WO2017155463A1

WO2017155463A1 - Self-cleaning osmosis process, apparatus and system

Info

Publication number: WO2017155463A1
Application number: PCT/SG2017/050089
Authority: WO
Inventors: Qianhong SHE; Rong Wang; Anthony Gordon Fane
Original assignee: Nanyang Technological University
Priority date: 2016-03-08
Filing date: 2017-02-27
Publication date: 2017-09-14

Abstract

The present application relates to a self-cleaning osmosis process and apparatus thereof, wherein the process comprises the steps of: (a) flowing a draw solution through a first channel of a membrane module while flowing a feed solution through a second channel of the membrane for a first duration, the draw solution having a higher solute concentration than the feed solution, the first channel and the second channel being separated by a semipermeable membrane having an active layer facing one of the first channel and the second channel; followed by (b) flowing the draw solution through the second channel while flowing the feed solution through the first channel for a second duration; and (c) repeating steps (a) and (b) for a number of cycles. A self-cleaning osmosis system comprising the said apparatus, a first energy recovery device and a second energy recovery device is also disclosed.

Description

SELF-CLEANING OSMOSIS PROCESS, APPARATUS AND SYSTEM CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority of Singapore Patent Application No. 10201601793P, filed March 8, 2016, the contents of it being hereby incorporated by reference in its entirety for all purposes.

FIELD

This invention relates to a self-cleaning osmosis process, apparatus and system.

BACKGROUND

Free energy released from mixing two solutions with unequal salinities (e.g., seawater and river water) has been identified as a type of renewable energy, known as salinity-gradient energy or osmotic energy. Pressure retarded osmosis (PRO) is one of the most promising technologies to harness this type of renewable energy for generating CO₂ emission-free electricity. In PRO, a semipermeable membrane separates a low salinity feed solution (FS) and from a high salinity draw solution (DS) that is applied with a hydraulic pressure lower than the osmotic pressure difference across the membrane. The free energy of mixing is converted into useful work with permeation of water from the FS into the DS. Electricity can be sustainably generated by running the volume-expanded DS through a hydroturbine.

In practical applications, the PRO performance can be significantly affected by membrane fouling. Membrane fouling is typically caused by the deposition of suspended particles or colloids, organic macromolecules, sparingly soluble inorganic compounds, microorganisms, or their mixtures in the FS onto the membrane surface and/or inside a porous support layer for the membrane [1]. In a typical PRO operation mode where the membrane active layer (AL) faces the draw solution (AL-DS), internal fouling within the support layer plays a dominant role in the PRO performance. Internal fouling occurs with the clogging of foulants in the pores of the porous support layer and/or the depositing of foulants on the back surface of the membrane active layer. In addition to internal fouling, external fouling, which occurs due to the deposition of foulants onto the membrane surface, also plays an important role in flux decline. Membrane fouling not only reduces the permeate water flux and osmotic power output, but also causes increased operating cost and shortened membrane life. To restore membrane performance and extend membrane life, periodic membrane cleaning has to be performed when the flux has declined to a certain level. It has been observed that backwashing, which is realized by reversing the direction of water flux, is one of the most efficient physical cleaning approaches against both internal and external fouling caused by various types of foulants [1]. In addition, chemical cleaning is also reported to be efficient to remove deposited foulants from the membrane [1]. However, conventional methods for implementing these membrane cleaning techniques could reduce system efficiency, increase the cost and energy consumption in the operation, and may have an adverse environmental impact due to the use of chemicals. To avoid these unexpected penalties arising from membrane cleaning, there is a need to develop novel cleaning techniques and processes for PRO applications.

SUMMARY

A self-cleaning osmosis (e.g. pressure retarded osmosis, PRO) apparatus and process are disclosed that can simultaneously ensure continuous operation of the apparatus and process for power production and provide effective membrane cleaning to maintain high performance. The apparatus and process can spontaneously perform membrane cleaning in continuous operation without downtime. This can significantly improve the efficiency of the system, reduce the system energy input, and facilitate system maintenance. The process can be achieved in operation through alternating the membrane orientation periodically, by switching the feed and draw flows by suitable valving in the apparatus. The apparatus and process can be incorporated into conventional osmosis systems and yield a novel osmosis system design with improved module connections. In addition, systems incorporating the present self-cleaning osmosis apparatus and process are provided to maximize energy recovery from the salinity-gradient resources. Methods to optimize module connections are also developed to improve the osmosis system efficiency.

A dual-stage PRO system is also developed according to an embodiment of the invention. The design of the dual stage PRO system can reduce the use of ERDs and pumps. This design can be extended to multiple-stage PRO system design.

Novel methods to connect the osmosis membrane modules in the apparatus are also developed. Particularly, the "cascade" method for connecting the modules provides great opportunities in multiple-pass and multiple-stage osmosis systems.

According to a first aspect, there is provided a self-cleaning osmosis process comprising the steps of:

(a) flowing a draw solution through a first channel of a membrane module while flowing a feed solution through a second channel of the membrane module for a first duration, the draw solution having a higher solute concentration than the feed solution, the first channel and the second channel being separated by a semipermeable membrane having an active layer facing one of the first channel and the second channel; followed by

(b) flowing the draw solution through the second channel while flowing the feed solution through the first channel for a second duration; and

(c) repeating steps (a) and (b) for a number of cycles. The feed solution and the draw solution may be always kept separate in different flow passages.

The first duration may comprise a length of time taken for flux across the semipermeable membrane from the second channel to the first channel to fall below a first predetermined level. The second duration may comprise a length of time taken for flux across the semipermeable membrane from the first channel to the second channel to fall below a second predetermined level.

According to a second aspect, there is provided a self-cleaning osmosis apparatus comprising: a membrane module having a first channel and a second channel, the first channel and the second channel separated by a semipermeable membrane, an active layer of the semipermeable membrane facing one of the first channel and the second channel; a draw solution inflow path being selectably divertable between an inlet of the first channel and an inlet of the second channel to flow a draw solution through only one of the first channel and the second channel at any one time; a feed solution inflow path being selectably divertable between an inlet of the first channel and an inlet of the second channel to flow a feed solution through only the other of the first channel and the second channel while the draw solution is flowed through the one of the first channel and the second channel; a draw solution effluent stream; and a feed solution effluent stream; wherein an outflow path from the first channel is selectably divertable between the draw solution effluent stream and the feed solution effluent stream in order to divert draw solution effluent from the first channel to the draw solution effluent stream and to divert feed solution effluent from the first channel to the feed solution effluent stream; and wherein an outflow path from the second channel is selectably divertable between the draw solution effluent stream and the feed solution effluent stream in order to divert draw solution effluent from the second channel to the draw solution effluent stream and to divert feed solution effluent from the second channel to the feed solution effluent stream.

The membrane module may comprise multiple membrane modules connected in series.

Alternatively, the membrane module may comprise multiple membrane modules connected in parallel.

Alternatively, membrane module may comprise multiple membrane modules connected to effect flow of the draw solution through each of the first channels of the multiple membrane modules in parallel, and flow of the feed solution through each of the second channels of the multiple membrane modules in series, and flow of the feed solution through each of the first channels of the multiple membrane modules in parallel and flow of the draw solution through each of the second channels of the multiple membrane modules in series.

Alternatively, membrane module may comprise multiple membrane modules connected to effect flow of the draw solution through each of the first channels of the multiple membrane modules in series and flow of the feed solution through each of the second channels of the multiple membrane modules in parallel, and flow of the feed solution through each of the first channels of the multiple membrane modules in series and flow of the draw solution through each of the second channels of the multiple membrane modules in parallel.

Alternatively, membrane module may comprise multiple membrane modules connected in series and in parallel.

Alternatively, membrane module may comprise multiple membrane modules connected in series and parallel to effect a tapered cascade flow path for the draw solution and the feed solution. The semipermeable membrane may comprise at least one of: thin-film composite (TFC) membranes and integrally asymmetric membranes.

According to a third aspect, there is provided a self-cleaning osmosis system comprising: the apparatus of the second aspect; a first energy recovery device provided along the draw solution inflow path upstream of where the draw solution inflow path is selectably divertable between the inlet of the first channel and the inlet of the second channel; and a second energy recovery device provided downstream of the draw solution effluent stream; wherein a portion of the draw solution effluent is diverted back to the first energy recovery device and a remainder of the draw solution effluent is passed through the second energy recovery device.

BRIEF DESCRIPTION OF FIGURES

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments of the present invention, the description being with reference to the accompanying illustrative drawings.

Fig. la is a schematic illustration of a first exemplary self-cleaning PRO process.

Fig. lb is a schematic illustration of a second exemplary self-cleaning PRO process.

Fig. 2a is a schematic illustration of a first exemplary co-current flow self-cleaning PRO

apparatus.

Fig. 2b is a schematic illustration of a first exemplary counter-current flow self-cleaning PRO apparatus.

Fig. 2c is a schematic illustration of a second exemplary counter-current flow self-cleaning PRO apparatus.

Fig. 3a is a schematic illustration of a second exemplary co-current flow self-cleaning PRO apparatus.

Fig. 3a is a schematic illustration of a third exemplary counter-current flow self-cleaning PRO apparatus.

Fig. 4a is a schematic illustration of a first exemplary embodiment of a connection of multiple

PRO modules of a large-scale self-cleaning PRO apparatus.

Fig. 4b is a schematic illustration of a second exemplary embodiment of a connection of

multiple PRO modules of a large-scale self-cleaning PRO apparatus.

Fig. 4c is a schematic illustration of a third exemplary embodiment of a connection of multiple PRO modules of a large-scale self-cleaning PRO apparatus.

Fig. 4d is a schematic illustration of a fourth exemplary embodiment of a connection of

multiple PRO modules of a large-scale self-cleaning PRO apparatus.

Fig. 4e is a schematic illustration of a fifth exemplary embodiment of a connection of multiple PRO modules of a large-scale self-cleaning PRO apparatus.

Fig. 4f is a schematic illustration of a sixth exemplary embodiment of a connection of multiple PRO modules of a large-scale self-cleaning PRO apparatus.

Fig. 5a is schematic illustration of an exemplary embodiment of a single-stage co-current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 5b is a schematic illustration of an exemplary embodiment of a single-stage counter- current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 6a- 1 is a schematic illustration of a first exemplary embodiment of a dual-stage co-current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 6a-2 is a schematic illustration of a first exemplary embodiment of a dual-stage counter- current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 6b- 1 is a schematic illustration of a second exemplary embodiment of a dual-stage co- current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 6b-2 is a schematic illustration of a second exemplary embodiment of a dual-stage

counter-current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 6c- 1 is a schematic illustration of a third exemplary embodiment of a dual-stage co-current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 6c-2 is a schematic illustration of a third exemplary embodiment of a dual-stage counter- current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 6d-l is a schematic illustration of a fourth exemplary embodiment of a dual-stage co- current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 6d-2 is a schematic illustration of a fourth exemplary embodiment of a dual-stage counter- current flow self-cleaning PRO system for osmotic energy harvesting.

Fig. 7a is a graph of experimental water flux with time.

Fig. 7b is a graph of experimental power density with time.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which a self-cleaning osmosis process 200, a self-cleaning osmosis apparatus 100 for the process 200, and an osmosis system 300 incorporating the process 200 and apparatus 100 may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the apparatus and process. Other embodiments may be utilized and structural, and logical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. The same reference numerals that are used throughout the figures refer to the same or similar parts of the invention for the various embodiments described. The following abbreviations may be used in the description below.

FS: feed solution

DS: draw solution

AL-DS: membrane orientation of active layer of the membrane facing the draw solution AL-FS: membrane orientation of active layer of the membrane facing the feed solution ERD: energy recovery device

The following description makes reference to pressure retarded osmosis (PRO) in the various embodiments of a self-cleaning osmosis process 200, self-cleaning osmosis apparatus 100 and system 300 by way of example only. It should be noted that the process 200, apparatus 100 and system 300 may be extended to other membrane systems with dual-channel flows, with one channel on each side of a semipermeable membrane, e.g., forward osmosis (FO) and pressure assisted osmosis (PAO), without being limited to only PRO.

Self-Cleaning PRO Process

Fig. 1 illustrates a basic concept of a self-cleaning PRO process 200. The self-cleaning PRO process 200 is realized through strategically alternating the membrane orientation in the self- cleaning PRO apparatus 100 without interrupting a continuous PRO operation. At each alternation between AL-DS and AL-FS, the direction of water flux is reversed accordingly. The reversed water flux not only maintains the continuous PRO operation but also plays a role in backwashing of the fouled membrane.

Fig. 1 illustrates one cycle of the self-cleaning PRO process 200 either starting from the AL- DS membrane orientation (Fig. la) or starting from the AL-FS membrane orientation (Fig. lb). Each cycle of the self-cleaning PRO process 200 can be divided into four phases, as will be described in greater detail below.

As shown in Fig. la, Phase 1 is when the semipermeable membrane 101 is operated in the AL- DS orientation, in which a feed solution 15 is input into a first channel of a PRO module 2 and a draw solution 9 is input into a second channel of the PRO module 2. A membrane 101 that generally comprises an active layer 110 and a support layer 130 separates the first channel from the second channel. In Phase 1, the active layer 110 of the membrane 101 faces the draw solution 9 in the second channel. Water flux (arrow 99) is in a direction from the first channel to the second channel as the feed solution 15 has a lower salinity or concentration of solute than the draw solution 9, and internal fouling 90 of the support layer 130 and membrane 101 occurs with progress of the PRO operation. As mentioned above, internal fouling 90 occurs with the clogging of foulants in the pores of the porous support layer 120 and/or the depositing of foulants on the back surface 112 of the membrane active layer 110. It is worthy to note that in Phase 1, external fouling would also occur on an exposed surface 132 of the support layer 130. External fouling is not explicitly illustrated in Fig. la since it plays a less significant role in the flux decline in most cases when compared to internal fouling.

When the membrane 101 is severely fouled and the flux has declined to below a first predetermined benchmark level, the PRO operation is switched into Phase 2 in which the membrane is operated in the AL-FS orientation. In Phase 2, the feed solution 15 is input into the second channel and the draw solution 9 is input into the first channel, so that the active layer 110 of the membrane 101 faces the feed solution 15 in the second channel. In Phase 2, the water flux has an opposite direction to that in Phase 1, i.e., water flux 99 in Phase 2 is from the second channel into the first channel. With the water permeating from the active layer 110 side to the support layer 130 side, the hydrodynamic drag force will help to detach and remove foulants 90 that are already deposited on the back surface 112 of the active layer 110 and clogged in the pores of the support layer 130. This indicates that the backwashing process initiates spontaneously in the PRO process after switching the membrane orientation, by switching the channels into which the feed solution 15 and the draw solution 9 are input. With the progress of the PRO operation in Phase 2, the foulants 90 will be removed from the membrane 101 and support layer 130, and the water flux 99 will be restored.

In Phase 3, with further progress of the PRO operation in the Phase 2 configuration, the foulants 90 on the feed side (i.e. the second channel through which the feed solution 15 is passed) will deposit on the active layer surface 110 and the flux 99 would again be declined. When the flux 99 has declined to below a second predetermined benchmark level, the PRO operation is switched into Phase 4 in which the membrane operation is switched to the AL-DS orientation by inputting the feed solution 15 into the first channel and inputting the draw solution 9 into the second channel. The second predetermined benchmark level of flux in Phase 3 may be the same as or different from the first predetermined benchmark level of flux in Phase 1 mentioned above. In Phase 4, the direction of water flux 99 is reversed again when compared with Phases 2 and 3. In Phase 4, foulants 90 on the membrane active layer surface 110 will be gradually moved away by the water flux 99 due to backwashing.

With the restoration of water flux 99 in the direction from the first channel to the second channel, the water convection would bring the foulants 90 from the feed solution 15 in the first channel towards and into the support layer 130 and eventually cause the internal fouling (as well as external fouling) again. This brings the process back to Phase 1. The whole self- cleaning PRO process is maintained by alternating the membrane orientation periodically between AL-DS and AL-FS where the four phases repeat again and again in different cycles. Notably, in the present self-cleaning osmosis process 200, the DS and FS are always kept separated in different flow passages and never mixed with each other in any proportion whatsoever.

Fig. lb shows the process 200 starting from the AL-FS orientation, although the whole process comprising the four phases is similar to that in Fig. la and thus is not described again in detail.

Self-Cleaning PRO Apparatus

Membranes applicable for the semipermeable membrane 101 of the apparatus 100 to perform the self-cleaning PRO process 200 should have (1) excellent selectivity and small structural parameter to provide high osmotic performance and (2) strong mechanical strength to resist the PRO hydraulic pressure, and work in both AL-DS orientation and AL-FS orientation. Such membranes can include but are not limited to thin-film composite (TFC) membranes and integral membranes (e.g. integrally asymmetric membranes). An integral membrane is preferred because it provides stronger mechanical stability in the AL-FS orientation during the PRO operation. The membrane 101 used in the self-cleaning PRO process 200 and apparatus 100 may be provided in modules and can comprise hollow fibre modules, spiral wound modules, or plate-and-frame modules, which are designed to take elevated pressure on either side of the membrane 101.

The DS 9 may include but is not limited to seawater, seawater desalination brine, industrial waste brine, or synthetic salt water, etc. The FS 15 may comprise impaired wastewater, pretreated wastewater, river water, brackish water, etc.

Alternation between the two orientations of the membrane 101 (i.e. between AL-FS and AL- DS) can be simply controlled by means including but not limited to valves installed in upstream and downstream flow passages of the PRO membrane module 2 respectively, as will be described in greater detail below with reference to Figs. 2a, 2b and 2c using two-way valves 17-24 and Figs. 3a and 3b using three-way valves 29-32 for different exemplary embodiments of the apparatus 100 of the PRO process 200.

Fig. 2a is an illustration of the PRO apparatus 100 configured in a co-current flow mode, while Fig. 2b shows the PRO apparatus 100 configured in a counter-current flow mode. For both flow modes as shown in Figs. 2a and 2b, the inflow of pressurized DS 9 into the PRO module 2 is controlled by two paralleled valves 17 and 18 which allow input of the DS 9 into the first channel A and the second channel B respectively of the PRO module 2. The inflow of the FS 15 into the PRO modules 2 is controlled by two paralleled valves 19 and 20 which allow input of the FS 15 into the second channel B and the first channel A respectively of the PRO module 2. The effluent 26 from the first channel A (either DS or FS) is directed to a corresponding DS or FS effluent stream (10 or 16 respectively) by two paralleled valves 21 and 22 respectively. The effluent 28 from the second channel B (either FS or DS) is directed to the corresponding FS or DS effluent stream (16 or 10 respectively) by two paralleled valves 23 and 24 respectively. The valves 17-24 can comprise ball valves, gate valves, needle valves, etc. The ball valve is preferred, according to an exemplary embodiment of the invention. In the operation of the self-cleaning PRO process 200, the paralleled valves 17 and 18, 19 and 20, 21 and 22, and 23 and 24 cannot both be open simultaneously.

Using the configurations shown in Figs. 2a and 2b, in Phase 1 (as described above with reference to Fig. la), valves 17, 19, 21, and 23 are open, while valves 18, 20, 22, and 24 are closed. The pressurized DS 9 flows through the valve 17 into the first channel A of the PRO module 2. Diluted DS 26 out of the first channel A flows through the valve 21 into the DS effluent stream 10 that will further flow to the ERDs (to be described in greater detail below). The FS 15 flows from valve 19 into the second channel B of the PRO module 2. Concentrated FS 28 out of the second channel B flows through valve 23 into the FS outlet stream 16.

During the switching from Phase 1 to Phase 2, the valves 18, 20, 22, and 24 are opened and the valves 17, 19, 21, and 23 are closed. When controlling the valves 17-24, the following sequence is preferred in the exemplary embodiment: open 18, close valve 17, open valve 20, close valve 19, open valve 24, close valve 23, open valve 22, close valve 21. In Phase 2, pressurized DS 9 flows through valve 18 via inlet pipe 27 into the second channel B of the PRO module 2. After passing through the PRO module 2, the diluted DS 28 coming from the second channel B flows through the valve 24 into the ERDs (to be described in greater detail below). The concentrated FS 26 coming from the first channel A flows through the valve 22 into the outlet of the FS stream 16.

During the switching from Phase 3 to Phase 4, valves 17, 19, 21, and 23 are opened again, while valves 18, 20, 22, and 24 are closed again. When controlling the valves 17-24, the following sequence is preferred in the exemplary embodiment: open valve 17, close valve 18, open valve 19, close valve 20, open valve 21, close valve 22, open valve 23, close valve 24. The flow of the DS 9 and FS 15 are the same as aforementioned for Phase 1.

Fig. 2c illustrates the counter-current flow mode in accordance with another exemplary embodiment of the PRO apparatus 100. However, the alternation of the membrane orientation in the self-cleaning process will change the stream flow direction in the first channel A and the second channel B. This configuration is specifically designed for the module connections shown in Fig. 6, described in greater detail below. The procedures to control the valves are similar to those in Figs. 2a and Fig. 2b except for a minor difference in the preferred sequence of adjusting the valves 17-24 when the switching the membrane orientation. When switching from Phase 1 to Phase 2, the preferred sequence is as follows: open valve 18, close valve 17, open valve 20, close valve 19, open valve 22, open valve 24, close valve 21, close valve 23.

The whole self-cleaning PRO process is realized by repeating the above operating procedures periodically.

Fig. 3 illustrates further exemplary embodiments of the apparatus 100, in which Fig. 3a is the apparatus 100 configured in a co-current flow mode, and Fig. 3b is the apparatus 100 configured in a counter-current flow mode. Compared to the embodiments in Fig. 2, the embodiments in Fig. 3 use three-way valves 29-32 to replace the valves 17-24. That is, a three- way valve 29 is used to replace the paralleled valves 17 and 18, another three way valve 30 replaces valves 19 and 20, another three way valve 31 replaces valves 21 and 22, and another three-way valve 32 replaces valves 23 and 24. The three-way valve 29-32 used in the self- cleaning PRO process has one inlet and two outlets. But two outlets cannot be open simultaneously. When one outlet is open, the other outlet is closed. In this way, the number of valves is reduced and the operation is simplified. The operating procedures are similar to those in Fig. 2 and will not be described again herein. Where the self-cleaning PRO apparatus 100 is configured for use in a large-scale PRO systems 300, the apparatus 100 will in practice comprise multiple PRO modules 2, and not just one PRO module 2 as shown in Figs. 2 and 3 Multiple PRO modules 2 may be connected as shown in Figs. 4a to 4f in the apparatus 100, in place of the single PRO module 2 depicted in Figs. 2 and 3. Proper connection of the multiple modules 2 in the apparatus 100 can maximize the water permeation rate and power output.

In principle, when multiple modules 2 are connected in the apparatus 100, the respective DS flow and FS flow can be in series, in parallel, or in both flow patterns. Fig. 4a illustrates that both the DS flow and FS flow are in series through the connected modules 2. Fig. 4b illustrates that both the DS flow and FS flow are in parallel through the connected modules 2. Fig. 4c illustrates that the DS flow is in parallel while the FS flow is in series through the connected modules 2. Fig. 4d illustrates that the DS flow is in series while the FS flow is in parallel through the connected modules 2. Fig. 4e illustrates that DS flow and FS flow are in both series and parallel through the connected modules 2.

It is worthwhile to note that while Figs. 4a to 4e only show the DS and FS in counter-current flow mode, the modes of co-current flow of DS and FS are also applicable. The DS flow and FS flow in parallel may help to use the osmotic driving force in the PRO process more efficiently. On the other hand, the series flow pattern may improve the inlet stream distribution in the modules 2 and facilitate system operation.

In one embodiment, Fig. 4f illustrates the design of a tapered cascade flow pattern for the module 2 connections. In this design, the DS and FS have to flow counter-currently. This design also includes both series and parallel flow pattern for both DS and FS. However, the number of modules 2 is in an ascending order in different passes from inlet to outlet. A pass is defined as the group of modules 2 through which DS flows in parallel. Fig. 4f shows a two pass design. The ratio of the number of modules 2 between the two passes is 3:4. In the DS flow line, the pressurized DS stream 25 from the main line is split into 3 ^χ branch lines flowing into the modules 2 in the first pass. The DS streams out of the first pass are then further increased to 4* branch lines towards the second pass, as the DS flow rate after the first pass is increased. The DS streams out of the second pass are then combined into the main stream line 26 for further energy harvesting. In the FS side, the FS stream from the main line 27 is split into 4x branch lines and flows into the second pass. The concentrated FS coming out from the second pass is further decreased to 3 ^χ branch lines flowing towards the first pass, as the FS flow rate from second pass is decreased. The effluent FS from the first pass is then combined in the main line 28. The ratio of module number between different passes is not limited to 3:4. It can be 1 :2, 2:3, 3:5, etc. The modules 2 can also be connected in three passes. The module number ratio can be 1 :2:3, 1 :3:5, 2:3:4, etc. The selected module number ratio between different passes is dependent on the designed DS dilution factor of the PRO system. The design in Fig. 4f can provide improved hydrodynamic conditions, enhanced osmotic efficiency in the system, optimized distribution of DS stream and FS stream in the module, and facilitated operation. In the different embodiments of the apparatus 100, the modules 2 connected in the apparatus 100 can have the same membrane orientation (i.e., AL-FS or AL-DS orientation) or they can be connected with mixed membrane orientations (i.e., both AL-FS and AL-FS orientation are formed). It is preferred to connect the modules 2 in the mixed membrane orientations with half of the modules 2 in AL-DS orientation and the other half in AL-FS orientation. This is because this connection can provide more stable permeate flow rate in a whole cycle of self-cleaning process, which can facilitate operation and maintenance.

The configurations to connect the modules 2 in Figs. 4a-4e can be directly incorporated into the self-cleaning PRO apparatus 100 as described above with reference to Figs. 2a, 2b and Fig. 3. To incorporate the module connection shown in Fig. 4f, the specific design of the apparatus 100 as described above with reference to Fig. 2c can be used.

Self-Cleaning PRO System

The various embodiments of the self-cleaning PRO apparatus 100 and process 200 as described above with reference to Figs. 1 to 4 can be incorporated into an energy recovering PRO system 300. Single-Stage System

A first exemplary embodiment of a single-stage PRO system 300 is shown in Fig. 5a, in which one of the embodiments of a co-current flow mode of the apparatus 100 is incorporated, where FS and DS flow co-currently through the PRO membrane module 2. The PRO module 2 has two flow channels A and B for DS flow and FS flow respectively, which are separated by the PRO membrane 101 inside the module 2. The first channel A where pressurised DS 9 is initially fed may be against either the membrane active layer 110 (e.g. as shown in Fig. la) or the support layer 130 (e.g. as shown in Fig. lb), and accordingly, the second channel B will be against the other side of the membrane 101 (i.e. the support layer 130 or the active layer 110 respectively).

The PRO system 300 includes a first energy recovery device (ERD) 3 that is used to recover the pressure energy from part of the diluted DS effluent and pressurize the DS 9 entering the apparatus 100. The first ERD 3 can include but is not limited to devices such as Francis Turbines (FT) (also known as reverse running pumps), Pelton Wheel, hydraulic turbocharger, recuperator, DWEER work exchanger, pressure exchanger, or close circuit device. A pressure exchanger is preferred due to its higher efficiency. A close circuit device also shows great promise according to the publication US20140007565A1 [3]. A second ERD 4 is provided for harvesting the energy from the PRO process 100. The second ERD 4 is typically a hydroturbine generator for electricity production. It can also be same ERD as the first ERD 3as aforementioned.

In the PRO system 300 as shown in Fig. 5a, low pressure DS 8 is passed through the first ERD 3 using a low pressure pump 5 for the intake of the low pressure DS 8. The DS 8 thus has low pressure before passing through the ERD 3. A booster pump 6 is used to boost the pressure of the DS 8 after passing through the first ERD 3 to the required pressure in the inlet DS stream 9 of the apparatus 100. Thus, the DS 9 is pressurized before entering the self-cleaning PRO apparatus 100 that performs the PRO process 200. After passing through the PRO apparatus 100 and process 200, DS 10 that is diluted and with increased flow rate is split into two streams 11 and 12. The first stream 11 has a flow rate similar to the permeating flow rate in the PRO module 2, while the second stream 12 has a flow rate similar to the flow rate of the inlet high pressure DS 9. The first stream 11 is run through the second ERD 4 and its pressure energy is recovered by the second ERD 4. The second stream 12 is flowed back to the first ERD 3 to transfer its pressure energy to the low pressure DS 8 intake stream 8. A low pressure pump 7 is used for the intake of FS 15 to the self-cleaning PRO apparatus 100. After passing through the PRO apparatus 100, the FS 16 is concentrated with reduced flow rate.

A second exemplary embodiment of the PRO system 300 is depicted in Fig. 5b in which any one of the embodiments of a counter-current flow mode of the apparatus 100 (e.g. as described above with reference to Figs. 2b, 2c or 3b) is incorporated, where FS and DS flow in opposite directions through the PRO membrane module 2. The embodiment of the self-cleaning PRO system 300 shown in Fig. 5b is otherwise similarly connected with respect to the first and second ERD 3, 4 as the embodiment shown in Fig. 5a.

Dual-Stage System

Dual-stage exemplary embodiments of the self-cleaning PRO system 300 are described below with reference to Figs. 6a- 1 to 6d-2. The designs shown in Figs. 6a- 1, 6b- 1, 6c- 1 and 6d-l are in a co-current flow mode where DS and FS flow co-currently, while the designs shown in Figs. 6a-2, 6b-2, 6c-2 and 6d-2 are in a counter-current flow mode where DS and FS flow counter- currently. Self-cleaning PRO apparatuses 100 and 100' are incorporated in the first stage 300-1 and second stage 300-2 respectively of the various embodiments of the dual-stage system 300 to perform the self-cleaning PRO process 200 in the system 300.

Compared to the single-stage PRO system 300 described above, the dual-stage PRO system 300 is to further utilise (in the second stage 300-2) the diluted DS generated in the first stage 300-1 to recover its osmotic energy. Therefore, more energy can be recovered from the dual- stage PRO system 300. Fig. 6a and Fig. 6b illustrate a dual-stage PRO system 300 with two identical but independent energy recovery systems 300-1 and 300-2. The diluted DSs from ERD 3 and ERD 4 in the first stage 300-1 converge and mix into stream 8' and are circulated to the second stage 300-2 that is a duplicate of the first stage 300-1. The applied hydraulic pressure in the DS stream 9' in the second stage 300-2 is lower than that in the DS stream 9 in the first stage 300-1. In the design of Fig. 6a, the FS is pumped to the two stages 300-1, 300-2 independently. In the design of Fig. 6b, the concentrated FS effluent from the first PRO stage 300-1 flows into the second stage 300-2 directly. Compared to the design in Fig. 6a, the design in Fig. 6b uses fewer pumps in the FS side, which can reduce the capital expenditure (CAPEX) for the whole system 300. However, the FS flowing into the second stage 300-2 has higher concentration, which may reduce the PRO performance.

Fig. 6c and Fig. 6d illustrate a dual-stage PRO system 300 using a transitional ERD 4 in between the two stages 300-1 and 300-2. The diluted DS stream 10 from the first stage 300-1 is directly passing through the ERD 4 without split-flow as illustrated in Fig. 6a and Fig. 6b. The diluted DS stream 10' from the second stage 300-2 is split into two streams 11 ' and 12'. Stream 12' with similar flow rate to stream 8 is recirculated to the ERD 3 in the first stage 300- 1 to pressurize the inlet DS 8. Stream 11 ' flows though ERD 4' for energy recovery. The transitional ERD 4 in these designs should provide the functions of both energy recovery and pressure adjustment for the next stage 300-2.

Compared to the designs in Fig. 6a and Fig. 6b, the designs in Fig. 6c and Fig. 6d can significantly reduce the CAPEX due to the use of fewer pumps (booster pumps/HP pumps/LP pumps) and ERDs. In addition, the fewer pumps and ERDs used in the designs of Fig. 6c and Fig. 6d may result in less frictional loss of energy and thus may offer higher energy output. Regarding the FS design in the system, Fig. 6c is identical to that in Fig. 6a and Fig. 6d is identical to that in Fig. 6b.

It will be understood that in large scale systems 300, the PRO modules 2 as shown in Figs. 5 a 5b, and 6a- 1 to 6d-2 may be alternatively replaced by any of the appropriate multiple module 2 connections described above with reference to Figs. 4a to 4f. Experimental Results

Using the apparatus 100 as embodied in Fig. 2c, experiments were performed to demonstrate the feasibility of the self-cleaning PRO process 200 in practical operation. Figs. 7a and 7b illustrate the experimental testing results. In the experiment, the FS 15 was reverse osmosis retentate (a real pretreated wastewater) from a wastewater reclamation plant in Singapore. It contained large quantities of organic foulants, inorganic minerals, colloidal particles, and microorganisms. The DS 9 was synthetic salt water of 1 M NaCl to simulate the seawater desalination brine. The membrane 101 is an integral cellulose triacetate (CTA) osmotic membrane. The effective applied pressure on the membrane was 10 bar. It should be noted that this pressure could be optimized to balance power density and backwash efficiency. The test started from the AL-DS membrane orientation (as shown in Fig. la - Phase 1). Obviously, a substantial decline of water flux and power density occurred in the first 1 hour (from -20 LMH to -11 LMH for flux, and - 5.6 W/m² to -3.1 W/m² for power density). In the subsequent 3 hours, the PRO performance decline became much slower (i.e., from -11.1 LMH to -6.6 LMH for water flux, and -3.1 W/m² to -1.8 W/m² for power density).

At the end of the 4 hours testing when the flux declined to -6 LMH, the membrane orientation was switched to the AL-FS orientation (as shown in Fig. la - Phase 2). The flux in AL-FS orientation experienced a gradual increase from -5 LMH to -10 LMH at the initial one hour. This is due to the self-cleaning of the fouled membrane 101, where foulant inside the membrane support layer 130 was moved away and the water permeability was restored. In the AL-FS orientation, the flux decline was marginal due to the inherent flux stability and less fouling tendency in this orientation.

After a period of operation, the operation was switched into the AL-DS orientation again (as shown in Fig. la - Phase 3). Obviously, the flux was nearly restored to the original level (-20 LMH). This is a complete cycle of the self-cleaning PRO process. By repeating the operation in the first cycle, the self-cleaning PRO process moved continuously. Two complete cycles are shown in Figs. 7a and 7b, demonstrating that the self-cleaning PRO process can be continuously operated. Advantages and Improvements

The presently disclosed self-cleaning PRO process 200, apparatus 100 and system 300 provide the following advantages over existing technology:

(1) Membrane cleaning is spontaneously carried out while ensuring continuous operation of the PRO system 300 without any downtime.

(2) The process 200, apparatus 100 and system 300 will not consume additional energy or fresh clean water in the cleaning process.

(3) The process 200, apparatus 100 and system 300 can achieve sufficient backwashing time and thus high cleaning efficiency without compromising energy production.

(4) The process 200 can thoroughly clean the membrane 101 in the whole module 2 without any dead space in the module 2.

(5) High salt concentration in the DS provides further help in the cleaning of microorganisms and mineral crystals on the membrane 101 during the alternation of the membrane orientation in the process 200. For example, when the bacteria of the biofilm on the membrane 101 are suddenly in contact with the high salt concentration, the water in their cell could be drawn out due to osmosis. The high salt concentration could also inhibit the transport of substances into the cell. All of these could help mitigate biofouling. The high salt concentration can also result in high ionic strength. It is able to increase the solubility product coefficient of mineral salts and help to destabilize the deposited colloidal and organic foulants. This would further help to remove foulants from the membrane 101.

(6) The process 200, apparatus 100 and system 300 can significantly improve the plant capacity, reduce the energy input in the operation, and improve the net energy production.

(7) The operation of the process 200 is simple, which could reduce the manpower in the operation and facilitate maintenance of the apparatus 100 and system 300.

The improved dual-stage design provides the following advantages:

(1) Reduce capital cost due to the use of fewer pumps and ERDs

(2) Reduce frictional energy loss in the operation with fewer ERDs and pumps

(3) Improve system energy output

The novel multiple module 2 connection provides the following advantages:

(1) Improve the hydrodynamic conditions on the membrane surface and module 2

(2) Optimize the distribution of DS and FS streams in the modules 2 Further improvements on the present invention can be considered.

(1) The optimized duration for alternating the membrane orientations should be predetermined for the self-cleaning PRO process 200.

(2) The optimized operating pressure should be determined to balance the power density and backwash efficiency.

(3) Intermittent injection of chemicals in the self-cleaning process 200 could be incorporated for helping the removal of tough foulants.

(4) The dual-stage PRO system 300 design could be further extended to multiple-stage design to improve the energy extraction from DS if the DS has very high concentration.

(5) Remote control system could be incorporated into the process 200 and system 300 designed to further facilitate control and maintenance.

(6) The designs in the present invention are not limited to PRO. They can be extended to other membrane systems with dual-channel flows on both sides of the membrane, e.g., forward osmosis (FO) and pressure assisted osmosis (PAO).

(7) Membranes that allow the operation in both AL-DS and AL-FS orientations and deliver high power density should be used. Integral membranes are particularly preferred. (8) The modules 2 should be designed to be able to withstand high pressure on both flow channels. Commercial Applications

The present process 200, apparatus 100 and system 300 have high potential for commercialization. A stand-alone PRO system 300 can be used to recover renewable osmotic energy from salinity-gradient pairs. The high salinity DS could include seawater, seawater desalination brine, waste industrial brine (e.g., oil & gas industry), synthetic salt water, etc. The low salinity FS could include river water, impaired wastewater, brackish water, etc.

In a hybrid system (e.g., PRO-RO), the present invention could be used in the following applications:

• Assisted seawater desalination

• Recover the osmotic energy from waste resources with salinity gradient

· Dilute the waste brine to reduce its environmental impact

• Concentrate the impaired wastewater for further treatment

Whilst there has been described in the foregoing description exemplary embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations and combination in details of design, construction and/or operation may be made without departing from the present invention.

References

1. She, Q.; Wang, R.; Fane, A. G.; Tang, C. Y., Membrane fouling in osmotically driven membrane processes: A review. J. Membr. Sci. 2016, 499, 201-233.

2. Thorsen, T.; Holt, T. Method and a System for Performing Maintenance on a Membrane Used for Pressure Retarded Osmosis. US2009/0008330A1, Jan. 8, 2009.

3. Efraty, A.; Adar, H. Power Generation by Pressure Retarded Osmosis in Closed Circuit without Need of Energy Recovery. US2014/0007564A1, Jan. 9, 2014.

Claims

1. A self-cleaning osmosis process comprising the steps of:

(c) repeating steps (a) and (b) for a number of cycles.

2. The process of claim 1, wherein the feed solution and the draw solution are always kept separate in different flow passages.

3. The process of claim 1 or claim 2, wherein the first duration comprises a length of time taken for flux across the semipermeable membrane from the second channel to the first channel to fall below a first predetermined level.

4. The process of claim 3, wherein the second duration comprises a length of time taken for flux across the semipermeable membrane from the first channel to the second channel to fall below a second predetermined level.

5. A self-cleaning osmosis apparatus comprising:

a membrane module having a first channel and a second channel, the first channel and the second channel separated by a semipermeable membrane, an active layer of the semipermeable membrane facing one of the first channel and the second channel; a draw solution inflow path being selectably divertable between an inlet of the first channel and an inlet of the second channel to flow a draw solution through only one of the first channel and the second channel at any one time;

a feed solution inflow path being selectably divertable between an inlet of the first channel and an inlet of the second channel to flow a feed solution through only the other of the first channel and the second channel while the draw solution is flowed through the one of the first channel and the second channel;

a draw solution effluent stream; and

a feed solution effluent stream;

wherein an outflow path from the first channel is selectably divertable between the draw solution effluent stream and the feed solution effluent stream in order to divert draw solution effluent from the first channel to the draw solution effluent stream and to divert feed solution effluent from the first channel to the feed solution effluent stream; and

wherein an outflow path from the second channel is selectably divertable between the draw solution effluent stream and the feed solution effluent stream in order to divert draw solution effluent from the second channel to the draw solution effluent stream and to divert feed solution effluent from the second channel to the feed solution effluent stream.

6. The apparatus of claim 5, wherein the membrane module comprises multiple membrane modules connected in series.

7. The apparatus of claim 5, wherein the membrane module comprises multiple membrane modules connected in parallel.

8. The apparatus of claim 5, wherein the membrane module comprises multiple membrane modules connected to effect flow of the draw solution through each of the first channels of the multiple membrane modules in parallel, and flow of the feed solution through each of the second channels of the multiple membrane modules in series, and flow of the feed solution through each of the first channels of the multiple membrane modules in parallel and flow of the draw solution through each of the second channels of the multiple membrane modules in series.

9. The apparatus of claim 5, wherein the membrane module comprises multiple membrane modules connected to effect flow of the draw solution through each of the first channels of the multiple membrane modules in series and flow of the feed solution through each of the second channels of the multiple membrane modules in parallel, and flow of the feed solution through each of the first channels of the multiple membrane modules in series and flow of the draw solution through each of the second channels of the multiple membrane modules in parallel.

10. The apparatus of claim 5, wherein the membrane module comprises multiple membrane modules connected in series and in parallel.

11. The apparatus of claim 5, wherein the membrane module comprises multiple membrane modules connected in series and parallel to effect a tapered cascade flow path for the draw solution and the feed solution.

12. The apparatus of any one of claims 5 to 11, wherein the semipermeable membrane comprises at least one of: thin-film composite (TFC) membranes and integrally asymmetric membranes.

13. A self-cleaning osmosis system comprising:

the apparatus of any one of claims 5 to 12;

a first energy recovery device provided along the draw solution inflow path upstream of where the draw solution inflow path is selectably divertable between the inlet of the first channel and the inlet of the second channel; and

a second energy recovery device provided downstream of the draw solution effluent stream;

wherein a portion of the draw solution effluent is diverted back to the first energy recovery device and a remainder of the draw solution effluent is passed through the second energy recovery device.