Patent Grant
7494572
This invention relates, in general, to water stills and is particularly, but not exclusively, applicable to water stills that provide potable water from a contaminated water resource, such as the sea or brackish water.
In arid or harsh environs, there is often difficulty in locating or providing, at low cost, potable water for immediate consumption, for cooking or for rehydrating, for example, food or drugs for human or animal consumption. For example, countries around The Gulf have vast seawater reserves, but little drinking water inland. Consequently, inhabited settlements are clustered around free-flowing rivers that provide a major source of fresh water or around ports that are either served by expensive desalination plants or by pipe networks from remote reservoirs. Also, saline rich or contaminated water cannot be used for irrigation since it is generally harmful to plant physiology, and so some form of purification is required if such water is to be utilized for food production. Consequently, crop production may be limited by any restrictions applied to the supply of water to the crop, with the restrictions arising from limited availability of suitably clean water or the cost of actually producing suitably clean water. In other words, although water in a generically “contaminated” form may be readily available, purification of the contaminated water to a level necessary for its intended use may not be economically viable.
Harsh environments, in terms of the securing of potable water for human existence, may be encountered at sea (when stranded in a life raft after a yachting accident), in the desert and even during inter-planetary exploration. Furthermore, natural disasters, including drought, flood and earthquake, can also significantly and adversely interrupt the supply of potable water to otherwise adequately serviced areas. Indeed, in these latter instances it may be necessary actually to ship potable water to the effected area in tankers, which is extremely expensive and logistically difficult to accomplish in a short time-frame.
It is also interesting to note that survival packs required by maritime law and used by armies actually include significant quantities of packaged potable water and that a significant proportion of the overall weight of the survival pack is therefore directly attributable to the volume of water carried. Consequently, survival packs are generally bulky and, if carried, cause additional stress to the bearer.
The re-cycling of water in space is also of considerable concern to space agencies, such as NASA. Moreover, the density of water imposes a small but limiting factor in space flights, with it essential that the amount of water carried by a space-craft be limited at blast-off to restrict weight and the requirement for additional thrust and hence more fuel. Re-cycling is therefore essential. Furthermore, with future potential inter-planetary exploration planned for after the turn of the new millennium, the identification of water and its effective conversion into either a potable supply or one suitable for hydroponics are significant issues.
Desalination mechanisms used on an industrial scale include evaporation techniques, electrolysis and osmosis; all are relatively expensive and often require vast systems and/or the supply of power. Clearly, in emergency situations (for example), such systems cannot just materialise and, even if present, may not function in view of the requirement to provide a constant and substantial power supply.
A need therefore exists in relation to the provision of a relatively simple and inexpensive water purification system.
According to a first aspect of the present invention there is provided a water still comprising: a chamber having a substantially impervious upper section and a base coupled to the substantially impervious upper section, the base formed from a membrane supporting a water pervaporation process therethrough; and a water collection well having an opening into which water droplets condensed within the chamber from the water pervaporation process collect, the water collection well sited within the base and generally extending outwardly therefrom.
Preferably, the membrane is non-porous.
A plurality of condensation trays are preferably formed within the chamber, wherein the plurality of condensation trays cause water droplets to be ferried into the water collection well.
In another aspect of the present invention there is provided a mushroom-shaped water still comprising: a humidity chamber formed from a substantially impervious domed upper section coupled to a membrane base supporting a water pervaporation process therethrough; a thermally conductive water collection well having an opening into which water droplets condensed within the humidity chamber from the water pervaporation process collect, the water collection well sited within the base and generally extending downwardly therefrom; and a tap coupled to the thermally conductive water collection well for drawing potable water from the mushroom-shaped water still.
In a preferred embodiment, the membrane comprises a copolyetherester elastomer with a water vapor transmission rate at least 400 g/m2/24 hr, and preferably a water vapor transmission rate of in excess 1000 g/m2/24 hr.
In a further aspect of the present invention there is provided a method of purifying contaminated water, comprising: forming a humidity chamber having a substantially impervious upper surface coupled to a membrane base that supports a water pervaporation process therethrough; providing a water collection well having an opening within the humidity chamber; providing a contaminated water supply juxtaposed the membrane base, thereby forming a vapor pressure gradient thereacross; and directing water condensed within the humidity chamber into the opening.
The method may optionally further comprise the step of using the temperature difference between the humidity chamber and the water source to condense the water vapor, by using the water collection well as a “heat sink.”
Preferably, the method also comprises channeling water droplets condensed within the humidity chamber away from the membrane base and into the water collection well.
In a further aspect of the present invention there is provided a water still comprising: a humidity chamber formed between a substantially impervious section and a non-porous membrane supporting a water pervaporation process therethrough; and a water collection well having an opening into which water droplets condensed within the chamber from the water pervaporation process collect, the water collection well sited towards a base of the humidity chamber.
Preferably, the substantially impervious section provides side walls and a base for the water still and thereby acts to funnel water droplets into the water collection well. In use, the entire water still is immersed.
In yet another aspect of the present invention there is provided a method of purifying contaminated water, comprising: forming a humidity chamber having a substantially impervious surface coupled to a non-porous membrane that supports a water pervaporation process therethrough; providing a water collection well having an opening towards a bottom of the humidity chamber; contacting a contaminated water supply with the non-porous membrane, thereby forming a vapor pressure gradient thereacross; and directing water condensed within the humidity chamber into the opening under the action of gravity.
Advantageously, the water still of the present invention provides a water purification system that has a simple mechanical construction and which can be deployed quickly. With a preferred embodiment potentially having no moving parts, the manufacturing costs and complexity are low. Indeed, the water still can be packaged into a low-weight, compact form that can readily be assembled. Scaling of the water still design of the preferred embodiments is arbitrary. A supply of potable water can therefore be obtained without the use of manufactured energy sources (batteries, generators, etc) as the process operates by the use of naturally occurring heat differentials and any optional moving parts can be driven, if desired, by natural forces such as solar, wind or wave energy.
Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which:
The underlying principles of construction of the water still 10 of the present invention are shown, in exemplary form, in the basic block diagram of
The water still 10 comprises a domed outer section 12 coupled to a hydrophilic membrane base 14. The hydrophilic membrane base 14 acts as a floor and co-operates with a water collection well 16 that provides stabilizing ballast to the water still 10 (in situ). Typically, the water collection well 16 is centrally located within the geometry of the water still 10, and extends downward in a stalk-like fashion to cause the water still 10 to have a mushroom-like cross-section. A tap 18, preferably incorporating a one-way valve, is generally located at or near the bottom of the water collection well 16. The domed outer section 12, which is preferably made from a durable, heat absorbing and substantially impervious layer of material (such as polyvinyl chloride, other plastics, metal and the like), the hydrophilic membrane base 14 and the water collection well 16 form a chamber 20. A central shaft 22 may extend between the water collection well and possibly beyond the domed outer section 12. The central shaft 22 can provide structural support to the water still 10 generally, and specifically the domed outer section 12 through appropriate coupling of the domed outer layer thereto.
In operation, the water still floats in a contaminated or otherwise non-potable water source 24, such as the sea, a lagoon, a river or in brackish water, with an intersection 26 between the domed outer section 12 and the hydrophilic membrane base 14 below the surface of the water source 24. The domed outer section 12 and the hydrophilic membrane base 14 are typically glued, welded or stitched together, although other securing mechanisms are clearly possible. The intersection 26 is, however, watertight. Once immersed in a water source 24, water vapor pervaporates through the hydrophilic membrane base 14 to cause the chamber 20 to have a relatively high humidity.
The inner surfaces of the chamber 20 act as condensation surfaces, with water droplets generally and principally directed into the water collection well 16. Preferably, the central shaft 22 (which also acts as a condensation surface) is made from a material with good thermal conduction properties, such as galvanized steel. Of course, the central shaft 22 could be made from a plastic material which can resist contact with water better than can metal, although strength and rigidity is generally inferior to that of metal, although metals may need to be treated to prevent and resist rusting and other forms of corrosion/fatigue. The water collection well 16 is also preferably made from the same material as the central shaft 22, although again the requirement is that the water collection well 16 has good thermal conduction properties. The water collection well 16, in use, therefore acts as a heat sink into the relatively cool water source 24, with any thermally coupled internal structure (such as the central shaft 22) therefore also benefiting from an ability to sink heat into the surrounding water source 24 via the water collection well 16. The rate of condensation is therefore increased through the use of the water collection still as a heat sink an d the cooling effect provided by the surrounding water.
As will be understood, condensation generally occurs on the coldest surfaces and so those having heat sinking capabilities (ultimately into the surrounding contaminated water supply) are most efficient. Furthermore, the efficiency of the condensation surfaces is extremely important in relation to the optimum performance of the water still since condensation reduces humidity within the chamber 20 and therefore enhances the vapor pressure gradient across the hydrophilic membrane, thereby increasing the rate of water vapor transmission through the membrane.
It has been found that the hydrophilic membrane base 14 operates more efficiently to pass more water vapor when a flow of air is able to encourage water pervaporate to be lifted from its surface 28. Consequently, it is preferable that the domed outer section 12 is an impervious heat absorbing layer to both prevent evaporation from potable water 29 collected (i.e. condensate) within the water collection well 16 or escape of water vapor from the chamber, generally. Moreover, with the domed outer layer 12 actually designed to absorb heat, a natural convection current may be established within the chamber 20 that encourages an increased rate in the pervaporation process from the surface 28 of the hydrophilic membrane base 14.
In order to encourage cooling within the chamber, a particular embodiment employs a water-absorbing surface 27 on the outside of the domed outer section 12, whereby evaporation of absorbed water by the action of the sun produces a cooling effect within the water still 10, generally, and more specifically in the upper regions of the chamber 20.
In a preferred embodiment, the portion of the water still 10 floating above the surface of the contaminated water source 24 is generally hemispherical in shape, with the entire hydrophilic membrane base 14 always totally submersed.
As regards a relative internal height profile within the water still 10, the water collection well 16 is raised relatively to the surface of the hydrophilic membrane base 12; as, with an increased lip, the clean potable water collected as condensate ultimately within the water collection well 16 does not significantly spill over the sides when the water still 10 is adversely affected by wave action or the like.
Turning briefly to the bottom plan view of the water still 10 shown in
It is preferable that water still 10 is weighted to ensure that hydrophilic membrane base 14 is always, in use, below the surface of the water source 24, with this achieved (at least in part) by the actual weight of the water collection well 16 and also the clean potable water that has been collected as condensate within the water collection well 16. Beneficially, with the hydrophilic membrane base 14 always below the surface, the likelihood of damage and puncture of the membrane is reduced since objects will generally collide against the edges of the water still 10 and hence contact the domed outer layer 12. Furthermore, with the hydrophilic membrane base 14 always below the surface, the efficiency of the membrane (i.e. its performance) is at least improved if not optimized.
The design of the water still 10 of the preferred embodiments of the present invention renders it suitable for use in a number of environments, including open seas, harbors, rivers, flooded plains and sheltered lagoons. The features of the design are such that is has an inherent capability of riding waves whilst remaining substantially upright. Alternatively, permanent or semi-permanent stills may be-secured in a fixed position with respect to the bed of the water body in which they are located, or they may be fixed to the bed but caused to move to adapt to changing water depths (for example, in tidal waters).
In summary, the condensate represents a supply of clean potable water which may be microbiologically clean.
Inflation of the chamber 20 can be achieved either using compressed gas or by blowing it up. Clearly, in the case of an industrially sized membrane that yields sufficient quantities of potable water, the former mechanism is preferred and also limits any chance of introducing microbes or the like into the chamber. However, in relation to a survival pack, since the individual is generally likely only to require the pack for a few (and at most several) hours or days before rescue, then self-contamination of the chamber 20 with air containing that individual's microbes is probably insignificant. Compressed gas, in terms of a survival kit, could be supplied by a small canister initially packaged within the water collection well 16. Alternatively, for permanent stills, the chamber 20 may be constructed in a fully expanded state.
A particular design option in relation to a relatively small and compact water still configuration utilizes a telescopic central shaft 22 that can be collapsed into the water collection well 16. The remaining domed outer section 12 and membrane base 14 is made semi-rigid with nylon-type rods, for example, maneuvered and clipped into place to provide any required structural rigidity.
As regards scales of size, this is dependent upon the expected area of deployment. For survival packs, a human requires anything from between about 0.5 liters to about eight liters of drinking water per day (dependent upon physical exertions, temperature and humidity). Consequently, the volume of the water collection well 16 must reflect this requirement, but may be constrained by packaging requirements. Preferably, the capacity of the water collection well in a survival kit will therefore be in the range of 0.5 liters to eight liters, more preferably in the range of one liter to four liters and most preferably around about two liters to three liters. Clearly, other sizes that are more cumbersome could be used for flood alleviation efforts, for example. Similarly, the size of the hydrophilic membrane base area and the condensation chamber should be selected to reflect the intended amount of water to be collected; and similar factors will also influence the nature and thickness of the chosen hydrophilic membrane and the incorporation or otherwise of the optional features disclosed herein.
For a water still system that can supply several cubic meters of potable water per day, a significantly larger water collection well 16 would be required. However, it is interesting to note that the natural buoyancy of the water still 10 of the present invention (arising from the effective air-tight chamber 20) still allows easy tapping-off of the potable water condensed and collected in the water collection well 16. Indeed, the water still remains buoyant (and becomes increasing stable by having a lowered center of gravity) with the condensation and subsequent collection of water in the water collection well 16 by virtue of the principles of Archimedes. With yet larger water stills constructed according to the preferred embodiments of the present invention, a periodic or continuous drawing-off process (employing a pump) could be employed, with the size of water collection well 16 adjusted accordingly.
In one particular embodiment, the water collection well 16 could be selectively detachable from the “canopy section” of the water still to allow easy maintenance and cleaning before re-use. The water collection well 16 could therefore contain a screw-thread and “o”-ring seal arrangement.
As regards the drawing off of potable water through tap 18, the potable water collected in the water collection well 16 provides some limited head of water that acts to purge interconnected drain pipes of accumulated debris and algae growths about the actual outlet port.
The water collection well 16 is further shown to include an outward-facing lip 42 to prevent any condensate inadvertently lying on the surface 38 of the hydrophilic membrane from entering the water collection well 16. Also, the water collection well 16 may include an internal lip 44, typically of a gauze-like construction, that limits spillage of water from the water collection well 16.
The major condensation tray 50 is shaped by a number of gentle curves that encourage water droplets to form and roll along both the upper and under sides of the major condensation tray 50, but not to fall onto the hydrophilic membrane base 14.
Co-operating with and partially underlapping the major condensation tray 50 is a feed tray 55 located proximate to an outer edge of the domed outer section 12. Again, in a similar fashion to the major condensation tray 50, the feed tray 55 is shaped by a number of gentle curves that encourage water droplets to form and roll along both its upper and under sides.
Water vapor will condense on an inner surface of the domed outer section 12, which may be smooth or, alternatively, may be designed to encourage vapor condensation (for example by the provision of a spiky, hairy or fuzzy surface). Water droplets that form on the inner surface and fall under the action of gravity are therefore either caught by the major condensation tray 50, the feed tray 55 or a gully 60 formed around the periphery of the domed outer section 12. The inner surface of the outer section 12 may, optionally, also be shaped to encourage condensed water to flow directly into any of these collection means. Clearly, water droplets that roll down the side of the domed outer section 12 fall directly into the gully 60. The feed tray 55 also terminates above the gully to ensure that water droplets condensed thereon fall into the gully 60. Water that enters the gully is communicated to the water collection well 16 via periodically placed narrow feed paths or channels 62, such as open troughs. The area of feed path is minimized to ensure that it does not act as an effective condensation surface, although it is shaped to encourage any water droplets forming on its underside to roll along into the water collection well 16 rather than to fall onto the hydrophilic membrane base 14.
For the sake of clarity, only one side of the internal structure of condensation trays is shown in
To improve overall structural stability of the water still, guide lines 70 may be coupled (e.g. selectively attached) between the periphery of the water still and the water collection well 16. Furthermore, with increasing size, it may be beneficial to provide a tether line 72 from the water still (such as from the bottom of the water collection well 16) to a sea anchor or the like. This tether line 72 may optionally act as a conduit to remove water from the water collection well 16.
Another possible design variant can be realized by the use of a rotating wind vane 74 mechanically coupled to the central shaft 22 and arranged to drive, via a suitable spindle, an internally located fan 76. Although the complexity of the mechanics of the overall structure increase, there is an improvement in the rate of circulation of the high humidity air within the chamber 20, and between the chamber 20 and the water collection well 16, that correspondingly encourages condensation and improves air flow over the surface of the hydrophilic membrane, hence increasing pervaporation therefrom. Such a mechanical arrangement requires no outside power supply and is merely reliant upon wind that is often experienced in coastal regions. Other forms of fan 76 could be employed, such as those benefiting from being driven by wave power or solar power. Generally, the internal fan 76 is arranged to circulate air throughout the chamber 20 and particular towards, across and through the condensation surfaces and the water collection well 16.
The optional features shown in
The inverted design of
As regards the hydrophilic membrane 204, its profile may be sloped, in either a convex or concave fashion, in order that condensate is guided from its surface towards the water collection well 16. The hydrophilic membrane 204 therefore remains substantially dry at all times, and is generally the warmest surface within the chamber 20.
In the context of the disclosure, hydrophilic membranes for use in the water stills of the various embodiments of the present invention are from hydrophilic polymers. The term “hydrophilic polymer” means a polymer that absorbs water when in contact with liquid water at room temperature according to International Standards Organization specification ISO 62 (equivalent to the American Society for Testing and Materials specification ASTM D 570).
The hydrophilic polymer can be one or a blend of several polymers. For example, the hydrophilic polymer could be a copolyetherester elastomer or a mixture of two or more copolyetherester elastomers, such as polymers available from E.I. du Pont de Nemours and Company under the trade name HYTREL®. Alternatively, the hydrophilic polymer could be polyether-block polyamide or a mixture of two or more polyether-block polyamides, such as the polymers from Elf-Atochem Company of Paris, France available under the name PEBAX™. Other hydrophilic polymers include polyether urethanes or a mixture thereof, homopolymers or copolymers of polyvinyl alcohol and mixtures thereof. The above list is not considered to be exhaustive, by merely exemplary of possible choices of hydrophilic membrane.
A particularly preferred polymer for water vapor transmission in this invention is a copolyetherester elastomer or mixture of two or more copolyetherester elastomers having a multiplicity of recurring long-chain ester units and short-chain ester units joined through ester linkages, said long-chain ester units being represented by the formula:
and said short-chain ester units are represented by the formula:
wherein:
a) G is a divalent radical remaining after removal of terminal hydroxyl groups from a poly(alkylene oxide)glycol having a number average molecular weight of about 400-4000;
b) R is a divalent radical remaining after removal of carboxyl groups from a dicarboxylic acid having a molecular weight less than about 300;
c) D is a divalent radical remaining after removal of hydroxyl groups from a diol having a molecular weight less than about 250; optionally
d) the copolyetherester contains 0-68 weight percent, based on the total weight of the copolyetherester, ethylene oxide groups incorporated in the long-chain ester units of the copolyetherester;
e) the copolyetherester contains about 25-80 weight percent short-chain ester units.
The preferred polymer film is suitable for fabricating into thin but strong membranes, films and coatings. The preferred polymer, copolyetherester elastomer and methods of making it our known in the art, such as disclosed in U.S. Pat. No. 4,725,481 for a copolyetherester elastomer with a WVTR of 3500 g/m2/24 hr, or U.S. Pat. No. 4,769,273 for a copolyetherester elastomer with a WVTR of 400-2500 g/m2/24 hr.
The use of commercially available hydrophilic polymers as membranes is possible in the context of the present invention, although it is clearly preferable to have as high a WVTR as possible since the water still is designed to supply potable water. Most preferably, the present invention uses commercially available membranes that yield a WVTR of more than 3500 g/m2/24 hr, measured on a film of thickness 25 microns using air at 23° C. and 50% relative humidity at a velocity of 3 ms−1.
The polymer can be compounded with antioxidant stabilizers, ultraviolet stabilizers, hydrolysis stabilizers, dyes, pigments, fillers, anti-microbial reagents and the like.
A useful and well-established way to make membranes in the form of films is by melt extrusion of the polymer on a commercial extrusion line. Briefly, this entails heating the polymer to a temperature above its melting point and extruding it through a flat or annular die and then casting a film using a roller system or blowing the film from the melt. Useful support materials include woven, non-woven or bonded papers, fabrics and screens and inorganic polymers stable to moisture, such as polyethylene, polypropylene, fiberglass and the like. The support material both increases strength and protects the membrane. The support material may be disposed on only one side of the hydrophilic polymer membrane, or on both sides. When disposed on only one side, the support material can be in contact with the water or away from it. Typically, the support material is formed on the outer side of the water still to best protect the membrane from physical damage.
Without being bound by any particular theory, it is believed that the purifying effect of the hydrophilic membrane, realized either in the form of a coating or an unsupported membrane, when in contact with water that may contain suspended or dissolved impurities, solids and emulsions, occurs because highly dipolar molecules, such as water, are preferentially absorbed and transported across the membrane or coating, compared to ions, such as sodium and chloride. When, in addition, a vapor pressure gradient exists across the membrane, water is released from the side not in contact with the source of water, which released water can be condensed to provide potable water and water for agricultural, horticultural, industrial and other uses.
In relation to the hydrophilic membranes used in the preferred embodiments of the present invention, the water transmission characteristics are generally determined using standard test procedure ASTM E96-95—Procedure BW (previously known and named as test procedure ASTM E96-66—Procedure BW). These standard test procedures are used to determine the water vapor transmission rate (WVTR) of a membrane, and use an assembly based on a water-impermeable cup (i.e. a “Thwing-Albert Vapometer”). The water-impermeable cup contains water to a level about nineteen millimeters below the top of the cup (specifically, 19 mm±6 mm). The opening of the cup is sealed watertight with a water-permeable membrane of the test material to be measured, leaving an air gap between the water surface and the membrane. In procedure BW, the cup is then inverted so that water is in direct contact with the membrane under test. The apparatus is placed in a test chamber at a controlled temperature and humidity, and air is then blown across the outside of the membrane at a specified velocity. Experiments are run in duplicate. The weights of the cups, water and membrane assemblies are measured over several days and results are averaged. The rate at which water vapor permeates through the membrane is quoted as the “water transmission vapor rate”, measured as the average weight loss of the assembly at a given membrane thickness, temperature, humidity and air velocity, as expressed as mass loss per unit membrane surface area and time. The WVTR of membranes or films according to ASTM E96-95—Procedure BW is typically measured on a film of thickness twenty five microns and at an air flow rate of three meters per second (3 ms−1), air temperature twenty three degrees Celsius (23° C.) and fifty percent (50%) relative humidity.
Advantageously, if a hydrophilic polymer such as HYTREL® (or the like) is used to provide the membrane base 16, then the water vapor pervaporate is sterile.
The water still of the various embodiments of the present invention is generally simple to manufacture and can be deployed quickly if need be. Furthermore, the simple mechanical design renders the water still relatively low cost. The various preferred and optional features can be mixed and matched between the various design variants shown in the accompanying figures.
The amount of water collected (over, say, a twenty-four hour period) by each water still embodying the inventive concepts of the present invention is unlikely to provide a yield per unit surface area as high as that in optimized laboratory conditions. However, this can be redressed by increasing the surface area of membrane in each water still while also ensuring that performance of the water still is optimized as far as possible, e.g. by providing effective removal of water vapor from the air within the chamber.
As regards collection techniques, it is contemplated that many water stills according to the various aspects of the present invention can be linked together such that there is either interconnecting passages between the individual chambers, or that the condensed potable water can be collected and drawn off, i.e. tapped, centrally.
Whilst it may be beneficial to have the water still of the present invention float, it may be equally desirable that it be held in place (and either fully or partially immersed in the contaminated water source 24). Indeed, with a permanently located water still, there is no need for the still to be made buoyant and so it may always be located below any tidal water line, for example. Indeed, with full immersion of the water still, the condensation process would benefit from the lower overall temperature achieved within the water still by virtue of the surrounding contaminated water supply 24.
It will, of course, be appreciated that the above description has been given by way of example only and that modifications in detail may be made within the scope of the present invention. For example, the water still of the present invention is neither restricted in shape nor size. Indeed, the tapping of water can be achieved using one or several outlet, with the design of outlet ideally ensuring that potable water purged from the water collection well 16 and external contaminants do not respectively re-enter or enter the water collection well 16. In this latter respect, the tap of piping-off could be double-vented. Should the water still be employed in an area of low water levels, it is envisaged that the water collection well could be considerably flattened to ensure that contact between the water source 24 and the hydrophilic membrane base 14 is optimized. Whilst it is clearly preferably to have the submerged base entirely made from membrane material to optimize water contact, this is clearly a design option since the hydrophilic membrane could be incorporated within an impervious framework (for strength purposes) so that merely a substantial proportion of the lower surface (i.e. the base) of the still is in contact with the water source 24.
It is contemplated that materials other than hydrophilic membranes, as defined, could be used in to context of the water still arrangement disclosed herein. However, other materials generally clog and block over time, and so a non-porous vapor permeable membrane is clearly preferred.
This application is a continuation of U.S. application Ser. No. 09/369,805, filed on Aug. 6, 1999, and now abandoned.
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| Number | Date | Country | |
|---|---|---|---|
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| Number | Date | Country | |
|---|---|---|---|
| Parent | 09369805 | Aug 1999 | US |
| Child | 10459827 | US |
