Modern methods of disinfecting indoor air
Description:
The incidence rate due to microbiological pollution of the indoor air environment remains high today. Most pathogens are transmitted by air and airborne droplets. This problem is especially acute in places with large concentrations of people and poorly ventilated indoor areas, as well as in rooms with air recirculation. Preventing the spread of disease - the main task of the process of air disinfection. The article discusses modern methods of combating pathogenic microflora in the premises.
Modern methods of disinfecting indoor air
The incidence rate due to microbiological pollution of the indoor air environment remains high today. Most pathogens are transmitted by air and airborne droplets. This problem is especially acute in places with large concentrations of people and poorly ventilated indoor areas, as well as in rooms with air recirculation. Preventing the spread of disease - the main task of the process of air disinfection. The article discusses modern methods of combating pathogenic microflora in the premises.
Ultraviolet radiation (ultraviolet, UV, UV) is electromagnetic radiation covering the wavelength range from 100 to 400 nm of the optical spectrum of electromagnetic waves, that is, between visible and X-ray radiation. Types of ultraviolet radiation are presented in table. one.
The use of ultraviolet energy at the present time is becoming increasingly relevant, since it is one of the main methods for inactivating viruses, bacteria and fungi. Under the inactivation of microorganisms understand the loss of their ability to reproduce after sterilization or disinfection [2].
Ultraviolet radiation with a wavelength range of 205–315 nm has a bactericidal effect; it causes destructive-modifying photochemical damage to the DNA of the microorganism cell nucleus. Changes in the DNA of microorganisms accumulate and lead to a slowdown in their reproduction rates and further extinction in the first and subsequent generations. As a result of a number of observations, it was noted that the impact of energy in the range of the UVC spectrum is most effective from a bactericidal point of view at a wavelength of 254 nm.
Live microbial cells respond differently to ultraviolet radiation, depending on wavelengths (Table 2).
Ultraviolet radiation equipment
Ultraviolet bactericidal exposure to air is effected using ultraviolet radiation equipment, the principle of which is based on passing an electrical discharge through a rarefied gas (including mercury vapor) inside the sealed enclosure, resulting in radiation.
Radiating equipment is bactericidal lamps, irradiators and installations. A bactericidal lamp is an artificial source of radiation, in the spectrum of which there is predominantly bactericidal radiation in the wavelength range of 205–315 nm. The most widespread, due to the highly efficient conversion of electrical energy into radiation, were low-pressure discharge mercury lamps, in which the process of electrical discharge in an argon-mercury mixture turns into radiation with a wavelength of 253.7 nm. These lamps have a long service life - 5 000– 8 000 hours. Known high-pressure mercury lamps, which, with small overall dimensions, have a large unit capacity - from 100 to 1,000 W, which allows in some cases to reduce the number of irradiators in a bactericidal installation. On the other hand, they are not very economical, have low bactericidal efficacy with a service life of 10 times less than low-pressure lamps, and therefore have not found wide application.
A number of the largest electric-bulb companies (Philips, Osram, Radium, Sylvania, etc.) are currently engaged in the development and production of UV lamps for photobiological installations.
In Russia, manufacturers are known: OJSC Lisma-VNIIIS (Saransk), NPO LIT (Moscow), OJSC SKB Xenon (Zelenograd), OOO VNISI (Moscow). The nomenclature of lamps is quite wide and varied. UV lamps are used to sterilize water, air and surfaces.
For a more rational use in practice of bactericidal lamps, it is advisable to embed them in bactericidal irradiators. A bactericidal irradiator is an electrical device consisting of a bactericidal lamp (s), a control gear, a reflective armature and a number of other auxiliary elements. By design, irradiators are divided into three groups: open, combined and closed. Open feeds are usually attached to the ceiling or wall, combined - to the wall and can be with or without reflectors. In open feeds, a direct bactericidal stream covers a wide zone in space up to the solid angle. They are intended for the process of disinfection of premises only in the absence of people or during their short-term stay. In closed irradiators, they are sometimes called recirculators, the lamps are located in a small closed enclosure of the irradiator and the bactericidal stream has no outlet outside the enclosure, therefore irradiators can be used when people are in the room. Bactericidal energy deactivates the majority of viruses and bacteria entering the indoor unit along with the air flow. Diffusers are provided in the irradiator housing, through which the air enters the device with the help of the built-in fan, where it falls under the source of UV radiation in the closed space of the indoor unit, and then returns to the room. Closed irradiators are placed, as a rule, on the walls of rooms, evenly around the perimeter, in the direction of the main air flow (often near heating devices) at a height of 1.5–2.0 m from the floor level.
Combined feeds are usually supplied with two bactericidal lamps, separated by a screen so that the flow from one lamp is directed only to the lower zone of the room, from the other - into the upper zone. Lamps can be switched on together and separately.
Bactericidal installation includes a group of bactericidal irradiators. It can also be a system of forced-air ventilation, the elements of which include germicidal lamps for supplying decontaminated air to the room. The level of bactericidal efficiency of the installation is set in accordance with the medical and technical tasks for its design.
The duration of the bactericidal installation, at which the required level of bactericidal efficiency is achieved, varies depending on the type of feed: for closed feeds, 1–2 hours; for open and combined 0.25–0.5 hours; for supply and exhaust ventilation systems for 1 hour or more.
A separate class of devices is bactericidal equipment as part of the installation of forced ventilation (air conditioning), which allows not to install devices in separate rooms, but to serve entire floors. These are the so-called air disinfection units. They are issued as a part of conditioners of common industrial, medical and hygienic execution. An air disinfection module consisting of a specific number of germicidal lamps and an air filter are usually included in the package of the disinfection unit.
For certain rooms there are requirements for the need for air disinfection. In tab. 3 lists the types of premises to be equipped with bactericidal installations for air disinfection, with an indication of bactericidal efficacy [3]. The most important objects from this position are hospitals, in which the need for air disinfection is strictly regulated [4]. Also issues of air disinfection in the premises of medical institutions are consecrated in [10].
The premises in which they place bactericidal plants are divided into two groups:
- in which the disinfection of air is carried out in the presence of people during the working day by ultraviolet installations with closed irradiators, excluding the possibility of exposure to people in the room;
- in which air disinfection is carried out in the absence of people with bactericidal installations with open or combined irradiators, and the limiting time of people in the room is determined by calculation.
The work of germicidal lamps may be accompanied by the release of ozone. The presence of ozone in the air in high concentrations is dangerous to human health, therefore the rooms where the facilities are located should be ventilated with either general exchange ventilation and exhaust ventilation, or through window openings with an air exchange rate of at least one krat in 15 minutes.
Table 3
Levels of bactericidal efficiency and bulk bactericidal dose (exposure) of Hv for S. aureus, depending on the categories of premises to be equipped with bactericidal installations for air disinfection
Bactericidal dose and bactericidal (antimicrobial) efficacy
The work of germicidal lamps is characterized by radiometric values. The main ones are bactericidal dose and bactericidal efficacy. The degree of disinfection of air or surfaces depends on the bactericidal dose. The bactericidal dose (ultraviolet radiation dose) or exposure should be understood as the density of the bactericidal radiation energy, or the ratio of the bactericidal radiation energy to the area of the irradiated surface (surface dose, J / m2) or the volume of the irradiated object (volume dose, J / m3) [3].
The effectiveness of microbial irradiation, or bactericidal (antimicrobial) efficiency, is the level of microbial contamination of the air environment or on any surface as a result of ultraviolet radiation. This value is estimated in percent - as the ratio of the number of dead microorganisms to their initial number before irradiation. The bactericidal efficiency of lamps depends mainly on the radiation dose (DUV, J / m2) supplied to microorganisms:
DUV = It, (1)
where I is the average intensity or dose of radiation, J / cm2;
t is the exposure time, s.
The application of this simple-looking equation is rather difficult when taking into account the dose for a particle passing through a device with a variable flux density. The equation describes the process of irradiating a particle with a dose received in a single pass through the device. With repeated exposure to microorganisms (recycling) bactericidal efficiency is doubled.
The survival rate of a microbial or colony forming unit (CFU) exposed to bactericidal exposure depends exponentially on the dose:
(2)
where k is the constant deactivation (inactivation), depending on the specific type of CFU m2 / J;
The resulting coefficient of inactivation of a particle in one pass (η) through the radiation field is used as an indicator of the overall radiation efficiency and shows the percentage or proportion of CFUs that are inactivated after one pass through the irradiation field, and also depends on S and is always less than 1:
η = 1 − S. (3)
The values of the parameter k for many species of bacteria, fungi, and mold have been experimentally obtained and may differ from each other by several orders of magnitude. This is due to the methods and conditions of measurement: they are produced in the air stream, in water or on the surface. The reading k is strongly influenced by the error in measuring the survival rate of the microbial culture. In this regard, it is very difficult to choose the right k values for the design conditions of bactericidal systems, and, as a rule, the average or maximum known k values are used for applying equation 2 depending on the goals of disinfection.
Standards for the design and maintenance of germicidal lamps
Despite the fact that the field of application of UV-irradiation technologies is constantly expanding and modern efficient systems are being developed, industry standards for the installation and maintenance of systems do not yet exist. In 2003, ASHRAE created a special group on ultraviolet treatment of air and surfaces, transformed in 2007 into a Technical Committee. In addition, a Standardization Committee was established to develop standards for testing air and surface disinfection systems. To date, two standards for the treatment of air and surfaces by UV radiation and the testing of air disinfection systems are under development. Also this year, a new section on disinfection with ultraviolet radiation has appeared in the ASHRAE manual for building systems and climate control equipment.
In our country in the early 1990s, a number of documents were developed on the rationing of technical requirements for medical equipment [5, 6, 7], and two documents were put into effect: in 2004, “Guidelines on the use of ultraviolet germicidal radiation for air disinfection in premises ”[3] and in 2002“ Guidelines for the design of ultraviolet germicidal plants for disinfecting air ”[8]. In 2004, the Ministry of Health of Russia adopted a Resolution “On the organization and conduct of cleaning and disinfection of ventilation and air conditioning systems” [9]. One of its main provisions is the requirement for equipping ventilation and air conditioning systems with bactericidal equipment based on modern ultraviolet technologies.
Duct air disinfection systems
Built-in bactericidal systems are recommended to be installed inside the air ducts or the casing of the air handling units for disinfection of internal surfaces and air supplied to the room (Fig. 1). In this case, there is either instantaneous inactivation of microorganisms, or a slowdown in the growth of their number. The zones of formation and accumulation of moisture, for example, drain pallets, are particularly dangerous. The use of ultra-fine filters is recommended (GOST R 51252-99. Air purification filters. Classification. Marking), despite the fact that they have high hydraulic resistance, cost and short service life.
Surface disinfection systems
Before starting the operation of the disinfection systems, surfaces should be cleaned, especially those that come into contact with moisture, from mold or microbial deposits. It is recommended to install germicidal lamps in close proximity to the cooling circuits in increments that allow even distribution of UV energy. To improve the efficiency of the lamps, reflecting devices are used (Fig. 2). The methods of installing the lamps can be different: before or after the cooling circuit and at any angle, it is only important that the UV energy penetrates to all points of the fins of the air coolers. The second method is often used because of the presence, firstly, of the available free space, and secondly, because of the possibility of open irradiation of the drain pan.
The principle of operation of a bactericidal installation with reflectors installed inside the duct
Figure 2.
The principle of operation of a bactericidal installation with reflectors installed inside the duct
Places for placement of lamps depend on the design of the air handling unit and the type of lamps used, the most common is the installation of lamps at a distance of 0.9–1.0 m from the cooling circuit during round-the-clock operation. Continuous exposure to UV irradiation provides a dose of ultraviolet radiation needed to prevent the development of microorganisms at low radiation intensity.
Air disinfection
The work of bactericidal systems, sufficient for disinfecting surfaces, is not always effective in the case of air disinfection. Although properly designed systems can handle both air and surfaces at the same time. They are usually not equipped with reflective devices that block the flow of ultraviolet energy (Fig. 3). It is possible to increase system performance by improving the overall reflectivity of the internal surfaces of ducts or inlet installations. This leads to an enhanced reflection of UV energy in the irradiation zone and an increase in the UV dose. The main purpose of using lamps is to evenly distribute the UV energy in all directions of engineering structures, regardless of their type.
The principle of operation of a bactericidal installation without reflectors
Figure 3.
The principle of operation of a bactericidal installation without reflectors
When designing bactericidal systems, the velocity of air in the ducts should be taken at a rate of 2.5 m / s. Under these conditions, the duration of exposure to UV radiation on the air flow is 1 s. Interestingly, the required dose of UV irradiation for the inactivation of microorganisms contained both on the surface and in the air flow is the same. To achieve the process of inactivation in a shorter time, higher levels of exposure are required. To do this, increase the reflectivity of the internal surfaces of the air ducts and (or) take to install a larger number of high-power lamps.
Air velocity of 2.5 m / s corresponds to the length of the irradiation zone not less than 0.6 m or the exposure time of the irradiation of microorganisms equal to 0.25 s. Usually bactericidal irradiators are placed in the inlet installations after the heating (cooling) circuits. There are cases of installation of lamps in front of the air heater (cooler), which leads to a decrease in the air flow rate or an increase in the exposure time of the irradiators, and the disinfection of the drain pan is also hampered.
Bactericidal systems with the joint operation of supply and exhaust ventilation systems are recommended to be used in premises with a constant stay of a large number of people or groups of people with a reduced immune barrier (hospitals, prisons, shelters) to prevent the spread of airborne infections (for example, staphylococcus, streptococcus, tuberculosis , flu, etc.) in the mode of permanent work. In rooms with no people at night, for example, in office buildings, shopping centers, etc., it is possible to use such systems in periodic mode, with switching off during off-hours to save energy and increase lamp life. Periodic operation should be foreseen at the design stage of the systems when equipment capacities are determined.
Systems for air disinfection of the upper zone of the premises
Radiation systems designed to disinfect the air in the upper zone of the room are attached to the ceiling or on the walls of the room at a height of at least 2.1 m above the floor level (Fig. 4).
Bactericidal plants for air treatment of the upper zone of the room
Figure 4.
Bactericidal plants for air treatment of the upper zone of the room
In this case, the lamps are equipped with screens to reflect the radiation upwards in order to intensify the UV irradiation of the upper zone of the room, while maintaining minimal levels of exposure in the working area (Fig. 5). Inactivation of microorganisms occurs during the irradiation of air passing above the lamps. There are bactericidal systems with built-in fans to improve the mixing of air, which greatly increases the overall efficiency of the systems.
The principle of operation of wall bactericidal installations for the treatment of air in the upper zone of the room
Figure 5.
The principle of operation of wall-mounted bactericidal installations for the treatment of air in the upper zone of the room. Depending on the height of the room, open-type lamps or with screens that prevent radiation from entering the upper zone are used. Open-type lamps provide intensive irradiation of the upper zone of the room, while maintaining a safe level of UV irradiation in the working area. The mechanical ventilation system mixes air in the irradiation zone. Ceiling type irradiators can also be used. 1 - disinfection system with indoor screens, 2.4–2.7 m high; 2 - disinfection system for rooms higher than 2.7 m
Ceiling or wall-type air disinfection systems should be used either independently in the absence of supply and exhaust ventilation systems with built-in irradiators, or in conjunction with it for more effective inactivation of microorganisms. Rules for the use and placement of UV lamps should be consistent with the passport of the equipment manufacturers. As the experience of using irradiators showed, the use of one lamp with a nominal power of 30 watts on average for every 18.6 m2 of the irradiated surface is sufficient, although it is known that lamps of such power do not always have the same efficiency, often it depends on the type of lamp manufacturer and many factors. As a result, a number of new studies have recommendations for the installation of lamps. The main requirement is to ensure uniform distribution of radiation in the upper zone of a room with a power in the range of 30–50 W / m2, which is considered sufficient for inactivation of cells containing Mycobacterium and most viruses. The efficiency of disinfection greatly increases with air mixing in the room, for which it is desirable to use mechanical ventilation systems or at least fans installed directly in the room.
The main parameters affecting the operation of disinfection systems
Relative humidity
At a relative humidity of more than 80%, the bactericidal effect of ultraviolet radiation decreases by 30% due to the screening effect of microorganisms. The dustiness of the flasks of lamps and reflectors of the irradiator reduces the value of the bactericidal flow by up to 10%. At room temperature and relative humidity up to 70%, these factors can be neglected. The influence of relative humidity on the behavior of microorganisms (k-value) is noted, although it is not fully substantiated, since studies do not provide permanent results. The relationship between relative humidity and susceptibility of microorganisms depends on their species, but nevertheless the best effect of inactivation is noted with an increase in relative humidity up to 70% and higher. Nevertheless, it is recommended to use these systems at a relative humidity of no higher than 60% from the condition of ensuring the required air quality and microbial seeding level. As a rule, systems for disinfecting indoor air work in conditions of low relative humidity, channel systems - at higher. The relationship between relative humidity and inactivation efficiency requires further study.
Air temperature and speed
The change in air temperature in the room affects the radiation power of the lamp and the UV dose. When the ambient temperature is less than or equal to 10 or 40 ° C or more, the value of the bactericidal flux of the lamps decreases by 10% of the nominal. As the room temperature falls below 10 ° C, ignition of the lamps becomes difficult and the sputtering of the electrodes increases, which leads to a reduction in the service life of the lamps. Also the lifetime is affected by the number of inclusions, each of which reduces the total lifetime of the lamps by 2 hours. The UV performance of duct systems varies from 100% to 60% depending on temperature changes and the air flow rate inside the duct, in particular, in systems with variable flow rates, where both parameters change simultaneously. The effect of temperature and air velocity should be considered when designing in-channel systems to maintain constant efficiency under all operating conditions. The susceptibility of microorganisms to radiation does not depend on temperature and air velocity.
The reflectivity of the irradiated surfaces
Improving the reflectivity of ducts increases the efficiency of the systems installed inside them and is a very economical way, since all the reflected energy is added to the direct energy when calculating the UV dose. Not every surface that reflects visible light reflects UV energy. For example, polished copper reflects most of the visible light, and ultraviolet - only 10%. The reflectivity of the galvanized steel from which the air ducts are made is approximately 55%. Also, to increase the efficiency of irradiation, it is advisable to veneer the air ducts with aluminum or other reflective materials.
The reflectivity of surfaces is useful for duct systems, but can be dangerous for ceilings, when applied, the surfaces of ceilings or walls should eliminate the reflection of UV rays from surfaces located at a distance of 3 m or less from the open side of the illuminator. Reflections from surfaces should be eliminated by applying low-reflective paints or coatings, but maintaining the required irradiation of the upper area of the room and at the same time reducing the impact of UV on people in the working area of the room.
The effect of UV rays on surface quality
Exposure to UV rays does not affect the physicochemical properties of inorganic materials, such as metal or glass, organic materials are destroyed fairly quickly. Thus, synthetic filter elements, gaskets, rubber, motor windings, electrical insulation, internal insulation of air ducts, plastic pipes located 1.8 m or less from the lamps inside the air handling units or air ducts should be protected from UV radiation in order to avoid damage. Otherwise, the safety of the entire system may be impaired.
Ceiling devices do not seriously harm the quality of building structures, with the exception of peeling paint or cracking coatings. Therefore, the irradiated surface is recommended to be made of materials resistant to UV radiation. Paper products: books, documents and various items stored in the upper part of the premises may discolor or dry out. There have been cases of negative effects of irradiators located in the upper zone of the room, on plants. These problems are completely eliminated by proper maintenance of the systems and removal of UV-sensitive objects from the irradiation zone.
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