Electrostatic Precipitation Of Bioaerosols

Bioaerosols in air usually consist of viruses, bacteria, fungal, as well as their aggregates and fragments, etc. Exposure to bioaerosols may cause health problems, including hypersensitivity, infections, asthma, lung disease and some airborne infectious diseases. The bioaerosol samplers are used to verify and quantify the presence of bioaerosols for exposure assessment and control. The electrostatic precipitation is a potential bioaerosol collection technique. In comparison with the conventional bioaerosol sampling techniques, the impact velocity towards the collection medium in electrostatic precipitator is typically several orders of magnitude smaller than that in bioaerosol impactor or impingers. Therefore, electrostatic technique could effectively collect bioaerosols with less injures. In this research, a home-made portable electrostatic precipitator was designed. The major factors affecting the electrostatic precipitation of bioaerosols were studied. The size distribution characteristics and average electric charges carried were monitored by Electrical Low Pressure Impactor (ELPI). The aerosol kinetics and factors affecting precipitation process were explored. The optimum collection conditions were determined, and will be used as reference for field bioaerosols sampling and novel electrostatic sampler development.The electrical mobility is a key factor affecting bioaerosols migration in electric field. It depends on both the size and electric charges of bioaerosols. The results showed that the Escherichia coli (E. coli) aerosols were in the range of 0.028-4.01 μm in aerodynamic diameter, and with an average electric charge per particle of 0-23 e.79% of cultivable E.coli aerosols were around 0.95-1.61μm. Candida albicans (C.albicans) aerosols were in the range of 0.028-9.96 μm, and with negative average electric charges of 0-45 e.40% of cultivable C.albicans aerosols were around 0.028-4.01 μm. Bioaerosols carry more electric charges than nonbiological NaCl aerosols, and could be better utilized for their electrostatic grade precipitation. The maximum collection efficiency of E.coli aerosols, C.albicans aerosols and NaCl aerosols were 52%,61% and 27%, respectively. The applied voltage, collection areas and flow rate were 10 kV,20 cm2 and 10 L/min, respectively. Besides, the grade collection efficiency was in positive relationwith electrode length and applied voltage.A pre-charger unit was used to enhance the electrostatic collection process. Both positive and negative corona discharges could increase electric charges on bioaerosols. After pre-charging, the average electric charges were 5-13 times and 3-20 times higher than the natural electric charges of E.coli aerosols and C.albicans aerosols, respectively. The electrical mobility can be increased about 10-20 times. The maximum precipitation efficiency of E.coli and C.albicans aerosols rose to 92% and 89%, respectively. The electric polarity of bioaerosols was determined by corona discharge. According to the grade collection theoretical model, the grade collection efficiency could be predicted via the specific collection area and average electric field intensity. It is useful in guiding the design of electrostatic sampler.The performance of biological collecting efficiency of ESP was compared with traditional impinger AGI-30 operated in parallel. An increase of collection area or a decrease of flow rate would significantly improve the bioefficiency. The increase of electric field intensity might cause gas discharge in the system and inactivation of the microorganism. The best collection result was gained when the collection areas, flow rate, electric field intensity were 20 cm2,10 L/min,4.3 kV/cm, respectively. The bioefficiencies of electrostatic precipitator were 1.2 times and 3.9 times of AGI-30 for E.coli and C.albicans aerosols, respectively. Comparison with the negative corona discharge, positive corona discharge was more suitable for bioaerosols pre-charging. When the pre-charging voltage was +5 kV, the maximum bioefficiencies were as 10 times and 6.9 times of AGI-30 sampled for E.coli and C.albicans aerosols, respectively. After pre-charging, bioaerosols would be unipolar charged and mostly collected on the electrode plate with opposite polarity, which would simplify the design of the sampler. In addition, collection medium and time need to be adjusted to different species of bioaerosols and environment.