The concentrations of bioaerosols near pretreatment facilities are often higher than those near activated-sludge aeration tanks in wastewater treatment plants (WWTP). The reason for this difference is not yet clear and the differences between the characteristics of suspended solids (SS) in these two processes might play a critical role. In this study, a lab-scale wastewater treatment system was tested with Escherichia coli (E. coli) in order to investigate the effects of the type and concentration of SS on the concentration and size distribution of emitted bioaerosols. Two types of SS, activated sludge from a hospital WWTP and kaolin clay, were selected to represent floc-type mixed-liquor SS (MLSS) and non-floc-type SS, respectively. An Andersen six-stage sampler was used to analyze the size distribution of the airborne E. coli aerosols. When the tested aeration rate was as low as 0.5 L min–1, it was found that the presence of bioaerosols slightly decreased in air/water ratio (AWR, CFU m–3 air/CFU 100 mL–1 water) when the SS increased from 500 to 2000 mg L–1 for both floc and non-floc SS. However when the aeration rates went up to 5–15 L min–1, the pattern of AWR vs. SS curve was different with an increase trend for non-floc SS and a decrease trend for floc SS, with an exception of increase trend for floc SS as the MLSS increased from 0 to 500 mg L–1. The major sizes of emitted bioaerosols ranged from 0.65 to 1.1 µm. A peak was observed at an aerodynamic diameter greater than 7 µm when the aeration rate was 15 L min–1. In conclusion, floc-type activated sludge can inhibit the emission of E. coli from an aeration tank. In contrast, the non-floc-type small-size kaolin clay can enhance the emission of E. coli from the pretreatment process.