Bioretention saturated zones: Do they work at the large-scale

Three large bioretention cells (each 955m2) were analysed for their removal of TSS, TN and TP. Two of the cells consisted of standard filter media, whereas the third incorporated a saturated zone intended to enhance TN removal via denitrification. The concentrations of total suspended solids (TSS), total nitrogen (TN) and total phosphorus (TP) measured in the runoff at the inlet to the bioretention cells were markedly lower than those typically recorded in urban catchments. Nevertheless, all cells performed well in terms of TSS removal, with significant reductions in outflow relative to inflow concentrations (pin=29.6 mg/L, μout=16.4 mg/L). In terms of TN, the two standard cells acted as sources (given the very low inflow concentrations) while the cell with the saturated zone significantly reduced concentrations (μin=0.98 mg/L, μout=1.05 mg/L). All cells significantly reduced TP concentrations (μin=0.087 mg/L, μout=0.059 mg/L), with the saturated cell performing the best. Given the difficulty in removing nitrogen, designers should focus on careful selection of filter media, plants and the possible use of a saturated zone with carbon, to enhance nitrogen removal processes. Even at this large-scale, a saturated zone effectively improves TN removal; these findings are consistent with those previously demonstrated by small-scale laboratory studies. Importantly, phosphorus leaching from the saturated zone does not appear to be an issue here. Large-scale trials in areas with inflows that have higher pollutant concentrations need to be investigated. A study that analyses event mean concentrations (EMC) as opposed to discrete concentrations is also required, as are investigations into the influence of vegetation type and composition on performance at this large-scale.