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Abstract
Biofilters, designed to facilitate the infiltration of stormwater into soil, are generally
ineffective in removing bacteria from stormwater, thereby causing pollution of groundwater
and receiving surface waters. The bacterial removal capacity of biofilters has been
shown to be lower in the presence of natural organic matter (NOM) and during intermittent
infiltration of stormwater. To improve the removal of fecal indicator bacteria (Escherichia
coli) under these conditions, we amended sand with 5% (by weight) biochar, a carbonaceous
geomedia produced by pyrolysis of biomass, and investigated the removal and remobilization
of E. coli. Three types of biochar were used to evaluate the role of biochar properties
on the removal. Compared to sand, biochar not only retained up to 3 orders of magnitude
more E. coli, but also prevented their mobilization during successive intermittent
flows. In the presence of NOM, the removal capacity of biochar was lower, but remained
higher than sand alone. The improved retention with the biochar amendment is attributed
to an increase in the attachment of E. coli at the primary minimum and to an increase
in the water-holding capacity of biochar-amended sand, which renders driving forces
such as moving air-water interfaces less effective in detaching bacteria from grain
surfaces. Biochars with lower volatile matter and polarity appear to be more effective
in removing bacteria from stormwater. Overall, our results suggest that a biochar
amendment to biofilter media has the potential to effectively remove bacteria from
stormwater.