WO2007057520A1

WO2007057520A1 - Method and apparatus for sterilizing gas, by measuring and controlling active oxygen content and uv-intensity

Info

Publication number: WO2007057520A1
Application number: PCT/FI2006/050501
Authority: WO
Inventors: Erkka Lehto; Ari Lehto
Original assignee: Oy Biocid Ltd
Priority date: 2005-11-17
Filing date: 2006-11-16
Publication date: 2007-05-24
Also published as: FI119679B; FI20051175A0; FI20051175A

Abstract

The present invention relates to a method and apparatus for sterilizing a microbe-containing gas. Such a method comprises steps in which the oxygen molecules of the gas are ionized for producing activated oxygen and the gas is exposed to radiation with ultraviolet light. According to the invention the active oxygen content of the gas and the intensity of the ultraviolet light is measured and the production of activated oxygen and the intensity of the ultraviolet light are adjusted on the basis of the measurement data. The invention produces a sterilizing apparatus having an optimal efficiency for destroying microbe stock in, for example, foodstuffs.

Description

Method and apparatus for sterilizing gas, by measuring and controlling active oxygen content and uv-intensity

The present invention relates to a method for sterilizing gas, especially air, as described in the preamble of claim 1. Typically such a need is encountered in food industry, medical industry, hospitals, airplanes and cabins of ships.

The present invention also relates to an apparatus suitable for applying the method.

Instead of traditional chemical disinfection, dry disinfection can be used in many applica- tions. The idea of dry disinfection is to destroy, among others, harmful bacteria, moulds, yeasts and their spores and viruses (hereafter also "microbes") from the area to be cleaned. Known dry disinfection techniques are use of UV-C spectrum ultraviolet light and activated oxygen. Hydroxyl radicals can additionally be used for destroying microbes.

In ultraviolet disinfection, UV-C radiation with a wavelength of about 260 nm is often used, this wavelength being well suitable for destroying and inactivating microbes. Such a radiation can be produced, for example, by vaporizing mercury in low pressure plasma, whereby ultraviolet light with a wavelength of 253,7 nm is produced. This industrial application of destroying harmful microbes has been studied since the 1930's, but only techno- logical developments in recent years have allowed its wide-scale use. The microbe- destroying effect of UV-C light is due to the fact that wavelengths in the vicinity of 260 nm penetrate physically into the microbes. This light energy makes permanent changes in the molecular bonds and the structure of the DNA, whereby it no longer can correctly react with the enzymes controlling cell division and proliferation. Soon, the microbes exposed to UV-C light will die and their amount will quickly decrease.

Radiation with a wavelength < 289 nm (at least 4.29 eV) is sufficient for breaking the C-H bond and radiation below 196 nm is sufficient for breaking C=C and C=N double bonds'. In contrast to chemical disinfection substances, micro-organisms can not develop an im- mune mechanism for protecting against UV radiation.

At the -260 nm range DNA has a maximum absorption (bases of nucleotides)¹¹. DNA consists of straight nitrogen-base chains, purines (adenine and guanine) and pyrimidines (thymine and cytosine). These compounds are linked into a chain by sugar-phosphate compounds. Purine and pyrimidine compounds are called base pairs and they are linked to each other by hydrogen bonds. With the germicidically most efficient wavelengths (263 - 275 nm) these hydrogen bonds break and thymine dimeres are formed in the DNA. When the cell tries to divide itself (mitosis), it can not replicate. Radiation with a wavelength below 200 nm affects the sugar-phosphate body of the DNA. The radiation absorption also has a devastating effect on the ability of the cell to regulate its internal osmotic pressure¹".

On the other hand, activated oxygen consists of very reactive ionized oxygen atoms and molecules in excitation state. The activated oxygen oxidizes molecules and removes gases and smells from the air. This chemical process is irreversible and it removes the gases and smells permanently. Electrically charged oxygen molecules also form oxygen clusters that attract, neutralize and remove particles, dusts and pollutants from air. Activated oxygen additionally breaks the cell membrane and prevents their growth and reproduction.

Activated oxygen can be produced in activators designed for this purpose. When the oxygen molecules of air pass the activator, more gaseous components are formed, the components being oxygen molecules and oxygen ions in excitation state. New components can also be formed in the reactions between ionized oxygen and the impurities of air. Typically, the ions are formed as follows:

Positive ions: e- + O₂ -» O₂+ + 2e- e- + N₂ -> N₂+ + 2e- e- + O₂ -_► O+ O +2e- e- + N₂ -» N + N+ 2e-

O₂ is electro negative, i.e. it has a tendency to pick electrons in a bonding reaction.

Negative ions: e- + O₂ + M -> O₂- + M e- + O₂ ^' + M -> O₂ ^" + M

Negative and positive ions are formed in approximately equal amounts. This phenomenon is called bipolar ionization. M is a gas molecule not participating in the reaction, the molecule removing the extra energy released during the bonding of the electron. Free radicals cause lipid peroxidation^ in the unsaturated lipids of the cells. The catalytic iron (Fe²⁺) and oxygen (dioxygen) in the cells initiates the lipid peroxidation.

Molecular ozone and its degeneration products, such as hydroxyl radicals, also have a powerful antimicrobial effect^v. Micro-organisms are quickly inactivated, because ozone reacts with the intra-cellular enzymes, nucleic material and cell membrane, spore coats and viral capsids.

Montie et al^vι have suggested three ways of low-pressure "cold plasma" having an effect on the death of a cell. Firstly, the sensitivity of unsaturated fatty acids for the adsorption of hydroxyl radicals leads to lipid peroxidation. Secondly, the sensitivity of amino acids to oxidation leads to oxidation of proteins. Thirdly, generation of base adducts leads to oxidation of DNA.

Laroussi et al³ and Montie et al^v" and noticed that with shorter applications (10-30 s) the outer membrane of the cell broke and the cytoplasm of the cell leaked out. With longer exposure times the whole cell disintegrated. This was thought to be due to changes in the lipides of the membrane caused by the peroxidation of the fatty acids.

No visible morphological changes were noted in Gram-positive bacteria, but the vitality of the cells was reduced⁵. This was thought to be due to the fact that some reactive radicals can be diffused straight through a membrane otherwise chemically and physically stable and react directly with intra-cellular matter. The radicals of the plasma can also affect the metabolism of the cells without the cell dying.

The main components of dry air are: oxygen (O₂) 21% and nitrogen (N₂) 78%. Usually air also contains some water vapour (H₂O). Water molecules are polar, i.e. their charge distribution is not spherically symmetric. As a whole the molecule is electrically neutral. Be- cause of the polarity, 10-15 H₂O molecules can bond to each ion in the air. These compounds are called hydrates.

As the humidity increases, hydroxyl radicals do also have an important part in the inactiva- tion of bacteria, because they chemically break the outer shell of the bacteria. If radicals are produced from air, NO- and NO_x-compounds will also be produced that increase the lethality of the process. Ozone (O₃) is also formed in this process, the ozone having a destroying effect on cellular respiration, among others. This takes place when O₂ is disintegrated into 2 O' s by means of electron collisions. A single oxygen atom (O) can react with O₂, thus forming ozone O₃: e- + O₂ -> 20 + e- O + O₂ + M -» O₃ + M

The amount of ozone produced in the activator depends on the voltage used. In an environment with people present the allowed ozone level is 0.02 ppm. Such amounts are not harmful to people, animals or plants. On a hot summer day natural ozone levels of 0.05 ppm are not uncommon outdoors. In an environment with no people or plants even higher levels can be accepted.

Ozone and activated oxygen have a bacteria-destroying effect. The effect depends on the concentration and exposure time. It has been noticed in research that the components produced by means of ionization have a destroying effect on a number of different types of coli bacteria. Research has also been conducted on the effect of ions on mould spores on surfaces.

Charged particles can also have a very important role in the disruption of the outer cell membrane of bacteria, resulting to leak of cytoplasm, and in the destruction of microbes". Bonding of charged particles on the cell membrane causes a great electrostatic force that can exceed the maximum tensile strength of the membrane. This will most probably take place with gram negative bacteria, since their surface is less even.

Moisan et al. propose that UV radiation cause erosion of genetic material atom by atom (photodesorption)'". Subsequent to this reactive atoms are adsorbed into the surface and etch it. Subsequent to this reactive matter reacts directly with the biomaterial of the cell forming volatile organic compounds. Thus, the simultaneous use of UV radiation and ions has a cell death increasing effect.

PMx (particulate Matter less than x micrometer diameter) particles are small air-borne, often electrically charged pollutants. Electric forces bond ions and hydrates to them and form larger particles. They can fall to the ground under the influence of gravity or they can attach to charged objects (walls, floor). Ionization is especially efficient with smaller particles that are transported deepest into the respiratory tracts and thus cause the largest health risk.

The following publications are presented as representative of state of the art.

The published application US 2003/0230477 discloses a wall-mounted air cleaning apparatus utilizing UV light and ozone as well as oxygen radicals, hydrogen peroxides and hy- droxyl radicals produced in the vicinity of the UV source.

The published application US 2003/003028 discloses an air cleaning apparatus having an air inlet and an air outlet and number of UV-C lamps arranged between them. The apparatus records the on-time of the lamps and reports the level of power loss due to the wear of the lamps on the basis of it. The apparatus is suitable for use, for example, as an air fresh- ener of large spaces.

One solution utilizing UV-C radiation for disinfecting food by destroying microbes is known from publication CA 2447310. In the solution, ozone and UV radiation are produced for destroying microbes and for producing hydroxyl radicals. Air can be humidified near the poles of the UV radiator for increasing the efficiency of the hydroxyl radicals. Publication CA 2455785 discloses a food cleaning tunnel operating with the same principle, in which there has been attempt to optimize the disinfection result by means of the design of the tunnel.

Publication GB 2405463 discloses an apparatus having a UV-C source and an air ionizator. The on-times of the UV source and the ionizator can be manually changed by choosing some of the pre-programmed operation codes.

Using UV-C radiation and photocatalytic surfaces together is known from publications WO 2005/053830 and US 2004/197243. The latter discloses a method in which UV radiation is directed against a surface containing titanium dioxide for producing peroxide radicals or oxygen oxides near the surface for destroying the organisms contained by the air. Publications US 2004/0041564 and US 2004/0061069 disclose methods and apparatuses for sterilizing the air passing through a channel by radiating it with UV light. The apparatus can comprise sensors for measuring the humidity/flow of the air and the performance of the UV lamp. Thus, the publications represent known UV sterilization technology.

A problem with the above-mentioned dry disinfection methods is that they do not take into consideration the currently prevailing microbe conditions and thus their efficiency can easily be too large or too small. Thus, their energy consumption and disinfection result are often far from optimal. Due to the "over-cleaning" the quality of the food can suffer as well. Because of the bad adjustment possibilities the known apparatuses are difficult to customize for different purposes and changing conditions.

In order to solve the above-mentioned problem publication US 2004/0231696 discloses a chamber having high-voltage electrodes for activating or ionizing the air. Electrodes can also be used for forming ozone, UV radiation and powerful electric or magnetic fields. The products to be sterilized are placed inside the chamber. The apparatus also comprises sensors for measuring the concentration of harmful substances such as bacteria, viruses, protozoa, yeasts, moulds, spores, insecticides, smoke and noxious gases. The concentration of the harmful substances can be used for controlling the operation of the system. Because there are very different harmful substances, there must also be a large number of sensors in order to produce an extensive view of the sterilization efficiency of the system. In order to at all be able measure the harmful substances from the matter to be cleaned, the system must be closed, which is a considerable restriction in many applications.

The aim of the invention is, therefore, to produce a new dry disinfection method and apparatus, more efficient and flexible than known methods and apparatuses.

The aim of the invention is especially to produce a method and apparatus, the cleaning efficiency of which can be adjusted to suit the current conditions and the cleaning effϊ- ciency of which is constant even over long periods of use. Such a method and apparatus is thus especially suitable for use in, e.g. production lines continuously running with a high capacity, even 24 hours a day, and in which reliable cleaning is very important and operation stoppages are economically very harmful. The invention is based on the idea that activated oxygen (oxygen radicals) is added to the gas to be cleaned while exposing the gas to UV radiation so that the amount of activated oxygen and the intensity of the radiation (disinfection parameters) are measured and, if necessary, further adjusted during the process. By means of a process controlled by pa- rameter measurement and measurement data the disinfection conditions can always be optimized as necessary. Especially when the amount of microbe populations in the target is known, the strengths and relative proportions of disinfection parameters can be set as desired.

According to one embodiment the gas is also humidified and the humidity of the gas is measured during the process. Thus the third disinfection parameter, humidity, can as well be adjusted for improving the efficiency of the sterilization. The reason for humidifying is especially that a humidified gas reacts with the produced radiation, whereby OH radicals (hydroxyl radicals) are formed that will further help in disinfection of the impurities. Thus, adjustment of humidifying offers a reliable way of adjusting the concentration of OH radicals in the gas. The humidity can be measured directly or via determining the concentration of OH radicals.

An apparatus according to the invention comprises an oxygen activator and a UV source. The apparatus further comprises means for determining the amount of activated oxygen and the intensity of UV radiation as well as an adjustment system that is sensitive to the units transmitted by the said means for achieving optimal disinfection conditions. The apparatus can further comprise a gas humidifier and a gas humidity detector that can also be connected to the said adjustment system. The humidifying can be effected by means of a suitable humidity generator.

According to one preferred embodiment photocatalytic surfaces have also been arranged in the space for further improving the disinfection effect. Photocatalytic surfaces are used especially in connection with the humidity generator integrated into the apparatus and within the range of the UV source, whereby it is especially preferably possible to control- lably produce OH radicals. The activation and possible humidifying of the oxygen can be made in any order. Preferably the radiation is effected on the UV-C wavelengths and especially preferably on humidified gas.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

On the other hand, the apparatus according to the invention is characterized by what is stated in the characterizing part of claim 13.

Considerable technical and economical advantages are achieved by means of the invention. Using activated oxygen, UV-C radiation and OH radicals together is more efficient than using them separately. Another important advantage of the invention is that the effect of the sterilization apparatus on the object can be optimized by changing the mutual relation- ships and exposure times of activated oxygen, OH radicals and UV radiation. When the system is adjusted according to prevailing microbe, radical and UV conditions, the most optimal disinfection combination can be chosen. This saves energy and avoids unnecessary exposure of food products to radiation or radicals.

One of the advantages of the invention is that by means of it the production of ozone, harmful to human metabolisms as well, can be reduced, as the sterilization process is efficiently optimized by means of other parameters, such as the amount of hydroxyl radicals. However, ozone is typically formed as a by-product in the activation process in small, harmless concentrations. Thus the method is also safe to humans working in the vicinity of the process space.

It is also to be noted that the measurement of activated oxygen according to the invention can be made from activated air already before it is directed to the object to be cleaned. Thus, one can, for example, avoid the drawbacks of a closed process as shown in US pub- lication 2004/0231696, and the sterilization apparatus can be constructed as an independent and/or an open unit, with the products to be cleaned outside of it. According to the invention the adjustment of sterilization efficiency is based on the measurement (and pre- adjustment) of the properties of activated and cleaned gas instead of measuring (and post- adjusting) the microbes and other harmful substances. Especially in food preparation lines the starting levels of microbes are well known even without a continuous measurement, whereby the adjustment of activated oxygen and/or UV light and/or humidity can be optimized according to the microbe species/levels of the object.

By means of the humidity generator, the humidity of the air passing through can be adjusted so as to be suitable for optimizing the combined effect of photocatalytic surface and the UV-C light. The OH radicals are very efficient microbe destroyers, especially together with UV-C light.

Experiments have shown that by means of the present method it has been possible to extend the shelf life of some product groups (applied in production: best before-date) by even two days in comparison with some traditional dry disinfection methods due to the lower microbe level attained. In grocery trade this is of great importance.

An apparatus according to the invention is reliable and inexpensive. It can be made to operate automatically for long operation periods, such as months, even over half a year without changes in its disinfection efficiency, i.e. the attained microbe level of the object. The apparatus can be made compact and it is thus especially suitable for small and cramped spaces. With it, the parts critical for the cleanliness of the process can be sterilized for achieving the necessary level of cleanliness. In case the cleanliness requirements, the product to be sterilized or the conditions of the environment change, the apparatus can automatically adjust itself or it can be reprogrammed. This can happen, e.g. in food industry production lines where different products are produced or processed on different days. In addition, change of seasons can also have an effect on the microbe stock, whereby also the sterilization needs change.

Some general advantages of a dry disinfection method over a traditional chemical disinfection are listed in the following:

- The method can be used continuously, for example during production or 24 hours / day, if necessary

- The method can be arranged so as to be automatic, thereby saving labour costs

- The method is efficient and it works also against spores

- The method reduces consumption of water and energy

- The method does not produce hazardous waste — The method does not produce resistant microbe stocks and

— The method is safe to employees (no exposure).

In the following, the embodiments of the invention are described in more detail and with reference to the appended drawings.

Figure 1 shows as flow diagram the process sequence according to one embodiment, Figure 2 shows an apparatus solution according to one embodiment of the invention as a schematic symbol drawing, Figure 3 shows an apparatus solution according to another embodiment of the invention as a schematic symbol drawing,

Figure 4 shows an apparatus solution provided with air cycling means as a schematic symbol drawing, Figure 5 shows an apparatus according to one embodiment connected to a production line as a schematic symbol drawing,

Figures 6 — 7 show two UV intensity measurement devices as symbol drawings, and Figure 8 shows one possible ion concentration measurement arrangement as a symbol drawing.

Let us study the figure 1 illustrating a solution for carrying out the method according to present method. The block 101 comprises oxygen activation, humidifying and radiating steps. According to the example, the air to be sterilized is directed into the activator for activating the air in step 111. The activated air is humidified at a humidifier in step 121. The humidified and activated air and is exposed to UV-C radiation subsequent to the acti- vation of oxygen in step 131 by means of a UV radiator or radiators. The block 103 comprises the measurement processes of the concentrations of oxygen and water vapour and the intensity of UV-C radiation (steps 113, 123 and 133, correspondingly), from which the measurement data is transmitted to the adjustment system 102. In the adjustment system the adjustment needs of each disinfection parameter are determined (steps 112, 122 and 132) and the necessary control commands are transmitted to the processes 111, 121 and 131 of block 101. The adjustments 112 and 122 of humidifying and radiation are preferably made in connection with each other, especially when the humidifying is related to the production of OH radicals by means of UV light in a photocatalytic coating located inside the apparatus. The partial processes 110, 120 and 130 of oxygen, humidity and UV radiation shown in figure 1 can be executed simultaneously in time. They can, however, be physically executed in different parts of the apparatus. Alternatively they can be physically totally or par- tially overlapping. The sequence shown in the figures thus illustrates the order in which the process parts appear when travelling with the flow of air. In this sense the process parts can be executed in an arbitrary order. It is, however, often preferable to have radiation as the last step pf the process or at least that it is preceded by the activation of oxygen, because the UV sensitivity of the microbes is then at its highest.

The activation of oxygen 111 is preferably carried out via a high electric field or a changing magnetic field. A dielectric discharge (surface discharge) or a corona discharge into the air can be electrically produced, the discharges ionizing the oxygen molecules on the discharge zone. The disadvantage of corona discharge is that ozone is produced therein.

Humidifying 121 is executed in a humidity generator, the operation principle of which can comprise, e.g. evaporation, vaporization or direct misting of water. According to one preferable embodiment ultra sound (ultra sound misting apparatus) is used therein.

The radiation 131 can be accomplished, for example, by means of a UV lamp based on low-pressure low-pressure mercury vapour or by means of suitable semiconductor sources, such as LED (light emitting diode) lamps. In some applications arc discharge can also be used.

The apparatus comprises, for example, means of the following types for measuring the concentration of oxygen ions and OH radicals and means for measuring the intensity of UV-C radiation.

The measurement 113 of the oxygen radicals is carried out by means of a suitable activated oxygen sensor. The sensor can preferably distinguish between different oxygen ions (O₂ ⁺, O⁺, O₂ ⁺, O₂ ^", O₂ ^2") and/or compounds containing these oxygen ions, but this is not necessary. The measurement can also comprise identifying hydroxyl radicals and/or other ions formed in the disinfection reaction. If necessary, the number of sensors can be higher. The identification can take place in the vicinity of the activator or the product to be cleaned, whereby a more reliable picture about the quality of air in the vicinity of the product can be obtained.

The measurement of oxygen radicals 113 can be done, e.g. by means of electromagnetic or electrostatic methods. Figure 8 shows a simple apparatus module 80 based on the measurement of ion flow. An ion flow 87 is formed between two electrodes 82, the magnitude of which is indicative of the total ion concentration between the electrodes. The signal is amplified in an amplifier 83 having a high input impedance and it is converted in an A/D converter 84. The digital information is further processed in a micro controller 85 for car- rying out the necessary adjustment. Suitable sensors are produced by, for example, Al- phaLab, Inc. {http:///www.trifieldmeter.com/Airlon.html). A simple ion flow meter can be made, for example, by means of absorption of particles formed by means of radioactivity or some other means, such as technology used in fire detectors.

The humidity measurement 123 can be carried out e.g. mechanically, electronically or spectroscopically. The measurement can be carried out at any place in the system, such as the cone of the radiator, in the vicinity of the photocatalytic surfaces or near the product or in a number of places, if necessary. The production rate of hydroxyl radicals is typically relative to the humidity of the gas. A direct humidity measurement means a measurement based directly on the concentration of water molecules, but the humidity can also be determined by means of, for example, OH radicals as shown in figure 1. Thus the humidity measurement 123 and measurement of oxygen radicals 113 can be carried out with only one apparatus. It is not necessary to calculate the absolute or relative value of humidity, but the adjustment can as well be made on the basis of lower level measurement information.

UV measurement 133 is carried out at chosen spot within the range of the UV source. In case the UV cone is directed in addition to the air passing through also directly to the product, the measurement can be carried out near the product. As the geometric properties and reflectivities (shape of cone) of the UV source are known, the radiation field can in principle be calculated at any point of the system. One of the most important tasks of the IV measurement is to determine the lowering of the efficiency of the UV source over time so that it can be compensated for by increasing the drive power of the source or by changing other disinfection parameters of the system for achieving the desired disinfection result. In addition to UV-C measurement other light wavelengths can be measured as well. A simple UV measurement and adjustment module 60 is shown in figure 6. The light of UV source 66 meets the light diode 62, which produces a signal that will be amplified in amplifier 63. The signal is further converted into an analogue signal in A/D converter 64 to be directed into the micro controller 65 controlling the UV source 66. The micro controller can be a separate one or the same that is used for adjusting the activated oxygen and/or humidity.

An exemplary connection of the UV measurement and adjustment module into the appara- tus is shown in figure 7. The module 70 is located beside the gas cavity and its light- sensitive area 72 is located within the range of the cone of the UV source 76. The UV cone can also comprise a ballast 74. If a LED-based UV source is used, the ballast is usually not needed, as the LEDs can be controlled directly by electronics.

The measurements 113, 123 and 133 are preferably regular and/or continuous for continuously monitoring the process status. The part of the apparatus carrying out the various part steps of the measurement system 103 is called the measurement unit.

The measurements 113, 123 and 133 the adjustment system 102 can be used for supplying the necessary information for controlling the sources 111, 121 and 131. The adjustment can be carried out separately for each process part or the adjustments of process parts can be carried out in relation with each other. Preferably the adjustment system 102 comprises an adjustment unit having a microprocessor. Thus the adjustment unit can comprise, for example, a computer with adjustment software. The software is continuously supplied with information via a data bus or buses from meters 113, 123 and 133 and it calculates the needed adjustment for each process part. The calculation is typically based on pre-input optimization algorithms that can be customized for each solution. The algorithms can take into account other system parameters than those specified above, such as, air temperature, air flow rate or geometric parameters of the process to be disinfected, such as the retention time of the products within the apparatus, the size of the products, type and so on. Preferably the apparatus comprises a user interface for changing the adjustment parameters or adjustment algorithms and for monitoring the status of the process. The apparatus can also be connected to a data transfer bus via which the data supplied by the apparatus can be read and the operation parameters of the apparatus can be changed from outside the apparatus.

According to one preferred embodiment the adjustment system 102 comprises an elec- tronic microbe library or it can be connected to such. The will make a new kind of inacti- vation-diagnosing application possible. The library comprises information about a number of microbes probably living in the air and/or on the surfaces of the production space. Preferred information includes the sensitivity of the microbes to UV light, hydroxyl radicals and oxygen ions (the dose needed for inactivation). In case of wavelength modulated UV light the microbe information can also include information about the inactivation sensitivity of wavelength dependencies. The microbe densities of air or surfaces can be estimated from samples taken in the room, with various estimation methods and/or on-line measurements. The effective amounts of UV, hydroxyl radicals and oxygen amounts are read from the measurement system, whereby the microbe density and microbe library can be utilized for further adjusting these values to a more optimal level. The microbe species of the object are input into the processor/software of the apparatus, the software or apparatus calculating the necessary optimal dose by means of the microbe library.

Preferably also the humidity data available from the humidity measurement and the proc- ess temperature are used in calculating the inactivation diagnostics. These parameters and their changes typically have the most essential effect on the proliferation of microbes and it is useful to constantly monitor them. Thus it is possible to calculate the inactivation of a certain microbe using a matrix having the following units: temperature, humidity, the known growth limits and speeds of the microbes, the (statistical) UV-C sensitivity of the microbes. The calculation matrix can further contain other variables, such as physical condition parameters, data about the process control (such as speed and washes).

The microbe library and the adjustment algorithms can preferably be updated via software when new research data about the prevailing microbe conditions and sensitivities of mi- crobes is available. For example, the necessary UVC radiation doses for inactivating various yeasts and fungi are referred to in appendix x.

The unit formed by the adjustment system 102 and the measurement system 103 is called a control system. In the following the operation of the apparatus is described in more detail with reference to figure 2. The figure is schematic and thus does not form a true description of the looks and design of the apparatus.

The apparatus preferably comprises a cavity 20, through which the air to be cleaned can be directed. The cavity is delimited by walls 22. The cavity preferably has a first end, open or semi-open to air flows (inlet end) and a second end, open or semi-open to air flows (outlet end). A fan 24 is located in the first end of the cavity, the fan directing air from the sur- roundings to the object. The fan can alternatively be located in the second end of the cavity. Subsequent to the inlet duct (in the direction of the air flow) the cavity contains an oxygen activator 26, a humidity generator 27 and one or more UV-C radiators 28. These can be located one after the other so that the operations take place subsequent to each other in the various part spaces of the cavity so that two or even all operations are executed in the same part space of the cavity. The walls 22 of the cavity 20 can consist at least partly of the structures of the activator 26, humidity generator 27 and the radiator 28.

The wall surfaces 23 within the range of the UV source 28 can be provided with a photo- catalytic coating, such as a ΗO₂ coating. In a solution according to figure 2 the UV sources 28 are located so that the radiation can be directed also outside the cavity, such as directly to the foodstuffs being disinfected or the surfaces of the cavity via the second end of the cavity. In a solution according to figure 3 a direct UV-C radiation to the outside of the cavity is prevented by means of a wall structure 32. This can also be accomplished by means of suitably placing of the UV unit 38 in the apparatus or by protecting the UV unit 38.

The air inlet duct can be directly connected to the room air or outdoors air. If necessary, the inlet air can be filtered or otherwise pre-cleaned. The sterilized air is directed through the air outlet duct to space 59, into which the product 51 to be sterilized can be arranged, as is shown in figure 5. The space can be arranged, for example, into a certain part of the pro- duction line so that all products of the line pass through it. The space can thus comprise means 53, for example a conveyor belt or the like, for moving the products. Large-scale entry of non-sterilized room air into the space is prevented by means of a sealed construction or by arranging a pressure higher than the ambient pressure. The space preferably comprises at least one air outlet duct 55. The duct 55 can contain a throttle for adjusting the pressure in space 59. Thus the apparatus makes it possible to sterilize the parts critical for the cleanliness of the plant so that it is not necessary to arrange the whole plant or production line as a clean line. Sterilizing can be made, for example, just before packaging the products or even during packaging in the space 59.

According to one preferable embodiment the radiators are located in the vicinity of the activator, whereby the radiation will take place immediately after the activation or simultaneously with the activation. The efficiency of the production of OH radicals is increased, for example, by humidifying the by directing the humidified air to the vicinity of the photocatalytic surface or by humidifying it near the activator.

According to one preferred embodiment the air to be sterilized is first directed to the air activator, in which the microbes are exposed to activated oxygen, subsequent to which the weakened microbes are exposed to UV-C radiation and further to OH radicals. The UV-C radiation can also be directed outside the apparatus for sterilizing the surfaces thereof instead of using it only inside the apparatus for production of OH radicals.

According to one preferred embodiment the oxygen activator 26, humidity generator 27, photocatalytic surfaces 23 and the UV-C unit 28 are integrated into one apparatus assem- bly. Thus it can preferably comprise a skeleton frame into which the said apparatuses can be mounted. The frame can also be formed of a wall structure 22. The frame can be installed into, for example, the desired place of a production line of an industrial plant as an independent operative sterilization unit. It can alternatively be connected to existing structures of industrial plants, such as a food packaging line or a packaging machine. The ad- justment system and/or the measurement system is/are preferably at least partly integrated into the same apparatus assembly. These systems can also be separate units, in which case they are operationally connected to each other by means of cables or wireless connections for carrying out the tasks described above and in the following. The apparatus is preferably connected at one or more places to the electric network.

The apparatus can be made modular so that its various parts, especially the activator, humidifier, UV source and/or photocatalytic surfaces can be easily changed or serviced. The service life of a typical mercury-based UV lamp is about 7000 h in normal conditions. However, even during this time a considerable decrease of UV power can take place. A continuous or intermittent measurement of UV radiation offers a possibility to take the power decrease into account when adjusting the apparatus. In this case a minimum UV power must be defined and it must be taken into account when programming the adjustment system so that the desired minimum level is attained during the whole planned service life of the lamp. When the efficiency of the lamp decreases, the input power can be increased so as to at least correspond with this minimum level.

The apparatus can also comprise a UV spectrum analyzer based on, for example, absorption spectrum which can be used via the adjustment system for controlling the power of the UV source and/or the wavelength. Absorption spectrum could also be used for determining the dust or concentration and possibly various organisms in the air. Also, lamps emitting different wavelengths could be used, whereby the inactivation of various bacteria and/or viruses can be more efficiently optimized. The UV absorption of DNA, for example, varies from one species to another on a scale that can be taken into account during radiation. The emission of different wavelengths can be accomplished by means of, for example, LED sources.

The apparatus can also comprise means for circulating air more than once through the apparatus prior to outlet for increasing the retention time in the apparatus and further for achieving a better sterilization result. Such a multiple air circulation can be activated, for example, when the above-mentioned UV spectrometer indicates an increase in the amounts of dust, particle or microbe concentration. A return flow duct 49 making air circulation possible is shown in figure 4. The return flow duct can comprise a separate fan for achieving the correct air circulation direction. For this, the cavity 40 can also comprise air guides or valves that can be adjustable.

The photocatalytic coating either on the inside surfaces of the apparatus or the UV lamps or ionization tubes of the activator is titanium dioxide derivative coating TiO₂M_x. This can improve the self-cleaning and germicidic properties of the surfaces.

The activated oxygen can be directed from the activator at least partly directly to the application point as well, i.e. to the vicinity of the product to be disinfected by means of a tubu- lar duct that can also comprise a separate fan. In this case, a separate UV source, such as LED lamps, can be arranged near the object, at the end of the duct of activated oxygen or adjacent it, for precise destroying microbes directly in the application point.

The apparatus can be programmed with an automatic self-testing, anticipation of service needs and reporting the status of the apparatus. The apparatus can also comprise means for sending an automatic message to the controller of the apparatus and/or its manufacturer and/or the service via a data transfer connection (such as a local area network or mobile telephone network or fixed telephone lines), when a predefined threshold value of a certain parameter is exceeded or the value is not reached.

(i) A. A. Bolshakov, B. A. Cruden, R. Mogul, M. V. V. S. Rao, S. P. Sharma, B. N. Khare, and M. Meyyappan Radio-Frequency Oxygen Plasma as a Sterilization Source AIAA Journal Vol. 42, No. 4, April 2004. NASA Ames Research Center, Moffett Field, California 94035

(ii) http://www.hanovia.com/uv_technology/uv-technology.htm

(iii)M. Laroussi. IEEE Transactions on Plasma Science, Vol. 30, NO. 4, August 2002 1409, Nonthermal Decontamination of Biological Media by Atmospheric-Pressure

Plasmas: Review, Analysis, and Prospects

(iv)Freya Q. Schafer, Steven Yue Qian and Garry R. Bbuettner., Cellular and Molecular Biology 46 (3), 657-662 0145-5680/00 Iron and free radical oxidations in cell membranes, Printed in France 2000 Cell. MoI. Biol.

(vii) S. Moreau, M. Moisan, J. Barbeau, J. pelletier, and A. Ricard, "Using the flowing afterglowof a plasma to inactivate bacillus subtilis spores: Influence of the operating conditions," J. Appl. Phys., vol. 88, pp. 1166-1174, 2000. (viii) D. A. Mendis, M. Rosenberg, and F. Azam, "A note on the possible electrostatic disruption of bacteria," IEEE Trans. Plasma ScL, vol. 28, pp. 1304-1306, Aug. 2000.

(ix)M. Moisan, J. Barbeau, S. Moreau, J. Pelletier, M. Tabrizian, and L'. H. Yahia, "Low temperature sterilization using gas plasmas: A review of the experiments, and an analysis of the inactivation mechanisms," Int. J. Pharmaceut, vol. 226, pp. 1-21, 2001.

(x) B. Hyllseth & H. Banrud, Literature on UVC (J/m²) microbe killing/inactivation (%).

Claims

Claims:

1. A method for sterilizing a microbe-containing gas, the method comprising the following steps: - ionizing oxygen molecules of the gas for producing activated oxygen,

- radiating the gas with ultraviolet light, characterized in that it further comprises steps of

- measuring the activated oxygen content of the gas and the intensity of the ultraviolet light, and - adjusting the production of activated gas or the intensity of the ultraviolet light on the basis of the measurement data.

2. A method according to claim 1, characterized hi that it further comprises a step in which gas is humidified by introducing water vapour into it and further steps in which the humidity of air is measured and the humidifying is adjusted, if necessary, on the basis of the measurement data.

3. A method according to claim 1 or 2, characterized in that the UV light is directed at least partly onto photocatalytic surfaces, such as TiO₂ surfaces, in the vicinity of which the gas to be sterilized is directed for producing OH radicals.

4. A method according to any of the previous claims, characterized in that the sterilized gas is directed to the vicinity of a foodstuff product for disinfecting it.

5. A method according to any of the previous claims, characterized in that also foodstuff product and/or the surfaces in its vicinity are radiated with the ultraviolet light.

6. A method according to any of the previous claims, characterized in that the input power of the UV source is adjusted on the basis of the intensity measurement of the UV light for compensating for the decrease of the efficient power of the source occurring over time.

7. A method according to any of the previous claims, characterized hi that the UV light is produced by means of mercury vapour.

8. A method according to any of the previous claims, characterized in that the UV light is produced by means of one or more semiconductor elements, such as an LED element.

9. A method according to any of the previous claims, characterized in that the adjustment of the production of activated oxygen and ultraviolet light comprises a step in which the value of at least one measured parameter is compared with an optimum value derived from a preset microbe library.

10. A method according to any of the previous claims, characterized in that it additionally comprises a step in which the microbe content of a gas is measured.

11. A method according to any of claims 2-10, characterized in that the step in which gas is radiated is accomplished after the oxygen activation and humidifying.

12. A method according to any of the previous claims, characterized in that the steps for producing activated oxygen and ultraviolet light are carried out in a cavity having a gas inlet duct and an outlet duct for sterilized gas, the outlet duct being connected to the space to be sterilized.

13. An apparatus for sterilizing a microbe-containing gas, the apparatus comprising

- a cavity having an inlet and an outlet for gas and into which gas can be directed via the inlet duct from the outside,

- an activator for activating the oxygen present in the gas directed into the cavity from the outside,

- at least one ultraviolet source for radiating the gas directed into the cavity, characterized in that the apparatus further comprises

- sensors for determining the activated oxygen concentration of the gas directed into the cavity and for measuring the intensity of the radiation, - means operationally connected to the sensors for adjusting the operation of the activator or ultraviolet source.

14. An apparatus according to claim 13, characterized in that it also comprises a humidity generator for humidifying the gas directed into the cavity, a sensor for measuring the hu- midity of the gas directed into the cavity and means operationally connected to the sensor for adjusting the operation of the humidity generator.

15. An apparatus according to claim 13 or 14, characterized in that one or more photo- catalytic surfaces are arranged within the range of the ultraviolet source for producing OH radicals.

16. An apparatus according to any of claims 13-15, characterized in that it comprises an ultraviolet source based on mercury vapour.

17. An apparatus according to any of claims 13-16, characterized in that it comprises a semiconductor-based ultraviolet source.

18. An apparatus according to any of claims 13-17, characterized in that the means for adjusting the operation of the activator and the ultraviolet source are arranged to read data from a pre-saved microbe library, to compare the data with the data transmitted from the said sensors and to make the adjustment on the basis of the comparison.

19. An apparatus according to any of claims 13-18, characterized in that it additionally comprises means for measuring the microbe content of the gas.

20. An apparatus according to any of claims 13-19, characterized in that the ultraviolet source is directed so that at least part of the ultraviolet light produced therewith is directed out from the outlet duct of the apparatus.

21. An apparatus according to any of claims 13-20, characterized in that the means for adjusting the operation of the ultraviolet source comprise means for adjusting the input power of the ultraviolet source for compensating for changes in the intensity of the measured radiation.

22. An apparatus according to any of claims 13-21, characterized in that the means for adjusting the operation of the activator and the ultraviolet source are arranged to maintain a predefined purity of the sterilized gas.