High water recovery hybrid membrane system for desalination and brine concentration

Abstract

The high water recovery hybrid membrane system for desalination and brine concentration combines nanofiltration, reverse osmosis and forward osmosis to produce pure water from seawater. The reject side of a nanofiltration unit receives a stream of seawater and outputs a brine stream. A permeate side of the nanofiltration unit outputs a permeate stream. A feed side of a reverse osmosis desalination unit receives a first portion of the permeate stream and outputs a reject stream. A permeate side of the reverse osmosis desalination unit outputs pure water. A draw side of at least one forward osmosis desalination unit receives the reject stream and outputs concentrated saline solution. A feed side of the at least one forward osmosis desalination unit receives a second portion of the permeate stream and outputs a dilute saline stream, which mixes with the first portion of the permeate stream fed to the reverse osmosis desalination unit.

Claims

1. A water recovery hybrid membrane system for desalination and brine concentration, comprising: a forward osmosis desalination unit including a forward osmosis desalination chamber having a semipermeable membrane defining a feed side and a draw side of the forward osmosis desalination chamber; a delivery pump for injecting a stream of seawater or brine into the feed side of the forward osmosis desalination chamber at a pressure of about 58 bar; a reverse osmosis desalination unit including a reverse osmosis desalination chamber having a semipermeable membrane, the membrane defining a feed side and a permeate side of the reverse osmosis desalination chamber, wherein the feed side of the reverse osmosis desalination chamber is in fluid communication with the feed side of the forward osmosis desalination chamber, such that a fluid drawn through the feed side of the forward osmosis desalination chamber is fed into the feed side of the reverse osmosis desalination chamber, wherein a rejected stream from the feed side of the reverse osmosis desalination chamber is received directly by the draw side of the forward osmosis desalination chamber and pure water extracted from a fluid from the permeate side is outputted from the reverse osmosis desalination unit; and a circulation pump for injecting the fluid from the feed side of the forward osmosis desalination chamber into the feed side of the reverse osmosis desalination chamber at a pressure greater than 60 bar.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a high water recovery hybrid membrane system for desalination and brine concentration.

(2) FIG. 2 is a schematic diagram of a conventional prior art reverse osmosis desalination unit.

(3) FIG. 3 is a schematic diagram of a conventional prior art forward osmosis desalination unit.

(4) FIG. 4 is a schematic diagram of an alternative embodiment of the high water recovery hybrid membrane system for desalination and brine concentration.

(5) FIG. 5 is a schematic diagram of another alternative embodiment of the high water recovery hybrid membrane system for desalination and brine concentration.

(6) Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) Referring to FIG. 1, the high water recovery hybrid membrane system for desalination and brine concentration 10 combines nanofiltration (NF), reverse osmosis (RO) and forward osmosis (FO) to produce pure water from seawater or other sources of saline solution or salt-contaminated water. As shown in FIG. 1, the feed side 20 of a nanofiltration unit 14 receives a stream of pretreated seawater S and outputs a reject brine stream B. It should be understood that the nanofiltration unit 14 may be any suitable type of nanofiltration system using a suitable nanofiltration membrane 16, such as that described above. A permeate side 18 of the nanofiltration unit 14 outputs a permeate stream. As discussed above, NF unit 14 rejects the divalent “hardness” ions from the seawater S, such as magnesium, calcium and bicarbonate ions. Thus, scaling and fouling of the RO and FO membranes 28, 40, respectively, will be reduced. It should be understood that the initial feed stream may include seawater, brine, brackish water, produced water, wastewater, or any other type of saline stream(s).

(8) A feed side 30 of a reverse osmosis desalination unit 26 receives a first portion of the permeate stream P.sub.1 output from the permeate side 18 of the nanofiltration unit 14 and outputs a reject stream R. A permeate side 32 of the reverse osmosis desalination unit 26 outputs the pure water product PW. It should be understood that reverse osmosis desalination unit 26 may be any suitable type of reverse osmosis desalination system using any suitable type of RO membrane 28, such as reverse osmosis unit 100 of FIG. 2, as described above.

(9) A draw side 36 of at least one forward osmosis desalination unit 34 receives the reject stream R output from the reverse osmosis desalination unit 26 and outputs a concentrated saline solution C. In FIG. 1, a single forward osmosis desalination unit 34 is shown. However, as will be described in greater detail below, a plurality of forward osmosis desalination units may be connected in series to one another. It should be understood that each forward osmosis desalination unit may be any suitable type of forward osmosis desalination system using any suitable type of FO membrane 40, such as forward osmosis unit 200 of FIG. 3, as described above.

(10) A feed side 38 of the at least one forward osmosis desalination unit 34 receives a second portion of the permeate stream P.sub.2 output from the permeate side 18 of the nanofiltration unit 14 and outputs a dilute saline stream D. The dilute saline stream D is recirculated to mix with the first portion of the permeate stream P.sub.1 output from the permeate side 18 of nanofiltration unit 14, prior to the receipt thereof by the feed side 30 of the reverse osmosis desalination unit 26.

(11) The fluid pressure within the draw side 36 of the at least one forward osmosis desalination unit 34 is greater than the fluid pressure within the feed side 38 of the at least one forward osmosis desalination unit 34. Although, according to the basic operation principles of FO, water should be transported from feed side 38 to draw side 36, the water is actually transported from draw side 36 to feed side 38 due to a higher hydraulic pressure existing in draw side 36 (i.e., pressure-assisted FO). As described above, the at least one forward osmosis desalination unit may be a single forward osmosis desalination unit, such as that shown in FIG. 1, or, alternatively, may be a plurality of forward osmosis desalination units serially connected to one another, such as the dual-FO system shown in FIG. 4.

(12) The high water recovery hybrid membrane system for desalination and brine concentration of FIG. 4 is similar to that of FIG. 1, but with an additional forward osmosis desalination unit 34′ connected in series to forward osmosis desalination unit 34. The concentrated saline solution C output from the draw side 36 of forward osmosis desalination unit 34 is received by the draw side 36′ of additional forward osmosis desalination unit 34′. The additional forward osmosis desalination unit 34′ may be any suitable type of forward osmosis desalination system using any suitable type of FO membrane 40′, such as forward osmosis unit 200 of FIG. 3, as described above. The draw side 36′ of the additional forward osmosis desalination unit 34′ outputs a highly concentrated brine solution C′. The dilute saline stream D output from the feed side 38 of forward osmosis desalination system 34 is received by the feed side 38′ of the additional forward osmosis desalination system 34′, which outputs a diluted water stream D′, which recirculates back to be mixed with the first portion of the permeate stream P.sub.1 output from the permeate side 18 of nanofiltration unit 14, prior to the receipt thereof by the feed side 30 of the reverse osmosis desalination unit 26, as in the previous embodiment.

(13) It should be understood that although FIG. 4 shows two forward osmosis desalination units 34, 34′ connected in series, any number of forward osmosis desalination units may be connected in the same manner. In experiments, a dual-FO system, such as that shown in FIG. 4, achieved a final FO brine total dissolved solids (TDS) of 128,493 ppm. The overall water recovery of this system was 60%. A comparison between the single-FO system of FIG. 1 and the dual-FO system of FIG. 4 is given in Table 2 below. A system with three forward osmosis desalination units achieved a final FO brine TDS of 154,191 ppm. The overall water recovery of this system was 70%.

(14) In Table 1, below, the labels of each fluid correspond to those of FIGS. 1 and 4, i.e., Table 1 provides results for the initial pretreated seawater stream S input into nanofiltration unit 14, the brine B output from nanofiltration unit 14, the pure water PW output from reverse osmosis desalination unit 26, and the reject output R from reverse osmosis desalination unit 26. Additional columns are provided for the permeate output by the NF unit 14 (prior to division into separate permeate streams P.sub.1 and P.sub.2), and the feed injected into the feed side 30 of the reverse osmosis desalination unit 26 (consisting of stream P.sub.1 and either dilute solution D of FIG. 1 or dilute solution D′ of FIG. 4). For each stream, Table 1 provides the total concentration of dissolved solids (primarily salt), the operating pressure of each stream in the process, and the overall water recovery (given as a percentage). Table 2 also provides the total concentration of dissolved solids (primarily salt), the operating pressure of each stream in the process, and the overall water recovery (given as a percentage), with specific comparisons being made between the concentrated outputs C, C′ between the systems of FIGS. 1 and 4, respectively, and the dilute outputs D, D′ between the systems of FIGS. 1 and 4, respectively.

(15) TABLE-US-00001 TABLE 1 Results for system of FIG. 1 Concentration, Pressure and Water Recovery for each Process Stream S B NF Permeate RO Feed PW R Concentration (ppm) 35,380 44,200 26,280 23,652 <280 65,700 Pressure (bar) 30 (at — 4 (at 70 (at 0.69 5.52 pump 12) pump 22) pump 34) Water recovery (%) 25.72 — — — 64.00 60.00

(16) TABLE-US-00002 TABLE 2 Results for systems of FIGS. 1 and 4 Concentration, Pressure and Water Recovery for each Process Stream C D C′ D′ Concentration (ppm) 95,189 16,800 128,493 10,517 Pressure (bar) 5.52 5.52 70.34 0.0 Water recovery (%) 45.00 — 34.99 —

(17) In the alternative embodiment of FIG. 5, the nanofiltration unit is removed from high water recovery hybrid membrane system for desalination and brine concentration 310 to desalinate the seawater S or brine B using only a reverse osmosis desalination unit 322 and a forward osmosis desalination unit 318. In this embodiment, the feed side 326 of the reverse osmosis desalination unit 322 is in fluid communication with the feed side 314 of the forward osmosis desalination unit, such that seawater S or brine B drawn through the feed side 314 of the forward osmosis desalination unit 318 is fed into the feed side 326 of the reverse osmosis desalination unit 322. A delivery pump 312 may inject the seawater S or brine B, under pressure, through the feed side 314 of the forward osmosis desalination unit 318. A circulation pump 320 delivers the seawater S or brine B from the feed side 314 of the forward osmosis desalination unit 318 to the feed side 326 of the reverse osmosis desalination unit 322. The circulation pump 320 is a high pressure pump, such that the seawater S or brine B injected into the feed side 326 of the reverse osmosis desalination unit 322 is injected at a pressure greater than 60 bar.

(18) It should be understood that reverse osmosis desalination unit 322 may be any suitable type of reverse osmosis desalination system using any suitable type of RO membrane 339, such as reverse osmosis unit 100 of FIG. 2, as described above. Similarly, it should be understood that the forward osmosis desalination unit 318 may be any suitable type of forward osmosis desalination system using any suitable type of FO membrane 332, such as forward osmosis unit 200 of FIG. 3, as described above.

(19) The reverse osmosis desalination unit 322 performs reverse osmosis desalination on the seawater S or brine B fed into the feed side 326, outputting pure water PW extracted from the seawater S or brine B from the permeate side 324. The feed side 326 of the reverse osmosis desalination unit 322 outputs a reject stream R. The draw side 316 of the forward osmosis desalination unit 318 is in fluid communication with the feed side 326 of the reverse osmosis desalination unit 322, such that the draw side 316 of the forward osmosis desalination unit 318 receives the reject stream R and outputs concentrated brine CB or highly saturated brine SB.

(20) The delivery pump 312 may be a low pressure pump, operating at about 1 to 2 bar. In experiments, using the above pressure values, no product water PW was observed for the initial two minutes of operation. The concentrated brine CB typically enters the draw side 316 of the forward osmosis desalination unit 318 at a hydraulic pressure of about 58 bar. Subsequently, water transport occurs from the draw side 316 to the feed side 314, across the FO membrane 332, due to the combined effect of hydraulic and osmotic pressures faced by the fluid in the draw side 316. As in the previous embodiment, pressure-assisted FO is taking place, such that pure water transports from the high osmotic pressure brine side 316 to the low osmotic pressure seawater side 314. This results in further dilution of the seawater or brine intake stream in the feed side 314 and a further concentration of the brine B.

(21) In experiments, system 310 attained a state of equilibrium after 2-4 minutes of operation, during which the TDS of the feed stream into the feed side 326 of the RO unit 322 dropped. Due to the reduction in the salinity of the RO feed seawater, the recovery of the RO unit 322 was drastically increased to ˜60% at an operating pressure of 60 bar to produce fresh water with a TDS of ˜135 ppm in the permeate side 324. Corresponding to the above experimental values, the initial seawater stream S had a TDS of 42,121 ppm with a flow rate of 600 lph, and a conductivity of 65,815 μS/cm. Pure water PW was output at a flow rate of 360 lph, with a conductivity of 192 μS/cm. For purposes of comparison, under similar conditions, a single RO unit would have a water recovery of 30.0%, whereas system 310 has a water recovery of 60.0%. Table 3 below compares the composition of seawater S and the pure water PW output by system 310.

(22) TABLE-US-00003 TABLE 3 Comparison between Seawater Feed and Pure Water Output Parameter (unit) Seawater feed Pure Water product pH 7.4 7.2 Conductivity (mS/cm) 55.4 0.29 TDS (ppm) 35801 135 Calcium (mg/L) 824 6.16 Magnesium (mg/L) 1154 5.83 Sulfate (mg/L) 3600 0 Chloride (mg/L) 26000 38 Sodium (mg/L) 14,800 65 Alkalinity (mg/L) 120 4.3 Boron (mg/L) 2.75 0.24 Nitrate (mg/L) 3.5 0.7 Copper (mg/L) <0.05 <0.05 Chromium (mg/L) <0.05 <0.05 Iron (mg/L) <0.05 <0.05 Silica (mg/L) 16.2 0.724 Phosphate (mg/L) 0.15 0.11 Fluoride (mg/L) 4.3 0.13

(23) As noted above, the seawater S may be pretreated, such as by passage through a coarse screen, use of a beach well or media filter, flocculation and clarification, ultrafiltration, dual media, and/or any other suitable pretreatment process(es) to remove suspended solids, such as silt particles, silica or organic matter. Further, as noted above, NF unit 14 rejects the divalent “hardness” ions from the seawater S, such as magnesium, calcium and bicarbonate ions, thus reducing scaling and fouling of the RO and FO membranes 28, 40. This treatment is also effective at reducing the salinity of the feed solution, thus reducing the pressure requirements in the subsequent FO and RO processes. The NF membrane 16 may be any suitable type of NF membrane, such as a spiral-wound membrane, a plate-and-frame membrane, hollow fiber modules, or a plurality of stacked or layered sheets or nanofillers incorporated into membranes or nanofibers. For example, NF membrane 16 may be formed from cellulose ester derivatives or other polyamide-type thin film composite membranes or nanocomposite membranes. The molecular weight cut-off of the NF membrane 16 may be in the range of 200-500 Da, with a typical operating pressure of about 70-400 psi, with a maximum operating pressure of about 600 psi. The maximum pressure drop of the NF unit 14 may be 12 psi over an element and 50 psi per housing. The typical operating flux of NF unit 14 is about 13-34 L/m.sup.2h.

(24) The RO membrane 28 (or membrane 339) may be a semipermeable membrane with any suitable geometric configuration, such as a spiral-wound membrane, a plate-and-frame, hollow fiber modules, or a plurality of stacked or layered sheets or nanofillers incorporated into membranes or nanofibers. For example, RO membrane 28 may be formed from cellulose ester derivatives or other polyamide-type thin film composite membranes or nanocomposite membranes. An RO membrane with a high salt rejection efficiency of >99% with a high operating pressure range of 60-80 bar would be suitable.

(25) The FO membranes 40, 40′ (or membrane 332) may be, for example, semipermeable membranes with suitable geometric configurations, such as spiral-wound membranes, plate-and-frame membranes, hollow fiber modules, or a plurality of stacked or layered sheets or nanofillers incorporated into membranes or nanofibers. The thickness of the FO membrane 40, 40′ is typically far less than that of RO membrane 28 due to the non-pressure requirement of the FO process. An operating pressure up to 70 bar and a salt rejection of >99% would be suitable. The FO membrane 40, 40′ is configured within the membrane module to attain high dispersion of dilute solution D, D′ and feed solution throughout the module to attain a high permeate flow. Additionally, the membrane 40, 40′ may be operated in any suitable configuration, such as cross flow, co-current, counter-current, axial or radial configurations.

(26) It is to be understood that the high water recovery hybrid membrane desalination system for desalination and brine concentration is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.