Removal of nitrogen from rainwater runoff by bioretention cells filled with modified collapsible loess

The construction of bioretention facilities in collapsible loess areas faces some problems such as low infiltration rate of rainwater runoff and system blockage, which seriously affect the treatment effect of bioretention cell on rainwater runoff. According to the different filler matrix, three bioretention cells with improved filler were constructed, the 1# bioretention system was filled with 50% of the construction waste mixed with 50% loess, and the 2# bioretention system was filled with 50% concrete sand mixed with 50% loess, the 3# bioretention system was filled with 45% concrete sand +10% wood chips +45% loess mixed in the bioretention cell. The results show that the removal rate of NH₄⁺-N is the highest when the height of the inundation zone of the bioretention cell is 600 mm (more than 98%). The change of hydraulic load and submerged zone height had a significant effect on the removal rate of NO₃⁻-N in bioretention cell (P < 0.05). The removal rate of NO₃⁻-N in the bioretention cell decreased with the increase of hydraulic load, and the bioretention cell had the highest removal rate of NO₃⁻-N (48.85%) under low hydraulic load conditions. The removal rate of NO₃⁻-N in the bioretention cell increased with the increase of the height of the inundation area, and the removal rate of NO₃⁻-N was the highest when the inundation area height of the bioretention cell was set at 600 mm (98.33%). The improved bioretention cell has a better effect on NH₄⁺-N treatment during the wet period, and the NH₄⁺-N concentration of the effluent is below 0.4 mg/L. The NO₃⁻-N concentration in the effluent of the three improved bioretention cells during the wet period differed greatly. Among them, the bioretention cells using sawdust modified fillers had the highest removal rate of NO₃⁻-N. The collapse amount of the three improved bioretention cells was below 0.5 cm, and the permeability coefficients were above 2 cm/h. The experiment proves that the three methods to improve the collapsible loess filler can be applied to the construction of the bioretention cell in the collapsible loess area so that the bioretention technology can be effectively promoted in the loess area. This experiment has great significance to the construction of bioretention cells in the loess area.