HK1119413A - Extended-life water softening system, apparatus and method - Google Patents

Description

Extended life water softening system, apparatus and method

Correlation reference

[0001] This application claims priority from U.S. provisional application No. 60/698,652, filed 7/2005/12, which is incorporated herein by reference in its entirety.

Technical Field

[0002] The present invention relates to a method and system for water treatment. In particular, the present invention relates to methods and systems for softening potable water, and to methods and systems for extending the operational life of water softening systems; more particularly, the present invention relates to methods and systems having lower water loss when removing ions from drinking water than conventional softening systems.

Background

[0003] Water containing higher concentrations of calcium and magnesium ions is called "hard water" because these two ions can combine with other ions or compounds to form hard, undesirable scale. Millions of households use hard water supplies, particularly those that use groundwater, for example, those that use residential water wells or as part of a municipal water supply. Hard water can form an undesirable film around the sink and utensils and can form hard water deposits on the clothing, causing discoloration and reducing the softness of the fabric. In addition, some soaps and detergents are not as effective when hard water is used as when soft water is used. In this case, an uncomfortable or unsightly thin film of soap may remain on the cleaned person or item.

[0004] About seven to twelve percent of private households use water softeners. In rural areas, the water softener is used at a higher rate than in cities, and it is estimated that about three percent of urban residents use water softeners. In the united states alone, it is estimated that one million ion-exchanged water softeners are sold annually, and that hundreds of millions of dollars are spent on salt. These softeners are often used in homes and small commercial units where groundwater is supplied as their water.

[0005] Although ion exchange softeners are suitable for a variety of applications, there are a number of significant limitations. In particular, the ion exchange water softening treatment results in a net increase in salt content in the discharged water because of the brine discharge. For areas where the reverse brine discharge regulations apply, the net increase in discharged salt content is a troublesome problem. These regulations are generally applicable to areas where the discharge water is utilized as agricultural water and it is desirable to avoid excessive salinity of the land due to the use of the discharge water. In addition, ion exchange softeners require frequent replacement of the sodium salt when the ion exchange resin is regenerated or recharged, thereby incurring maintenance costs associated with purchasing salt.

[0006] In view of the important problems associated with hard water and the limitations of ion-exchanged water softeners, the development and manufacture of water softeners for softening domestic water using nanofiltration elements (nanofiltration elements) is currently ongoing due to the relatively low pressures and relatively high efficiencies. Of particular interest in this field is U.S. patent application No. 09/909488, entitled nanofiltered water softening apparatus and method, to Muralidhara et al. However, despite the significant advances in current softening treatment technologies, there remains a need for improvements in methods and systems for softening water using nanofilter elements, and in particular for membrane elements that have a reduced frequency of replacement and a longer life.

Disclosure of Invention

[0007] Some embodiments of the present invention relate to methods and systems for softening water, and more particularly, to methods and systems for softening water that prevent an increase in ions in a wastewater stream. The system employs a nanofilter element to selectively remove hard ions, particularly large ions (e.g., divalent ions of calcium and magnesium), to achieve water softening without increasing salinity in the wastewater stream.

[0008] In addition, other embodiments of the present invention provide methods and systems for extending the operating life of nanofilter elements in softening treatment systems, and methods and systems for improving the performance of softening systems. These methods and systems are particularly beneficial for multi-element nanofiltration systems having one, two, and particularly three or more nanofiltration (membrane) elements connected together in series. In these nanofiltration softening systems, the drinking water enters a first nanofiltration element and is separated into a softened filtered water stream and a concentrated water stream containing retained calcium and magnesium ions. The softened filtered water stream may be used while the concentrated water stream from the first nanofiltration element is sent to the second nanofiltration element. At the second nanofiltration element, the concentrate water from the first nanofiltration element is again separated into a softened filtered water stream and a concentrate water stream containing the retained calcium and magnesium ions. At the third element system, the concentrated water stream from the second nanofiltration element is sent to the third nanofiltration element and separated again into a softened filtered water stream and a concentrated water stream containing retained calcium and magnesium ions.

[0009] The use of multiple nanofiltration elements is advantageous because it allows for more efficient water utilization and thus allows less water to be discharged into the wastewater stream. However, each subsequent nanofiltration element receives increasing concentrations of calcium and magnesium. This causes a number of different problems, the most significant of which is fouling of the film with calcium and magnesium deposits. Thus, for example, in a three-element system, the third element can form significant calcium precipitates on the membrane surface of the nanofiltration element, thereby significantly reducing the flux through the membrane. In some cases, these precipitates can cause fouling on the membranes to a certain extent and result in premature replacement of the nanofiltration element.

[0010] As described above, some embodiments of the present invention provide methods and systems for extending the operating life of nanofilter elements used in softening systems, and methods and systems for improving the performance of softening systems. These methods and systems are particularly beneficial for multi-element nanofiltration systems having one, two, and particularly three or more nanofiltration elements connected together in series. The improvement resides in a method of periodically reversing the flow of water through a nanofiltration softening system to reduce scaling and fouling of the membranes. In addition, this embodiment provides a flushing mode of operation in which each nanofiltration membrane is flushed with potable water to remove excess calcium and magnesium from the nanofiltration element. In certain embodiments, the rinsing process includes dissolving calcium and magnesium precipitates in the nanofiltration element with a weak acid. These precipitates are then removed from the system and discarded in a wastewater stream.

[0011] Some embodiments of the present invention provide a number of different improvements over previous softening systems, including obtaining soft water that is stable in nature, while having a lower level of bacteria and pyrogens relative to ion exchange softening. In addition, it does not require the addition of salt to the water supply and is therefore more environmentally friendly.

[0012] Generally, the mean pore size of the nanofilter element allows water and most monovalent ions to pass through, but substantially prevents most divalent ions from passing through. Thus, the softener does not add ions to the water stream, but removes at least some of the ions from the input water stream and discharges them into the unfiltered output water stream. A variety of different nanofilter elements, including filter elements having positively charged membranes, are suitable for use in the present invention.

[0013] The above summary of some embodiments of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The figures and the detailed description that follow more particularly exemplify these embodiments.

Drawings

[0014] Specific embodiments of the invention are described in the following description and are shown in the figures. Like numbers refer to like parts throughout.

[0015] Fig. 1 is a simplified schematic diagram of a nanofiltration water softening system comprising three nanofiltration elements according to an embodiment of the present invention.

[0016] Figure 2 is a simplified schematic diagram of a nanofiltration water softening system according to an embodiment of the present invention comprising three nanofiltration elements, which system works with a standard forward input water flow.

[0017] Figure 3 is a simplified schematic diagram of the operation of the nano-filtration water softening system of figure 2 operating with the input water flow reversed.

[0018] Figure 4 is a simplified schematic diagram of a nano-filtration water softening system according to an embodiment of the present invention, which operates in a flush mode with a bypass water flow.

[0019] FIG. 5 is a simplified schematic of a nanofiltration water softener according to an embodiment of the present invention, the system being configured and operated in an acid flush mode to remove sediment from the nanofiltration element.

[0020] FIG. 6 is a graph showing the effect of acid rinse on water softening system flux.

[0021] Figure 7 is a graph showing the effect of flushing a nanofiltration element on the water flux through a softening system.

[0022] Figure 8 is a graph showing the effect of flushing and reversing water flow on water flux through a softening system.

[0023] Figure 9 shows the effect of acid rinse on water flux through the softening system.

[0024] Figure 10 shows the effect of time on filtration flux and interception.

[0025] Figure 11 shows the effect of time on filtration flux and hardness.

[0026] Fig. 12 shows the effect of time on the filtered flow rate of boiler feed water.

[0027] Figure 13 shows the effect of time on filtration flux and hardness.

[0028] Figure 14 shows the effect of time on filtration flux and interception.

Detailed Description

[0029] The following description of the invention is intended to illustrate various embodiments of the invention. Also, the specific transformations described below are not to be construed as limiting the scope of the invention. Numerous and varied equivalents, changes and modifications will occur to those skilled in the art without departing from the scope of the invention, and it is therefore contemplated that such equivalent embodiments are also within the scope of the invention.

[0030] An embodiment of the present invention provides an apparatus and method for softening water, and in particular, an apparatus and method for softening water without increasing the number of ions in the wastewater stream. The present embodiments provide methods and systems for extending the operational life of nanofilter elements in softening systems, and methods and systems for improving the performance of softening systems. The improvement is to periodically reverse the flow of water through the nanofiltration softening system to avoid scaling and fouling of the membranes.

[0031] In addition, the present embodiment provides a rinsing mode of operation, wherein the potable water rinses each nanofiltration membrane to remove excess calcium and magnesium from the nanofiltration element. In certain embodiments, such washing includes dissolving all calcium and magnesium precipitates with a weak acid, followed by removing these precipitates from the system and discarding them in a wastewater stream.

[0032] The present embodiments provide methods and systems for extending the operational life of nanofilter elements in softening systems, and methods and systems for improving the performance of softening systems. These methods and systems are particularly beneficial for multi-element nanofiltration systems having at least one, typically two, and particularly three or more nanofiltration elements connected together in series. In these nanofiltration softening systems, the drinking water enters a first nanofiltration element and is separated into a softened filtered water stream and a concentrated water stream containing retained calcium and magnesium ions.

[0033] The softened filtered water stream is directed for use, while the concentrate from the first nanofiltration element is transported to the second nanofiltration element. At the second nanofiltration element, the concentrate water from the first nanofiltration element is again separated into a softened filtered water stream and a concentrate water stream containing the retained calcium and magnesium ions. At the third element system, the concentrated water stream from the second nanofiltration element is sent to a third nanofiltration element and is again separated into a softened filtered water stream and a concentrated water stream containing the retained calcium and magnesium ions.

[0034] Having multiple nanofiltration elements is advantageous because it allows for efficient water utilization, particularly in further reducing the amount of water discharged into the wastewater stream. Each subsequent nanofiltration element receives increasing concentrations of calcium and magnesium. This causes a number of different problems, the most serious being fouling of the film with calcium and magnesium precipitates. Thus, for example, in a three-element system, the third element can form more calcium precipitates on the membrane surface of the nanofiltration element, thereby significantly reducing flux. In some cases, these deposits can cause fouling of the membrane to the extent that it can only be replaced prematurely.

[0035] Figure 1 shows a simple schematic diagram of a first embodiment of the invention. As shown in fig. 1, system 10 includes three nanofiltration elements 12, 14, and 16 in series. As noted above, the system of the present invention can include more or less than three nanofiltration elements. Thus, for example, in some embodiments, system 10 includes only two nanofiltration elements, while in other embodiments, system 10 includes four, five, or more elements. Also, certain forms of the invention, such as flushing the nanofiltration element with a low pH solution, are also suitable for use when there is only one nanofiltration element.

[0036] The system 10 of fig. 1 includes a water supply 70 from a source, such as a residential water well or from a municipal water supply. Fig. 1 and the following figures are simplified to clearly illustrate the main elements and the arrangement of these elements. For example, the system 10 generally includes a plurality of valves that can effect a change in flow direction. These valves are not shown in the figures but can be deduced from the description of the water flow.

[0037] The water from the water supply 70 is typically first subjected to one or more pre-filters or treatment steps, such as a particulate filter 60 and an activated carbon filter 62. These filters 60, 62 are generally optional and can significantly increase the operational life of the nanofiltration elements 12, 14, 16. After passing through the prefilters 60, 62, the water flows along the pipe 20 (typically a plastic or metal pipe or tube) and into the first nanofiltration element 12. The water entering the nanofiltration element 12 is separated into two streams: a softened filtered water stream and an un-softened concentrated water stream, the concentrated water stream having a higher hardness than the water entering the nanofiltration element 12. The filtered water stream exits the nanofiltration element 12 and is directed by the pipe 30 to a reservoir 40 or is carried directly for end use, such as directly into a residential water supply.

[0038] The concentrated water stream exits the nanofiltration element 12 and is directed by the conduit 22 to the second nanofiltration element 14. The water stream entering the second nanofiltration element 14 is again separated into a filtered water stream and a concentrated water stream. The filtered water stream is directed by the conduit 32 into the reservoir 40 or is carried directly for final use. Typically the filtered water streams from conduits 30 and 32 are both treated the same, i.e. either to the same storage or directly to the water supply. The concentrated water stream from the nanofiltration element 14 exits the element 14 through a conduit 24, and the conduit 24 carries the water stream to the nanofiltration element 16. The nanofiltration element takes this concentrated water stream from element 14 and delivers it to nanofiltration element 16, which is more concentrated than the concentrated water stream from element 12. The nanofiltration element 16 again separates the incoming water stream into two distinct output water streams. The first stream is a softened filtered water stream which exits the element 16 through conduit 34 and either enters the reservoir 40 directly or is used as softened water. The concentrated water stream from nanofiltration element 16 is discharged through conduit 26 to a discharge terminal 50, which discharge terminal 50 is typically a sanitary drain or other waste water terminal.

[0039] Fig. 2 shows a nanofiltration system similar to that of fig. 1, with the addition that the nanofiltration system 10 has the ability to reverse the flow of water through the nanofiltration elements 12, 14, 16 in order to prevent or reduce salt precipitation, particularly calcium and magnesium salts, on the nanofiltration elements. The arrows in fig. 2 indicate the direction of water flow within the system 10. Nanofiltration water softening system 10 comprises an additional conduit 25 which allows water flow from water source 70 up into conduit 26, then into nanofiltration element 16, then into nanofiltration element 14, and finally into nanofiltration element 12, exiting nanofiltration element 12 and being directed by conduit 27 back to discharge conduit 31, which discharge conduit 31 directs it to discharge terminal 50. Conduits 34, 32 and 30 carry the softened filtered water stream sequentially away from the nanofiltration element, while conduits 24 and 22 connect the nanofiltration element.

[0040] The benefit of the operation of the system shown in figure 2 is that it allows the water flow to circulate so that the sequence of water flow through the membranes is periodically reversed. During a first time period, the water flows in a first direction, and during a second time period, the water flows in the opposite direction. This avoids excessive concentration of calcium and magnesium ions on the last nanofiltration membrane resulting in precipitation of ions on the membrane. Depending on the nature of the input water stream, the reversed water stream may even remove some of the precipitate from the nanofiltration membrane.

[0041] Fig. 3 shows the same nanofiltration softening system as fig. 2, but with the water flow sequence through the nanofiltration elements 12, 14, 16 reversed as indicated by the water flow arrows.

[0042] The present invention may be employed with a variety of different nanofilter elements. The filter element should be suitable for softening hard water, which can provide a suitable high flow rate and recovery rate at low pressure. Thus, not all nanofiltration elements can provide suitable hard ion rejection ratios, water flow, and water recovery ratios. Suitable nanofiltration elements are described in more detail below.

[0043] The dimensions of the nanofiltration element are generally selected according to the requirements of the application in which it is to be used. Thus, the length, width, and surface area of the nanofiltration element can be selected to enhance the suitability of the softening device for a particular application. Nanofiltration elements come in a variety of different configurations, including spiral membrane elements, hollow fiber elements, and tubular elements. Typically the nanofiltration element is a spiral membrane element.

[0044] The nanofiltration element generally has a surface area greater than 2.0 square meters but less than 40 square meters, and typically 7 to 40 square meters. The nanofiltration element should not be too long or it would require the production of a large housing that is not suitable for residential use. In general, the softening unit should be selected to be a nanofiltration element suitable for the area of home use. For example, a suitable element may be a filter having an overall length of 40 to 125 centimeters. Typical nanofiltration elements suitable for use in the present invention have a diameter of 5 to 25 cm.

[0045] For example, nanofiltration membranes suitable for use in water-softening units include the following: the Dow Film Tec NF90, which is a polyamide Film synthetic Film; the Dow Film TeCnF270, which is a polyamide Film synthetic Film; the Dow Film Tec NF 200, which is a polyamide Film composition; the Trisep TS 83, which is an aromatic polyamide film; the Trisep TS 80, which is an aromatic polyamide; PTI-AFM NPs, which are polyamide thin film compositions; and Koch film TFC-SR1, which is a thin film synthetic polyamide film. NF90 has proven to be a particularly useful membrane having approximately five to fifteen percent solute channels, and 21.4LMH flux, fifteen parts per million (15ppm) total hardness, three parts per million calcium ions, and two parts per million magnesium.

[0046] Table 1 below shows the results of using six different membranes and hardness analysis of the filtered and input water when municipal water supply was used. All experiments used flat films at room temperature and 70 pounds per square inch (psi) pressure.

TABLE 1

Sample(s)	Flow-through volume (LMH)	Total hardness (ppm)	Calcium (ppm)	Magnesium (ppm)
					Initial input	N/A	182	45	17
NF 90	21.4	15	3	2
					NF 270	38	117	32	9
NF 200	9.5	101	32	5
					TRISEP TS83	15.8	61	16	5
TRISEP TS80	18.8	40	16	0
					PTI-AFM NP	26.4	117	32	9

[0047] In general, nanofiltration elements suitable for use in the present invention have a high rejection ratio for divalent ions which, together with a sufficient flow of relatively low pressure water through the nanofiltration element, provide a sufficiently high flow-through ratio and recovery ratio to meet the needs of most domestic consumers. These divalent ions include many hard ions such as calcium and magnesium. The average peak flow rate through the filter is expressed as the flow rate. The percentage of input water that is recovered as softened water relative to the total amount of water entering the water softener is expressed as the recovery ratio. While these specific parameters are important individually, the combination of these parameters is particularly important for providing a water softener suitable for residential or small commercial use.

[0048] Typical nanofilter elements have an average pore size that allows water and monovalent ions to pass through but can block divalent ions, particularly divalent ions related to the hardness of water. A number of different ions can be used to measure the blocking ratio, one of which is suitable for such measurements is calcium ion. Typical nanofilter elements suitable for the present invention can generally limit more than eighty percent of calcium particles passing through the filter element under operating conditions. More preferred filter elements may limit more than eighty-five percent of the calcium ions passing through the filter under operating conditions. More suitable filter elements can block more than ninety percent of the calcium ions under operating conditions. The nanofiltration element must have sufficient water filtration flux. For example, in certain embodiments, the flow-through of deionized water through the nanofiltration element is about 30 liters per square meter per hour (liters per square meter per hour, 1mh) per square meter of filtration membrane at a pressure of 30-60 pounds per square foot (psi).

[0049] Generally suitable nanofiltration elements typically have a filtration cutoff diameter of 20 to 500 molecular weight, more commonly 100 to 400, and most commonly 200 to 300. As used herein, filtration cut-off (expressed in molecular weight) follows the convention of filtration measurement, i.e., the range of molecular weights expressed as species that can be cut-off at a high rate. However, often small amounts of material with molecular weights in the filter cut-off range still pass through these membranes. In addition, it may also happen that molecules outside the blocking range are blocked at a relatively high rate, but this blocking rate is usually lower relative to that within the blocking range. The use of a higher molecular weight cut-off filter increases the water flux. In this case, the filter element having a molecular weight cutoff range of 200 to 300 can ensure sufficient cutoff of calcium ions and passage of an appropriate amount of water.

[0050] The configuration of the device is advantageous in that it does not substantially increase the total amount of salt relative to the incoming water flow. Thus, the softener does not add ions to the water stream, but rather removes at least some of the ions from the input water stream and discharges them into the un-softened output water stream. A variety of different nanofilter elements are suitable for use with the present invention, including filter elements having positively charged membranes, as such membranes generally repel positive and divalent hard ions and thereby restrict their passage through the membrane.

[0051] The hard water softener of the present invention is typically designed to provide high quality water softening treatment for small scale requirements of residential (and similar) applications. Water softeners typically provide sufficient flow of water so that a reservoir or pressure vessel is no longer required to hold the softened and stored water. Water softeners are therefore generally able to provide adequate instantaneous water softening to meet the needs of the average household. It is beneficial for the consumer to avoid the use of a water reservoir because the likelihood of microbial contamination of the reservoir is reduced. In addition, avoiding the use of a water storage tank reduces the size and cost of the water softener. However, in some applications containers are still used to store at least a portion of the softened water to meet peak demand.

[0052] The present invention may also employ a variety of different prefilters to improve the performance and life of the nanofiltration element. For example, prefilters are used to remove large suspended matter that may clog the nanofilter elements. The present invention may also be used with other prefilters, an iron prefilter to remove iron from the input water source, a sediment prefilter to remove sediment from the input water source, a chlorine prefilter to remove chlorine from the input water source, and a biological prefilter to remove bacteria, protozoa, and other microorganisms.

[0053] In addition to the use of a pre-filter, the water may be pre-treated to improve its performance, or heated sufficiently to increase its flow rate without scale formation, or magnetically pre-treated to suppress scale in the input water. Other pretreatment steps, such as chemical pretreatment, are also suitable for use with embodiments of the present invention.

[0054] The softened water of the present invention is typically potable water, such as from a ground water source. For example, the water may be from a private residential well, or from a municipal water supply (typically containing groundwater), or other source. Although the water supply is generally potable, non-potable water may be used in certain embodiments by employing a prefilter that removes contaminants such as Cryptosporidium (Cryptosporidium).

[0055] The water softener of the present invention has a general size such that it can be placed in a space equal to or smaller than that required by conventional ion exchange water softeners. This allows the present softener to be used as a replacement for existing softeners. In particular embodiments, the softener of the present invention is designed to be significantly smaller than an ion exchange softener having the same softening capacity as it. This reduction in size is possible because ion exchange media or water storage is not necessary here.

[0056] As noted above, the water softeners of the present invention are generally formulated to operate at relatively low pressures, typically less than 250 pounds per square inch gauge (psig). This low pressure avoids the use of expensive pressure boosting equipment. Certain embodiments of the present invention provide devices that are manufactured and formulated to have a filtered water output of 200 gallons or more per 24 hours. Typically the device may have a peak filtered water output of less than 10 gallons per minute, more typically 5 to 10 gallons per minute. The softening unit is generally highly efficient, producing output filtered water containing more than eighty percent of the input water volume. In some embodiments, the output filtered water comprises more than ninety percent of the input water volume. For example, the output filtered water typically has a hardness of less than 1.5 grains per gallon (grains).

[0057] In some embodiments, the function of the membrane elements may be improved by reversing the flow of water through the membrane elements and flushing the concentrate with input water, thereby improving performance and reducing scaling problems, thereby maintaining a certain level of throughput.

[0058] Embodiments of the present invention also relate to regenerating nanofiltration softening elements by rinsing the membranes with an acidic solution to dissolve calcium and magnesium precipitates. Acidic rinse is typically used when the nanofiltration system can no longer soften the water for end use, so it is preferable to schedule several hours of acidic rinse function at lower water usage, such as at a later time in the evening. At the same time, the nanofiltration element that is generally flushed is easily isolated from the rest of the water system, so that the acid can be flushed through the nanofiltration element in a closed loop without transporting the acidic water to the end user. After flushing through the nanofiltration element with acid, the acidic water can be discharged through a waste line, i.e. generally the same line that carries the concentrated water away from the nanofiltration element.

[0059] The acid used to regenerate the nanofiltration element is preferably approved for human use by the Food and Drug Administration (FDA) and is food grade. Suitable acids include, for example: acetic acid, hydrochloric acid, lactic acid, and combinations thereof. Other suitable acids include phosphoric acid, citric acid, nitric acid, sulfuric acid, and the like. For example, suitable mixtures include: 2% to 3% acetic acid, 3% to 5% hydrochloric acid and 0.05% to 0.1% lactic acid.

[0060] Suitable pH values include, for example, pH values of 2 to 2.5. Acceptable pH values are generally below 6.0, typically below 5.0, and may also be below 4.0, and in some embodiments below 3.0. The acid solution is more effective at increasing temperatures and therefore the system may also include a heater to heat the acid solution prior to directing it through the nanofiltration element. Suitable temperatures for acidic rinsing are, for example: above 25 ℃, above 30 ℃, above 40 ℃ and below 50 ℃. Likewise, a temperature range of 25 ℃ to 45 ℃ may be used, as may a temperature of 30 ℃ to 40 ℃ and a temperature of 40 ℃ to 45 ℃.

[0061] Figure 6 shows the effect of using an acidic rinse through the nanofiltration membrane to promote increased nanofiltration element flux. The experiments shown in FIGS. 9, 10 and 11 employ Dow Filmtec NF90-4040 membranes which have a membrane area of about 22.3 square meters. The municipal water supply pressure for Savage (Savage), minnesota was 47 psig and the temperature was 18 degrees celsius. The membranes had an initial input (d.i.) water flux of 2.25 gallons per minute, but after 160 hours of use, i.e., 14,250 gallons of water were softened, the membranes fouled to some extent and their flux was reduced to approximately.75 gallons per minute. The throughput was increased to 1.25 gallons per minute by flushing the fouled membranes with 10 gallons of 3-5% strength hydrochloric acid solution for 30-45 minutes. The input (d.i.) water flux was increased to 2.2 gallons per minute by flushing the fouled membranes with 10 gallons of 3-5% strength hydrochloric acid along with 0.05-0.1% strength lactic acid solution for 30-45 minutes. Figure 10 shows the effect of time on filtration flux and interception, illustrating that although flux diminishes with time, the interception remains above 95%. While figure 11 shows the effect of time on filtration flux and stiffness, it shows that the total filtration stiffness remains below fifteen parts per million despite the flux becoming smaller over time. Fig. 10 and 11 show that embodiments of the present invention are particularly well suited for long-term softening applications.

[0062] In some embodiments, the nanofiltration membrane is washed with an acidic solution having a pH of 4 to 4.5 at a temperature of at least 30 ℃ for 5 minutes every 100 hours. In other embodiments, the nanofiltration membrane is washed with an acidic solution having a pH of 3 to 3.5 at a temperature of at least 25 ℃ for 5 minutes every 100 hours. In yet other embodiments, the nanofiltration membrane is washed with an acidic solution having a pH of 2 to 2.5 at a temperature of at least 20 ℃ for 5 minutes every 100 hours.

[0063] Another embodiment of the present invention provides for removing hardness from boiler feed water to achieve long term effective use of the boiler. By reducing the hardness of the boiler input water, the life of the boiler can be extended and the energy costs and chemical treatment costs of boiler operation can be reduced. This embodiment employs any one or a combination of the preceding embodiments to treat boiler input water. Additionally, the boiler input water may be pretreated with carbon or other filters or other treatment methods known in the art, depending on the composition of the boiler input water, prior to the aforementioned nanofiltration. Referring to fig. 12, the effect of time on the filtration throughput is shown. As shown in fig. 12, after prolonged use, the flux had decreased by 33% over 800 hours of uninterrupted operation. The initial flux can be restored by treatment with mineral acids or the like. Referring to fig. 13, the effect of time on filtration flux and hardness is shown. As shown in fig. 13, the hardness remained below eight parts per million for over 800 hours of uninterrupted operation after extended use, indicating that the present method and apparatus is suitable for boiler feed water applications. Referring to fig. 14, the effect of time on filtration flux and hold-up is shown. As shown in fig. 14, after extended use, the hold-off remained above about 95% for over 800 hours of uninterrupted operation, again indicating that the present method and apparatus is suitable for boiler feed water applications.

[0064] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The specification is to be considered as exemplary only, with the following claims being indicative of the full scope and spirit of the invention.

[0065] In the foregoing specification, the invention has been described in terms of certain preferred embodiments and numerous details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic spirit of the invention.

Claims (25)

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

(i) providing at least one first nanofiltration element;

(ii) providing at least one second nanofiltration element, which is connected in series with the first nanofiltration element;

(iii) providing a drinking water source;

(iv) make the drinking water

a) First passing through the first nanofiltration element for a first period of time to form a first softened filtered water stream having a lower hardness than the potable water source and a first concentrated water stream having a higher hardness than the potable water source, and

b) subsequently, passing the first concentrated water stream through the second nanofiltration element, forming a second softened filtered water stream having a lower hardness than the drinking water source and a second concentrated water stream having a higher hardness than the drinking water source;

(v) reversing the flow of the drinking water so that drinking water from the drinking water source

a) First passing through the second nanofiltration element for a second time period, forming a softened filtered water stream having a lower hardness than the drinking water source and a concentrated water stream having a higher hardness than the drinking water source, and

b) subsequently passing said concentrated water stream through said first nanofiltration element, forming a softened filtered water stream having a lower hardness than said source of potable water; and

(vi) repeating steps (iv) and (v).

2. The method of softening water of claim 1, wherein the first nanofiltration element is configured to reject at least 80% of calcium ions.

3. The method of softening water of claim 1, wherein the first nanofiltration element is configured to reject at least 80% of calcium ions.

4. The method of claim 1, further comprising a third nanofiltration element between the first and second nanofiltration elements.

5. The method of claim 1, wherein the first period is less than 2 hours.

6. The method of claim 1, wherein the first period is less than 1 hour.

7. The method of claim 1, wherein the first period is less than 30 minutes.

8. The method of claim 1, wherein the first period is at least 10 minutes.

9. The method of claim 1, wherein the second period is less than 2 hours in time.

10. The method of claim 1, wherein the second period is less than 1 hour in time.

11. The method of claim 1, wherein the second period is less than 30 minutes in time.

12. The method of claim 1, wherein the second period is at least 10 minutes in time.

13. The method of claim 1, further comprising cleaning the nanofilter element for at least 30 seconds.

14. The method of claim 1, further comprising cleaning the nanofiltration element in less than 5 minutes.

15. The method of claim 1, further comprising cleaning the nanofiltration element in less than ten percent of the softening cycle.

16. The method of claim 1, further comprising cleaning the nanofiltration element for less than five percent of the softening cycle.

17. The method of claim 1, further comprising cleaning the system with a composition of an acid.

18. The method of claim 1, wherein the acid is selected from the group consisting of: hydrochloric acid, acetic acid, lactic acid, and combinations thereof.

19. The method of claim 1, wherein the acid is selected from the group consisting of: phosphoric acid, sulfuric acid, citric acid, and combinations of the foregoing.

20. A method of softening water, the method comprising:

(i) providing a first nanofiltration element capable of intercepting at least 80% of calcium ions;

(ii) providing a second nanofiltration element capable of intercepting at least 80% of calcium ions, the nanofiltration element being connected in series with the first nanofiltration element;

(iii) providing a drinking water source;

(iv) passing the potable water through the first nanofiltration element and then into the second nanofiltration element for a first period of time;

(v) reversing the flow of the potable water through the second nanofiltration element and then into the first nanofiltration element for a second time period, wherein the second time period is shorter than the first time period;

repeating steps (iv) and (v) during the performance of the method.

21. The method of claim 20, further comprising a third filter element disposed between the first and second filter elements such that water flow between the first and second filter elements passes through the third filter element.

22. The method of claim 20, wherein the first time period is 20 to 30 minutes and the second time period is 20 to 30 minutes.

23. The method of claim 20, further comprising adding an acid.

24. The method of softening water in accordance with claim 20, wherein the input water stream has a pressure of 10 to 200 psig.

25. The method of softening water in accordance with claim 20, wherein the input water stream has a pressure of 25 to 50 psig.