BR102020002545A2 - high efficiency water treatment process - Google Patents

Abstract

high efficiency water treatment process. is a process for treating water, comprising delivering feedwater to an inlet chamber, passing pressurized feedwater from the chamber through at least one semipermeable membrane to produce a product water and a brine, and moderately removing salts. present in the brine by passing it through a salt removal unit, the brine being subjected to a series of recirculation steps, which comprise: (i) returning the brine to the inlet chamber in a circulation step to passing the brine through the semipermeable membrane to reconcentrate the brine; and prepare it for a next step of circulation to remove moderately soluble salts; and (ii) carry out a series of recirculation steps (i) for the continuous reconcentration of the brine and crystallization of poorly soluble salts from the recirculated brine in order to increase the recovery by membrane separation, in which the saturation indices of salts are little solubles during these steps are kept within predefined ranges within a controlled crystallization zone. the process also includes steps of adding and/or deactivating antifoulings.

Description

HIGH EFFICIENCY WATER TREATMENT PROCESS

[001] This invention relates to an improved water treatment process, in particular an improved method for cleaning feedwater such as industrial effluent, mining impacted effluent, BWRO brine and seawater.

FIELD OF TECHNIQUE

[002] Water treatment processes are known in the art to provide clean water from sea water, brackish water or other industrial effluents. For example, there are desalination processes to provide desalinated water from seawater. Reverse osmosis (RO) occurs when the salt water solution is compressed against semipermeable membranes at a pressure greater than its osmotic pressure. An example of this process is the “Plug-Flow Desalination” method which involves passing the pressurized feed stream through pressure vessels with semi-permeable membranes. The feed then separates into an unpressurized stream of desalinated permeate (“product water”) and a pressurized stream of brine effluent (“waste product”).

[003] Filtration methods can also be used to clean water. Nanofiltration (NF) also involves a method based on filtering a semipermeable membrane that uses cylindrical pores the size of nanometers. Nanofiltration can be used to treat all types of water including soil, surface and wastewater. Nanofiltration membranes have the ability to remove a significant fraction of dissolved salts.

[004] However, the efficiency of these types of processes is generally low and is highly dependent on the quality of the feedwater, which can also be highly variable.

[005] The treatment of fluids such as sea water, brackish water, industrial effluents, waste water etc. usually involve the use of semi-permeable membranes. During the process, the fluid introduced into the system is compressed against the semi-permeable membrane at a pressure greater than the osmotic pressure of the fluid. As a result, the fluid separates into two parts, a first part comprising a product stream, which is part of the fluid that passes through the semipermeable membrane, and a second part comprising a brine stream, which is part of the fluid that does not pass through the semi-permeable membrane. The degree of separation, or the ratio between the volume of the product and the volume of fluid introduced into the semipermeable membrane, depends on the osmotic pressure and the chemical composition of the fluid. Generally, the chemical composition of the fluid is the limiting factor on the maximum degree of separation that can be achieved in the semi-permeable membrane. It is desirable to provide an improved process (or method) that minimizes or overcomes the limitation of fluid composition.

[006] It is an object of the present invention to provide an improved process for the treatment of fluids, in particular water, which overcomes or at least alleviates the aforementioned disadvantages.

SUMMARY OF THE INVENTION.

[007] The present invention relates to an improved process for treating fluids by semipermeable membranes that overcomes or minimizes the limitation of the chemical composition of fluids mentioned above by gradual crystallization of low solubility salts, thus allowing the fluid to reach the degree separation that is governed by the osmotic pressure of the fluid. The present invention is based on the combination of a gradual concentration of fluids, passing the fluid through semipermeable membranes, and the removal of low solubility salts by heterogeneous crystallization of the low solubility salts in seeds.

• (i) retornar a saída de salmoura para a primeira câmara de entrada na etapa de circulação para passar a saída de salmoura através da membrana semipermeável para reconcentração da saída de salmoura; e prepará-la para a próxima etapa de circulação de remoção de sais moderadamente solúveis; e
• (ii) realizar uma série de etapas de circulação passo a passo (i) para reconcentração contínua da saída de salmoura e cristalização de sais pouco solúveis a partir da salmoura recirculada com o objetivo de aumentar a recuperação por separação de membrana, em que durante essas etapas de índices de saturação de um ou mais dos seguintes sais de baixa solubilidade contidos na entrada da salmoura é mantido dentro da faixa:
  • (1) o logaritmo de carbonato de cálcio do índice de saturação (Log SI) é na faixa de 0,5 a 2,0;
  • (2) o índice de saturação sulfato de cálcio é na faixa de 100% a 400%;
  • (3) o índice de saturação de sílica é na faixa de 100% a 220%;
  • (4) o índice de saturação de sulfato de bário é na faixa de 100% a 5.000%;
  • (5) o índice de saturação de fluoreto de cálcio é na faixa de 100% a 400%; e

- fornecer uma zona de cristalização controlada que compreende condições ligeiramente saturadas em que cristais de sais crescem apenas em uma superfície de baixa energia e não pode nuclear, a zona de cristalização controlada que é abaixo de uma zona instável e metaestável e acima da zona estável.[008] The present invention provides a process for treating water, the method comprises:
(a) distributing feed water to a first inlet chamber;
(b) passing pressurized feed water from the first inlet chamber through at least one semi-permeable membrane to produce a product water outlet and a brine outlet; and
(c) remove moderately soluble salts present in the brine outlet, passing the brine through a crystallizing unit in a series of recirculation steps, comprising:

• (i) returning the brine outlet to the first inlet chamber in the circulation step to pass the brine outlet through the semipermeable membrane for reconcentration of the brine outlet; and preparing it for the next circulating step of removing sparingly soluble salts; and
• (ii) carry out a series of step-by-step circulation steps (i) for continuous reconcentration of the brine output and crystallization of sparingly soluble salts from the recirculated brine in order to increase recovery by membrane separation, during which saturation index steps of one or more of the following low solubility salts contained in the brine input is kept within range:
  • (1) the calcium carbonate logarithm of the saturation index (Log SI) is in the range of 0.5 to 2.0;
  • (2) the calcium sulfate saturation index is in the range of 100% to 400%;
  • (3) the silica saturation index is in the range of 100% to 220%;
  • (4) the barium sulfate saturation index is in the range of 100% to 5,000%;
  • (5) the calcium fluoride saturation index is in the range of 100% to 400%; and

- provide a controlled crystallization zone comprising slightly saturated conditions in which salt crystals only grow on a low energy surface and cannot nuclear, the controlled crystallization zone which is below an unstable and metastable zone and above the stable zone.

[009] The maintenance of the predetermined degree of supersaturation keeps the salt in a controlled crystallization zone.

[010] Preferably, the process includes the step of reducing the brine outlet pressure before step (c).

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

[012] Optionally, the fluid can be provided with an anti-scalant to prevent or delay the crystallization of salts. In the present invention, the process preferably includes the step of deactivating any anti-scalant in the brine output that may interfere with the salt crystallization process. The deactivating step may comprise passing the brine output through an anti-scalant deactivation unit.

[013] Preferably, the process includes the step of injecting ferric chloride or ferric sulfate to deactivate precipitation that can interfere with antifouling components, which are preferably supplied at a dosage rate between 0 to 10 mg/l as Fe, more preferably 0.05 to 0.5 mg/l as Fe.

[014] Any suitable apparatus can be provided to carry out the controlled crystallization step of the salts in the brine outlet, but preferably the brine outlet is recirculated through a fluidized bed reactor or crystallizing unit, in which the brine outlet is pumped through a bed of seed particles, the seed particles acting as sites of crystallization.

[015] Preferably, the hydraulic load in the fluidized bed crystallizer is kept in the range between 40 to 120 m/h, preferably 60 to 80 m/h and/or the resistance time in the fluidized bed crystallizer is kept in the range of 1, 0 to 12 minutes, more preferably 3.0 to 8.0 minutes.

[016] Periodically, without interrupting the reactor operation, a lower portion of the bed is preferably discharged, and fresh seed material is introduced.

[017] The process can also include the step of removing the brine outlet from the first inlet chamber when it reaches a predetermined osmotic pressure in the brine outlet and delivers fresh feed water to that chamber. For example, a reorientation can occur by detecting a predetermined reduction in the efficiency of the semipermeable membrane, such as by detecting a predetermined maximum salt concentration corresponding to the maximum osmotic pressure that the membrane can operate.

[018] The process may further comprise cleaning the inlet chamber during brine removal.

[019] The process of the present invention preferably combines (1) fluid concentration by semipermeable membranes, (2) deactivation of the anti-scalant, (3) removal of low solubility salts by crystallization on the surface of the seeds in a controlled crystallization zone and, where necessarily, (4) addition of antifouling.

• • a salmoura produzida na membrana semipermeável é recirculada de volta através da desativação do antiincrustante, remoção de sal e unidade de adição de antiincrustante.
• • o produto é produzido continuamente na membrana semipermeável.
• • o fluido bruto/fresco é adicionado continuamente para substituir o volume de produto que é retirado do sistema, para manter o volume constante do fluido no sistema.
• • a água bruta/fresca é misturada com a salmoura produzida antes da introdução na membrana semipermeável.
• • Todo o volume do sistema é substituído pelo fluido bruto/fresco após atingir a limitação da pressão osmótica na membrana semipermeável.

[020] The process preferably operates in semi-batch mode, which means:

• • the brine produced in the semi-permeable membrane is recirculated back through the antifouling deactivation, salt removal and antifouling addition unit.
• • the product is produced continuously on the semi-permeable membrane.
• • raw/fresh fluid is continuously added to replace the volume of product that is withdrawn from the system, to maintain a constant volume of fluid in the system.
• • the raw/fresh water is mixed with the brine produced before introducing it into the semi-permeable membrane.
• • The entire system volume is replaced by the raw/fresh fluid after reaching the osmotic pressure limitation in the semi-permeable membrane.

[021] In the manner described above, the fluid is gradually concentrated every time it passes through the semi-permeable membrane, the concentration of high solubility salts gradually increases. Low solubility salts, whose concentration reaches the controlled crystallization zone, are crystallized on the surface of the seeds in the salt removal unit so that their concentration is maintained within the controlled crystallization zone.

[022] As long as the degree of saturation of low solubility salts is maintained in the controlled crystallization zone and never exceeds this zone, the anti-scalant, dosed into the circulated fluid, is kept at a very low level, much lower compared to standard semipermeable membrane process without gradual removal of low solubility salts. In the case of anti-scalant based phosphonate, the anti-scalant concentration is maintained in the range of 0 to 1.5 mg/l as phosphate, preferably in the range of 0.5 to 1.0 mg/l as phosphate.

[023] Since the fluid is gradually concentrated by the semi-permeable membrane, preferably no chemicals are needed to initiate the crystallization process, in contrast to common crystallization processes in which chemicals are used to initiate crystallization. If the ions that make up the low solubility salt are in a non-stoichiometric ratio, the ion whose concentration is lower can be added to increase the extent of removal of the low solubility salt.

[024] The deactivation of the anti-scalant is done on the surface of the seeds, so no additional deactivating agent is needed. For cases where the low saturation degree of the final brine is required, additional deactivating agents can be used, such as oxidizers (ozone, hydrogen peroxide etc.) or precipitation agents (ferric chloride, ferric sulfate, aluminum chloride, sulphate aluminum etc.), preferably ferric chloride or ferric sulfate at a concentration of 0.05 to 0.5 mg/l as Fe.

[025] To make the process continuous, some of the units can be provided with redundancy, to allow enough time for drainage when the osmotic pressure limit is reached and to replenish with raw/fresh fluid.

BRIEF DESCRIPTION OF THE DRAWINGS.

[026] Modalities of the invention shall not be described, by way of example only, with reference to the attached drawings in which:

[027] Figure 1 is a schematic diagram of a water treatment system suitable for carrying out the process according to an embodiment of the present invention;

[028] Figure 2 is a schematic diagram of a water treatment system suitable for carrying out a process according to another embodiment of the present invention;

[029] Figure 3 is a graph of concentration in relation to the temperature of the saline solution, which illustrates the different zones of crystallization;

[030] Figure 4 is a schematic diagram illustrating the process steps of an embodiment of the present invention; and

[031] Figure 5 is a graph of concentration versus process time for a saline solution, which illustrates a controlled crystallization zone.

DETAILED DESCRIPTION.

[032] The present invention provides an improved water treatment process to improve the efficiency of water treatment, particularly to allow feedwater of varying quality to be used with different recovery rates.

[033] Referring to Figure 1 of the attached drawings, the basic components of a system that can be used to carry out a process of the invention for the treatment of feedwater are illustrated. The Figure shows a reverse osmosis system and process, but the nanofiltration membrane can be used as an alternative to the reverse osmosis membrane. The feed water or seawater (FW) is introduced into a single feed chamber 2 from which it is directed through a manifold 2i to a high pressure pump 6. The high pressure pump 6 then pressurizes the water of feed before its passage through a reverse osmosis membrane 8 from which product water PW is produced, together with a stream of concentrated brine CW. A pre-treatment unit, such as a filter unit (not shown), optionally can be provided to pre-treat the feed water before it passes through the membrane.

[034] Conventionally, a brine refuse stream would then be discarded. In the present invention, repeated recycling of this waste product occurs. The concentrated brine stream CW is distributed back to the feed chamber 2 via an optional deactivation unit 40 and a desaturation unit 20 comprising a fluidized bed reactor. The deactivation unit deactivates components within the CW that should interfere with a salt precipitation process and the desaturation unit reduces the saturation level of sparingly soluble salts contained in the CW brine stream. The operating conditions to carry out this process are particularly important to provide improved recovery and these are discussed in more detail below. Ideally, the chamber is open to atmosphere to provide an open loop system that allows for a reduction in concentrated brine pressure to near atmospheric pressure. Alternatively, a pressure exchanger or energy recovery system (not shown) can be provided to reduce the overall pressure of the brine stream.

[035] The concentrated brine stream distributed back to feed chamber 2 is mixed with additional feed water FW which is still distributed to the chamber and then recycled back through the system to provide more PW product water and concentrated brine CW. This recycling circuit is repeated a number of times. The system can be dosed with antifouling (not shown) to prevent membrane peeling.

[036] The system is equipped with a detector (not shown) to verify the efficiency of the reverse osmosis process. In this regard, it should be considered that repeated recycling of the brine stream will reduce the efficiency of the process over time as the salt concentration of the feed water increases. To solve this problem, the system is provided with a series of gate valves 12, 14, 16 and 18. During normal recycling of concentrated water (CW) through the system, valves 12, 14 and 18 are opened and the valve 16 closed. Once a predetermined reduction in process efficiency is detected, these valves are closed and valve 16 opened. This temporarily closes the system/process to empty chamber 2. Once empty, valve 16 is closed and the other valves opened to allow fresh water to enter chamber 2 and a new recycling process is started.

[037] Figure 2 of the attached drawings illustrates an alternative example of a system to carry out the process of the present invention. Identical features already discussed in relation to Figure 1 are given the same reference numbers and only the differences will be discussed in detail. The system includes both a first feed chamber 2 and a second feed chamber 4 with the process shifting to the second feed chamber during emptying of the first feed chamber. When the concentration of feed water in the first chamber 2 reaches a predetermined level, the distribution tube 2i is closed and the feed water is introduced into the system from a second chamber 4 by means of a distribution tube 4i. This feed water is then passed through desaturation unit 20, pumped through reverse osmosis membrane 8 to provide concentrated brine CW and product water PW. The concentrated brine is recycled back to the second chamber 4 via deactivation unit 40 and desaturation unit 20 and a return tube 4R to recycle through the system with more feed water.

[038] While feed water is introduced from the second chamber, the highly concentrated CW brine water in the first chamber is removed by means of a 2o outlet pipe. The chamber is cleaned and fresh feed water is introduced into chamber 2.

[039] Once the system detects a predetermined reduction in the efficiency of the reverse osmosis process, the system returns to the use of the first chamber 2. In this regard, as the first feed chamber over time, the feed water of the second chamber reaches a predetermined concentration, preferably being around the maximum osmotic pressure at which the reverse osmosis membrane can operate, at which the inlet point 4i of the second chamber is closed and feed water is again distributed through the system from from the first chamber 2 back to the first chamber via pressure exchanger 40 and return tube 2R. The concentrated brine in the second chamber is removed by an outlet 4o and the fresh water is distributed in the second chamber 4.

[040] Any appropriate number and arrangement of associated supply and distribution chambers and return tubes can be provided in the system. Camera groups can work simultaneously.

[041] Various pre- and post-treatments can be provided. In the case of a desaturation unit (20), this can only be operational when a predetermined concentration of salt is reached. An example of a desaturation unit is a fluid bed type crystallizer as sold under the name Crystalactor®.

[042] The controller for the system automatically redirects concentrated water distribution from the first to the second chamber or vice versa, preferably upon detection of the predetermined reduction in overall process efficiency. Alternatively, the controller can automatically shut down the system as described in Figure 1.

[043] The basic parts of the system component shown in Figure 1 and 2 carry out the process of the invention under predefined conditions to provide optimized results. Concentration of the fluid is done gradually by passing the fluid through the semi-permeable membrane. A semipermeable membrane is a type of biological or synthetic membrane that will allow certain molecules or ions to pass through it by diffusion or occasionally by specialized processes of facilitated 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 for each solute. Depending on the membrane and solute, permeability may depend on solute size, solubility, properties, or chemicals.

[044] When fluid is introduced into the semipermeable membrane and is compressed against the semipermeable membrane at a pressure higher than the osmotic pressure of the fluid, the fluid is separated into two parts. 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 of the fluid that does not pass through the semi-permeable membrane, which part is called brine.

[045] As salts are generally rejected by the semi-permeable membrane, the brine part contains more salts than the product part. Salts in brine can be divided into two parts – high solubility salts like sodium chloride, potassium chloride etc., and low solubility salts like calcium carbonate, calcium sulfate, barium sulfate, silica etc.

[046] During the concentration of fluid in the semipermeable membrane, salts of low solubility can pass the solubility limit of these salts and as a result can start to crystallize. The "unsaturated salt" is a salt that has a concentration lower than the solubility limit, the "supersaturated salt" is a salt that has a concentration greater than the solubility limit, and the "saturated salt" is a salt that has concentration equal to the solubility limit.

[047] The crystallization process, among other things, depends on the degree of supersaturation of the salt. Below saturation (below the solubility limit), there is not enough energy to initiate any type of crystallization (stable zone). At a high degree of supersaturation, there is enough energy to start the crystallization process everywhere (unstable zone) – on all available surfaces as well as in solution. At a low degree of supersaturation, there is enough energy only for crystallization on available surfaces and not for crystallization in solution (metastable zone). At very low supersaturation, crystallization will only start on some of the surfaces available with the lowest surface energy (zone of controlled crystallization). These zones are illustrated in Figure 3 of the attached drawings.

[048] Anti-scalant is generally used to prevent or delay the crystallization process of low solubility salts on the surface of the semi-permeable membrane. There are different types of anti-scalants on the market specifically designed to prevent or delay the crystallization of different low solubility salts. The anti-scalant is limited to a certain supersaturation limit, above this limit, immediate crystallization will occur, below this limit the crystallization process will start after a certain period of time, called “induction time”.

[049] Seed material is generally used to reduce the concentration of salts of low solubility in solution by crystallizing the salts from the surface of the seeds. To control the crystallization process on the surface of the seeds, a certain degree of supersaturation of the low solubility salts must be maintained. The supersaturation should be high enough to start the crystallization process immediately with lower induction time and should be low enough to prevent the crystallization process in solution and on unwanted surfaces with high surface energy. Therefore, the supersaturation of low solubility salts must be in the “controlled crystallization zone” as shown in Figure 3 (and Figure 5).

[050] Consequently, the process of the present invention carefully controls the crystallization of salts present in the brine stream by maintaining certain conditions, in particular to keep salts of low solubility in the controlled crystallization zone as the brine output is recirculated through the process. To control the crystallization process of calcium carbonate on the surface of the seeds, the degree of supersaturation (log of the saturation index) of the calcium carbonate is kept between 0.5 and 2.0, preferably between 1.0 and 1.5 . To control the crystallization process of calcium sulphate on the surface of the seeds, the degree of saturation (saturation index) of the calcium sulphate is kept between 100% and 400%, preferably between 150% and 300%. To control the silica crystallization process on the seed surface, the degree of supersaturation (saturation index) of the silica is kept between 100% and 220%, preferably between 130% and 175%.

[051] These parameters differ from those used in all high recovery reverse osmosis systems where recovery is limited by the chemistry of sparingly soluble salts. Conventionally, these systems operate in the following saturation index range:

[052] Calcium sulfate limited systems – the calcium sulfate saturation range 400% to 450%;

[053] Calcium carbonate limited systems – the calcium carbonate saturation range (Log SI) is 2.2 to 2.9;

[054] Barium Sulphate Limited Systems – Barium Sulfate Saturation Index range is 6,000% to 8,000%;

[055] Calcium Fluoride Limited Systems – Calcium Fluoride Saturation Index range is 8,000% to 12,000%; and

[056] Silica-Limited Systems – The silica saturation index range is 200% to 250%. For example, see “Scale formation and control in high pressure membrane water treatment systems: A review”. Journal of Membrane Science 383 (2011) 1 to 16, see in particular page 11. The present invention maintains one or more of these parameters at a lower level in a controlled crystallization zone, between the AD and CF curves shown in Figure 1, more preferably being kept between the BE and CF curves, whose values for each salt are given in the Table below:

[057] If the anti-scalant is used to prevent or delay the crystallization process on the surface of the semipermeable membrane, in order to control the crystallization process on the surface of the seeds, the degree of saturation of low solubility salts must be maintained at the same track as mentioned above. However, due to the presence of antifouling, the activity of the antifouling must be eliminated immediately prior to the contact of the fluid containing the low solubility salts and the antifouling with the surface of the seeds. The antifouling activity can be eliminated by a number of different methods, including (1) addition of chemicals to capture the antifouling (such as ferric chloride, ferric sulfate, aluminum chloride, aluminum sulfate, etc.), (2 ) adding chemicals to destroy the anti-scalant (such as ozone, hydrogen, peroxide, etc.), or (3) providing a surface to absorb the anti-scalant (such as seed surface, etc.) etc.

[058] The process of the present invention can use different types of equipment to crystallize the salts of low solubility in seeds, such as chemical reactor followed by clarifier, fluidized bed reactor etc. A fluidized bed reactor is preferred and has an advantage over other crystallization methods due to the high flow rate and low residence time. The operating principle of the fluidized bed reactor is as follows: the reactor is partially filled with suitable seed particles; the fluid, (in the present case, the brine produced by the semi-permeable membrane) is pumped upward through the bed of particles to keep it in a fluidized state. Seed particles are used as crystallization sites; they provide the high surface area that reduces the energy needed for precipitation. As the crystals become progressively heavier, they gradually travel towards the bottom of the bed. Periodically, without interrupting reactor operation, the lower portion of the bed is discharged, and fresh seed material is introduced. The applied upward speed is in the range of 40 to 120 m/h, preferably between 60 to 80 m/h. The residence time of the liquid in the reactor is between 2.0 and 12.0 minutes, preferably between 3.0 and 8.0 minutes.

[059] The process of the present invention combines the following criteria to provide optimized conditions; (1) fluid concentration by semipermeable membranes, (2) antifouling deactivation, (3) removal of low solubility salts by crystallization on the seed surface, (4) addition of antifouling as illustrated in Figure 4 of the drawings attached.

• • a salmoura produzida na membrana semipermeável é recirculada de volta através da desativação anti-incrustante, remoção de sal e unidades de adição de anti-incrustante.
• • o produto é produzido continuamente na membrana semipermeável.
• • o fluido bruto/fresco é adicionado continuamente para substituir o volume do produto que é retirado do sistema, para manter o volume constante de fluido no sistema.
• • a água bruta/fresca é misturada com a salmoura produzida antes da introdução na membrana semipermeável.
• • todo o volume do sistema é substituído pelo fluido bruto/fresco após atingir a limitação da pressão osmótica na membrana semipermeável.

[060] The process operates in a semi-batch mode. A semi-batch mode means that:

• • the brine produced in the semi-permeable membrane is recirculated back through the antifouling deactivation, salt removal and antifouling addition units.
• • the product is produced continuously on the semi-permeable membrane.
• • raw/fresh fluid is continuously added to replace the volume of product that is withdrawn from the system to maintain a constant volume of fluid in the system.
• • the raw/fresh water is mixed with the brine produced before introducing it into the semi-permeable membrane.
• • the entire volume of the system is replaced by the raw/fresh fluid after reaching the osmotic pressure limitation in the semi-permeable membrane.

[061] In the manner described above, the fluid is gradually concentrated every time it passes through the semi-permeable membrane, the concentration of high solubility salts gradually increases. Low solubility salts, whose concentrations reach the controlled crystallization zone, are crystallized on the surface of the seeds in the salt removal unit so that their concentrations are maintained within the controlled crystallization zone.

[062] As long as the degree of saturation of low solubility salts is maintained in the controlled crystallization zone and never exceeds that zone, the anti-scalant, dosed into the circulated fluid, is kept at a very low level, much lower compared to the standard semipermeable membrane process without the gradual removal of low solubility salts. In the case of anti-scalant based phosphonate, the anti-scalant concentration is maintained in the range of 0 to 1.5 mg/l as phosphate, preferably in the range of 0.5 to 1.0 mg/l as phosphate.

[063] Since the fluid is gradually concentrated by the semi-permeable membrane, no chemicals are needed to initiate the crystallization process, in contrast to common crystallization processes where chemicals are used to initiate crystallization. If the ions that make up the low solubility salt are in a non-stoichiometric ratio, the ion whose concentration is lower can be added to the system to increase the extent of removal of the low solubility salt.

[064] Antifouling deactivation is done on the surface of the seeds so no additional deactivating agent is needed. For cases where a low degree of saturation of the final brine is required, additional deactivating agents can be used as oxidizers (ozone, hydrogen, peroxide etc.) or precipitation agents (ferric chloride, ferric sulfate, aluminum chloride, sulphate aluminum etc.), preferably ferric chloride or ferric sulfate at a concentration of 0.05 to 0.5 mg/l as Fe.

[065] To make the process continuous, some of the units can be provided with redundancy, to allow enough time for its drainage when the osmotic pressure limit is reached and replenished with raw/fresh fluid.

Claims (21)

Process to treat water, the method being characterized by comprising:
(a) delivering feed water to a first inlet chamber;
(b) passing pressurized feed water from the first inlet chamber through at least one semi-permeable membrane to produce a product water outlet and a brine outlet; and
(c) removing sparingly soluble salts present in the brine outlet, passing the brine through the salt removal unit, the brine outlet being subjected to a series of recirculation steps comprising;

• (i) returning the brine outlet to the first inlet chamber in a circulation step to pass the brine outlet through the semipermeable membrane for re-concentration of the brine outlet; and preparing it for the next circulating step of removing sparingly soluble salts; and
• (ii) carry out a series of step-by-step circulation steps (i) for continuous reconcentration of the brine output and crystallization of sparingly soluble salts from the recirculated brine in order to increase recovery by membrane separation, in which during these steps, the saturation indices of one or more of the following low solubility salts contained in the brine inlet are kept within range:
  • (1) the logarithm of the calcium carbonate saturation index (Log SI) is in the range of 0.5 to 2.0;
  • (2) the calcium sulfate saturation index is in the range of 100% to 400%;
  • (3) the silica saturation index is in the range of 100% to 220%;
  • (4) the barium sulfate saturation index is in the range of 100% to 5,000%; and
  • (5) the calcium fluoride saturation index is in the range of 100% to 400%;

- to provide a controlled crystallization zone comprising slightly saturated conditions in which salt crystals only grow on a low surface energy surface and cannot nucleate the controlled crystallization zone which is below an unstable and metastable zone and above the zone stable. Process according to claim 1, characterized in that it further comprises the step of deactivating any anti-scalant in the brine output that may interfere with the salt crystallization process. Process according to claim 1 or claim 2, characterized in that it further comprises reducing the pressure of the brine outlet before step (c). Process according to claim 1, claim 2 or claim 3, characterized in that the logarithm of the calcium carbonate saturation index (Log SI) is kept in the range of 1.0 to 1.5. Process according to any one of the preceding claims, characterized in that the calcium sulfate saturation index is kept in the range of 150% to 300%. Process according to any one of the preceding claims, characterized in that the silica saturation index is kept in the range of 130% to 175%. Process according to any one of the preceding claims, characterized in that the barium sulfate saturation index is kept in the range of 150% to 1,000%. Process according to any one of the preceding claims, characterized in that the calcium fluoride saturation index is kept in the range of 150% to 300%. Process according to any one of the preceding claims, characterized in that the fluid is treated with an anti-scalant to prevent or delay crystallization of the salts. Process according to claim 9, characterized in that the process includes the step of adding an anti-scalant based on phosphonate, which is preferably supplied in the range of 0.5 to 1.0 mg/l as phosphonate. Process according to claim 9 or 10, characterized in that it further comprises the step of deactivating any anti-scalant in the brine output that may interfere with the salt crystallization process. Process according to claim 11, characterized in that the deactivation step comprises passing the brine output through an antifouling deactivation unit. Process according to claim 11 or claim 12, characterized in that the process includes the step of injecting ferric chloride or ferric sulfate for deactivation, which is preferably supplied at a dosage rate between 0 to 10 mg/l as Fe. Process according to any one of the preceding claims, characterized in that the brine outlet is recirculated through a fluidized bed crystallizer unit, wherein the brine outlet is pumped through a bed of seed particles, the seed particles which act as crystallization sites to carry out the controlled crystallization step of the salts in the brine output. Process according to claim 14, characterized in that the crystallization unit also deactivates any anti-scalant in the fluid. Process according to claim 14 or 15, characterized in that the hydraulic load in the fluidized bed crystallizer is kept in the range between 40 to 120 m/h. Process according to claim 14, 15 or 16, characterized in that the residence time in the fluidized bed crystallizer is kept in the range of 1.0 to 12 minutes. Process according to any one of claims 14 to 17, characterized in that it further comprises periodically, without interrupting the operation of the crystallizer unit, discharging a lower portion of the bed and introducing the fresh seed material. Process according to any one of the preceding claims, further comprising the continuous addition of fresh feed fluid to replace the volume of product that is withdrawn from the process to maintain a constant volume of fluid in the process. Process according to claim 19, characterized in that it comprises periodically replacing the entire volume of fluid with fresh feed fluid after reaching the osmotic pressure limitation in the semipermeable membrane. Process according to any one of the preceding claims, characterized in that no chemical is added to start the crystallization process step.

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