Nitrogen retention and purification efficiency from rainfall runoff via retrofitted bioretention cells

Abstract

Nitrogen processing in the bioretention media is often variable, and the contribution to nitrogen retention and loss remains unclear. Bioretention cells including 12 medium combinations were constructed following ten intermittent, simulated storm events, meanwhile, the contents of pollutants and related enzyme activities in the media were detected. Among these medium combinations, C-04 ∼ C-12 were retrofitted media, which comprised a mixture of modifiers and traditional bioretention soil mixing (BSM). Among them, BSM + Water treatment residual (WTR), BSM + Green zeolite, BSM + Flyash, BSM + Vermiculite, and BSM + Turfy were recommended for reducing and purifying stormwater runoff, especially for nitrogen pollutants (more than 63.4%). Considering the inflow/outflow pollutant load, the total nitrogen (TN) load reduction rates of retrofitted media were 10% higher than that for BSM, however, the changes of TN mass in these media were significantly different before and after the tests, and TN losses for BSM + Flyash, BSM + Turfy, BSM + Coconut husk, BSM + Medical + Turfy, and BSM + Green zeolite + Turfy were more pronounced than that for others. Under ammonization, nitrification, denitrification, or leaching, the mass of TN and total organic nitrogen (TON) in the media decreased by 0.54% to 58.3% (median = 8.1%) and 5.3% to 98.6% (median = 52.8%) during the tests, respectively. On the contrary, the mass of ammonium (NH4+-N) increased in the media from 1.2% to 5.4% (median = 2.2%). Under the impact of simulated rainfall, the activities of urease, protease, dehydrogenase, and catalase increased, however, the activities of invertase and acid phosphatase decreased. This study will contribute to a greater understanding of the runoff pollutants purification mechanism in bioretention systems.