## 2 Materials and Methods
### Experimental Design  
After planting the _Ophiopogon japonicus_, the plants were watered every three days. The concentrations of TP and TN were 0.062 \(\pm\) 0.021 mg/L and 0.87 \(\pm\) 0.26 mg/L, respectively. After two months of cultivation as the plants gradually grew, the bioretention system was tested and monitored in July 2018. The removal efficiency of pollutants was investigated based on rainfall intensity, sunny weather, and concentration. Based on the empirical model of rainfall intensity for Nanjing (Jiangsu Province, China) in Equation (1), we simulated the rainfall patterns using the Chicago Storm Theory, as provided by Equations (2) and (3) [30]. The rainfall duration (\(t\)) was assumed to be 360 min with rain peak using a rain peak coefficient of 0.4. Three different recurrence intervals in years (1a, 5a, 10a) were used, as presented in Table 3 and Figure 2.  
\[i=\frac{64.3+53.8\times\log P}{(t+32.9)^{1.001}} \tag{1}\] \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Media Layer**} & \multicolumn{3}{c}{**Filling Format**} & \multirow{2}{*}{**Height**} \\ \cline{2-2} \cline{4-4}  & **Control** & & & \\ \hline Water-retention & & & \\ layer & & Local soil (20\%) mixed with sand (80\%) & 10 cm \\  & & Zeolite: Volcanic rock: Sponge iron = & Zeolite: Volcanic rock: Sponge & \\ Adsorption layer & 1:1:1 (volume ratio), layered filling, & iron = 1:1:1 (volume ratio), mixed & 60 cm \\  & & grain size of 1–2 mm & filling, grain size of 1–2 mm & \\ Filter layer & & A non-woven permeable geotextile membrane & / \\ Catchment layer & Crushed limestone gravel (grain size of 5–10 mm, void space of 34\%) & 5 cm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Filling method for media layer in the infiltration column. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Project**} & \multicolumn{5}{c}{**Components**} \\ \cline{2-7}  & **TP (mg/L)** & \
\begin{tabular}{c} **NH\({}_{4}\)\({}^{+}\)-N** \\ **(mg/L)** \\ \end{tabular} &
\begin{tabular}{c} **NO\({}_{3}\)\({}^{-}\)N** \\ **(mg/L)** \\ \end{tabular} & **COD (mg/L)** & **pH** & **Temp (\({}^{\circ}\)C)** \\ \hline Typical concentration & \(1.2\pm 0.1\) & \(3.0\pm 0.3\) & \(1.5\pm 0.1\) & \(100.0\pm 3.0\) & \(7.0\pm 0.3\) & \(22.0\pm 2.0\) \\ High concentration & \(2.4\pm 0.1\) & \(6.0\pm 0.3\) & \(3.0\pm 0.4\) & \(200.0\pm 6.0\) & \(7.0\pm 0.3\) & \(22.0\pm 2.0\) \\ Source & \(\text{KH}_{2}\text{PO}_{4}\) & NH\({}_{4}\)Cl & KNO\({}_{3}\) & NaNO\({}_{2}\) & NaOH/HCl & / \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Two types of concentrations for synthetic runoff used in this study. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Recurrence**} & **Percent Dance of** & **Mean Rainfall** & **Rain Intensity of** & **Rainfall in Six** \\
**Interval (Year)** & **Occurrence in Any Year** & **Intensity (mm/min)** & **Six Hours (L/s-ha)** & **Hours (mm)** \\ \hline
1a & 100 & 0.1532 & 25.5436 & 55.1689 \\
5a & 20 & 0.2429 & 40.4865 & 87.4333 \\
10a & 10 & 0.2815 & 46.9206 & 101.3289 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Six-hour rainfall in different return periods. Before Peak:  
\[i(t_{b})=\frac{A[\frac{(1-n)t_{b}}{r}]+b}{\left[\frac{b}{r}+b\right]^{n+1}} \tag{2}\]  
After Peak:  
\[i(t_{a})=\frac{A[\frac{(1-n)t_{a}}{1-r}+b]}{\left[\frac{b}{1-r}+b\right]^{n+1}} \tag{3}\]  
where \(i\) (mm/min) is the rainfall intensity, \(t\) (min) is the rainfall duration, \(p\) is the recurrence interval, and \(A\), \(b\), and \(n\) are the constants dependent on the units employed and the recurrence interval of the storm, \(r\) is the coefficient of rain peak. Next, we calculated the rainwater runoffs associated with different recurrence intervals, based on the drainage criterion (GB50014-2006) in China. The rainwater design flow rate was calculated using Equation (4).