# Multi-Level Control and Utilization of Stormwater Runoff
## 4 Conclusions  
The novel PBT-GR system proposed in the current study shows promise for the collection, treatment and reuse of road rainwater runoff. Consequently, we can reach the following conclusions:  
1. The hydraulic load of the green roof should be 0.3 m\({}^{3}\).(m\({}^{2}\).d)\({}^{-1}\) to maximize the utilization of nitrogen and phosphorus resources. Under a typical rainfall, the initial interception of road rainfall-runoff by the porous pavement and the secondary purification by the bioretention system can significantly improve water quality. The removal rates of SS, N, P, Pb and Zn by the porous pavement of 63.9%, 18.8%, 27.8%, 36.8% and 30.4%, respectively, improved to 90.5%, 53.3%, 73.8%, 80.4% and 72.1%, respectively, after treatment by the bioretention system.
2. Removal of N, P and heavy metals was significantly higher in the bioretention unit compared to the porous pavement and green roof units (\(p<0.05\)), whereas the porous asphalt pavement achieved significantly higher SS removal compared to the bioretention and green roof units (\(p<0.05\)). The PBT-GR system significantly reduced the pollution level of road rainfall-runoff, with the concentrations of SS, N, P and heavy metals in the effluent of the system as 10.1 mg/L, 1.2 mg/L, 0.11 mg/L, 0.08 mg/L and 0.1 mg/L, respectively, within the class V surface water standards in China. The heavy metal contents of water spinach planting using the effluent of the PBT-GR system met the national standard.
3. The estimation of investment required for the PBT-GR system indicated that the investment cost can be recovered within 3 years, thereby indicating the good economic viability of the system. The current study showed that the PBT-GR system was able to effectively treat and utilize road rainfall-runoff and that the system has the advantages of low operation cost, a small volume and density and is environmentally friendly. Therefore, the system is likely to be widely used.
4. However, use of the PBT-GR system should consider the source of irrigation rainwater during a drought period, the appropriate selection of vegetables to be planting hydroponically, the specific mechanisms of degradation of pollutants and the physiological and ecological changes experienced by vegetables planting using this system.  
Figure 8: Concentrations of various pollutants in the effluent of a porous asphalt pavement-bioretention storage tank green roof (PBT-GR) system that was used to treat domestic grey water (n = 6).  
**Author Contributions:** Conceptualization, H.L. and Y.Z.; methodology, H.L.; software, B.H.; validation, M.S.; formal analysis, B.C.; investigation, M.Y.; resources, H.L.; data curation, H.L.; writing--original draft preparation, H.L.; visualization, B.H., Y.Z. and Y.Z.; supervision, B.C.; project administration, M.Y.; funding acquisition, W.Z. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was supported by the National Science and Technology Support Program (2015 BAL02B04); the Technology Project of China Housing and Urban Rural Development Ministry (2015-K7-012); a Project Funded by the National First-class Disciplines (PNFD), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The datasets used during the current study are available from the corresponding author on reasonable request.  
**Acknowledgments:** The authors would like to thank [PERSON] and [PERSON] for their assistance in the laboratory experiment.  
**Conflicts of Interest:** The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.