Cartridge for photocatalytic purification of fluids

The present invention relates to a replacable cartridge for use in a photocatalytic fluid purification system. The cartridge is capable of modifying impurities in a fluid flowing through the cartridge in the presence of light. The cartridge includes a flexible, porous element having a semiconductor coating associated with it and a rigid support structure which supports the element. The cartridge may be used in a variety of fluid purification applications where photocatalysis has not heretofore been utilized.

FIELD OF THE INVENTION 
The invention relates generally to photocatalytic fluid purification 
systems, and, more particularly, to a novel cartridge for use in a 
photocatalytic fluid purification system. 
BACKGROUND OF THE INVENTION 
Photocatalysis is a process in which a chemical composition may be modified 
by bringing the composition into the vicinity of a semiconductor material 
in the presence of light having an energy level greater than or equal to 
the bandgap of the material. This process may be used, for example, to 
break down a harmful chemical substance into a number of inert components. 
Because of its ability to transform harmful substances, photocatalysis is 
known for being useful in fluid purification systems. An example of such a 
system can be found in U.S. Pat. No. 4,888,101 issued to the present 
inventor and hereby incorporated by reference. 
Fluid purification systems are used in a vast number of different 
applications. Photocatalysis, however, has only been used in a limited 
number of these applications. One reason for the limited use of this 
technology is that photocatalytic purification systems are generally more 
expensive to implement than other types of systems. Another reason is that 
known photocatalytic systems have not been successfully developed or 
adapted for many potential applications. For example, known photocatalytic 
systems lose their effectiveness over time due to a buildup of surface 
contaminants on the photocatalyst and this has limited their usefulness in 
many applications. 
A need therefore exists for a method and apparatus for implementing 
photocatalytic fluid purification in a practical manner in a wider variety 
of applications than have been known before. 
SUMMARY OF THE INVENTION 
The present invention relates to a replaceable cartridge for use in a 
photocatalytic fluid purification system. The cartridge is of simple 
construction and is relatively inexpensive to manufacture. The cartridge 
can be removably inserted into a system, used for a period of time, and 
then replaced after its effectiveness has been reduced below an acceptable 
level. The disposable nature of the cartridge makes it a practical 
alternative for fluid purification applications where photocatalysis has 
not heretofore been utilized. The invention is useful in a variety of 
different fluid purification applications including, for example, 
purifying the water in an aquarium. 
In one aspect of the present invention, a replaceable cartridge for use in 
a fluid purification system is provided. The cartridge includes a 
flexible, porous element having a semiconductor coating bound to it and a 
rigid support structure for supporting the element. The flexible, porous 
element can include a fibrous mesh, made of a material such as fiberglass 
or glass wool, which may be woven or unwoven. The element is impregnated 
with a semiconductor coating which may include one or more of the 
following: TiO.sub.2, ZnO, SnO.sub.2, SrTiO.sub.3, WO.sub.3, or Fe.sub.2 
O.sub.3. The semiconductor coating may be associated with the flexible, 
porous element by a process which includes contacting the element with a 
slurry made of a liquid and a semiconductor powder and then evaporating 
the liquid from the element. It may also be deposited on the flexible, 
porous element by other means such as chemical reaction, vapor deposition 
or chemical-vapor deposition. To operate effectively and efficiently, the 
photocatalyst coated, flexible, porous element should be capable of 
absorbing a substantial portion of the light produced by a light source 
placed near the element, wherein the light reaches a depth in the element 
sufficient to provide adequate photocatalytic cross-section. Adequate 
photocatalytic cross-section means that the photocatalyst is distributed 
on the element in a manner such that substantially all of the fluid moving 
through the element comes into intimate contact with the photocatalyst and 
that simultaneously a large part of the photocatalyst is able to be 
illuminated with light. 
The rigid support structure is located next to the flexible, porous element 
for providing support to the element, which may be structurally weak. The 
rigid support structure may be made of a material such as polypropylene, 
polyethylene, flouroelastomers, or other similar polymeric material. The 
rigid support structure may also be made of glass or other rigid material. 
The cartridge may further include a component for attaching the rigid 
support structure to the flexible, porous element for providing further 
support to the element. In addition, the cartridge may include a fluid 
seal structure for facilitating the flow of fluid through the element. 
In another aspect of the present invention, a photocatalytic fluid 
purification system is provided. The system includes a photocatalytic unit 
into which a cartridge, such as the cartridge described above, having a 
flexible, porous element with a semiconductor coating associated with it 
and a rigid support structure for supporting the element, may be removably 
inserted. The photocatalytic unit includes a fluid inlet, for carrying 
impure fluid to the cartridge, and a fluid outlet, for carrying purified 
fluid away from the cartridge. The photocatalytic unit also includes a 
structure for conducting fluid from the fluid inlet, through the element, 
and to the fluid outlet. 
In addition, the photocatalytic unit includes a light source for 
illuminating the flexible, porous element during operation of the system. 
The light source must be capable of emitting light having an energy level 
greater than or equal to the bandgap energy of the semiconductor material 
associated with the element. The light source may be located in close 
proximity to the cartridge, but it does not have to be physically coupled 
to the cartridge. In one embodiment, a tubular light source is mounted 
within a tubular pocket in a cylindrically wound element. In another 
embodiment, a planar photocatalytic element lends itself to use with solar 
radiation or reflected illumination from an external source. 
The system of the present invention may also include: a fluid reservoir 
containing a fluid to be purified, a structure for conducting fluid from 
the reservoir to the fluid inlet of the photocatalytic unit, a structure 
for conducting fluid from the fluid outlet of the photocatalytic unit to 
the reservoir, and an apparatus for creating pressurized fluid flow 
through the cartridge. The fluid reservoir may include, for example, an 
aquarium, a cistern of drinking water, a holding tank filled with high 
purity water, air in a room, or a gas containing vessel. The structures 
for conducting fluid may include, for example, pipes, tubes, or hoses 
connected between the reservoir and the unit. The apparatus for creating 
pressurized fluid flow through the cartridge may include, for example, a 
pump, fan, or similar device, connected somewhere in the fluid flow path. 
As an alternative to the above configuration, the system may include two 
reservoirs, a first one containing impure fluid and a second one 
containing purified fluid, in which case a structure is provided for 
conducting impure fluid from the first reservoir to the fluid inlet of the 
photocatalytic unit and a structure is provided for conducting purified 
fluid from the fluid outlet of the photocatalytic unit to the second 
reservoir. 
In a third aspect of the present invention, a process is provided for 
purifying the water in an aquarium using photocatalysis. The process 
comprises the steps of delivering water from an aquarium to a 
photocatalytic unit, contacting the water with a photocatalytic element 
within the photocatalytic unit in the presence of light, and returning the 
water to the aquarium. The process is capable of reducing the 
concentration of contaminants in the water by one-half in a period of 2 
days or less. The process may be carried out using the photocatalytic 
fluid purification system of the present invention. 
In another application, the present invention may be part of a series of 
water purification steps. For example, discrete canisters or housings 
containing activated carbon, a woven filter, and mixed-bed ion-exchange 
resins are the basic components of many high purity process water systems. 
For the production of even higher purity water, the photocatalytic unit 
could be installed at the end of the series and would be one more 
component of the water purification system. In this example, it is 
providing a final polish to purified water by removing trace organic 
compounds. Pressure for fluid flow is derived from the municipal water 
line or an auxiliary pump.

DETAILED DESCRIPTION 
The present invention relates to a replaceable cartridge for use in a 
photocatalytic fluid purification system. The cartridge is of simple 
construction and is relatively inexpensive to manufacture. The cartridge 
can be removably inserted into a system, used for a period of time, and 
then replaced after its effectiveness has been reduced below an acceptable 
level. The disposable nature of the cartridge makes it a practical 
alternative for fluid purification applications where photocatalysis has 
not heretofore been utilized. The cartridge also has health advantages. 
One such advantage is related to the reduced need for handling the 
flexible, porous element once it is installed in the cartridge. Handling 
of the element creates stresses on the mesh that can dislodge and 
aerosolize the semiconductor particles thereon which may be harmful if 
inhaled. A reduction in the handling of the element, therefore, results in 
a reduction in the possibility of inhaling these particles. 
The invention is useful in a variety of different fluid purification 
applications including, for example, purifying the water in an aquarium. 
Other possible applications include home air purification, decontamination 
of the air in hospitals for those with impaired immune systems, the 
destruction of organic contaminants in industrial air vents and 
work-spaces, purification of municipal drinking water, the destruction of 
microorganisms and trace organic toxicants in home drinking water, 
provision of drinking water in remote areas, and the production of process 
water for pharmaceutical production, beverage manufacturing, and 
microelectronics. Another possible application is in the production of 
ultra high purity water for molecular biology and biotechnology. A current 
commercial photocatalytic system provides such ultra high purity water, 
however the equipment is relatively costly and complex, and replacement of 
the photocatalyst is difficult and time consuming. The present invention 
provides a photocatalytic system which is relatively inexpensive and 
allows for rapid replacement of the photocatalyst. The invention is 
particularly useful in applications where frequent replacement of the 
photocatalyst is required. 
The present invention permits significant (approximately 1000-2000%) 
improvement in performance to the existing UV water purification industry 
for a relatively insignificant (approximately 1%) additional cost. 
Applications of conventional UV units can range from purifying municipal 
drinking water to purifying aquariums. However, the process of irradiating 
with UV light alone to purify water and air is fundamentally inefficient 
in comparison to photocatalysis. The UV lamps utilized in this industry 
can drive photocatalytic reactions and the UV reactors have similar 
geometries to photocatalytic reactors. Prior to the present invention, 
however, there was enough discrepancy in the required geometry to make 
retrofitting these UV purifies with a photocatalytic element impractical. 
The photocatalytic cartridge of the present invention allows easy 
retrofitting of many UV systems with a photocatalytic element. 
The photocatalytic cartridge of the present invention can be used to 
destroy a wide range of impurities. For example, photocatalysis is capable 
of treating organic compounds, such as pesticides, hydrocarbons, DNA, 
proteins, endotoxins, alcohols, ketches, and aldehydes. Photocatalysis is 
also capable of destroying microorganisms, such as bacteria, yeast, algae, 
viruses, and possibly spores, cysts, and protozoa. To treat fluids 
contaminated by organic compounds and microorganisms, the cartridge 
photocatalytically oxidizes and degrades the impurities until all that 
remains are relatively harmless degradation products. In addition, 
photocatalysis is capable of removing certain metal ions, such as ions of 
lead, mercury, copper, and chromium. It is believed that the ions are 
photoreduced to the metal or an insoluble oxide which is deposited on the 
surface of the photocatalyst and is thus removed from the fluid stream. 
A simple method of implementing photocatalysis for fluid purification is by 
immobilizing a photocatalyst on a substrate. Because it is important to 
achieve three dimensional distribution of the photocatalyst in the fluid 
during purification, porous substrates are preferred. Fiber meshes such as 
fiberglass or woven fiberglass are very desirable substrate materials 
because of their highly dispersed, three dimensional structure, their 
chemical inertness, and their low cost. However, because of their lack of 
rigidity, fiber meshes are messy, somewhat dangerous, and inconvenient to 
handle and replace. The present invention provides a safe, inexpensive, 
convenient, and standardized method of introducing and replacing a 
three-dimensional porous substrate having a semiconductor coating into a 
photocatalytic fluid purification system. 
The replaceable cartridge of the present invention may be used in a 
photocatalytic fluid purification system 10 as illustrated in FIG. 1. It 
should be appreciated that the word fluid, as used herein, refers to both 
liquids and gases. The system includes: a photocatalytic unit 12 having an 
inlet 14, an outlet 16, a replaceable cartridge 18, a retention structure 
19, and a light source 20; a fluid reservoir 22; a first channel 24 for 
delivering fluid from the reservoir 22 to the inlet 14; a second channel 
26 for delivering fluid from the outlet 16 to the reservoir 22; a fluid 
pump 28; and a power source 30 for providing power to the light source 20. 
Photocatalytic purification of fluids is accomplished by creating fluid 
flow through the cartridge 18 using the fluid pump 28, while at the same 
time illuminating the element in the cartridge 18 using light source 20 
and power source 30. For solar applications there is no need for light 
source 20 or power source 30. 
FIG. 2 is a partially cut-away, perspective view of one embodiment of the 
replaceable photocatalytic cartridge of the present invention. For 
convenience, this embodiment will be referred to by the reference numeral 
40. As illustrated in FIG. 2, cartridge 40 includes: a flexible, porous 
element 42, a rigid support structure 44, and two end-caps 46. The 
flexible, porous element 42 is comprised of a fibrous mesh sheet 
impregnated with a semiconductor coating, which is wound into a 
cylindrical shape. The rigid support structure 44 surrounds and supports 
the element 42. It should be understood that the word "rigid", as used 
herein, can mean "stiff, yet flexible" and is not limited in connotation 
to the meaning "completely inflexible". The end-caps 46 cover the ends of 
the support structure 44 and the element 42 for securing the element 42 to 
the support structure 44 and for making the cartridge 40 more structurally 
sound. Because of the flexibility of the element 42, it is unable to be 
handled in such a manner that it is easily replaceable as a disposable 
element in a water purification system. Thus, by providing a rigid 
support, the flexible photocatalytic element can now be readily used as a 
replaceable component. In this manner, photocatalytic systems can be 
feasible in a variety of applications which before were unsuitable because 
of the cost, potential health danger, and technical expertise required for 
replacing photocatalytic elements. 
The flexible, porous element 42 is made of any suitable material to which a 
semiconductor coating will adhere and which allows a fluid to pass through 
having sufficient surface area contact with the semiconductor coating to 
obtain acceptable rates of purification. This material can be, for 
example, fiberglass or glass wool. The material may be woven, like a 
cloth, non-woven, like fiberglass insulation, or in other suitable 
configurations. The mesh is impregnated with a semiconductor coating using 
any material having photocatalytic properties. The coating may be 
comprised of, for example, oxides and mixed oxides of transition metals or 
Group IA and Group IIA metals. These materials may be doped with trace 
impurities or surface coated with ordinary catalytic metals, such as 
platinum, copper, and ruthenium to enhance activity. The material used may 
be one or more of the following: TiO.sub.2, ZnO, SnO.sub.2, SrTiO.sub.3, 
WO.sub.3, and Fe.sub.2 O.sub.3. Preferred materials include strontium 
niobate, potassium tantalate, tantalum oxide, and, most preferred, 
titanium dioxide. As used herein, the words semiconductor and 
photocatalyst are interchangeable. 
The semiconductor coating may be associated with the flexible, porous 
element using any one of a number of processes. In a preferred process, a 
slurry is first created by combining a semiconductor powder and a liquid, 
such as water. The flexible, porous element is then contacted with the 
slurry, for example, by dipping the flexible, porous element into a vessel 
containing the slurry and then removing it. The coated flexible, porous 
element is then allowed to dry, leaving the semiconductor powder 
associated with the element. Another process for associating the 
semiconductor coating with the flexible, porous element is disclosed in 
U.S. Pat. No. 4,892,712 to Robertson et al. It should be appreciated that 
the present invention may utilize any technique to associate the 
semiconductor coating with the flexible, porous element and is not limited 
to any one process. The specific nature of the association is not known to 
be critical. For example, the association can be in the nature of a 
covalent bond, some charge or ionic interaction, an agglomeration, or 
general entrapment. The flexible, porous element 12 may be formed into its 
final shape either before or after the semiconductor coating is associated 
with it. 
As mentioned above, the flexible, porous element 42 may be woven or 
non-woven. If a woven cloth material is used, several layers of mesh are 
generally required to achieve an acceptable photocatalytic cross-section, 
i.e., cross-sectional volume of the element in which photocatalytic 
activity takes place. If several layers of mesh are used, the mesh 
material contains a high percentage of void, i.e., holes through which 
light can pass, so that light reaches a sufficient number of layers to 
achieve the desired photocatalytic cross-section. If a non-woven material 
is used for the element, a single layer of mesh may be used while still 
obtaining adequate photocatalytic cross-section. A non-woven mesh material 
is chosen which has a fiber density that allows light to penetrate into 
the mesh to a depth adequate to create the desired cross-section. 
The function of a rigid support structure in the present invention is to 
maintain the flexible, porous element having the semiconductor coating in 
a configuration adequate to achieve sufficient contact between the fluid 
to be purified and the semiconductor coating. A second function is to 
allow for the simple and efficient replacement of photocatalytic 
cartridges in systems. Thus, such cartridges can be replaced by 
individuals without special technical training or equipment being 
necessary. A variety of specific rigid supports are described throughout. 
With reference to FIG. 2, it is seen that in this embodiment the rigid 
support structure 44 surrounds the flexible, porous element 42 for giving 
structural support to the element 42. The support structure 44 is made of 
a material which is relatively stiff compared to the mesh material so that 
easy handling in removal and replacement of the cartridge can be achieved. 
For example, the rigid support structure 44 may be constructed of 
polypropylene sheet material or other polymeric sheet material. The rigid 
support structure 44 is wound into a cylindrical shape having an inner 
surface adjacent to and abutting the outer surface of the flexible, porous 
element 42. The support structure 44 is then secured in a cylindrical 
shape by using fasteners, adhesives, or by welding. If a non-woven mesh 
material is used for the element 42, the mesh and the support structure 44 
may be wound together in a single step. The unwound mesh is laid on top of 
the unwound support structure 44 and the two are then rolled into a 
cylindrical shape at the same time. The support structure 44 is then 
secured in this cylindrical shape as described above. 
Depending upon the fluid flow configuration which will be utilized, the 
rigid support structure 44 may be porous or nonporous, i.e., perforated or 
non-perforated. For example, if radial fluid flow is to be used, the rigid 
support structure 44 must be porous to allow fluid to flow radially 
outward from the interior of the flexible, porous element 42 through the 
element 42 and the support structure 44. If axial fluid flow is to be 
used, however, the rigid support structure 44 should be non-porous so that 
the fluid is contained within the element as it travels axially from one 
end of the cartridge to the other. For convenience, the balance of the 
description of this embodiment of the present invention will be with 
respect to a cartridge adapted for radial fluid flow. 
The flexible porous element of the present invention and the rigid support 
are interrelated such that the rigid support provides support to the 
flexible porous element for the reasons identified above. The 
interrelationship can be achieved in a variety of ways depending on the 
particular structure of the elements and supports. A particular end cap 
for use in a cylindrical configuration is described below with reference 
to FIG. 3. In addition, an element and support can be associated such as 
by encasement as is shown in FIGS. 6A and 6B which are discussed in detail 
below. 
FIG. 3 is a partially cut-away perspective view of one of the end-caps 46 
used in the embodiment of the present invention illustrated in FIG. 2. As 
seen in FIG. 3, the end-cap 46 includes a ring-shaped body 48 having a 
circular aperture 50 through its center. The end-cap 46 can also include a 
rubber O-ring 52 disposed within the circular aperture 50 which acts as a 
fluid seal, as will be discussed in more detail later in the 
specification. In addition, the end-cap includes a first interior surface 
54 and a second interior surface 56 which form the boundaries of an 
internal hollow 58 within the end-cap. The end-cap 46 is placed over the 
end of the element/support assembly 42/44 so that the edge of the element 
42 and the edge of the rigid support 44 enter the internal hollow 58 of 
the end-cap 46. When the end-cap is properly installed, interior surface 
54 will be engaging a portion of support structure 44 and interior surface 
56 will be engaging a portion of the flexible, porous element 42 for 
securing the support structure 44 to the element 42. The aperture 50 in 
end-cap 46 is large enough to allow a tubular light source to be placed 
through it and into an internal tubular pocket of the cylindrical element. 
The replaceable cartridge of the present invention is adapted to be capable 
of being removably inserted into a photocatalytic system in a position 
adjacent to a light source and in a fluid flow path. A variety of 
structural configurations are suitable to meet those requirements. As 
basic requirements for a photocatalytic system, the substrate having 
photocatalytic material must be in contact with fluid being purified in 
the presence of light. With particular regard to the present cartridge and 
system, the components are adapted and structured so that the fluid flow 
path can be readily, such as without tools or equipment, disrupted. A 
spent cartridge is then removed, and a fresh cartridge inserted into the 
fluid flow path which is then re-connected. 
As illustrated in FIG. 4, element 40 is placed within a housing 60 to 
effect fluid purification. Housing 60 includes a cylindrical body 62, a 
first end unit 64 having an inlet 66, and a second end unit 68 having an 
outlet 70 and an aperture 72. The cylindrical body 62 is made of a rigid, 
preferably lightweight material and has an interior diameter greater than 
the largest exterior diameter of the cartridge. As illustrated in FIG. 5, 
first end unit 64 includes a rigid tube structure 74 protruding from an 
interior wall and having an exterior surface 76. Rigid tube structure 74 
is operative for receiving fluid from inlet 66 and delivering it to 
cartridge 40. First end unit 64 also includes an interior surface 78. 
First end unit 64 is mounted onto one end of the cylindrical body 62, 
whereby interior surface 78 engages an exterior surface of the body to 
create a fluid seal between the first end unit 64 and the body 62. 
Cartridge 40 is installed in the housing 60 by sliding it into the 
opposite end of the body until it engages the end of rigid tube structure 
74. Slight pressure is then applied to the cartridge so that rigid tube 
structure 74 enters the aperture 50 in the corresponding end-cap 46 of the 
cartridge 40. Once the rigid tube structure 74 is firmly within the 
aperture 50 of the end-cap 46, a fluid seal is created by the exterior 
surface 76 of the rigid tube structure 74 engaging the O-ring 52 mounted 
within the aperture 50. 
After the cartridge is properly in place within the body 62 of the housing 
60, second end unit 68 is installed on the opposite end of the body 62 
from first end unit 64. As with the first end unit 64, a fluid seal is 
created between the second end unit 68 and the body 62 of the housing 60. 
After the second end unit 68 has been installed, a tubular light source 
(not shown in the drawing) is inserted through the aperture 72 of the 
second end unit 68 and through the aperture 50 of the corresponding 
end-cap 46 of the cartridge 40 into an internal tubular pocket of the 
flexible, porous element 42. Art exterior surface of the tubular light 
source engages the O-ring 52 within the aperture 50 of the corresponding 
end-cap 46 for creating a fluid seal between the tubular light source and 
the corresponding end-cap 46. 
In practice, the housing 60 will be permanently installed in a 
photocatalytic fluid purification system, such as the one illustrated in 
FIG. 1. The cartridge 40 will be periodically removed and replaced 
whenever its effectiveness has declined below an acceptable level. The 
inlet 66 of the housing 60 will be connected to the first channel 24 for 
receiving impure fluid from the fluid reservoir 22. The outlet 70 of the 
housing 60 will be connected to the second channel 26 for delivering 
purified fluid to the fluid reservoir 22. The second channel 26 is 
preferably comprised of a flexible tubing which is readily removed from 
the outlet 70 of the end-cap 68 without need for special tools or skills. 
The pump 28 is operative for providing pressurized fluid flow to the 
cartridge 40. 
In operation, the housing 60 receives impure fluid through inlet 66. This 
fluid flows through rigid tube structure 74 into an interior region of the 
cartridge between the flexible, porous element 42 and the tubular light 
source. Because a fluid seal exists between the rigid tube structure 74 
and the corresponding end-cap 46 at one end of the cartridge and between 
the tubular light source and the corresponding end-cap 46 at the other end 
of the cartridge, a large percentage of the fluid will be forced through 
the cylindrical element 42 in a radial direction. In a preferred 
embodiment, the amount of the liquid being forced through the element 42 
will exceed 50 percent. In a more preferred embodiment, the amount of the 
fluid being forced through the element will exceed 75 percent. In a most 
preferred embodiment, the amount of the fluid being forced through the 
element will exceed 99 percent. 
At the same time that the fluid is being forced through the element, the 
tubular light source will be delivering light to the element. This light 
activates the semiconductor coating on the flexible, porous element 42 to 
act as a catalyst in a reaction which modifies an impurity in the fluid. 
After the fluid passes through the element, it travels through an area 
between the support structure 44 and the body 62 of the housing 60 until 
it reaches outlet 70 of second end-unit 68. From there, the purified fluid 
returns to the reservoir through second channel 26. 
FIGS. 6A and 6B illustrate another embodiment of the cartridge of the 
present invention. For convenience, this embodiment will be referred to by 
the reference numeral 80. As illustrated in the FIG. 6A, cartridge 80 
includes a transparent, water-tight covering 82 surrounding a 
substantially flat, flexible, porous element 84. The covering includes an 
inlet 86 and an outlet 88 for allowing fluid to enter and exit the 
cartridge 80, respectively. As illustrated in FIG. 6B, the cartridge 80 
may have the appearance of a pillow when viewed from the side. It should 
be understood that the cartridge of this embodiment can be manufactured in 
a number of different shapes, such as a cylindrical shape, and is not 
limited to the shape illustrated in FIG. 6B. 
The element 84 in cartridge 80 may be comprised of the same mesh materials 
used for the element 42 of the previous embodiment. A non-woven mesh, 
however, is preferred because it is inherently compressible and is more 
likely to fill the entire void within the covering 82 than is a woven 
mesh. Also, the element 84 will be associated with a semiconductor 
coating, such as those mentioned in the description of the previous 
embodiment. 
The covering 82 of the cartridge 80 is made of a material which is 
transparent to the particular wavelength of light which is required to 
activate the semiconductor material which is associated with the element 
84. The material may include, for example, teflon or other UV transmitting 
flouroelastomers, or similar polymeric material. The material used for the 
covering 80 may also have structural rigidity adequate to perform the same 
two general functions as the rigid support structure 44 of the previous 
embodiment. Alternatively, a bottom portion 83 of the covering 80 may have 
the required structural rigidity while an upper portion 85 does not. In 
some very low pressure applications, the cartridge may be provided 
sufficient structural support by simply resting it on a rigid surface. In 
higher pressure applications, the cartridge may require a rigid housing 
with a transparent window. The covering 82 is water-tight, except for an 
inlet 86 and an outlet 88 through which a fluid can flow. 
The cartridge may contain a plurality of compartments 90 through which the 
fluid must flow in order to get from the inlet 86 to the outlet 88. Each 
compartment 90 is filled with a portion of the element 84 and therefore 
contributes to the purification of the fluid by the cartridge 80. The use 
of compartments 90 in the cartridge 80 increases the distance that the 
fluid must flow through the element 84 in the presence of light and 
therefore increases the overall effectiveness of the cartridge 80. The 
compartments 90 also help reduce the effect of fluid channelling and 
provide additional structural strength via a multiplicity of welds. 
The cartridge 80 may be constructed in a relatively simple and inexpensive 
manner. In one method of construction, a thin sheet of plastic material is 
first laid on a flat surface. Next, a flat mesh having an area smaller 
than the plastic sheet is laid on top of the sheet. Then, another thin 
sheet of plastic having the same dimensions as the first sheet is laid on 
top of the mesh and the top sheet is welded to the bottom sheet all around 
the perimeter of the mesh, leaving a small opening for the inlet and the 
outlet. Alternatively, the three layer stack can be placed into a 
compression mold which can simultaneously create the entire seal around 
the perimeter of the mesh and the separate compartments 90 of FIG. 6A. 
In practice, cartridge 80 is removably inserted into a photocatalytic fluid 
purification system 10, such as the one illustrated in FIG. 1. The 
cartridge 80 is placed in a support housing within the system 10 which 
holds the cartridge in place during operation of the system 10. After the 
cartridge 80 is in place, inlet 86 of cartridge 80 is connected to first 
channel 24, for receiving impure fluid from the fluid reservoir 22, and 
outlet 86 of cartridge 80 is connected to second channel 26, for 
delivering purified fluid to the fluid reservoir 22. The cartridge 80 can 
be periodically removed and replaced when its effectiveness has dropped 
below an acceptable value. 
When inserted into system 10, the cartridge 80 is placed in close proximity 
to a light source 20. The operation of the system 10 using cartridge 80 is 
substantially the same as the operation of the system using the cartridge 
40 of the previous embodiment. The impure fluid is forced through the 
element in the presence of light, during which time impurities in the 
fluid are modified by the catalytic action of the semiconductor coating. 
As illustrated in FIGS. 7A and 7B, any number of different methods of 
illuminating the cartridge 80 may be utilized. For example, FIG. 7A shows 
a cartridge 80 being illuminated by solar radiation. FIG. 7B shows a 
cartridge 80 being illuminated by both direct and reflected light from a 
light source 94 and an associated reflector 96. 
The following examples and test results are provided for the purpose of 
illustration and are not intended to limit the scope of the invention. 
EXAMPLES 
Example 1 
An experiment was performed to compare the performance of water 
purification systems using photocatalytic elements to that of a system 
which uses only ultra-violet light to perform purification, such as are 
commonly used in aquariums. Systems using both axial fluid flow and radial 
fluid flow through the photocatalytic elements were tested. The experiment 
involved separately connecting each system to a reservoir containing a 
solution of water and a red dye consisting of an organic compound to 
determine the rate of decomposition of the red dye achieved by each 
system. To normalize the results of the comparison, each test used the 
same ultra-violet light source and each test provided an equivalent rate 
of fluid flow through the corresponding system. 
The experiment tested three systems. The first system included a 
photocatalytic element adapted for axial fluid flow. The element was fit 
snugly into the annular volume between an internal tubular light source 
and an external cylindrical jacket. The second system included a 
photocatalytic cartridge adapted for radial fluid flow. The cartridge was 
placed into a housing and the light source was placed in an internal 
tubular pocket in the cartridge. The third system used the same housing as 
the second system, and the same light source, but with the photocatalytic 
cartridge removed. 
To perform each test, a purification system was first connected to the 
reservoir containing the red dye solution. A pump was then turned on 
forcing solution from the reservoir, through the subject system, and back 
to the reservoir. Periodically during each test, samples of solution were 
taken from the reservoir and measured for dye concentration. To determine 
the concentration of the dye in each sample, a spectrophotometer was used 
to measure the light absorption of the sample at a wavelength of 492 nm, 
the wavelength of maximum light absorption for the dye. The concentration 
of dye in each sample, as a percentage of the initial concentration of dye 
at the time the test was begun, was then plotted versus time to 
graphically display the rate of decomposition of the red dye in the 
solution produced by each system. 
FIG. 8 is a graph illustrating the rate of decomposition of the red dye 
produced by each system tested. Curve 100 illustrates the rate of 
decomposition for the system using only ultra-violet radiation. Curve 102 
represents the rate of decomposition for the system using the radial flow 
cartridge. Curve 104 represents the rate of decomposition for the system 
utilizing axial fluid flow. As seen in FIG. 8, the half-lives of the dye 
in the solution are 123, 10, and 7.5 minutes, respectively. This result 
indicates that systems using photocatalytic fluid purification are at 
least 12-17 times more effective than systems using ultra-violet treatment 
alone. In addition, additives may be added to the photocatalytic elements 
in the photocatalytic systems which have the potential to double the 
efficiency of these systems. Thus, the present invention which provides 
for the rapid, efficient use of photocatalytic systems in a variety of 
applications, such as aquariums, allows for significant improvements in 
effectiveness compared to technology currently in use. 
Example 2 
An experiment was performed to determine the effectiveness of a 
photocatalytic cartridge in removing impurities from a gas. With reference 
to FIG. 9, the cartridge used in the experiment was made in the following 
manner. First, a photocatalyst coating was applied to a woven fiberglass 
cloth 106 having a leno weave with a high degree of void and to one side 
of a piece of cardboard. Next, an edge of the cloth 106 was attached to an 
edge of the piece of cardboard 108. The assembly was then rolled around a 
mandrel with the cloth 106 on the inside and the cardboard 108 on the 
outside. The photocatalyst coated surface of the cardboard 108 faced the 
inside. The resulting cartridge had a diameter less than the inner 
diameter of a cylindrical reactor housing 110 for easy insertion of the 
cartridge into the housing 110. The reactor housing 110 was a 
stainless-steel tube containing a low pressure mercury lamp 112 at its 
center. Upon insertion of the cartridge into the housing 110, the 
restoring force of the rolled cardboard 108 caused the cartridge to expand 
snugly against an inner wall of the reactor housing 110. Simultaneously, 
the woven fiberglass cloth 106 unrolled slightly. The layers of the 
fiberglass cloth 106 distributed themselves throughout the annular volume 
defined by the outer surface of the lamp 110 and the inner surface of the 
housing 108. The large number of wraps ensured complete light absorption, 
and the looseness of the wraps ensured a low pressure drop and good 
three-dimensional photocatalytic cross-section. The complex structure 
provided good convective mixing without the use of turbulent flow 
velocities. 
As illustrated in FIG. 10, cologne and orange cleaner, both highly 
odoriferous, were individually dissolved in water contained in an air 
bubbler 114. Using a pump 116, a valve 118, and an activated charcoal 
filter 120, clean air was pumped through the bubbler 114, passed through 
the cartridge in the photocatalytic reactor housing 110, and then 
collected in a large plastic bag 122. With the lamp 112 off, the odors of 
the agents were readily detected in the bag 122. When the experiments were 
repeated with the lamp 112 on, neither the odor of cologne nor orange was 
detected. 
Example 3 
Another experiment was performed to determine the effectiveness of a 
photocatalytic cartridge in removing impurities from a gas. The experiment 
used gas chromatography to measure a cartridge's ability to decompose 
ethanol. The experimental set-up is shown in FIG. 11. For comparison, a 
reactor using a cartridge as described in the previous example and a 
reactor using a photocatalyst merely as a surface coating on glass tubes 
(two-dimensional reactor) were tested. The results of the experiment are 
shown in FIG. 12 which illustrates the percentage of ethanol decomposed by 
each of the two reactors as a function of the ethanol concentration. Curve 
124, for the two-dimensional reactor, shows the efficiency of ethanol 
decomposition diminishing rapidly with increasing ethanol concentration. 
However, curve 126, for the cartridge reactor, shows the efficiency of 
ethanol decomposition remaining at approximately 99% at the highest 
concentration measured. 
Although the present invention has been described in conjunction with its 
preferred embodiment, it is to be understood that modifications and 
variations may be resorted to without departing from the spirit and scope 
of the invention as those skilled in the art readily understand. Such 
modifications and variations are considered to be within the purview and 
scope of the invention and the appended claims.