Patent Application
20200398223
The present invention relates to design and operation of facilities associated with generation of potable water, and in particular to a system and method for efficient and economical recovery of minerals and re-mineralization of water using minerals generated as a byproduct of operation of a divalent ion-selective filtration process.
Distillate water produced by a desalination process (i.e., desalinated water) is known to typically be slightly acidic, typically in the range of 6.0-6.5 pH in the case of distillate from a thermal evaporator process. This is also the case for permeate (desalinated water) produced using a reverse osmosis (RO) membrane, because bicarbonate ions (HCO3−) are largely rejected at the RO membrane and discharged in concentrated form in the reverse osmosis retentate outlet stream, while most of the CO2 in the feed water passes through the RO membrane in the permeate outlet stream.
Due to the slight acidity of the desalinated water from such processes, the water needs to be treated to raise its pH in order to protect the downstream water transportation and distribution system from corrosion, as well as to meet drinking water guidelines. In the prior art, this has been accomplished in thermal and membrane desalination plants with post-treatment re-mineralization (also referred to as potabilization or re-carbonation) in a system that treats the desalinated water to produce potable water for civil consumption. A typical re-mineralization treatment includes increasing the concentrations of calcium ions (Ca++) and bicarbonate ions (HCO3−). Usually total dissolved solids (TDS) and pH are increased through the re-mineralization, resulting in the final potable water having a better taste and a slightly positive saturation index.
An example design guide for potable water quality to be considered in designing the re-mineralization system of a desalination plant is summarized in Table 1.
An example of a common re-mineralization process in the desalination industry is schematically illustrated in
In this system an inlet flow of desalinated water 1 enters the re-mineralization system, whereupon approximately 20-50% of the desalinated water is diverted via branch 2 to a re-mineralization process train. The first step of the re-mineralization process is to inject CO2 gas 3 into a portion of the diverted desalinated water in a CO2 absorber 4. The CO2 gas may be supplied, for example, in the form of released CO2 gas generated in an upstream thermal evaporator, CO2 received from a CO2 generation plant, or from some other source. The chemical reaction in the CO2 absorption process is CO2+H2O→H2CO3.
Following the water acidifying process, a re-carbonation process is performed in a limestone filter 5, where the portion of the diverted desalinated water that passes through the CO2 absorber 4 rejoins the remainder of the diverted desalinated water. In this re-carbonization step the acidified water reacts with limestone (CaCO3), according to the relationship H2CO3+CaCO3→Ca(HCO3)2. The stoichiometry of the reactions provides for a mole of calcium bicarbonate (Ca(HCO3)2) to be produced for each mole of carbon dioxide (CO2) that reacts.
Following re-carbonization, the output from the limestone filter 5 passes to a degassifier 6 which removes remaining free CO2, in preparation for the final stages of the re-mineralization process.
After degasification the diverted portion of the desalinated water is rejoined with the portion that bypasses the re-mineralization process train. The entire volume of water may then be further treated by injection of chemicals that complete preparation of the desalinated water for potable use, such as injection of chlorine gas 7 for disinfection purposes and injection of sodium hydroxide (NaOH) to meet pH and LSI parameter objectives (the absorbed CO2 reacts with the NaOH according to the relationship NaOH+CO2→Na++HCO3−). The previous removal of free CO2 in the degasification process minimizes the amount of NaOH that would otherwise be depleted by the excess CO2, thus minimizing the amount of NaOH that must be supplied to the remineralization process. The resulting potable water 9 is then ready for downstream transport and consumption.
Example parameters at different locations along the
Another common re-mineralization practice in the desalination industry is to use lime milk (Ca(OH)2). In this process, the acidified diverted portion of the desalinated water from the CO2 absorber reacts with lime in the re-carbonization step to reach a CaCO3 saturation point according to the relationships of 2CO2+Ca(OH)2→2Ca++2HCO3−, and Ca(HCO3)2+Ca(OH)2→2CaCO3+2H2O.
Other re-mineralization processes are also known, although they usually are less economic than the above examples. These processes include use of hydrated lime+sodium carbonate (Ca(OH)2+Na2CO3→CaCO3+2NaOH), sodium bicarbonate (CaSO4+2NaHCO3→Ca(HCO3)2+Na2SO4), and calcium chloride (CaCl2+2NaHCO3→Ca(HCO3)2+2NaCl).
In a re-mineralization process, the higher the desalinated water's saturation index and pH, the easier the desalinated water is to treat, which saves operational and capital costs.
An object of the present invention is to reduce or eliminate the need for procuring and using external chemicals (i.e., chemical supplies that must be brought in from outside sources) for re-mineralization of desalinated water. This and other objectives are achieved in association with operation of a desalination facility using nanofiltration (NF) technology, such as the system described in U.S. patent application Ser. No. 16/371,816, the disclosure of which is incorporated herein by reference.
The output from nanofiltration of a saline source water is a concentrated brine (i.e., retentate) that is rich in concentrated divalent ions, i.e., a concentration several times higher than naturally occurring in source saline water (e.g., seawater). A portion of the NF retentate output stream is used as a source of minerals for re-mineralization, thereby reducing or eliminating the need to use commercial re-mineralization chemicals such as lime or limestone, and providing the opportunity for beneficial reuse of a portion of the brine generated by the desalination plant. Both of these advantages can lead to substantial reductions in the overall cost of operating a desalination plant.
In an example embodiment of the invention, a nanofiltration system is located upstream of a main desalination process (for example, a thermal or membrane desalination process). The NF system may treat all or a portion of the source saline water that is supplied to a main desalination process. Because of the pore size of NF, NF membranes are ion-selective, with higher rejection of divalent ions (including minerals such as calcium) and lower rejection of monovalent ions (such as sodium and chloride). The nanofiltration system therefore removes ions associated with undesired mineral deposits and scaling in downstream desalinators, while generating a reject (retentate) stream that is rich in divalent ions such as calcium and magnesium and low in monovalent ions. The lower-concentration NF permeate stream also benefits the desalination system by permitting lower boiling point elevation loss in thermal desalination processes and lower feed pressure requirements in membrane desalination processes.
The NF permeate stream is supplied to a downstream desalination system, while the NF retentate stream is discharged from the NF system. A portion of the NF retentate stream may be branched off to be fed into a re-mineralization system, where it is mixed with the desalinated water produced by the desalination system to re-mineralize the desalinated water and produce potable water. The ratio of NF retentate mixed with the desalinated water may be adjusted to obtain a desired potable water quality. NF retentate input to the re-mineralization system at a rate on the order of 1% of the flow of the desalinated water stream may be sufficient to treat the desalinated water.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
In this example, saline source water 110 passes through an initial pre-treatment process 120, such as debris removal, prior to some or all of the source water being input into the NF unit 130. If only a portion of the source water is to be treated by the NF unit 130, the remainder may bypass the NF unit via line 131 to be supplied to the downstream desalination unit 140. The desalination unit 140 further separates the saline water it receives into a desalination retentate stream 141 and a desalination permeate (desalinated water) stream 142.
In the NF unit 130, the source water is separated by membrane into a high divalent ion-concentration retentate stream that is rejected from the unit via discharge line 132, and into a low divalent ion-concentration permeate stream 133 that is input to the desalination unit 140, along with any bypass source water flow through line 131. A portion of the NF retentate stream 134 that is discharged via line 132 may be diverted via line 135 for use in the post-treatment re-mineralization unit 150.
At the re-mineralization unit 150 the desalination unit permeate stream 142 is mixed with the portion of the NF retentate discharge stream diverted via line 135 to treat the desalinated water to generate the output potable water stream 160.
The fluid flows and energy consumed in this desalination process should be balanced as necessary to maximize potable water production at the lowest operating cost. In one embodiment of the present invention, the desalinated water stream from the desalination unit 140 has a system TDS of 50 ppm and a pH of 5.2, with an estimated LSI value of −6.7. The NF retentate stream 132 has a TDS of 47,895 ppm (the dissolved solids being very rich in divalent ions useful in re-mineralizing desalinated water, either in the same desalination plant or a different desalination or in another water reclamation/wastewater treatment facility). In this example, a typical 1.1% volume mix of NF retentate 132 to the desalinated water 142 results in the mixed desalinated water and NF retentate having at least: (i) a better LSI value to protect receiving water piping against corrosion; (ii) elevated mineral content adequate to meet the drinking water quality requirements of the World Health Organization, US EPA drinking water regulations, and European Union drinking water standards; and (iii) an increase of the total amount of water produced by the desalination facility.
Example operating parameters in this embodiment are shown in Table 3, below. Note that the pH and LSI of the re-mineralized water in Table 3 could be further adjusted by adding CO2 (from thermal desalination evaporator or other source), or other known methods, to reach the pH range of 6.5 to 9.0, and an LSI value close to neutral or a bit positive (e.g., +0.1-+0.3).
In a further embodiment, NF may be used to treat only a small portion of the source seawater, either after pretreatment or installed separately and independently of the desalination system, in order to produce NF retentate desirably rich in calcium and magnesium to be utilized in the re-mineralization process.
The present invention's use of an NF retentate discharge stream which is rich is divalent ions for re-mineralization of desalinated water is applicable to desalination plants that process source brackish water, seawater and/or wastewater, and is not limited to NF processing solely upstream of a desalination process. For example, in another embodiment of the present invention as illustrated in
In this embodiment saline source water 210 passes through an initial pre-treatment process 220 prior the source water being directed to a desalination unit 240. The desalination unit 240 separates the saline water it receives into a desalination retentate stream 241 and a desalination permeate (desalinated water) stream 242. At least a portion of the desalination retentate stream 241 enters a NF unit 230, which separates the desalination retentate into an NF high divalent ion retentate stream that is rejected from the unit via discharge line 232, and into a low divalent ion-concentration NF permeate stream 233. The NF permeate stream 233 may be separately utilized in other processes, or may be recycled to the inlet of the desalination unit 240 to maximize desalinated water production and, due to its low divalent ion concentration, to assist in decreasing deposit and scale formation in the desalination unit. If only a portion of the source water is to be treated by the NF unit 230, the remainder may bypass the NF unit via line 243.
A portion of the NF retentate stream 234 that is discharged via line 232 may be diverted via line 235 for use in the post-treatment re-mineralization unit 250. At the re-mineralization unit 250, the desalination unit permeate stream 242 (i.e., the desalinated water) is mixed with the portion of the NF retentate discharge stream diverted via line 235 to treat the desalinated water to generate the output potable water stream 260.
Another embodiment similar to the
Another embodiment of the present invention is arranged as in the
In the present invention, the desalinated water quality is highly dependent on the desalination system performance, and the quality of the water being input into the system. Similarly, the NF retentate quality is also highly dependent on the NF membrane performance and the NF system configuration and operating conditions. Consequently, the 1.1% mixing of NF retentate in the foregoing embodiments is only an example. The mixing rate may be adjusted as necessary to meet the desired end-product drinking water qualities.
The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Because such modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.
