# Multi-Level Control and Utilization of Stormwater Runoff
## 3 Results and Discussion
### The Underlying Processes Driving Runoff Pollution Load Reduction by the PBT-GR System  
Figure 4 shows the pollutant concentration of each unit of the combined PBT system. The removal of pollutants by the PBT-GR system mainly relied on the combined action of the porous pavement, bioretention system and the hydroponics green roof. The removal rates of different pollutants increased as water moved through the different units. Water quality achieved through secondary purification by the bioretention system improved significantly compared to that achieved by primary purification through the porous pavement. Compared to that achieved by the porous pavement, the bioretention system increased SS, N, P, Pb and Zn removal rates from 63.9 \(\pm\) 2.1%, 18.8 \(\pm\) 0.5%, 27.8 \(\pm\) 1.2%, 36.8 \(\pm\) 3.4%, 30.4 \(\pm\) 2.7% to 90.5 \(\pm\) 6.7%, 53.3 \(\pm\) 4.3%, 73.8 \(\pm\) 4.7%, 80.4 \(\pm\) 7.1% and 72.1 \(\pm\) 6.9%, respectively. Figure 5 shows the contributions of different components of the PBT-GR system to the overall removal of different pollutants. The removal rates of N, P and heavy metals achieved by the bioretention unit (>35%) were higher than those achieved by the porous pavement (<36.1%) and green roof (<27.1%). The green roof achieved lower removal rates compared to the bioretention unit since rainwater entering the green roof had relatively stable pollution loads. The aerobic and anaerobic environment of the green roof wetland provided an environment suitable for sustaining the growth of plants and microorganisms [30], and therefore the green roof was effective at removal of nitrogen pollutants. The removal rate of SS by the porous pavement (>63.9%) was significantly higher than those of the bioretention system (17.6%) and green roof (13.9%), respectively. This was due to the presence of unique internally connected and semi-connected cavities of the porous asphalt pavement, which formed a dense network structure. The majority of particulate pollutants were intercepted by the upper structure of the porous pavement, with only a small portion of sediment with a small sediment particle size passing through the porous asphalt pavement into the bioretention system and reservoir. This result is consistent with that obtained by [PERSON] [25] and [PERSON] [30]. Within the entire PB system, the role of the reservoir was mainly to regulate water storage, while the removal of SS through precipitation in the reservoir was limited. This was because rainwater had a short residence time in the reservoir and because the majority of particulate matter had been intercepted by the porous asphalt pavement.  
Figure 4: Removal of pollutants across the PBT-GR system. The sampling points 1 to 5 represent the porous asphalt pavement, the bioretention outlet, the storage tank and two sampling points within the hydroponics green roof, respectively. (**a**) suspended solids (SS); (**b**) ammonium nitrogen; (**c**) total nitrogen (TN); (**d**) total phosphorus (TP); (**e**) Pb; (**f**) Zn.  
Figure 5: Contributions of each unit within the PBT-GR system to the removal of pollutants.