# Multi-Level Control and Utilization of Stormwater Runoff
## 3 Results and Discussion  
### The Effect of Hydraulic Loading Rate (HLR) on Pollutant Removal by the Hydroponic Green Roof  
As shown in Figure 2a, the TN removal rate generally decreased with increasing _HLR_. The TN removal rate was highest at an _HLR_ of 0.1 m\({}^{3}\).(m\({}^{2}\cdot\)d)\({}^{-1}\), reaching 63.2%, and was lowest (only 31.7%) when _HLR_ increased to 0.4 m\({}^{3}\).(m\({}^{2}\cdot\)d)\({}^{-1}\). The reason for this observation was that the increase in _HLR_ resulted in a decrease in the hydraulic retention time, leading to a short contact time between the substrate hosting microorganisms and pollutants and consequently reducing the treatment effectiveness. As _HLR_ increased, the removal load first increased and then decreased. The removal load was lowest at an _HLR_ of 0.1 m\({}^{3}\).(m\({}^{2}\cdot\)d)\({}^{-1}\) at 0.054 g.(m\({}^{2}\cdot\)d)\({}^{-1}\). The removal load reached a maximum when _HLR_ was 0.3 m\({}^{3}\).(m\({}^{2}\cdot\)d)\({}^{-1}\) at 0.135 g.(m\({}^{2}\cdot\)d)\({}^{-1}\). The removal load reduced to 0.011 g.(m\({}^{2}\cdot\)d)\({}^{-1}\) when _HLR_ increased to 0.4 m\({}^{3}\).(m\({}^{2}\cdot\)d)\({}^{-1}\). These results indicate that increasing _HLR_ can increase the removal load within a certain range. However, when the _HLR_ exceeds a certain value, dissolved pollutants in rainwater cannot be effectively degraded due to excessive water flow velocity. The changes observed in the TP removal rate were similar to that of the TN removal rate (Figure 2b). The TP removal rate first increased and then decreased with an increase in _HLR_ (\(p<0.1\)). The removal load reached a maximum of 0.024 g.(m\({}^{2}\cdot\)d)\({}^{-1}\) when _HLR_ was 0.3 m\({}^{3}\).(m\({}^{2}\cdot\)d)\({}^{-1}\).  
Figure 2: The influence of hydraulic loading on pollutant removal (mean \(\pm\) standard deviation). (**a**–**d**) are TN, TP, Zn and Pb respectively.  
The metal removal rate reached a maximum above 65% when _HLR_ was 0.1 m\({}^{3}\cdot\)(m\({}^{2}\cdot\)d)\({}^{-1}\), following which the metal removal rate gradually decreased. The metal removal rate was lowest below 50% when _HLR_ was 0.4 m\({}^{3}\cdot\)(m\({}^{2}\cdot\)d)\({}^{-1}\). These results indicate that a low _HLR_ was conducive to the adsorption of heavy metal ions on the substrate surface. Pb\({}^{2+}\) and Zn\({}^{2+}\) in runoff were precipitated as Pb(OH)\({}_{2}\) and Zn(OH)\({}_{2}\) and adsorbed onto the substrate surface. They may also have reacted with CO\({}_{3}\)\({}^{2-}\) to form heavy metal carbonate precipitation complexes [24].  
\[\text{Pb}^{2+}+\text{CO}_{3}^{-2}\Leftrightarrow\text{Pb}(\text{CO}_{3})_{2} \tag{6}\]  
\[\text{Zn}+\text{CO}_{3}^{-2}\Leftrightarrow\text{Zn}(\text{CO}_{3})_{2} \tag{7}\]  
The Pb removal rate gradually increased with increasing _HLR_ (_p_ < 0.1), with the maximum load of Pb removed being 0.012 g-(m\({}^{2}\cdot\)d)\({}^{-1}\). The load of Zn removed with the change in _HLR_ was similar to that of TN and TP, showing first an increase followed by a decrease. The Zn removal rate reached a maximum when the _HLR_ was 0.3 m\({}^{3}\cdot\)(m\({}^{2}\cdot\)d)\({}^{-1}\) at 0.028 g-(m\({}^{2}\cdot\)d)\({}^{-1}\). As such, nutrients should be removed in as short a time as possible to achieve full utilization of nitrogen and phosphorus nutrients. The test results showed that the maximum loads of TN, TP and Zn were reached when _HLR_ was 0.3 m\({}^{3}\cdot\)(m\({}^{2}\cdot\)d)\({}^{-1}\). Heavy metals were effectively treated in the initial runoff. These results indicated that the _HLR_ of a hydroponics green roof should be 0.3 m\({}^{3}\cdot\)(m\({}^{2}\cdot\)d)\({}^{-1}\) to ensure the maximum utilization of nitrogen and phosphorus resources.