Patent Application
20240359138
Saline brines produced from desalination and industrial activity represent an environmental hazard if not properly disposed of. They can pollute the ground or fresh groundwater and surface water sources. Often the ideal solution from an environmental standpoint would be evaporation to dryness and internment so that the brines would not spread and contaminate land and water resources. In addition, there could be methods of obtaining valuable minerals by treating and purifying the concentrates. The classical methods of treating saline brines with thermal methods (multi-effect distillation and mechanical vapor compression evaporators) are highly energy intensive. If membrane processes could be used, this would represent a significant energy savings. One of the barriers to carrying out very high recovery reverse osmosis (RO) of brines (brackish groundwater, saline industrial effluents, seawater) is that the brines that would be generated from such operation would have osmotic pressures that exceed greater than 100 bar (1,450 psi) and would require the use of pressures and that are beyond the reach of conventional high salt rejection RO membranes and pumps that would be exceedingly expensive.
A number of approaches have been suggested using counter-current osmotic cascades and feeding the less saline streams on the permeate side to reduce the osmotic pressure difference (Osmotic assisted RO, or OARO). These approaches suffer from the internal concentration polarization that occurs in the membrane support. Another approach has been to apply electrodialysis (ED) to the treatment of RO concentrates produced for desalting brackish groundwater and combining the ED diluate with the RO permeate (Oren et al., Desalination, Vol. 261, 3, 321-330, 2010). This can result in recoveries of 97-98% with concentrate concentrations that approached 80-100 g/L. There are two problems with this approach. The first is when the concentrate concentration in ED gets too high, there is back diffusion and a drop in the electrical efficiency of the ED. The second problem is that the divalent scaling species (Ca, Ba, Mg, SO4) that are transferred from the diluate stream to the concentrate stream can cause scaling, that can only be partially treated by electrodialysis reversal (EDR) and sidestream concentrate crystallizer (Oren et al., Desalination, Vol. 261, 3, 321-330, 2010).
Thus, there is a need in the art for improved desalination methods. This invention satisfies this unmet need.
In one aspect, the current invention relates to a reverse osmosis system comprising: a feed source input, a high pressure feed pump fluidly connected to the feed source input, and a reverse osmosis (RO) cascade fluidly connected to the high pressure feed pump; wherein the RO cascade comprises: at least one low salt rejection reverse osmosis (LSRRO) stage including a LSRRO membrane, and a seawater reverse osmosis (SWRO) stage including a SWRO membrane fluidly connected to the at least one LSRRO stage.
In one embodiment, the LSRRO stage further comprises a pump configured to boost the pressure of and to recirculate a portion of a concentrate output by the LSRRO stage to the input of said LSRRO stage. In one embodiment, the LSRRO stage further comprises a pressure exchanger configured to recirculate at least a portion of a permeate output by the LSRRO stage to the input of said LSRRO stage or to the SWRO stage. In one embodiment, the LSRRO stage further comprises a booster pump configured to boost the pressure of at least a portion of a permeate output by the LSRRO stage to the input of said LSRRO stage or to the SWRO stage. In one embodiment, feed source input comprises a feed tank. In one embodiment, the system further comprises a monovalent selective electrodialysis (MSED) stage fluidly connected between the feed source and the RO cascade.
In one embodiment, the MSED stage comprises an electrodialysis stack of alternating monovalent selective cation exchange membranes (CEM) and monovalent selective anion exchange membranes (AEM). In one embodiment, an input of the MSED stage includes an input configured to receive raw groundwater of low ion content water. In one embodiment, the system further comprises a scaling ion scavenging stage fluidly connected between the MSED stage and the RO cascade.
In one embodiment, the SWRO stage comprises the first stage of the RO cascade. In one embodiment, the RO cascade further comprises an antiscalant input fluidly connected to an input of the at least one LSRRO stage. In one embodiment, the antiscalant comprises a low molecular weight antiscalant.
In one embodiment, the at least one LSRRO stage comprises two or more LSRRO stages. In one embodiment, the two or more LSRRO stages are connected in series. In one embodiment, the two or more LSRRO stages are connected in parallel.
In one embodiment, the system further comprises at least one permeate storage tank fluidly connected to at least a portion of one or more permeate outlets of the RO cascade. In one embodiment, the permeate storage tank is fluidly connected to one or more inputs of RO cascade.
In one embodiment, the SWRO is downstream of the at least one LSRRO.
In one aspect, the current invention relates to a reverse osmosis method, comprising: providing the reverse osmosis system of claim 1, inputting a high salinity fluid into the feed source input, increasing the pressure of the high salinity fluid via the high pressure feed pump, performing reverse osmosis via the RO cascade, and collecting at least one of a concentrate and a permeate. In one embodiment, the reverse osmosis comprises performing at least one low salt rejection reverse osmosis (LSRRO) and a seawater reverse osmosis (SWRO).
The following detailed description of embodiments of the invention will be better understood when read in conjunction with the appended drawings. It should be understood that the invention is not limited to the precise arrangements and instrumentalities of the embodiments shown in the drawings.
It is to be understood that the Figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, many other elements used in mining influenced water purification. Those of ordinary skill in the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. The disclosure herein is directed to all such variations and modifications to such elements and methods known to those skilled in the art.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
As used herein, each of the following terms has the meaning associated with it in this section.
The articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.
“About” as used herein when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of ±20%, ±10%, ±5%, ±1%, or ±0.1% from the specified value, as such variations are appropriate to perform the disclosed methods.
Ranges: throughout this disclosure, various aspects of the invention can be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 2.7, 3, 4, 5, 5.3, and 6. This applies regardless of the breadth of the range.
The present invention relates to a purification system comprising a cascade of low salt-rejection reverse osmosis membranes.
The present invention relates in part to a purification system 100 comprising a cascade 103 of low salt-rejection reverse-osmosis membranes. In an aspect of the invention, the purification system 100 comprises one or more stages. In some embodiments, the purification system comprises a feed source input 101. In some embodiments, the reverse osmosis cascade 103 comprises one or more low salt rejection reverse osmosis (LSRRO) stages 104 including one or more LSRRO membranes. In some embodiments, the reverse osmosis cascade 103 comprises one or more seawater reverse osmosis (SWRO) stages 105 including one or more SWRO membranes, fluidly connected to the one or more LSRRO stages 104. In some embodiments, a high pressure feed pump 102 is fluidly connected to the feed source input 101. In some embodiments, the feed source 101 comprises a feed tank. In some embodiments, the system 100 comprises a monoselective electrodialysis (MSED) stage 109 fluidly connected between the feed source 101 and the RO cascade 103. In one embodiment, the MSED stage 109 comprises an electrodialysis stack of alternating cation exchange membranes (CEM) and anion exchange membranes (AEM). In one embodiment, an input of the MSED stage 109 includes an input configured to receive raw groundwater of low ion content water. In one embodiment, the system 100 further comprises a scaling ion scavenging stage 110 fluidly connected between the MSED stage 109 and the RO cascade 103. In one embodiment, the SWRO stage 105 comprises the first stage of the RO cascade 103. In one embodiment, RO cascade 103 further comprises an antiscalant input fluidly connected to an input of the at least one LSRRO stage 104. In one embodiment, the antiscalant comprises a low molecular weight antiscalant. In one embodiment, the at least one LSRRO stage 104 comprises two or more LSRRO stages. In one embodiment, the two or more LSRRO stages 104 are connected in series. In one embodiment, the two or more LSRRO stages 104 are connected in parallel. In one embodiment, the system 100 further comprises at least one permeate storage tank 111 fluidly connected to at least a portion of one or more permeate outlets of the RO cascade 103. In one embodiment, the permeate storage tank 111 is fluidly connected to one or more inputs of RO cascade 103.
In one embodiment, each stage of the RO cascade 103 comprises an inlet, one or more outlets, and a semipermeable membrane. In one embodiment, the stages comprise a low salt-rejection reverse-osmosis membrane. In one embodiment, the low salt-rejection reverse-osmosis membrane comprises an inlet and one or more outlets. In one embodiment, the stages comprise a standard high salt rejection module. In one embodiment, the outlet of the feed source input is fluidly connected to the inlet of the first stage with a low salt-rejection reverse-osmosis membrane. In one embodiment, the outlet of a low salt-rejection reverse-osmosis membrane is fluidly connected to the inlet of another low salt-rejection reverse-osmosis membrane. In one embodiment, the outlet of a low salt-rejection reverse-osmosis membrane is fluidly connected to an intermediate container. In one embodiment, the outlet of a low salt-rejection reverse-osmosis membrane is fluidly connected to the feed source input. In one embodiment, the outlet of a low salt-rejection reverse-osmosis membrane is fluidly connected to a standard high salt rejection module. In one embodiment, the stages comprise pumps. In one embodiment, the stages comprise one more recirculation pumps 106. In one embodiment, the stages comprise one more pressure exchangers 107. In one embodiment the stages comprise one or more high pressure pumps. The pressure exchangers 107 comprise an inlet A and an outlet B. In one embodiment, contaminated fluid from the system feed tank 101 is pumped through the stages. In one embodiment, pumping the contaminated fluid through the stages desalinizes the fluid. In one embodiment, purified fluid is collected from any one of the stages. In one embodiment, brine is collected from any one of the stages. In one embodiment, concentrate is collected from any one of the stages.
In one embodiment, the feed source input flows into the inlet of a stage n and a permeate flows out of the outlet of a stage n. In one embodiment, a concentrate flows out of the outlet of a stage. In one embodiment, purified water flows out of the outlet of a stage. In one embodiment, brine flows out of a stage.
In one embodiment, the stages operate in parallel. In one embodiment, the stages comprise a first chamber and a second chamber separated by the semipermeable membrane. Examples of membranes include but are not limited to, ion exchange membranes, monovalent selective cation exchange membranes, monovalent selection anion exchange membranes, low salt rejection reverse osmosis membranes, reverse osmosis membranes, standard seawater reverse osmosis membrane, normal osmosis membranes, forward osmosis membranes, nanofiltration membranes, semi-transparent membranes, and ultrafiltration membranes.
The system feed source input may comprise a feed stream. The feed stream may comprise any type of fluid. In particular, the fluid may comprise seawater, brackish groundwater, wastewater, or recycled water from any one of the stages in the system. In one embodiment, the feed stream is a concentrate. The feed stream may comprise any type of cation and anion. Exemplary cations and anions include, but are not limited to calcium, aluminum, magnesium, sodium, potassium, strontium, barium, ammonia, carbonate, sulfate, chloride, nitrate, boron, silicon dioxide, and iron. In one embodiment, the feed stream comprises salts such as sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl2), magnesium carbonate (MgCO3), magnesium sulfate (MgSO4), calcium chloride (CaCl2)), calcium sulfate (CaSO4), calcium carbonate (CaCO3), potassium acetate (KAc) or calcium magnesium acetate (CaMgAc).
In a particular embodiment, the fluid at any one of the stages is saline. In various embodiments, the salinity of the fluid is in the range of 1 (g/L) to 500 (g/l). In one embodiment, the salinity of the fluid is between 50 (g/L) and 400 (g/L). In one embodiment, the salinity of the fluid is between 100 (g/L) and 300 (g/L). In one embodiment, the salinity of the fluid is at least 200 (g/L).
The stages described herein may comprise any suitable hollow vessel appropriate for transportation of fluid. In one embodiment, the vessels are tanks. In one embodiment, the vessels are pipes. In one embodiment, the vessels are reservoirs. In one embodiment, the vessels are barrels. In one embodiment, the vessels are containers.
Suitable material for the vessels may comprise any material known in the art including but not limited to organic polymers, inorganic polymers, homopolymers, copolymers, thermoplastics, thermosets, glass, quartz, ceramic, silica, alloy, metal alloy, stainless-steel, stainless-steel alloy, aluminum, aluminum alloy, aluminum oxide, copper, copper, alloy, titanium, titanium alloy, brass, plastic, or any combination thereof. Exemplary plastics include, but are not limited to, polyolefins, polyethylene, high-modulus polyethylene (HMPE), polypropylene, polybutylene, polybutene, polybutadiene, polybutylene terephthalate (PBT), polyethylene terephthalate (PET), 30 polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polycyclopentadiene (PCP), hydrogenated polycyclopentadiene (HCPC), polyetherimide (PEEK), polystyrene (PS), polyurethane (PU), polycarbonate (PC), polyacrylate, polymethacrylate, poly (methyl) methacrylate, polyoxymethylene, polylactic acid, polyether ether ketone, polyvinyl ether, polyvinyl chloride (PVC), chlorinated polyvinyl chloride, acrylonitrile butadiene styrene (ABS), polyethylene vinyl acetate (PEVA), styrene-butadiene copolymer, fluorinated polymer, and combinations thereof. In some embodiments, the material may be the same throughout the slag reactor, or the material may be varied to accommodate various specifications, such as transparency for monitoring or to meet requirements for the fluid to move slowly and/or quickly.
In one embodiment, the vessels described herein comprise valves to control fluid levels. In one embodiment, the valves are drain valves. In one embodiment the valves are venting valves. In one embodiment, the valves are shut-off valves. In one embodiment, the valves are transfer valves. In one embodiment, the valves are pressure relief valves. The valves may comprise any type of valve known in the art which include, but are not limited to, tec valves, ball valves, butterfly valves, diaphragm valves, gate valves, pinch valves, piston valves, plug valves, globe valves, needle valves, swing check valves, multi-port valves, float valves, foot valves, knife gate valves.
The vessels may comprise a drainage layer. The drainage layer may comprise at least 1% of the total volume of the reactor. In one embodiment, the drainage layer comprises between 2% and 10% of the total volume of the reactor. In one embodiment, the drainage comprises 8.6% total volume of the reactor. In one embodiment, the drainage layer comprises at least 25% of the total volume of the reactor. Suitable material for the drainage layer may comprise any material known in the art including, but not limited to, charcoal, concrete, glass, perlite, vermiculite, sand, clay, gravel, pea gravel, river rock, quartz sand crushed stone, and plastic beads, In one embodiment the drainage layer is a filter disc. In one embodiment, the drainage layer is a mesh filter. In one embodiment, the drainage layer is a grid filter. In one embodiment, the drainage layer is a cotton filter. In one embodiment, the drainage layer is stainless steel filter.
In various embodiments, the vessels have a circular, rectangular, triangular, elliptical, or rectilinear cross-section. In one embodiment, the vessels have a uniform cross-section area. In one embodiment, the cross-sectional area is substantially the same along the length of the reactors. In one embodiment, the vessels have a non-uniform cross-section area. In one embodiment, the cross-sectional area is not the same along the length of the vessels.
In one embodiment, a pump may be employed to drive the feed stream through all or parts of the system. In a particular embodiment, a pump may be applied across one or more of the stages which returns a recycle stream from the outlet of stage n to the feed inlet of stage n. In one embodiment, the magnitude of the total flow into the stage n membrane elements exceeds the minimum requirements by the membrane manufacturer. In one embodiment, a booster pump is used to pressure the permeate of a low salt rejection reverse osmosis membrane. In one embodiment, the system comprises high pressure pumps. In one embodiment, the system comprises booster pumps. In one embodiment, the system comprises recirculation pumps. In one embodiment, the LSRRO stage further comprises a pump configured to boost the pressure of and to recirculate a portion of a concentrate output by the LSRRO stage to the input of said LSRRO stage.
In a particular embodiment, LSRRO stage 104 further comprises a booster pump 106 configured to boost the pressure of at least a portion of a permeate output by the LSRRO stage 104 to the input of said LSRRO stage 104 or to the SWRO stage 105. In one embodiment, the system comprises a centrifugal pump. In one embodiment, the system comprises a submersible pump. In one embodiment, the system comprises a positive displacement pump. In one embodiment, the pump is a screw pump. In one embodiment, the pump is a reciprocating pump. In one embodiment, the pump is a radial piston pump. In one embodiment, the pump is a hydraulic pump. In one embodiment, the pump is a rotary vane pump. In one embodiment, the pump is a piston pump. In one embodiment, the pump is an axial flow pump. In one embodiment, the pump is a gear pump. In one embodiment, the pump is a plunger pump. In one embodiment, the pump is a dynamic pump. In one embodiment, the pump is a diaphragm pump. In one embodiment, the pump is a lobe pump. In one embodiment, the pump is a gear pump. In one embodiment, the pump is a metering pump. In one embodiment, the pump is a vacuum pump. In one embodiment, the pump is a peristaltic pump. In one embodiment the stages comprise a power source that can be used to operate the pumps and monitoring equipment in order to operate the system and vessels discussed herein. In one embodiment, a pump is not required, and flow of fluid is based on gravity.
In a particular embodiment, the LSRRO stage 104 further comprises a pressure exchanger 107 configured to recirculate at least a portion of a permeate output by the LSRRO stage 104 to the input of said LSRRO stage 104 or to the SWRO stage 105. In one embodiment, the system 100 comprises a pressure exchanger 107 installed between stages n and n−1 which reduce energy requirements and provide a volume balance. In one embodiment, the permeate of stage n is fed into the inlet A of a pressure exchanger and is pressurized and exits from outlet B of the pressure exchanger and is fed into the feed source input of stage n−1. In one embodiment, a pressure exchanger transfers water to the feed source input of any upstream or downstream stage. In one embodiment, a concentrate from stage n is fed into the pressure exchanger at an inlet and exits from the outlet of the pressure exchanger at atmospheric pressure after transferring its energy to the permeate stream of stage n. In one embodiment, a pressure exchanger is used to pressurize the permeate of a low salt rejection reverse osmosis membrane before permeate enters a standard high salt rejection module. In one embodiment, the system operates multiple pressure exchangers in parallel. In one embodiment, the system operates multiple pressure exchangers in series.
In a particular embodiment, the first stage is low salt rejection reverse osmosis membrane combined with a standard high salt rejection module. In one embodiment, the permeate of a low salt rejection reverse osmosis membrane is fed into one or more standard high salt rejection modules. In one embodiment, a pressure exchanger is used to pressurize the permeate of a low salt rejection reverse osmosis membrane in order to calibrate it to the operating pressure of the standard high salt rejection module. In one embodiment, the concentrate from the standard high salt rejection module is blended with the stream in the system feed tank. In one embodiment, purified water is collected from the standard high salt rejection module. In one embodiment, the permeate from a stage with a low salt rejection reverse osmosis membrane is fed into a stage with a standard high salt rejection module. In a particular embodiment, the concentrate of the standard high salt rejection module is passed through a pressure exchanger and the reject is returned to the original feed of the low salt rejection reverse osmosis cascade after transferring its energy through the pressure exchanger into the feed of the stage with a standard high salt rejection module before the high-pressure pump.
In one embodiment, antiscalant is fed to the permeate stream in the last stage. In one embodiment, a low molecular weight antiscalant is fed to the concentrate stream in the last stage so that the antiscalant crosses the membrane barrier into the permeate. Exemplary scalants include but are not limited to polyacrylic acids, carboxylic acids, polymaleic acids, organophosphates, polyphosphates, phosphonates, anionic polymers, phosphonates such as 1-hydroxyethylidenc 1,1-diphosphonic acid (HEDP) or other proprietary silica anti-scalants such as Vitec 4000 (Avista Technologies Inc) Genesys SI (Genesys International Ltd), calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, calcium fluoride, iron, colloidal material, silica and other organic contaminants.
In one aspect, the present invention relates in part to a method of purifying contaminated fluid. The method comprises the steps of providing any of the reverse osmosis systems 100 described herein; inputting a high salinity fluid into the feed source input 101; increasing the pressure of the high salinity fluid via the high pressure feed pump; performing reverse osmosis via the RO cascade 103; and collecting at least one of a concentrate and a permeate.
The process may be operated in any manner desired, e.g. as continuous, semi-continuous. The process may be controlled using known equipment and control schemes. For example, hydraulic retention time, desired feed rates of input fluid, anti-scalants, etc. may be determined by routine experimentation. In one embodiment, the flow rate in the process is controlled by setting a pump to the desired flow rate. In one embodiment, the ratio of volume or flow rate of fluid into the feed stream of a stage n to the volume or flow rate of the concentrate feed of stage n is 4 or higher. In one embodiment, the ratio of volume or flow rate of fluid into the permeate of a stage n to the volume or flow rate of the concentrate feed of stage n is 4 or higher.
The method described finds use for the desalination of fluid. The reverse osmosis system may comprise a feed source input 101. The feed source input 101 may comprise any type of fluid. In particular, the fluid may comprise seawater, brackish groundwater, wastewater, or recycled water from any one of the stages in the system. In one embodiment, the feed stream is a concentrate. The feed stream may comprise any type of cation and anion. Exemplary cations and anions include but are not limited to calcium, aluminum, magnesium, sodium, potassium, strontium, barium, ammonia, carbonate, sulfate, chloride, nitrate, boron, silicon dioxide, and iron. In one embodiment, the feed stream comprises salts such as sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl2), magnesium carbonate (MgCO3), magnesium sulfate (MgSO4), calcium chloride (CaCl2)), calcium sulfate (CaSO4), calcium carbonate (CaCO3), potassium acetate (KAc) or calcium magnesium acetate (CaMgAc).
In one embodiment, the stages operate in parallel. In one embodiment, the stages comprise a first chamber and a second chamber separated by the semipermeable membrane. Examples of membranes include but are not limited to, low salt rejection reverse osmosis membranes, reverse osmosis membranes, standard seawater reverse osmosis membrane, normal osmosis membranes, forward osmosis membranes, nanofiltration membranes, semi-transparent membranes, and ultrafiltration membranes.
In one embodiment, the step of pumping the feed source input through the stages desalinizes the fluid. In one embodiment, the step of pumping the fluid from the vessel to a stage with a low salt rejection reverse osmosis membrane is done by a high-pressure feed pump. In one embodiment, the step of pumping the fluid through the low salt rejection membrane desalinizes the fluid. In one embodiment, the permeate is recirculated with a booster pump or a recirculation pump. In one embodiment, the permeate is pumped to the standard high salt rejection module with a high-pressure feed pump. In one embodiment, a pressure exchanger, with the help of a booster pump, is used to pressurize the permeate of a low salt rejection reverse osmosis membrane before it enters the standard high salt rejection module.
In one embodiment, antiscalant is fed to the permeate stream in final stage. In one embodiment, a low molecular weight antiscalant is fed to the concentrate stream in the last stage so that the antiscalant crosses the membrane barrier into the permeate. Exemplary scalants include but are not limited to polyacrylic acids, carboxylic acids, polymaleic acids, organophosphates, polyphosphates, phosphonates, anionic polymers, phosphonates such as 1-hydroxyethylidene 1,1-diphosphonic acid (HEDP) or other proprietary silica anti-scalants such as Vitec 4000 (Avista Technologies Inc) Genesys SI (Genesys International Ltd), calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, calcium fluoride, iron, colloidal material, silica and other organic contaminants.
The invention is further described in detail by reference to the following experimental examples. These examples are provided for purposes of illustration only, and are not intended to be limiting unless otherwise specified. Thus, the invention should in no way be construed as being limited to the following examples, but rather, should be construed to encompass any and all variations which become evident as a result of the teaching provided herein.
Without further description, it is believed that one of ordinary skill in the art can, using the preceding description and the following illustrative examples, make and utilize the present invention and practice the claimed methods. The following working examples therefore are not to be construed as limiting in any way the remainder of the disclosure.
It is the purpose of this invention to overcome the twin barriers to near liquid zero discharge (near-ZLD) of saline streams using non-thermal membrane processes that include at least one pressure driven membrane process. These two barriers are osmotic pressure and the accumulation of scaling species in the concentrate that can scale and foul the pressure driven membrane surface. While the process of low salt rejection RO cascades (LSRRO) can overcome high osmotic pressure limitations, scaling species in the brine being treated can reach concentrations that would result in very high supersaturations leading to precipitation and scaling of the membrane surface. By first treating the feed brine with monovalent selective electrodialysis (MSED), a secondary brine at even higher concentration can be fed to the LSRRO process in which the concentration of scaling species (divalent salts of sulfate, silica, and calcium carbonate) is greatly reduced relative to the initial feed brine and a diluate is produced that can be mixed with RO permeate streams to produce a water that can be used for both potable and agricultural purposes.
A way to overcome the issue of osmotic pressure of reverse osmosis (RO) concentrates and other industrial brines is called low salt-rejection RO (LSRRO) in which downstream RO stages use membranes with substantially lower salt rejections and the permeate from a downstream stage is fed back to the feed of the previous upstream stage. This reduces the required applied pressure to generate a positive net driving pressure for stage (NDPi) to drive flux by three different mechanisms as illustrated in equation 1:
where P is the hydraulic pressure. The stage reflection coefficient (σ) and the stage rejection (R) are significantly less than 1 while the average bulk concentration on the feed side is less than it would be without applying the cascade approach. Simple modelling using a two-parameter model (assuming reflection coefficient is unity) has already been carried out and shown that very high salinities can be reached (12-20% TDS) with modest energy consumptions (4-7 kWh/m3) (Wang, Z. et al., Water Res., 170, 115317, 2020; Du, Y., et al., Water Res., 209, 2022).
An idealization of the approach is illustrated in
There will be a problem in that any scaling species fed to the LSRRO cascade will be concentrated in the final LSRRO brine by the volume concentration factor effected by the LSRRO cascade. For example an RO concentrate that is 20,000 mg/L TDS that contains 500 mg/L calcium, 500 mg/L of sulfate, 1500 mg/L of bicarbonate and 1 mg/L of barium, that is further concentrated 5-fold in the LSRRO cascade to reach 100,000 mg/L TDS will now contain 2500 mg/L of calcium, 2500 mg/L of sulfate, 7500 mg/L of bicarbonate and 5 mg/L of barium. This will be highly supersaturated with respect to both calcium sulfate and barium sulfate and to calcium bicarbonate, and may not be controllable by standard antiscalants. A brackish groundwater was desalinated by reverse osmosis and because of the presence of silica in the raw brackish groundwater, the recovery in the RO step was only 82%. The composition of the brackish water feed and RO concentrate are provided in table 1.
If high pressure RO desalination were used to concentrate this stream to 200,000 mg/L the final osmotic pressure to overcome would be 130 bar, far beyond the reach of standard RO systems but well within the capability of LSRRO. However, if the RO concentrate were fed directly to the LSRRO, the final silica concentration would be 4655 mg/L which is ˜40 times the saturation concentration.
The use of monovalent selective electrodialysis (MSED) produces a concentrate stream that is significantly lower in divalent content and silica content. By combining MSED and LSRRO the shortcomings of both (limitations of concentrate concentration for MSED, and scaling issues with LSRRO) can be overcome (Sata, T. et al., J. Memb. Sci., 93, 117-135, 1994; Cohen, B. et al., Desalination, 431, 126-139, 2018; Ahdab, Y. D. et al., ACS EST Water, 1, 1, 117-124, 2021).
The saline feed stream (1) originates from RO concentrate or other saline stream from industry and contains scaling ions. It is split into two streams to feed to the diluate and concentrate side of the electrodialysis stack equipped with pairs of alternating monovalent selective cation exchange membrane (CEM) and monovalent selective anion exchange membrane (AEM). A detailed presentation of the MSED stack is shown in
The MSED stack removes predominantly monovalent ions from diluate stream to produce a diluate product stream of reduced salinity but retaining most of the divalent ions and practically all the silica that was in the stream. The concentrate exit stream that received the ions that cross the membranes in the ED stack is concentrated with respect to the original saline feed stream. Its divalent content depends on the CEM and AEM selectivity and the extent of removal of the monovalent ion (particularly sodium chloride). If monovalent selectivity is high enough (and thus divalent cation content low enough) then it can be fed directly to the LSRRO cascade where it is concentrated to its highest level as LSRRO concentrate. If the divalent content in the stream is still somewhat high, then this can be dealt with by feeding it to a scaling ion scavenging process where the divalent ions are reduced. The outlet with reduced calcium content can then be fed to the LSRRO. Embodiments of this process can include cation exchange columns for removal of divalent cations, or a crystallization process (fluidized bed, mixer settler crystallizer, etc.) to remove calcium carbonate or calcium fluoride or other scaling species that may be slightly higher than saturated in stream. However the size of this process will be greatly reduced since the bulk of such scaling species remain in the diluate product stream.
Since the feed to the concentrate side of the MSED stack is already concentrated by the previous RO process, there will be an elevated concentration of scaling species in the ED concentrate even before more ions are pushed into the concentrate during the ED process. This means a higher level of such scaling species in stream with which the LSRRO will have to contend. A way of reducing this load, is to use raw groundwater or some other low ion content water as the receiving solution fed to the ED concentrate side as stream. This can greatly reduce the scaling ion load, especially when the MSED is highly selective and the initial concentrate feed strongly influences the final scaling species content.
Many additional variations can be made on the principles enunciated here. The rest of the description will provide illustrations of these principles.
If a RO concentrate as defined in table 1 is fed to a process flow stream as described in
In which the subscripts on the sodium concentrations refer to streams in
Q refers to the volumetric rate of the flow. Y refers to the volumetric yield of product water and the subscript refers to the unit operation. The explicit definitions of Y are given in the equations below:
Then:
Where j refers to the particular aqueous species in the stream. If no scaling species removal step is applied then can set C4=C3b.
Then one can write to reach a target concentration of sodium ion in the LSRRO concentrate we have:
Applying equations 1-5 we can generate the profiles of required recovery in the LSRRO step to get a concentration of 2.2 M sodium ion in the final stream. For many compositions, depending on the MSED selectivity, this will give a final concentrate which has an osmotic pressure of 120-160 bar. The required recovery in the LSRRO step as a function of the recovery in the ED step and the extent of sodium ion removal from the diluate stream.
Similarly using the equations 1-5 for calculating calcium concentration in the final LSRRO brine stream as a function of the cation exchange membrane selectivity of sodium over calcium for and MSED recovery rate of 80%.
Even for the worst case (sodium removal in ED step of only 50% and Sodium/Calcium selectivity of only 4 (C5, Ca2+=420 meq/L), the final LSRRO brine has a calculated LSI of 2.4 for calcite which can be controlled by antiscalant chemicals.
If a calcium removal step involving a cation exchange column, then the calcium level can be reduce by 80-90% and then the highest calcium level in the LSRRO concentrate will not exceed 45-90 meq/L. The demands on such a column will be <25% of those if the same column were applied to the RO concentrate because at a sodium/calcium selectivity of 4 in the ED step, when 90% of sodium is removed, only 22.5% of the calcium is moved to the concentrate stream.
If the same composition of stream 1 is used, but raw groundwater is used as concentrate feed instead of RO concentrate. then we can reduce the concentration of scaling ions even further. The options of feed stream from the RO concentrate is compared to feed stream from the raw groundwater. As shown in the table 2 below, silica levels in the LSRRO concentrate are much lower when the groundwater is used as the MSED feed. As can be seen, the level of silica in the stream is only near or below the saturation limit of silica (˜120 mg/L) when raw groundwater is used as stream and YED is at 85% or higher.
A different realization can be provided in which a truly once-through steady-stage operation can be realized even/especially in small systems by inserting recycle pumps within each stage (
A further realization can be obtained by using pressure exchangers (PX) instead of high-pressure pumps to move permeate stream from stage n to the feed stream of stage n−1 (
In
A further example has been envisaged as illustrated in
Since there is a risk of scaling minerals on the different stages of the LSRRO cascade, normal practice would be to dose antiscalant on the feed/high-pressure side of the LSRRO cascade. However, because the membranes are low rejection, they may leak scaling species into the permeate and there can be scaling potential there as well. If the scaling species are partially passed but the antiscalants (which are often polymers of 1000-2000 Da) are completely rejected, then scaling waters on the permeate side could plug the back side of the membrane. Therefore, an additional novel step will be to feed antiscalant to the permeate of the last stage to protect the permeate side of the cascade. Alternatively, a low molecular weight antiscalant (Cl-C3) can be fed to last stage feed that can be expected to partially pass the membrane and get into the last stage permeate.
The disclosures of each and every patent, patent application, and publication cited herein are hereby incorporated herein by reference in their entirety. While this invention has been disclosed with reference to specific embodiments, it is apparent that other embodiments and variations of this invention may be devised by others skilled in the art without departing from the true spirit and scope of the invention. The appended claims are intended to be construed to include all such embodiments and equivalent variations.
This application claims priority to U.S. provisional application No. 63/463,026 filed on Apr. 30, 2023, and to U.S. provisional application No. 63/463,030 filed on Apr. 30, 2023, each of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63463026 | Apr 2023 | US | |
| 63463030 | Apr 2023 | US |
