Limited Bacterial Removal in Full-Scale Stormwater Biofilters as Evidenced by Community Sequencing Analysis

In
urban areas, untreated stormwater runoff can pollute downstream
surface waters. To intercept and treat runoff, low-impact or “green
infrastructure” approaches such as using biofilters are adopted.
Yet, actual biofilter pollutant removal is poorly understood; removal
is often studied in laboratory columns, with variable removal of viable
and culturable microbial cell numbers including pathogens. Here, to
assess bacterial pollutant removal in full-scale planted biofilters,
stormwater was applied, unspiked or spiked with untreated sewage,
in simulated storm events under transient flow conditions, during
which biofilter influents versus effluents were compared. Based on
microbial biomass, sequences of bacterial community genes encoding
16S rRNA, and gene copies of the human fecal marker HF183 and of the
Enterococcus spp. marker Entero1A, removal of bacterial
pollutants in biofilters was limited. Dominant bacterial taxa were
similar for influent versus effluent aqueous samples within each inflow
treatment of either spiked or unspiked stormwater. Bacterial pollutants
in soil were gradually washed out, albeit incompletely, during simulated
storm flushing events. In post-storm biofilter soil cores, retained
influent bacteria were concentrated in the top layers (0–10
cm), indicating that the removal of bacterial pollutants was spatially
limited to surface soils. To the extent that plant-associated processes
are responsible for this spatial pattern, treatment performance might
be enhanced by biofilter designs that maximize influent contact with
the rhizosphere.