Measurement of electrical charges on airborne bacteria generated using a single-pass bubbling aerosolizer

Laboratory-generators are likely to produce electrostatically charged particles. The amount of electrical charges carried by airborne particles, including microorganisms and other biological airborne entities, can greatly influence their behaviour (deposition, filtration, etc.). Microorganisms are more complex in their composition than inert particles and naturally contain electrical charges. Electrical charges carried by laboratory-generated bacteria consist of two components: their own natural charge, which can be high, and the charge imparted by the aerosolization process. Inert non-biological particles, in contrast, carry only the charges induced by the dispersion process. We previously developed and characterized the performances of a ‘Liquid Sparging Aerosolizer’-type bioaerosol generator where microorganisms are dispersed by bubbling compressed air through a film of microbial suspension. We assumed that the generated microorganisms will carry few electrical charges; such a scenario would imply that a neutralization step, with expected additional negative effect on the viability of sensitive microorganisms, is not necessary. The objectives of this study were to measure the fraction of neutral particles and the number of elementary charges per particle as a function of the aerodynamic diameter of the airborne gram-negative sensitive bacteria (E. coli and E. hirae), for different operating conditions. The experimental setup includes the tested bioaerosol generator, an electrostatic precipitator (ESP), a custom-built sampling volume with 6 parallel probes and real-time devices to characterize the airborne particles (Aerosol Electrometer IONER EL-5030, Ramem S.A - Aerodynamic Particle Sizer APS, TSI model 3321 - Electrical Low Pressure Impactor ELPI, Dekati). The fractions of neutral particles ranged from around 30% to 50%, depending on the operating conditions. The higher the bubbling airflow, the greater the fraction of neutral particles in the generated bioaerosol. Whatever the species or the operating condition considered, the net electrical charge carried by the aerosolized bacteria is negative, what is consistent with previous published works. The average number of elementary charges carried per particle varied from around -10 to -50 elementary units. The higher the bubbling airflow or the aerodynamic diameter, the greater (absolute value) the number of elementary charges carried by bacteria. In such wet dispersion method, different physical factors and charging mechanisms are responsible for the electrical charges carried by airborne microorganisms: liquid disrupting forces, contact charging effect, initial size of the droplets, initial electrical charges carried by these droplets and by microbial particles, etc. Given the number of elementary charges carried by the generated bacteria, a neutralization step seems not to be necessary. Nevertheless, if an equilibrium state of charge is required, our results also suggest that the use of the neutral fraction downstream an ESP may be advantageous instead of using neutralizers.