CN110902765A

CN110902765A - High-efficiency water treatment process

Info

Publication number: CN110902765A
Application number: CN201911344178.7A
Authority: CN
Inventors: 亚历克斯·德拉克; 托马尔·埃弗拉特
Original assignee: IDE Technologies Ltd
Current assignee: I D E 技术有限公司; IDE Technologies Ltd
Priority date: 2019-11-14
Filing date: 2019-12-24
Publication date: 2020-03-24
Also published as: WO2021095018A1; GB201918123D0; BR102020002545A2; MX2020001510A; PE20210967A1; CN110845043A; GB201916571D0; CL2019003771A1; GB2588925A; GB2588977A

Abstract

A method of treating water comprising delivering feed water to an input chamber, passing pressurized feed water from the chamber through at least one semi-permeable membrane to produce product water and brine, and passing the brine through a desalination unit to remove sparingly soluble salts present in a brine output, the brine output being subjected to a series of recirculation steps comprising: (i) returning the brine to the input chamber in a recirculation step to pass the brine through the semi-permeable membrane to re-concentrate the brine; and preparing for the next cycle step of its use for the removal of slightly soluble salts; and (ii) sequentially and progressively performing a series of recycling steps (i) to continue to re-concentrate the brine output and to crystallize low soluble salts from the recycled brine to increase recovery through membrane separation, wherein the saturation index of the low soluble salts in these steps is maintained within a predefined range of the controlled crystallization zone. The method further comprises an anti-fouling agent addition and/or deactivation step.

Description

High-efficiency water treatment process

Technical Field

The present invention relates to an improved Water treatment process, in particular an improved method for cleaning feed Water, such as industrial wastewater, mining contaminated wastewater, BWRO (Brackish Water Reverse Osmosis) brine and seawater.

Background

Water treatment processes are known in the art for providing clean water from seawater, brackish water or other industrial wastewater. For example, desalination processes exist to provide desalinated water from seawater. Reverse Osmosis (RO) occurs when a saline solution is compressed against a semi-permeable membrane at a pressure higher than its osmotic pressure. One example of such a process is the "plug flow desalination" method, which involves passing a pressurized feed liquid through a pressure vessel having a semi-permeable membrane. The feed liquor is then separated into non-pressurized desalted permeate ("product water") and pressurized brine waste liquor ("waste").

Filtration methods can also be used to clean water. Nanofiltration (NF) also relates to a method based on semi-permeable membrane filtration using cylindrical through-holes of nanometer size. Nanofiltration can be used to treat a variety of waters including ground water, surface water and sewage. Nanofiltration membranes have the ability to remove most of the dissolved salts.

However, these types of processes are often inefficient and highly dependent on the quality of the feedwater, which may also vary widely.

The treatment of fluids such as seawater, brackish water, industrial waste water, sewage, etc., typically involves the use of a semi-permeable membrane. In this process, fluid introduced into the system is pressed against the semi-permeable membrane at a pressure above the osmotic pressure of the fluid. As a result, the fluid is divided into two portions, the first portion comprising a product stream that is part of the fluid passing through the semi-permeable membrane, and the second portion comprising a brine stream that is part of the fluid not passing through the semi-permeable membrane. The separation or product amount to the amount of fluid introduced into the semipermeable membrane depends on the osmotic pressure and chemical composition of the fluid. In general, the fluid chemistry is the limiting factor on the maximum separation achievable in a semi-permeable membrane. It is desirable to provide an improved process (or method) that minimizes or overcomes the limitations of the chemical composition of the fluid.

It is an object of the present invention to provide an improved process for treating fluids, particularly water, which overcomes or at least alleviates the above disadvantages.

Disclosure of Invention

The present invention relates to an improved process for treating fluids through a semi-permeable membrane that overcomes or minimizes the above-mentioned limitations of the chemical composition of the fluid by gradually crystallizing low solubility salts to achieve a maximum separation of the fluid as controlled by the osmotic pressure of the fluid. The invention is based on a combination of progressive concentration of the fluid by passing it through a semi-permeable membrane and removal of low solubility salts by heterogeneous crystallization of the low solubility salts on seed crystals.

The present invention provides a method of treating water, the method comprising:

(a) delivering feed water to a first input chamber;

(b) passing pressurized feed water from a first input chamber through at least one semi-permeable membrane to produce a product water output and a brine output; and

(c) removing sparingly soluble salts present in a brine output by passing the brine through a desalination unit, the brine output undergoing a series of recirculation steps comprising:

(i) returning the brine output to the first input chamber in a recirculation step to pass the brine output through the semi-permeable membrane to re-concentrate the brine output; and preparing for the next cycle step of its use for the removal of slightly soluble salts; and

(ii) (ii) sequentially performing a series of recycling steps (i) in which the saturation index of one or more low soluble salts contained in the brine input is maintained within the following range, to continue to re-concentrate the brine output and to crystallize low soluble salts from the recycled brine to increase recovery through membrane separation:

(1) the logarithm (Log SI) of the saturation index of calcium carbonate is within the range of 0.5-2.0;

(2) the saturation index of calcium sulfate is within the range of 100-400%;

(3) the silica saturation index is within the range of 100-220%;

(4) the barium sulfate saturation index is within the range of 100-5000%; and

(5) the calcium fluoride saturation index is in the range of 100% to 400% to provide a controlled crystallization zone comprising micro-saturation conditions, wherein salt crystals grow only on low energy surfaces and cannot nucleate, below the unstable and metastable regions, and above the stable region.

The salt maintained at a predetermined supersaturation may maintain the salt in a controlled crystallization zone.

Preferably, the method comprises: a step of reducing the brine output pressure prior to step (c).

Preferably, the logarithm of the saturation index (Log SI) of calcium carbonate is kept in the range of 1.0 to 1.5. Preferably, the calcium sulfate saturation index is in the range of 150% to 300%. Preferably, the silica saturation index is in the range of 130% to 175%, and preferably, the barium sulfate saturation index is in the range of 150% to 1000%. Preferably, the calcium fluoride saturation index is in the range of 150% to 300%.

Optionally, the fluid may be provided with an anti-fouling agent to prevent or delay crystallisation of the salt. In the present invention, preferably the method includes the step of deactivating any anti-fouling agent in the brine output which may interfere with the salt crystallisation process. The inactivation step may comprise passing the brine output through an antiscalant inactivation unit.

Preferably, the method includes the step of injecting ferric chloride or sulfate to inactivate precipitates that may interfere with the scale inhibitor component, preferably provided at a feed rate of 0 to 10mg/L iron (Fe), more preferably at a feed rate of 0.05 to 0.5 mg/L.

Any suitable means may be provided for performing the step of controlled crystallisation of salt in the brine output, but preferably the brine output is recycled through the fluidized bed reactor or crystalliser unit, with the brine output being pumped through the bed of seed particles, i.e. the seed particles act as crystallisation sites.

Preferably, the hydraulic load in the fluidized bed crystallizer is maintained between 40 and 120 m/hr, preferably between 60 and 80m/hr, and/or the residence time in the fluidized bed crystallizer is maintained within the range of 1.0 to 12 minutes, more preferably within the range of 3.0 to 8.0 minutes.

Preferably, the lower portion of the bed is periodically drained and fresh seed material is introduced without interrupting reactor operation.

The method may further comprise the steps of: when a predetermined osmotic pressure is reached in the brine output, brine is drained from the first input chamber and fresh feed water is delivered to the chamber. For example, redirection may occur upon detection of a predetermined decrease in the efficiency of the semi-permeable membrane, such as by detection of a predetermined maximum salt concentration corresponding to the maximum osmotic pressure at which the membrane may operate.

The method may further include cleaning the input chamber during the removing of the brine.

Preferably, the process of the invention combines (1) fluid concentration through the semi-permeable membrane, (2) deactivation of the anti-fouling agent, (3) removal of low solubility salts by crystallization on the surface of the seed in a controlled crystallization zone, and if necessary (4) addition of the anti-fouling agent.

Preferably, the process is carried out in a semi-batch mode, which means that:

the brine produced in the semi-permeable membrane is recycled back through the anti-scaling agent deactivation, desalting and anti-scaling agent addition means.

Continuous production of product in the semipermeable membrane.

Continuous addition of raw/fresh liquid to replace the amount of product withdrawn from the system to maintain a constant amount of fluid in the system.

Mixing raw/fresh water with the produced brine before introduction into the semipermeable membrane.

After reaching the osmotic limit in the semi-permeable membrane, the entire system volume will be replaced by the stock/fresh.

In the above manner, the fluid is gradually concentrated each time it passes through the semipermeable membrane, and the concentration of the high-solubility salt is gradually increased. The low solubility salt, having reached a concentration in the controlled crystallization zone, will crystallize on the surface of the seed in the desalination unit to maintain its concentration in the controlled crystallization zone.

Since the saturation of the low solubility salts is maintained in the controlled crystallization zone and never beyond, the antiscalant dosing to the circulating fluid is maintained at a very low level, much lower than in standard semi-permeable membrane processes, without the need for gradual removal of the low solubility salts. If a phosphonate-based anti-scaling agent is used, the concentration of the anti-scaling agent (in terms of phosphate) should be maintained in the range of 0 to 1.5mg/L, preferably 0.5 to 1.0 mg/L, in terms of phosphate.

Since the fluid is gradually concentrated by the semi-permeable membrane, preferably no chemicals are needed to initiate the crystallization process, as opposed to the normal crystallization process where chemicals are used to initiate crystallization. If the ions making up the low solubility salt are in a non-stoichiometric ratio, a lower concentration of ions may be added to increase the degree of removal of the low solubility salt.

The deactivation of the anti-fouling agent is carried out at the surface of the seed crystal, so that no further deactivating agent is required. If low saturation of the final brine is required, other deactivating agents such as oxidizing agents (ozone, hydrogen peroxide, etc.) or precipitating agents (ferric chloride, ferric sulfate, aluminum chloride, aluminum sulfate, etc.) may be used. Preferably ferric chloride or ferric sulfate, wherein the concentration of the iron is 0.05-0.5 mg/L.

In order to allow the process to continue, redundancy may be provided for certain units so that when the osmotic pressure limit is reached, sufficient time is allowed for filling in the dope/new liquor to drain.

Description of the drawings.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a water treatment system suitable for performing a method according to one embodiment of the present invention.

FIG. 2 is a schematic diagram of a water treatment system suitable for performing a method according to another embodiment of the invention.

FIG. 3 is a graph showing the concentration of salt solution versus temperature in different crystallization zones.

FIG. 4 is a schematic diagram illustrating the process steps of an embodiment of the present invention. And

FIG. 5 is a graph showing concentration of salt solution versus treatment time in a controlled crystallization zone.

Detailed Description

The present invention provides an improved water treatment process to increase the efficiency of water treatment, particularly enabling the use of variable quality feed water at different recoveries.

Referring to FIG. 1 of the drawings, there is shown the basic components of a system that may be used to carry out the method of the present invention for treating feedwater. The figure shows a reverse osmosis process and system, but a nanofiltration membrane can be used instead of a reverse osmosis membrane. Feed water or brine (FW) is introduced into a single feed water chamber 2 from which it is directed through a delivery pipe 2i to a high pressure pump 6. The high pressure pump 6 then pressurizes the feed water before it passes through the reverse osmosis membrane 8, producing product water PW from the reverse osmosis membrane 8, as well as a concentrated brine stream CW. A pre-treatment unit, such as a filter unit (not shown), may optionally be provided to pre-treat the feed water before it passes through the membrane.

Typically, the brine waste stream is then discarded. In the present invention, the waste is recycled. The concentrated brine stream CW is fed back to the feed chamber 2 through an optional deactivation unit 40 and a desaturation unit 20 comprising a fluidized bed reactor. The deactivation unit deactivates components of the CW that may interfere with the salt precipitation process, while the desaturation unit reduces the saturation of slightly soluble salts in the brine stream CW. The operating conditions under which the process is carried out are particularly important for improving the recovery, which will be discussed in detail below. Ideally, the chamber is open to the atmosphere to provide an open loop system capable of reducing the pressure of the concentrated brine to near atmospheric pressure. Alternatively, a pressure exchanger or energy recovery system (not shown) may be provided to reduce the overall pressure of the brine stream.

The concentrated brine stream delivered back to the feed chamber 2 is mixed with additional feedwater FW still being delivered to the feed chamber and then circulated back through the system to provide more product water PW and concentrated brine CW. This cycle is repeated a number of times. The system may be dosed with an anti-fouling agent (not shown) to prevent fouling of the membrane.

The system is provided with a detector (not shown) for checking the efficiency of the reverse osmosis process. In this regard, it should be appreciated that as the feedwater salt concentration increases, the repeated recirculation of the brine flow will become less efficient over time. To solve this problem, the system is provided with a series of

gate valves

12, 14, 16 and 18. During normal recirculation of Concentrate Water (CW) through the system,

valves

12, 14 and 18 are open and valve 16 is closed. These valves are closed and valve 16 is opened once a predetermined decrease in process efficiency is detected. This temporarily shuts down the system/method to evacuate the chamber 2. Once empty, valve 16 is closed and the other valves are opened to allow fresh water to enter the chamber 2 and start a new recirculation process.

Figure 2 of the accompanying drawings shows an alternative example of a system for performing the method of the invention. The same features already discussed in relation to fig. 1 are given the same reference numerals and only the differences will be discussed in detail. The system comprises both a first feeding chamber 2 and a second feeding chamber 4, the method changing to the second feeding chamber during evacuation and refilling of the first feeding chamber. When the concentration of the feed water in the first chamber 2 reaches a predetermined level, the delivery pipe 2i is closed and feed water is introduced into the system from the second chamber 4 via the delivery pipe 4 i. The feed water is then passed through a desaturation unit 20 and pumped through a reverse osmosis membrane 8 to provide concentrated brine CW and product water PW. The concentrated brine is recirculated back to the second chamber 4 through the deactivation unit 40 and desaturation unit 20 and return line 4R for recirculation through the system along with additional feedwater.

When feed water is introduced from the second chamber, the highly concentrated brine CW in the first chamber is removed through the outlet tube 2 o. The chamber is cleaned and fresh feed water is introduced into the chamber 2.

Once the system detects a predetermined decrease in the efficiency of the reverse osmosis process, the system resumes use of the first chamber 2. In this regard, over time, as the first feed chamber acts, the feed water from the second chamber reaches a predetermined concentration, preferably around the maximum osmotic pressure at which the reverse osmosis membrane is operable, with the inlet 4i of the second chamber closed, and the feed water is again delivered from the first chamber 2 back to the first chamber through the system via the pressure exchanger 40 and return line 2R. The concentrated brine in the second chamber is removed through the outlet 4o and fresh water is delivered to the second chamber 4.

Any suitable number and arrangement of feed chambers and associated delivery and return conduits may be provided in the system. Several sets of chambers may be operated simultaneously.

Various pre-and post-treatments may be provided. In the case of a desaturation unit (20), this is only operable when a predetermined salt concentration is reached. An example of a desaturation unit is a fluidized bed type crystallizer, such as that sold under the name Crystalactor @.

Preferably, the controller of the system automatically transfers the redirected concentrated water from the first chamber to the second chamber and vice versa upon detection of a predetermined decrease in the efficiency of the overall process. Alternatively, the controller may automatically shut down the system as shown in FIG. 1.

The basic components of the system shown in fig. 1 and 2 perform the process of the present invention under predetermined conditions to provide the best results. Fluid concentration is gradually accomplished by passing the fluid through a semi-permeable membrane. A semi-permeable membrane is a biological or synthetic membrane that will allow certain molecules or ions to pass through by diffusion or occasionally by more specifically processes that promote diffusion, passive transport or active transport. The rate of passage depends on the pressure, concentration and temperature of the molecules or solutes on both sides, as well as the permeability of the membrane to each solute. Depending on the membrane and solute, permeability may depend on the size, solubility, nature, or chemical nature of the solute.

When a fluid is introduced into the semipermeable membrane and pressed against the semipermeable membrane at a pressure above the osmotic pressure of the fluid, the fluid is divided into two portions. The first part is the part of the fluid that passes through the semi-permeable membrane, which part is called the product. The second part is the part where the fluid does not pass through the semi-permeable membrane, this part being called saline.

The saline fraction contains more salt than the product fraction because normally the salt is rejected by the semi-permeable membrane. The salt in the brine can be divided into two parts: high solubility salts, such as sodium chloride, potassium chloride, and the like, and low solubility salts, such as calcium carbonate, calcium sulfate, barium sulfate, silica, and the like.

During concentration of the fluid in the semipermeable membrane, low solubility salts may exceed the solubility limit of these salts, and as a result, crystallization may begin. "Low saturated salt" is a salt having a concentration below the solubility limit, "supersaturated salt" is a salt having a concentration above the solubility limit, and "saturated salt" is a salt having a concentration equal to the solubility limit.

The crystallization process depends, among other things, on the degree of supersaturation of the salt. Below saturation (below the solubility limit), there is not enough energy to start any type of crystallization (stable region). In the case of high supersaturation, there is sufficient energy to initiate the crystallization process (in the unstable region) anywhere (unstable region) -all available surfaces being the same as in the solution. In the case of low supersaturation, there is only enough energy on the available surface for crystallization, while there is not enough energy in the solution (transferable zone) for crystallization. At very low supersaturation, crystallization will only begin on some of the available surfaces with the lowest surface energy (controlled crystallization zones). These regions are shown in figure 3 of the drawings.

Antiscalants are commonly used to prevent or retard the crystallization process of low solubility salts on the surface of semi-permeable membranes. Various antiscalants have been developed specifically on the market to prevent or delay the crystallization of different low solubility salts. The anti-fouling agent is limited to a certain supersaturation threshold above which crystallisation will occur immediately, below which crystallisation will start after a certain period of time (referred to as the "induction time").

Seed materials are commonly used to reduce the concentration of low solubility salts in solution by crystallizing the salt at the surface of the seed. In order to control the crystallization process on the seed surface, a degree of supersaturation of the low solubility salt should be maintained. The supersaturation should be high enough to start the crystallization process immediately with the shortest induction time and low enough to prevent crystallization processes in solution and on poor surfaces with high surface energy. Therefore, the supersaturation of the low solubility salt should be in the "controlled crystallization zone" as shown in fig. 3 (and fig. 5).

Thus, the process of the present invention carefully controls the crystallization of salts present in the brine stream by maintaining certain conditions, particularly to maintain low solubility salts in the controlled crystallization zone as the brine output is recycled through the process. In order to control the crystallization process of calcium carbonate on the surface of the seed crystal, the supersaturation degree (logarithm of saturation index) of calcium carbonate is maintained between 0.5 and 2.0, preferably between 1.0 and 1.5. In order to control the crystallization process of calcium sulphate on the surface of the seed, the saturation degree (saturation index) of the calcium sulphate is kept between 100% and 400%, preferably between 150% and 300%. In order to control the crystallization process of the silica on the surface of the seed crystal, the supersaturation degree (saturation index) of the silica is maintained between 100% and 220%, preferably between 130% and 175%.

These parameters are different from those used in all high recovery reverse osmosis systems where recovery is limited by the chemistry of sparingly soluble salts. Typically, these systems operate in the following saturation index ranges:

in the system limited by calcium sulfate, the saturation range of the calcium sulfate is 400-450%;

a calcium carbonate-limited system, wherein the calcium carbonate saturation range (Log SI) is 2.2-2.9;

a system limited by barium sulfate, the barium sulfate saturation index ranging from 6,000% to 8,000%;

a calcium fluoride-limited system having a calcium fluoride saturation index in the range of 8,000% to 12,000%; and

the silica saturation index of the system is limited by silica, and the silica saturation index ranges from 200% to 250%. For example, please refer to journal of membrane science (2016) "scale formation and control in high pressure membrane water treatment systems: pages 1 to 16, especially page 11, of the overview ". The present invention maintains one or more of these parameters at a low level within the controlled crystallization zone between curves A-D and C-F shown in FIG. 1, and more preferably between curves B-E and C-F, with the values for each salt provided in the table below:

if a scale inhibitor is used to prevent or delay the crystallization process on the surface of the semipermeable membrane, the supersaturation degree of the low-solubility salt should be maintained in the same range as described above in order to control the crystallization process on the surface of the seed crystal. However, due to the presence of the anti-fouling agent, the anti-fouling agent activity should be eliminated before the liquid containing the low-solubility salt and the anti-fouling agent contacts the surface of the seed crystal. The scale inhibitor activity can be eliminated by a number of different methods including: (1) chemical additives to trap the scale inhibitor (e.g., ferric chloride, ferric sulfate, aluminum chloride, aluminum sulfate, etc.), (2) chemical additives to destroy the scale inhibitor (e.g., ozone, hydrogen peroxide, etc.), or (3) provide surfaces to adsorb the scale inhibitor (e.g., seed surfaces, etc.), etc.

The process of the present invention may use different types of equipment to crystallize the low solubility salt on the seed, such as a chemical reactor followed by a clarifier, a fluidized bed reactor, and the like. Fluidized bed reactors are preferred and have advantages over other crystallization processes due to high flow rates and short residence times. The principle of operation of a fluidized bed reactor is as follows: the reactor section is filled with suitable seed particles; a fluid, in this case brine produced by the semipermeable membrane, is pumped up through the bed of particles to maintain it in a fluidized state. The seed particles serve as crystallization sites; they provide a high surface area that can reduce the energy required for precipitation. As the crystals become progressively heavier, they move progressively towards the bottom of the bed. The lower part of the bed was periodically drained without interrupting the operation of the reactor and fresh seed material was introduced. The upward velocity applied is in the range of 40 to 120 m/hr, preferably 60 to 80 m/hr. The residence time of the liquid in the reactor is from 2.0 to 12.0 minutes, preferably from 3.0 to 8.0 minutes.

The method of the invention incorporates the following criteria to provide optimized conditions. (1) Concentrating the liquid by the semipermeable membrane; (2) the scale inhibitor is deactivated; (3) removal of low solubility salts by crystallization on the seed surface, (4) addition of anti-fouling agent, as shown in figure 4 of the accompanying drawings.

The process is carried out in a semi-batch mode. Semi-batch mode means:

the brine produced in the semi-permeable membrane is recycled back through the antiscalant deactivation, desalting and antiscalant addition unit.

Continuous production of product in the semipermeable membrane.

In the above mode, the fluid is gradually concentrated each time it passes through the semipermeable membrane, and the concentration of the high-solubility salt is gradually increased. The low solubility salt, having reached a concentration in the controlled crystallization zone, will crystallize on the surface of the seed in the desalination unit to maintain its concentration in the controlled crystallization zone.

Since the saturation of the low solubility salts is maintained in the controlled crystallization zone and never beyond, the antiscalant dosing to the circulating fluid is maintained at a very low level, much lower than in standard semi-permeable membrane processes, without the need for gradual removal of the low solubility salts. If a phosphonate-based scale inhibitor is used, the concentration of scale inhibitor (in terms of phosphate) should be maintained in the range of 0 to 1.5mg/L, preferably 0.5 to 1.0 mg/L, in terms of phosphate.

Since the fluid is gradually concentrated by the semi-permeable membrane, no chemicals are needed to initiate the crystallization process, as opposed to the normal crystallization process where chemicals are used to initiate crystallization. If the ions comprising the low solubility salt are in a non-stoichiometric ratio, a lower concentration of ions may be added to the system to increase the degree of removal of the low solubility salt.

The deactivation of the anti-fouling agent is carried out at the surface of the seed crystal, so that no further deactivating agent is required. In the case where low saturation of the final brine is required, other inactivating agents such as oxidizing agents (ozone, hydrogen peroxide, etc.) or precipitating agents (ferric chloride, ferric sulfate, aluminum chloride, aluminum sulfate, etc.) may be used, and ferric chloride or ferric sulfate is preferred, with the concentration of iron being 0.05 to 0.5 mg/L.

In order to allow the process to continue, redundancy may be provided for certain units to allow sufficient time for drainage when the osmotic pressure limit is reached and the dope/new is filled.

Claims

1. A method of treating water, the method comprising:

(a) delivering feed water to a first input chamber;

(b) passing pressurized feedwater from the first input chamber through at least one semi-permeable membrane to produce a product water output and a brine output; and

(i) returning the saltwater output to the first input chamber in a recirculation step such that the saltwater output passes through a semi-permeable membrane to re-concentrate the saltwater output; and preparing for the next cycle step of its use for the removal of slightly soluble salts; and

(2) the saturation index of calcium sulfate is within the range of 100-400%;

(3) the silica saturation index is within the range of 100-220%;

(4) the barium sulfate saturation index is within the range of 100-5000%; and

2. The method of claim 1, further comprising the step of deactivating any scale control agent in the brine output that may interfere with the salt crystallization process.

3. The method of claim 1 or 2, further comprising reducing the pressure of the brine output prior to step (c).

4. The process according to claim 1 or 2 or 3, wherein the logarithm of the saturation index of calcium carbonate is kept in the range of 1.0 to 1.5.

5. The process according to any of the preceding claims, wherein the calcium sulphate saturation index is maintained in the range of 150% to 300%.

6. The process according to any of the preceding claims, wherein the silica saturation index is maintained in the range of 130% to 175%.

7. The process according to any one of the preceding claims, wherein the barium sulfate saturation index is maintained in the range of 150% to 1,000%.

8. The method according to any one of the preceding claims, wherein the calcium fluoride saturation index is maintained in the range of 150% to 300%.

9. The method of any preceding claim, wherein the fluid is treated with an anti-fouling agent to prevent or delay crystallisation of the salt.

10. A method according to claim 9, wherein the method comprises the step of adding a phosphonate based anti-fouling agent, preferably providing phosphate in the range of 0.5-1.0 mg/L.

11. The method of claim 9 or 10, further comprising the step of deactivating any anti-scaling agent in the brine output that may interfere with the salt crystallization process.

12. The method of claim 11, wherein the deactivating step comprises passing the brine output through an antiscalant deactivation unit.

13. A method according to claim 11 or claim 12, wherein the method includes the step of injecting ferric chloride or sulphate for deactivation, preferably providing Fe at a dose rate of between 0 and 10 mg/L.

14. The method according to any one of the preceding claims, wherein the brine output is recycled through a fluidized bed crystallizer unit, wherein the brine output is pumped through a bed of seed particles, the seed particles serving as crystallization sites, to perform a controlled crystallization step of salt in the brine output.

15. The method of claim 14, wherein the crystallizer unit also deactivates any anti-fouling agent in the fluid.

16. The method according to claim 14 or 15, wherein the hydraulic load in the fluidized bed crystallizer is maintained between 40-120 m/hr.

17. The process according to claim 14, 15 or 16, wherein the residence time in the fluidized bed crystallizer is maintained in the range of 1.0 to 12 minutes.

18. The method of any one of claims 14 to 17, further comprising periodically withdrawing a lower portion of the bed and introducing fresh seed material without interrupting operation of the crystallizer unit.

19. The process of any one of the preceding claims, further comprising continuously adding fresh feed fluid to replace an amount of product withdrawn from the process to maintain a constant amount of fluid in the process.

20. The method of claim 17, further comprising periodically replacing the entire amount of fluid with fresh feed fluid after the osmotic pressure limit is reached in the semi-permeable membrane.

21. The method of any one of the preceding claims, wherein no chemicals are added to initiate the crystallization process step.