Bioretention has been widely used in urban non-point source (NPS) pollution management
for effectively reducing downstream pollution loads and peak flows. However, nitrogen
(N) removal in conventional bioretention systems has been uniformly unstable and highly
variable due to a lack of anaerobic denitrification. To improve the stability and
effectiveness of N removal, two bioretention columns with bilayer media (C1 and C2)
were designed. High permeability quartz sand (~2 mm diameter) was used as the upper
media, and low permeability modified media (~0.6 mm diameter, adding 5% organic substance)
as the lower media. The bilayer media structure formed an anaerobic zone for promoting
denitrification processes. The results showed that the retrofitted columns performed
well and that the removal efficiencies of various forms of N were considerably enhanced
to 76.8%-95.3%, 85.1%-98.3%, and 87.5%-97.4% for TN, NH4+-N, and NO3--N, respectively.
Additionally, copying numbers of the denitrification functional genes detected via
FQ-PCR in the lower media of C1 and C2 were accounted for 46.06% and 44.16% of the
16S rDNA gene, respectively. These results confirmed the presence of anaerobic denitrification
processes. Consequently, bilayer media bioretention systems are worth promoting in
cities where nitrogen in urban runoff poses a threat to the receiving surface water,
due to the systems' remarkable performance in nitrogen removal, simple structure,
and easy implementation.