter representative of the salinity of the mixed flow, the first control system and the second control system being configured to regulate respectively the recirculated permeate flow and the portion of the pressurized water flow circulating upstream of the low pressure membrane filtration unit from said measurement of the parameter representative of the salinity of the mixed flow;

[0022] - the parameter representing the salinity of the mixed flux is the electrical conductivity;

[0023] - the recirculation loop includes a branch on the supply line, the sensor of a parameter representative of the salinity of the mixed flow being placed upstream of the high pressure pump and downstream of the branch of the recirculation loop;

[0024] - the low pressure membrane filtration device comprises respectively a plurality of low pressure membranes, and the high pressure reverse osmosis filtration device comprises a plurality of high pressure reverse osmosis membranes, the permeability of each low pressure membrane being greater than the permeability of each high pressure reverse osmosis membrane;

[0025] - at least one low-pressure membrane filtration device is a nanofiltration or low-pressure reverse osmosis filtration device;

[0026] - at least one low pressure membrane filtration device is a nanofiltration filtration device, the low pressure membrane filtration unit comprising a first low pressure membrane filtration device and a second low pressure membrane filtration device, fluidly connected in series in each other, the first low pressure membrane filtration device being intended to filter the high pressure concentrate stream and to provide a permeate stream intended to be filtered by the second low pressure membrane filtration device;

[0027] - the pressure of the concentrate flow from the high-pressure stage is greater than or equal to the minimum operating pressure of the low-pressure filtration device to produce the permeate flow from the low-pressure stage and the concentrate flow from the low-pressure stage;

[0028] - the sodium chloride retention rate of the low pressure membrane filtration device is lower than the sodium chloride retention rate of the high pressure reverse osmosis filtration device;

[0029] - the unit further includes an energy recovery device configured to pressurize at least a portion of the raw water using at least one concentrate stream from the low-pressure stage;

[0030] - the operating pressure of the high-pressure reverse osmosis filtration device is between 10 bar and 120 bar, and the operating pressure of the low-pressure membrane filtration device is between 3 bar and 40 bar; and

[0031] - the unit further includes an osmotic energy exploitation device configured to produce electrical energy from the salinity gradient between at least one permeate flow from the low-pressure stage and at least one concentrate flow from the low-pressure stage.

[0032] The invention also relates to a process for treating raw water, said process comprising the following steps:

[0033] - to supply raw water to a high-pressure pump to provide a flow of pressurized water,

[0034] - filter the pressurized water stream with a high-pressure reverse osmosis device and provide a permeate stream from the high-pressure stage and a concentrate stream from the high-pressure stage,

[0035] - filter the concentrate stream from the high-pressure stage with a low-pressure membrane filtration unit comprising at least one low-pressure membrane filtration device to provide at least one permeate stream from the low-pressure stage and at least one concentrate stream from the low-pressure stage, said permeate stream from the low-pressure stage having a salinity less than or equal to the salinity of the raw water,

[0036] - recirculate at least a part of at least one of the permeate streams from the low-pressure stage, upstream of the high-pressure pump so as to provide at the inlet of the high-pressure pump a mixed stream formed by a mixture between the raw water and at least a part of at least one of the permeate streams from the low-pressure stage, the process further comprising a step of maintaining the salinity of the pressurized water stream substantially constant over time and equal to a setpoint value by means of a control unit, by regulating the permeate stream from the low-pressure stage recirculated upstream of the high-pressure pump and / or by circulating a part of the pressurized water stream directly to the inlet of the low-pressure membrane filtration unit.

[0037] The method according to the invention may comprise one or more of the following features, taken individually or in any technically feasible combination:

[0038] - when the salinity of the raw water decreases, the flow of permeate recirculated upstream of the high-pressure pump decreases;

[0039] - when the salinity of the raw water increases, part of the pressurized water flow is sent directly to the inlet of the low pressure membrane filtration unit to increase the flow of recirculated permeate upstream of the high pressure pump;

[0040] - the concentrate flow from the high-pressure stage is filtered by the low-pressure membrane filtration unit without additional pressurization of said concentrate flow from the high-pressure stage;

[0041] - the process further includes a pressurization step of at least a portion of the raw water with an energy recovery device using at least one concentrate stream from the low-pressure stage; and

[0042] - the process further includes a step of producing electrical energy with a device for exploiting osmotic energy from the salinity gradient between at least one permeate flow from the low pressure stage and at least one concentrate flow from the low pressure stage.

[0043] The invention will be better understood upon reading the following description, given solely by way of example, and made with reference to the accompanying drawings, among which: a is a schematic representation of a raw water treatment unit not covered by the present invention; a is a schematic representation of a raw water treatment unit not covered by the present invention; a is a schematic representation of a raw water treatment unit according to the invention; a is a schematic representation of a raw water treatment unit not covered by the present invention; a is a schematic representation of a raw water treatment unit not covered by the present invention; and a is a schematic representation of a raw water treatment unit not covered by the present invention.

[0044] On the figure, a treatment unit 10 of raw water 12 is represented according to a first embodiment not covered by the present invention.

[0045] The treatment unit 10 includes a raw water supply line 14, a high-pressure pump 16, a high-pressure reverse osmosis filtration device 18, a low-pressure membrane filtration unit 20 and a recirculation loop 21.

[0046] Raw water 12 is typically seawater which has, for example, a salinity, that is to say a content of dissolved salts, between 30 g / L and 50 g / L.

[0047] The raw water supply line 14 12 is hydraulically connected to the high pressure pump 16, and in particular to an inlet 22 of the high pressure pump 16.

[0048] The high-pressure pump 16 is designed to provide a flow of pressurized water 24. The high-pressure pump 16 is capable of pressurizing a fluid to a pressure between 10 bar and 100 bar.

[0049] The high-pressure pump 16 is, for example, a piston pump or a diaphragm pump.

[0050] The high-pressure reverse osmosis filtration device 18 comprises a plurality of pressure tubes arranged in parallel and supplied with a flow of pressurized water 24. Each pressure tube comprises a plurality of membranes arranged in series one after the other along a principal direction of elongation of the pressure tube.

[0051] The sodium chloride rejection rate of each of the membranes is greater than 99%, meaning that more than 99% of the dissolved salts present in the pressurized water are retained by the membrane.

[0052] Preferably, the sodium chloride rejection rate is between 99.6% and 99.9%.

[0053] The permeability of each membrane is between 0.5 L / h / m² / bar and 1.5 L / h / m² / bar. Permeability is defined as the permeate flow rate normalized by the feed pressure and the membrane surface area, but does not take into account the salinity of the feed solution.

[0054] The rejection rate and permeability are calculated for example under the following standard conditions: sodium chloride concentration of the feed solution: 32000 ppm, boron concentration of the feed solution: 5 ppm, test pressure: 55 bar, temperature of the feed solution: 25°C, pH: 8, conversion rate: 8% and membrane surface area: 440 ft².

[0055] The membrane cutoff threshold, that is, the critical molar mass for which 90% of the solutes are retained by the membrane, is for example between 30 Da and 150 Da.

[0056] The minimum supply pressure at which the filtration device 18 produces permeate and concentrate is, for example, between 10 bar and 50 bar. The minimum supply pressure depends in particular on the characteristics of the membrane, especially the material, the salinity of the water and the water temperature.

[0057] In steady state, i.e. an operation in which the permeate flow rate and the conversion rate are constant over time, the operating pressure is higher than the minimum supply pressure and is for example between 10 bar and 120 bar, preferably between 30 bar and 120 bar.

[0058] The membranes of the high-pressure reverse osmosis filtration device 18 are made of polymer, composite materials and preferably in the form of a spiral module.

[0059] The high-pressure reverse osmosis filtration device 18 forms a high-pressure filtration stage of the treatment unit 10. The high-pressure reverse osmosis filtration device 18 is intended to filter the pressurized water stream 24 to provide a permeate stream from the high-pressure stage 26 and a concentrate stream from the high-pressure stage 28.

[0060] In particular, the high-pressure reverse osmosis filtration device 18 includes a permeate line 30 and a concentrate line 32. The concentrate line 32 is hydraulically connected to the low-pressure membrane filtration unit 20, and in particular to the inlet of said filtration unit 20.

[0061] The permeate flow from the high-pressure stage 26 is intended for the production of drinking water. For this purpose, the treatment unit 10 may include a permeate remineralization system (not shown) fluidly connected to the permeate line 30, and located downstream of the high-pressure reverse osmosis filtration device 18.

[0062] The efficiency of the high-pressure reverse osmosis filtration device 18 is, for example, between 40% and 50%, that is to say, the permeate flow from the high-pressure stage 26 represents between 40% and 50% of the pressurized water flow 24. The concentrate flow from the high-pressure stage 28 represents the remainder.

[0063] The relative pressure of the permeate flow from the high-pressure stage 26 is typically between 0 bar and 0.3 bar.

[0064] The relative pressure of the concentrate flow from the high-pressure stage 28 is typically between 8 bar and 118 bar.

[0065] The high-pressure reverse osmosis filtration device 18 forms a first filtration stage of the treatment unit 10.

[0066] The low-pressure membrane filtration unit 20 includes at least one low-pressure membrane filtration device 34. The filtration unit 20 is intended to filter the concentrate stream from the high-pressure stage 28 to provide a concentrate stream from the low-pressure stage 36 and a permeate stream from the low-pressure stage 38.

[0067] Each low-pressure membrane filtration device 20 comprises a plurality of pressure tubes arranged in parallel and supplied with a flow of concentrate from the high-pressure stage 28. Each pressure tube comprises a plurality of low-pressure membranes arranged in series one after the other along a principal direction of elongation of the pressure tube.

[0068] According to the invention, the permeate flow from the low pressure stage 36 has a salinity less than or equal to the salinity of the raw water 12.

[0069] The permeability of each low-pressure membrane is greater than the permeability of each high-pressure reverse osmosis membrane.

[0070] According to a particular embodiment, the low pressure membrane filtration device 34 is a low pressure reverse osmosis filtration device.

[0071] In this embodiment, the low pressure membranes are brackish water type membranes.

[0072] Preferably, the sodium chloride rejection rate is between 99.6% and 99.9%.

[0073] The permeability of each of these membranes ranges from 2.0 L / h / m² / bar to 10 L / h / m² / bar. As mentioned above, permeability is defined as the permeate flow rate normalized by the feed pressure and membrane surface area, but does not take into account the salinity of the feed solution.

[0074] The rejection rate and permeability are calculated for example under the following standard conditions: sodium chloride concentration of the feed solution: 1500 ppm, boron concentration of the feed solution: 5 ppm, test pressure: 7 bar, temperature of the feed solution: 25°C, pH: 8, conversion rate: 15% and membrane surface area: 440 ft².

[0075] Low-pressure membranes, for example, are made of polymer materials, composites, and preferably in the form of a spiral module.

[0076] In steady state, i.e. an operation in which the permeate flow rate and the conversion rate are constant over time, the operating pressure of these membranes is for example between 10 bar and 40 bar.

[0077] Alternatively, the low pressure membrane filtration device 34 is a nanofiltration filtration device.

[0078] For membranes in this type of device, the rejection rate for divalent ions is higher than for monovalent ions. For example, the rejection rate for sodium chloride ranges from 20% to 97%. The rejection rate for MgSO4, for instance, ranges from 97% to 99.9%, and from 91% to 93% for CaCl2.

[0079] The permeability of each of these membranes ranges from 2.5 L / h / m² / bar to 20 L / h / m² / bar. As mentioned above, permeability is defined as the permeate flow rate normalized by the feed pressure and membrane surface area, but does not take into account the salinity of the feed solution.

[0080] The membrane cutoff threshold, that is, the critical molar mass for which 90% of the solutes are retained by the membrane, is for example between 200 Da and 3000 Da.

[0081] The rejection rate and permeability are, for example, calculated under the following standard conditions: sodium chloride concentration of the feed solution: 500 ppm, magnesium sulfate concentration of the feed solution: 2000 ppm, test pressure: between 5 and 11 bar, temperature of the feed solution: 25°C, pH: between 6.5 and 7.5, conversion rate: 15% and membrane surface area: 440 ft².

[0082] Nanofiltration membranes, for example, are made of polymer and inorganic materials such as ceramics. They can be composite or asymmetrical. They are implemented in spiral modules as well as in tubular geometries and hollow fibers.

[0083] In steady state, i.e. an operation in which the permeate flow rate and the conversion rate are constant over time, the operating pressure of these membranes is for example between 3 bar and 20 bar.

[0084] Advantageously, the pressure of the concentrate flow from the high-pressure stage 28 is greater than or equal to the minimum operating pressure of the low-pressure filtration device 34 to produce the permeate flow from the low-pressure stage and the concentrate flow from the low-pressure stage.

[0085] In the example of the, the low pressure membrane filtration unit 20 comprises a single low pressure membrane filtration device 34, referred to as the first low pressure membrane filtration device 40. The first membrane filtration device 40 forms a second filtration stage of the treatment unit 10.

[0086] The first low-pressure membrane filtration device 40 is intended to filter the concentrate stream from the high-pressure stage 28 and to provide a first concentrate stream from the low-pressure stage 42 and a first permeate stream from the low-pressure stage 44.

[0087] Thus, in the first embodiment, the concentrate flow from the low pressure stage 36 is formed by the first concentrate flow from the low pressure stage 42 and the permeate flow from the low pressure stage 44 is formed by the first permeate flow from the low pressure stage 44.

[0088] The first low-pressure membrane filtration device 40 includes a first feed conduit 45 fluidically connected to the concentrate conduit 32 of the high-pressure reverse osmosis filtration device 18.

[0089] The recirculation loop 21 is intended to recirculate at least part of one of the permeate streams from the low pressure stage 38, upstream of the high pressure pump 16 so as to provide at the inlet 22 of the high pressure pump 16 a mixed stream 46 formed by a mixture between the raw water 12 and at least part of at least one of the permeate streams from the low pressure stage 38.

[0090] In the example of the, the recirculation loop 21 is intended to recirculate at least part of the first permeate flow from the low pressure stage 44, upstream of the high pressure pump 16 so as to provide at the inlet 22 of the high pressure pump 16 a mixed flow 46 formed by a mixture between the raw water 12 and at least part of the first permeate flow from the low pressure stage 44.

[0091] Preferably, the entire first permeate flow 44 is recirculated upstream of the high-pressure pump 16.

[0092] The efficiency of the high-pressure stage, for example, is between 40% and 60%.

[0093] The efficiency of the low-pressure stage, for example, is between 10% and 90%.

[0094] Thus, the processing unit 10 makes it possible to increase the overall yield.

[0095] If the salinity of the first permeate stream from the low-pressure stage 44 is lower than the salinity of the raw water 22, the mixed stream 46 has a lower salinity than the raw water. This is particularly advantageous because it reduces the osmotic pressure and thus decreases the required inlet pressure for the high-pressure reverse osmosis filtration unit 18.

[0096] Advantageously, the treatment unit 10 further includes an energy recovery device 48 configured to pressurize at least a portion of the raw water 22 using at least one concentrate stream from the low pressure stage 36.

[0097] In the example of the, the energy recovery device 48 is configured to pressurize at least part of the raw water 12 using the first concentrate stream from the low pressure stage 42.

[0098] Typically, the pressure drop PCh generated by the passage of the pressurized water flow 24 to P alim in the high pressure reverse osmosis filtration device 18 is between 1 bar and 2 bars.

[0099] The first permeate flow from the low-pressure stage 44 is at atmospheric pressure P atm .

[0100] The first concentrate flow from the low-pressure stage 42 is at a pressure P c =P alim -P Ch .

[0101] For example, the energy recovery device 48 is a pressure exchanger, as illustrated in the.

[0102] As an alternative (not shown), the energy recovery device 48 is an energy recovery turbine.

[0103] A process for treating raw water 12 according to the invention will now be described.

[0104] The process first includes a raw water supply step 12 to a high-pressure pump 16 to provide a pressurized water flow 24.

[0105] The relative pressure of the raw water 12 at the inlet of the high-pressure pump 16 is typically less than 5 bar.

[0106] The relative pressure of the pressurized water flow 24 at the outlet of the high-pressure pump 16 is, for example, between 10 bar and 120 bar.

[0107] The process then includes a step of filtering the pressurized water stream 24 with a high-pressure reverse osmosis device 18 to provide a high-pressure permeate stream 26 and a concentrate stream from the high-pressure stage 28.

[0108] Next, the concentrate stream from the high-pressure stage 28 is filtered with a low-pressure membrane filtration unit 20 comprising at least one low-pressure membrane filtration device 34. The filtration unit 20 provides at least one permeate stream from the low-pressure stage 38 and at least one concentrate stream from the low-pressure stage 36. The permeate stream from the low-pressure stage 38 has a salinity less than or equal to the salinity of the raw water 22.

[0109] In the example of the, the low pressure membrane filtration unit 20 comprises a single first low pressure membrane filtration device 40 providing a first permeate flow from the low pressure stage 44 and a first concentrate flow from the low pressure stage 42. Thus, the filtration unit 10 provides a single permeate flow from the low pressure stage 38 formed by the first permeate flow from the low pressure stage 44 and a single concentrate flow from the low pressure stage 36 formed by the first concentrate flow from the low pressure stage 42.

[0110] The permeate flow from the low pressure stage 38, i.e. the first permeate flow from the low pressure stage 44 in the example of the, is recirculated at least in part upstream of the high pressure pump 16 so as to provide at the inlet of the high pressure pump 16 a mixed flow 46 formed by a mixture between the raw water 12 and the permeate flow from the low pressure stage 26, i.e. the first concentrate flow from the low pressure stage 42.

[0111] Advantageously, the concentrate flow from the high-pressure stage 28, i.e. the first concentrate flow from the low-pressure stage 42 in the first embodiment, is filtered by the low-pressure membrane filtration unit 20 without additional pressurization of said concentrate flow from the high-pressure stage 28.

[0112] Advantageously, the process further includes a pressurization step of at least part of the raw water 12 with an energy recovery device 48 using the concentrate flow from the low pressure stage 36.

[0113] Laillume illustrates a second embodiment not covered by the present invention. This embodiment will be described by its differences from the first embodiment.

[0114] In this embodiment, the treatment unit 10 further includes an osmotic energy exploitation device 50 configured to produce electrical energy from the salinity gradient between at least one permeate flow from the low pressure stage 38 and at least one concentrate flow from the low pressure stage 36.

[0115] In particular, in the example of the, the osmotic energy exploitation device 50 is configured to produce electrical energy from the salinity gradient between the first permeate flow from the low pressure stage 44 and the first concentrate flow from the low pressure stage 44 supplied by the first low pressure membrane filtration device 40.

[0116] The osmotic energy exploitation device 50 is, for example, a reverse electrodialysis device (“reverse electrodialysis” in English or RED).

[0117] Alternatively, the osmotic energy exploitation device 50 is a pressure-retarded osmosis device (PRO).

[0118] Preferably, in the case where the processing unit 10 also includes an energy recovery device 48, the first concentrate flow from the low pressure stage 42 first feeds the energy recovery device 48 before feeding the osmotic energy exploitation device 50.

[0119] The first permeate flow from the low pressure stage 44 first feeds the osmotic energy exploitation device 50 before recirculating with the recirculation loop 21 upstream of the high pressure pump 16.

[0120] The treatment process is identical to the process described above. It includes a step of producing electrical energy with the osmotic energy exploitation device 50 from the salinity gradient between the first permeate flow from the low pressure stage 44 and the first concentrate flow from the low pressure stage 42.

[0121] Laillustrates an embodiment covered by the present invention. This embodiment is described in relation to the first embodiment, that is to say that unless explicitly stated otherwise, this embodiment may include all the features described in the first embodiment.

[0122] In this embodiment, the treatment unit 10 includes a control unit 61. The control unit 61 includes a first control system 52 configured to regulate the permeate flow from the low pressure stage 26, i.e. the first permeate flow from the low pressure stage 44, recirculated upstream of the high pressure pump 16, and a second control system 63 configured to circulate a portion of the pressurized water flow 24 directly to the inlet of the low pressure membrane filtration unit 20, so as to maintain the salinity of the pressurized water flow 24 substantially constant over time and equal to a setpoint value.

[0123] Put another way, the second control system 63 is configured to allow part of the pressurized water flow 24 not to pass through the high pressure reverse osmosis filtration device 18.

[0124] Unit 61 allows the inlet of the high-pressure reverse osmosis filtration device 18 to have a substantially constant osmotic pressure over time by taking into account and compensating for the temporal variations in salinity of the raw water 14, and therefore allows the high-pressure pump to operate at substantially constant pressure over time.

[0125] Preferably, the control unit 61 includes at least one sensor 60 for a parameter representative of the salinity of the mixed flow 46, configured to acquire at least one measurement of a parameter representative of the salinity of the mixed flow 46. The first control system 52 and the second control system 63 are configured to regulate, respectively, the recirculated permeate flow and the portion of the pressurized water flow 24 circulating upstream of the low-pressure membrane filtration unit 20, based on said measurement of the parameter representative of the salinity of the mixed flow 46. The sensor 60 is located upstream of the high-pressure pump 16 and downstream of the recirculation loop branch 21.

[0126] Advantageously, the parameter representing the salinity of the mixed flow is electrical conductivity. This is particularly interesting because it allows us to take into account both the variations in salinity and temperature of the raw water, two parameters that directly influence electrical conductivity and osmotic pressure.

[0127] The process includes, in addition to the steps mentioned in the first embodiment, a step of maintaining the salinity of the pressurized water flow 24 substantially constant over time and equal to a setpoint value, by means of the control unit 61, by regulating the flow of permeate from the low pressure stage recirculated upstream of the high pressure pump or by circulating part of the pressurized water flow 24 directly to the inlet of the low pressure membrane filtration unit 20.

[0128] In particular, when the salinity value of the raw water decreases and falls below the setpoint value, the first control system 52 reduces the flow of recirculated permeate from the low pressure stage 26 in order to increase the salinity of the mixed flow 46 entering the high pressure pump 16 so as to maintain the salinity of the pressurized water flow 24 substantially constant over time, substantially equal to the setpoint value.

[0129] When the salinity value of the raw water increases and goes above the setpoint value, the second control system 63 circulates part of the pressurized water flow 24 directly to the inlet of the low pressure membrane filtration unit 20, so as to increase the flow of recirculated permeate upstream of the high pressure pump, and thus decrease the salinity of the pressurized water flow 24.

[0130] According to a particular embodiment, the processing unit 10 according to the third embodiment may also include an osmotic energy exploitation device 50 as described in the second embodiment.

[0131] Laillume illustrates a fourth embodiment not covered by the present invention. This embodiment will be described by its differences from the second embodiment.

[0132] In this embodiment, the low-pressure membrane filtration unit 20 further comprises a second low-pressure membrane filtration device 54. The second membrane filtration device 54 forms a third filtration stage.

[0133] Put another way, the low pressure stage comprises a first low pressure stage including the first low pressure membrane filtration device 40, and a second low pressure stage including the second low pressure membrane filtration device 54. The second low pressure stage forms the third filtration stage of the treatment unit 10.

[0134] In particular, the second low-pressure membrane filtration device 54 is intended to filter the first stream of concentrate from the low-pressure stage 42 produced by the first low-pressure membrane filtration device 40 and to provide a second stream of concentrate from the low-pressure stage 56 and a second stream of permeate from the low-pressure stage 58.

[0135] Thus, in this embodiment, the low pressure membrane filtration unit 20 provides two permeate streams from the low pressure stage 38 formed by the first permeate stream from the low pressure stage 44 and the second permeate stream from the low pressure stage 58, and two concentrate streams from the low pressure stage 42 formed by the first concentrate stream from the low pressure stage 42 and the second concentrate stream from the low pressure stage 56.

[0136] Each of the permeate streams from the low pressure stage 38 has a salinity lower than the salinity of the raw water 22. The salinity of the second concentrate stream is higher than the salinity of the first concentrate stream.

[0137] According to a particular embodiment, the first low-pressure membrane filtration device 40 and the second low-pressure membrane filtration device 54 are identical.

[0138] Alternatively, the permeability of the second low-pressure membrane filtration device 54 is greater than the permeability of the first low-pressure membrane filtration device 40.

[0139] The operating pressure of the second low-pressure membrane filtration device 54 is lower than the operating pressure of the first low-pressure membrane filtration device 40.

[0140] The recirculation loop 21 is intended to recirculate at least part of the first permeate flow from the low pressure stage 44 and at least part of the second permeate flow from the low pressure stage 58 upstream of the high pressure pump 16.

[0141] Preferably, the entire first permeate flow from the low-pressure stage 44 and the second permeate flow from the low-pressure stage 58 is recirculated.

[0142] In this embodiment, the energy recovery device 48 is configured to pressurize at least part of the raw water 12 using the second concentrate stream from the low pressure stage 56.

[0143] The relative pressure of the second concentrate flow is, for example, between 5 bar and 75 bar.

[0144] The relative pressure of the second permeate flow is, for example, between 0 bar and 0.3 bar.

[0145] The osmotic energy exploitation device 50 is configured to produce electrical energy from the salinity gradient between the second permeate stream from the low pressure stage 58 and the second concentrate stream from the low pressure stage 58 supplied by the second low pressure membrane filtration device 54.

[0146] The second permeate flow from the low pressure stage 58 is recirculated upstream of the high pressure pump 16 after passing through the osmotic energy exploitation device 50.

[0147] Advantageously, the first concentrate stream from the low-pressure stage 42 is filtered by the second low-pressure membrane filtration device 54 without additional pressurization of said first concentrate stream from the low-pressure stage 42.

[0148] Of course, according to a particular embodiment, the processing unit 10 may not include an osmotic energy exploitation device 50.

[0149] Alternatively, according to a particular embodiment, the processing unit 10 may include a control system 52 as described in the third embodiment.

[0150] Laillustrates a fifth embodiment not covered by the present invention. It is a variant embodiment of the fourth embodiment.

[0151] In this embodiment, the osmotic energy exploitation device 50 is configured to produce electrical energy from the salinity gradient between the first permeate flow from the low pressure stage 44 supplied by the first low pressure membrane filtration device 40 and the second concentrate flow from the low pressure stage 56 supplied by the second low pressure membrane filtration device 54.

[0152] Three-stage filtration embodiments are particularly advantageous. When the permeate feeds the osmotic power extraction device 50, a transfer of salinity occurs between the concentrate and the permeate. Consequently, the permeate has a higher salinity at the outlet. When the permeate is recirculated upstream of the high-pressure pump 16, the dilution of the seawater is therefore less significant. The third stage addresses this drawback.

[0153] Laillustrates a sixth embodiment not covered by the present invention. It is a variant embodiment of the first embodiment.

[0154] In this embodiment, the low pressure membrane filtration unit 20 comprises two low pressure membrane filtration devices 34, namely a first low pressure membrane filtration device 34A and a second low pressure membrane filtration device 34B.

[0155] The low pressure stage comprises a single first low pressure stage including the first low pressure membrane filtration device 34A and the second low pressure membrane filtration device 34B.

[0156] Each of the low pressure membrane devices 34A, 34B is formed by a nanofiltration filtration device.

[0157] The first filtration device 34A and the second filtration device 34B are fluidically connected in series so that the first filtration device 34A filters the high-pressure concentrate stream 26 to provide a permeate stream which feeds the second filtration device 34B.

[0158] In the example of the, the concentrate streams from the first filtration device 34A and the second filtration device 34B are mixed and feed the energy recovery device 48.

[0159] The permeate flow from the second filtration device 34B is recirculated upstream of the high-pressure pump 16 by the recirculation loop 21.

[0160] The first low-pressure filtration device 34A and the second low-pressure filtration device 34b form respectively a first filtration pass of the low-pressure stage and a second filtration pass of the low-pressure stage.

[0161] The first low-pressure filtration device 34A and the second low-pressure filtration device 34b are similar or different.

[0162] Of course, according to alternative embodiments, in the second and third embodiments, the single low-pressure stage may include a first low-pressure membrane filtration device 34A and a second low-pressure membrane filtration device 34B, as described above.

[0163] Similarly, alternatively (not shown), in the fourth and fifth embodiments, one of the first and second low-pressure stages may include a first low-pressure membrane filtration device 34A and a second low-pressure membrane filtration device 34B, as described above.

Claims

Raw water (12) treatment unit (10), said treatment unit (10) comprising: - a raw water (12) supply line (14), - a high-pressure pump (16) comprising an inlet (22) fluidly connected to the raw water (12) supply line (14), said high-pressure pump (16) being intended to provide a pressurized water flow (24), - a high-pressure reverse osmosis filtration device (18) forming a high-pressure filtration stage, intended to filter the pressurized water flow (24) to provide a permeate flow from the high-pressure stage (26) and a concentrate flow from the high-pressure stage (28), - a low-pressure membrane filtration unit (20) forming a low-pressure stage comprising at least one low-pressure membrane filtration device (34),said filtration unit (20) being intended to filter the high-pressure concentrate stream (26) to provide at least one permeate stream from the low-pressure stage (38) and at least one concentrate stream from the low-pressure stage (36), said permeate stream from the low-pressure stage (38) having a salinity less than or equal to the salinity of the raw water (12), - a recirculation loop (21) intended to recirculate at least a part of at least one of the permeate streams from the low-pressure stage (38), upstream of the high-pressure pump (16) so as to provide at the inlet (22) of the high-pressure pump (16) a mixed stream (46) formed by a mixture between the raw water (22) and at least a part of at least one of the permeate streams from the low-pressure stage (38),- a control unit (61) comprising a first control system (52) configured to regulate the permeate flow from the low-pressure stage (38) recirculated upstream of the high-pressure pump (16), and a second control system (63) configured to circulate a portion of the pressurized water flow (24) directly to the inlet of the low-pressure membrane filtration unit (20), so as to maintain the salinity of the pressurized water flow (24) substantially constant over time and equal to a setpoint value. Processing unit (10) according to claim 1, wherein the control unit (61) comprises at least one sensor (60) of a parameter representative of the salinity of the mixed flow (46) configured to acquire at least one measurement of a parameter representative of the salinity of the mixed flow (46), the first control system (52) and the second control system (63) being configured to regulate respectively the recirculated permeate flow and the portion of the pressurized water flow (24) circulating upstream of the low pressure membrane filtration unit (20) from said measurement of the parameter representative of the salinity of the mixed flow (46). Processing unit (10) according to claim 2, wherein the parameter representative of the salinity of the mixed stream is the electrical conductivity. Treatment unit (10) according to claim 2 or 3, wherein the recirculation loop (21) includes a tap on the supply line (14), the sensor (60) of a parameter representative of the salinity of the mixed flow (46) being disposed upstream of the high pressure pump (16) and downstream of the tap of the recirculation loop (21). Processing unit (10) according to any one of claims 1 to 4, wherein the low pressure membrane filtration device (34) comprises respectively a plurality of low pressure membranes, and the high pressure reverse osmosis filtration device (18) comprises a plurality of high pressure reverse osmosis membranes, the permeability of each low pressure membrane being greater than the permeability of each high pressure reverse osmosis membrane. Processing unit (10) according to any one of claims 1 to 5, wherein at least one low pressure membrane filtration device (34) is a low pressure nanofiltration or reverse osmosis filtration device. Processing unit (10) according to claim 6, wherein at least one low pressure membrane filtration device (34) is a nanofiltration filtration device, the low pressure membrane filtration unit (20) comprising a first low pressure membrane filtration device (34) and a second low pressure membrane filtration device (34), fluidly connected in series in each other, the first low pressure membrane filtration device (34) being intended to filter the high pressure concentrate stream (26) and to provide a permeate stream intended to be filtered by the second low pressure membrane filtration device (34). Processing unit (10) according to any one of claims 1 to 7, wherein the pressure of the concentrate flow from the high-pressure stage (28) is greater than or equal to the minimum operating pressure of the low-pressure filtration device (34) to produce the permeate flow from the low-pressure stage and the concentrate flow from the low-pressure stage. Processing unit (10) according to any one of claims 1 to 8, wherein the sodium chloride retention rate of the low pressure membrane filtration device (34) is lower than the sodium chloride retention rate of the high pressure reverse osmosis filtration device (18). Processing unit (10) according to any one of claims 1 to 9, further comprising an energy recovery device (48) configured to pressurize at least a portion of the raw water (12) using at least one concentrate stream from the low pressure stage (36). Processing unit (10) according to any one of claims 1 to 10, wherein the operating pressure of the high-pressure reverse osmosis filtration device (18) is between 10 bar and 120 bar, and the operating pressure of the low-pressure membrane filtration device (34) is between 3 bar and 40 bar. Processing unit (10) according to any one of claims 1 to 11, further comprising an osmotic energy exploitation device (50) configured to produce electrical energy from the salinity gradient between at least one permeate stream from the low pressure stage (38) and at least one concentrate stream from the low pressure stage (36). A process for treating raw water (12), said process comprising the following steps: - supplying raw water (22) to a high-pressure pump (16) to provide a pressurized water flow (24), - filtering the pressurized water flow (24) with a high-pressure reverse osmosis device (18) and providing a permeate flow from the high-pressure stage (26) and a concentrate flow from the high-pressure stage (28), - filtering the concentrate flow from the high-pressure stage (28) with a low-pressure membrane filtration unit (20) comprising at least one low-pressure membrane filtration device (34) to provide at least one permeate flow from the low-pressure stage (38) and at least one concentrate flow from the low-pressure stage (36), said permeate flow from the low-pressure stage (38) having a salinity less than or equal to the salinity of the raw water (22),- recirculating at least a portion of at least one of the permeate streams from the low-pressure stage (38), upstream of the high-pressure pump (16) so as to provide at the inlet (22) of the high-pressure pump (16) a mixed stream (46) formed by a mixture of raw water (22) and at least a portion of at least one of the permeate streams from the low-pressure stage (38), the process further comprising a step of maintaining the salinity of the pressurized water stream (24) substantially constant over time and equal to a setpoint value by means of a control unit (61), by regulating the permeate stream from the low-pressure stage (38) recirculated upstream of the high-pressure pump (16) and / or by circulating a portion of the pressurized water stream (24) directly to the inlet of the low-pressure membrane filtration unit (20). A treatment process according to claim 13, wherein when the salinity of the raw water decreases, the flow of permeate recirculated upstream of the high-pressure pump (16) decreases. Process of claim 13 or 14, wherein when the salinity of the raw water increases, a portion of the pressurized water flow (24) is sent directly to the inlet of the low pressure membrane filtration unit (20) to increase the flow of recirculated permeate upstream of the high pressure pump (16). Processing method according to any one of claims 13 to 15, wherein the concentrate stream from the high-pressure stage (28) is filtered by the low-pressure membrane filtration unit (20) without additional pressurization of said concentrate stream from the high-pressure stage (28). Processing method according to claim 13 or 16, further comprising a pressurization step of at least a portion of the raw water (12) with an energy recovery device (48) using at least one concentrate stream from the low pressure stage (36). Processing method according to any one of claims 13 to 17, further comprising a step of producing electrical energy with a device for exploiting osmotic energy (50) from the salinity gradient between at least one permeate flow from the low pressure stage (38) and at least one concentrate flow from the low pressure stage (36).

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