Generation of Biological Aerosols in Natural and Industrial High Temperature Processes

Abstract

A comprehensive survey conducted in Australia among 4878 participants in order to assess a national level as to the number of people exposed to specific asthmagens in workplaces revealed that about 47% men and 40% women were exposed to at least one asthmagen at their workplace. Moreover, the most common exposure was related to bioaerosols (29% males, 16% females) originating from a wide range of activities (Fritschi et al. 2014). Most airborne microorganisms are sensitive to high temperature. Furthermore, ultra-high temperature treatment is considered to be one of the most rapid and efficient methods for inactivation of bioaerosols (Grinshpun et al. 2010). However, a possibility of viable microorganisms’ aerosolisation from the processes with a temperature higher than ambient was demonstrated for cooling towers (Nguyen et al. 2006, Walser et al., 2014), water systems (Wadowsky et al. 1982), lime slaking (Barth et al. 2009) and biomass combustion (Semple et al. 2010). The objective of this research programme is to assess the possibility of viable microorganisms’ aerosolisation in high temperature processes that have not been investigated earlier. The research was focused on two high temperature processes: (i) liquid interaction with hot surface and (ii) biomass combustion. Both these processes can take place in residential, occupational and natural environment. The objective was achieved by addressing a number of research questions as follows: i. Is the process of droplet-surface collision capable of generating a viable bacterial aerosol? ii. Does the temperature interval that represents the fragment of the boiling curve with the lowest heat transfer provide the most favourable conditions for microbial survival? iii. What is the numerical model of the mechanism of water-surface interaction? iv. What is the size of a water droplet capable of departing from the hot surface and the rate of droplet cooling based on the ambient air parameters? v. What is the influence of the droplet cooling rate on bacterial survival? vi. What is the possibility of bacteria and fungi survival during combustion of different types of contaminated organic materials under controlled laboratory conditions? vii. Is there a possibility of microbial aerosolisation during and post biomass combustion processes occurring in controlled and natural environments? The research included both laboratory and field investigations. The laboratory investigations involved bioaerosol generation using the predicted mechanisms, bioaerosol collection and analysis. The field investigations were focused on biomass combustion processes taking place during natural and prescribed biomass burning events. The outcomes of this research programme provide new knowledge that can be summarised as follows: i. The process of interaction between water and a hot surface can be associated with establishment of viable bacterial aerosol when the water is contaminated with bacteria. ii. The violent film boiling regime represents the best conditions for bioaerosol formation. iii. The bacterial concentration in the contaminated water does not affect the aerosolisation of microorganisms. The aerosolisation of bacteria is predominantly affected by the surface temperature. iv. The microbial survival was found to be strain specific. The recovery ratio of B. subtilis was significantly higher than E. coli. v. The highest temperature of 480°C revealed significantly lower bacteria recovery ratio due to formation of levitating drops. Those drops are capable of remaining on the surface over extended periods of time that limits the generation and release of fine droplets containing microbes. vi. A mathematical model has been developed to consider droplet shooting off conditions and following airborne droplet evolution due to cooling. vii. The critical size of the droplet capable of taking off was modelled as a function of wall temperature and droplet size. viii. Following the departure from hot surface, droplet cooling time mainly depends on the initial droplet radius while the influence of the ambient temperature is marginal. ix. The shortest cooling time was associated with higher survival rates of both B. subtilis and E. coli. x. Laboratory experiments did not demonstrate aerosolisation of microorganisms during biomass combustion. xi. A relatively high survivability rate of bacteria in the combustion products was shown in the laboratory experiments. The survived microorganisms can be aerosolised from the post-combustion materials by high velocity natural air flows. xii. Field investigations demonstrated significant increase in the bioaerosol concentration above natural background during and post biomass combustion. Overall, results of this study make an original contribution to the existing knowledge regarding the mechanisms of bioaerosol generation. The processes of droplet-surface interaction are generally related to industrial procedures such as cooling of surfaces in metallurgical processes and combustion. However, it can have important applications in everyday life. Collision of contaminated water with the surface of hot rocks in a hot spa, steam generation in the iron-press, water cleaning of the hot barbeque facilities can potentially result in generation of viable biological aerosols. The research also validates the capability of viable microorganisms to survive in the material remaining after biomass combustion. Viable microorganisms are able to aerosolise if an additional source of aerosolisation is being present. Elevated concentrations of fungi and bacteria were observed during biomass combustion and prescribed burnings as well as in post combustion environment. The two possible changes in the ambient conditions caused by the fire and relevant to variations in the bioaerosol concentration include elevated levels of all types of combustion related air pollutants and turbulent air flow created by the burning process.