EP2356078A1

EP2356078A1 - Method and device for treating water by uv radiation

Info

Publication number: EP2356078A1
Application number: EP09829412A
Authority: EP
Inventors: David Stenman; Haidong Liao
Original assignee: Wallenius Water AB
Current assignee: Wallenius Water AB
Priority date: 2008-11-28
Filing date: 2009-11-27
Publication date: 2011-08-17
Also published as: SE533527C2; CN102300816A; EP2356078A4; SE0850107A1; WO2010062253A1; KR20110105786A

Abstract

The present invention relates to a device for treating water, comprising an enclosure having UV radiating means, and catalysts comprising a number of plates arranged in stacks with a certain distance between them and generally parallel to each other, characterised in that it further comprises an electric DC power source connectable to said catalysts such that every second plate of the stack is connected to the plus side of the power source and every second plate of the stack is connected to the minus side of the power source.

Description

Method and device for treating water by UV radiation

TECHNICAL AREA

The present invention relates to a method for treating liquids, and in particular purifying liquids in order to remove or destroy harmful organisms in the liquid with photo-catalytic reactions.

TECHNICAL BACKGROUND

There is a greater and greater demand on the environmental effects of polluted liquids and in particular water. The access to clean and unpolluted water has become a major issue in the world. This entails both fresh water as well as salt water. The fresh water supply in many areas of the world is limited at the same time as many of the fresh water sources are polluted by man.

The biological balance in the seas has also been affected by man due to ballast water handling. Ships are arranged with ballast water tanks that are filled in order to stabilize them when the ships are not fully loaded with cargo. It is well known that ballast water contains species that have been recognised as major ecological problem if spread are cholera, kelp, toxic algae and mussels, just to mention a few. It is estimated that about 3-5 billion tonnes of ballast water are transported around the world. It is thus not surprising that this has become a major issue where the International Maritime Organisation of UN has issued a convention that with start from 2009 will put demand on all commercial ships to be equipped with and use special systems for handling ballast water.

Many systems have been developed for treating and purifying water such as with chemicals where chlorine is commonly used. In order to reduce the negative impact that many chemicals have on the environment, systems have been developed that do not use chemicals but rely on other effects in order to kill organisms in water in order to purify it, such as methods for purifying water with ozone (O3) in drinking water installations and bathing facilities, and also ozone dissolved in water for cleaning, disinfection and sterilization of articles. By means of its oxidizing effect, the ozone acts rapidly on certain inorganic and organic substances.

Another method is to radiate the created ozone with UV light of certain wave lengths in order to break down the ozone and create radicals, which are more aggressive than ozone. Such a method is disclosed in EP 0 800 407, in which the medium which is to be treated is introduced into some form of enclosure. In the enclosure, the medium is exposed to UV radiation with a spectral distribution within the range of 130 - 400 nm.

The wavelengths below 200 nm, in particular, convert the oxygen in the medium to ozone molecules (O3). The ozone molecules formed are at the same time decomposed by radiation within the above-mentioned wavelength range, especially at wavelengths of - 400 nm. At the same time, the O2 formed is broken down to form atomic oxygen.

In order to increase the efficiency during generation of free radicals, in particular HO' radicals, catalysts are utilized, arranged in the zone where the ozone is decomposed to free radicals.

A development of the above mentioned method utilizing radicals is disclosed in document PCT/ SE2007/ 050676, by the same applicant as the present application. It discloses a liquid treatment enclosure or reactor having an inlet and an outlet. Inside the reactor a number of elongated UV-generating means are arranged generally perpendicular tot the liquid flow through the reactor. A number of catalytic plates are also arranged in stacks inside the reactor and generally parallel with the liquid flow. The elongated UV-generating means runt through the stacks of catalysts. The arrangement of the catalysts and the UV- generating means provides a very thorough mixing of the liquid and a very good exposure of organisms in the liquid to the radicals formed close to the catalysts. In all a very thorough and complete treatment of the entire volume of liquid passing through the reactor is obtained.

The above described design with stacks of plates could provide additional treatment properties, which forms the subject of the present invention.

BRIEF DESCRIPTION OF THE INVENTION

The aim of the present invention is to further enhance the treatment capabilities and properties of photo-catalytic purification.

This aim is obtained by the features of the independent patent claims. Preferable embodiments of the present invention are found in the dependent patent claims.

According to a main aspect of the invention it is characterised by a device for treating water, comprising an enclosure having UV radiating means, and catalysts comprising a number of plates arranged in stacks with a certain distance between them and generally parallel to each other, characterised in that it further comprises an electric DC power source connectable to said catalysts such that every second plate of the stack is connected to the plus side of the power source and every second plate of the stack is connected to the minus side of the power source.

According to a further aspect of the invention, the DC power source is capable of providing a voltage in the range of -5 - +5.

According to yet an aspect of the invention, the DC power source is capable of providing a current in the region of 1 mA - 1 A. According to yet an aspect of the invention, the catalysts comprise metal, metal oxides or both, such as noble metals, aluminium oxide, titanium oxide, silicon oxide and mixtures thereof.

According to a further aspect of the invention, said UV generating means comprises UV lamps, that said UV lamps are arranged in elongated UV permeable tubes, and that said tubes are arranged generally transversal to the direction of flow of the liquid.

The present invention has a number of advantages in comparison with the known devices in this technical area.

By applying a voltage between two adjacent catalytic plates, a number of positive effects are to be expected: enhancement of the generation of radicals in the treatment process; enhancement of the photocatalytic ability of the catalysts in that the spectral interval of light that can provide photocatalysis is enhanced; to enable negatively or positively surface-charged organisms to stay for longer periods adjacent the charged catalytic surfaces in order to improve the process; change the surface charge (z-potential) of the photocatalysts in order to change the chemistry of the surfaces; change the semi-conductor properties in different directions of the photocatalyst.

Also the properties of the process are improved in situ for: - the activation of ozone to reactive radicals, conversion of the ozonide anion ( O₃ ^" ) to ozone, conversion of ozone to the ozonide anion ( O₃ ^" ),

- conversion of superoxide ( O₂ ) to oxygen ( O₂ ),

- conversion of oxygen ( O₂ )to superoxide ( O₂ ), - conversion of superoxide ( O₂ ) to hydroperoxide ( O₂ ^" (+2H+ ->

H₂O₂)), conversion of H₂O₂ to hydroxyl radicals,

- conversion of H₂O₂ to superoxide, - decomposition of hydroperoxides (ROOH),

- decomposition of organic peroxides (R1OOR2).

Further, because at least selected parts of the interior surfaces are arranged with reflection increasing means, the UV radiation emitted from the UV radiation generating means is used to a much higher degree than if some of the UV radiation is absorbed, which thus leads to a more efficient treatment process. Also, the required power is reduced.

The inner surfaces could be covered by suitable materials, that have reflection increasing properties. Preferably the materials also have properties to withstand the tough conditions inside the treatment unit and the aggressive effects from the liquid to be treated. The materials should also be effective against scaling, which otherwise would reduce the reflection effect during use.

These and other aspects of and advantages with the present invention will become apparent from the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description, reference will be made to the accompanying drawings, of which

Fig. 1 shows schematically one feasible embodiment of a treatment unit utilizing the present invention,

Fig. 2 shows an example of design of a stack of catalytic plates comprised in the present invention,

Fig. 3 shows another example of design of catalytic plates, and Fig. 4 shows yet an example of a stack of catalytic plates of a certain shape.

DETAILED DESCRIPTION OF THE INVENTION According to the embodiment shown in Fig. 1 , a purifier that could use the present comprises a housing 20, in the shown embodiment as a generally elongated enclosure with a rectangular cross-section and with in- and outlets 22, 24 at each end of the enclosure. When water is flowing in the enclosure it will flow in the direction of the elongated enclosure between the inlet and the outlet. In the enclosure a number of UV radiating light sources 26 are arranged in elongated tubes of quartz glass 28, which extend between the opposite walls of the enclosure. The light sources are connected to suitable power supply. The UV radiating light sources are chosen such that they emit wave lengths in the region of 130 - 400 nm for converting oxygen in the medium to ozone molecules (O3) and for decomposing the ozone molecules.

Further, a number of plates 30, at least two, are arranged in the enclosure, the extension of which generally coincide with the direction of flow and thus perpendicular to the extension of the lamps. The plates are arranged in stacks with a certain distance between them. The plates act as catalysts for the treatment process thus boosting the amount of radicals produced. The plates are thus made of a material with catalytic properties to increase the number of radicals produced in the reactive zones. The material could include metal and/or metal oxides, such as noble metals, aluminium oxide, titanium oxide, silicon oxide and mixtures thereof.

According to the present invention the plates of the stacks are connected to a DC-power source 36 such that every other plate in a stack is connected to the positive connection of the power source and every second plate is connected to the negative connection of the power source, Fig. 2. A suitable voltage is applied, which could be in the range -5 - +5 V and with a suitable current, which could be in the range 1 mA - 1 A.

With this arrangement a few positive effects are obtained. The application of the voltage on the plates alters the response to light of the surface of the catalyst plates and thereby the efficiency of the plates for creating radicals or other chemical components by photo-catalysis. Thus the spectral interval of the light that can provide photo-catalysis is broadened and the efficiency in the transformation of light energy to chemical energy is improved. Other positive effects are the mass transport providing improved selectivity, to have negatively or positively charged organisms stay longer periods at the charged catalytic surfaces in order to improve the process and enhance the surface charge (z- potential) of the photocatalyst in order to alter the chemistry in the layer closest to the catalytic surfaces.

The number of plates and the distance between them are chosen such that an optimization is obtained regarding e.g. transportation of light from the lamps to the active surfaces of the plates; transportation of organisms in the vicinity of the surfaces; and transportation of free radicals from the surfaces into the liquid volume.

The glass tubes are arranged substantially perpendicular to the direction of flow. In the embodiment shown in Fig. 2 the lamps are arranged in two rows, but there could be only one row as well, or more than 2 rows depending on the energy demands.

The catalyst plates are preferably designed to increase and/ or promote the turbulence in the reactive zones as well as designed to increase the surface area. There are a number of different designs, configurations and combinations of these that could be used. According to Fig. 4 the catalyst plates 30 are made of expanded metal, thus creating a number of perforations or holes 34 through the plates. One advantage with expanded metal is that the edges of the holes are sharp, thus increases the turbulence. Other types of designs could be punching, structure pressings, corrugations, grooves and the like. It is also conceivable to use nets, woven or non-woven fabrics, wire mesh and the like. These could further be made in light permeable material such as quartz glass, glass fibre or other materials having the right properties. The design of the surfaces of the plates and/ or structure of the plates ensure that the boundary layer becomes very thin, which otherwise would prevent fluid exchange adjacent the photo catalytic surfaces of the plates, creating flow dead zones close to the surface where the radicals are the most potent. Other ways of decreasing the boundary layer could be to increase the surface rawness of the catalysts, by for example applying quartz sand to the surfaces.

There are further measures that can be made in order to increase the turbulence and mixing. Fig. 5 shows an embodiment where, in contrast to Fig. 2, the plates do not extend all through the enclosure but are

"interrupted", providing uninterrupted spaces 36 between the stacks of catalytic plates. This causes turbulence in the liquid when leaving a stack and further turbulence when hitting the subsequent stack so that a process, ->photo catalysis -> mixing -> photo catalysis ->mixing, is obtained.

To even further enhance the turbulence when leaving a stack, the plates could have a cross-sectional design where the leading edge of each plate, i.e. facing the flow, is sharp, and where the trailing edge is blunt, Fig. 6. The interior surfaces of the enclosure may be arranged with reflection enhancing means. Either selected parts of the interior surfaces are provided with reflection enhancing means or all inner surfaces. The reflection enhancing means provides a "reuse" of the UV light that is emitted from the lamps. This provides the effect that there is a much better effect in that light that hits the interior of the treatment unit is reflected and continues to treat the liquid. There is thus no absorption of light, whereby the power required for the UV lamps is reduced.

There are a number of materials that might be suitable as reflection enhancing means. One important factor is that the material has to be able to withstand the rather aggressive conditions inside the unit, such as corrosion resistant properties and the like.

Materials that have proven successful are some polymeric materials, and in particular fluoroplastic such as polytetrafluoro ethylene (PTFE). PTFE has very high reflection capabilities and is thus suitable as a reflection enhancing material. Besides that, PTFE displays very low friction coefficient and is also resistant against aggressive liquids such as seawater. This will reduce or even eliminate the scaling and will also reduce the hydraulic friction trough the treatment unit. In this context, it is to be understood that other polymeric materials displaying similar properties can be used instead of PTFE. Polymeric materials are also much cheaper than steel or other metals. Further, the polymeric material could be prepared with catalytic material in for example powder form dispersed in the polymer, such as for example metal and/ or metal oxides, such as noble metals, aluminium oxide, titanium oxide, silicon oxide and mixtures thereof.

It is to be understood that the embodiments of the invention described above and shown in the drawings are to be regarded only as non- limiting examples of the invention and that it may be modified in many ways within the scope of the patent claims.

Claims

PATENT CLAIMS

1. A device for treating water, comprising an enclosure having UV radiating means, and catalysts comprising a number of plates arranged in stacks with a certain distance between them and generally parallel to each other, characterised in that it further comprises an electric DC power source connectable to said catalysts such that every second plate of the stack is connected to the plus side of the power source and every second plate of the stack is connected to the minus side of the power source, wherein the DC power source is capable of providing a voltage in the range of -5 - +5.

2. A device according to claim 1 or 2, wherein the DC power source is capable of providing a current in the range of 1 mA - 1 A.

3. A device according to any of the preceding claims, wherein it further is provided with turbulence and mixing generating means.

4. A device according claim 4, wherein the catalysts having turbulence and mixing generating means selected from one or more of the means from the group consisting of perforations, holes, punchings, structured pressings, corrugations, and grooves.

5. The device according to claim 1 , wherein the catalyst plates are arranged such that the UV radiating means are going though the catalyst plates.

6. The device according to claim 1 , wherein the UV radiating means are radiating light within the range from about 130 to about 400 nm.

7. The device according to claim 7, wherein the UV radiating means are radiating light in at least the regions of 187 nm and of 254 nm.

8. The device according to any one of the preceding claims, wherein the catalysts comprise metal, metal oxides or both, such as noble metals, aluminium oxide, titanium oxide, silicon oxide and mixtures thereof.

9. The device according to any one of the preceding claims, wherein the device also comprises UV light reflecting means.

10. The device according to claim 10, wherein the UV light reflecting means are means made of PTFE.

1 1. The device according to claim 10 or 1 1 , wherein the UV light reflecting means further comprises catalytic material.

12. Device according to claim 1 , wherein said UV generating means comprises UV lamps, that said UV lamps are arranged in elongated UV permeable tubes, and that said tubes are arranged generally transversal to the direction of flow of the liquid.

13. Device according to any of the preceding claims, wherein said lamps are arranged through said stacks, and wherein the extension of said plates generally coincide with the direction of flow of the liquid.

14. Device according to any of the preceding claims, wherein there are a number of lamps arranged in said enclosure, that each lamp is arranged through a stack of plates, and that there is a distance between each stack, enabling turbulence and mixing of the liquid when entering and leaving said stacks.

15. Device according to claim 15, wherein said plates have a cross-sectional design such that the leading edges are sharp and the trailing edges are blunt.

16. A method for treating water comprising radiation of the ballast water with ultra violet light (in presence of oxygen, and catalysts for formation of ozone and free radicals in reactive zones, wherein the catalysts comprise a number of plates, which plates are arranged in stacks with a certain distance between them and generally parallel to each other, characterised in that it further comprises applying electric DC power to said catalysts such that every second plate of the stack is connected to the plus side of the power source and every second plate of the stack is connected to the minus side of the power source, and wherein the DC power is providing a voltage in the range of -5 - +5.

17. A method according to claim 17 or 18, wherein the DC power is providing a current in the range of 1 mA - 1 A.