2.2. Sampling and analysis

Six bioaerosol sampling campaigns were conducted on 21th November 2019, 5th December 2019, 16th December 2019, 23rd December 2019, 7th January 2020, and 8th January 2020 in plants A and B by using an Andersen six-stage cascade impactor (FA-1, Hongchangxin Inc., Beijing, China) (^{Hung et al., 2010}). Sterile agar media Egg-Yolk Mannitol Salt Agar Base and MacConkey-Agar-Medium were used as the collection media for culturing and colony enumeration of S. aureus and E. coli, respectively (^{Oppliger et al., 2005; Szyłak-Szydłowski et al., 2016; Nasir et al., 2018; Wang et al., 2019}). A 27 mL aliquot of this sterile agar media (autoclaved at 121 °C for 15 min) was pipetted into sterile glass Petri dishes equipped with the cascade impactor (^{Jahne et al., 2015; Jahne et al., 2016}).

The sampling point was set at 1.5 m above each aeration tank's ground (^{Szyłak-Szydłowski et al., 2016}). The cascade impactor was operated for 10 min at a flow rate of 28.3 L/min (^{Hung et al., 2010; Kowalski et al., 2017}). Each stage of the Andersen six-stage cascade impactor was decontaminated with 75% alcohol before and after use for air sampling on site (^{Hung et al., 2010}). All samples were in triplicate and transported to the laboratory in a cold box before being cultivated in incubators for 24–48 h at 37 °C.

After cultivation, the samples were enumerated as colony-forming unit (CFU) by using an automatic colony enumeration instrument (HICC-B, Wanshen Inc., Hangzhou, China). The positive hole method was used to correct and then obtain the actual number of colonies measured at the each Petri dish stage on the basis of the enumeration results (^{Hung et al., 2010; Delort and Amato, 2017}). Bioaerosol concentrations of S. aureus and E. coli in CFU/m³ were estimated by dividing the number of colonies in CFU by the sampled air volume in m³ (^{Hung et al., 2010}). Then, the bioaerosol concentration was the sum of the concentrations of the six Petri dish stages of the Andersen six-stage cascade impactor (^{Katsivela et al., 2017}).