Multistage and multi-pass separation process for high-recovery water treatment

Abstract

A method for operating a tapered flow pressure driven arrangement containing a plurality of membrane modules, which comprises positioning said membrane modules in at least a first and a second successively arranged flow passes and at least a first and a second successively arranged flow stages in said 1st pass, passing a feed stream through said first flow stage to generate a permeate and a concentrate, directing said concentrate to the said consecutive flow stage and passing said concentrate therethrough, while passing at least part of the combined 1st pass permeate through said second pass, while periodically replacing one or more of the membrane modules belonging to said second flow stage with one or more membrane modules belonging to any flow stage in said consecutive flow pass, wherein the periodicity of said replacement is such that the surface of membranes disposed in said consecutive flow stages of the first pass is exposed to super-saturation conditions associated with the passage of said concentrate for a period of time that is shorter than the time required for said concentrate to precipitate therefrom one or more sparingly soluble salts and / or minerals, onto the surface of the membranes in said consecutive flow stages under said saturation or super-saturation conditions, or before deposit of organic substances can occur in the last of said two or more successively arranged flow stages.

Description

[0001] Multistage and Multi-pass Separation Process for High-Recovery Water Treatment

[0002] Field of Invention

[0003] The present invention generally relates to improving the recovery and prevention of precipitation fouling, in liquid filtration process. More particularly, the invention relates to method and system for preventing precipitation fouling in reverse osmosis and nanofiltration pressure-driven filtration processes and for increasing recovery by precipitation of supersaturated minerals from concentrate stream and preventing biofouling.

[0004] Background of Invention

[0005] Constant growth in water demand for potable and industrial use, along with growth in need for smaller physical footprint, lower carbon footprint and brine management, makes membrane filtration an attractive solution. Since traditional treatment methods are not always able to meet the requirements of environmental and drinking water regulations and quality, membrane filtration processes are becoming preferable in such applications.

[0006] Pressure-driven membrane process are defined as processes in which the feed stream is fed at a volumetric rate Qf into a membrane device (e.g., pressure vessel) equipped with membranes that divide the device space into a feed side and a permeate side (rear and front) and in which a pressure difference across the membranes causes the solvent (usually water) to pass from the feed space to the permeate space at a volumetric rate denoted as Qp. The remaining solution, which is now concentrated in the rejected solutes, leaves the feed space of the membrane device as a concentrate stream as a volumetric rate denoted as Qb. the fraction of feed that leaves the membrane device as permeate is referred to as the membrane recovery, %RR:

[0007] %RR = (1)

[0008]

[0009] As recovery is increased the concentration of rejected solutes in the concentrate stream, Cb, is given by mass balance as:

[0010] r>r1-%RR(1-R)

[0011] C» h C« f

[0012] °11-%RR

[0013]

[0014] Pressure-driven membrane processes are distinguished from each other by the types of solutes they reject or pass to the permeate side. For a given process this selectivity can be defined by the empirical solute rejection for each solute i, Ri, defined by:

[0015]

[0016] Where Cfj, Cp,i refer to the feed and permeate concentrations respectively of component i. For highly rejected species, Ri=l and then the relation defined in equation 2, between its concentration in the concentrate stream, Cb, and the recovery, %RR reduces to:

[0017] Cb= CfWhere R = 1 (4)

[0018]

[0019] DJ 1—%RRv 7

[0020] The rate at which solvent together with non-rejected solutes passes through the membrane per unit membrane area is defined as the membrane flux (denoted Jv) with units of volume / (unit area-time). During the filtration process the membranes become fouled and as a result less effective. Membrane fouling has become one of the primary impediments to their acceptance in water treatment applications. The occurrence of membrane fouling in reverse osmosis and nanofiltration processes leads to reduction in the production rate and sometimes to loss of solute rejection. There are several kinds of fouling which typically occur in such processes:

[0021] colloidal fouling;

[0022] organic fouling (adsorption of soluble organics on membrane surface); biofouling — formation of a biofilm which by itself or in combination with other kinds of fouling causes deterioration of membrane performance; and precipitation fouling (or scaling) due to precipitation of sparingly soluble salts and minerals.

[0023] Approaches to control membrane fouling involve hydrodynamic and chemical methods, periodic backwashing, chemical cleaning, changing operating conditions, and reducing the operating flux. Another solution for controlling membrane fouling proposes changing the flow direction to reduce concentration polarization and fouling in general (“Ultrafiltration Membranes and Applications”, Breslau, B. R. at al, Polymer Science and Technology, Plenum Press, Vol. 13; “Flux Enhancement Using Flow Reversal in Ultrafiltration”, Hargrove, S C andllias, S., Sep. Sci. Technol., 34 (6&7), 1319).

[0024] A flow reversal process and device are described in U. S. Pat. No. 5,690,829 (to Lauer), which particularly relates to the cleaning of the membrane from dirt particles. Another possible solution for reducing membrane fouling is described in U. S. Pat. No. 5,888,401 (to Nguyen), which suggests periodically increasing the permeate pressure next to the membrane by partially closing a valve on the permeate side, which results in reductions in the permeate flow rate. This last method reduces the rate of overall permeate recovery, which is a disadvantage. Another possible solution for reducing membrane fouling is described in U. S. Pat. No. 9649598, U. S. Pat. No. 8,137,539, and U. S. Pat. No. 8,632,682.

[0025] As will be apparent to those skilled in the art, an efficient solution for preventing precipitation fouling has also implications for biofouling since the stagnant layers and surfaces of scale layers can allow biofilms to attach and develop with less shear forces to remove them.

[0026] Melo and Bott (1997) Suggested that velocity and turbulence could play a role in preventing biofouling growth due to Reynolds number changes. Experiments conducted with Pseudomonas Fluorescens confirmed this hypothesis. Conductivity, TDS and pH differences along limitation of nutrients were also suggested as factors effecting prevention of biofouling (Melo and Bott, 1997).

[0027] In many processes sparingly soluble salts can limit the recovery of desalination processes as their concentration increases in the brine as more product water is pulled out of the feed flow. Different techniques have been used to cope with this problem (Section 9.4 in Water Treatment Membrane Processes, Mallevialle, J., Odendaal, P, Wiesner, M. eds., McGraw-Hill, 1996). Chemical softening has been proposed to precipitate sparingly soluble salts most of which are salts of alkali earth metals (Ca, Sr, Mg, Ba). The problem with this approach is that it requires stoichiometric amounts of chemicals to precipitate all of the metal ions of sparingly soluble salts, which is often costly. For example, brackish water containing 100 mg / L of calcium, 30 mg / L of magnesium, and 150 mg / L of carbonate alkalinity as bicarbonate will require 91 g / m3 of hydrated lime and 135 g / m3 of soda ash to completely remove the calcium. At 80 $ / ton for hydrated lime and 180 $ / ton for soda ash this would involve a chemical cost of 1.8 cents / m3. In addition, the sludges formed in lime softening are often voluminous and hard to remove. This can be prevented by using advanced precipitation processes that combine precipitation softening with microfiltration, also known as Membrane Assisted Crystallization (MAC) or filtering through a filter cake of calcium carbonate seeds which is also known as Compact Accelerated Precipitation Softening (CAPS). Another method would be fluidized bed reactor (pelleting).

[0028] Alternatively, the pH can be reduced by adding acid and removing the carbonic acid formed by air stripping. This type of treatment eliminates the carbonate scales problems, but the problems associated with sulfate scales will still remain.

[0029] Alternatively, the pH can be increased by adding NaOH or other base or by removing CO2 from the water, for elimination of silica scales problems, however most water does have some sort of carbonate system which might then form a scale since saturation will be high in high pH.

[0030] Consequently, the most common approach today is to use antiscalants which allow operation at various values of super-saturation. However, even with the most advanced antiscalants used today, there are limits on the super-saturation ratios (for example '2.60-3.0 for calcium sulfate, a Langelier Saturation Index (LSI — log 10 of supersaturation ratio) of 2.8 for calcium carbonate, and supersaturation ratio of 2.0 for silica. This means that recoveries are often limited to 75-90%.

[0031] Sea-water desalination recovery rates are often limited to 35-55% due to energy efficiency or structure cost or complexity and not due to mineral scaling, however biofouling growth and colloidal fouling are known to affect sea-water desalination systems as well, causing increase in operational costs (OPEX) and downtime for CIP / Flush (Matin et al., 2011).

[0032] In view of the aforementioned problems there is a need for filtration processes capable of effectively and efficiently controlling and preventing / minimizing precipitation fouling in pressure-driven membrane desalination processes, and of improving the recoveries of such processes, especially with reduced use of antiscalants. It is an object of the present invention to provide a high-flux filtration process for controlling and preventing precipitation fouling in pressure-driven membrane processes.

[0033] It is another object of the present invention to provide a method and system for efficiently controlling flow reversal in a membrane filtration process for preventing precipitation fouling in pressure-driven membrane processes.

[0034] It is still another object of the present invention to provide a method and system for preventing precipitation fouling in a membrane filtration process wherein the composition of the solution next to the active membrane surface exceeds the effective saturation limit of sparingly soluble salts.

[0035] An additional object of the invention is to provide a method and system for achieving high recoveries in a membrane filtration process operating with high local super-saturations.

[0036] An additional object of the invention is to provide a method and system for achieving high recoveries in a membrane filtration process operating with higher quality permeate.

[0037] An additional object of the invention is to provide a method and system for achieving lower operational costs from chemical use, CIP events and downtime events in a membrane filtration process.

[0038] Other objects and advantages of the invention will become apparent as the description proceeds.

[0039] The Invention

[0040] In membrane separation systems, hollowfiber or spiral wound reverse osmosis (RO) or nanofiltration (NF) membrane modules are connected in series inside pressure vessels (usually up to 8 membrane modules per pressure vessel). The pressure vessels can be assembled in blocks. A pressurized aqueous stream to be treated flows through the pressure vessels, to produce permeate and concentrate streams. Permeate is often treated again by series of membrane modules referred to as " Second Pass" in a double pass system (Figure 1).

[0041] The invention provides a multistage and / or multi-pass flow design, managing the flow of concentrate streams produced by individual blocks installed in RO and NF systems.

[0042] To this end, the blocks are divided into two (or more) groups, corresponding to different flow stages denoted Si (i=l, 2,...). The number of blocks designated to each distinct flow stage is denoted Bk (k =1, 2,...). The stages can be divided into passes denoted Pn(n=l,2,...) corresponding to different permeate required quality. Stages in different pass are denoted as S[n,i] as Stage Si in Pass Pn(e.g. Stage Si in Pass P2 denoted as Sp.i]). In its most general form, the invention is based on directing the aqueous stream from a feed supply unit to the blocks designated to the first stage Si and circulating the concentrate stream produced by the blocks operating at stage Si through one or more blocks designated to the consecutive stage Si+i, with reversal of flow direction between stages and occasional or regular block rotation between stages and passes.

[0043] The invention relates to a multistage membrane separation process for water treatment using membrane modules connected in series inside pressure vessels assembled in blocks, comprising:

[0044] dividing the blocks into two or more stages Si (i =1, 2,... ) within two or more passes Pn; passing a pressurized aqueous feed stream from a feed supply unit through the blocks designated to stage S[i,i], wherein the aqueous feed stream flows in a first direction through each of the blocks operating at stage S[1,1], to produce S[i,i] permeate streams and collect S[i,i] concentrate streams;

[0045] circulating a joined concentrate stream of stage S[n,i-1]through one or more blocks designated to successive stage S[n,i] or S [n+1,1], wherein the concentrate stream flows in stage S[n,i]or S[n+1,1]in a direction opposite or similar to the flow direction of S[n,i-1];

[0046] joined or separated permeate streams from stages S[n,1]to S[n,i]are either collected or flow as feed to pass P[n+i] to one or more blocks designated to stage S[n+i,i]; concentrate (reject) streams produced in the various passes Pnare either withdrawn as a reject stream or partially or fully recirculated back as a feed to any of the upstream passes, wherein the process comprises periodically transitioning blocks between passes and stages.

[0047] In some cases, the process and systems of the prevent invention may render superfluous the use of antiscalants, and as such, the process of the invention may be the multistage membrane separation process for water treatment as disclosed herein, wherein no antiscalant is added. In other words, the present invention also relates to said process wherein the process does not comprise adding an antiscalant to the water feed.

[0048] In some embodiment, the present invention relates to a method for operating a tapered flow pressure driven arrangement containing a plurality of membrane modules connected in series inside pressure vessels optionally assembled in blocks, which comprises positioning said membrane modules in at least a first and a second successively arranged flow passes and at least a first and a second successively arranged flow stages in said 1stpass, passing a feed stream through said first flow stage to generate a permeate and a concentrate, directing said concentrate to the said consecutive flow stage and passing said concentrate therethrough, while passing at least part of the combined 1stpass permeate through said second pass, while periodically replacing one or more of the membrane modules belonging to said second flow stage with one or more membrane modules belonging to any flow stage in said consecutive flow pass, wherein the periodicity of said replacement is such that the surface of membranes disposed in said consecutive flow stages of the first pass is exposed to super-saturation conditions associated with the passage of said concentrate for a period of time that is shorter than the time required for said concentrate to precipitate therefrom one or more sparingly soluble salts and / or minerals, onto the surface of the membranes in said consecutive flow stages under said saturation or super-saturation conditions, or before deposit of organic substances can occur in the last of said two or more successively arranged flow stages.

[0049] In some embodiments, the method further comprises concurrently with the repositioning of membrane modules in the sequentially arranged flow passes, reversing the flow direction within at least a portion of the newly positioned membrane modules. In some embodiments, an energy recovery device (ERD) is positioned at the concentrate stream flowing from the first stage to the second stage of the 1stpass (stream QR1S2) or downstream of the overall reject stream (stream QRT).

[0050] In some embodiments, a permeate - flush tank is accepting permeate to periodically flush the last stage of the 1stpass.

[0051] In some embodiments, the method comprises adjusting the pH of the feed to the last stage of 1stpass.

[0052] In some embodiments, the method comprises changing the blocks position between stages in 1stpass in order to keep an undersaturated environment, such as by modifying the flow routing.

[0053] In some embodiments, the overall reject stream QRtis at least partially circulated back to feed a stage of an upstream pass and the ratio of recirculation is regulated using a control valve.

[0054] In some embodiments, individual Cleaning in Place (CIP) could be performed for one or multiple blocks during operation.

[0055] In some embodiments, brine from last stage of 1stpass (QRT) is directed to the 1stpass feed. In some embodiments, the method does not comprise adding an antiscalant to the feed water.

[0056] The basic design of a membrane separation process and system according to the invention is described conceptually in reference to Figures 3 and 6, showing an example of two passes system configuration with two stages in 1stpass (while n=l, i=2) and two stages in 2ndpass (while n=2, i=2) with 6 blocks (k=6) of pressure vessels, denoted by capital letters A to F. In the example illustrated in Figures 3 and 4, n=2, ii =2, ii=2, B=1 (number of passes is 2, number of stages in 1stpass is 2, number of stages in 2ndpass is 2, number of pressure vessels in each block is 1). The blocks in S[i,i] are supplied with an aqueous stream from a feed water supply unit, pressurized by pump PFI (Figures 3 and 6) to produce a feed flow. The pressurized feed solution consisting of the water to be treated, for example, wastewater, brackish water or sea water, is split into subsidiary streams which are directed (via a manifold) to the individual blocks, or in fact, most of the blocks: only blocks operating at S[i,i] receive their incoming stream from the original feed solution. The direction of flow in blocks operating at S[i,i] is defined herein as the regular, or normal, direction, shown by three parallel arrows, pointing from left to right, whereas the outgoing concentrate streams produced by blocks A and B, operated as Pass S[i,i] are joined to form a combined concentrate stream, which is guided to flow through block C which is operated as S[i,2], in a same or opposite direction to the normal flow direction with or without pressure increase by pump or permeate back pressure (throttling). Only blocks operating at S[i,2] receive their incoming stream from S[i,i] concentrate solution, with or without pressure increase. The permeate streams produced by S[i,i] and S[i,2] are collected in a conventional manner (not shown) or straightly directed to S[2,i], whereas the outgoing concentrate streams produced by S[i,2] is guided to flow to reject stream, drain, further treatment or partially back to pass Pi feed as circulation stream.

[0057] The combined permeate stream from Pass Pi (whole or partially) is normally pressurized before treated in Pass P2, normally with pump Ppi, forming Sp,i] feed. Sp.i] feed is split into subsidiary streams which are directed (via a manifold) to the individual blocks, or in fact, most of the blocks: only blocks operating at Sp,i] receive their incoming stream from the original pass P2 feed solution. The direction of flow in blocks operating at S[2,i] is shown by three parallel arrows in Figures 3 and 4, pointing from left to right, whereas the outgoing concentrate streams produced by blocks D and E, operated as Sp,i] are joined to form a combined concentrate stream, which is guided to flow through block F which is operated as Sp,2], normally in opposite direction to the normal flow direction with or without pressure increase by pump or permeate back pressure (throttling). The concentrate from S[2,2] is directed to either pass Pi feed, Sp,2] or to reject stream (Optional - regulated between few options). The permeate streams produced by pass P2 (all stages) are collected in a conventional manner (not shown).

[0058] A key feature of the invention is that during the treatment, blocks are periodically transitioned between passes and stages. That is, the block operating in Sp,2] (e.g., block C), is exchanged after a while (before induction time achieved or slightly after) with one of the blocks operating in pass P2, say, Sp,i] as block D shift to operate in Sp,i], and Block C shift to operate as Sp,i] and so forth, in a cyclic manner (i.e., C«-> D, then D^E then E«-> C). Another option could be block rotation between Sp,2] and Sp,2], as Block C, operates in S[i,2] is shift to operate as Sp,2] and Block F is shifted to operate as Sp,2] in a cyclic manner (i.e., C«-> F, then F«-> C). The redesignation of the two blocks replacing each other is achieved with the aid of an array of valves. A useful array of valves is shown in Figures 3 and 4 and shows both options of shifting blocks though normally only one option is used. Both shifting - rotating manners are explained in a tabular manner below. Briefly, each block C and F and / or C, D and E are equipped with three pairs of valves; a first pair which regulates a flow of a stream supplied by the feed solution in the normal direction across the block, a second pair of valves, which regulates the flow of the joined concentrate in the opposite direction across the block and third pair which regulates the flow of the permeate direction (to pass P2 feed or to system permeate). Based on the state of these valves, each block can be disconnected from the supply of the feed solution and connected so as to receive the concentrate as a feed solution, transitioning between S[i,2] and Sp,2] (Or Sp.i]), and vice versa. The timing of the transition of blocks between stages is determined based on considerations described below.

[0059] The treatment process involves cycles consisting of distinct steps corresponding to Figures 3 to 6 for both options as valve VFI is open and pump PFI is in operation. QFI is inlet to Blocks A and B which are operating as 1stPass - 1ststage in conventional manner.

[0060] Total concentrate stream from 2ndPass (QR2S2) is directed to combine with raw feed stream (QRF) creating feed flow QFI.

[0061] Option 1: S[1,2] rotates with S[2,1]:

[0062] In the first step, which is shown in Figure 3, Booster pump Ppis2 is pressurizing the concentrate stream from blocks A and B to stream QRIS2 that feeding Block C.

[0063] Combined permeate from Blocks A, B and C is feeding Blocks D and E (2ndpass - 1ststage) after pressurized by Pump Ppi. Reject (Concentrate) stream from Block C is directed to drain as total system reject.

[0064] Blocks D and E are now operating as 2ndpass - 1ststage.

[0065] Concentrate stream from Blocks D and E combined to stream Q2F2 which feeds Block F. In the second step which is shown in Figure 4, Booster pump Ppis2 is pressurizing the concentrate stream from blocks A and B to stream QRIS2 that feeding Block D. Combined permeate from Blocks A, B and D is feeding Blocks C and E (2ndpass - 1ststage) after pressurized by Pump Ppi. Reject (Concentrate) stream from Block D is directed to drain as total system reject.

[0066] Blocks C and E are now operating as 2ndpass - 1ststage.

[0067] Concentrate stream from Blocks C and E combined to stream Q2F2 which feeds Block F. In the third step which is shown in Figure 5, Booster pump Ppis2 is pressurizing the concentrate stream from blocks A and B to stream QRIS2 that feeding Block E.

[0068] Combined permeate from Blocks A, B and E is feeding Blocks C and D (2ndpass - 1ststage) after pressurized by Pump Ppi. Reject (Concentrate) stream from Block E is directed to drain as total system reject.

[0069] Blocks C and D are now operating as 2ndpass - 1ststage.

[0070] Concentrate stream from Blocks C and D combined to stream Q2F2 which feeds Block F.

[0071] After the third step, stage one is repeated and so on.

[0072] The status of the valves in option 1 is reported in table 1 below.

[0073] Option 1

[0074] Valve Status (O – open, X – Closed)

[0075] Step 1 Step 2 Step 3

[0076] VF1 0 0 0

[0077] VF2C2 X X X

[0078] VR2C2 X X X

[0079] VF1C2 0 X X

[0080] VF2C1 X 0 0

[0081] VR1C2 0 X X

[0082] VR2C1 X 0 0

[0083] VP1C2 0 X X

[0084] VP2C2 X 0 0

[0085] VF1D2 X 0 X

[0086] VF2D1 0 X 0

[0087] VR1D2 X 0 X

[0088] VR2D1 0 X 0

[0089]

[0090] VP1D2 X 0 X VP2D1 0 X 0

[0091] VF1E2 X X 0

[0092] VF2E1 0 0 X

[0093] VR1E2 X X 0

[0094] VR2E1 0 0 X

[0095] VP1E2 X X 0

[0096] VP2E1 0 0 X

[0097] VF1F2 X X X

[0098] VF2F2 0 0 0

[0099] VR1F2 X X X

[0100] VR2F2 0 0 0

[0101] VP1F2 X X X

[0102]

[0103] VP2F1 0 0 0

[0104] Table 1 - Valves status by step in Option 1

[0105] Option 2: S[1,2]rotates with S[2,2]:

[0106] In the first step (similar to first step in Option 1), which is shown in Figure 3, Booster pump PpiS2 is pressurizing the concentrate stream from blocks A and B to stream QRIS2 that feeding Block C.

[0107] Combined permeate from Blocks A, B and C is feeding Blocks D and E (2ndpass - 1ststage) after pressurized by Pump Ppi. Reject (Concentrate) stream from Block C is directed to drain as total system reject.

[0108] Block C is now operating as 1stpass - 2ndstage.

[0109] Block F is now operating as 2ndpass - 2ndstage.

[0110] Concentrate stream from Blocks D and E combined to stream Q2F2 which feeds Block F. In the second step, which is shown in Figure 3, Booster pump Ppis2 is pressurizing the concentrate stream from blocks A and B to stream QRIS2 that feeding Block F.

[0111] Combined permeate from Blocks A, B and F is feeding Blocks D and E (2ndpass - 1ststage) after pressurized by Pump Ppi. Reject (Concentrate) stream from Block F is directed to drain as total system reject.

[0112] Block F is now operating as 1stpass - 2ndstage.

[0113] Block C is now operating as 2ndpass - 2ndstage.

[0114] Concentrate stream from Blocks D and E combined to stream Q2F2 which feeds Block C. The status of the valves in option 2 is reported in table 2 below.

[0115] Option 2

[0116] Valve Status (0 - open, X - Closed)

[0117] Step 1 Step 2 VF1 0 0 VF2C2 X 0 VR2C2 X 0 VF1C2 0 X VF2C1 X X VR1C2 0 X VR2C1 X X VP1C2 0 X VP2C2 X 0 VF1D2 X X VF2D1 0 0 VR1D2 X X VR2D1 0 0 VP1D2 X X VP2D1 0 0 VF1E2 X X VF2E1 0 0 VR1E2 X X VR2E1 0 0 VP1E2 X X VP2E1 0 0 VF1F2 X 0 VF2F2 0 X VR1F2 X 0 VR2F2 0 X VP1F2 X 0

[0118]

[0119] VP2F1 0 X Table 2 - Valves status by step in Option 2 Additionally, Figures 3 -6, disclose the role of the valves used in the multistage and multipass process.

[0120] In a typical set-up, the pumps may be of centrifugal type with flow-pressures specs as calculated for feed water flow rate, recovery rate and osmotic pressure and are based on design calculations.

[0121] Pneumatic ball valves designed for pressures and diameter are typically installed for flow control and transition. Membranes may be spiral wound membranes and are typically installed in GRP or stainless-steel pressure vessels.

[0122] Another aspect of the invention is a multiple stage RO or NF system comprising:

[0123] A feed water supply unit operated by a first pump and connected by a feed line and a feed manifold to the feed ports of pressure vessels assembled in blocks, said pressure vessels housing membrane modules;

[0124] a permeate recovery line connected to permeate ports of the pressure vessels;

[0125] a concentrate booster pump on second pass;

[0126] an array of valves associated with the blocks of pressure vessels;

[0127] a control unit determining the designation of blocks to flow stages and passes, by connecting / disconnecting the pressure vessels to and from the feed line and the concentrate flow line by adjusting the states of the valves associated with the pressure vessels; optionally, an energy recovery device (ERD). ERD could be Isobaric type (Pressure exchange), Work exchange or Turbo Charge types as energy converted from brine (concentrate) stream to feed stream of one of the previous stages.

[0128] The present invention provides a membrane separation process which separates an original solution into a volume with higher concentration of soluble and / or insoluble material and a volume of lower concentration of soluble and / or insoluble material. This process consists of 1 to 9 spiral wound or hollow fiber reverse osmosis (RO) or nanofiltration (NF) membranes inside of pressure vessels that are structured in blocks that form multiple stages and multiple passes of separation. A given block may alternate in its position as part of the first stage or last stage of separation in either pass of separation treatment. When in the first stage it can be fed from the opposite end of the pressure vessel compared to when it is in the last stage.

[0129] Number of blocks, stages and passes can vary depending on recovery rate, TDS, saturated ion or mineral, system size, flow rate and water quality. Stages can be separated by pumps for required pressure adjustments. " Stage" is defined as configuration where multiple RO units are connected in series (Figure 2). In this setup, the brine concentrate from the first unit becomes the feedwater for the second unit, and so on. Each additional stage increases the desalination efficiency and water recovery but also requires more energy and infrastructure. " Pass" is defined as configuration where multiple RO units are connected in series n this setup, the permeate from the first unit becomes the feedwater for the second unit (Figure 1).

[0130] The present invention, through the alternation of the blocks and redirecting the feed direction allows membranes to alternate in their position from being exposed to the highest concentrations to the lowest concentrations. These adjustments may be used to maximize the amount of the concentration factor, while minimizing the effects of fouling and scaling on the membrane surface.

[0131] The present invention provides advantages over previous inventions as it allows flexible design and significantly reduces the need for chemicals (such as antiscalants) use during operation by simultaneously rotating the blocks so that the membranes in the last stage are alternated with membranes in the second pass to reduce the effects of fouling and scaling. Sheering forces produced by different pressures and flow rates along the different passes, which normally operate in different fluxes, recovery rates and beta-polarization conditions, could contribute to such an effect as well. The present invention is also well suited for the treatment of seawater.

[0132] The time required for a supersaturated aqueous solution to start precipitating sparingly soluble salts and / or minerals therefrom (in the absence of seed crystals or other appropriate pre-existing crystal growth surface) is known as the “induction time” (T). Most preferably, the process according to the present invention comprises the estimation of the induction time associated with the composition of the water to be treated and the nature of the membrane surface, to permit the flow reversal to be carried out at the desired periodicity.

[0133] According to one preferred embodiment, the induction time is estimated experimentally, for example, by carrying out a preliminary experiment, in which the water stream corresponding to the final concentrate composition is passed along a small membrane unit under flow conditions generating the concentration polarization that would be expected in a full-scale unit, without reversing the flow direction. The period of time measured until sparingly soluble salts from said water stream precipitate onto the membrane surface as reflected in a drop in membrane water permeability, or until a drop in the bulk concentration of the sparingly soluble salt is detected, is subsequently used to schedule the flow reversal accordingly. The occurrence of the precipitation of sparingly soluble salts onto the membrane surface may be conveniently traced by measuring changes to the membrane water permeability, Lp, as measured by the flux, Jv, divided by the net pressure driving force (NDP) as defined by:

[0134] ^

[0135]

[0136] Lp= Jv / NDP = Jv / (ΔP−Δπ) (5)

[0137] Where AP and An are the transmembrane pressure and the osmotic pressure difference between feed and permeate solutions respectively.

[0138] Alternatively, the induction time is estimated using the following equation:

[0139] ln(τ) = A / [ln(S)]2+ B (6)

[0140] wherein:

[0141] A and B are constants related to the salt, the hydrodynamics and the membrane surface, since the nucleation that must be prevented is the nucleation at the membrane surface. Constant A includes effect of the surface energy at the nucleating surface and molar volume of the salt. B includes the frequency factor for the nucleation rate. Values of A and B have already been determined, for example, for calcium sulfate and polyamide low pressure RO membranes (e.g., D. Hasson at al, “Induction times induced in an RO system by antiscalants delaying CaSO4 precipitation”, Desalination, v.157 (2003), p 193; Alimi, F. et al, Desalination V 157, pp 9-16 (2003); He, S. et al, J. Colloid and Interface Science, V 162 pp. 297-303 (1994)). A and B may also be readily obtained experimentally. S is the ratio between the maximum actual activity product of a given sparingly salt in the solution next to the feed side of the membrane and the thermodynamic solubility product, or, for an undissociated mineral (e.g. silica), it is the ratio of the actual maximum mineral concentration to the saturation concentration of that mineral for the given composition. It increases when the product water recovery is increased, also increasing the concentration polarization. This ratio can be calculated from commercial desalination process simulation programs available in the open market (such as IMSdesign from Hydranautics, Inc or WAVE from Dupont Inc. etc), or independently as described in D. Hasson at al, “Inception of CaSO4 scaling on RO membranes at various water recovery levels”, Desalination, 139, 73-81 (2001). S may be also calculated upon measuring data related to the concentration level of salts / mineral in the water stream, and data related to the permeate and concentrate flows. This data can be obtained utilizing appropriately placed flow meters and sensing devices within the system, in order to provide corresponding signals (e.g., conductivity) for calculating S. Typically, induction times are above 10 minutes.

[0142] Having obtained and / or experimentally determined the values of A, B and S, for the relevant salt and system, one may easily estimate the induction time, T. The periodic reversal is then set to take place at some time less than the T, for example at 0.1 to 0.9 the value of the induction time. In the event that there is more than one sparingly soluble salt, the salt with the shortest induction time is the one used for determining the period for flow reversal.

[0143] Alternatively, the induction time may be determined during system operation via appropriately situated sensors for monitoring permeation rate (flow meter or weigh scale) or scaling ion concentration (e.g. using calcium sensor, or conductivity sensor) as a function of time, to see when these indicators change as a result of precipitation in the system. Example

[0144] An experiment was conducted using an RP03 system, which has been upgraded to allow operation in two-pass system. The goal of the experiment was to evaluate a configuration according to the present invention.

[0145] System Description

[0146] The system operates as a double pass with two -stage configurations using 4" -membranes. In each stage, two pressure vessels (PV), each containing four membranes, are connected in series, labeled and referred to as a " Block". This configuration effectively creates a pressure vessel consisting of eight membranes.

[0147] The flexible pilot system can operate in different modes of operation. For this trial, an FR-2P configuration was tested (two-pass system with flow reversal reverse osmosis).

[0148] The FR-2P configuration was tested under controlled conditions to assess its impact on system performance parameters.

[0149] The objectives of the experiment were to:

[0150] 1. Test the system's operation in FR-2P mode.

[0151] 2. Test operation in high concentration risk with no added antiscalant.

[0152] The instrumentation included the following sensors:

[0153] • Feed: Flow, pressure, conductivity, temperature, ORP and pH

[0154] • Permeate: Flow, conductivity, and pressure

[0155] • Pressure transmitters between stages, at pump suction and discharge

[0156] The systems included one main pump Pl and two booster pumps; one booster pump P2 was stationed between stages 1 and 2, and the second booster pump P3 was stationed between stages 2 and 3 which in turn, can operate as a 2ndpass.

[0157] The flow direction was periodically reversed between the 2ndstage of the 1stpass and the 2ndpass which in this case, consisted of a single stage. The transition between Block F as 1stpass 2ndstage and Block E as 2ndPass was operated by opening the valves: EFI, EFO, FRO and FRI and closing the valves: ERO, ERI, FFI, FFO and vice versa. According to the standard test, the membranes used un the system have a total rejection of -95%.

[0158] Feed water profile (as estimated by water quality in the area as well as direct measurements) is presented in Table 3.

[0159] Table 3- Feed water profile

[0160] Parameter Value Unit

[0161] pH 7.6 - Temperature 25 °C

[0162] Conductivity 860 μS / cm

[0163] TDS 568 Mg / 1

[0164] Ca 19 Mg / 1

[0165] Na 200 Mg / 1

[0166] Cl 332.4 Mg / 1

[0167] Alkalinity 82 Mg / 1 as CaCO3

[0168]

[0169] For the experiment, desalinated water from the local RO system was used, with added minerals to achieve the water profile for required saturation. The saturation achieved is shown in Figure 7.

[0170] Since no antiscaling agent was used, the water is untreated and saturation levels of CaCO3are 116.69% at a recovery rate of 80%. Concentrate stream was calculated to have LSI >0. The membrane elements used were 4" in diameter type BW30-4040 and the water: 7 m3 / h FR-2P Operation

[0171] During the experiment, the system operated in 2-pass configuration. Blocks A-B were operated as lst-pass-lst-stage, Blocks C-D were closed, and one block from the blocks E-F was operated as 1St-pass-2nd-stage, and the other one as 2nd-pass. The rotation between 1st-pass-2nd-stage and 2nd-pass was controlled by valves ERO, EFI, ERI, EFO, FRO, FFI, FRI, FFO. During rotation, 1ststage 1stpass block remained in position while 2ndstage operated as 2ndpass (one pass with two PV on 1stpass and one pass with two PV on 2ndpass configuration). This configuration only lasted few minutes to allow rotation.

[0172] lst-pass and 2nd-pass permeates were joined and then split; part of this water was directed back to an intermittent tank and then fed back to the 2nd-pass. The 2nd-pass brine was also circulated back to the feed tank (Figures 8A-B - The FR-2P system in the two modes of operation, before (A) and after (B) rotation). For the purpose of the experiment, all the outputs were eventually fed back to the feed tank to reconstitute the original feed.

[0173] The overall recovery rate was 80%

[0174] Results and discussion

[0175] While water temperature was stable around 35 °C during the experiment. Recovery rate setpoint was 80% (Figure 9 - Pressures and Recovery Rate (%) during system operation). Fluctuation can be explained by the valves’ operation during the experiment for rotation of 2ndpass with 2ndstage of 1stpass.

[0176] During the 3 days of operation, the feed pressure did not significantly increase in either stages or passes (Figure 9).

[0177] Measured differential pressure showed a slight increase during the last hours of operation however it remained stable during 3 days of operation at 80% recovery (Figure 10 -Differential Pressures (DP) during system operation).

[0178] Average system flux remained stable throughout the operation around 21.2 l / m2 / h (Figure 11- Permeate flux and Temperatures during system operation).

[0179] Conclusions

[0180] Though operation without antiscalant agent was calculated to limit recovery rates to below 65% (using Genesys Membrane Master 5 software), the FR-2P system successfully operated at 80% recovery and remained stable for 3 days of operation without either scaling or hydraulic issues.

[0181] Valves were successfully rotated and insured rotation of 2ndpass block with 1stpass 2ndstage block.

Claims

Claims1) A method for operating a tapered flow pressure driven arrangement containing a plurality of membrane modules connected in series inside pressure vessels optionally assembled in blocks, which comprises positioning said membrane modules in at least a first and a second successively arranged flow passes and at least a first and a second successively arranged flow stages in said 1stpass, passing a feed stream through said first flow stage to generate a permeate and a concentrate, directing said concentrate to the said consecutive flow stage and passing said concentrate therethrough, while passing at least part of the combined 1stpass permeate through said second pass, while periodically replacing one or more of the membrane modules belonging to said second flow stage with one or more membrane modules belonging to any flow stage in said consecutive flow pass, wherein the periodicity of said replacement is such that the surface of membranes disposed in said consecutive flow stages of the first pass is exposed to supersaturation conditions associated with the passage of said concentrate for a period of time that is shorter than the time required for said concentrate to precipitate therefrom one or more sparingly soluble salts and / or minerals, onto the surface of the membranes in said consecutive flow stages under said saturation or supersaturation conditions, or before deposit of organic substances can occur in the last of said two or more successively arranged flow stages.2) A method according to claim 1, further comprising concurrently with the repositioning of membrane modules in the sequentially arranged flow passes, reversing the flow direction within at least a portion of the newly positioned membrane modules.3) A method according to claim 1, wherein an energy recovery device (ERD) is positioned at the concentrate stream flowing from the first stage to the second stage of the 1stpass (stream QR1S2) or downstream of the overall reject stream (stream QRT).4) A method according to claim 1, wherein a permeate - flush tank is accepting permeate to periodically flush the last stage of the 1stpass.5) A method according to claim 1, comprising adjusting the pH of the feed to the last stage of 1stpass.6) A method according to claim 1 comprising changing the blocks position between stages in 1stpass in order to keep an undersaturated environment, such as by modifying the flow routing.7) A method according to claim 1, wherein the overall reject stream QRtis at least partially circulated back to feed a stage of an upstream pass and wherein the ratio of recirculation is regulated using a control valve.8) A method according to claim 1, wherein individual Cleaning In Place (CIP) could be performed for one or multiple blocks during operation.9) A method according to claim 1, wherein brine from last stage of 1stpass (QRT) is directed to the 1stpass feed.10) The method of any one of claims 1 -9, wherein the method does not comprise adding an antiscalant to the feed water.11) A multistage membrane separation process for water treatment using membrane modules connected in series inside pressure vessels assembled in blocks, comprising:a. dividing the blocks into two or more stages Si (i =1, 2,...) within two or more passes Pn;b. passing a pressurized aqueous feed stream from a feed supply unit through the blocks designated to stage S[1,1], wherein the aqueous feed stream flows in a first direction through each of the blocks operating at stage S[1,1], to produce S[1,1]permeate streams and collect S[1,1]concentrate streams; c. circulating a joined concentrate stream of stage S[n,i-1]through one or more blocks designated to successive stage S[n,i]or S[n+1,1], wherein the concentrate stream flows in stage S[n,i]or S[n+1,1]in a direction opposite or similar to the flow direction of S[n,i-1];d. periodically transitioning blocks between passes and stages;wherein joined or separated permeate streams from stages S[n,1]to S[n,i]are either collected or flow as feed to pass P[n+1]to one or more blocks designated to stage S[n+1,1]; concentrate (reject) streams produced in the various passes Pnare either withdrawn as a reject stream or partially or fully recirculated back as a feed to any of the upstream passes.

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