## 3 Results and Discussion
### The Removal Effect of a Single Filter Material
Coconut shell activated carbon's superior adsorption and retention of NH\({}_{3}\)-N can be attributed to its high specific surface area and the abundance of oxygen-containing functional groups on its surface. Although volcanic rock has a smaller specific surface area, its high concentrations of iron, magnesium, and calcium enable it to engage in ion exchange reactions with NH\({}_{4}\)\({}^{+}\) in solution, resulting in slightly weaker adsorption and retention compared to coconut shell activated carbon. As the system's operation time increased, the bioadsorption and degradation effects of the biofilm intensified. NH\({}_{3}\)-N removal occurred through the nitrification reactions of nitrifying bacteria, leading to an upward trend in removal rates [26]. On the 26 th day of operation, the NH\({}_{3}\)-N removal rates for all three single filter materials stabilized. Coconut shell activated carbon maintained the highest NH\({}_{3}\)-N removal rate (98.90 \(\pm\) 0.42%), followed closely by volcanic rock (98.77 \(\pm\) 0.99%) and quartz sand (97.55 \(\pm\) 1.25%). Previous studies have indicated that elements such as iron and Mg can promote the growth and reproduction of microorganisms, accelerating the synthesis of enzymes related to autotrophic bacteria and thereby enhancing the removal capacity of nitrifying bacteria [27; 28]. Elemental analysis of the filter materials revealed that volcanic rock contains significantly higher levels of calcium than coconut shell activated carbon. Theoretically, volcanic rock should perform better in removing NH\({}_{3}\)-N. However, the lower concentration of NH\({}_{3}\)-N in the water may account for the similar removal rates of both materials, each approaching 100%. 3.1.3. The TOC Removal Effect  
Figure 5 illustrates the removal effect of a single filter material on TOC over 36 days. In the initial operation stage, due to limited biological growth, TOC removal primarily relies on the adsorption and retention capabilities of the filter material [22]. The TOC removal rates  
Figure 4: The removal effect of a single filter material on NH\({}_{3}\)-N.  
of coconut shell activated carbon (51.80 \(\pm\) 0.21%) and volcanic rock (43.34 \(\pm\) 1.25%) were both significantly (\(p\) < 0.05) higher than those of quartz sand (29.60 \(\pm\) 0.32%). Among them, the average removal rate of coconut shell activated carbon was the highest, 8.46 percentage points higher than that of volcanic rock. The adsorption and retention of TOC by filter materials are primarily achieved through electrostatic and non-electrostatic effects [28]. Coconut shell activated carbon's superior adsorption and retention capabilities stem from its high specific surface area. In contrast, volcanic rock's smaller specific surface area results in a relatively weaker adsorption and retention of TOC. With the extension of the system operation time, the bioadsorption of the biofilm and the degradation of microorganisms gradually increase, and the TOC removal rate continues to rise. After 26 days of operation, the TOC removal rates of all three filter materials stabilized. At this time, the TOC removal rates of volcanic rock (70.55 \(\pm\) 0.42%) and coconut shell activated carbon (68.60 \(\pm\) 0.42%) were both significantly (\(p\) < 0.05) higher than that of quartz sand (49.62 \(\pm\) 0.70%). Among them, the average removal rate of volcanic rock was the highest, 1.95 percentage points higher than that of coconut shell activated carbon. Previous studies have indicated that elements such as calcium, iron, and magnesium can enhance biofilm density, promote microorganism growth and reproduction, and accelerate the synthesis of related enzymes, thereby improving the adsorption and degradation capacity of biofilms [24; 27; 29]. Elemental analysis of the filter materials revealed that volcanic rock contains significantly higher levels of calcium, iron, and magnesium than does coconut shell activated carbon.