WO2021074688A1

WO2021074688A1 - Improved desalination systems and methods using carbon dioxide

Info

Publication number: WO2021074688A1
Application number: PCT/IB2020/000859
Authority: WO
Inventors: Thomas Altmann; Ratul DAS
Original assignee: International Company For Water And Power Projects
Priority date: 2019-10-18
Filing date: 2020-10-15
Publication date: 2021-04-22

Abstract

The present invention relates to improved desalination systems and methods for mixing or injecting carbon dioxide (CO₂) gas into raw water, such as seawater or brackish water, prior to pretreatment and reverse osmosis membrane separation processes. These systems and methods reduce the pH of raw water below its natural pH, allowing one to maintain or increase recovery ratios while reducing the need for industrial acids (e.g., H₂SO₄, HCl, or other inorganic acids) and/or anti seal ant additives. One aim is to reduce or eliminate the use of expensive industrial acids for acidification of seawater during RO pretreatment processes; instead carbon dioxide (CO₂) is injected. The injection of CO₂ into seawater essentially reduces the carbon footprint of the RO process. CO₂ addition reduces scaling potential and allows a higher recovery operation, it will also make acid and anti seal ant dosing obsolete. The dissolved CO₂ due to injection in seawater passes through the RO membranes. Consequently, the CO₂ addition also lowers the pH of the RO permeate and brine, the presence of additional CO₂ in RO permeate reduces the need of food grade CO₂ in the post-treatment process. Low pH brine stream is an ideal condition for further brine concentration processes.

Description

IMPROVED DESALINATION SYSTEMS AND METHODS USING CARBON

DIOXIDE

FIELD OF THE INVENTION

The present invention relates to improved desalination systems and methods for mixing or injecting carbon dioxide (CO2) gas in a reverse osmosis desalination process. More particularly, the present invention relates to systems and methods where a desalination plant is coupled to an industrial power plant, and the CO2 that is mixed or injected is recovered from flue gas emissions or exhaust gas from the industrial power plant, reducing overall CO2 emissions from the industrial power plant.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Patent Application No. 62/923,294, filed October 18, 2019, the content of which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

Water demand globally has increased with growing global populations and rising standards of living. The need for more fresh water for domestic use, agricultural irrigation, and industrial processes has resulted in new methods to create alternative sources of fresh water. Oceans provide a nearly inexhaustible supply of seawater; thus, methods for desalinating water have developed over the years.

Desalination is a separation process used to filter fine solids and reduce the dissolved salt and mineral concentrations of raw water, such as seawater or brackish water, so that higher quality, permeate water is generated for use. Raw water is separated by the desalination process into, for example, two output streams: permeate water (low-salinity product water) and brine water (concentrated salt solution). The desalination process also typically removes organic chemicals and microbial contaminants from the raw water. Permeate water is suitable for most domestic, industrial, and agricultural uses, but requires post-treatment or re-mineralization before it is suitable for human consumption as potable water. Thermal desalination and membrane desalination are two known desalination technologies. Thermal desalination uses heat in multistep distillation processes (evaporating and condensing water) and requires higher energy consumption and costs more than membrane desalination. Membrane desalination usually does not require heat; it instead involves the passage of raw water through a semipermeable membrane. The most common membrane desalination technology is reverse osmosis (RO) desalination. In RO desalination, raw water is passed through a semipermeable membrane unit under pressure. Such a membrane unit comprises one or more membrane elements connected in series.

A less common membrane desalination process is forward osmosis desalination, which involves use of a “draw” solution with higher ionic strength than the raw water to be treated so it does not require pressure to transfer the raw water across the semipermeable membrane. After the raw water passes through the semipermeable membrane, heat is applied to separate permeate water from the draw solution through phase separation. Thus, a source of heat is necessary in a forward osmosis desalination process. In addition, there is a concern that undesirable traces of draw solution remain in the permeate water, which requires another treatment step to verify that no “draw” solution is present in the permeate water. Although there has been some research and development into forward osmosis desalination technology in lab- scale systems, it has not yet been applied to large-scale industrial plants. In addition, the RO desalination process is more desirable because it is more energy efficient and requires less energy consumption than the forward osmosis desalination process. Furthermore, it is well known that forward osmosis desalination processes are less efficient with warmer seawater such as Red Sea or Arabian Gulf seawater.

A current, conventional reverse osmosis desalination system typically carries out four major steps: (1) pretreatment, (2) pressurization, (3) membrane separation, and (4) post-treatment stabilization. During the pretreatment step, the raw water is typically pretreated to: (a) remove suspended solids and destabilize colloids by means of coagulation (including flocculation and filtration process (e.g., membrane filtration, ultra-filtration, and nano-filtration)); (b) adjust the pH; and (c) add antisealant additives. The coagulation process to remove suspended solids works more efficiently at lower pH levels between 5.0 to 6.0, or between about 5.0 to about 6.0. This is an important step for the pretreatment of seawater. The antisealant additives prevent scaling, a process whereby salts deposit on the RO membrane, resulting in reduced raw water flow through the RO membrane or blockage, and requiring the RO membrane to be cleaned frequently.

During the pressurization step, a pump raises the pressure of the treated raw water to an operating pressure appropriate for the RO membrane and the salinity of the treated raw water. The typical range for the operating pressure in a reverse osmosis desalination process is about 50-80 bars.

In the membrane separation step, the RO membrane inhibits the passage of dissolved salts, minerals, and fine solids while permitting the desalinated, permeate water to pass through.

Finally, the post-treatment stabilization step produces product water. Post-treatment stabilization steps may involve, for example, remineralizing the permeate water by adding calcium (e.g., calcium carbonate or calcium hydroxide), magnesium, and/or carbonates, adding chlorine to disinfect the permeate water, adding sodium fluoride, or adjusting the pH by adding food-grade CO2. Depending on the specific post-treatment steps used, potable water may be produced for human consumption. In applications in which generating potable water is not the goal (such as water for irrigation), post-treatment stabilization may be skipped or streamlined.

One measure of the efficiency of a desalination plant is the recovery ratio. This quantity is calculated based on the ratio of Ff which is the feed flow rate, and P f which is the permeate flow rate. The feed flow rate, Ff, may be measured using a feed flow meter placed along the feed flow path, whereas the permeate flow rate, P f, may be measured using a permeate flow meter placed along the permeate flow path, after RO membrane separation but before post treatment. More formally, the recovery ratio is defined as follows:

Recovery Ratio = — ^Pf X 100

F ft

The recovery ratio of current, conventional reverse osmosis systems that use sea water as an input and which utilize antisealant and/or an industrial acid is typically 35-45%, which corresponds to about 35-45 liters of output desalinated, permeate water for every 100 liters of input raw water. In current, conventional reverse osmosis systems, antisealant additives are added during pretreatment in order to achieve a recovery ratio of 35-45%, depending on the seawater salt content. Antisealant additives are organic compounds that keep potential precipitants in solution so they pass through the RO membrane, reducing scaling on the RO membrane and extending the time between RO membrane cleanings. Examples of typical antisealants include, but are not limited to, Accepta 2651 (Accepta^®), RPI-4900 (Ropur RPI^®), AMI AS-102 (ami chemicals^®), Flocon^® 135 (BWA™), Flocon^® 190 (BWA™), Flocon^® 260 (BWA™), Flocon^® Plus N (BWA™), or Titan ASD™ 200 (PWT™).

However, increased use of these antisealant additives may cause biofouling — the formation of biofilm on the RO membrane. Although antisealant additives may prevent scaling, the organic nature of these additives encourages the growth of microorganisms on the RO membrane. RO membranes have large surface areas, increasing the chances that a single bacterium will reach the membrane surface and colonize to form a biofilm. This process is encouraged by the addition of antisealant additives, which are a source of nutrients for these microorganisms. Biofouling causes severe losses in performance of RO membranes and requires costly cleaning procedures to remove the biofilm, resulting in reduced desalination plant production or output, or increased energy consumption.

Alternatively or additionally, skilled artisans may use industrial acids such as sulfuric acid (H2SO4), hydrochloric acid (HC1), or other inorganic acids in the pretreatment stage to achieve a recovery ratio of 35-45%, depending on the seawater salt content. The addition of industrials acids reduce the pH of raw water below its natural pH, which reduces scaling and allows for increase in the recovery ratio. However, the use of sulfuric acid (H2SO4) or hydrochloric acid (HC1) is not desirable because they are expensive, dangerous to handle, difficult to store, and often contain traces of heavy metals due the nature of their manufacturing process that have a catalytic effect on compounds present in the raw water and forms free radicals that damage the RO membrane.

Within the ranges discussed above, the recovery ratio for a given RO desalination system depends on numerous factors such as feed water chemistry and the pretreatment process. Such systems are designed using design software that optimizes the recovery ratio based on such factors. If the recovery ratio is too high for a RO desalination system, it can lead to problems due to scaling and fouling. The recovery ratio is also limited by the osmotic pressure of the brine water that is produced. Typically, the recovery ratio cannot be increased to greater than 50% due to these considerations.

Thus, there exists a need in the art for a desalination process that can maintain or increase the recovery ratio and improve efficiency of the coagulation process, while reducing the need for antisealant additives and industrial acids (e.g., H2SO4, HC1, or other inorganic acids) during pre-treatment processing.

SUMMARY OF THE INVENTION

The inventors have discovered improved desalination systems and methods that reduce or eliminate the need for industrial acids (e.g., H2SO4, HC1, or other inorganic acids) and/or antisealant additives, while simultaneously improving the efficiency of the coagulation process and maintaining or increasing the recovery ratio, by mixing or injecting carbon dioxide (CO2) gas into the raw water, such as seawater or brackish water, prior to the pretreatment and reverse osmosis membrane separation processes. These systems and methods reduce the pH of raw water below its natural pH before pretreatment, allowing for more efficient coagulation of colloidal matter and maintaining or increasing recovery ratios, while reducing the need for industrial acids and/or antisealant additives.

In preferred embodiments of the invention, a system for desalination of raw water comprises a) a source of carbon dioxide (CO2) gas, b) a gas transmission pipeline coupled to the source of carbon dioxide gas, and c) a desalination unit. The term “coupled’ includes both direct and indirect coupling.

In certain embodiments of the present invention, a system for desalination of raw water comprises a source of carbon dioxide gas; a gas transmission pipeline coupled to the source of carbon dioxide gas; and a desalination unit. The desalination unit includes a feed water unit connected to a source of raw water; a mixing unit (including a mixing unit output); pretreatment unit (including a pretreatment unit output), a RO membrane (including a first output and a second output), a permeate water unit (including a permeate water unit output), and a brine water unit (including a brine water unit output). The mixing unit is coupled to the feed water unit and the gas transmission pipeline. The pretreatment unit is coupled to the mixing unit output. The RO membrane is coupled to the pretreatment unit output. The permeate water unit is coupled to the first output of the RO membrane, and the brine water unit is coupled to the second output of the RO membrane. The permeate water unit provides permeate water at the permeate water unit output. The brine water unit provides brine water at the brine water unit output. CO2 from a CO2 source may be dosed into the brine water before transmission out of the plant or further processing.

In certain embodiments of the present invention, a system for desalination of raw water comprises a source of carbon dioxide gas; a gas transmission pipeline coupled to the source of carbon dioxide gas; and a desalination unit. The desalination unit includes a feed water unit connected to a source of raw water, wherein the feed water unit includes a first output and a second output; a mixing unit (including a mixing unit output); pretreatment unit (including a pretreatment unit output), a RO membrane (including a first output and a second output), a permeate water unit (including a permeate water unit output), and a brine water unit (including a brine water unit output). The mixing unit is coupled to the gas transmission pipeline. The pretreatment unit is coupled to the mixing unit output. The first output of the feed water unit is also coupled to the mixing unit, and the second output of the feed water unit is coupled to the mixing unit output at a point that is upstream of the pretreatment unit. The RO membrane is coupled to the pretreatment unit output. The permeate water unit is coupled to the first output of the RO membrane, and the brine water unit is coupled to the second output of the RO membrane. The permeate water unit provides permeate water at the permeate water unit output. The brine water unit provides brine water at the brine water unit output. CO2 from a CO2 source may be dosed into the brine water before transmission out of the plant or further processing.

In certain embodiments of the present invention, a system for desalination of raw water comprises a gas transmission pipeline and a desalination unit. The gas transmission pipeline is configured to transmit carbon dioxide gas. The desalination unit includes a feed water unit connected to a source of raw water; a mixing unit (including a mixing unit output); pretreatment unit (including a pretreatment unit output), a RO membrane (including a first output and a second output), a permeate water unit (including a permeate water unit output), and a brine water unit (including a brine water unit output). The mixing unit is coupled to the feed water unit and the gas transmission pipeline. The pretreatment unit is coupled to the mixing unit output. The RO membrane is coupled to the pretreatment unit output. The permeate water unit is coupled to the first output of the RO membrane, and the brine water unit is coupled to the second output of the RO membrane. The permeate water unit provides permeate water at the permeate water unit output. The brine water unit provides brine water at the brine water unit output. CO2 from a CO2 source may be dosed into the brine water before transmission out of the plant or further processing.

The raw water may be any saline water, including but not limited to seawater or brackish water.

The source of carbon dioxide gas may be any source of CO2, including but not limited to food- grade CO2, industrial grade CO2, or CO2 recovered from flue gas or from air, or exhaust gas emissions from an industrial power plant. For example, the industrial power plant may be located in close proximity (e.g., within one, two or three miles from), or adjacent to, the improved desalination system. In other embodiments, the industrial power plant may be located more distantly from the improved desalination system with a longer gas transmission pipeline connecting them.

In certain embodiments, the mixing unit may comprise, for example, one or more of a section of pipe, a chamber, a tank, a static mixer, and a high-pressure injection unit.

The pH of the acidified raw water can be measured immediately after the CO2 is mixed or injected into the raw water, or immediately before the reverse osmosis membrane. A skilled artisan understands that the pH of the raw water may be monitored throughout the desalination process. In certain embodiments, the pH of the raw water at the mixing unit output is below the natural pH of the raw water. In certain embodiments, the pH of the raw water at the mixing unit output is between 5.0 and 8.0, with a preferred pH of 6.5 to 7.5. In other embodiments, the pH of the raw water measured immediately before it enters the reverse osmosis membrane is below the natural pH of the raw water. In certain embodiments, the pH of the raw water measured immediately before it enters the reverse osmosis membrane is between 5.0 and 8.0, with a preferred pH of 6.5 to 7.5.

In certain embodiments the system for desalination uses less than 2 mg/L, less than 1 mg/L, less than 0.5 mg/L, or no antisealant additive. In other embodiments the system for desalination uses less than 50 mg/L, less than 30 mg/L, or no industrial acid. In other embodiments the system for desalination uses less than 50 mg/L, less than 30 mg/L, or no sulfuric acid (H2SO4). In other embodiments the system for desalination uses less than 50 mg/L, less than 30 mg/L, or no hydrochloric acid (HC1).

In certain embodiments, the permeate water unit output is coupled to a post-treatment unit, wherein the post-treatment unit provides product water at an output of the post-treatment unit. The product water may be potable water, depending on the post treatment process that is carried out. For example, food-grade CO2 may be added during post-treatment to produce potable water or other product water. For example, less than 50 mg/L, less than 30 mg/L, or less than 10 mg/L food-grade CO2 may be added during post-treatment to produce potable water or other product water.

In certain embodiments, a high-pressure pump is downstream of the mixing unit and upstream of the reverse osmosis membrane. Such a pump may be used to increase the pressure of raw water impinging on the reverse osmosis membrane, which increases the effectiveness of the reverse osmosis filtering through that membrane.

In certain embodiments, the system for desalination has a recovery ratio of at least 40%; at least 45%, at least 50%, or at least 55%. Such recovery ratios may be attained while reducing the need for industrial acids (e.g., H2SO4, HC1, or other inorganic acids) and/or antisealant additives compared to known reverse osmosis desalination systems.

In certain embodiments, the brine water unit output is coupled to a brine water post-treatment unit, wherein the brine water post-treatment unit provides concentrated brine water at a first output and permeate or product water at a second output. The post-treatment unit may include, for example, its own filtering mechanism for producing such outputs, as will be known to those of ordinary skill in the art. The post-treatment unit may also include an acidification system, in which, for example, CO2 may be dosed into the water stream in that unit.

In preferred embodiments of the invention, an improved method for desalination of raw water comprises the steps of a) mixing or injecting carbon dioxide gas into raw water to produce acidified raw water, b) treating the acidified raw water through a pretreatment process to produce treated raw water, and c) passing the acidified raw water through a reverse osmosis membrane to produce permeate water at a first output and brine water at a second output. In certain embodiments of the present invention, a method for desalination of raw water includes the steps of: mixing or injecting carbon dioxide gas into raw water to produce acidified raw water; then providing the acidified raw water to a pretreatment unit to produce treated raw water; then passing the treated raw water through a reverse osmosis membrane; then separating the output of the reverse osmosis membrane into two streams, permeate water and brine water.

In certain embodiments of the present invention, a method for desalination of raw water includes the steps of: 1) mixing or injecting carbon dioxide gas into a first raw water stream to produce a first acidified raw water stream; 2) mixing the first acidified raw water stream with a second raw water stream to produce a combined acidified raw water stream; 3) then providing the combined acidified raw water stream to a pretreatment unit to produce treated raw water; 4) then passing the treated raw water through a reverse osmosis membrane; 5) then separating the output of the reverse osmosis membrane into two streams, permeate water and brine water.

The source of carbon dioxide gas may be any source of CO2, including but not limited to food- grade CO2, industrial grade CO2, or CO2 recovered from flue gas or exhaust gas emissions from an industrial power plant. For example, the industrial power plant may be located in close proximity (e.g., within one, two or three miles from), or adjacent to, the improved desalination system. In other embodiments, the industrial power plant may be located more distantly from the improved desalination system with a longer gas transmission pipeline connecting them.

In certain embodiments, the mixing unit comprises, for example, one or more of a section of pipe, a chamber, a tank, a static mixer, and a high-pressure injection unit.

The pH of the acidified raw water can be measured immediately after the CO2 is mixed or injected into the raw water, or immediately before the reverse osmosis membrane. A skilled artisan understands that the pH of the raw water may be monitored throughout the desalination process. In certain embodiments, the pH of the acidified raw water is below the natural pH of the raw water. In certain embodiments, the pH of the acidified raw water is between 5.0 and 8.0, with a preferred pH of 6.5 to 7.5. In other embodiments, the pH of the acidified raw water measured before it enters the reverse osmosis membrane is below the natural pH of the raw water. In certain embodiments, the pH of the acidified raw water measured before it enters the reverse osmosis membrane is between 5.0 and 8.0, with a preferred pH of 6.5 to 7.5.

In certain embodiments, the method for desalination uses less than 2 mg/L, less than 1 mg/L, less than 0.5 mg/L, or no antisealant additive. In other embodiments the system for desalination uses less than 50 mg/L, less than 30 mg/L, or no industrial acid. In other embodiments the system for desalination uses less than 50 mg/L, less than 30 mg/L, or no sulfuric acid (H2SO4). In other embodiments the system for desalination uses less than 50 mg/L, less than 30 mg/L, or no hydrochloric acid (HC1).

In certain embodiments, the permeate water is provided to a post-treatment unit, wherein the post-treatment unit produces product water at an output of the post-treatment unit. The product water may be potable water, depending on the post-treatment process that is carried out. For example, food-grade CO2 may be added during post-treatment to produce potable water or other product water. For example, less than 50 mg/L, less than 30 mg/L, or less than 10 mg/L food-grade CO2 may be added during post-treatment to produce potable water or other product water. However, the need for addition of CO2 during post-treatment may be reduced or eliminated by the dosing of CO2 at the front end. Additionally, the use of a split partial second pass reverse osmosis plant with blending of output form the first few elements of the first RO membrane and output from the second RO membrane further reduces or eliminates the need to dose CO2 at the output end of the plant. In particular, usually the front elements or membranes of a RO pressure vessel produces good quality permeate with low TDS when compared with the elements towards the end of the pressure vessel. Thus, part of the front permeate, for example, approximately 40%, may be blended with the product water to meet water quality requirements. This increases the overall system recovery and reduces the size of the facility and the energy consumption in the RO process during operation.

In certain embodiments, a high-pressure pump is upstream of the reverse osmosis membrane. Such a pump may be used to increase the pressure of water impinging on the reverse osmosis membrane, which increases the effectiveness of the reverse osmosis filtering through that membrane. In certain embodiments, the method for desalination has a recovery ratio of at least 40%; at least 45%, at least 50%, or at least 55%. Such recovery ratios may be attained while reducing the need for industrial acids (e.g., H2SO4, HC1, or other inorganic acids) and/or antisealant additives compared to known reverse osmosis desalination systems.

In certain embodiments, the brine water unit output is acidified with C02 and provided to a brine water post-treatment unit, wherein the brine water post-treatment unit produces concentrated brine water at a first output and permeate or product water at a second output. The post-treatment unit may include, for example, its own filtering mechanism for producing such outputs, as will be known to those of ordinary skill in the art. The post-treatment unit may also include an acidification system, in which, for example, CO2 may be dosed into the water stream in that unit.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to improved desalination systems and methods that involve mixing or injecting CO2 into raw water prior to the pretreatment and reverse osmosis processes. An important aspect of the present invention is the reduction of pH before the reverse osmosis membrane. Thus, in certain embodiments the improved desalination systems and methods can be carried out in which CO2 is added before the reverse osmosis membrane, and before pretreatment. In certain other embodiments, the CO2 is added before the reverse osmosis membrane but after one or all steps of pretreatment, and still obtain many of the benefits described herein. Yet in certain other embodiment, CO2 is added before the reverse osmosis membrane but after pretreatment, and still obtain many of the benefits described herein.

The CO2 may be mixed or injected into the raw water using any suitable, known method for mixing CO2 with raw water that is known to skilled artisans. For example, the same methods used to add CO2 during post-treatment that are known to skilled artisans may be used to mix or inject CO2 with raw water prior to the pretreatment and reverse osmosis processes.

The addition of CO2 prior to pretreatment improves the efficiency of the coagulation process and maintains or increases the recovery ratio, while reducing the pH of the raw water and the need for industrial acids (e.g., H2SO4, HC1, or other inorganic acids) and/or antiscaling additives that can lead to biofouling. In preferred embodiments of the invention, a system for desalination of raw water comprises a) a source of carbon dioxide gas, b) a gas transmission pipeline coupled to the source of carbon dioxide gas, and c) a desalination unit. The term “coupled’ includes both direct and indirect coupling. The desalination unit may, in an embodiment, comprise a feed water unit, mixing unit, pretreatment unit, reverse osmosis membrane, permeate water unit, brine water unit, and post-treatment unit.

In preferred embodiments of the invention, a method for desalination of raw water comprises the steps of a) mixing or injecting carbon dioxide gas into raw water to produce acidified (i.e., lower pH) raw water, b) treating the acidified raw water using a pretreatment process to produce treated raw water, and c) passing the acidified raw water through a reverse osmosis membrane to produce permeate water at a first output and brine water at a second output. CCh from a CCh source may be dosed into the brine water before transmission out of the plant or further processing.

In the present invention, the raw water can be any source of water that needs to be desalinated. For example, sources of raw water may include, but are not limited to, any of, or a combination of any of, sea water, brackish water, and any other saline water that has been partially desalinated through another process.

Any suitable source of CCh can be used in the present invention. For example, sources of CCh may include, but are not limited to, food-grade CCh, industrial grade CCh, or CCh recovered from flue gas or exhaust gas emissions from an industrial power plant. Those of skill in the art will recognize that various industrial power plants emit CCh that could be used as the source of CCh in the present invention. Thus, any industrial power plant that emits CCh may be used. For example, the industrial power plant may include, but is not limited to, a coal-buming power plant, an oil-burning power plant, a syngas (synthesis) gas-buming plant, or gas-buming power plants. Carbon capture and sequestration (“CCS”) methods and technology are also known to skilled artisans, and any CCS method may be used to absorb and recover CCh from gas stream emissions from industrial power plants. Using CCh recovered from flue gas or exhaust gas emissions from an industrial power plant as the CCh source may have the added benefit of resulting in tradable carbon credits, in jurisdictions that have implemented a suitable regulatory structure. Any suitable gas transmission pipeline known to skilled artisans may be used in connection with the present invention to convey CO2 from a source to the improved system for desalination of raw water. The gas transmission pipeline may be short, for example, when conveying CO2 from a source (such as an industrial power plant or pressurized tank within the desalination plant) that is in close proximity to the improved desalination system. In other embodiments, where the distance between the source of CO2 and the improved desalination system is great, the length of the gas transmission pipeline will be correspondingly great.

Any suitable feed water unit known to skilled artisans may be used in connection with the present invention. The feed water unit receives the raw water from its source, for example an ocean or a sea. The feed water unit may comprise one or more units comprising, for example, a section of pipe, a chamber, a tank, or a combination thereof. Other examples of a feed water unit that are used in known fluid transport systems or reverse osmosis desalination systems will be known to the skilled artisan and may also be used.

Any suitable pretreatment unit known to skilled artisans may be used in connection with the present invention. The pretreatment unit receives the raw water and is where pretreatment processes such as addition of an industrial acid and/or antisealant agents may be carried out. As such, the pretreatment unit may comprise one or more units comprising, for example, a section of pipe, a chamber, a screen, a tank, a filter, an air floatation unit, or a combination thereof. For example, in one embodiment, the pretreatment unit may comprise a first chamber or section of pipe in which an industrial acid is added, and a second chamber or section of pipe coupled downstream in which an antisealant is added. Other examples of a pretreatment unit that are used in known reverse osmosis desalination systems will be known to the skilled artisan and may also be used.

Any suitable permeate water unit known to skilled artisans may be used in connection with the present invention. The permeate water unit receives the permeate water from the reverse osmosis membrane. The permeate water unit may comprise one or more units comprising, for example, a section of pipe, a chamber, a tank, or a combination thereof. Other examples of a feed water unit that are used in known reverse osmosis desalination systems will be known to the skilled artisan and may also be used. Any suitable brine water unit known to skilled artisans may be used in connection with the present invention. The brine water unit receives the brine water from the reverse osmosis membrane. The brine water unit may comprise one or more units comprising, for example, a section of pipe, a chamber, a tank, or a combination thereof. Other examples of a brine water unit that are used in known reverse osmosis desalination systems will be known to the skilled artisan and may also be used.

Any suitable post-treatment unit known to skilled artisans may be used in connection with the present invention. The post-treatment unit receives either permeate water from the permeate water unit or brine water from the brine water unit, and is where post-treatment processes such as addition of minerals and/or food-grade CO2 may be carried out. As such, the post-treatment unit may comprise one or more units comprising, for example, a section of pipe, a chamber, or a combination thereof. For example, in one embodiment, the post-treatment unit may comprise a first chamber or section of pipe in which minerals such as calcium is added, and a second chamber or section of pipe coupled downstream in which food-grade CO2 is added. Other examples of a post-treatment unit that are used in known reverse osmosis desalination systems will be known to the skilled artisan and may also be used.

In certain embodiments, the mixing unit comprises a section of pipe, a chamber, a tank, a static mixer, or a high pressure injection unit, or any combination thereof. For example, the mixing unit may comprise a section of pipe and a chamber in a case in which the CO2 is mixed with the raw water along both of these units.

Seawater typically has a pH between about 7.8 to about 8.2, for example between 7.8 to 8.2. By mixing or injecting CO2 into the raw water, the pH may be decreased below the natural pH of seawater. Natural pH refers to the pH of the raw water in its natural state (the sea or ocean) as delivered from the original source to the RO desalination system, prior to any treatment or physical change to the raw water. In certain embodiments, the pH may be decreased to about 5.0 to about 8.0, for example between 5 to 8. In the present invention, the raw water is mixed or injected with CO2, prior to pretreatment, and in certain embodiments the pH of the acidified raw water is decreased to at least about 8.0 (for example at least 8), to at least about 7.5 (for example at least 7.5), to at least about 7.0 (for example at least 7.0), to at least about 6.5 (for example at least 6.5), to at least about 6.0 (for example at least 6.0), to at least about 5.5 (for example at least 5.5), or to at least about 5.0 (for example at least 5.0). In certain embodiments, the pH is preferably between 6.5 and 7.5. The pH of the acidified raw water can be measured immediately after the CCh is mixed or injected into raw water, or immediately before the reverse osmosis membrane. A skilled artisan understands that the pH of the raw water may be monitored throughout the desalination process.

In the split partial second pass reverse osmosis plant embodiment that is discussed further below, in addition to measurement of the pH of the acidified water in advance of the first pass reverse osmosis membrane, the pH may also be measured before the feed pump for the second pass reverse osmosis membrane. For example, the pH may be measured at some point along the path between element 524 and element 526 of the Fig. 5 embodiment.

Solely as a non-limiting example, we analyzed, using several different methods, the amount of CCh that needs to be mixed or injected into raw water to reduce its pH to 6.5 in the SHUQAIQ 2 IWPP plant. SHUQAIQ 2 IWPP is an integrated water and power plant and is located in Shuqaiq, Saudi Arabia. Split partial second pass reverse osmosis (SPSP RO) plants and the SHUQAIQ 2 IWPP plant are discussed in further detail further below in this disclosure.

Based on characteristics of raw seawater available at the SHUQAIQ 2 IWPP plant, including parameters including temperature, water hardness, alkalinity and mineral content, we calculated the amount of CCh or H2SO4 that needs to be mixed or injected into raw seawater to reduce its pH to 6.5. In particular, we used (1) Toray Industries Inc.’s Toray Design System (TorayDS/DS2): Design software for Reverse Osmosis Process Design, (2) Toyobo’s calculation tool, (3) Aqion’s Hydrochemistry & Water Analysis tool, (4) WinWASFs WinWASI 5.0 tool, (5) Dupont’s Wave Software For Water Treatment Plant Design, and (6) ACWA’s proprietary tool for these calculations.

Table 1 depicts the raw seawater characteristics at the SHUQAIQ 2 IWPP plant that were used for the calculation. Table 2 depicts the results of these calculations. The last two rows in Table 2 respectively depict (i) the concentration of H2SO4 currently added to reduce the pH of the raw seawater to 6.5 at SHUQAIQ 2 IWPP and, (ii) the results from a lab scale test performed using Air Products’ Products Halia^® pH neutralization system bench test equipment. The results of these calculations are generally consistent with one another, apart from the Air Products lab scale test, which appears to be an outlier and which was not used in calculating the average figures depicted in the last row of Table 2.

Table 1

Table 2

Based on the finding that approximately 40.65 mg/L of CO2 should be added to the available raw seawater at SHUQAIQ 2 IWPP, we expect that a volume of 595,000 m³/day of raw seawater being treated at that plant will require approximately 24-25 tons/day of CC to reduce the pH of the raw water from 8.1 to 6.5.

By mixing or injecting CCh into raw water prior to pretreatment and RO membrane separation processes, there are a number of benefits that result. By reducing the pH of the raw water, the recovery ratio can be maintained or increased while reducing the amount of industrial acids (e.g., H2SO4, HC1, or other inorganic acids) and/or antisealant additive needed to keep salt from precipitating on the RO membrane, compared to conventional reverse osmosis processes. In certain embodiments the improved desalination process uses less than 2 mg/L, less than 1 mg/L, less than 0.5 mg/L, or no antisealant additive. In other embodiments the system for desalination uses less than 50 mg/L, less than 30 mg/L, or no industrial acid. In other embodiments the system for desalination uses less than 50 mg/L, less than 30 mg/L, or no sulfuric acid (H2SO4). In other embodiments the system for desalination uses less than 50 mg/L, less than 30 mg/L, or no hydrochloric acid (HC1). In certain embodiments, the improved desalination process has a recovery ratio of at least 40%; at least 45%, at least 50%, or at least 55%. Furthermore, as discussed above, the coagulation process is more efficient at a lower pH. For example, in one embodiment, a recovery ratio of at least 50% is reached with less than 2 mg/L of antisealant additive (for example, in one embodiment, with no antisealant additive), while using less than 50 mg/L of sulfuric acid and less than 50 mg/L of hydrochloric acid (for example, in one embodiment, with no sulfuric acid or hydrochloric acid). In another embodiment, a recovery ratio of at least 40% is reached with less than 0.5 mg/L of antisealant additive (for example, in one embodiment, with no antisealant additive), while using less than 50 mg/L of sulfuric acid and less than 50 mg/L of hydrochloric acid (for example, in one embodiment, with no sulfuric acid or hydrochloric acid). Other embodiments will also be apparent to the skilled artisan based on these disclosures.

In certain embodiments, the CO2 is mixed or injected into a portion of the raw water first, and then this portion of acidified seawater is mixed with the rest of the raw water prior to pretreatment and RO membrane separation processes. By mixing or injecting the CO2 with a portion of the raw water first, more effective overall mixing of the CO2 into the raw water may be achieved. After the raw water passes through the RO membrane, there are two output streams: permeate water (low-salinity product water) and brine water (concentrated salt solution). The permeate water may be corrosive and has been stripped of minerals such as calcium and carbonates by the RO membrane, which results in a higher pH than the raw water upstream impinging on the RO membrane. Thus, during the post-treatment stage of the conventional desalination process, in addition to such minerals, food-grade CO2 may also be added to the permeate water to adjust the pH and produce product water, such as potable water. Food-grade CO2 is expensive to purchase and store, thus it is desirable to reduce the amount of food-grade CO2 used in desalination systems and methods. CO2 from a CO2 source may be dosed into the brine water before transmission out of the plant or further processing.

In the improved desalination process, since CO2 is mixed or injected into raw water prior to the pretreatment and RO membrane separation processes, the dissolved CO2 goes through the RO membrane, and the dissolved CO2 is present in the permeate water and brine water streams. Thus, the pH of the two output streams, permeate water and brine water, will be lower than the analogous output streams which result from the conventional desalination process. Therefore, the improved desalination process reduces the amount of costly food-grade CO2 that needs to be added during post-treatment compared to the conventional desalination process, resulting in cost-savings. In certain embodiments less than 50 mg/L, less than 30 mg/L, or less than 10 mg/L food-grade CO2 is added during post-treatment.

In addition, the pH of the brine water produced in the improved desalination process is lower than that obtained using the conventional desalination process. Lower pH brine water is advantageous when attempting to further concentrate the brine water and obtain additional permeate or product water. Brine water may be further concentrated by means of a counter flow reverse osmosis (CFRO) process as described, for example, in U.S. Patent Nos. 9,206,060 and 9,427,705, the disclosures of which are incorporated by reference herein in their entireties. Examples of CFRO processes include, but are not limited to a counter-flow configuration from Gradiant Corp., Osmotically Assisted Reverse Osmosis from Hyrec, or by crystallization with heat supplied from a fossil power plant, solar power plant, or other industrial heat source. The lower pH brine water from the improved desalination process herein allows for higher recovery of permeate or product water from the brine water while decreasing the amounts of CO2 or antisealant required. Thus, the environmental impact of the improved desalination process is reduced since the brine is more easily concentrated to produce additional permeate or product water. CO2 from a CO2 source may be dosed into the brine water before transmission out of the plant or further processing.

Typically, the pH of total brine output from a RO water desalination plant in which CO2 is dosed upstream of the RO membrane unit in accordance with the invention is within the range 3.0 to 6.0, or about 3.0 to about 6.0. Such pH values for the total brine output may be obtained without any additional acidification or dosing of CO2 after the RO membrane unit.

As will be apparent to one of ordinary skill in the art based on the current disclosure, additional CO2 may be added to the brine in, for example, a brine water post-treatment unit to lower the pH of the brine water. Doing so is advantageous when attempting to further concentrate the brine water and obtain additional permeate or product water, as discussed above. Alternatively, this is advantageous when brine is used for multi-media filter backwashing, as the pH of the brine is similar to the pH of acidified seawater. Thus, such backwashing will not expose the filter - which is operated as a biofilter during the backwashing - to a sudden change of pH value that might otherwise negatively impact the effective removal of nutrient by bacteria. Considerations related to brine management and mitigation of fouling in desalination plants are known to those of skill in the art. See, e.g., US20130193077A1, entitled “Neutralization and Precipitation of Silica from High pH Brines”; US8062530B2, entitled “Method for treatment of high pH/silica brines”; US5254257A, entitled “Reclaiming of spent brine”; D.M. Warsinger et al., “Inorganic fouling mitigation by salinity cycling in batch reverse osmosis”, Water Research, Vol. 137, 384-394 (2018); J.S. Ho et al, “Inline coagulation-ultrafiltration as the pretreatment for reverse osmosis brine treatment and recovery”, Desalination, Vol. 365, 242-249 (2015); J. Morillo et al, “Comparative study of brine management technologies for desalination plant”, Desalination, Vol. 336, 32-49 (2014); E. Drioli et al., “Integrated system for recovery of CaC03, NaCl and MgS04 7H20 from nanofiltration retentate”, Journal of Membrane Science, Vol. 239, Issue 1, 27-38 (2004); K.D. Vos, “Lifetime of Cellulose Acetate Reverse Osmosis Membranes”, Ind. Eng. Chem. Prod. Res. Dev., 5, 3, 211-218 (1966); S.Y. Sun et al, Separation of magnesium and lithium from brine using a Desal nanofiltration membrane, Journal of Water Process Engineering, Vol. 7, 210-217 (2015), each of whose disclosures are herein incorporated in their entirety. In summary, by mixing or injecting CO2 into raw water and reducing its pH prior to the pretreatment and RO membrane separation processes, the improved desalination process of the present invention reduces the need for expensive antisealant chemicals and industrial acids during pretreatment, maintains or increases the recovery ratio of the process, improves the efficiency of the coagulation process, decreases the amount of costly food-grade CO2 needed during post-treatment, and reduces the environmental impact of the brine water, resulting in an overall process that is more economical and beneficial for the environment.

As discussed above, an important aspect of the present invention is the reduction of pH before the reverse osmosis membrane. Thus, in certain embodiments the improved desalination systems and methods can be carried out in which CO2 is added before the reverse osmosis membrane, and before pretreatment. In certain other embodiments, the CO2 is added before the reverse osmosis membrane but after one or all steps of pretreatment, and still obtain many of the benefits described herein. Yet in certain other embodiment, CO2 is added before the reverse osmosis membrane but after pretreatment, and still obtain many of the benefits described herein.

EXAMPLES

The following examples serve only to illustrate the invention and practice thereof. The examples are not to be construed as limitations on the scope or spirit of the invention.

The present invention is directed, in certain embodiments, to improved systems for desalination of raw water. With reference to Figure 1, one exemplary embodiment of the invention is depicted. The improved system for desalination of raw water comprises a source of carbon dioxide gas 20; a gas transmission pipeline 22 coupled to the source of carbon dioxide gas 20; and a desalination unit 24. The desalination unit includes a feed water unit 26 connected to a source of raw water; a mixing unit 28 (including a mixing unit output 30); pretreatment unit 32 (including a pretreatment unit output 34), a RO membrane 36 (including a first output 38 and a second output 40), a permeate water unit 42 (including a permeate water unit output 44), and a brine water unit 46 (including a brine water unit output 48). The mixing unit 28 is coupled to the feed water unit 26 and the gas transmission pipeline 22. The pretreatment unit 32 is coupled to the mixing unit output 30. The RO membrane 36 is coupled to the pretreatment unit output 34. The permeate water unit 42 is coupled to the first output of the RO membrane 38, and the brine water unit 46 is coupled to the second output of the RO membrane 40. The permeate water unit 42 provides permeate water at the permeate water unit output 44. The brine water unit 46 provides brine water at the brine water unit output 48. CO2 from a CO2 source may be dosed into the brine water before transmission out of the plant or further processing.

With reference to Figure 2, another exemplary embodiment of the invention is depicted. The system for desalination of raw water is substantially similar to Figure 1, except that the feed water unit 26 includes a first output 50 and second output 52. The first output 50 is coupled to the mixing unit 28, and the second output 52 is coupled to the mixing unit output 30 at a point that is upstream of the pretreatment unit 32. In this embodiment, the stream from feed water unit 26 is split into two streams (50, 52) with CO2 mixing occurring in only one of the streams (50). Afterwards, the streams are recombined, for example, upstream of pretreatment unit 32. By mixing or injecting the CO2 into the first output 50 first, more effective overall mixing of the CO2 into the recombined stream is achieved.

In the present invention, the mixing unit is not limited to any specific embodiments, and may comprise, among others, a section of pipe, a chamber, a tank, a static mixer, or a high pressure injection unit, or any combination thereof. For example, the mixing unit may comprise a section of pipe and a chamber in a case in which the CO2 is mixed with the raw water along both of these units. For example, the mixing unit could be the Pressurized Solution Feed System by TOMCO, a CO2 Carbonic Acid Feed System such as TETRApHix^® by JCI, or Dif-Jet™ gas injector by Fortrans Inc. The mixing unit is where the carbon dioxide gas is mixed or injected into the raw water.

In the present invention, the gas transmission pipeline as used in connection with the invention may be short, for example, when conveying CO2 from a source (such as an industrial power plant or pressurized tank within the desalination plant) that is in close proximity to the improved desalination system. In other embodiments, where the distance between the source of CO2 and the improved desalination system is great, the length of the gas transmission pipeline will be correspondingly great. In present invention, the feed water unit receives the raw water from its source, for example an ocean or a sea. The feed water unit may comprise any of, for example, a section of pipe, a chamber, a tank, or a combination thereof. Other examples of a feed water unit that are used in known fluid transport systems will be known to the skilled artisan and may also be used.

In the present invention, the pretreatment unit receives the raw water that has been mixed with CO2 and is where pretreatment processes such as addition of an industrial acid and/or antisealant agents may be carried out. The pretreatment unit may comprise one or more units comprising, for example, a section of pipe, a chamber, a screen, a tank, a filter, an air floatation unit, or a combination thereof. For example, in one embodiment, the pretreatment unit may comprise a first chamber or section of pipe in which an industrial acid is added, and a second chamber or section of pipe coupled downstream in which an antisealant is added. Other examples of a pretreatment unit that are used in known reverse osmosis desalination systems will be known to the skilled artisan and may also be used.

In the present invention, the permeate water receives the permeate water from the reverse osmosis membrane. The permeate water unit may comprise one or more units comprising, for example, a section of pipe, a chamber, a tank, or a combination thereof. Other examples of a feed water unit that are used in known reverse osmosis desalination systems will be known to the skilled artisan and may also be used.

In the present invention, the brine water unit receives the brine water from the reverse osmosis membrane. The brine water unit may comprise one or more units comprising, for example, a section of pipe, a chamber, a tank, or a combination thereof. Other examples of a brine water unit that are used in known reverse osmosis desalination systems will be known to the skilled artisan and may also be used.

The present invention may also include a high-pressure pump that is downstream of the mixing unit and upstream of the reverse osmosis membrane. Such a pump may be used to increase the pressure of the acidified raw water that includes CO2 that impinges on the reverse osmosis membrane.

The present invention may further include a post-treatment unit that is coupled to the permeate water unit. As described above, the post-treatment unit may add minerals and food-grade CO2 to the permeate water to adjust the pH and produce product water, which may be potable. A brine water post-treatment unit may also be coupled to the brine water unit to further concentrate the brine water and produce additional permeate, product, and/or potable water, as will be known to skilled artisans. In certain embodiments, CO2 can be added to the brine water before entering the brine water post-treatment unit, in order to obtain more permeate water in the cost-effective, efficient manner as described for the improved desalination process for raw water.

The post-treatment unit or brine water post-treatment unit may comprise one or more units comprising, for example, a section of pipe, a chamber, or a combination thereof. For example, in one embodiment, the post-treatment unit may comprise a first chamber or section of pipe in which minerals such as calcium is added, and a second chamber or section of pipe coupled downstream in which food-grade CO2 is added. Other examples of a post-treatment unit that are used in known reverse osmosis desalination systems will be known to the skilled artisan and may also be used.

The present invention is directed, in certain embodiments, to improved methods for desalination of raw water. With reference to Figure 3, in step 110, carbon dioxide gas is mixed or injected into raw water to produce acidified raw water. In step 120, the acidified raw water is provided to a pretreatment unit to produce treated raw water. Then, in step 130, the treated raw water is forced through a reverse osmosis membrane, which produces two output streams, a permeate water stream and a brine water stream. CO2 from a CO2 source may be dosed into the brine water before transmission out of the plant or further processing.

With reference to Figure 4, another exemplary embodiment of the invention is depicted. In the improved method for desalination of raw water depicted in Figure 4, in step 210, carbon dioxide gas is mixed or injected into a first raw water stream to produce a first acidified raw water stream. In step 220, the first acidified raw water stream is mixed with a second raw water stream to produce a combined acidified raw water stream. The second water stream may not have been treated or mixed with another substance or fluid. In step 230, the combined acidified raw water stream is provided to a pretreatment unit to produce treated raw water. Then in step 240, the treated raw water is forced through a reverse osmosis membrane, which produces two output streams, a permeate water stream and a brine water stream. Mixing or injecting carbon dioxide gas into the first raw water stream and combining the two streams afterwards allows for more efficient mixing of the carbon dioxide gas into the combined acidified raw water stream. CC from a CC source may be dosed into the brine water before transmission out of the plant or further processing.

Embodiments of the present invention, as depicted in Figures 3 and 4, may further include a step wherein the permeate water is provided to a post-treatment unit to produce product water. As described above, the post-treatment unit adds minerals and food-grade CCh to the permeate water to adjust the pH and produce product water, which may be potable. The present invention, as depicted in Figures 3 and 4, may also include an additional step wherein the brine water is provided to a brine water post-treatment unit to further concentrate the brine water and produce additional permeate or product water. In certain embodiments, CCh can be added to the brine water before entering the brine water post-treatment unit, in order to obtain more permeate water in the cost-effective, efficient manner as described for the improved desalination process for raw water.

In another exemplary, non-limiting embodiment, CCh can be added to the raw feed water in a split partial second pass reverse osmosis (“SPSP RO”) plant. The design of an SPSP RO plant exploits the fact that the first few elements of a reverse osmosis membrane unit produce higher quality permeate then the later elements. In this design, the plant includes two reverse osmosis membrane units. Permeate from the first few elements (for example, the first two, or the first three elements) of the first pass reverse osmosis membrane unit is removed and blended back downstream as explained next. Permeate from the later elements of the first pass reverse osmosis membrane unit is input into the second pass reverse membrane unit. The permeate output from the second pass reverse osmosis membrane unit is blended with the permeate from the first few elements of the first pass reverse osmosis membrane unit to form the permeate stream. SPSP RO plants produce high-quality permeate streams at lower cost compared to other types of RO plants. See, e.g. , Rybar, S., Boda, R., & Bartels, C., Split partial second pass design for SWRO plants , Desalination and water treatment, 13(1-3), 186-194 (2010); Saif, Y., Almansoori, A., & Elkamel, A., Optimal design of split partial second pass reverse osmosis network for desalination applications, AIChE Journal, 60(2), 520-532 (2014); Kim, J., Park, K., & Hong, S., Application of two-stage reverse osmosis system for desalination of high- salinity and high-temperature seawater with improved stability and performance, Desalination, 492, 114645 (2020); Chu, K. H., Lim, I, Kim, S. J., Jeong, T. U., & Hwang, M. H., Determination of optimal design factors and operating conditions in a large-scale seawater reverse osmosis desalination plant, Journal of Cleaner Production, 244, 118918 (2020); U.S. Patent No. 4,046,685; and Korea Patent No. 101319412B1, whose disclosures are herein incorporated by reference in their entireties. Thus, in this embodiment, CCh is added, for example, in a CCh mixing unit upstream of both reverse osmosis membranes of the SPSP RO plant.

Figure 5 depicts a typical RO plant that is exemplary and non-limiting. As depicted in Fig. 5, CO2 is added from CO2 source 510 to pre-treated raw seawater 512 to reduce its pH, in accordance with the earlier discussion. After transmission through feed unit 514, the treated seawater stream 516 impinges on the section of the plant that includes the reverse osmosis units. In Fig. 5, elements 518, 526 and 538 are the first pass reverse osmosis unit, first stage of the second pass reverse osmosis unit and second stage of the second pass reverse osmosis unit, respectively.

As depicted in Fig. 5, permeate 554 from the front elements of the first pass reverse osmosis unit 518 is of higher quality compared to the permeate 520 from the back elements of first pass reverse osmosis unit 518. Part of permeate 554 may be removed and combined with downstream permeate 530 (for example in a permeate mixing unit) which forms the permeate output 532. For example, 40% of the permeate from these front elements (for example, the first two elements, or the first three elements) of the first pass reverse osmosis unit 518 may be removed for combination with downstream permeate 530. A part or percentage of a stream of water, such as a permeate stream or a brine stream, may be directed through another flow path using, for example, control valves, as will be apparent to one of ordinary skill in the art based on the current disclosure. In this configuration, permeate output 532 is separately coupled to each of (i) the front elements of the first pass reverse osmosis unit 518, and (ii) downstream permeate 530. Permeate output 532 is separately coupled to (i) and (ii), because there are separate paths connecting each of (i) and (ii) to permeate output 532. CCh, for example food grade CCh, may be added to the combined permeate stream as part of post-treatment stabilization in producing potable permeate output 532. In an alternative embodiment for recombining permeate from the first pass reverse osmosis unit with downstream permeate (not depicted in Fig. 5), permeate is not removed from the front elements of the first pass reverse osmosis unit 518 for combination with downstream permeate 530. Instead, part of the permeate 520 from the back elements of first pass reverse osmosis unit 518 is diverted to be combined with downstream permeate 530. In this alternative embodiment, permeate output 532 is separately coupled to downstream permeate 530, and permeate 520 from the back elements of first pass reverse osmosis unit 518.

In yet another alternative embodiment (not depicted in Fig. 5), part of the permeate from the front elements of the first pass reverse osmosis unit 518 and part of the permeate 520 from the back elements of first pass reverse osmosis unit 518 are diverted for combination with downstream permeate 530. In this additional alternative embodiment, permeate output 532 is separately coupled to downstream permeate 530, permeate 520 from the back elements of first pass reverse osmosis unit 518, and permeate from the front elements of the first pass reverse osmosis unit 518.

CC is added to the permeate 520 from the back elements of first pass reverse osmosis unit 518. Further, as described further below, a feedback permeate stream 542 from the output of the second stage of second pass reverse osmosis unit 538 may also be added to permeate 520. The mixture, after passing through feed unit 522, impinges as permeate stream 524 on the first stage of the second pass reverse osmosis unit 526. The permeate output 528 from the first stage of the second pass reverse osmosis unit 526 may be combined with part of the high quality permeate output 540 from the first few elements (for example, from the second or third elements) of the second stage of the second pass reverse osmosis unit 538 to form the second pass reverse osmosis permeate 530, as discussed above in connection with the embodiment depicted in Fig. 5. (As also discussed above, in alternative embodiments, part of the permeate 520 from the back elements of first pass reverse osmosis unit 518 may be diverted for combination with downstream permeate 530 instead of, or in addition to, diversion of permeate from the front elements of the first pass reverse osmosis unit 518.) The permeate output 542 from the second stage of the second pass reverse osmosis unit 538 is the feedback permeate stream 542, as briefly mentioned above. In another embodiment, part or all of permeate output 542 may be diverted for combination with the second pass reverse osmosis permeate 530, with the remainder (if any) forming feedback permeate stream 542. In the embodiment depicted in Fig. 5, brine output 544 from the second stage of the of the second pass reverse osmosis unit 538 is blended into stream of pre-treated raw seawater 512. This provides the benefit of reduction of the overall salinity of the first pass feed water. As depicted, CO2 from CO2 source 550 may be added to brine output 546 from the first pass reverse osmosis unit 518 before transmission out of the plant.

In another embodiment (not depicted), the brine outputs 546 and 544 from, respectively, the first pass reverse osmosis unit 518 and the second stage of the second pass reverse osmosis unit 538 are combined to form the total brine output from the plant. CO2 from CO2 source 550 may be added to the total brine output before transmission out of the plant. Alternately, or additionally, the total brine output, or at least part of it, may be processed further to extract minerals or additional water, as will be apparent to one of ordinary skill in the art based on the current disclosure.

The depicted CO2 sources 510, 524, 550 and 534 may be the same source of CO2 or different sources.

The SHUQAIQ 2 IWPP plant is another non-limiting example of the use of the present invention. SHUQAIQ 2 IWPP is an integrated water and power plant located in Shuqaiq, Saudi Arabia. The plant is owned by SqWEC (Shuqaiq Water & Electricity Company), and ACWA Power is one of the major shareholders. The plant is operated by the First National Operation & Maintenance Company (NOMAC), which is a wholly-owned subsidiary of ACWA Power. The SWRO plant at SHUQAIQ 2 IWPP can produce 216,000 m³/d of desalinated drinking water. At SHUQAIQ 2 IWPP, HOLLOSEP membranes (cellulose triacetate hollow fiber membranes) from Toyobo are used in the first-pass RO process and FilmTec™ Brackish water membranes, BW30HR-440i from DuPont are used in the second pass. The plant is operated at a 40% first-pass recovery ratio and 90% recovery ratio in the second pass. The plant processes 600,000 m³/d of seawater collected from the Red Sea. The entire RO process at the unit consists of 16 similar trains.

In certain embodiments, the invention may be used in forward osmosis water desalination plants (“FO plant”.) In particular, CO2 may be added to the water stream (e.g., the raw water stream) upstream of the forward osmosis membrane unit. Such pre-treatment provides benefits over the long-term operation of an FO plant. Although considerations for implementing such an FO plant will vary in accordance with the varying local seawater conditions at the location of the plant, implementing the FO plant to take account of such local conditions will be apparent to one of ordinary skill in the art based on the current disclosure. Similar to the situation with RO plants that include CO2 dosing upstream of the RO membrane, pH reduction of the feed water in a FO plant will improve coagulation and reduce scaling potential of sparingly soluble salts. This can be achieved by using CO2 in the pretreatment step for FO. The use of CO2 in this manner will also result in additional CO2 concentration in the permeate and brine outputs. This will aid in further processing of such output streams. For example, the additional CO2 concentration in permeate output will aid in additional processes directed to production of usable, desalinated water.

As described above, all of the embodiments of the present invention decrease the pH of the raw water prior to pretreatment, which results in improved efficiency of the coagulation process and maintains or increases the recovery ratio of the desalination system, while reducing the need for industrial acids (e.g., H2SO4, HC1, or other inorganic acids) and/or antiscaling additives.

While this invention has been particularly shown and described with references to preferred embodiments thereof, in light of the present disclosure it will be understood by persons skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

CLAIMS What is claimed is:

1. A system for desalination of raw water, the system comprising: a source of carbon dioxide gas; a gas transmission pipeline coupled to the source of carbon dioxide gas; and a desalination unit, wherein the desalination unit includes: a feed water unit connected to a source of raw water; a mixing unit coupled to the feed water unit, wherein the mixing unit is additionally coupled to the gas transmission pipeline, and wherein the mixing unit includes a mixing unit output; a pretreatment unit coupled to the mixing unit output, wherein the pretreatment unit includes a pretreatment unit output; a reverse osmosis membrane coupled to the pretreatment unit output, wherein the reverse osmosis membrane includes a first output and a second output; a permeate water unit coupled to the first output of the reverse osmosis membrane, wherein the permeate water unit provides permeate water at a permeate water unit output; and a brine water unit coupled to the second output of the reverse osmosis membrane, wherein the brine water unit provides brine water at the brine water unit output.

2. The system of claim 1, wherein the raw water is seawater or brackish water.

3. The system of claim 1, wherein the source of carbon dioxide gas is at least one from the group comprising emissions from an industrial power plant, a gas storage tank and a commercial provider of carbon dioxide gas.

4. The system of claim 3, wherein the industrial power plant emits the carbon dioxide gas as flue gas.

5. The system of claim 1, wherein the mixing unit is a section of a pipe, a chamber, a tank, a static mixer, or a high pressure injection unit.

6. The system of claim 1, wherein the pH of the raw water at the mixing unit output is below the natural pH of the raw water.

7. The system of claim 6, wherein the pH of the raw water at the mixing unit output is between about 5.0 and about 8.0.

8. The system of claim 6, wherein the pH of the raw water at the mixing unit output is between 6.5 and 7.5.

9. The system of claim 1, wherein the pH of the raw water measured immediately before it enters the reverse osmosis membrane is below the natural pH of the raw water.

10. The system of claim 9, wherein the pH of the raw water measured immediately before it enters the reverse osmosis membrane is between about 5.0 and about 8.0.

11. The system of claim 9, wherein the pH of the raw water measured immediately before it enters the reverse osmosis membrane is between 6.5 and 7.5.

12. The system of claim 9, wherein less than 50 mg/L of an industrial acid is added to the raw water in the pretreatment unit.

13. The system of claim 12, wherein no industrial acid is added to the raw water in the pretreatment unit.

14. The system of claim 1, wherein less than 2 mg/L of antisealant additive is added to the raw water in the pretreatment unit.

15. The system of claim 14, wherein no antisealant additive is added to the raw water in the pretreatment unit.

16. The system of claim 1, wherein the permeate water unit output is coupled to a post treatment unit, wherein the post-treatment unit provides product water at an output of the post-treatment unit.

17. The system of claim 16, wherein less than 50 mg/L of food-grade carbon dioxide gas is added to the permeate water in the post-treatment unit.

18. The system of claim 16, wherein a high-pressure pump is downstream of the mixing unit and upstream of the reverse osmosis membrane.

19. The system of claim 16, wherein the brine water unit output is coupled to a brine water post-treatment unit, wherein the brine water post-treatment unit provides concentrated brine water at a first output and additional permeate or product water at a second output.

20. The system of claim 1, wherein a recovery ratio of the system is at least 40%.

21. The system of claim 12, wherein a recovery ratio of the system is at least 40%.

22. The system of claim 14, wherein a recovery ratio of the system is at least 40%.

23. The system of claim 1 further comprising a post-treatment unit coupled to the output of the permeate water unit output, and a brine-treatment unit coupled to the output of the brine water output unit.

24. A method for desalination of raw water comprising the following steps: mixing carbon dioxide gas with raw water to produce acidified raw water; then providing the acidified raw water to a pretreatment unit to produce treated raw water; then passing the treated raw water through a reverse osmosis membrane; then separating the output of the reverse osmosis membrane into a permeate water stream and a product water stream.

25. The method of claim 24, wherein the raw water is seawater or brackish water.

26. The method of claim 24, wherein the source of carbon dioxide gas is at least one from the group comprising an industrial power plant, a gas storage tank and a commercial provider of carbon dioxide gas.

27. The method of claim 25, wherein the industrial power plant emits the carbon dioxide gas as flue gas.

28. The method of claim 24, wherein the step of mixing carbon dioxide gas with raw water is conducted in a section of a pipe, a chamber, a tank, a static mixer, or a high pressure injection unit.

29. The method of claim 24, wherein the pH of the acidified raw water is below the natural pH of the raw water.

30. The method of claim 29, wherein the pH of the acidified raw water is between about 5.0 and about 8 0

31. The method of claim 29, wherein the pH of the acidified raw water is between 6.5 and 7.5.

32. The method of claim 24, wherein the pH of the treated raw water measured before it enters the reverse osmosis membrane is below the natural pH of the raw water.

33. The method of claim 32, wherein the pH of the treated raw water measured before it enters the reverse osmosis membrane is between about 5.0 and about 8.0.

34. The method of claim 32, wherein the pH of the treated raw water measured before it enters the reverse osmosis membrane is between 6.5 and 7.5.

35. The method of claim 32, wherein less than 50 mg/L of an industrial acid is added to the acidified raw water in the pretreatment unit.

36. The method of claim 35, wherein no industrial acid is added to the acidified raw water in the pretreatment unit.

37. The method of claim 24, wherein less than 2 mg/L of antisealant additive is added to the acidified raw water in the pretreatment unit.

38. The method of claim 37, wherein no antisealant additive is added to the acidified raw water in the pretreatment unit.

39. The method of claim 24, wherein the permeate water stream is provided to a post-treatment unit, wherein the post-treatment unit produces product water.

40. The method of claim 35, wherein less than 50 mg/L of food-grade carbon dioxide gas is added to the permeate water stream in the post-treatment unit.

41. The method of claim 35, wherein a high-pressure pump is used to pass the treated raw water through the reverse osmosis membrane.

42. The method of claim 35, wherein the brine water stream is provided to a brine water post treatment unit, wherein the brine water post-treatment unit produces a concentrated brine water stream at a first output and an additional permeate or product water stream at a second output.

43. The method of claim 24, wherein a recovery ratio of the method for desalination is at least 45%.

44. The method of claim 35, wherein a recovery ratio of the method for desalination is at least 45%.

45. The method of claim 37, wherein a recovery ratio of the method for desalination is at least 45%.

46. A reverse osmosis water desalination plant comprising at least one reverse osmosis membrane unit and a CO2 mixing unit upstream of the at least one reverse osmosis membrane unit.

47. The reverse osmosis water desalination plant of claim 46, wherein the total brine output from the plant has a pH in the range of 3.0 to 6.0.

48. A split partial second pass reverse osmosis water desalination plant comprising at least two reverse osmosis membrane units, wherein each membrane unit comprises one or more elements, and a CO2 mixing unit upstream of each of the at least two reverse osmosis membrane units.

49. The split partial second pass reverse osmosis water desalination plant of claim 47 further comprising a permeate mixing unit separately coupled to each of (i) the output from a plurality of elements of a first reverse osmosis membrane unit, and (ii) a permeate output of a second reverse osmosis membrane unit.

50. A forward osmosis water desalination plant comprising at least one front osmosis membrane unit and a CO2 mixing unit upstream of the at least one forward osmosis membrane unit.