## 3 Results
### Slow Sand Filtration
#### 3.2.2 Optimal Ratio between Sand and Gravel  
The experiment measures and compares the turbidity values of three water filtration systems (the composition of the materials inside the filter is described in the methods chapter, part 2.2). Each system starts with a turbidity of 12,000 NTU at the beginning of each day (Figure 6a,b).  
Figure 5: (a) Turbidity values over three days—proof of concept system (the value shown each day is the lowest recorded for that day); (b) water quality parameters comparison after filtration.  
The results show significant reductions in turbidity across all three systems, with varying degrees of effectiveness. System 1 shows a rapid and substantial decrease in turbidity values, quickly dropping from 12,000 NTU to much lower levels within a few repetitions, indicating high filtration efficiency. System 2 also demonstrates a sharp reduction in turbidity, effectively lowering the values from 12,000 NTU to minimal levels, though with slight variations. System 3 is the most efficient, achieving the greatest reduction in turbidity (Figure S3). It consistently reduces the turbidity from 12,000 NTU to much lower levels, demonstrating superior filtration performance. Due to its superior performance, the gravel-sand ratio from System 3 was adopted for subsequent systems. Further tests were conducted to evaluate its water purification capabilities in the following days (Figure 7).  
Figure 6: (**a**) Comparison of turbidity values after one day (the value shown for each system is the system’s lowest recorded); (**b**) comparison of turbidity removal efficiency after one day.  
The data reveal a significant reduction in water turbidity after the filtration process, with initial turbidity levels at 12,000 NTU. On Day 1, turbidity decreased markedly to 25 NTU, followed by a slight increase to 35.50 NTU on Day 2. By Day 3, the turbidity value further decreased to 5.15 NTU. These results underscore the system's effectiveness in significantly reducing water turbidity over time, with a clear improvement in water quality observed across the testing period.  
The increase in turbidity values observed on Day 2 using System 3, compared to Day 1, is primarily due to biofilm development. As the biofilm matures, it may cause temporary detachment of particles, resulting in higher turbidity readings. Research shows that initial biofilm growth can disrupt the stability of trapped particles in filtration systems, leading to this phenomenon [7,8].  
The superior performance of System 3, which had a higher sand-to-gravel ratio, can be attributed to several key factors. First, the increased ratio likely reduced hydraulic conductivity, slowing down the water flow through the filter. This, in turn, increased contact time, which is crucial for both mechanical filtration and biological processes, such as biofilm development [6,7]. A longer contact time enables the biofilm to more effectively metabolize contaminants and capture fine particles [7,8]. Additionally, a higher sand content provides a greater surface area for biofilm formation, which is essential for biological filtration efficiency [7]. However, there may be a trade-off between achieving optimal filtration and maintaining system longevity. For example, while a higher sand-to-gravel ratio improves contaminant removal, it can also increase the likelihood of clogging due to biofilm accumulation, which could reduce the filter's lifespan [7,9]. Thus, while System 3 demonstrated higher efficiency, regular maintenance, such as scraping off the top layer of the biofilm, would be necessary to prevent clogging and ensure long-term functionality [7,10].