Forward osmotic and water hammer method of membrane cleaning

Abstract

Apparatus and method for semi-permeable membrane cleaning in particular, applying series of pulsed water stroke, made simultaneously with osmosis backward flow causing superposed membrane directional shaking and fouling detachment. Pulsed water stroke provided by water stroke generator as several momentum sharp changes in gauge pressure and induce velocity pulse of residual brine flow. The pulsed water strokes ideally induce resonance in the membrane. Osmosis backward wash may be provided either by injection for predetermined injection time, additional solution selected in such way that net driving pressure becomes opposite to normal osmotic operation thereby providing a backward flow of permeate towards to the side opposite to normal operation mode, so as to lift said foulant, or by throttling permeate exiting from the permeate enclosure, until the net driving pressure value become equal to zero, during application of precise synchronized and opposing brine and permeate pressure strokes thereby providing a plurality of quick RO-FO-RO process changes. These procedures allow a membrane to be kept continuously clean and operate at higher recovery.

Claims

1. A method for cleaning a semi-permeable membrane in a pressure retarded osmosis module, the method comprising: providing a pressure retarded osmosis module comprising: an enclosure, the enclosure comprising a semi-permeable membrane with a first side and a second side opposite the first side, with foulant located on at least the second side, a raw saline solution having an osmotic pressure POr at a gauge pressure PGr for entering the first side of the semi-permeable membrane, a fluid stream having a gauge pressure PGp and an osmotic pressure POp for entering the second side of the semi-permeable membrane, wherein at least part of a fluid from the fluid stream penetrates from the second side of the semi-permeable membrane to the first side according to a net driving pressure defined by a balance of pressures PGr, POr, POp and PGp, wherein a remainder of the fluid exits as a residual fluid stream from the second side of the semi-permeable membrane via an outlet, wherein the raw saline solution and the penetrated fluid exits as a residual brine stream from the first side of the semi-permeable membrane via a residual brine outlet; applying, at least periodically, a plurality of directional pressure strokes having PGp and/or PGr directed from at least one of the fluid stream or the residual brine stream to the first side of the semi-permeable membrane, thereby effecting mechanical shaking of the semi-permeable membrane for detachment of the foulant; applying a pulsed-flow regime in the fluid stream, thereby increasing shearing force for enhancing evacuation of the foulant; and periodically feeding an Additional Solution (AS) having an osmotic pressure POs into the first side of the semi-permeable membrane, the osmotic pressure POs being less than the osmotic pressure POr of the raw saline solution, thereby creating a net driving pressure that is opposite to pressure retarded osmosis, thereby inducing a reverse osmosis (RO) process to create backward flow towards the second side of the semi-permeable membrane, thereby lifting the foulant and enhancing evacuation of the foulant, wherein a generator of pressure strokes installed in an inlet and/or the outlet of the second side is configured to release flow from the second side, wherein the release occurs from the inlet, from the outlet, or simultaneously from the inlet and the outlet.

2. A method for cleaning a semi-permeable membrane in a pressure retarded osmosis module, the method comprising: providing a pressure retarded osmosis module comprising: an enclosure, the enclosure comprising a semi-permeable membrane with a first side and a second side opposite the first side, with foulant located on at least the second side, a raw saline solution having an osmotic pressure POr at a gauge pressure PGr for entering the first side of the semi-permeable membrane, a fluid stream having a gauge pressure PGp and an osmotic pressure POp for entering the second side of the semi-permeable membrane, wherein at least part of a fluid from the fluid stream penetrates from the second side of the semi-permeable membrane to the first side according to a net driving pressure defined by a balance of pressures PGr, POr, POp and PGp, wherein a remainder of the fluid exits as a residual fluid stream from the second side of the semi-permeable membrane via an outlet, wherein the raw saline solution and the penetrated fluid exits as a residual brine stream from the first side of the semi-permeable membrane via a residual brine outlet; and periodically feeding an Additional Solution (AS) having an osmotic pressure POs into the second side of the semi-permeable membrane, the osmotic pressure POs being greater than the POp, thereby creating a net driving pressure that is opposite to pressure retarded osmosis, thereby inducing a reverse osmosis (RO) process to create backward flow from the first side of the semi-permeable membrane to the second side, thereby lifting the foulant from the second side and enhancing evacuation of the foulant, wherein a generator of pressure strokes installed in an inlet and in the outlet of the second side is configured to pump in the AS to the inlet and to simultaneously release fluid flow from the outlet of the second side.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1, 1b and 1c are schematic and expanded views of an RO system according to an embodiment of the present invention;

(2) FIG. 2 is a graph illustrating an alternating permeate PGp pressure below and above NPSH by pump 10;

(3) FIG. 3 is a graph illustrating alternating permeate PGp pressure by pump 10 and PRV 13;

(4) FIG. 4 is a Flow and pressure graph for selection permeate pump 10 and PRV 13;

(5) FIG. 5 is a schematic drawing illustrating the present invention incorporated into the entire scheme of water treatment;

(6) FIG. 6 is a general view of a membrane module incorporated into an arrangement for AS preparation and injection;

(7) FIG. 7 is a graph showing “Amplitude-Frequency” area; and

(8) FIG. 8 is a schematic view of a system according to another embodiment of the present invention having a water stroke generator for pressure and flow pulsation.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) FIG. 1 is a schematic view of a RO unit comprising a reverse osmosis (RO) separation module 100. Tank 20 contains Raw Saline solution (RS) 21. Tank 23 contains additional solution 24. Feed pump 22 pumps raw saline solution into a feed channel 5 and toward a membrane feed side 8. Raw saline solution which comprises a solvent and dissolved salts, moves along the membrane feed side 8 under gauge pressure PGr and has an osmotic pressure POr. Residual brine is removed via outlet (RB) 28.

(10) Semi-permeate membrane 14 has a feed side 8 and a permeate side 15. Feed spacer 6 maintains a gap between two opposing feed sides 8 of two opposing membranes 14 and defines the feed channel 5. During a RO separation process, permeate (PR) 27 has an osmotic pressure POp and a gauge pressure PGp, and is driven by a positive net driving pressure defined by the balance of the four pressures PGr, POr, POp and PGp, and penetrates via membrane 14 into the permeate channel 4 which is shown in detail in FIG. 1c.

(11) Basic unit 300 in FIG. 1c is defined by a pair of adjacent solid permeate spacer ribs 7 which maintain gap 4 between two opposing permeates sides 15 of two opposing free membrane portions 16 of membrane 14. As mentioned above, membrane 14 tends to get a wave-like geometry due to a higher PGr than PGp and due to the mechanical support provided by the grid of the rigid ribs of the permeate spacer 7.

(12) A typical spiral membrane element may have an about 40,000 basic units. A basic unit 300 may have a dimension of an about 0.5 mm in width and 1,000 mm in length. A drumhead of a stretched membrane over a void space 4 of the permeate channel consists of stretched free membrane portion 16 located between two opposing solid fibers of permeate spacer 7. According to the present invention, periodic oscillations of the pressure difference between PGr and PGp, cause an oscillating displacements of free membrane portions 16.

(13) As shown in FIG. 1b and FIG. 1c, membrane 14 and free membrane elements 16 may be displaced from a first position 80 to a second position 80′. It is one aspect of the present invention, to induce such displacements by changing the ratio between PGr and PGp temporarily, in a synchronized way or in an unsynchronized way, in a way which may cause a quick pulsed displacement characterized by a certain direction. Such a direction is preferably from the permeate channel toward the feed channel.

(14) Due to the wave-like geometry of membrane 14, as mentioned above, flow of the raw solution in the feed channel experiences velocity changes. The feed flow velocity tends to be higher in narrow spaces in feed channel 5, such as the spaces above the solid ribs 7 of the permeate spacer which gives no room to membrane 14 to bend (sag) toward the permeate channel. These areas are characterized as supported membrane portions 18 which has no room to bend (sag) toward the permeate side. The feed flow velocity tends to be lower in wider spaces of the feed channel 5, such as in the feed channel spaces located above void permeate channels spaces 4 which has no rigid support. Such places are characterized by free membrane portions 16 which have room to be displaced and bend (sag) toward the permeate side and toward the feed side, as a function of the ratio and/or changes of this ratio, of the PGr and PGp. Moreover, due to these changes of feed flow velocity, and due to other factors, in general fouling 9 tends to be thicker above free membrane portions 16 and thinner above supported membrane portions 18.

(15) As shown in FIG. 1b, feed spacer 6 and permeate spacer 7 maintain gaps between opposing membranes. Gap 5 between two opposing feed sides 8 of two opposing membranes 14, in which feed spacer 6 is located, is defined as the feed channel 5 and it is in a fluid communication with feed pump 22 and residual brine outlet 28. Gap 4, which is located between two opposing permeate sides 15 of two opposing membranes 14, in which permeate spacer 7 is located, is defined as the permeate channel and it is in a fluid communication with permeate pipe 29.

(16) As shown in FIG. 1, permeate channel 4 is in fluid communication with tank 26 contains permeate 27 through valve 12, and pipeline 31 with pressure relief valve 13 and pump 10 with valve 11. An arrangement, for example, such as pump 10, valve 11 and a pressure relief valve 13 may be configured to create a pulsed water stroke of an increased PGp pressure.

(17) Other arrangements may include pressure relive valve 109 that takes all or part of the residual brine stream via line 128 from pipeline 28 and discharge it via pipeline 129. Pressure relive valve 109 may be configured to create a pulsed water stroke in different frequencies, smoothly change frequency, and adjust water stroke frequency to membrane natural frequency, and reach membrane resonance oscillation. Such pressure relief valve or water stroke generator may be controlled by spring, diaphragm, solenoid or other means. It can be adjusted manually or automatically. In multi stage RO systems the pressure relief valve may be installed in interstate residual brine stream.

(18) FIGS. 2 and 3 illustrate changes in PGp and PGr pressure during cleaning process FO&WS and have to be considered together with FIG. 1. FIGS. 2 and 3 show a smooth sinusoidal waveform, including half-wave increase and half-wave decrease, of gauge pressures and also illustrate sequences in which different devices are activated.

Process Description

(19) The present invention covers a variety of options for membrane mechanical shaking without, but preferably with resonance, by means of PGp and/or PGr gauge pressure strokes, optionally combined with reversing osmotic processes from RO to FO or opposite, by means of injecting additional solution that changes the osmotic pressure from POr to POs that starts an osmotic pump. The osmotic pump may provide continuous backward permeate flow or more preferably, in pulsation form following NPSH PGp pressure. Due to wide variety of options, only a few process examples will be described below and with reference to the accompanying drawings.

(20) In accordance with one embodiment of the invention, during a RO process, a cleaning procedure consisting of FO&WS can be initiated by closing valve 12, starting pump 10 which pumps permeate 27 from tank 26 into permeate channel 4 via pipeline 30 and 29. The beginning of cleaning procedure may be seen as timeline 77 on FIG. 2. Pulse water strokes may be generated by series of quickly opening and closing valve 11 which move free membrane portions 16 of a basic unit 300 displacement (shaking) from a position 80 to a position 80′. Continuation of opening and closing valve 11 that causes set of pulse water strokes by frequency in the range about 0.2 to 0.1 Hz, by amplitude about ±4 bar, during cleaning procedure is presented on FIG. 2 as PGp smooth sinusoidal pulsation line 76. Note that this PGp pressure stroke has no possibility of, and is not intended to change NDP balance from RO to FO, but only to mechanically shake membrane. Finishing point of cleaning procedure takes place when pump 10 stops, valve 11 is closed, valve 12 is opened and the separation module 100 continues its normal RO separation process, as may be seen as timeline 82 on FIG. 2. The actual time passing between timelines 77 and 82 is about three to five minutes. Fouling 9 which is located on membrane 14 may experience mechanical pulsed directional displacement force 1 that causes its detachment from membrane 14 and its removal with residual brine 28.

(21) In accordance with one embodiment of the invention, at the same timeline 77 on FIG. 2, instantaneously with pulse water stroke described above, a pulse of additional solution 24 from tank 23 may be fed into pump 22 instead of the raw solution 21 coming from tank 20 or in a combination with it. This may be done by opening valve 25 for about 3-50 seconds. The AS injection switches the osmotic pressure on the feed side 8 of membrane 14 from POr to POs. As a result, the new balance of the net driving pressure across membrane 14 experiencing the AS pulse, temporarily reverses the process from RO to FO and backward flow 2 of permeate 27 toward feed channel 5 takes place.

(22) Fouling 9 which is located on membrane 14 may experience three forces: a first force 1, which may be generated due to a mechanical membrane pulsed directional displacement (shake) as a result of the pulsed water stroke; a second force 2, which may be generated by a permeate backward flow due to a temporarily backward permeate flow across membrane 14 caused by a pulse of an AS; and a third shearing force 3, due to the longitudinal feed and residual brine flow in the feed channel. This third shearing force 3 may increase during such a pulsed water stroke due to a simultaneous increased velocity of the feed flow. Such a simultaneous velocity increase may happen due to the following reasons: (a) The feed flow may increase due to fact that permeate is not squeezed out of the feed channel at that time into the permeate channel rather than the opposite, permeate flows backward from the permeate side into the feed side and increasing its flow; (b) The cross section of the feed channel 5, at least in the relevant area which experiences the pulsed water stroke, becomes narrower due to the fact that free membrane portions 16 are displaced toward the feed channel from a first position 80 to a second position 80′. The superposition of the above factors and forces act together to enhance the detachment of fouling 9 from membrane 14 and removal with residual brine.

(23) According to an alternative embodiment of the invention, permeate backward flow 2 may take place not continuously as explained above, but only in the microseconds when membrane mechanical shaking 1 is in its maximum amplitude. The synchronization may take place when PGp fluctuation, line 76, cross about in the middle of the line 83 that may be equal to NPSH value of osmotic pump (FIG. 2).

(24) When PGp pressure is above NPSH value 83 the FO process provides backward permeate flow proportional to NDP-FO, when PGp pressure is below NPSH value 83 FO process cannot provide backward permeate flow proportional to NDP-FO, lines 76 and 83 (FIG. 2). Therefore, the accumulative effect is: a pulse of mechanical shaking and a pulse of backward permeate flow for fouling separation which may be stronger than a pulse of mechanical shaking and a continuous backward permeate flow.

(25) Pulsating permeate backward flow 2 during FO&WH cleaning process has an additional benefit over continuous forward osmotic backward flow 2. In the pulsating form, a smaller amount of permeate crosses the membrane. Since permeate which crosses the membrane into the feed channel dilutes the additional solution 24, a smaller amount of permeate which crosses the membrane the lower the dilution and the concentration of the additional solution 24 lasts longer and may still be effective toward the end of the FO&WH cleaning process. Alternatively, due to the lowered dilution effect, it may be possible to use an additional solution having a lower osmotic pressure POs or generating shorter injection pulses of an additional solution in order to get the same cleaning effect.

(26) Pulsating PGp to provide backward permeate flow 2 may be made in “Amplitude-Frequency” area “A” shown on FIG. 7. The frequency is about 0.02 to 0.1 Hz and amplitude up to ±4 bar measured on the feed end of pressure vessel. The upper limit of the gauge pressure change velocity, 10 psi per second, should not be exceeded. This limiting value 10 psi per second is shown as line 500 on FIG. 7.

(27) As mentioned above, the cleaning process based on this invention is ecologically friendly because it is based on mechanical energy rather than chemicals.

(28) Additional embodiments of this invention may increase the membrane cleaning effect by creating membrane oscillation by gauge pressure stroke PGp shown on FIG. 1b as arrowhead 1, having a frequency approximately equal to natural frequency of about 2.5 Hz of the free membrane portions 16 in order to get a resonance and enhanced displacement. Membranes oscillating in a frequency proportional to their natural frequency may increase the amplitude of their vibration (resonance effect). Opening and closing valve 11 cannot be made at 2.5 Hz frequency. Water Stroke Generator such as Pressure relief valve (PRV) 13 may be implemented to provide such frequent oscillation of PGp.

(29) In accordance with one embodiment of the invention, during a RO process, a cleaning procedure FO&WS can be initiated by closing valve 12, starting pump 10 and keeping valve 11 continuously open. Pump 10 pumps permeate 27 from tank 26 into permeate channel 4 via pipeline 29, 30, 31 and via PRV 13 discharging it to tank 26 as a pulse flow. The beginning of cleaning procedure may be seen as timeline 86 on FIG. 3.

(30) The configuration of PRV 13 may be selected by the ability to discharge permeate 27 as a series of pulses and create flow-induced vibrations.

(31) Pulse water strokes generated by PRV 13 motivate free membrane portions 16 of a basic unit 300 resonance displacement (shaking) from a position 80 to a position 80′, frequency about 2.5 Hz. Continuation of PRV flow-induced vibrations cause pulse water stroke during cleaning procedure shown on FIG. 3 as smooth, sinusoidal PGp pulsation line 92 around certain pressure 116. Free membrane portions 16 resonance oscillation for different membrane types may be in range of frequency 0.1 to 5 Hz with best results reached with 2.5 Hz for certain membrane types. The amplitude in this frequency range should be low and not exceed ±3 bar measured on feed end of pressure vessel. This recommended area of membrane resonance is marked its area “B” on FIG. 7. This PGp pressure stroke has no possibility or intention of changing NDP balance from RO to FO, but only provides for membrane resonance shaking. The finishing point of the cleaning procedure takes place when pump 10 stops, valve 11 is closed and valve 12 is opened, with the separation module 100 continues its normal RO separation process as may be seen as timeline 89 on FIG. 3. The actual time passes between timeline 86 and 89 is about three to five minutes. Fouling 9 which is located on membrane 14 may experience mechanical pulsed directional displacement force that causes its detachment from membrane 14 and its removal with residual brine 28.

(32) In accordance with one embodiment of the invention at the same timeline 86 on FIG. 3, instantaneously with the pulse water stroke described above, a pulse of additional solution 24 from tank 23 may be fed into pump 22 instead of the raw solution 21 coming from tank 20 or in a combination with it.

(33) FIG. 4 of the accompanying drawings shows an example of selection permeate pump 10 and pressure relief valve 13 by flow and pressure according to one embodiment of the present invention. Such a relationship may demonstrate, at least in one way among possible other ways, how to define pump 10 characterized by a curve 71. FIG. 4 further shows some aspects related to the requirements of pressure relief valve 13 according to this one embodiment.

(34) In general, the flow output of permeate pump 10 and PRV 13 may be selected to meet the RO module nominal permeate production rate which is defined in FIG. 4 as point “A”. The pressure output of permeate pump 10, at its shut-off point 70, may be selected so that it is about 20% below the pressure rate of permeate pipe 29. Because of implementation small PGp amplitude, the present invention allows the use of low-pressure plastic permeate lines. Permeate pump 10 may have a flat pressure-flow characteristic. Permeate pump 10 may be equipped with a VSD to simplify the adjustment between pump duty pressure and the PRV's pressure set point “A”.

(35) Pressure relief valve or other water stroke generator 13 may be selected to meet the nominal permeate out flow 73 of RO module 100, and pressure relief set point “A”. Above this point “A” PRV relieves permeate flow 27 at frequency between 0.1 and 5 Hz. The frequency of such pressure pulsations may be smoothly adjusted, for example, by adjusting spring tension or changing water passes to diaphragm valve or by PROGRAMMABLE LOGIC CONTROLLER changing the frequency automatically or manually until highest turbidity of reject brine flow will be reached. An increase in turbidity indicates that resonance was reached. It is recommended to make variations of frequency near by or close to the best frequency.

(36) Pumps 10 and 22 should hydraulically and/or electrically communicate in order to control and stop pump 10 immediately if pump 22 has stopped for any reason. This safety precaution has to be duplicated and even triplicated to ensure that permeate pressure PGp will never be more than the feed pressure PGr as known to the skilled man in the art.

(37) In accordance with one embodiment of the invention, membrane oscillation can be initiated by pulse water strokes PGr generated by PRV 109 positioned in residual brine stream 28. The pulse water strokes PGr may have sinusoidal pulsation 93 around nominal PGr pressure 116. The pressure presented as line 116 on FIG. 3 may have very different value for PGp and PGr pulsation.

(38) Membrane mechanical shaking 1 may be made by gauge pressure stroke PGr and/or PGp in “Amplitude-Frequency” range “B” shown on FIG. 7. The frequency is in range of about 0.1 to 5 Hz. It is recommended not to exceed gauge pressure amplitude more than ±3 bar, measured on common manifold at the feed end of pressure vessels. It is recommended not to exceed the 45 psi/second oscillation velocity, line 501 on FIG. 7.

(39) For seawater applications, it is recommended to apply PGr gauge pressure stroke amplitude below ±5% of the normal operation range. To be effective, PGr pulsation has to be smooth sinusoidal, the water strokes generator has to have smoothly adjustable frequency. As described above, an indication of resonance is demonstrated by an increase in turbidity of the residual brine stream.

(40) FIG. 5 of the accompanying drawings shows a standard UF module operating as Ultra-Filtration Batch Absorber 110 positioned down stream of RO module 100 and receives permeate 29 as feed. Sorbent injects via dosing system 115 continuously or as a pulse in to UF module 110, at the rate according to required water quality. Sorbent accumulates in UF module during dead-end filter cycle, and performs mass transfer and diffusion with organic matter presented in permeate stream 29. Dead-end filtration cycle keeps sorbent in UF fibers until it sorption capacity is exhausted. During periodic backwash, sorbent 115 may be discharged via outlet 112. The high purity product without organic matter is delivered from outlet 113.

(41) FIG. 6 shows a general view of membrane module and arrangement for options of additional solution preparation and injection.

(42) Raw saline solution 21 passes through micronics filter 50. Feed Pump 22 pumps raw saline solution 21 into the RO module 100. Product leaves module 100 via pipeline 29, residual brine is removed via pipeline 28. According to one embodiment of the invention, additional solution 24 may be fed into the membrane from a syringe type pump 51. This syringe pump may have a vertical arrangement of pipes. The syringe pump may be charged from the bottom by an additional solution 24 from tank 52 and by a pump 53 via filter 54. Syringe pump 51 operates by pressure drop on micronics filter 50. No piston is needed in the syringe pump, because difference in specific gravity between the raw saline solution 21 and the additional solution 24 is enough to prevent a mix of the two. Additional solution 24 may be prepared from a residual brine removed via pipeline 28 during previous FO&WS cleaning procedure. Additional solution 24 does not chemically interact with the fouling. The aim of the additional solution is only to serve as an osmotic source of energy to operate the forward osmotic pump during a FO&WS cleaning. The concentration of the additional solution may be diminished due to dilution by permeate backward flow during cleaning, however its concentration at the end of the process is still quite high, and may save salt for next additional solution preparation. Accordingly, the FO&WS cleaning procedure according to the present invention is ecologically friendly.

(43) Residual Brine 28 is collected during previous FO&WS cleaning sessions in tank 65. Residual brine 28 may be filtrated by micronics filter 54 and may be added to tank 52 for re-concentration. Tank 52 comprises of a wall 55. Wall 55 does not reach the bottom of tank 52. Wall 55 start from an about a height of 50 mm above the bottom of tank 52 allowing water to flow and circulate below it but prevents dry salt to fill Pump 53 suction compartment. Pump 53 circulates the additional solution for the purpose to re-concentrate it until its saturation level. Dry salt 56 is loaded into tank 52. During the additional solution circulation, the additional solution passes gaps 61 between one or several magnets 58 and 59. In accordance to one aspect of the invention, magnetic treatment based on permanent magnets may be made of FeNdB, NdFeB, Neodymium (Nd), Dysprosium (Dy), or Praseodymium (Pr) having for example 0.5 to 10 Tesla.

(44) In accordance with an embodiment of the invention a magnetic treatment device 400 may consist of a tower of multiple magnets allowing water to pass through their magnetic field. According to one embodiment, two types of magnets may be used. Ring shape magnets 58 with plugged holes, and ring shape magnets 59 with holes and peripheral plugs. These magnets may be arranged in pipe 60 in such a way that gaps 61 between adjacent magnets is created and water may pass one magnet through it internal hole and the next magnet from the outside.

(45) Gaps 61 between adjacent magnets are the areas where the magnetic field 57 is the strongest. During dissolution of the dry salt and after its dissolution, the additional solution 24 may be circulated continuously via the magnetic treatment device. According to one embodiment, charging the syringe pump 51 with the additional solution 24 should be made immediately before the additional solution 24 is to be fed into the membrane module 100 during FO&WS cleaning procedure.

(46) This magnetic treatment device increases the osmotic coefficient and actual osmotic pressure of additional solution 24.

(47) The mechanical shaking of the membrane with periodic feeding of additional solution to the feed side of the membrane as described with reference to FIGS. 1 to 7 is carried out periodically, such as once a day for approximately 10 seconds. It does require a temporary halt in the reverse osmosis process but provides enhanced fouling detachment and hence, cleaning of the membrane.

(48) In an alternative embodiment of the present invention, cleaning may be carried out all the time with no feeding of additional solution (AS) to the feed side. In this embodiment, known as “KEEPING CLEAN PROCEDURE” (“KCP”), mechanical shaking of the free membrane portions to detach foulant is again carried out by applying, for a predetermined period of time, a plurality of directional pressure strokes PGp and/or PGr on the permeate and/or residual brine membrane side. In addition, a pulse-wise flow regime is applied in the residual brine stream to increase the shearing force to the membrane thereby achieving enhanced fouling evacuation.

(49) In this embodiment, the pressure strokes are precisely synchronized with flow pulsation, preferably in the alternating sequence: PGr pressure decreasing, shearing force increasing; PGp increasing; followed by PGr pressure increasing, shearing force decreasing, PGp pressure decreasing, thereby providing synergetic enhancement of fouling detachment and evacuation. The required cleaning effect is achieved when the free membrane portions shaking provided by residual brine gauge pressure PGr sharp decrease is precisely synchronized with permeate gauge pressure PGp sharp increase and precisely synchronized with residual brine flow velocity increase.

(50) As a non-limiting example, precise synchronization may be reached by connecting the water generator to a 3-way valve connected to the residual brine stream, permeate stream and to drain. A flexible diaphragm or sealed piston or similar means may be provided between the synchronized streams. The 3-way valve may switch the water stroke generator between two positions to provide the required sequence, as follows:

(51) Position 1: The 3-way valve opens brine flow to water stroke generator causing PGr decrease, shearing force increase and permeate pressure PGp increase; and

(52) Position 2: The 3-way valve closes brine flow and connects water stroke generator to drain, causing PGr increase, shearing force decrease and PGp decrease.

(53) FIG. 8 of the accompanying drawings illustrates one example of an apparatus for achieving the aforementioned Keeping Clean Procedure. Water stroke generator 600 has a moveable sealed piston 601. The piston 601 may have an equal diameter in the residual brine and permeate sides or have a different diameter on each side. One side of water stroke generator 600 is connected to permeate pipeline 29 via pipeline 31. The other side is connected to the residual brine stream 128 or to the drain via a 3-way valve 602. The stroke generator may also be provided with a vent 604. The 3-way valve 602 has an activator controlled by a Programmable Logic Control (PLC) (not shown on FIG. 8). The activator serves to change the 3-way valve position by the required frequency. The 3-way valve is movable between two positions (Position 1 and 2 above) wherein in Position 1 valve 602 connects line 128 to the water stroke generator 600 and disconnects drain line 610 and Position 2 wherein valve 602 disconnects line 128 from the water stroke generator 600 and connects the drain line 610 to it.

(54) As shown in FIG. 8, the arrangement 701 has similar connections with the modification that one side of the water stroke generator 600 is connected via pipeline 609 to feed line, or to interstate residual brine stream pipeline in multistage RO systems, or to residual brine pipeline 28 instead of the permeate line 29. Osmosis separation modules 100 may have residual brine stream valve 603 which opens and closes synchronically with valve 602. Permeate exiting from the permeate enclosure may be throttled by valves 12.

(55) Thus, during normal reverse osmosis process through the osmosis separation module 100, the keeping clean procedure (KCP) is carried out with the stroke generator 600 being switched between the two positions by means of the 3-way valve 602, providing precise synchronization of pressure and flow alteration. The frequency movement of valve 602 between the two positions may be tuned in to the vicinity of free membrane portions natural frequency for achieving constructive and/or beating interference oscillation of said membrane portions measurable as increase in residual brine stream turbidity. The pattern of movement may be continuous, rotational or include fast movement when valve changes between positions with some holdback in each open position. The holdback may not be equal in Position 1 and Position 2.

(56) For large flow modules, continuous operation of arrangement 700 may be combined with arrangement 701, and may be combined with continuous operation of valve 603. Arrangement 700 and 701 may work synchronically, asynchronically or each of them alone. Several arrangements 700 and/or 701 may be installed in different positions of large flow osmosis modules.

(57) The aforementioned 3-way valve arrangement is one example only of different arrangements that may provide precise synchronization between PGr decrease, shearing force increase, PGp increase. Additional valves may be installed in the exit of the residual brine stream and/or permeate stream synchronized with the water stroke generator for pulse-wise discharge of brine in large modules. Such a valve in the exit of the residual brine stream may operate alone, without stroke generator; closing and opening residual brine for providing pulse-wise flow. A single valve arrangement is likely to produce a reduced cleaning effect than combined with the stroke generator but be better than standard continuous residual brine flow approach.

(58) The pressure pulses PGr, and/or PGp synchronized “in phase” on feed and/or permeate sides respectively may provide “Constructive Pressure Wave Interference” or Beating Pressure Wave Interference with free membrane portion resonance oscillations. To reach such effect, the frequency of pressure pulsed have to be tuned to value equal, proportional or close to natural frequencies of free membrane portions.

(59) The beating phenomenon may be used as instrument of tuning gauge pressure pulsation frequency into vicinity of membrane portions natural frequency. Because the same membrane element may have different sizes of free portions membrane, beating phenomenon may take place in different frequencies and tuning gauge pressure pulsation frequency may be required in wide range. Measurement of reject flow turbidity may be good indicator for selection of the right frequency range.

(60) The Keeping Clean Procedure that may be performed all the time may be subjected to an additional activity at periodic time intervals as part of the procedure or as an independent procedure. The additional cleaning activity comprises a periodic osmotic backwash (POB) which may be performed with or without oxidation. This activity is made on-line without stopping the feed pump, with very limited interruption in the normal desalination process. The periodic osmotic backwash may take a few minutes and may be implemented daily or every few hours. The backwash may employ an extremely small amount of cleaning solution that need not be discharged after use.

(61) The periodic osmotic backwash is based on high frequency (several times a minute) changes from Reverse Osmosis (RO) to Forward Osmosis (FO). This is brought about without feeding of Additional Solution to the feed side of the membrane but rather involves extremely fast and precise synchronized changes of the residual brine and permeate gauge pressures; decrease PGr, increase PGp and vice versa. In this respect, the process changes from RO to FO when the sign of net driving pressure, defined by the balance of osmotic and gauge pressures PGr, Por, Pop and PGp, changes.

(62) The POB procedure is based on our new understanding that the process change between Reverse Osmosis to Forward Osmosis may be extremely quick based on simultaneously, opposite sign, change of gauge pressures PGr and PGp. A quick change may take place because the osmotic process does not have massive inertial parts, and therefore change in flow direction may take place immediately. This is not the case if process change between Reverse Osmosis to Forward Osmosis is based on injection of Additional Solution and a change osmotic pressure from POr to POs.

(63) Additionally, the POB procedure is based on our further understanding that the backwash process on the membrane may be extremely short in time, because the distance on which fouling has to be moved from membrane surface may be few microns, if the shearing force of residual brine increases precisely in these microseconds when both pressures PGr and PGp shack membrane in the same direction and backward flow of permeate take place.

(64) Furthermore, field experiments that have been carried out demonstrate that dozens of fast and frequent changes back and forth between RO and FO and dozens of short backwashes are more effective in fouling removal than one single change from RO and FO and one long backwash.

(65) A non-limiting numerical example to show this POB process is as follows:
NDP (FO or RO)=+PGr−POr−PGp+POp
NDP (RO)=+12−3−1+0.1=+8.1 bar.
Sign (+) means the process is RO.

(66) Thus, the difference between Keep Clean Procedure (KCP) and the periodic backwash procedure (POB) is in adding one-step: throttling permeate exiting from permeate enclosure, increasing permeate gauge pressure PGp until the NDP value become equal to zero. In non-limiting numerical example, PGp increases from 1 bar to 9.1 bar.
NDP (Neutral)=+12−3−9.1+0.1=0.0 bar
Precisely synchronized directional strokes with oppose change of pressure: PGp (between 11.5 and 12.5 bar and PGr (between 9.6 and 8.6 bar) providing plurality of quick RO-FO-RO process changes.
NDP (FO)=+11.5−3−9.6+0.1=−1.0 bar.
Sign (−) means the process is FO
NDP (RO)=+12.5−3−8.6+0.1=+1.0 bar.
Sign (+) means the process is RO

(67) In an alternative embodiment, the quick back and forth movement of permeate across the membrane caused by changing the process between RO and FO as described above is enhanced by the inclusion of a strong oxidizing agent. This procedure is termed “Periodical Oxidation and Osmotic Backwash” (PO&OB) and involves the addition of another step, being to inject cleaning solution into the permeate enclosure, preferable before the permeate throttling step. PO&OB may be implemented for membranes such as graphene, zeolite, carbon, ceramic, nanostructures, mix matrix etc. that are able to withstand a high concentration of strong oxidizers.

(68) Different types of cleaning-solutions may be used in the PO&OB process. Preferably, the cleaning solution is able to pass via semipermeable membrane in both directions. A non-limiting example of a suitable cleaning solution is a high concentration of oxygen dissolved in water for organic fouling removal, or a high concentration of carbon dioxide dissolved in water for calcium carbonate scaling removal.

(69) The PO&OB cleaning of membrane requires a hundred-fold less amount of chemicals compared to standard CIP process, where chemical solution circulates in feed-brine stream, external piping, CIP tanks, filters, and pumps.

(70) Such a small amount of cleaning solutions is enough because the cleaning solution is located only in small permeate spacer area, not circulating through the system; it only goes back and forth in the distance of few microns passing the membrane. Most of cleaning solution may be directed back after cleaning session into the same tank from which it was injected for cleaning. This repeated use of chemicals is possible because cleaning solutions acts in feed membrane side when it is in FO mode and cleans itself when it is coming back through membrane in RO mode.

(71) The POB may include implementation of cleaning solution PO&OB as option. The six steps POB procedure with PO&OB option, presented below is non-limiting example. It uses the same water stroke generator, and the same 3-way valve Positon 1 and 2 that is presented in the previous embodiment “KCP” and shown in FIG. 8.

(72) Step 1 (option with cleaning solution PO&OB): Cleaning solution injected in one side of permeate enclosure and fill it up, when separation module is in normal RO operation.

(73) Step 2: Throttling permeate exiting from permeate enclosure. Reaching NDP (Neutral).

(74) Step 3: The 3-way valve in Position-1 opens brine flow to water stroke generator, causing: PGr decreasing; PGp Increase. Process changed from Reverse Osmosis to Forward osmosis. Backwash by permeate takes place and fouling evacuation by high shearing force. (Fouling oxidizes or dissolves option PO&OB).

(75) Step 4: The 3-way valve in Position-2 closes brine flow and connects water stroke generator to drain. PGr increasing due to water hammer caused by sudden valve closing: permeate pressure PGp decreasing, caused by sudden opening brine side of water stroke generator to drain; Shearing force in feed membrane side decreasing. Process changed from Forward osmosis to Reverse Osmosis. Permeate goes back to permeate area. In PO&OB option chemical solution filtrate itself by this back movement via membrane.

(76) Steps 3 and 4 are repeated frequently causing “back and forth” dozens of times backwash and, optionally, dozens of fouling oxidation, or scaling dissolution.

(77) Step 5 (option with cleaning solution PO&OB): Cleaning solution moves back to storage tank for re-concentration and reuse.

(78) Step 6: Permeate enclosure opens by valve and begin normal RO operation.

(79) Practical implementation of POB procedure may request two field adjustments for each specific osmosis separation module. The first adjustment is throttling permeate exiting from permeate enclosure, for reaching NDP neutral. Although the value of PGp equal to neutral NDP may be calculated, it worth to make some variation around this value, until maximum increase of brine turbidity may be measured. The second adjustment is tuning frequency of pressure stroke alteration into vicinity of free membrane portions natural frequency also measurable as increase in residual brine stream turbidity.

(80) In accordance with one embodiment of the invention for heavy fouling condition, shearing force may be increase even more if other water stroke generators will be installed in residual brine line between stages of osmosis separation modules and/or outlets from them, with modification, that instead of permeate line, the residual brine line will constantly be connected to it.

(81) Thus, the embodiment of the invention shown in FIG. 8 may be carry out different cleaning operations, either continuously or at predetermined time intervals. Osmotic separation module 100 performs normal reverse osmosis process, which may include as non-limiting example several options of operation:

(82) Option A. Normal RO process which may include Keeping Clean Procedure.

(83) Option B. Normal RO process which may include Keeping Clean Procedure and intermittently applied “Periodical Osmotic Backwash”

(84) Option C. Normal RO process may include Keeping Clean Procedure, and intermittently applied “Periodical Osmotic Backwash” and some of “Periodical Osmotic Backwash” may be performed as Periodical Oxidation & Osmotic Backwash” (PO&OB).

(85) Each of mentioned above procedure KCP, POB and PO&OB may be combined as described above or applied as separate procedure in any configuration.

(86) The six steps below presents POB and PO&OB procedure. The steps required for PO&OB procedure marked (option PO&OP):

(87) Step 1 (option PO&OP): Cleaning solution 606 injected from tank 605 and fills up permeate enclosure 29.

(88) Step 2: Valve 12 throttles permeate exiting from permeate enclosure 29. Pressure PGp increases until neutral net driving pressure NDP reached.

(89) Step 3: The 3-way valve 602 in Position-1 opens residual brine stream 28 exiting from the module 100 towards water stroke generator 600 causing precise synchronically: PGr decreasing; PGp increase, increasing shearing force 3, and providing short FO process.

(90) Step 4: The 3-way valve 602 in Position-2 closes brine flow from line 28, and connect water stroke generator to drain 610. PGr increasing due to water hammer caused by sudden valve 602 closing; permeate pressure PGp decreasing, caused by sudden opening brine side of 3-way valve 602 to drain 610. Precise synchronically PGr increase; PGp decrease provide short RO process. Shearing force 3 in feed membrane side decreasing. Permeate goes back to permeate area. (Option PO&OP chemical solution filtrate itself by this back movement via membrane).

(91) Steps 3 and 4 repeated frequently causing “back and forth” dozens of times changes RO-FO-RO process, and backwash. If cleaning solution option included dozens of fouling oxidation, or scaling dissolution will take place.

(92) Step 5 (option PO&OP): Cleaning solution 606 moves back to storage tank 605 for re-concentration and reuse.

(93) Step 6: Valve 12 opens module 100 returns to normal RO operation.

(94) POB and PO&OB cleaning in addition to mentioned above KCP procedure executes:

(95) Enhanced foulant detachment and evacuation, due to applying plurality quick RO-FO-RO backwash procedures.

(96) Enhanced fouling oxidation and scaling dissolution due to applying chemical treatment of fouling during plurality cleaning solution penetration via membrane in RO-FO-RO process.

(97) Enhanced foulant evacuation may have more technological benefits such as increase in recovery of separation module operation.

(98) Arrangement 701 intended to provide plurality of directional pressure strokes in residual brine stream between stages of osmotic separation modules by using remainder pressure of final residual brine stream, and applying pulse-wise flow regime to increase shearing force achieving enhanced fouling evacuation. In some applications arrangement 701 may use pressure in residual brine for pulse-wise pumping in separation module raw saline solution to, increase-shearing force.