## 2 Materials and Methods
### Test Water  
Rainwater was collected from the roof of a building in Hainan Province, which was divided into accessible and non-accessible sections. It was collected and analyzed multiple times, with the collection time being the first hour after each rainfall. Water quality analysis was conducted in accordance with the \"Hygienic Standard for Drinking Water\" of China (GB 5749-2022) [20]. The collected rainwater quality parameters are shown in Table 3.  
Analysis revealed that the indicators for turbidity, NH3-N, TOC, and Cr(VI) levels in the water exceeded standard limits. Theoretically, due to the influence of both human and natural factors, the total number of colonies and Escherichia coli in rainwater should have exceeded standard levels. However, possibly due to frequent rainfall at the test site, good air quality, and sampling within one hour after the rain began, the accumulation of pollutants on the roof surface was relatively minimal. Additionally, test results for heavy metals and toxic metals indicated that all indicators, except Cr(VI), met the standards for \begin{table}
\begin{tabular}{c c c c} \hline
**Filter Media Characteristics** & **Quartz Sand** & **Volcanic Rock** & **Coconut Shell Activated Carbon** \\ \hline Particle size & 0.5–1 mm & 0.5–1 mm & 0.5–1 mm \\ Specific surface area & - & 7.0 m\({}^{2}\)/g & 712.1 m\({}^{2}\)/g \\ Average aperture & - & 6.06 nm & 3.45 nm \\ Iodine adsorption value & - & - & 700 mg/g \\ \hline \end{tabular}
\end{table} 
Table 1: Characteristics of filter media. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Element wt\%** & **O** & **Si** & **C** & **Ca** & **Fe** & **Mg** \\ \hline Quartz sand & 23.06 & 76.94 & - & - & - & - \\ Volcanic rock & 36.68 & 21.81 & - & 9.34 & 27.28 & 4.89 \\ Coconut shell activated carbon & 7.52 & 0.05 & 85.3 & 4.41 & 1.1 & 1.62 \\ \hline \end{tabular}
\end{table} 
Table 2: The main elements of the filter material. To ensure the stability of the daily test water and the accuracy of the test data, synthetic water was used as the test water. The concentration of the test water is shown in Table 4. Using tap water that had been exposed to sunlight for a day as the base, we added the following reagents in the stated proportions:  
1. analytical-grade pure ammonium chloride (NH4 Cl) to achieve an NH3-N concentration of 1.2-1.4 mg/L (calculated as N);
2. sieved (<0.1 mm) air-dried soil to achieve a turbidity of 14-15 NTU;
3. analytical-grade potassium dichromate (K2 Cr2O7) solution to achieve a Cr(VI) concentration of 0.05-0.06 mg/L;
4. analytical-grade humic acid to achieve a TOC concentration of 15.0-16.0 mg/L. A magnetic stirrer was used to stir the mixture for 10 min every 2 h, taking the supernatant as the test water. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Water Quality** & **Unit** & **Rainwater** & **Test Water** \\
**Indicator** & & **Concentration** & **Concentration** \\ \hline Turbidity & NTU & 9.79–15.09 & 14–15 \\ NH3-N & mg/L & 0.8–1.46 & 1.2–1.4 \\ TOC & mg/L & 15.21–16.04 & 15.0–16.0 \\ Cr(VI) & mg/L & 0.052–0.067 & 0.05–0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Quality indicators of the collected rainwater and experimental water.