## 1 Introduction
The EPS matrix plays a crucial role in heavy metal adsorption due to its high content of negatively charged functional groups. Carboxyl, hydroxyl, and phosphate groups are key to binding positively charged metal ions such as copper (Cu\({}^{2+}\)), nickel (Ni\({}^{2+}\)), and lead (Pb\({}^{2+}\)). These interactions occur primarily through ion exchange, where the metal ions replace cations like calcium (Ca\({}^{2+}\)) or magnesium (Mg\({}^{2+}\)) naturally present in the EPS [8]. In addition, complexation mechanisms also occur, where metal ions form stable complexes with the functional groups in the EPS, further enhancing the ability of the biofilm to immobilize and retain heavy metals [8]. Microorganisms within the biofilm metabolize organic pollutants, converting them into harmless byproducts, effectively removing dissolved organic carbon, nitrates, and phosphates from the water [7; 8; 9; 10]. As the biofilm matures, its filtration efficiency increases due to a more complex and diverse microbial community, enhancing its ability to capture and degrade a broader range of contaminants [9; 10; 11]. However, the formation of biofilms also leads to a significant decrease in filtration rate and, over time, the system may become clogged. To restore the medium and maintain system activity, the top layer of the biofilm can be scraped off, allowing a new biofilm layer to form [10]. Research shows that filtration quality is expected to be 30-70% removal of pathogens before the biofilm layer forms, and with biofilm formation, filtration quality rises to 99% [7; 8; 9; 11]. In addition, a longitudinal field study in Kenya highlighted the efficiency of SSFs in reducing pathogen levels and improving water clarity. The study underscored the method's suitability for rural environments, where water sources are often contaminated with various pollutants [12]. Another evaluated technology is sedimentation following chemical coagulation using local coagulants. During coagulation, chemicals such as alum, ferric chloride, or poly-aluminum chloride are added to water to destabilize suspended particles, causing them to aggregate into larger flocs. These flocs, formed through the bridging and charge neutralization mechanisms, are subjected to gravity-driven sedimentation. During sedimentation, the denser flocs settle to the bottom, separating from the clear water above. This process effectively reduces turbidity and removes many contaminants, including suspended solids, microorganisms, and organic matter [13; 14]. Local materials found in the remote villages, such as chalk for boards (calcium carbonate) and limestone (calcium hydroxide), have been used as coagulant aids. Adding these local materials impacts precipitation by several mechanisms, including charge neutralization by calcium ions (Ca\({}^{2+}\)), which can neutralize the negative charges on the surfaces of suspended particles, reducing the repulsive forces between them and promoting aggregation of particles into larger flocs. In addition, calcium ions also bridge negatively charged particles, aiding in the formation of larger, more stable flocs. Finally, Calcium can form insoluble precipitates with various anions present in water, such as carbonate or phosphate. These precipitates can incorporate suspended particles, enhancing their removal during sedimentation [15]. While using local coagulants can be highly effective, efficiency may vary depending on the specific water chemistry and the concentration of suspended particles. Moreover, a reliance on local materials can significantly reduce costs and environmental impact, as it eliminates the need for transporting and handling conventional chemical coagulants. However, further research is needed to optimize the use of these local coagulants, ensuring consistent performance across different water sources and varying conditions. Additionally, the potential for local coagulants to introduce unwanted byproducts or influence the pH balance of treated water must be carefully considered to maintain water quality and safety standards.