Patent Application
20070045187
The present technique relates generally to water-purification systems and methods, and, in certain exemplary embodiments, to techniques for reducing biofouling of a membrane in a membrane-based water-purification system, for example.
Various types of water-purification systems are known and in use. In general, a membrane water-purification system serves to remove all ions in water, decreasing the likelihood of the formation of scaling that often manifests itself as an unattractive film around sinks and dishes, for instance. As an example of a traditional technique, employing a reverse osmosis membrane separates ions and minerals from water received at high pressure, thus reducing the presence of scaling. Another traditional technique employs nanofiltration membranes to purify water for residential use and is generally located at a “point of entry” or a “point of use” of the residence. Nanofiltration membranes typically include a semi-permeable membrane that relies on surface charges to selectively reject divalent and polyvalent ions (i.e. hardness ions) while allowing passage of monovalent ions, thus, again, reducing the presence of scaling in the water supply.
Unfortunately, these membrane separation processes are prone to fouling by microbes. For example, microbes, over time, accumulate on the membrane (often referred to as “biofouling”), and this biofouling causes a decrease in permeate effluent and an increase in pressure differential, both of which are generally undesirable. In addition, continued operation of a water-purification system in such conditions generally requires an increased number of cleanings over the lifetime of the membrane, thereby decreasing the membrane life and increasing maintenance costs, for instance. Typically, performance of membrane-based water purifiers is reduced by membrane biofouling. Further, the performance of the membrane is also degraded by the presence of chlorine and chloramines in the influent water. Therefore, it is advantageous to remove the chlorine or chloramines from the influent water prior to treating the water with the membrane-based systems. It is also advantageous to remove chlorine and chloramines from the influent water to improve the potable quality of the water.
In some conventional membrane systems, activated carbon filters, hereafter carbon filters, are disposed upstream of the membrane to remove chlorine and chloramines from the influent water. Unfortunately, use of activated carbon filters results in growth of bacteria on the surface of the carbon filter. In summary, chlorine and chloramines prevent the growth of microbes and bacteria, and filtering out chlorine and chloramines leaves the water supply downstream of the carbon filter and upstream of the membrane susceptible to bacterial growth. Further, bacterial build-up is sloughed off from the activated carbon and is released into the effluent water from the filter. As a result, the concentration of bacteria in the effluent water may exceed the concentration of bacteria in the influent water. The high levels of bacteria in the effluent water accelerates the biofouling of any membrane used downstream of the carbon filter.
Therefore, there is a need for an improved membrane-based water-purification technique. Particularly, there is a need for an improved technique for reducing biofouling of a membrane of a membrane-based water-purification system.
In accordance with one exemplary embodiment, the present technique provides a water-purification system. The water-purification system includes a bacteriostatic filter including a bacteriostatic agent therein and a membrane filter fluidically coupled downstream of the bacteriostatic filter and configured to block the passage of cations and anions therethrough.
In accordance with another exemplary embodiment, the present technique provides a method for purifying water. The method includes receiving an influent flow of water and routing the influent flow of water through a filter having a bacteriostatic agent to produce a first effluent flow of water having a concentration of bacteria not greater than that of the influent flow of water. The method also includes routing the first effluent flow of water through a membrane filter to produce a second effluent flow of water comprising purified water by rejecting cations and anions in the first effluent flow of water.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As discussed in detail below, embodiments of the present invention function to provide techniques for reducing biofouling of a membrane-based water purification system. It should be noted that, the term “biofouling,” as used herein, refers to the accumulation and growth of bacteria on a surface, such as those of a membrane or a carbon filter, for example. Although the present discussion focuses on point of entry (POE) and point of use (POU) membrane-based water purification systems, the present technique is applicable to any membrane-based water purification systems for use in facilities that utilize or produce potable water or that desire to reduce the concentration of bacteria in the effluent water supply of a filter upstream of the membrane filter. Accordingly, the appended claims should not be limited to or by the exemplary embodiments provided in the following discussion.
Referring now to the drawings,
In one embodiment, the water-purification membrane 12 is a nanofiltration membrane. As an alternative, the water-purification membrane 12 can be a reverse osmosis membrane. Of course, the particular kind of water purification membrane 12 employed by the water-purification system 11 depends upon the desired operating parameters and conditions, and the present technique is applicable to any number of membrane types.
The exemplary water-purification system 11 also includes a bacteriostatic filter 16 disposed upstream of the water-purification membrane 12. As used herein, the bacteriostatic filter 16 contains a bacteriostatic agent to prevent growth of bacteria in the filter 16, which would subsequently be discharged in high concentrations in the filter's effluent. Further, discharge of high concentrations of bacteria in the effluent would cause biofouling of the membrane filter 12. In one embodiment, the filter 16 contains activated carbon, which acts to remove chlorine from the feed water 14. Removal of the chorine, however, leaves the egressing water from the filter 16 susceptible to bacterial growth. In this embodiment, the filter 16 includes a bacteriostatic agent to retard the growth of bacteria both on filter surfaces and downstream of the filter 16. As used herein the term “bacteriostatic” agent refers to an agent configured to inhibit the growth and increase in numbers of bacteria within the water-purification system 11. The bacteriostatic agent inhibits the growth of bacteria in the filter 16 upstream of the membrane filter 12 and, in turn, reduces biofouling of the water-purification membrane 12.
In one embodiment, the exemplary filter 16 is activated carbon filter, and employs silver as the impregnated bacteriostatic agent. In certain embodiments, the concentration of the bacteriostatic silver in the filter 16 ranges from about 0.1 weight % to about 1 weight %. Of course other kinds of filters with various kinds of bacteriostatic agents are envisaged. It should be noted that any biocidal agent may also be used to achieve the same bacteriostatic properties, provided it is used in sufficiently low concentrations and it has been demonstrated to be safe for use in drinking water. Further, the concentration of the bacteriostatic agent in the filter 16 may be varied depending upon the operating conditions of the water-purification system 11 for substantially reducing the biofouling of the water-purification membrane 12.
During operation, the filter 16 receives the flow of feed water 14 having a first concentration of bacteria from the water source 18, such as a source of water, including a municipal water supply, for example. Once the feed water 14 is processed, the filter 16 discharges a first effluent flow of water 20 having a second concentration of bacteria that is not greater than the first concentration of bacteria in the influent flow of water 14. That is, the filter 16 precludes the concentration of bacteria in the first effluent flow of water 20 from exceeding the concentration of bacteria in the influent flow of feed water 14.
The silver in the filter 16 functions as the bacteriostatic agent to prevent the growth of bacteria in the filter 16. In operation, silver from the filter 16 binds to sulfhydryl groups of proteins of the bacteria and renders the proteins inactive, thereby preventing the growth of bacteria. By reducing bacterial growth in the filter 16, biofouling of the water-purification membrane 12 is also reduced.
Moreover, the water-purification system 11 may include a pump 22 located fluidically between the filter 16 and the water-purification membrane 12, to boost the pressure of the first effluent flow of water 20 to the water-purification membrane 12. As will be appreciated by those skilled in the art, the amount of pressure boost can vary, based on whether the water source 18 is a pressurized municipal supply, groundwater or well water, for example. Typically, the pump 22 will boost the water pressure to a determined performance level for the water-purification membrane 12.
In the present embodiment, the water-purification membrane 12 rejects the cations and anions in the first effluent flow 20 and discharges a flow of purified water 24. Subsequently, the purified water 24 may be supplied to one or more points of use such as a faucet or other point of use device (e.g., a refrigerator), for example. In addition, the retained uncharged component, divalent and multivalent ions are removed from the water-purification membrane 12 as membrane reject stream 25. The membrane reject stream 25 includes concentrate water 26 that may be subsequently discarded or recycled. For example, the concentrate water 26 may be discarded, such as through discharge into a sewer 28, or used for purposes in which hardness ions are not problematic. In a present embodiment, the bacteriostatic agent discharged from the filter 16 is blocked from passing through the water-purification membrane 12 into the flow of purified water 24 and is instead recycled back through the water-purification membrane 12 via the membrane reject stream 25 and recycle loop 30. More specifically, the bacteriostatic agent rejected into the concentrate water 26 is routed upstream toward the filter 16 via a feedback conduit 32 for further reducing the biofouling of the water-purification membrane 12. It should be noted that various configurations may be envisaged to achieve the recycling of the concentrate water 26 through the water-purification membrane 12. The components required for such recycling operation, such as check valves and conduits, are not presently discussed in detail for the ease of explanation. In certain embodiments, the recycled water can pass through the water-purification membrane 12 to increase water recovery of the water-purification system 11.
The effluent flow of water from the prefilter 36 is then supplied to the bacteriostatic filter 16. In a present embodiment, the bacteriostatic filter includes a silver impregnated carbon filter. As discussed above, the influent flow of feed water 14 to the bacteriostatic filter 16 may include bacteria or other microorganisms in the influent flow of feed water 14 such as those originating from the water source 18. For example, in this embodiment, the influent flow of feed water 14 to the filter 16 includes a first concentration of bacteria from the water source 18. The bacteriostatic filter 16 receives the influent flow of feed water 14 and discharges a first effluent flow of water having a second concentration of bacteria in the first effluent flow of water. In one embodiment, the bacteriostatic filter 16 operates such that the second concentration of bacteria in the first effluent flow of water is not greater than the first concentration of bacteria in the influent flow thereby reducing the biofouling of the water-purification membrane.
The first effluent flow of water from the bacteriostatic filter 16 is then directed to the water-purification membrane 12. Further, the pump 22 may be employed to boost the pressure of first effluent flow to the water-purification membrane 12. During operation, the pressurized flow of water from the pump 22 is purified by the water-purification membrane 12 to generate the flow of purified water 24 by rejecting the cations and anions in the water. In certain embodiments, a distribution pump 38 may be employed to deliver the purified water 24 to a point of use 40 at a desired pressure. In one embodiment, the purified water 24 from the water-purification membrane 12 is supplied to a residential application.
Moreover, the water-purification membrane 12 also discharges the effluent flow of concentrate 26 that may be subsequently discarded, such as through discharge into the sewer 28. In one embodiment, a portion of the concentrate water 26 may be recycled back through the water-purification membrane 12 as illustrated by the recycle loop 30. Particularly, at least a portion of the bacteriostatic agent blocked by the water-purification membrane 12 is routed upstream, toward the filter 16 for use in further reducing biofouling of the water-purification membrane 12. Advantageously, the partial recirculation of the concentrate 26 facilitates the reduction of biofouling of the water-purification membrane 12 by augmenting the existing concentration of the bacteriostatic agent in the influent water to the water-purification membrane 12.
As illustrated in
Moreover, the concentration of silver at the purified water outlet 70 maintains a desired concentration of silver over a period of time to achieve pre-determined quality standards of the purified water for use in an application such as a point-of-entry residential application. In fact, as illustrated in
As noted above, the use of a bacteriostatic agent such as silver in the filter 16 precludes the concentration of bacteria in the effluent water from substantially exceeding the concentration of bacteria in the influent water. In particular, the silver impregnated carbon filter 16 disposed upstream of the water-purification membrane 12 prevents the increase in bacterial concentration in the effluent flow of water from the carbon filter 16.
In the illustrated embodiment, the bacteria present in the purified water 24 and concentrate 26 from the water-purification membrane 12 are enumerated through the spread plate technique by employing R2A Agar as the medium. As will be appreciated by one skilled in the art, R2A Agar is a medium with a low nutrient content, which, in combination with a low incubation temperature and an extended incubation time, is suitable for the recovery of bacteria from water. In the present embodiment, R2A agar plates are inoculated and subsequently incubated in the dark for about 6 days to about 8 days at room temperature prior to counting colonies of bacteria. Further, each colony counted is considered to have originated from one bacterial cell present in the original samples of water. In certain embodiments, the samples of water may be diluted to achieve plates with an appropriate colony density of bacteria.
In the illustrated embodiment, the distribution of the bacterial concentration for the purified water 24 with and without the use of the bacteriostatic agent is represented by reference numerals 90 and 92 respectively. Similarly, the distribution of the bacterial concentration for the concentrate 26 with and without the use of the bacteriostatic agent is represented by reference numerals 94 and 96 respectively. In the illustrated embodiment, to evaluate the effect of use of the bacteriostatic agent such as silver in the filter 16 on the bacterial concentration in water, the silver impregnated carbon filter 16 is replaced with a standard activated carbon filter as represented by reference numeral 98 after a predetermined time period. As illustrated, the concentration of bacteria 90 in the purified water with the use of the bacteriostatic agent is negligible. Further, once the silver impregnated carbon filter 16 is replaced with a standard activated carbon filter, the concentration of bacteria 92 in the purified water 24 increases over a period of time. Similarly, the bacterial concentration in the concentrate 26 with the use of the bacteriostatic agent is substantially lesser than the bacterial concentration in the concentrate 26 without the use of the bacteriostatic agent as represented by curves 94 and 96.
As noted above, the use of silver impregnated carbon filter 16 upstream of the water-purification membrane 12 minimizes the biofouling of the water-purification membrane 12. As a result, a pressure drop across the water-purification membrane 12 due to biofouling of the membrane 12 is substantially reduced, thereby preventing decreased flow rate through the water-purification membrane 12.
Referring now to
The various aspects of the technique described hereinabove have utility in water-purification systems such as membrane-based water-purification systems. As will be appreciated by those skilled in the art, the present technique provides a mechanism of reducing biofouling of a membrane of the membrane-based water-purification system, while maintaining the desired quality of water from the water-purification system.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
