EPA Contract Number:
Title:
Investigator:
Small Business:
3700 Koppers Street
Baltimore, MD 21227
Telephone Number: (410) 368-7275
EPA Contact:
Phase: I
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Relative to outdoor air, the quality of indoor air can be much worse. For example,
On a day with the highest pollution index, indoor air can be worse to breathe than the air outdoors. Over half of our homes and offices are suffering from a form of sick building syndrome.
The seriousness of biological contamination in indoor air is demonstrated by Legionnaires Disease, a pneumonia which attacks 2-5% of those exposed. Between 5-15% of those who contract legionella die from it. Factors influencing susceptibility include the elderly and those with suppressed immune systems, and others with weak lungs or constitutions. It incubates in human hosts and will not abate without medication. Estimates of the number of cases vary from 25,000 to 50,000 a year in the US. There have been over 50 separate outbreaks.
Tuberculosis is spread via the air through inhalation. Mycobacterium tuberculosis is carried in airborne particles known as droplet nuclei. The droplet nuclei are so small that they can be suspended indefinitely in the air and be spread throughout a facility by the HVAC system. In recent years, the transmission of tuberculosis in health care facilities has reached epidemic proportions. These transmissions have included outbreaks of multidrug-resistant strains of Mycobacterium tuberculosis that have produced many deaths. A 1992 study found that 10% of patients in a large hospital's HIV unit had TB, and that half had acquired the infection since admission. More than half the nurses working on the same floor had a positive tuberculin test, indicating they were infected with the bacterium.
The filtration of bacteria and viruses from indoor air is hindered by two characteristics of the organisms - extremely small size and the ability to propagate. The typical diameter of bacteria is a few micrometers, but viruses can be 1/100 this diameter. It is well known that the effective filtration of particles less than one micrometer is difficult. The HEPA filter used for this service is effective, but requires a high-pressure drop. It is also known that the organisms that are captured by the filter can flourish on the filter surface and migrate through the filter, necessitating frequent filter changes.
The use of electric fields and electric discharges can address these challenges. Enhancement of filter capture efficiency through the application of electrostatic fields is well established. Polarization effects brought about by a DC electric field produce an attractive force between particles and filter fibers resulting in significantly enhanced filter efficiency, allowing a porous, low-pressure drop filter to be used for high efficiency service. The sterilization of surfaces through exposure to the University of Tennessee's One Atmosphere Uniform Glow Discharge Plasma (OAUGDP) has been demonstrated to be very effective. Microbe destruction occurs through attack by atomic oxygen and oxygen radicals created by the plasma.
The OAUGDP can be produced by attaching electrodes to both sides of a flat filter and energizing the electrodes with a RF source. The plasma produced by the periodic energization of the electrodes kills the captured organisms. Furthermore, when a DC voltage is applied across these electrodes below the discharge onset, then a particle capture-enhancing field is applied across the filter media. A filter combining these effects would be field-enhanced, plasma-sterilized (FEPS) filter. A demonstration of this concept was the purpose of this effort.
The test program was carried out at the EEC Laboratory in Baltimore, MD, the University of Tennessee Plasma Sciences Laboratory, and the UTK Department of Microbiology. The UTK team constructed a one square foot FEPS filter. EEC constructed an airflow assembly that was used to hold and operate the FEPS filter. The complete filter assembly was tested at UTK by EEC technicians and UTK faculty and students. The system was composed of a HEPA filtered room air intake duct leading to the FEPS filter. A DC power supply provided a continuous electric field for filtration enhancement, and an RF power supply was used to provide intermittent plasma sterilization. The airflow passed through the filter and was subsequently exhausted outside through a fan. Microorganisms were introduced upstream of the filter through an atomizing nebulizer. The microorganisms penetrating the filter were measured through isokinetic sampling and capture on a secondary sterile filter.
Two microorganisms (S. aureus and Bacteriophage FX174) challenged the capture and sterilization capabilities of a field-enhanced, plasma-sterilized, one square foot area filter. Parameters that were varied were field strength and plasma exposure time. It was found that the microorganism capture efficiency of a low-pressure drop filter could be increased from 93% to 99.99% with the imposition of an electric field. It was shown that the viable concentration of microorganisms captured by the filter could be reduced by four orders of magnitude through exposure to the OAUGDP plasma. Finally, it was found that the polypropylene fabric used for the efficiency tests was strengthened by plasma exposure.
The tests have thus demonstrated that the filtration of submicron-sized microbes can be significantly enhanced by the application of a DC electric field across the filter, such that a low pressure drop filter can obtain HEPA-like capture efficiency. It was furthermore shown that 99.99% of the captured microorganisms could be destroyed by the short-term application of the One Atmosphere Uniform Glow Discharge Plasma.
Future work will be directed toward the design, construction and testing of a prototype, commercial FEPS Filter. A commercial scale panel will be constructed and tested. Preliminary testing will take place in the laboratory, followed by long term testing in several rooms of an occupied building. The air quality of these rooms will be monitored, and the microbe species populations on the filter surface will be monitored.
The FEPS Filter panel can simply be composed of a polyethylene felt, sandwiched between electrode grids. In this form, it can be assembled in the same way as conventional filter panels are currently assembled. The panels can be arranged in a sawtooth pattern to accommodate a wide range of flow rates. It was found that if the cost of a 1000 CFM FEPS Filter can be kept below $1000, then it will be competitive with UV-sterilizing filters that may soon be marketed.
The identified application area is where high efficiency filtration and filter sterilization are both needed and where lower pressure drop operation is desired. The principal application is hospital ventilation with secondary applications in office buildings, schools, and other facilities. Hospitals are a priority because they have the need and an existing demand for high efficiency filtration for infection control. Office building and school market segments offer potential for the general avoidance of sickness and the need for low-pressure drop filtration.
Last Updated: September 9, 1999
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