Porous media filters are used widely to remove bacteria from contaminated water, such as stormwater run-off. Biofilms that colonize filter media during normal function can significantly alter performance, but it is not clear how characteristics of individual populations colonizing porous media combine to affect bacterial retention. We assess how four bacterial strains isolated from stormwater and a laboratory strain, Pseudomonas aeruginosa PAO1, alter Escherichia coli retention in experimental sand columns under conditions of stormwater filtration relative to a clean-bed control. Our results demonstrate that these strains differentially affect E. coli retention, as was previously shown for a model colloid. To determine whether E. coli retention could be influenced by changes in relative abundance of strains within a microbial community, we selected two pairs of biofilm strains with the largest observed differences in E. coli retention and tested how changes in relative abundance of strain pairs in the biofilm affected E. coli retention. The results demonstrate that E. coli retention efficiency is influenced by the retention characteristics of the strains within biofilm microbial community, but individual strain characteristics influence retention in a manner that cannot be determined from changes in their relative abundance alone. This study demonstrates that changes in the relative abundance of specific members of a biofilm community can significantly alter filter performance, but these changes are not a simple function of strain-specific retention and the relative abundance. Our results suggest that the microbial community composition of biofilms should be considered when evaluating factors that influence filter performance.

Stormwater biofilters have variable colloidal particle removal efficiencies that may be due to different characteristics of microorganisms colonizing them during normal operation. Little is known about how naturally occurring bacteria modify the surface properties of the filtration media, or how these modifications affect colloidal microplastic removal. We used simulated infiltration through model sand columns to study the impact of colonizing microorganisms on the retention of model colloidal microplastic particles, carboxyl-modified-latex (CML) beads. Four naturally occurring stormwater bacterial strains inhibited bead retention compared to a clean bed in a cell-density independent manner, while a model biofilm forming microorganism (Pseudomonas aeruginosa) enhanced retention likely due to physical straining. Atomic force microscopy (AFM) was applied to analyze interaction energy between CML and each bacterial strain biofilm surface. We found that interaction energy between CML and each strain was significantly different, and we propose that changes in the CML bead retention behavior are related to these differences. In particular, we propose that larger repulsive energy generated by some biofilms prevents close contact of CML to surface required for adhesion. Our evidence suggests that extra-cellular polymeric substances modify media surfaces to alter both adhesion and repulsion of beads. Overall, the work demonstrates how AFM can be used to characterize colloidal-collector interactions and provide insights into mechanisms that could be exploited to improve EIS design and operation. The purpose of this collection is to make available to the public the material necessary to validate the research findings.