## 2 Materials and Methods
### Test Scheme  
During the early operation stage of the BSF reactor, there is less biological growth on the surface of the filter material. The pollutants in the water are mainly removed through the physical adsorption and chemical reactions of the filter material itself [9]. As pollutants are continuously adsorbed, a substantial amount of nutrients is provided for microorganisms, leading to the gradual formation of a biofilm on the filter material's surface. The efficiency of pollutant removal is enhanced through the adsorption and degradation of pollutants by microorganisms in the biofilm [10]. As the operation time of the BSF reactor increases, the bioadsorption and degradation effects of microorganisms gradually dominate [12]. This paper analyzes the decontamination performance of the filter material in two stages: the early operation stage of the BSF reactor (0-6 days, with the first sampling time being 48 h) and the stable removal rate (after 26 days). The factors influencing the decontamination effect of the BSF reactor mainly include filter material type, hydraulic load, temperature, oxygen, filtration rate, and filter layer thickness [11]. This paper primarily discusses the influence of filter material types and mixed filter materials on the decontamination effect, aiming to eliminate the interference of other factors on the test results. Under conditions of the same influent water quality, a constant temperature of 24 \({}^{\circ}\)C, and a filtration rate of 0.2 m/h, the decontamination effect \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Water Quality** & **Detection Method** & **Detection Instrument** & **Manufacturer** \\ \hline Turbidity & Spectrophotometry & AE86065 & Dongguan Frank Technology \\  & & turbidity meter & Co., Ltd. (Guangdong, China) \\ TOC & Spectral water quality detection & Water Detective & Shenzhen Bit Atom Technology \\  & & Type 3 & Co., Ltd. (Shenzhen, China) \\ NH\({}_{3}\)-N & Nessler’s reagent & PT-001B & \\  & spectrophotometry & multi-parameter & Xiamen Pantian Biotechnology \\ Cr(VI) & Diphenylcarbazide & water quality detector & Co., Ltd. (Xiamen, China) \\  & spectrophotometric method & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Detection methods and instruments. of the BSF reactor was tested simultaneously. The pollutant indicators detected included turbidity, NH\({}_{3}\)-N, TOC, and Cr(VI). In this study, the height of the filter-media layer in the BSF reactor was 0.9 m, and the filtration rate was 0.2 m/h. Therefore, the hydraulic retention time (HRT) was calculated as follows: HRT = Height of the filter-media layer/Filtration rate = 0.9 m \(\div\) 0.2 m/h = 4.5 h.  
The pollutant loading rate was calculated based on the concentration of the test water and the filtration rate, as follows:  
(1) Turbidity loading rate: 14-15 NTU \(\times\) 0.2 m/h = 2.8-3.0 NTU\(\cdot\)m/h  
(2) NH\({}_{3}\)-N loading rate: 1.2-1.4 mg/L \(\times\) 0.2 m/h = 0.24-0.28 mg/(L\(\cdot\)h)  
(3) TOC loading rate: 15.0-16.0 mg/L \(\times\) 0.2 m/h = 3.0-3.2 mg/(L\(\cdot\)h)  
(4) Cr(VI) loading rate: 0.05-0.06 mg/L \(\times\) 0.2 m/h = 0.01-0.012 mg/(L\(\cdot\)h)  
The mixed filter material was prepared using the uniform mixing method: volcanic rock and coconut shell activated carbon were thoroughly stirred in a container in a mass ratio (e.g., 1:3) and then filled layer by layer into the filter material layer of the BSF reactor to ensure uniform particle distribution. Among them, 1\({}^{\#}\) served as the control group with quartz sand as the filter material; 2\({}^{\#}\), 3\({}^{\#}\), 4\({}^{\#}\), 5\({}^{\#}\), and 6\({}^{\#}\) were the experimental groups, employing single and mixed filter media. The parameters of the filter media are shown in Table 6. The single filter media reactor is depicted in Figure 2a, with coconut shell activated carbon, volcanic rock, and quartz sand arranged from left to right. The mixed filter media reactor is shown in Figure 2b, with the mixing ratios from left to right being 1:3, 1:1, and 3:1, respectively.