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Continuous Ionization System for Electrostatic Collection of Bioaerosols in Building Protection Applications


OBJECTIVE: Develop a system capable of continuous ionization of airborne bioaerosols in the 0.5-5 m size range that does not generate ozone. The system should be designed for use in electrostatic removal of bioaerosols in HVAC environments at reduced operational costs compared to HEPA filtration. DESCRIPTION: Continually operating, or"always on,"removal of airborne particulates provides not only day-to-day treatment of air in building environments but also provides the most rapid response in the event of a biological contamination event, such as an aerosolized anthrax attack. These systems require no sensors or activation delays since they are already up and running. Systems currently used for high removal efficiency of particulate contaminants are limited to fabric filters and electrostatic precipitation. Filters such as HEPA are effective at removing particles, but induce a large pressure drop (1-6"W.G. depending on loading) that is expensive from an energy standpoint. While electrostatic precipitators are widely used in particulate removal in flue gas streams, typical ionization methods utilize a corona discharge that may generate significant (>10 ppm) levels of ozone which is undesirable in an indoor environment. Standard air filtration relies on impaction and interception of particles on filter fibers. Prolonged use requires cleaning or replacement of filter media. To achieve high particle removal efficiencies, more tightly woven filters are utilized, increasing energy costs associated with the pressure drop across the filter. Additionally, filters contaminated with biological material are hazardous and must be properly disposed of to avoid spread of contaminants. Ionization-based filtration processes charge particulate matter in the air stream which is subsequently collected by electrostatic precipitation, eliminating the pressure drop associated with fabric filters. Ionization of the airborne particulates is traditionally obtained by corona discharge. However, work funded under a previous SBIR project has demonstrated that an effective ionization region can be established using an electrospray wick array. As with industrial-type electrostatic precipitation systems, it is possible to remove contaminants without the need to directly contact the collection surface, thus eliminating the difficulties associated with disposal of fabric filters. Electrospray ionization has been used for laboratory chemical analysis and has recently been proposed for incorporation into low-pressure drop air filtration units demonstrating a potential reduction in O&M costs while providing particle removal rates similar HEPA filtration. One significant benefit of electrospray ionization is that ozone is not formed as a byproduct of the process. Rather than direct ionization of particles by an applied voltage difference, electrospray induced ionization of airborne particulates is achieved by charge transfer to the particle from a water based aerosol. Implementation of electrospray into"always on"air filtration applications is currently limited by the lack of reliable, continuous-feed fluid delivery mechanisms for ionization. Recently, bipolar ionization (BPI) has been demonstrated as an effective method for charging of indoor aerosols, inducing agglomeration and settling of particles, as well as destruction of harmful VOCs. Additionally, BPI has been used as a charging mechanism for a newly designed aerosol time-of-flight mass spectrometer coupled with a differential mobility analyzer and charged plates. BPI can generate high levels of negative and positive ions in an indoor environment without producing ozone and has potential for use in electrostatic precipitation applications. This topic seeks the development of a novel system for bioaerosol ionization to be used in an electrostatic collection system for building protection applications. PHASE I: Conduct research on ionization systems that provide uniform and continual delivery of the ions across the entire volume of influent air. The system should be designed for autonomous operation requiring minimal maintenance or user operation (service less than once per month). Additionally, power consumption should be targeted to achieve less than 1.0 watts per cubic feet (35.3 W/m3) of treated air, to align with Department of Defense goals to reduce energy costs. The system should be capable of adapting to treatment of a high concentration spike of bioaerosols that would occur during a biological attack. By the end of Phase I, continual operation of the proposed ionization system should be demonstrated for periods of at least one week without losing effective ionization capabilities or having to replace consumables. The system should be capable of operating for at least two weeks without need for maintenance or replenishing consumables. It should be able to ionize 2,000 particles per cm3 at 50 CFM (1.42 m3/min) to a level adequate for removal of 99.97% of 0.3-1.5 um (micron) diameter particles in a 20 kV downstream collection region with no measurable ozone generation and minimal temperature increase (<5 degrees C). Particular attention should be paid to ionization of biological particles of this size range using surrogate spores. The specific surrogate to be employed must be proposed and approved by the DoD Technical Point of Contact. It is not necessary to demonstrate the collection region in Phase I. PHASE II: Design and test a filtration system building upon the ionization concepts developed in Phase I. Technology for electrostatic collection of the charged particles will be explored and tested in combination with the most promising ionization mechanism. Candidate integrated ionization and collection systems should be refined with regards to energy consumption, particle removal efficiencies, ease of operation, and effectiveness in the presence of biological materials contamination in addition to normal particle loadings. Extended duration tests should be accomplished over a variety of temperature and humidity levels. The culmination of Phase II should result in a validated, pilot-scale prototype capable of operation at 2,000 CFM (56.6 m3/min) and a plan for full size scale-up. Effective ionization and collection of surrogate bioaerosols with particle removal efficiencies greater than 99.9% of the 0.3-1.5 m (micron) size particles should be demonstrated. Additionally, energy consumption in the final system should be favorable when compared to HEPA filtration. PHASE III: Production of a full size filtration system (>20,000 CFM [>566 m3/min]) for demonstration at an DoD installation or other relevant facility. Product performance should be verified to meet the metrics targeted for the bench scale system. At this stage, attention should be paid to parameters such as operational noise, product footprint, and material costs and modifications made as necessary to increase commercialization and manufacturing potential. PHASE III DUAL USE APPLICATIONS: Electrostatic air filtration technology has potential markets both within and outside of the government for buildings in which there is an elevated threat of chemical and biological contamination. In addition, the technology has the potential to be used in hospitals, schools, and other buildings where high levels of indoor air purification are desired as an alternative to HEPA filtration.
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