## 3 Results and Discussion  
### The Removal Effect of a Single Filter Material  
3.1.1. The Turbidity Removal Effect  
Figure 3 illustrates the turbidity removal effect of a single filter material over 36 days. In the early stage of operation, biological growth in the system is minimal, and turbidity removal primarily relies on the filtration and retention capabilities of the filter material [22]. The removal effects of volcanic rock (91.46 \(\pm\) 0.99%) and coconut shell activated carbon (86.47 \(\pm\) 0.36%) on turbidity were much higher than those of quartz sand (83.87 \(\pm\) 1.11%). The removal rate was analyzed using a one-way analysis of variance (ANOVA) and [PERSON]'s multiple comparison test. The results showed that \(p\) < 0.05. This indicates that in the initial stage of operation, the characteristics of the filter material are crucial for achieving the turbidity removal effect. Meanwhile, the average removal rate of volcanic rock is 4.99 percentage points higher than that of coconut shell activated carbon. The porous structure of volcanic rock enables it to retain suspended particles more effectively, thereby enhancing removal efficiency. In contrast, the microporous structure of coconut shell activated carbon can lead to clogging when retaining larger suspended particles, negatively impacting its removal rate [23]. Consequently, during the early stage of operation, volcanic rock outperformed coconut shell activated carbon in turbidity removal. As the system's operation time increases, the growth of microorganisms on the surface of the filter material gradually increases, and a dense biofilm is gradually formed on the surface layer of the filter material. Through the flocculation and adsorption effect of the biofilm, suspended particles are efficiently removed, and the removal rate is continuously improved [22]. After 26 days of system operation, it was observed that the turbidity removal rates of the three filter materials all reached a stable state. Among them, the turbidity removal rates of volcanic rock (97.36 \(\pm\) 0.08%) and coconut shell activated carbon (94.93 \(\pm\) 0.26%) were significantly (_p_ < 0.05) higher than that of quartz sand (93.28 \(\pm\) 0.25%). Among them, the average removal rate of volcanic rock was the highest, 2.43 percentage points higher than that of coconut shell activated carbon. Previous studies have indicated that the presence of calcium can enhance the coagulation effect of microorganisms, significantly increasing biofilm density and improving the retention capacity for suspended particles [24]. Elemental analysis of the filter materials revealed that volcanic rock contains a much greater amount of Ca than coconut shell activated carbon. Therefore, during the stable period, the removal effect of volcanic rock on turbidity is better than that of coconut shell activated carbon. Figure 3: The turbidity removal effect of single filter material. 3.1.2. The NH\({}_{3}\)-N Removal Effect  
Figure 4 presents the NH\({}_{3}\)-N removal effect of a single filter material over 36 days. In the early stage of operation, biological growth in the system is limited, and the removal of NH\({}_{3}\)-N mainly relies on the adsorption and interception capabilities of the filter material [22]. The removal effects of volcanic stone (80.10 \(\pm\) 2.63%) and coconut shell activated carbon (86.82 \(\pm\) 2.53%) on NH\({}_{3}\)-N were significantly better than those of quartz sand (69.07 \(\pm\) 1.43%) (_p_ < 0.05), indicating that in the initial stage of operation, the characteristics of the filter material are crucial for the removal of NH\({}_{3}\)-N. In addition, the average removal rate of coconut shell activated carbon is 6.72 percentage points higher than that of volcanic rock. The adsorption and retention of NH\({}_{3}\)-N by filter materials depend mainly on physical adsorption, supplemented by ion exchange [25].