GB2380994A

GB2380994A - Photocatalytic treatment of fluids

Info

Publication number: GB2380994A
Application number: GB0222082A
Authority: GB
Inventors: Malcolm Robert Snowball
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-09-24
Filing date: 2002-09-24
Publication date: 2003-04-23
Also published as: WO2003027021A2; GB0222082D0; AU2002334077A1; GB0122933D0; WO2003027021A3

Abstract

A fluid treatment apparatus comprising a surface 15 which is coated with a photo catalytic oxidising semiconductor doped with a metal. This surface is placed in the flow path of the apparatus and means 13 for exciting the doped semiconductor material to form electron-hole pairs is provided. The photocatalytic material may be the anatase form of titanium dioxide doped with a noble metal such as gold, silver, palladium or platinum. The electron hole pairs produce hydroxyl and oxygen free radicals 22 which are highly reactive and will break down complex molecules and, in the case of micro-organisms, will destroy them by breaking down the cell walls. The semiconductor material may be coated onto a plurality of porous carrier elements. The means for exciting the doped semiconductor material may be arranged to irradiate the material with light, ionised particles, ultra-sonic waves, a magnetic field, an electromagnetic field, or an electrostatic field. There may also be provided means for supplying chemicals, gasses, or compounds into the fluid to stimulate the semiconductor material to produce oxidising species to further enhance the efficiency of the apparatus.

Description

Fluid Treatment Apparatus

This invention relates to an apparatus for the treatment of fluid and more particularly but not solely to the disinfection and removal or reduction of pollutants from water and effluents.

In traditional water and effluent treatment there have always been separate and distinct processes for the disinfection of water and for the removal of pollutants from water.

An object of the present invention is to provide a 10 single technology, which will perform both tasks.

When titanium dioxide (TiO2) or any other photo catalytic oxidising semiconductor is irradiated with ultra-violet (UV)light having a wavelength 380nm or less, its surface becomes activated and, in the presence of water and 15 oxygen, produces hydroxyls and oxygen free radicals. Hydroxyls and oxygen free radicals are highly reactive and will break down complex molecules and, in the case of micro-organisms, will destroy them by attacking and breaching their cell walls.

Titanium dioxide in activated powder form has been used 20 in laboratory experiments to kill micro-organisms and to remove pollutants from water. The technique, whilst of interest, is unpredictable and impracticable. To provide an efficient practical process, the TiO2 powder must present a large activated surface area and remain in the reaction chamber 25 whilst the water is continuously flowing through it. To achieve this, the powder must remain in suspension in the water and must be irradiated with UV light the whole time. However, if the process is a batch process, whereby the water does not flow through the chamber but remains in the chamber and is treated 30 by adding the TiO2 powder and then activating it, there remains the problem of removing the TiO2 powder after treatment.

A further problem that adds to the unpredictability of the technique, is that of the particles of TiO2 shade each other from the light and hence become deactivated. Previous 35 attempts to provide a solution to these problems have involved wrapping a TiO2 coated or impregnated glass fibre mat around a lamp and passing the fluid to be treated through the irradiated mat. Unfortunately this is self-defeating, since the surface area of the TiO2 undoubtedly goes up but is negated by the large amount of shading of the TiO2 coated fibres in the mat 5 construction. A further undesirable feature of this technique is the fact that the mat acts like a filter and gathers debris from the treated fluid, thereby curtailing its useful life.

Other attempts have involved ceramic filters whose surface and pores are coated with TiO2 and whose surface is 10 then illuminated to activate the TiO2. This technique is ineffective because of depth shading.

The major problem of using titanium dioxide as a photo catalytic oxidising semiconductor is that its practical efficiency is poor.

In the process of photo catalysis generally, the illumination of the surface of a photo-catalyst, such as an oxide semiconductor, produces chemically active sites associated with excited electron-hole pairs. The positive holes migrate to the semiconductor surface and participate in 20 oxidation reactions. These reactions require the excited electron-hole pairs to remain apart long enough for the random chance of an oxidising reaction to take place. Unfortunately, in the case of titanium dioxide, the charge carrier recombination occurs within nanoseconds (usually within 30 25 nanoseconds) making it of little practical use in this application.

We have now devised a fluid treatment apparatus which alleviates the above-mentioned problems.

In accordance with this invention, there is provided a 30 fluid treatment apparatus comprising a surface of a photo catalytic oxidising semiconductor material doped with a metallic material disposed in a flow path of the apparatus and means for exciting the doped semiconductor material to form electron-hole pairs.

It has been found that by doping the crystalline lattice of the photo catalytic oxidising semiconductor with certain metals, metallic oxides and/or metallic compounds, the charge carrier recombination time can be extended to minutes or even hours. This transforms the practical efficiency of photo catalytic oxidising semiconductor. Certain dopants also have the effect of extending the activation wavelengths to 400nm and beyond.

Preferably the photo catalytic oxidising semiconductor material comprises titanium dioxide.

Preferably the photo catalytic oxidising semiconductor material is the anatase form of titanium dioxide.

Preferably the metallic material is a noble metal such as gold, silver, palladium or platinum. Noble metals have a relatively high electrochemical potential which helps to increase the carrier recombination time.

The doped photo catalytic oxidising semiconductor 15 material may be formed in one of the following ways: 1. A substrate preferably of metal is coated with the metallic material and then with titanium e.g. by plating.

The titanium is then converted to titanium dioxide using any of the processes known to those skilled in the art, such as 20 anodising; 2. Titanium and the metallic material are deposited onto a substrate by plating, sputtering, metallic evaporation, arc or plasma spraying or some other suitable process. The titanium is then converted to titanium dioxide using any of the 25 processes known to those skilled in the art.

A feature of this invention is to produce large surface areas of continuously activated photo catalytic oxidising semiconductor and then flow the fluid over and/or through these activated surfaces, thereby providing a very efficient and 30 predictable technology for the disinfection and removal of pollutants from fluids.

Preferably, the apparatus comprises an element disposed in the flow path, the photo catalytic oxidising semiconductor material forming a surface coating on the element.

Preferably the element is porous, the fluid being arranged to flow through pores in the element.

Preferably means are provided to encourage fluid to flow over and/or through the coated element.

Preferably a plurality of coated elements are provided, the elements preferably being sufficiently spaced from each other such that minimal shading of the activated photo catalytic oxidising semiconductor material thereon occurs and such that any particles in the fluid are not trapped by the 10 elements.

In one embodiment, the means for exciting the doped semiconductor material is arranged to irradiate the material with light having of a wavelength chosen to excite the material.

In one embodiment, the photo catalytic oxidising semiconductor material is irradiated by a photo-luminescent material, such as phosphor.

Preferably means are provided to cause the photo-luminescent material to fluoresce.

In an alternative embodiment, the means for exciting the doped semiconductor material is arranged to radiate ionised particles, ultrasonic waves, microwaves, a magnetic field, an electromagnetic field, an electrostatic field or any combination thereof.

Preferably means are provided to introduce chemicals, gasses or compounds into the fluid which stimulate the photo catalytic oxidising semiconductor material to produce oxidising species to further enhance the efficiency of the apparatus.

Embodiment of this invention will now be described by 30 way of example only and with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a first embodiment of fluid treatment apparatus in accordance with this invention; Figure 2 is schematic diagram to illustrate the 35 principle of operation of the apparatus of Figure 1; Figure 3 is a sectional view through a second embodiment of fluid treatment apparatus in accordance with this invention; Figure 4 is a sectional view through a third embodiment of fluid treatment apparatus in accordance with this invention; Figure 5 is a sectional view along the line V-V of Figure 3; Figure 6 is a sectional view through a fourth embodiment of fluid treatment apparatus in accordance with this invention; Figure 7 is a sectional view through a fifth embodiment 10 of fluid treatment apparatus in accordance with this invention; Figure 8 is a sectional view through a sixth embodiment of fluid treatment apparatus in accordance with this invention; Figure 9 is a sectional view through a seventh embodiment of fluid treatment apparatus in accordance with this 15 invention; and Figure 10 is a sectional view along the line X-X of Figure 9.

Referring to Figure 1 of the drawings, there is shown a fluid treatment apparatus comprising a sealed rectangular 20 chamber 10 having inlet and outlet ducts 11,12 disposed on respective opposite side walls 18,19 thereof. A plurality of elongate lamps 13 extend longitudinally of the chamber 10 between opposite end walls 20,21 thereof and parallel with the side walls 18,19. The lamps 13 emit light having a wavelength 25 which is selected to suit the particular photo catalytic semiconductor being used. In the case of titanium dioxide, the lamps emit light having a wavelength of 400nm or less.

The lamps 13 are each mounted inside a glass or quartz sleeve 14 which are sealed at their opposite ends to the end 30 walls 20,21 of the chamber 10, so that each lamp 13 is isolated from the fluid in the chamber 10 but allowed to irradiate the inside of the chamber 10 through the sleeve 14. A plurality of wire mesh grids 15 extend longitudinally of the chamber 10 and parallel to the lamps 13 on opposite sides thereof. The grids 35 15 are sealed along their respective longitudinal side edges to the upper and lower walls of the chamber 10 respectively.

The grids 15 are coated with the anatase form of TiO2, which has been doped with certain metals, metallic oxides and/or metallic compounds to improve its reactivity.

A series of equally spaced baffles 16 extend perpendicular to the grids 15 across the chamber 10. The 5 baffles 16 alternately extend across the chamber 10 from opposite side walls 18,19 thereof to form a tortuous flow path between the inlet and outlet 11, 12. In use, fluid flows from the inlet 11 through each grid 15 on one side of the first baffle 16 and then returns though each grid 15 on the opposite 10 side of the baffle 16 and so on until it reaches the outlet 12, thereby passing through each grid n+ i times, where n is the number of baffles 16.

A gas or other oxygenating chemical is injected into the flow at the inlet 11 through a port 17.

Referring to Figure 2 of the drawings, the lamps 13 illuminate opposite sides of the grids 15 of doped Titanium Dioxide which, combined with the oxygen in the fluid, produces a continuous stream of oxidising elements 22 at the surface and through the pores of the grid 15. In effect, what is created 20 is a series of oxidising agent zones in the vicinity of each grid 15. These highly reactive elements 22 break chemical bonds. If for instance complex chemical molecules are present in the fluid, the oxidising species 22 will break them down into harmless basic elements. If for example a micro-organism 25 is present then the oxidants will attack the cell wall of the micro-organism and destroy it. There are no micro-organisms that can resist this process.

The fluid has to flow through each irradiated grid 15 several times therefore receiving high exposure to the 30 oxidising species 22. The baffles 16 also ensure that all of the fluid and any matter contained therein has to flow through each irradiated grid 15 several times. None of the parts of the grid 15 are shaded from the light and thus the grid 15 is fully effective at disinfecting and removing pollutants from the 35 fluid. The number of grids 15, baffles 16 and the length of the chamber 10 can be chosen to suit the fluid being processed and the degree of disinfection.

Referring to Figure 3 of the drawings, there is shown an alternative embodiment of fluid treatment apparatus which is similar in operation to the previous embodiment. The apparatus comprises an elongate tubular treatment chamber 23 5 sealed at both ends by respective end walls 24,25. A lamp 26 or a series of lamps extend axially of the chamber 23 inside a glass or quartz sleeve 27 which is sealed at its respective opposite ends to the end walls 24,25 of the chamber 23.

A series of circular grids 28 are mounted normal to the 10 lamp 27 at intervals along the length of the chamber 23 by means of clamps 30 attached around the lamp 27. The pore size of the grids 28 is sufficient to allow the matter to be treated to pass through. The surface of each grid 28 is coated with the anatase form of TiO2 doped with metals, metallic oxides and/or 15 metallic compounds to improve its reactivity.

The radially outer edge of the grids 28 are connected to the inner wall of the chamber 23 by respective seals 29. The seals 29 provide a seal between the grids 28 and the chamber wall, thereby encouraging any fluid flowing in the chamber 23 20 to flow through the grids 28. The circular grids could instead be conical to provide more surface area, providing they do not shade each other from the irradiation of the lamp 26.

Inlet and outlet ducts 31,32 are respectively extend from the end walls 31,32. A port 33 is provided on the inlet 25 31 for injecting oxygenated gas into the fluid.

The apparatus operates in a similar manner to the apparatus of Figure 1. The efficiency of the apparatus can be improved by placing ultra-sonic transmitters (not shown) in the spaces between the grids 28, so that the doped crystalline 30 lattice of the TiO2 surfaces are mechanically excited as well as illuminated.

Referring to Figures 4 and 5 of the drawings, there is shown an alternative embodiment of fluid treatment apparatus which is similar in operation to the previous embodiments. The 35 apparatus comprises an elongate annular treatment chamber having inner and outer tubular walls 34,37, sealed at their respective opposite ends by end walls 35,36. A plurality of lamps 38 are arranged at circumferentially-spaced positions around the annular chamber and extend axially thereof between the end walls 35,36. A further lamp 39 extends along the central longitudinal axis of the chamber inside the inner tubular wall 37 thereof.

The lamps 38 are is enclosed in glass sleeves 40 which are sealed at both ends of the chamber, so that the lamps are isolated from the fluid in the chamber but allow the lamps 38 to illuminate the inside of the chamber through the glass 10 sleeves 40. Likewise the lamp 39 illuminates the inside of the chamber through the inner tubular wall 37 thereof, which is formed of a transparent material. A tubular grid 41 is disposed around each of the lamps 38 within the chamber, with the grids 41 being resiliently compressed between the inner and outer 15 tubular walls 34,37.

The surface of the grids 41 have been chemically treated, as discussed in the previous embodiments to preferably produce the anatase form of TiO2 doped with certain metals, metallic oxides and/or metallic compounds.

Internal of the annular chamber are a series of baffles 42 which extend normal to the lamps 38 and the grids 41. The baffles 42 are generally c-shaped, as shown by the hatched shading in Figure 5, and completely divide the chamber into longitudinally spaced zones, apart from a small semi-circular 25 gap 43 which allow the fluid to flow between adjacent zones.

The rotational position of the gap 43 varies by 1800 between adjacent baffles 42, such that the fluid flows in a tortuous path between the inlet and outlet ducts 44,45. Again, an inlet port 46 is on the inlet 44 for injecting gas into the fluid. 30 The internal surface of each of the tubular grids 41 is illuminated its respective lamp 38, whilst the outer surfaces are irradiated by the central lamp 39. The grids 41 are tubular and thus present a large surface area, with the resulting increase in the contact time between the contaminants/pollutants and the oxidising agent zones. As the fluid flows through the first part of the grid 41 it splits into two paths in opposite senses around the annular chamber, as shown in Figure 5, the fluid then flows in and out of the grids 41 as it moves around the annular chamber. The baffles 42 encourage the fluid to move in a tortuous path, providing a good mixing effect and maximising the exposure to the grids 5 41. The oxidising species kill any microorganisms present in the fluid and pollutants are reduced to harmless basic elements.

Referring to Figure 6 of the drawings, there is shown an alternative embodiment of fluid treatment apparatus 10 comprising a cylindrical chamber having an external wall 47 formed of a material which is substantially transparent to light of wavelengths 400nm or less. Opposite ends of the chamber are closed by end walls 48,49. An elongate rotational shaft 50 extends along the central longitudinal axis of the 15 chamber (4) . The shaft 50 is supported at its opposite ends by bearings 51, mounted to the respective end walls 48,49.

Attached to the shaft are a series of impellers 52 having a surface which is coated with or converted to the anatase form TiO2 and doped with certain metals, metallic 20 oxides and/or metallic compounds to improve its reactivity. A gap is provided between the radially outer ends of the impellers 52 and the inner surface of the tubular wall 47.

A plurality of circumferentially-spaced lamps 53 extend axially of the chamber around the transparent tubular wall 47 25 thereof.

In use, the fluid to be treated passes through inlet 54 and is aerated via injector port 55. The fluid passes into the chamber and flows through the impellers to the outlet 56. The lamps 53 illuminating the impellers 52 through the transparent 30 wall 47 of the chamber, thereby activating the TiO2 surface of the impellers 52. The fluid flow causes the impellers 52 to rotate. The contact efficiency with the impellers 52 is improved by inhibiting the shaft rotation using a restrictive damper 57, so that the fluid rolls over the impellers 52. The 35 fact that the impellers 52 rotate provides a very low pressure drop across the apparatus and virtually no filtering action.

To improve the mixing effect, the impellers 52 can be slowly driven in reverse against the flow with the aid of a motor (not shown) attached to the shaft 50. Alternatively, the impellers can be rotated at a sufficient speed to create cavitation. The number of impellers 52 and the length of the 5 chamber can be chosen to suit the fluid being processed and the degree of microbiological or pollutant contamination therein.

The efficiency of the system can also be improved by exciting the TiO2 surfaces with Ultra Sound and/or microwaves.

Referring to Figure 6 of the drawings, there is shown 10 an alternative embodiment of fluid treatment apparatus comprising an annular chamber 58 having inner and outer tubular walls 59,60 formed of glass or other material which is substantially transparent to light having a wavelength of 400nm or less. The annular chamber 58 is filled with spheres 61 or 15 other particulate material. A layer of doped titanium dioxide is disposed on the surface of the spheres 61. The spacing between the inner and outer walls 59,60 is greater than the diameter of the spheres 61 but less than twice the diameter, so that the spheres 61 stack on top of each other in a column: 20 This arrangement allows for good exposure to the illumination from the lamps and provides a large activated doped TiO2 surface area.

A plurality of lamps 62 are arranged at circumferentially-spaced positions around the annular chamber 25 and extend axially thereof. A further lamp 63 extends along the central longitudinal axis of the apparatus inside the inner tubular wall 59 thereof.

A helical baffle 64 is disposed inside the annular chamber 58, to cause the fluid to flow in a circular and 30 downwards spiralling motion as it flows from the inlet 65 to the outlet 66. Thus, the baffle 64 effectively lengthens the flow path and increases the contact time between the spheres 61 and any fluid flowing through the column 58.

A port 67 is provided for aerating or oxygenating the 35 fluid flowing through the inlet, as described in previous embodiments. Preferably the port 67 injects air which is ionised by a device 68.

In use, the spheres 61 are substantially fully irradiated by the lamps 62, 63. The column of spheres 61 form a natural regular lattice or matrix and thus the fluid flows in a turbulent manner through this lattice creating high 5 contact times with the surface of the spheres 61. Using hollow perforated or porous spheres increases the contact time.

Another feature of the invention is that the column of spheres 61 is substantially self-cleaning because the surfaces of the spheres are so active that they prevent anything 10 adhering or bonding to them.

A magnetron 69 can be provided for irradiating the chamber 58 with microwaves. These microwaves excite the surfaces of the spheres 61 at the same time as they are irradiated by the lamps 62.

It has been found that the reactivity of the doped TiO2 surface is greatly enhanced when it is simultaneously irradiated with light having a wavelength of 400nm or less and microwaves. Preferably the frequency of the microwaves is selected for minimal absorption by the fluid. Preferably the 20 microwave magnetron is partially or fully modulated with ultrasonic frequencies in the range 20KHz to 4MHz to achieve an additional increase in the doped TiO2 reactivity. This produces an ultrasonic effect in the surfaces of the sphere which mechanically vibrates the doped TiO2 crystalline lattice 25 whilst it is being irradiated with light of wavelength 400nm or less.

Several phosphor crystals have been shown to emit light when activated by microwave radiation when modulated to produce an Ultra sonic vibration in their crystalline lattice and in 30 particular some emit wavelengths in the 350 - 400nm range.

Accordingly, the spheres 61 could also comprise a phosphor coating which emits light to activate the doped TiO2.

This overcomes the problems of shading and allows porous spheres to be used. The overall efficiency of the system is 35 increased due to the large surface area produced by the porous spheres. Also, fluids having a poor transmissivity do not attenuate the light and adversely effect the disinfection performance.

Referring to Figure 8 of the drawings, there is shown an alternative embodiment of fluid treatment apparatus comprising a treatment chamber 70 having tubular side wall 73 5 and end walls 74,75. An inlet and an outlet 71, 72 for the fluid to be treated respectively extend from the end wall 74 and the side wall 73. The opposite end wall 75 is made from a material which provides very little absorption of microwave energy e.g., glass, ceramic or plastic. Fixed to the end wall is the output 10 from a resonating cavity 76, which in turn is fed from one or more magnetrons 77.

A pair of mesh grids 78 extend across the chamber between the inlet and outlet 71,72. The grids 78 are also formed of a material which provides very little absorption of 15 microwave energy. The space between the meshes 78 is filled with a media 79 in the form of granules or particles. The particles 79 are in the form of regular or irregular shapes with each particle being first coated with a phosphor, which emits wavelengths of light in the range 350 - 400nm when 20 activated by microwave/ultra sonic radiation. Then the particles are coated with doped Ti02.

Fluid is aerated or oxygenated by gas injector 80 as it flows through the inlet 71 and into the chamber 70. The magnetron 77 is generating microwaves of a frequency which is 25 selected and amplified by the resonating cavity 76. These microwaves pass through the end wall 75 and into the chamber, thereby exciting the phosphor coating on the media 79. The resultant light from the phosphor activates the doped TiO2 providing a large surface area of activated photo catalytic 30 oxidising semiconductor.

The magnetron 77 is partially or fully modulated with ultrasonic frequencies in the range 20KHz to 4MHz to achieve an additional increase in the doped TiO2 reactivity. This produces an ultrasonic effect on the surfaces of the media 35 which mechanically vibrates the doped TiO2 crystalline lattice on the media particles whilst it is being irradiated with light from the phosphor.

If the media 78 becomes clogged, it can be cleaned by back flushing. To achieve this, the treated fluid is made to flow back through the media via outlet 81 and exits via the waste disposal port 82 and valve 83. Injecting air into the 5 reverse flowing fluid, to induce turbulence enhances the effect. Any collected debris is washed away leaving the media ready to treat more fluid.

Recent developments in electroluminescent materials comprising a chromophoric polymeric composition have been 10 demonstrated to emit radiation in the 350 - 500nm wavelength ranges depending on their composition. They are excited to luminesce by the application of an electrical potential across the material.

Referring to Figures 9 and 10 of the drawings, there is 15 shown an alternative embodiment of fluid treatment apparatus comprising a porous block 84 of a substrate material, which is formed such that the central potion 85 of the block 84 is porous but the region 86 around the edge of the block 84 is solid and impervious.

A conductive layer is deposited onto the surface and pores of the block 84. Then a chromophoric polymeric layer 87 is deposited onto the surface and through the pores of the porous part of the block 84. On top of this layer 87 is deposited a transparent conductive layer. The two conductive 25 layers provide means for applying a potential across the chromophoric polymeric layer 87. On top of this layer is deposited a transparent waterproof layer 88, which protects the chromophoric polymeric layer 87. The transparent layer 88 is an electrical insulator and is transparent to the wavelengths 30 emitted by the chromophoric polymeric layer 87. A layer 89 of doped TiO2 is deposited on top of the transparent layer 88 using methods described in previous embodiments.

Electrical contacts 90 are provided on the block 84 for applying electrical potential to respective ones of the two 35 conductive layers sandwiching the chromophoric polymeric layer 87.

The block 84 is mounted inside a treatment chamber 91 between an inlet 92 and an outlet 93 thereof. When an electrical potential is applied across the chromophoric polymeric layer 87, the layer luminesces and emits light of a wavelength which activates the TiO2 layer 89. The fluid to be 5 treated flows through the inlet pipe 92 which is attached to gas injection port 95, as described in previous embodiments.

Fluid flows through the porous block 84 and the activated TiO2 surfaces in the block 84 combine with the oxygen in the fluid to produce oxidising species zones in the vicinity of the doped 10 titanium dioxide surface. The block 84 may be further excited with microwaves.

In all of the described embodiments, the fluid to be treated is past over a surface which is coated with a photo catalytic oxidising semiconductor doped with a metal. The metal 15 doping increases the life of electron hole pairs which are generated when the photo catalytic oxidising semiconductor is irradiated with light having a wavelength in the UV range.

These electron hole pairs produce hydroxyls and oxygen free radicals. Hydroxyls and oxygen free radicals are highly 20 reactive and will break down complex molecules and, in the case of micro-organisms, will destroy them by attacking and breaching their cell walls.

Claims

1. A fluid treatment apparatus comprising a surface of a photo catalytic oxidising semiconductor material doped with a metallic material disposed in a flow path of the apparatus and 5 means for exciting the doped semiconductor material to form electron-hole pairs.

2. A fluid treatment apparatus as claimed in claim 1, in which the photo catalytic oxidising semiconductor material comprises titanium dioxide.

3. A fluid treatment apparatus as claimed in claim 2, in which the photo catalytic oxidising semiconductor material is the anatase form of titanium dioxide.

4. A fluid treatment apparatus as claimed in claim 1, in which the metallic material is a noble metal such as gold, 15 silver, palladium or platinum.

5. A fluid treatment apparatus as claimed in claim 1, in which the apparatus comprises an element disposed in the flow path, the photo catalytic oxidising semiconductor material forming a surface coating on the element.

6. A fluid treatment apparatus as claimed in claim 5, in which the element is porous, the fluid being arranged to flow through pores in the element.

7. A fluid treatment apparatus as claimed in claim 5, in which a plurality of coated elements are provided.

8. A fluid treatment apparatus as claimed in claim 1, in which the means for exciting the doped semiconductor material is arranged to irradiate the material with light.

9.

A fluid treatment apparatus as claimed in claim 8, in which the means for exciting the doped semiconductor material is arranged to irradiate the material with light irradiated by a photo-luminescent material.

10. A fluid treatment apparatus as claimed in claim 9, in 5 which means are provided to cause the photo-luminescent material to fluoresce.

11. A fluid treatment apparatus as claimed in claim 1, in which the means for exciting the doped semiconductor material is arranged to irradiate the material with ionised particles.

12. A fluid treatment apparatus as claimed in claim 1, in which the means for exciting the doped semiconductor material is arranged to irradiate the material with ultra-sonic waves.

13. A fluid treatment apparatus as claimed in claim 1, in which the means for exciting the doped semiconductor material 15 is arranged to irradiate the material with a magnetic field.

14. A fluid treatment apparatus as claimed in claim 1, in which the means for exciting the doped semiconductor material is arranged to irradiate the material with an electromagnetic

field.

15. A fluid treatment apparatus as claimed in claim 1, in which the means for exciting the doped semiconductor material is arranged to irradiate the material with an electrostatic

field.

16. A fluid treatment apparatus as claimed in claim 1, in 25 which means are provided to introduce chemicals, gasses or compounds into the fluid to stimulate the photo catalytic oxidising semiconductor material to produce oxidising species to further enhance the efficiency of the apparatus.

17.

A fluid treatment apparatus substantially as herein described with reference to Figures 1 and 2, Figure 3, Figures 4 and 5, Figure 6, Figure 7, Figure 8 and Figures 9 and 10 of the drawings.