# Multi-Level Control and Utilization of Stormwater Runoff
## 1 Introduction  
The rapid acceleration of urbanization has resulted in large areas of surface water area, grassland and other natural permeable ground being gradually replaced by impervious pavement, which has consequently resulted in a series of environment problems including deterioration of the urban water environment, frequent flood disasters and serious pollution of rainfall-runoff [1; 2; 3]. Urban non-point source pollution is one of the most important drivers of degradation of urban water quality. Roads are an important component of the urban catchment surface, and since they are always closely connected with drainage facilities, they are a key source of urban non-point source pollution. Source-based, decentralized, small-scale and low-impact development (LID) rainwater control measures have been piloted across China to effectively address the challenges presented by urban road stormwater systems. Studies in both China and abroad [4; 5; 6] show that regardless of whether centralized infiltration technology, such as infiltration pool/pond, porous pavement and rainwater gardens, or horizontal transmission technology such as grass ditches or reservoirs are used, the high accumulation of nitrogen, phosphorus and heavy metals by the long-term operation of technology alone or in combination poses serious pollution risks to surface water, groundwater and soil [7; 8]. Integration of urban stormwater management into agriculture and forestry could facilitate the transformation of problematic nitrogen, phosphorus and metal elements in road stormwater runoff intovaluable fertilizer resources. However, the agricultural and forestry industries require large areas of land, whereas urban land resources are extremely limited. Within this context, the present study proposes the concept of combining a stormwater treatment unit comprising a porous pavement combined with bioretention ponds with a stormwater utilization unit comprising a green aquatic vegetable roof, which promotes sustainable development through stormwater recycling. A hydroponic green roof (GR) is an ecological landscape garden constructed on a roof, terrace or balcony. GR is an effective technology for reducing rainfall-runoff from the source [9; 10; 11] and is usually used for rainwater collection, storage and treatment. However, traditional green roofs are associated with various disadvantages such as excessive load bearing, limited rainwater-runoff interception capacity and the need to irrigate during the dry season [12]. The improvement in current methods of stormwater storage and purification is a component of stormwater resource utilization technology. A hydroponic vegetable wetland has strong nitrogen and phosphorus removal capacity [13; 14; 15]. Water treated by a combined LID facility can provide a nutrient solution for a hydroponic green roof. The large root system of a hydroponic green roof can absorb nitrogen, phosphorus and some other water quality variables for plant growth [16]. Income can be gained from a green roof by harvesting vegetables, thereby realizing the utilization of nitrogen and phosphorus [17; 18; 19]. The use of a horizontal vegetable wetland on top of an urban building roof can not only relieve shortages of urban land, but also facilitate the purification of road rainfall-runoff with the additional economic benefits. The present study developed a system (PBT-GR) comprised of porous asphalt pavement (PAP), a bioretention system (BS), a storage tank (T) and a hydroponic green roof (GR), and the relevant parameters of segmented units were optimized. The contributions of each unit to the removal of different pollutants were determined through changes in pollutant concentrations throughout the treatment period. The present study also discusses the benefits of the system from the perspectives of water quality safety, the economic feasibility of the system and the safety of vegetables produced through the system for human consumption. The PBT-GR system integrates a green roof with a stormwater ecological treatment unit.