A microbial risk assessment was carried out for the rotating-brush aerator used during summer in a wastewater treatment plant (WWTP) of Xi'an, China. Bacterial aerosols were first collected by an Andersen cascade impactor at selected sampling sites near the rotating-brush aerator. The concentrations of airborne bacteria were used to obtain microbial emission rate by back calculation. The downwind concentrations of airborne bacteria were then calculated by a modified Gaussian dispersion model accounting for environmental impact and microorganism decay. Subsequently, the exposure parameters suitable for Chinese people were incorporated into a risk assessment model to evaluate non-carcinogenic risks of airborne mesophilic bacteria to sewage workers and surrounding residents. The results indicate that both mean bacterial concentrations at ground level and the exposure hazard quotient decrease rapidly with downwind distance. The exposure hazard quotient by inhalation route is over 10 5 times more than by dermal contact route for both children and adults, suggesting that inhalation route is the major exposure pathway of microbial aerosol intake for surrounding people. Although the present model gives acceptable low risk values at various downwind distances, it is worth noting that health risks of microbial aerosols associated with rotating-brush aeration for children are generally much more than those for adults.

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Observations of atmospheric air ions at most geographic areas have been undertaken, but there are still very few observations at middle-latitude semi-arid areas throughout the world. To quantify characteristics of small air ions in semiarid areas of China, measurements of small air ions and particulate matter (PM 10 ) were conducted at 16 different sites in Xi'an (34°26′N, 108°94'E), China, from August 2009 to July 2010 along with meteorological parameters. The influences of meteorological parameters and PM 10 concentration on small air ions as well as their influence mechanisms were discussed in detail. The results showed that both spatial and temporal distributions of small ions were uneven in Xi'an city. Spatially, the concentration of small ions increased gradually from city center to the suburbs. The mean annual concentration of small negative ions was in the range of 200-400 cm -3 with the mean value of 261 ± 39 cm -3 at urban areas, while more than 500 cm -3 at suburban areas. Significant seasonal variations of small ions were observed with highest level in summer, followed by spring and autumn, and lowest in winter at various types of sites. The positive correlations were found between small ions concentration and temperature as well as relative humidity, while negative correlation between the small ions concentration and PM 10 concentration. Air ions concentration was also applied to the assessment of air quality in Xi'an. The assessment results indicated that the air quality was better at suburban areas than at urban areas, suggesting that the unipolarity model is reasonable to indicate air cleanness.

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