WO2006087754A1

WO2006087754A1 - Device and method for preventing aeromicrobial growth in refrigerating equipment and air conditioning apparatus

Info

Publication number: WO2006087754A1
Application number: PCT/IT2006/000076
Authority: WO
Inventors: Luca Poletti
Original assignee: Sereco Biotest S.N.C.
Priority date: 2005-02-18
Filing date: 2006-02-14
Publication date: 2006-08-24
Also published as: EP1861128A1; ITRM20050073A1

Abstract

The invention concerns a microwave diffuser, for trays to. The invention concerns a regenerable air filtering system for control ling aeromicrobial growth, which enables the permanent removal and deacti vation of micro-organisms suspended in confined air spaces by having a lethal action on them. The system consists of an air filtering device composed of a filter element, which can be of a modular type, having a granular bed based on aluminosilicate metallized with one or more metal ions having an antibacte rial activity and non-toxic for the body, such as silver, zinc, copper or their mixtures, and a particle size ranging between 1 μm and 50 mm, preferably between 50 μm and 50 mm. The filter element may be combined with one or more pre-filter ele- ments based on chitosan and/or its functionalised derivatives, metallized with one or more ions having antimicrobial activity and non-toxic for the body. The device may also include separate means for forcing the air undergoing treat¬ ment through the device.

Description

DEVICE AND METHOD FOR PREVENTING AEROMICROBIAL GROWTH IN REFRIGERATING EQUIPMENT AND AIR CONDITIONING APPARATUS

DESCRIPTION

The present invention concerns a device and method for preventing aeromicrobial growth in refrigerating equipment and air conditioning apparatus. More specifically, the invention concerns an air filtering system for abating microbes contained in air and providing for the dispersal and permanent deac- tivation of micro-organisms suspended in confined air spaces by exerting a lethal action on them. The system may be adapted to air spaces of various dimensions and can be regenerated indefinitely in a simple and cost-effective manner.

The presence of bacterial and fungus micro-organisms in the air of confined working or living spaces is a considerably important and topical hygiene-health issue. It is widely held scientifically that many infective and allergic pathologies are caused by micro-organisms present in the air. Fungus basidiospores, for example, have been related to the onset of various forms of allergies, ranging from those of Type I (hay fever and asthma) to Type III (pu- lomonary hypersensitivity). The microbial decontamination of working and domestic environments is thus a very important objective for health policies of all Western countries, so much so that certain criteria have been drafted (European Collaborative Action Report no. 12) establishing the critical levels of total bacterial contamination and of mycete toxinogens (Stachybotris, As- pergillus, Penicillium, Fusarium, Cladosporium) in the air.

Preventing aeromicrobial pollution is of primary importance also in the food industry, where contamination of foodstuffs is one of the most serious hygiene-health problems. According to epidemiological data reported by the World Health Organisation, every year a great many people are affected by disorders caused by food which is contaminated by micro-organisms or their toxins (food-borne diseases, FBD).

Another element of considerable concern is that, despite the undis- puted scientific and technological progress in the field of food hygiene and microbiology, the number of episodes of FBD of unknown aetiology and origin are surprisingly on the rise. This makes it difficult to identify and control the risk factors, which are largely unknown or difficult to assess in terms of docu- mentary evidence.

Considering concrete applicative cases, the problem of bacterial decontamination is found, for example, in refrigerating chambers and even in household refrigerators, which are normally equipped with air circulation systems (often induced by thermal gradients) in order to guarantee a high degree of uniformity in cooling every part of the refrigeration space. The circulation of unsuitably decontaminated air can result in a high level of microbes owing to the presence of biodegradable organic material representing a potential source of microbial input. This condition may involve the risk of contaminating foodstuffs - above all, if the parameters of the refrigerator temperature set- tings are not respected (for instance, owing to an electricity black-out or to wrong thermostat settings).

Similarly, in refrigerated counters of cafeterias and food stores the re- circulating air involves an inevitable risk of contamination by airborne microbes. Moreover, the degradation of the protein contents of foodstuffs gives rise to foul smelling compounds such as mercaptans and other sulphur compounds, ammonia, amines and other nitrogen compounds whose presence negatively affects the commercial presentation of products on sale and leads to foodstuffs undergoing undesirable changes at an organoleptic level.

In the food processing industry the possibility to effectively prevent microbial contamination is fundamental. For example, dairy producers have concentration limits for mycetes in cheeses (the main agents responsible for the contamination of products) of 200-700 CFU (colony forming units) per m³ of air. In beverage bottling plants (mineral waters, wines and beers), preventing airborne microbial contamination is crucial in order to avoid serious or- ganoleptic problems that the presence mycetes causes in beverages. In meat processing plants, the refrigeration chain may be one of the most critical points of the entire process, since any interruption in such chian may lead to the proliferation of micro-organisms inside the food, also due to the surface depositing of airborne bacterial cells. Any internal air space, even one that appears of the utmost cleanliness, if not suitably treated with filter systems, can arrive at containing thousands of bacteria cells per cubic metre. In meat storage rooms, the humidity can be very high - as much as 75-80%; this condition allows the proliferation of germ cells on the room's walls, thus increasing the risk of contamination to the meat itself.

At present, prevention of aeromicrobial growth is carried out with different techniques, also depending on the kind of air space to be treated. In domestic and working environments, and particularly in hospital environments such as in operating theatres and intensive care units, highly efficient air sterilisation systems are used - called HEPA (high efficiency particulate arrestors, or high efficiency particulate air filters) - based on forcing air through special tissue filters designed to trap particulates of dimensions greater than 0.3 μm, and thus micro-organisms as well. However, these filters cannot be regenerated and thus require regular replacement and thermal destruction of the used filter. Moreover, as regards the prevention of aeromicrobial growth, these filters do not rule out the problem of recontamination due to bacteria cell "repair". The biological "repair" mechanisms activated to deal with the cell dam- age suffered by micro-organisms (bacterial stress) are such that in many cases the bacteria trapped in the filter can recover their viability.

Even sterilisation with ultraviolet rays or with oxidising agents (such as ozone or even chlorine or formaldehyde-based products) are subject to losses of efficiency caused by these biological repair mechanisms and, moreover, can present problems of secondary toxicity. In cases where the environment undergoing treatment is designed for foodstuffs, their use has many more critical contra-indications, since these systems have an oxidising capacity that can alter and degrade many constituents present in foods, such as fats, proteins and natural colourings. In this case, the degradation may easily lead to the production of free radicals that can cause undesirable changes in the foods themselves.

For the treatment of air, not in order to decontaminate it from bacteria - A -

but in order to remove odorous compounds which could themselves turn into dangerous sites of bacterial proliferation - for example, in waste treatment or compost plants, active carbon filters are sometimes also used. These are often used together with HEPA filters in air conditioning and purification sys- terns. Active carbon filters mechanically trap germs inside their pores but do not deactivate them, and they are themselves subject to the bacterial repair mechanism. That is why active carbon filters are only proposed for removing smells.

For regeneration, the active carbon must be sterilised at a tempera- ture of at least 180-200⁰C, but there is the risk that the heat can alter the carbon structure thus volatising part of the organic component. The methods used for regenerating active carbon in industrial plants envisage the flowing of inert gas in a controlled atmosphere or under vacuum, or the use of superheated steam. None of these procedures can be used at home, also owing to their cost.

Patent literature suggests various methods and equipment for controlling the quality of air in confined spaces, and particularly in indoor human spaces and in food storage areas - including both domestic and industrial refrigeration systems. International patent application No. WO 86/05120, for example, describes an air filter device wherein the filter material is composed of a mixture, in given proportions, of silica gel, active carbon and zeolite. The use of three different adsorbent materials together is considered a necessary expedient for the proper functioning of the device. Since none of the three components per se has germicide properties, the primary function of the above filter is as a deodorant for environmental air and for removing the dust and particulate present in the air. If germicide properties are desired, then the filter material must be soaked in a germicide liquid, such as a quaternary nitrogen-based preservative. For the efficient treatment of environmental air, the aforesaid device may be combined with a small fan in order to obtain a forced ventilation filter system. Owing to the kind of filter material used, the filter must be periodically replaced in order to eliminate the filtering material which, at least for the active carbon component, cannot be regenerated for the reasons mentioned above.

US patent No. 5253488 describes a system that is placed into the vegetable compartment of home refrigerators. The system functioning is based on the adsorption of undesirable compounds by zeolite material. According to the description, the system does is not intended to prevent bacterial growth but to eliminate the ethylene-based gases arising from the degradation of vegetable material, and envisages a plate-shaped absorption element containing zeolite powder placed on the upper surface of the vegetable compart- ment. This absorption element is associated with an UV ray lamp placed above the element itself and which periodically turns on to regenerate the zeolite. It is evident that the sole purpose of the aforesaid device is that of eliminating undesirable gases and odours, since zeolite alone does not have any germicide properties. Another similar device for similar purposes is dsclosed in international patent application No. WO 95/15187, which describes an odour-absorption device to be placed inside a refrigerator. The device is composed of a perforated container inside which there is some granular material based on zeolite and active carbon. The document prescribes pre-established proportions of zeolite and active carbon inside the device, and here too the action envisaged is only of the removal of odours.

As is known, zeolites are generally hydrated aluminosilicates, either natural or synthetic, of one or more alkali or alkaline-earth metals, more frequently sodium, potassium, magnesium, calcium, strontium and barium. These aluminosilicates feature a lattice with large pores and regularly arranged channels in which the cations are weakly linked and can thus easily exchange with other cations found in solution. Moreover, the zeolites are available in a large variety of three-dimensional crystalline structures, all characterised by a large surface area whose channels and pores are permeable to molecules of various sizes and shapes. Because of their structure, the zeolites show a selective permeability for molecules of a particular size and shape, and act like real molecular sieves. Examples of natural zeolites include sodalite, mordenite, analcime, clinoptilolite, faujasite and cabasite, while examples of synthetic zeolites are zeolite A₁ zeolite Y and zeolite X.

The properties of zeolites to act like molecular sieves, in applications as the ones hitherto described, make them useful for removing undesirable odours from the air, but they do not have any real microbial decontamination activity. In order to prevent aeromicrobial growth it is possible to use the germicide properties of metal ions, such as the silver ion, which has long been known to have germicide and disinfectant properties - for example, in its con- ventional use made as silver nitrate with antiseptic properties. The silver ion is believed to complex the sulphhydryl groups of the bacterial membrane proteins, denaturising these proteins and thus causing the destruction of the cell membranes of the microbes. Other hypothesised actions are the inhibition of bacteria cell respiration and replication. Similar activities are also found with other metal ions, such as the zinc ion and copper ion, with a greater activity towards eucariote cells (fungi, moulds and yeasts), while silver is more active on procariote cells (bacteria).

Instead of the conventionally used silver nitrate, it has thus been suggested to load the silver ion onto zeolite material in order to obtain a silver zeolite equipped with germicide properties as well as having a molecular sieving action. In this regard, US patent No. 4911898 proposes a polymeric item having antibacterial properties owing to the incorporation of an antibacterial agent composed of particles of zeolite loaded by ion exchange with silver ions. Similarly, US patent No. 4938958 describes a resin composition with antibiotic properties which comprises an amount of zeolite loaded through ion exchange with silver and with ammonium ions.

In both the aforesaid cases, as well as in all the known literature on the subject, the antiseptic compound composed of zeolite loaded with silver ions (or with ions of another metal with similar properties) is made by a fine powder that is incorporated within a polymeric substrate in much the same way as a pigment. The polymeric substrate makes up the product with the aforesaid antibacterial properties - be it a fibre, a moulded plastic item, a paint or other coating material. Therefore, the fine granules of metallized zeolite are in any case incorporated in a polymer matrix from which they must be able to be released or, at least, to whose surface they must be exposed, in order to carry out their antibacterial action. In a possible application of this system for preventing aeromicrobial growth in an air space such as the one enclosed in a refrigerator, for example, the aforesaid system is applied as a coating layer on the internal walls of the refrigerator. The layer contains dispersed particles of micron dimensions (2-10 μm) of silver zeolite incorporated in a polymer matrix. In order for the silver ion to perform its antibacterial action, the silver zeolite particles must not to be completely covered by the polymer matrix and the bacteria suspended in the air inside the refrigerator must come into direct contact with the particles themselves on colliding with the refrigerator walls. Besides being of uncertain effectiveness, this system also has the inconvenience of not being regenerable, since the silver zeolite particles contained in the inside coating of the refrigerator gradually deplete without it being possible to replenish the metallized zeolite material, thus determining a non-renewable lifecycle of the system.

On the basis of this prior art, the object of the present invention is to provide an effective system for the prevention of aeromicrobial growth in con- fined spaces, with particular reference to applications in the food processing and preservation industry as well as in air conditioning equipment, which system shows a powerful antimicrobial activity enabling the permanent deactivation of bacteria cells. Such system is intended to allow a repeated regeneration of the material eployed with a simple and inexpensive procedure, and to be esasily adapted for treating environments of different sizes. Moreover, the system should not negatively affect the organoleptic properties of foodstuffs placed inside the treated environment.

To this end, the present invention proposes to use a zeolite-based granular material as a filtering material for the air undergoing treatment, such zeolite being suitably loaded with a metal ion having germicide properties, such as silver zeolite, and being arrenged to form a bed of suitable particle size in the absence of other carriers or binders - the said bed being possibly divided into two or more serial modular elements. In this way, the metallized zeolite bed is directly passed through by the air to be sterilised, possibly with the aid of devices for forcing the air through the filter. The metallized zeolite granules thus come into immediate contact with the air undergoing treatment and the germicide activity of the bed can be periodically restored without losing any material, simply by making the granular bed undergo heat treatment in a heater at around 180°C, in conditions that are normally possible inside an ordinary domestic oven.

In view of the fact that, for greater effectiveness, the proposed mate- rial consisting of zeolite loaded with a germicide metal ion is left in a granular form, the aforesaid filtering and sterilising bed can be integrated with a pre- filter element acting as containment of the granular material and protection for the zeolite against dust or aerosols. This pre-filter can be made of fibres of chitosan or its derivatives, metallized with a metal ion having germicide prop- erties. Chitosan fibres have already been used in making fabric gauzes or non-woven fabric feltsin filter devices and also in bandages in the medical field. When treated with silver ions, chitosan fibres have per se an antibacterial activity that goes to enhance that of the silver zeolites.

Thus, the present invention specifically provides an air filtering device for controlling aeromicrobial growth comprising a filter element made up of a granular bed based on aluminosilicate material metallized with one or more metal ions having germicide activity and non-toxic for the human body, with a particle size ranging between 1 μm and 50 mm, preferably between 50 μm and 50 mm. The germicide metal ion is preferably selected from among silver, zinc, copper and their mixtures.

The aluminosilicates used according to the present invention have a very high capacity to capture the metal ion in the outermost parts of their crystalline pores. By being available for bacterial exposure, the metal ion will have a powerful antibacterial activity. It is thus possible to achieve abatement per- centages approaching 100%, as will be evident from the experimental data presented below.

As already noted, the sterilising action of the device according to the present invention may be further enhanced by introducing one or more additional filter elements based on chitosan and/or its functionalised derivatives, metallized with one or more non-toxic ions having antimicrobial activity. Preferably, these additional filter elements are made of gauzes, felts or non-woven fabric elements arranged in layers crosswise to the air flow to be treated.

More specifically, the additional filter element is a pre-filter composed of a silver chitosan gauze placed upstream of the granular bed filter element and along the air flow direction.

According to some preferred embodiments of the present invention, the granular bed filter element, with or without any additional metallized chito- san-based filter elements, can be made in the form of a flat panel to be placed inside a domestic refrigerator, for example, on the inside of the door of a refrigerator of the no-frost type.

According to further embodiments of the present invention, the pro- posed device also includes equipment for forcing the air to be treated through the granular bed of the aforesaid filter element. This equipment may, in particular, consist of a fan of suitable size or of a peristaltic or pneumatic pump, or even an ejector.

Preferably, the granular bed filter element consists of one, two or more serial sections that can be added in a modular way to the device according to system specifications.

As already noted, the aluminosilicate used for the purposes of the present invention, which can be either natural or synthetic aluminosilicate but is by preference a natural zeolite, is preferably metallized with silver, whose germicide properties are already well known. Silver is also the metal ion that is preferably used with the chitosan-based additional filter elements when they are present.

The aluminisilicate form on which the best results have been achieved in the experimental phase is clinoptilolite, and particularly Cuban clinoptilolite, which is found in nature in a considerably pure state.

It has also been found that by subjecting the aluminosilicate to a preliminary exchange with sodium or hydrogen cations, for example, by pre- treating the clinoptilolite with a solution of sodium chloride and then washing to eliminate the excess Cl^" ions, it is possible to increase the metallization yields, with an increase in the quantity of silver that can be uptaken through ionic exchange in the zeolite. The pre-treatment can also be carried out with other alkali or alkaline-earth cations.

A range of particlesizes particularly preferred for the metallized alumi- nosilicate-based granular bed according to the present invention is between 1 mm and 5 mm, as will be evident from the experimental data reported below. A first possible embodiment of an air filtering device for controlling aeromicrobial growth according to the present invention, to be placed for operation inside a refrigerator, consists of provinding a layer of metallized zeolite filter material according to the present invention, preferably contained by one or two layers of metallized chitosan band, in panel form on the inside door of a refrigerator of the no-frost type. Since this kind of refrigerator is equipped with its own ventilation system for air circulation inside the refrigerator itself, the device according to the present invention does not require further fans in order to obtain the forced circulation of air through the filter bed.

A second embodiment of the proposed device, based on driving the air to be treated through the metallized zeolite filter bed and suitable for opera- tion inside a refrigerator, consists of a central tubular body with a filter element composed of one, two or more serial sections, each filled with a metallized aluminosilicate-based granular bed, an air-pervious closing end element attached to one end of the central body and another end element with a fan attached on the other end of the central body. This form of device, which has been called MotoVentilatore Z (MVZ), preferably has a natural zeolite-based granular bed of particle size between 1 mm and 5 mm, metallized with silver, zinc, copper or their mixtures. According to some embodiments thereof, the device also includes a pre-filter based on chitosan and/or its functionalised derivatives metallized with one or more metal ions having antimicrobial activity and non-toxic for the body. More specifically, the pre-filter may be composed of layers of metallized chitosan gauze, in particular metallized with silver.

The MotoVentilatore Z turns out to be suitable for the sterilisation of confined spaces and, when in a small size, it acts as a real portable element, suitable for the sterilisation of domestic refrigerators and shop refrigerated counters.

The experimentation carried out has confirmed that devices with a granular filter bed based on metallized aluminosilicates according to the present invention are extremely effective in preventing aeromicrobial growth. The bacterial cells trapped in the granular bed are deactivated for good and cannot be reactivated through bacterial repair mechanisms. Moreover, the device is easy to regenerate and is economical: placing the central body (containing the zeolite) of the MotoVentilatore Z inside a heating unit (such as a domestic oven) for three hours at 180⁰C results in a complete sterilisation of the material, which can then be reused in a new cycle. The heat treatment does not alter the crystalline bond with the metals, and the concentration of the metals in the aluminosilicate remains unaltered following such treatment.

By varying the sizes and relative air flows, the device according to the present invention can be adapted to the treatment of larger environments, not ruling out the sterilisation of indoor spaces requiring a high degree of hygiene (such as hospital operating theatres and the like), as well as stables and animal cages, where the presence of a high number of animals, together with their metabolic waste materials, can create situations of infection risk both for the workers and for the animals themselves. Other applications correlated to a scaling-up of the device according to the present invention may be the decon- tamination of air spaces that have suffered a bio-terrorist attack and their preventive treatment, as well as the filtering of air entering living quarters through centralised air conditioning systems, in order to reduce the risk of contracting infections.

According to a further aspect thereof, the present invention specifically provides the use of an aluminosilicate metallized with silver, zinc, copper or their mixtures, with a particle size between 1 μm and 50 mm, and preferably between 50 μm and 50 mm, for the production of a granular filter bed for treat- ing air in order to eliminate the microbes it contains. As already noted, the granular filter bed is preferably associated with devices for the forced passage of the air undergoing treatment through the aforesaid granular bed - the said devices preferably consisting of a fan. The manufacturing and functional features of the present invention, as well as its advantages and relative operational modalities, will be more evident with reference to the detailed description of one of its specific embodiments, presented merely for exemplification purposes below, together with the results of the experiments conducted on it. This solution is illustrated also in the at- tached drawings, wherein:

Figure 1 shows a perspective view of one embodiment of the Mo- toVentilatore Z device, according to the present invention;

Figure 2 is a longitudinal cross-sectional view of the same device devoid of its granular contents, taken along the B-B plane; Figure 3 is a side view of the same device shown in Figure 1 , partially sectioned to show the granular filling;

Figure 4 is a side view of the various elements making up the device of Figure 1 ;

Figure 5 is a front view of the same elements shown in Figure 4; Figure 6 is a side view of the various elements making up the second embodiment of the MotoVentilatore Z device of the invention, wherein there is a second modular section of the filter element;

Figure 7 is a front view of the same elements shown in Figure 6;

Figure 8 is a side view of a third embodiment of the MotoVentilatore Z device of the present invention, partially sectioned to show the filling, suitable for conducting laboratory trials, in which there is a second modular section equipped with a borehole for connection to experimental equipment;

Figure 9 is a side view of the various elements making up the device of Figure 8; and Figure 10 is a front view of the same elements shown in Figure 9. Preparation and characterisation of zeolites metallized with Ag

In trials on the metallization of Cuban clinoptilolite with silver, an established quantity of natural zeolite, of a given granular size, was put into contact with an established quantity of AgNO₃ solution of a given concentra- tion. The suspension thus obtained was placed inside an airtight container under mechanical stirring at room temperature. After a set time, the suspension was filtered on a Buchner device under vacuum and the filter was washed with ultrapure water to eliminate any excess silver ions adsorbed on the zeolite surface. The exchanged zeolite thus obtained was dried in air and in darkness for 3 days.

As a variation to the aforesaid procedure, the process started from a zeolite preventively exchanged with sodium (called Z-Na), obtained by contacting the starting zeolite with a solution of NaCI 0.5 N for 2 days under stirring and then filtered and washed to eliminate the excess Cl^" ions. The subse- quent trials were carried out still using given quantities of zeolite of various particle size, and with solutions of various concentrations in silver nitrate for given contact times. The washing methods and drying times were the same as in the previous case.

On the silver zeolite samples thus obtained, the concentration Of Ag⁺ was dosed by atomic absorption spectrometry (AAS), operating according to the following procedures.

A small quantity of zeolite-Ag (~0,5 g) was broken down with a mixture of HF 50%, HNO₃ 65%, H₂SO₄ 10% on a microwave digester. Digestion was carried out in 3 phases: Phase 1 - Zeolite + 2O mI HF 50% + 10 ml H₂SO₄ 10

Microwave: in 10 min. rising to 100⁰C and then resting for 15 min. Cooling Phase 2 - + 10 ml H₂SO₄ 10% + 3 ml HNO₃ 70%

Microwave: in 10 min. rising to 120⁰C and then resting for 11 min. Cooling

Phase 3 - + 4 ml HNO₃ 70% + 35 ml ultrapure water

Microwave: in 10 min. rising to 160⁰C and then resting for 10 min. Cooling and then taking to a volume of 100 ml.

The digested sample underwent analysis for Ag determination through atomic absorption spectrophotometry on a flame.

Table 1 summarises the operating conditions and the results of various metallization trials carried out with non-pretreated zeolites. For each series of trials, with operating conditions being equal, only the minimum and maximum values are given, in terms of silver quantity loaded onto the zeolite.

TABLE 1 Metallization with Ag on non-pretreated zeolite samples

The data relative to similar trials carried out with zeolite preventively exchanged with sodium are reported in the following table. Here, too, for each series of trials, only the minimum and maximum values of metallization obtained are reported.

TABLE 2 Metallization with Ag of zeolite samples pretreated with Na

By comparing the values reported in the two tables, it is evident that the zeolite pre-exchanged with sodium allows increasing the quantity of silver exchanged. In fact, with equal particle sizes, the best result in terms of silver loaded is given by the zeolite preventively exchanged with sodium. The aver- age metallization without pre-treatment with Na is of 53.4 mg/g, while with pre- treatment the average metallization rises to 72.7 mg/g.

Moreover, it is noted that the particle size from which the best result is obtained in terms of exchanged silver is between 2 mm and 3.35 mm.

Other considerations emerging from the above tables are that the quantity of silver exchanged seems to depend only on the absolute quantity of

Ag⁺ ions in solution, and that for the zeolite-Na or decrease in contact time seems to have a negative effect in terms of decrease in Ag exchanged, while it does not appear to have any effect for normal zeolite.

Realization and experimentation of the MotoVentilatore Z device

Since the primary application of the device is the sterilisation of domestic refrigerators and small industrial refrigerators, the device has been designed so that its size is small and adaptable to the dimensions of the space it is used for. To this end, the MVZ device has a filter element that may be made up of one, two or more serial modular sections, each of which in turn can be filled with several layers of granular material with different functions.

Further requirements of the device, besides adaptability and modularity, are mechanical strength, the possibility of easy maintenance and user- friendliness. In the specific case illustrated in Figures 1-5, the device is applied to a domestic refrigerator with a volume of 200 litres, and is designed such that in one hour it can treat all the air inside the refrigerator for a number of times ranging between 25 and 50, more than enough to guarantee an efficient sterilisation. The device includes a central body consisting of a single filter ele- ment (1) 85 mm long with an external diameter of 50 mm and internal diameter of 46 mm, with a tubular shell (2) filled with a granular bed (3) (only shown in Figure 3) based on metallized aluminosilicate, and specifically silver zeolite. The chosen length for the filter element is the one which turned out to be the most suitable for assuring a good sterilisation efficiency while at the same time optimising the silver zeolite with respect to pressure drop.

The closure of the filter element (1) is made out by an end element (4) consisting of a square-shaped end block (5) 60 mm per side, equipped with a round aperture 50 mm in diameter, enabling insertion in the tubular shell (2), which closes onto the end block (5). The end element (4) also includes a round steel containment mesh (6) (50 mm in diameter) that fits tightly into the central aperture of the end block (5) and is held in place by the O-ring (7) or by a copper washer. The containment mesh (6) has a mesh width of about 1 mm, adequate for the containment of the granular bed (3) of zeolite material compatibly with the particle size used. The function of the mesh is to prevent the outflow of the silver zeolite from the filter element (1), while allowing the incoming air to penetrate the filter section without excessive pressure drop. The end element (4) is attached to the filter element (1) by means of a screw (8) inserted in the upper section.

The opposite side of the end element (4) has a connection element (9) for the fan, which has similar features to those of the end element (4), being composed of a square connection block (10), a containment mesh (6a) and an airtight O-ring (7a), and attached to the filter element (1) via a screw (8a) inserted in the upper section.

The suction module uses a fan (11) with a nominal flow capacity of 20 m³/h - more than enough to reach the specification flow of 5-10 m³/h irrespective of any pressure drop. For the fan (11) connection, the connection block (10) has four boreholes (12) for four connection screws. The fan (11) works by suction (but it can also work by delivery or compression), causing a positive flow inside the device from the end element (4) to the connection point of the fan itself. The fan (11) operates with 12 Volts power and is connected to the main power supply via a transformer. The device obtained in this way, which is compact and easy to assemble, has an overall length of about 10 cm. The individual components of the apparatus are made of brass, which is easy to manufacture, but can also be made of any other metal or polymer.

The MotoVentilatore Z device in its basic configuration, shown in Figures 1-6, may be extended by adding further sections of the filter element (1), in the form of modules of an axial length of 30 mm each, which increase its capacity to filter and abate airborne germs. The side and front views of a device modified in this manner, subdivided into its various constitutive elements, are shown in Figures 7 and 8, wherein the elements corresponding to those of Figures 1-6 have been given the same reference numbering.

Instead of the connection element (9), in this case the device envis- ages a junction element (13) with two annular collars (15 and 16), which can be attached to the rest of the device via two screws (17a and 17b) inserted in the upper section. The additional section of the filter element consists of the junction element (13) associated with the tubular shell (18), with the relative granular filling (not shown), while the connection element (9) for the fan, with the connection block (10) for the fan (11), and the fan itself are the same.

The system with this configuration is versatile, with volumetric capacities that are modular and can be selected according to one's particular needs. Since the central body containing the zeolite can be of variable length, the distance that the air to be sterilised must move through the silver zeolite bed and the length of contact time between air and zeolite (and thus the necessary quantity of zeolite), can be varied according to the volumes of air undergoing treatment, the microbial quantities present in the air and the number of air treatment cycles one wishes to carry out.

To conduct some experiments on the device of the present invention, the configuration used was the one illustrated in Figures 8-10 (the system assembled for the experiments), where only the central body is present, in the configuration of Figures 1-6. The latter was connected to a module similar to the additional section of the filter element shown in Figures 7 and 8, with an axial length of 30 mm but with a lateral borehole (19) enabling the insertion of a connector for connecting up to the necessary equipment for each trial. In each of Figures 8-10, the elements corresponding to those of Figures 1-7 have been assigned the same reference numbering. To measure the air flowing through the device and to determine the pressure drop, a probe was inserted in the specific borehole (19). The probe is a hot wire anemometric probe connected to a velocity measuring device. This equipment was used to assess the pressure drop incurred by the air flow under forced circulation as a function of variations in particle size and in the quantity of zeolite introduced in the filter element of the device. The data obtained are useful in order to assess the most suitable system configuration according to its application.

Biological experimentation on the effectiveness of abatement of airborne microbes - Laboratory trials

Still using a configuration similar to the one illustrated in Figures 8-10 (the system assembled for experiments), but without a fan on one end, a system for measuring the bacteria decontaminating effectiveness of the device was set up by using aerosols to pass airborne bacterial suspensions through the device and then assessing the level of microbial pollution of the air exiting from the filter element of the device.

The experimental device included an upside-down vacuum beaker acting as an accumulation of the bacterial aerosol. The beaker aperture was in direct contact with the end element (4) of the device configured as described above (i.e. with the basic filter element, followed by the module with a lateral borehole), held vertically. The lower edge of the borehole module, upon flaming, was dipped for 1-2 mm into the agar of a 60 mm Petri dishes (agar TSA, Tryptic Soy Agar). The lateral borehole of the module contained a rubber stopper in which ran a tube for connecting to a peristaltic pump that was used to put the whole system into depression, going to replace the fan (11) as a means for forcing the air through the antimicrobial filter of the invention. The peristaltic pump was set at a speed of 200 rpm, and the whole apparatus was placed under a laminar flow hood.

The bacterial aerosol used derived from suspensions having a title ranging from 10⁵ to 10⁸ CFU/ml. The bacterial suspensions were grown in enrichment broths coming from colonies grown on TSA agarised media. The medium typologies used for the various bacteria strains are summarised in the following table.

The bacterial aerosol was inputted intermittently (10" + 10" pause) into the system via a normal aerosol system for therapeutic use. Each trial lasted 5 minutes. The dishes were left to incubate in a thermostat according to the conditions indicated for each type of micro-organism in the above table.

The regrowth trials were carried out qualitatively, by placing about 1 g of the zeolite granules in tubes containing TSB (Tryptone Soya Broth) and then noting the turbidity of the broth with respect to a control.

The results obtained are summarised in the following two tables.

TABLE 3 TRIALS WITH MIXED MESOPHYLIC FLORA

TRIAL 1

Petri dish from empty MVZ: CFU uncountable

Petri dish from MVZ with AgZ (silver zeolite): 151 CFU

AgZ bacterial growth trials: negative

TRIAL 2

1st trial (empty tube) 2nd trial (tube with AgZ) Dish 1 : 60 CFU Dish 1 : 0 CFU Dish 2: 42 CFU Dish 2: 0 CFU Dish 3: 19 CFU Dish 3: 0 CFU AVERAGE: 40 AVERAGE: 0

STERILIZATION EFFECTIVENESS: 100% AgZ bacterial growth trials: negative

TRIAL 3

1st trial (empty tube) 2nd trial (tube with AgZ) Dish 1 : 421 CFU Dish 1 : 2 CFU Dish 2: 320 CFU Dish 2: 11 CFU Dish 3: 460 CFU Dish 3: 12 CFU AVERAGE: 400 AVERAGE: 8

STERILIZATION EFFECTIVENESS: 98% AgZ bacterial growth trials: negative

TRIAL 4

1st trial (empty tube) 2nd trial (tube with AgZ) Dish 1 : 920 CFU Dish 1 : 4 CFU Dish 2 : 748 CFU Dish 2 : 3 CFU Dish 3 : 642 CFU Dish 3 : 3 CFU AVERAGE: 770 AVERAGE: 4

STERILIZATION EFFECTIVENESS: 99.5% AgZ bacterial growth trials: negative TRIAL 5

1st trial (empty tube) 2nd (tube with AgZ) Dish 1 : 143 CFU Dish 1 : 0 CFU Dish 2 : 222 CFU Dish 2 : 0 CFU Dish 3 : 254 CFU Dish 3 : 1 CFU AVERAGE : 206 AVERAGE : 1

STERILIZATION EFFECTIVENESS: 99.5% AgZ bacterial growth trials: negative

TRIAL 6

1st trial (empty tube) 2nd trial (tube with AgZ) Dish 1 : CFU un. Dish 1 : 0 CFU Dish 2 : CFU un. Dish 2 : 1 CFU Dish 3 : CFU un. Dish 3 : 0 CFU

STERILIZATION EFFECTIVENESS: 99.97%* AgZ bacterial growth trials: negative un.: uncountable (> 1500)

* considering that CFU = un. « 1000

TRIAL 7 (with active carbon)

1st trial (empty tube) 2nd trial (tube with AC 3rd trial (tube with AC 1<0<2 mm) 2<0<3.35mm)

Dish i : 216 CFU Dish 1 : 0 CFU Dish 1 : 3 CFU Dish 2 : 400 CFU Dish 2 : 7 CFU Dish 2 : 5 CFU Dish 3 : 888 CFU Dish 3 : 10 CFU Dish 3 : 13 CFU AVERAGE: 501 AVERAGE: 6 AVERAGE: 7 DECONTAMINATION DECONTAMINATION EFFICIENCY* = 99% EFFICIENCY* = 99%

Growth trials : positive TRIAL 8 (zeolite without silver) 1st trial (empty tube) 2nd trial (tube with Z) DisM (1'): 284 CFU Dish i (1'): 11 CFU Dish 2 (3'): 548 CFU Dish 2 (3'): 15 CFU Dish 3 (5'): 920 CFU Dish 3 (5'): 36 CFU AVERAGE: 584 AVERAGE: 21

DEBACTERISATION EFFICIENCY* : 99.5% Growth trials: positive

* In these cases, the efficiency is of "debacterisation" since the bacteria are not killed by the filter device, as with the AgZ trials (positive growth trials)

The aforesaid trial 7 shows that, although granular active carbon is efficient in trapping the microbial cells and in ensuring that the exiting air con- tains a significantly lower concentration of bacteria with respect to the incoming air, it is not as efficient in permanently deactivating the germs themselves. Similarly, trial 8 shows that even zeolite without silver is able to reduce airborne microbes but here too the material does not permanently deactivate the bacteria, which can then regenerate when in contact with a suitable growth medium.

TABLE 4 TRIALS WITH SPECIFIC MICRO-ORGANISMS

TRIAL 1 - Escherichia coli 1st trial (empty tube) 2nd trial (tube with AgZ) Dish 1 : 112 CFU Dish 1 : 18 CFU Dish 2: 196 CFU Dish 2: 19 CFU Dish 3: 221 CFU Dish 3: 11 CFU AVERAGE: 176 AVERAGE: 16

STERILIZATION EFFICIENCY: 91% Bacterial growth trials of AgZ: negative

TRIAL 2 - Escherichia coli

1st trial (empty tube) 2nd trial (tube with AgZ)

Dish i : 181 CFU Dish 1 : 5 CFU

Dish 2: 197 CFU Dish 2: 4 CFU

Dish 3: 152 CFU Dish 3: 6 CFU

AVERAGE: 177 AVERAGE: 5

STERILIZATION EFFICIENCY: 97%

TRIAL 3 - Pseudomonas aeruginosa 1st trial (empty tube) 2nd trial (tube with AgZ)

Dish 1 : 451 CFU Dish 1 : 15 CFU

Dish 2 : 572 CFU Dish 2 : 25 CFU

Dish 3 : 401 CFU Dish 3 : 21 CFU

AVERAGE : 176 AVERAGE : 16

STERILIZATION EFFICIENCY: 91 % Bacterial growth trials of AgZ: negative

TRIAL 4 - Pseudomonas aeruginosa

1st trial (empty tube) 2nd trial (tube with AgZ)

Dish 1 : 325 CFU Dish 1 : 51 CFU

Dish 2 : 488 CFU Dish 2 : 18 CFU

Dish 3 : 412 CFU Dish 3 : 26 CFU

AVERAGE : 408 AVERAGE : 32

STERILIZATION EFFICIENCY: 92% TRIAL 5 - Staphylococcus aureus

1st trial (empty tube) 2nd trial (tube with AgZ)

Dish i : 195 CFU Dish 1 : 65 CFU

Dish 2 : 161 CFU Dish 2 : 75 CFU

Dish 3 : 102 CFU Dish 3 : 62 CFU

AVERAGE : 153 AVERAGE : 67

STERILIZATION EFFICIENCY: 66 %

Bacterial growth trials of AgZ: negative

TRIAL 6 - Staphylococcus aureus

1st trial (empty tube) 2nd trial (tube with AgZ)

Dish 1 : 205 CFU Dish 1 : 98 CFU

Dish 2 : 265 CFU Dish 2 : 65 CFU

Dish 3 : 195 CFU Dish 3 : 102 CFU

AVERAGE : 222 AVERAGE : 88

STERILIZATION EFFICIENCY: 60%

TRIAL 7 - Candida albicans

1st trial (empty tube) 2nd trial (tube with AgZ)

Dish 1 : 95 CFU Dish 1 : 25 CFU

Dish 2 : 105 CFU Dish 2 : 5 CFU

Dish 3 : 112 CFU Dish 3 : 10 CFU

AVERAGE : 104 AVERAGE : 13

STERILIZATION EFFICIENCY: 87 5%

Bacterial growth trials of AgZ: negative

The laboratory trials with both generic mesophylic micro-organisms and with specific germ strains showed the effectiveness of the filtering treatment using the system according to the present invention, with bacterial abatements up to 99.97%, starting from bacterial concentrations, expressed in CFU/dish, of about 1500 CFU/dish.

Here too, the results obtained with specific micro-organisms showed the effectiveness of the present invention, with better results achieved with gram-negative micro-organisms (E. coli and Ps. aeruginosa).

An important indication emerging from the experimentations is that filtering with AgZ guarantees the permanent elimination of micro-organisms. This is demonstrated by the absence of any bacterial regrowth from the zeolite granules subjected to infected air. Other filtering mediums, such as zeolite without silver and active carbons, have a purely chemical-physical filtering action devoid of any germicide activity, as the granules of these materials - after undergoing filtering with infected air - give rise to positive bacterial re- growth. In these cases, the term "debacterisation" should be uses instead of "sterilisation".

Biological experimentation on the effectiveness of microbe abatement - Trials inside a real system (a refrigerator)

The system according to the present invention was also tested on real systems, in particular on a refrigerator of about 200 litres capacity, suitably modified to enable access to its contents via two sterile gloves connected to two specially designed holes on the refrigerator door. Another borehole was used in order to inject the bacteria-infected air used in each trial. The refrigerator had been appropriately washed and sterilised before the experimenta- tion. It was then used to contain TSA agar dishes for suitable lengths of time, later assessing the number of bacteria colonies depositing on the dishes either in the presence or absence of the action of the MotoVentilatore Z device according to the present invention, and also in the presence or absence of the silver-zeolite based granular filling in the device. The trials showed the effectiveness of the treatment according to the present invention, with high statistical significance. The system guarantees an average 50% abatement of airborne microbes by keeping the MotoVentilatore Z device in operation for just one hour. An abatement of 50% guarantees a significant reduction of the quantity of airborne microbes that can deposit on foodstuffs and on the refrigerator surfaces, thus leading to a considerable decrease in the risk of contamination of foodstuffs and in the resulting risk of infection on the part of users and consumers, with a decrease in the probabil- ity of contracting diseases, above all, of the gastro-intestinal system.

The present invention has been disclosed with reference to some of its specific forms of realisation, but it is understood that variations or modifications thereof can be made by experts in the art without departing from the relative sphere of protection.

Claims

1. An air filtering device for controlling aeromicrobial growth comprising a filter element (1) having a granular bed (3) based on aluminosilicate metallized with one or more metal ions having antimicrobial activity and nontoxic for the body, with a particle size ranging between 1 μm and 50 mm.

2. An air filtering device according to claim 1 , wherein the said particle size ranges between 50 μm and 50 mm.

3. An air filtering device according to claims 1 or 2, wherein the said aluminosilicate is metallized with silver, zinc, copper or mixtures thereof.

4. A device according to any one of claims 1-3, also including one or more additional filter elements based on chitosan and/or its functionalised derivatives metallized with one or more ions having antimicrobial activity and non-toxic for the body.

5. A device according to claim 4, wherein the said one or more additional filter elements consist of gauzes, felts or non-woven fabric elements arranged in layers crosswise to the air flow to be treated.

6. A device according to claim 5, wherein the said additional filter element is a pre-filter of silver chitosan gauze placed upstream of the said granularbed-based filter element along the direction of the air flow to be treated.

7. A device according to each of claims 1-6, wherein the said granular bed based on metallized aluminosilicate (3) is arranged in the form of a flat panel inside a domestic refrigerator.

8. A device according to claim 7, wherein the said granular bed based on metallized aluminosilicate (3) is covered with one or more band layers of metallized chitosan.

9. A device according to any one of claims 1-6, also including means (1 1) for the forced input of air undergoing treatment through the bed (3) of the said filter element (1 ).

10. A device according to claim 9, wherein the said means for forced air input consist of a fan (1 1 ), a peristaltic or pneumatic pump, or an ejector.

1 1. A device according to any one of claims 1-10, wherein the said granular bed based filter element (1) consists of one, two or more sections in series, that can be added in modular fashion to the device according to the system specifications.

12. A device according to any one of claims 1-1 1 , wherein the said aluminosilicate is metallized with silver.

13. A device according to any one of claims 4-12, wherein the said chitosan and/or its functionalised derivative is metallized with silver.

14. A device according to any one of claims 1-13, wherein the said aluminosilicate is pretreated with sodium or hydrogen cations, or with other alkali or alkaline-earth cations, before metallization.

15. A device according to any one of claims 1-13, wherein the said aluminosilicate is a natural aluminosilicate.

16. A device according to claim 15, wherein the said natural alumi- nosilicate is Cuban clinoptilolite.

17. A device according to any one of claims 1-16, wherein the particle size of the said granular bed (3) based on metallized aluminosilicate ranges between 1 mm and 5 mm.

18. An air filtering device according to any one of claims 1-4 suitable to be operated inside a refrigerator, consisting of a central tubular body with a filter element (1) composed of one, two or more serial sections, each filled with a granular bed (3) based on metallized aluminosilicate, an air-permeable end closure element (4) attached to one end of the said central body, and an end element with a fan (1 1) attached to the other end of the said central body.

19. A device according to claim 18, wherein the said granular bed (3) is based on zeolite metallized with silver, zinc, copper or mixtures thereof.

20. A device according to claim 19, wherein the said zeolite is natural zeolite and the particle size of the said granular bed (3) ranges between 1 mm and 5 mm.

21. A device according to any one of claims 18-20 also including a pre-filter based on chitosan and/or its functionalised derivatives metallized with one or more ions having antimicrobial activity and non-toxic for the body.

22. A device according to claim 21 , wherein the said pre-filter consists of layers of metallized chitosan gauze.

23. A device according to claim 22, wherein the said metallized chitosan gauze is metallized with silver.

24. Use of an aluminosilicate metallized with silver, zinc, copper or their mixtures, with a paricle size ranging between 1 μm and 50 mm, for producing an air-filtering granular bed (3) suitable to control aeromicrobial growth.

25. Use according to claim 24, wherein the said particle size ranges between 50 μm and 50 mm.

26. Use according to claims 24 or 25, wherein the said filtering granular bed (3) is associated with additional filter elements based on chitosan and/or its functionalised derivatives metallized with silver, zinc, copper or mixtures thereof.

27. Use according to any one of claims 24-26, wherein the said filter- ing granular bed (3) is associated with means (11) for forcing the air undergoing treatment through the said granular bed (3).

28. Use according to claim 27, wherein the said means for forcing air through the device consist of a fan (11).