# Multi-Level Control and Utilization of Stormwater Runoff
## 3 Results and Discussion
### Effect of Rainfall Intensity on Pollutant Removal by the Porous Parement Bioretention (PB) System  
Figure 3 shows the effectiveness of the PB system in removing pollutants from stormwater runoff under three rainfall intensities. There was a gradual increase in the concentrations of TN, TP, Pb and Zn with an increasing duration of rainfall, but not for SS. The results showed that the SS removal rate within the PB system decreased with increasing rainfall intensity. The greatest SS removal rate was achieved at a rainfall intensity of 1a. The concentrations of SS in the effluents of the porous asphalt pavement and bioretention systems were 35.8 mg/L and 21.1 mg/L, respectively. However, an increase in rainfall intensity to 5a resulted in the SS removal rate from the effluent of porous asphalt pavement and bioretention tank systems decreasing by 22.5% and 16.1%, respectively. This result was consistent with research by [PERSON] [25] and [PERSON] [26]. The removal of SS by the porous asphalt pavement and bioretention pond mainly relies on interspace interception and surface adsorption. Due to the high rainfall intensity and obvious scour effect under heavy rainfall, rainwater runoff will rapidly flush SS through the porous asphalt pavement and bioretention ponds. The concentration of NH\({}_{4}\)\({}^{+}\)-N increased with increasing rainfall duration, thereby showing an opposite trend to that of SS. The NH\({}_{4}\)\({}^{+}\)-N removal rate by the PB system decreased by 10-13% during the later period compared with that in the earlier stage of rainfall. The NH\({}_{4}\)\({}^{+}\)-N removal rate gradually decreased with increasing rainfall intensity. The NH\({}_{4}\)\({}^{+}\)-N concentration of the effluent of the PB system was 1.18 mg/L when the rainfall intensity was 1a, falling within the class IV standard for surface water in China. An increase in the rainfall intensity to 5a resulted in the NH\({}_{4}\)\({}^{+}\)-N removal rate of the PB system decreasing by 15.8%, with the effluent NH\({}_{4}\)\({}^{+}\)-N concentration reaching 1.68 mg/L. The change in TN was similar to that of NH\({}_{4}\)\({}^{+}\)-N, although the NH\({}_{4}\)\({}^{+}\)-N removal rate in the PB system reached >30% under a rainfall intensity of 1a, the TN removal rate reached a maximum of only 15.3%. This result showed that there was a change in nitrogen forms in the PB system without overall removal, while the bioretention unit greatly improved the TN removal rate. The inclusion of a bioretention unit can increase the TN removal rate by 27.1-32.9% compared to use of a single porous asphalt pavement unit, mainly due to its strong water retention and adsorption capacity [27]. In addition, the presence of an anoxic region in the lower part of the bioretention unit may facilitate denitrification to remove nitrate nitrogen, thereby improving the TN removal rate. These results are  
consistent with those of [PERSON] et al. [9]. Some differences were evident compared to those by [PERSON] et al. [28] mainly due to the high voidage of the porous asphalt pavement in the present study and the low pollutant concentrations of artificial rainwater [27]. The TN concentrations of the effluent in the PB system under a rainfall intensity of 1a-5a were 3.2 mg/L-3.8 mg/L, thereby exceeding the class V surface water limit level in China. The direct discharge of effluent with this TN concentration range would have serious effects on the ecological environment. Removal of nitrogen from the effluent could provide fertilizer resources needed for agriculture. The TP removal efficiencies under different rainfall intensities remained low, ranging between 65.3-74.2%, with the treated water TP concentration ranging between 0.2 mg/L-0.28 mg/L, falling within the class IV surface water standard. Adsorption and chemical precipitation may occur on the surfaces of different media [29].