Outflow risks of antibiotic-resistant bacteria in stormwater bioretention cells: understanding roles of adsorption and transmission

In this study, lab-scale bioretention cells were designed for the investigation of antibiotic-resistant bacteria (ARB) outflow profiles at different depths, effects of adsorption and transmission, as well as modelling evaluation of ARB outflow risks using the common decay models (e.g., first-order decay models). ARB outflow was first found in the upper layers (after 100 days of the operation) with the lowest transmission frequencies of antibiotic resistance. Although the adsorption of ARB onto the substrate and its surface biofilms was effective with the maximum amount of ARB adsorbed (Q(max)) reaching 108 CFU/g of the substrate and 107 CFU/g of biofilms, ARB outflow was detected in the bottom outlets after over 4 months of operation, reflecting that there was still a risk of antibiotic resistance through the treatment of bioretention cells. ARB outflow for both upper and middle outlets could be well described by third-order polynomial equations with correlation coefficients 0.9067 (p = 0.0002) and 0.9780 (p < 0.0001), respectively, where there were both positive and negative relationships between outflow ARB and inflow ARB, conforming that the combined action of mechanisms blocking ARB outflow (e.g., substrate adsorption) and promoting ARB outflow (like transmission). These suggested two potential controlling approaches for ARB outflow from stormwater bioretention cells.