Thermoelectric integrated membrane evaporation system

A urine-water recovery system is described which provides efficient potable water recovery from waste liquids. The design allows use over extended durations such as encountered in space flights. The system has advantages such as low power consumption, compactness, and gravity insensitive operation. The system comprises a vacuum distillation system combining a hollow fiber polysulfone membrane evaporator with a thermoelectric heat pump and condenser. With the system of the present invention, water purified from urine can be produced at a rate of more than 0.5 kg/hr at a total system energy of less than 400 w-hr/kg.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of art to which this invention pertains is distillation apparatus 
and process including vapor treating devices. 
2. Description of the Prior Art 
The reclamation of water from human urine has presented aerospace 
engineering with a challenging task. The systems required on long duration 
space missions for water recovery of about 220 kg per man per month from 
crude generated waste water has resulted in many zero gravity system 
design attempts. Although some credible and efficient design systems have 
been developed, all have serious design and operational problems. 
Accordingly, an acceptable system is still being sought. 
Raw urine contains approximately 4% by weight of urea and various salts in 
solution. As water is removed from the urine, solids rapidly concentrate 
and when, for example, 95% of the original water content of the urine is 
removed, the residual solids concentration of the remaining fluid is in 
the order of 30%. And while the requirement for a distillation product 
from precipitate-forming corrosive fluids is routinely encountered in the 
chemical industry in a gravity environment, the task of the aerospace 
engineer is complicated by the fact that a similar task must be 
accomplished in a zero gravity environment and with severe launch weight, 
volume and power consumption restrictions imposed. 
Three of the more prominent waste recovery systems presently known are "The 
Air Evaporation Subsystem" (AES), "The Vapor Diffusion Reclamation System" 
(VDR), and "The Vapor Compression Distillation System" (VCD). In the AES 
system, electrical energy is used to provide the heat for evaporation of 
urine contained in a wick. Evaporated "pure" water is carried away by 
recirculated air, and subsequently condenses downstream of the wick on a 
conventional plate fin condenser cooled by a low temperature fluid. Wicks 
are periodically replaced when the solids content builds up and prevents 
sufficient wick feeding. In the VDR system, electrically heated urine is 
evaporated through a flat membrane sheet that is held in place by a 
pressurized diffusion gap between the membrane and condensed on a porous 
cold plate condenser, also using a low temperature cooling fluid. A 
recirculated urine-brine solution becomes concentrated as water is 
evaporated and is contained in a replaceable recirculation tank. While the 
system does have some advantages, it requires high energy input. The VCD 
system also uses a recirculation loop and a replaceable brine concentrate 
tank but employs a rotating drum to generate a gravity field that allows 
the evaporator and condenser surfaces to function. Steam evaporating over 
the inner drum surface is compressed and allowed to condense due to its 
elevated saturation temperature over the backside of the rotating 
evaporator drum. The elevated saturation temperature provides the 
potential required for conversion and transfer of the heat of condensation 
to the heat of evaporation. This is a regenerative system that requires 
approximately 25% of the energy requirements of the two previous systems. 
While AES is a relatively simple system which provides for good liquid 
separation, the problems of wick blockage, efficient wick feeding, and the 
heating requirements are significant disadvantages. While VDR has the 
advantages of good liquid separation and bacterial control, the necessity 
for a flat membrane and its heating requirements are significant problem 
areas. And while VCD does have the advantage of being a low power system, 
it has the disadvantages of being complex, noisy, and produces poor liquid 
separation, especially during shutdown periods. 
Accordingly, what has been lacking in the art is a low-power, compact and 
gravity insensitive distillation system which has a relatively simple 
design and gives good component separation and bacterial control. 
BRIEF SUMMARY OF THE INVENTION 
Apparatus for producing microbiologically safe potable water from human 
urine, especially in a zero gravity environment, which is relatively light 
in weight, small in volume, and has low power requirement (e.g., less than 
1/3 that of the latent heat of evaporation of water) is described. The 
apparatus comprises a hollow fiber membrane evaporator, a thermoelectric 
heat pump to provide the heat for evaporation of the urine water, and a 
condensing surface to condense the water distilled from the urinal waste. 
The components are located relative to one another to keep heat transfer 
resistance to a minimum. In operation, the urinal waste is heated at the 
hot junction surface of the heat pump, evaporated through the hollow fiber 
membrane to separate the water from the waste solids, and the evaporated 
water condensed at the cold junction surface of the heat pump on a 
condensation surface. 
Another aspect of the invention is the method of processing raw urine into 
potable water by passing it through the system described above. 
Another aspect of the invention is the thermoelectric heat 
pump-condensation surface distillation-condensation apparatus of the 
present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Preferably the thermoelectrically integrated membrane evaporation system of 
the present invention will be used in a vacuum distillation process 
operating at about 17.5 kPa absolute (2.5 psia) pressure level. It is also 
preferably used in conjunction with conventional feed, pretreatment and 
posttreatment apparatus, much of which is known in the art. Reference is 
made to FIG. 1 for a demonstration of a typical system according to the 
present invention. 
Of course it is understood that while the system of the present invention 
is primarily for urine-water recovery, it can be used with any 
impurity-containing water where a potable end product is desired. 
In FIG. 1 urinal flush indicated as 1, is conveyed to mixing tank 2, where 
a pretreating liquid 3, e.g., containing chromic acid, is metered to 
mixing tank 2 by pretreat metering valve 4. The mixed urinal flush and 
pretreating liquid is then conveyed to waste storage tank 5. From the 
storage tank the treated urinal flush is heated by preheaters 6 and 
conveyed to the recycle tank 7. From the recycle tank the flush is then 
pumped by recirculation pumps 8 to the hot junction surface 9 of the 
thermoelectric heat pump where the waste fluid is heated to a 
pasturization temperature of about 65.degree. C. (150.degree. F.) within a 
finned heat exchanger thermally contacting the hot junction surfaces of 
the thermoelectric elements. When the fluid is heated to evaporation 
temperature as detected by temperature sensor 10, the recirculated 
pretreated waste fluid is then passed through the hollow fiber membrane 
module 11 containing a plurality of hollow fiber membranes of about 0.05 
cm (20 mils) inner diameter (preferably polysulfone tubes). The tube 
outsides are exposed to low pressure causing steam originating from the 
urinal fluid to evaporate at the tube outer walls, the heat of evaporation 
provided by the heated pretreated urinal flush flowing at a rate of about 
175 kg per kg of product water through the tubes. A waste fluid 
recirculation rate is selected which limits the temperature decrease 
through the evaporator to a few degrees (Centigrade), typically 3.degree. 
C. The limited temperature decrease provides for efficient evaporator and 
thermoelectric heat pump operation. The slightly cooled (approximately 
3.degree. C. or 5.4.degree. F.) and concentrated urine is returned to the 
recycle tank 7 after passing through the module 11 passing by a 
concentration sensor 12 and filtered by an approximately thirty micron 
conventional fiber-wrapped precipitate filter 13. It is then pumped back 
to the hot junction surface 9 by recirculation pumps 8 where it is 
reheated and recycled. The evaporated steam 14 condenses on the 
condensation surface 15 thermally contacting the cold junction surface of 
the thermoelectric element 15. Valve 14a controls vacuum source 14b which 
may be either a conventional vacuum pump or space vacuum and can assist in 
evaporating steam 14 from the exterior of the hollow fiber membranes 
contained in module 11. Preferably, the temperature differential A 
detected between the end of the hot junction surface and the end of the 
module is approximately 3.degree. C. and the temperature differential B 
between the hot junction surface and condensing surface is approximately 
9.degree. C. In the particular embodiment of FIG. 1, a 9.degree. C. 
temperature differential between the cold junction surface and the hot 
junction surface of the thermoelectric elements were detected and this was 
found to produce operation of the system in a performance range that 
maintained a favorable heat pump coefficient of performance (COP). This is 
defined as the refrigeration effect divided by the electrical power input. 
The pretreated and diluted urinal flush passed through the 9.degree. C. 
temperature difference of the thermoelectric elements provides a 6.89 kPa 
(1 psia) saturation pressure difference across the membrane. Due to the 
urea and salt contents of the urine, this pressure differential increases 
at high concentrations. The steam condenses on the wet, cooled 
condensation surface 15, e.g., a porous plate which acts like a sponge and 
the condensate is withdrawn through the porous plate to a thermally 
conducting water passage between the porous plate and the thermoelectric 
element cold junction surface where it is collected for removal. The 
removed condensate is pumped by pump 16 through conductivity sensor 17 to 
a small charcoal and ion exchange bed 18 where it is filtered and 
eventually pumped to a potable water storage tank 19 for crew use. 
FIG. 2 demonstrates the high efficiency thermoelectric regenerator of the 
present system better seen in the cutaway of FIG. 3. The apparatus of FIG. 
2 represents a highly efficient apparatus designed to maximize heat use. 
In this design, 31 indicates the thermoelectric heat pump elements with 23 
being the hot junction surfaces and 24 the cold junction surfaces. Heat 
conducting fins 25 bridge the hot junction surfaces of the thermoelectric 
elements maximizing heat transfer surface efficiency. The fins are so 
sized and located, between the hot junction surface pair in FIG. 2, to 
minimize temperature loss due to conduction. Porous or channeled 
condensation plates 32 are attached to the cold junction surfaces of the 
thermoelectric elements through screens 26 to allow bleed off of the 
condensed vapor at screens 26. The steam evaporated from the urine source 
enters the condensation plates through headers 27. This unit represents an 
efficient heat transfer design maximizing heat use with a minimal space 
requirement. Such design also allows a series of such units to be used 
based on the system size desired by simply attaching a series of such 
units side-by-side with steam header contact, further minimizing heat 
losses and maximizing heat use in the units. While the thermodynamic 
calculations herein are based on a two-unit system, additional units can 
be used to improve power efficiency and a three-unit system has been found 
to be particularly suitable. 
In FIG. 3, the thermoelectric elements are indicated as 31 and condenser, 
here porous plates, as 32. The urinal flush enters through ports 33 and 
exits at port 34. The urine is heated at passages 35. The urinal steam 
enters steam headers 36 through ports 37 and is distributed through 
passages 38 and is passed over cold plates 39. The condensate is withdrawn 
through port 40. Cooling water enters through port 41 during high voltage 
periods of operation to limit the maximum temperature of urine waste water 
to about 65.degree. C., and thus preventing urea breakdown to amonia. A 
high thermal conductance is maintained between the porous plate and the 
cold junction surface by means of a screen 43, or pin fin type or 
equivalent surface so as not to impede condensate and coolant flow. 
Character 42 indicates conventional elastomer fluid seals. 
The tubular hollow fiber membrane shown by FIG. 4 having internal surface 
43 and external surface 44 bridged by fibular microstructure 45 allows the 
liquid feed to operate at atmospheric pressure by capillary action. 
Initially, a vacuum is pulled on the steam side, i.e., the outside of the 
fiber, to initiate fluid flow. Free and dissolved gases contained in the 
hot recirculating urine stream pass through the membrane and increase 
pressure in the steam passages. Although noncondensables in the steam 
passages will inhibit steam flow, testing of the system has shown the 
process to be tolerable of moderate amounts of noncondensables in the 
steam passages, such as oxygen and nitrogen, in the order of 10% by 
volume. Periodically, a vacuum purge is used to cleanse the system of 
noncondensables. 
The thermoelectric section is preferably designed to operate on a 29.+-.2.5 
volt DC space vehicle power supply. If this is used in conjunction with a 
solar cell power system, the line voltage will vary from 26.5 volts for a 
high vehicle load condition to 31.5 volts during low load conditions. 
Although the thermoelectric device power draw will increase at the higher 
line voltages, no additional penalty is assumed for this low load case. 
Design power input for the heat pump is about 127 watts at 26.5 volts DC 
applied voltage with about 0.68 kg/hr (1.5 lb/hr) of dilute urine 
processed. 
The thermoelectric elements are commercially designed to operate at 
relatively fixed electrical resistances, and power input and heat 
generated varies with the square of the voltage. For example, at 31.5 
volts DC a resulting 40% increase in electrical heating relative to 26.5 
volt DC operation would raise the temperature of the recirculating urine 
higher than 65.degree. C. (150.degree. F.). This would result in a urea 
breakdown and require extensive posttreatment to control ammonia 
production. This can be avoided by limiting temperature in the system to 
about 65.degree. C. by means of a spoiler loop. The condensed steam water 
is recirculated through a cooler indicated as 20 in FIG. 1 where heat is 
absorbed and the cooled condensate returned back to the condenser when 
temperature exceeds 65.degree. C. This additional cooling effect reduces 
the cold junction temperature of the thermoelectric elements and thus 
limits the amount of heat pumped to the hot junction of the thermoelectric 
element contacting the urine in the heating loop of the heat exchanger. A 
temperature control valve sensing urine loop temperature can be used to 
regulate the condensed product water recirculation flow by governing 
whether or not the condensed water is sent through the cooling loop or 
not. This is indicated in FIG. 1 as character 10. Although close tolerance 
voltage regulation would also prevent over-temperature, utilizing this 
control approach, the cost, weight, and power inefficiency of a close 
tolerance DC voltage regulator is avoided. 
The product water delivered by the condensate pump 16 in FIG. 1 is passed 
through a conductivity sensor 17. If unacceptable product water 
conductivity is detected, based on impurity content, a three-way valve 21 
that normally delivers the product water to the filtration module 18, will 
automatically activate and recycle the unacceptable water through a 
bacteria filter-trap 22, such as an acrylic polymer hollow fiber membrane, 
to the waste storage tank outlet at the end of tank 5. The filter-trap is 
used here to prevent bacteria in the waste recycle loop from contaminating 
the product water section. During normal operation, product water is 
delivered through the multi-filtration module 18 containing charcoal and 
ion exchange resins where the total organic carbon and ammonia impurities 
content is reduced to less than 25 parts per million. 
The Table presents results of chemical analysis of four water samples run 
through the system of the present invention. Also included is the number 
of hours the membrane had been in operation with pretreated urine and the 
number of continuous hours of operation at 75.degree. C. (167.degree. F.). 
In viewing the results, it should be noted that urea breakdown to ammonia 
is known to increase rapidly at temperatures in the 75.degree.-100.degree. 
C. range (167.degree.-212.degree. F.). Solids concentration of the 
processed brine at the time of sample collection is also noted. These test 
samples were of collected condensate with no posttreatment or filtering of 
any kind. 
While any hollow fiber membranes may be used in the system of the present 
invention which have the requisite properties of an ability to contain a 
liquid in a zero gravity environment with capability to withstand moderate 
pressure differentials, for example in excess of 1 atmosphere, polysulfone 
hollow fiber membranes SM-1 from Amicon having anisotropic structure and a 
20 mil inner diameter were found particularly suitable. 
TABLE 
__________________________________________________________________________ 
Potable Water 
Sample 
Sample 
Sample Sample 
Characteristic 
**Spec. SE-S-0073 
1 2 3 4 
__________________________________________________________________________ 
Total Hours on 
Membrane 7 120 40 210 
Continuous Hours 3 24 40 210 
% Solids 5 6.5 10 7 
Total Organic Carbon 
1.0 ppm 6.0 ppm 
240 ppm* 
15.1 ppm 
12.0 ppm 
Total Kjeldahl 4.0 mg/l 
11.0 mg/l 
2.3 mg/l 
3.3 mg/l 
Ammonia 2.5 ppm 
Cadmium 0.01 mg/l None Detected 
Chromium (hexavalent) 
0.05 mg/l None Detected 
Copper 1.0 mg/l 0.005 mg/l 
Iron 0.3 mg/l 0.035 mg/l 
Lead 0.05 mg/l 0.01 mg/l 
Manganese 0.05 mg/l None Detected 
Mercury 0.005 mg/l None Detected 
Nickel 0.05 mg/l 0.004 mg/l 
Selenium 0.01 mg/l None Detected 
Silver 0.1 mg/l None Detected 
Zinc 5.0 mg/l 0.107 mg/l 
__________________________________________________________________________ 
*The sample was contaminated with oil from the vacuum pump. 
**NASA water purity specification. 
Such fibers enable the module containing a plurality of these fibers, e.g., 
1400 fibers, to operate at high permeabilities without the use of a 
pressurizing diluent gas as is required, for example, with flat sheet 
membranes. The hollow fiber membranes are compact, light, may be used in a 
vacuum distillation mode and automatically provide deaeration of the urine 
in the recirculation loop. 
The pretreatment tank (character 3 of FIG. 1) of the system of the present 
invention contains a concentrated mixture of sulfuric and chromic acid 
which is added to the urinal flush to inhibit bacterial growth and to 
inhibit urea breakdown. The polysulfone hollow fiber membranes are 
compatible with this acid pretreated urine. Extremely high surface areas 
for evaporation are also obtained using the 0.05 cm (20 mil) inner 
diameter fibers. In the preferred module of the present invention 
containing such fibers, 1400 fiber elements, approximately 9.7 cm (3.8 
inches) long containing 2787 cm.sup.2 (432 in.sup.2) of area for transport 
were strung longitudinally in a cartridge with the ends potted in end 
rings. The membrane bundle is mounted in a stainless steel brace member 
and inserted in a replaceable cartridge. The cartridge assembly also 
contains a screened liquid trap and a conductivity sensor which can detect 
contamination in the event of a urine breakthrough. 
Membrane performance has been characterized by a permeability factor P 
equal to the evaporated throughout divided by membrane inner area and by 
the saturation pressure differential across the membrane. FIG. 5 shows 
graphically a performance run of an Amicon SM-1 300 cm.sup.2 (46.5 
in.sup.2) module evaporating a 4-30% concentrated urine solution. The 
element gradually wears into a P value of about 30 kg/hr-cm.sup.2 -kPa 
which is a preferred value for system sizing. 
The actual evaporation occurs at the tube outer diameter and a 
concentration gradient occurs across the tube wall structure. Solids left 
at the evaporation interface diffuse back to the recirculation stream 
through the tube wall and concentrate in the recirculation circuit volume. 
During testing, back flushing removed the deposited solids and the 
measured permeability returned to the initial value approaching 61.23 
kg/hr-cm.sup.2 -kPa (1.0 lb/hr-ft.sup.2 -psi). The repeated regenerations 
indicate that the solids do not permanently damage the membrane. Batching 
of the process, whereby loop solids are allowed to concentrate to an 
excess of 30%, followed by a fresh urine change, will produce a periodic 
dilute flush of the membrane fibers, limiting performance degradation and 
insuring many hours of useful operation per cartridge. 
It is anticipated that periodic membrane replacements may be required in a 
long duration operating system, and module cartridges can be designed 
accordingly. Such cartridges are preferably designed with an operating 
life exceeding one month per cartridge. 
The combination of the solid state thermoelectric elements and the porous 
plate condensors as demonstrated, for example, in FIG. 2, provide a 
particularly attractive subsystem for distillation systems wherever water 
purification is desired. The heat pump aspect of the subsystem provides an 
exceptionally energy efficient distillation subsystem which is also light 
in weight. The solid state thermoelectric element enables the urine water 
to be distilled at a power consumption of less than about 1/3 of the 
latent heat of evaporation of the water. In terms of vapor cycle or heat 
pump machinery performance this corresponds to a coefficient of 
performance (COP) of about three (COP=refrigeration effect/power input). 
Rather than using a vapor compression cycle, solid state thermoelectric 
devices are used in the system of the present invention to recover the 
heat of steam condensation at 57.degree. C. (135.degree. F.) and pump it 
to 65.degree. C. (150.degree. F.) to provide the heat for evaporation of 
the urine-based water at the hollow fiber outer surface. 
The use of the thermoelectric devices in the system of the present 
invention provides an efficient, simple means of heat pumping because of 
the small temperature differentials required in the system. Although the 
device's efficiencies are not favorable in comparison to conventional 
refrigeration vapor cycle machinery where large hot-to-cold junction 
temperature differentials are required, they integrate quite 
satisfactorily with the hollow fiber membrane evaporator of the system of 
the present invention to produce a simple, compact, quiet, gravity 
insensitive, and efficient distillation process and apparatus. 
The thermoelectric element is composed of thermocouples connected in series 
and grouped in small modules. A preferred module selected for the system 
of the present invention is a Cambion element manufactured by the 
Cambridge Thermionics Corporation of Cambridge, Mass. Each couple contains 
a P and an N material typically made of bismuth telluride. When voltages 
impress across the couple, a temperature differential occurs. As current 
flows through the series circuit, a heat flow is induced from the lower 
temperature side (cold junction) to the hot temperature side (hot 
junction) and thus produces a heat pump effect. Electrical and 
thermodynamic performances of the devices can readily be expressed in 
mathematical formula established by Seebeck, Peltier, and Thompson. The 
cold junction heat flow may be expressed as: 
EQU Q.sub.C =amTcI-(RmI.sup.2 /2)-Km .DELTA.T 
where: 
I=(V-am .DELTA.T)/Rm 
Q.sub.C =cold junction heat flow, w 
am=average Seebeck coefficient, v/deg K 
Tc=cold junction temperature, deg K 
I=current, amps 
V=voltage 
Rm=average electrical resistance, ohms 
Km=average thermal conductivity, w/deg K 
.DELTA.T=temperature difference between the hot and cold junction surfaces 
of the thermoelectric element. 
In the present application, it is most convenient to impose a DC voltage 
across a stack of thermoelectric elements and relate the heat transfer to 
generated temperature differentials. Rearranging the foregoing equations, 
the resultant form is: 
##EQU1## 
Using Cambion Thermionics Corporation published data for 65.degree. C. 
(149.degree. F.) properties of am, Rm, and Km, (The Cambion Thermoelectric 
Handbook, 2nd Edition, Cambridge Thermionics Corporation, Cambridge, Mass. 
1972), a performance map for a thermoelectric heat pumping device 
according to the present system has been worked out as is demonstrated by 
FIG. 6. 
Characteristics of the device give a heat pumping capacity which varies 
directly with imposed voltage and inversely with the imposed temperature 
differential. Competitive COP's occur at relatively low differentials and 
heat fluxes. Optimization of the distillation unit, therefore, requires 
the matching of the thermoelectric element characteristics for the 
available voltage with the integrated membrane characteristics and the 
fluid properties. 
The condensation surface (15 in FIG. 1 and 32 in FIGS. 2 and 3) can be any 
heat conducting material, preferably a metal such as stainless steel. Also 
preferred is that the plate be in the form of a porous condensation plate 
which can be any permeable, heat conducting material, preferably a metal 
such as a sintered stainless steel plate, having a water permeability of 
at least 1 cc/hr-cm.sup.2 -kPa. Exemplary plates can be obtained from 
Martin Kurz & Co., Inc., Mineola, N.Y. In addition to the novel 
thermoelectric element-condensation plate subsystem, the coupling of this 
subsystem with the hollow fiber membrane evaporator produces a system of 
surprising power efficiency. The membrane area and number of 
thermoelectric elements are selected by balancing power requirements with 
component sizes. Based on the particular thermoelectric elements selected 
for the subsystem, in order to attain the desired COP greater than 3.0, 
the design temperature differential must be restricted to 10.degree. C. 
(18.degree. F.) and the element voltage held below 1.0 volt. Power 
efficiency increases at decreasing values of these parameters. Low system 
volume and weight requirements make selection of the design point in this 
range or lower desirable. A 10.degree. C. temperature differential allows 
for heat transfer losses and also enables the evaporator to be operated at 
a 6.89 kPa (1 psia) saturation pressure differential which results in an 
acceptable membrane module size. 
FIG. 7 displays graphically a thermoelectric device performance with the 
membrane evaporator characteristics superimposed. About 2787 cm.sup.2 (432 
in.sup.2) of membrane area and about 968 cm.sup.2 (150 in.sup.2) of 
thermoelectric element area are required for each approximately 3.31 kg 
(1.5 lb) per hour of water evaporated. Typical system sensitivity to power 
supply voltage variations and recycle loop concentrations is demonstrated 
by FIGS. 8 and 9. 
As referred to above, urinal flush waste water to be fed to the recycle 
loop system is pretreated with chromic acid, at a rate of about 4 ml of 
pretreat per liter of waste water. Sulfuric acid and chromium trioxide is 
present in the pretreat solution as a concentrated solution, i.e., about 
44% by weight H.sub.2 SO.sub.4, 11% by weight CrO.sub.3, the balance being 
water. The acid pretreat solution, in addition to assisting in the 
prevention of urea breakdown, assists in bacteria control. The system of 
the present invention contains a holding tank, for example indicated as 5 
on FIG. 1, instrumented for high and low fluid levels. As water evaporates 
from the constant volume recycle loop, make-up pretreated urine is added 
from the holding tank. If the holding tank content is low, the system may 
be automatically shut down or be placed in a recycle mode with the 
condensate returned to the concentrated urine recycle loop via the acrylic 
polymer hollow fiber membrane, bacteria trap indicated as 22 on FIG. 1. 
The recycle tank indicated as 7 in FIG. 1 will gradually increase in solids 
concentration, eventually having its contents in a mud-like state. It is 
preferably designed to be of such a size for replacement at two-week 
intervals based on a three-person load and urine concentration to 50% 
solids (representing a 98% water recovery). 
Incorporation of a bellows design can also be used in the recycle tank for 
use, e.g., in a flight system, to eliminate the requirement for tank 
replacement. Thus, when the solids concentration in the urine 
recirculation loop reaches maximum concentration, the fluid can be 
discharged to a conventional vacuum dried waste commode system with the 
concentrated fluid being further processed in the same manner as crew 
solid waste. By bellows design is of course meant any design system which 
could be emptied as bellows by the simple application of pressure 
resulting in the shrinking and substantial elimination of the inner space 
of the bellows chamber in response to such pressure and the forcing out of 
the contents of such bellows in response to such shrinking space. Further, 
in response to the elimination of such pressure, the bellows-type chamber 
would return to its normal size having been emptied of its internal 
contents. The recycle tank contains a solids filter at its entrance to 
trap precipitates forming in the concentrated recycle loop. This is 
indicated as 13 in FIG. 1. It is located in the coolest section of the 
loop since that is where precipitation would first occur. A bellows tank 
design would require a separate filter cartridge that would have to be 
located at the recycle tank outlet line and would require periodic 
replacement. 
The small, highly efficient evaporator and thermoelectric regenerator 
modules allow for a small process section package of only about 0.03 
M.sup.3 (1.0 ft.sup.3). FIG. 9 is a pictorial package of an exemplary 
system according to the present invention. In this exemplary system, a 
two-subsystem pack of thermoelectric modules and porous condensation 
plates (FIG. 2) are used, indicated as 91 in FIG. 9. The membrane module 
is indicated as 92. Headers 93 insure relatively smooth flow. In 
operation, the pretreated urinal flush enters at conduits 94 and exits at 
conduits 95. The water condensate exits at conduits 96. Preferably, 
forty-eight thermoelectrical elements are included in each subsystem pack. 
Accordingly, in the exemplary system shown ninety-six units are present. 
The entire system design and location of component parts is made in such a 
way as to maximize all heat transfer. Accordingly, the urine loop heat 
exchanger is centrally located to minimize hot side heat loss and to 
minimize heat transfer temperature losses through the heat exchanger fins. 
Condenser steam passages are cooler and are located on the outer sides 
where they will be insulated to control temperature levels. 
Handling of the concentrated urine has proved to be difficult due to the 
abrasive and corrosive nature of the concentrated urine brine. In fact, 
the concentrate is so hostile that it even inhibits bacterial growth. In 
view of this fact, periodic concentration of the liquid plus the 
75.degree. C. (150.degree. F.) operating temperature provides excellent 
bacterial control. The possibility of bacterial build-up in the cooler 
condenser plate must also be considered. Accordingly, if desired, a 
sterilization mode can be built into the system whereby the thermoelectric 
module voltage polarity is periodically reversed causing the hot junction 
to occur at the condenser plate, thereby elevating the normally cooler 
condenser plate to sterilization temperature. 
Pump selection in other urine reclamation systems has always proved 
difficult because of the corrosive nature of the urinal fluids. 
Peristalsis (undulated tube) pumps previously used are limited life items 
and therefore failure prone. While any pump with the requisite thermal and 
corrosive properties may be used, a well-proved positive displacement gear 
pump driven by a 28 volt DC permanent magnet motor is preferred. A 
magnetically coupled motor/pump drive provides complete leak-proof 
separation of the urine loop fluid from the driving motor. No dynamic 
sealing is required. Positive displacement is selected over centrifugal 
non-positive displacement type pumps because of the relative insensitivity 
to viscosity changes (resulting from the increasing solids concentration 
in the recirculating urine loop) of the positive displacement pumps. 
While any material which has the requisite temperature and corrosion 
resistant properties for use in the system of the present invention may be 
used for the various storage chambers and conduits of the present system, 
300 series stainless steel (AISI) and compatible brazing and welding 
materials are especially preferred because of their anticorrosive 
properties, especially toward chromic acid. 
Another advantage of the system of the present invention is that the 
components of the system are selected and so placed in conjunction with 
one another to keep heat transfer resistance to a minimum and heat 
conductivity to a maximum in the system, further contributing to the 
improved efficiency of the system. 
From a hardware standpoint, the thermoelectric integrated membrane 
evaporation system of the present invention presents an approach to 
urine-water reclamation that integrates proven components to produce a 
design exhibiting exceptional weight, power and volume properties not to 
mention gravity insensitivity. For example, a system suitable for flight 
on long duration space missions in addition to consistently producing 
potable water and being able to be maintained with safety and within 
acceptable crew time limitations will also have the advantages of being 
light in weight, for example, approximately 85 kg (187 lbs), have low 
power requirements, for example, 167 watts at 26.5 vdc, and be of small 
volume, for example, 0.5 M.sup.3 (17.7 ft.sup.3). The system produces 
microbiologically safe potable water by a sequential application of 
definite control procedures. At each component and interface, the change 
of entrance or growth of bacteria is checked. Procedures designed into the 
system including chemical pretreatment, pasteurization, distillation, 
vapor duct liquid entrapment, and final bacteria filtration ensure that at 
each component and component interface the chance of bacterial growth is 
eliminated or severely reduced. For example, reversing the polarity of the 
thermoelectric elements present an effective condenser sterilization mode. 
Accordingly, while the system provides important weight, power and volume 
advantages, its efficient production of quality potable water is 
paramount. 
Although this invention has been shown and described with respect to a 
preferred embodiment, it will be understood by those skilled in this art 
that various changes in form and detail thereof may be made without 
departing from the spirit and scope of the claimed invention.