Ndiebene Gandiol is a commune in Senegal located 20 kilometers from the city of Saint-Louis, near the mouth of the Senegal River. Geographically, Ndiébène lies at the heart of the historic Gandiol region. Since 2014, with the adoption of Act 3 of decentralization, Ndiébène Gandiol has become the capital of the Gandiol commune. Within the Gandiol commune, several schools were explored. However, our study will primarily focus on the Gandiol School, which is considered for implementing an ecological wastewater treatment system. The location and size of the establishment are illustrated in Figure 1. The school accommodates 505 students, aged between 7 and 14 years, who are distributed across 12 classrooms. Despite this substantial number, the school has only 4 latrines for students and one reserved for teachers. Unfortunately, none of these

latrines are operational. Moreover, the school lacks any water points, being devoid of taps. Its sanitation system is considered non-compliant. Finally, the condition of the toilets is concerning: they are in poor condition, frequently obstructed, and thus unusable for students and teachers most of the time.

The configuration of the horizontally flowing planted filter is presented in 3D in Figure 2. The filter consists of two filtering bed cells, each measuring 12 × 8.5 m, preceded by a septic tank. A tank collects the treated water. Both cells have the same depth; the difference lies in the choice of plants, as we aim to determine their influence on water treatment. Similarly, flint gravel is used throughout but at different diameters. At the entrance and exit, over a width of 0.5 m, large gravel covers the pipes for the distribution and recovery of wastewater. In the middle, between the two layers of large gravel, there are smaller 5 - 15 mm diameter gravels. The filter’s characteristics are presented in Table 1. Or this study, only the results concerning Vetiver are presented.

The analysis of the results in Table 4 demonstrates a significant improvement in water quality after passing through the Vetiver filter. Initially, the values of COD and BOD5 at the entrance of the septic tank are 1347 mg/L and 259 mg/L, respectively. After treatment by the septic tank, these values decrease drastically to 291 mg/L and 78 mg/L, respectively. However, after passing through the Vetiver filter, an even more pronounced decrease is observed, with average values of 74 mg/L for COD and 11 mg/L for BOD5.

Regarding Total Suspended Solids (SS), a significant decrease is observed from 76 mg/L to 35 mg/L after the septic tank and a spectacular reduction to only 3 mg/L after the Vetiver filter. The concentrations of ammonium ( N-NH 4 + ) and nitrate ( N-NO 3 − ) also show a decreasing trend after each treatment step. In particular, the ammonium concentration decreases from 248 mg/L to 53 mg/L after the septic tank and reaches 7 mg/L after the Vetiver filter. The phosphate concentration ( P-PO 4 3 − ) undergoes a marked decrease from 48 mg/L to 47 mg/L after the septic tank, but it is after passing through the Vetiver filter that its concentration drops drastically to 4.5 mg/L. Microbiologically, the concentration of fecal coliforms decreases significantly from 5.9 log10 FCU/100mL at the septic tank outlet to as low as 2.2 log10 after treatment by the Vetiver filter. It is also noteworthy that helminth eggs, especially anguillule larvae, are not detected at the filter outlet, indicating that the system effectively eliminates these pathogens. In summary, these results indicate that the Vetiver filter plays a crucial role in water purification, resulting in improved water quality in terms of physico-chemical and microbiological parameters.

Table 5 presents the percentage removal of pollutants by the Vetiver filter after treatment in the septic tank. COD shows a reduction of 74.57%. BOD5 exhibits a reduction of 85.90% after passing through the Vetiver filter. For Total Suspended Solids (SS), the Vetiver filter successfully removed 91.40% of the particles present. The removal rate of ammonium ( N-NH 4 + ) by the filter is 86.70%. Finally, in terms of fecal coliforms, the filter achieved a reduction of 2.3 log units.

The analysis of water quality treated by the Vetiver planted filter, based on the information in Table 4 and Table 5, clearly demonstrates that the Vetiver filter offers a significant improvement in water quality, especially in terms of pollutant removal. Comparing these results with existing data can provide insight into the

effectiveness of the Vetiver filter in regional and similar contexts. [11] studied a constructed wetland wastewater treatment system for the treatment and reuse of municipal wastewater for agricultural purposes in Senegal. While their study does not specifically focus on Vetiver, it provides a solid basis for comparing the efficiencies of different treatment methods. Regarding COD, which is an indicator of the amount of oxygen required to oxidize organic matter, our study revealed a drastic reduction from 1347 mg/L to 74 mg/L thanks to the Vetiver filter. This performance appears to be more effective than those reported by [11] although precise values are needed for an accurate comparison. The reduction in BOD5 from 259 mg/L to 11 mg/L is also noteworthy, indicating a significant decrease in the amount of oxygen required for biochemical degradation over 5 days. These results seem to be in line with or even surpass those in the literature, highlighting the efficiency of the Vetiver filter for wastewater treatment. The substantial decrease in Total Suspended Solids (SS) from 76 mg/L to only 3 mg/L demonstrates the filter’s efficiency in capturing and retaining solid particles. Compared to existing data for the region, this performance is impressive and may even outperform other wetland treatment systems. The reduction in concentrations of ammonium ( N-NH 4 + ) and phosphate ( P-PO 4 3 − ) lso indicates effective nutrient removal, which is essential to prevent eutrophication when treated water is discharged into the environment. The values obtained in this study appear to be comparable to, if not better than, those reported by [11] for the same geographical area. Finally, from a microbiological perspective, the drastic reduction in fecal coliforms from 5.9 log10 FCU/100mL to 2.2 log10 demonstrates the filter’s efficiency in eliminating pathogens. This performance is essential to ensure that treated water is safe for potential reuse, especially for agricultural purposes. In conclusion, the Vetiver filter stands out as a promising solution for wastewater treatment. However, like any study, it has limitations. A more in-depth analysis of operational conditions and specific constraints related to Vetiver would be beneficial for large-scale implementation. Based on target values for effective treatment or acceptable discharge established by [12] , the performance of the Vetiver filter can be evaluated. Regarding 5-day Biochemical Oxygen Demand (BOD5), the post-treatment results show a concentration of 11 mg/L, well below the standard of 25 mg/L. This demonstrates the filter’s effectiveness in removing biologically degradable organic matter. For Chemical Oxygen Demand (COD), the post-treatment concentrations are at 74 mg/L, lower than the target value of 125 mg/L, highlighting the efficient removal of the majority of organic matter. Post-treatment Total Suspended Solids (SS) displays a concentration of 3 mg/L, well below the standard of 35 mg/L, indicating effective particle removal. Nitrate concentrations after treatment are at 27 mg/L, meeting the limit of 50 mg/L. As for Total Kjeldahl Nitrogen (TKN), although it is not explicitly in our data, the concentration of ammonium nitrogen after treatment is 7 mg/L, thus meeting the standard of less than 30 mg/L. However, for phosphates, the post-treatment concentration of 4.5 mg/L exceeds the standard of 2 mg/L, which may require specific attention. Regarding fecal coliforms, they display a concentration of 180 FCU/100mL after treatment, significantly lower than the standard of 1000 FCU/100mL. In summary, the Vetiver filter demonstrates excellent performance for most of the analyzed parameters, meeting or even exceeding established standards. However, phosphate treatment may require optimization or additional measures to fully comply with standards. The strengths and weaknesses of water quality results and pollutant removal percentages by the Vetiver filter, which uses flint gravel as the filter medium, are evident. The Vetiver filter demonstrates a strong ability to reduce BOD5 and COD, indicating effective removal of biologically degradable organic matter. These findings align well with the standards established by [12] , highlighting the optimal performance of the Vetiver filter in this situation. Additionally, post-treatment SS concentrations are remarkably low, indicating that flint gravel plays a crucial role in physically filtering suspended particles, as noted by [13] . The filter’s efficiency in nitrogen treatment is also commendable, with significantly reduced levels of ammonium nitrogen after treatment, suggesting effective prevention of eutrophication risks. Furthermore, the low concentrations of fecal coliforms after treatment are a positive indicator of improved microbiological water quality, making it safer for applications like irrigation. However, despite these strengths, the Vetiver filter has limitations, especially in phosphate removal. Flint gravel, as a filter medium, may not have ideal surface chemistry for phosphate adsorption. Over time, the flint could potentially reach a saturation point, further reducing its effectiveness in phosphate removal [14] [15] . Other factors, such as the nature of phosphates, adsorption capacity, the presence of other ions, high initial concentration, and issues related to hydraulic residence time or excessive flow, can also contribute to this limitation [3] [16] [17] [18] . In particular, the specific adsorption capacity of flint or its granulometry can influence this efficiency [13] [15] . In summary, while the Vetiver filter with flint gravel has notable advantages for water treatment, especially for the removal of organic matter, suspended solids, and nitrogen, there are areas, such as phosphate removal, that require additional attention and optimization.

Table 6 provides a comparison of the quality of effluents from Vetiver planted filters with standards and recommendations from various entities: the Senegalese standard [19] , old WHO recommendations [20] , and European legislation [21] . This comparison focuses on the elimination and reuse of water for irrigation. The analysis of Table 6 reveals crucial information regarding the quality of effluents from Vetiver planted filters compared to the standards and recommendations in place in Senegal, the EU, and those from WHO. Overall, the water treated by the Vetiver filter appears to have a significantly higher quality than the requirements of the Senegalese standard for discharge into the environment, suggesting diverse opportunities for reuse. For the parameter of Total Suspended Solids (SS), the treated water shows a concentration of 3 mg/L, which is well below the Senegalese standard of 40 mg/L and also falls within the range

a, b, c, d Classes of recycled water quality and agricultural use and permitted irrigation method: Class A: All food crops, including raw-consumed roots and food crops where the edible part is in direct contact with the recycled water. All irrigation methods are allowed; Class B: Raw-consumed food crops when the edible part is produced above ground and is not in direct contact with the recycled water, processed food crops and non-food crops, including crops for feeding milk or meat-producing animals. All irrigation methods are permitted; Class C: The same category of crops irrigable with class B quality water. Only drip irrigation is allowed; Class D: Industrial, energy crops, and seeds. All irrigation methods are allowed.

recommended by European legislation, ranging from 10a to 35 b, c, d, depending on the class and intended agricultural use. The levels of BOD5 and COD, which are 10 mg/L and 98 mg/L, respectively, are well below the thresholds set by the Senegalese standard and European legislation. This indicates the efficiency of the Vetiver filter in treating organic matter. The level of fecal coliforms (FC) at 176 FCU/100mL is significantly lower than the Senegalese standard of 2000 FCU/100mL and falls within the range of European legislation, allowing for water reuse in various crop classes. The absence of helminth eggs in the treated water is also a positive point, in line with WHO recommendations for unrestricted irrigation water reuse. The conclusions drawn from Table 6 suggest that water treated by the Vetiver filter is of a sufficiently high quality to be safely reused in agricultural applications, especially for irrigation. Taking into account the water quality classes established by European legislation, this treated water would be suitable for a wide range of crops, from raw food crops to industrial and energy crops. However, it is important to note that while water quality meets current standards, additional considerations, such as the long-term impact of reused treated wastewater on soil health and crop quality, should be taken into account. Furthermore, the implementation of such a system on a large scale would require a detailed study of technical and economic feasibility, as well as acceptance by local stakeholders. In summary, the Vetiver filter, with flint gravel as the filter medium, appears to offer a viable solution for treating wastewater to a level that allows for reuse in agriculture. The opportunities it presents could help meet growing water needs while ensuring sustainable water resource management.