Ndiebene Gandiol is a Senegalese municipality situated approximately 20 kilometers from Saint-Louis city, near the Senegal River estuary. It forms part of the Rao district within the Saint-Louis department and region. Its geographical location places Ndiebene at the historical center of the Gandiol area. Following the implementation of the third act of decentralization in 2014, Ndiebene Gandiol has been designated as the administrative center of the Gandiol municipality. Within the Gandiol municipality, several schools have been examined. However, our study will focus primarily on the Gandiol school, which is considered for the implementation of an ecological wastewater treatment system. The specifics of the selected site are recorded in Table 1. This table highlights the sanitary situation and the infrastructure of the “Ndiebene Gandiol 1” school. The institution accommodates 505 students, aged between 7 and 14 years, distributed across 12 classrooms. Despite this significant number of students, the school has only 4 latrines for student use and one reserved for teachers. The latrines are often non-operational. Moreover, the school lacks any functioning water points. Its sanitation system is deemed non-compliant. Lastly, the toilets are in poor condition, frequently blocked, and thus unusable for students and teachers most of the time. To design an appropriate wastewater treatment system, it is imperative to carry out sampling to determine the quality of the wastewater.

To determine the wastewater quality of the studied site, a characterization methodology was developed. The goal is to assess the main physicochemical and microbiological characteristics of these waters to propose suitable treatment solutions.

According to [27] , the Population Equivalent (PE) is a standardized unit used to compare the pollutant load produced by various users in an area (residents, workers, students, etc.) to that of a typical inhabitant. This unit is frequently used to size sanitation facilities. To determine the Population Equivalent for our study site, it is crucial to quantify the pollutant load generated by the students and staff, in relation to the activities on site. However, for an accurate assessment, detailed information on waste generation, water consumption, and other factors is required. In the absence of this information, we can still establish an estimate based on certain criteria stated in Table 1.

When considering an ecological wastewater treatment, such as CWs, it is essential to have references for permissible pollutant loads to ensure optimal system efficiency. These reference values can vary depending on national or local regulations, environmental conditions, and the type of ecological system being considered. However, here are some general and recommended values for various parameters, widely recognized in the sanitation field (cf Table 2) according to [28] . It is important to note that these values are general and may vary based on local regulations, environmental context, and other factors. Furthermore, several organizations and publications provide guidelines on this matter, such as the World Health Organization (WHO) [29] , the United States Environmental Protection Agency (EPA) [15] , or European standards [30] . For specific applications, it is recommended to consult local regulations and the recommendations of experts in the field.

Following observations and site visits at the Ndiebene Gandiol 1 school, wastewater samples were collected for in-depth analysis. The results of these analyses are presented in Table 3. For biological oxygen demand (BOD5), the values obtained were 439 mg/l for the first sample and 293 mg/l for the second, with an overall average of 366 mg/l. Regarding chemical oxygen demand (COD), the first sample had a concentration of 1070 mgO₂/l, while the second showed 1480 mgO₂/l, for an average of 1275 mgO₂/l. For Suspended Solids (SS), the concentrations were 254 mg/l and 162 mg/l for samples 1 and 2 respectively, with an average of 208 mg/l. Nitrate levels were 60 mg NO 3 − /l for the first sample and 40 mg NO 3 − /l for the second, leading to an average of 50 mg NO 3 − /l. In terms of Total Kjeldahl Nitrogen (TKN), the concentrations observed were 486.2 mg/l and 390.3 mg/l for samples 1 and 2 respectively, giving an average of 438.25 mg/l. Phosphates showed a concentration of 344 mg PO 4 3 − /l in the first sample and 420 mg PO 4 3 − /l in the second, for an average of 382 mg PO 4 3 − /l. Finally, regarding Fecal Coliforms, the first sample recorded a high concentration of 1.03E+06 CFU/100 ml, while the second reached an even more concerning level of 4.00E+07 CFU/100 ml. The average for this parameter for the two samples was 2.05E+07 CFU/100 ml. These results come directly from the analyses performed at [31] and are essential for understanding the wastewater quality within the Ndiebene Gandiol 1 school.

Based on the data from Table 1, the school has 505 students. It can be assumed that a student produces a pollutant load less than that of an adult (let’s say 50% of an adult’s load). If one adult corresponds to 1 PE (Population Equivalent), then a student would be rated at 0.5 PE. The fact that all the latrines are out of service, combined with the dilapidated state of the toilets, means that the wastewater is not receiving proper treatment. While this does not directly influence the calculation of the PE, it highlights the inadequacy of current infrastructure in relation to the defined number of PEs. The lack of a water point suggests restricted water consumption or supply from other sources. This could reduce the volume of wastewater produced, but it is difficult to assess its impact on the PE without additional data. Assuming one teacher per class, which makes 8 teachers, and considering that a teacher generates a pollutant load equivalent to that of an adult, each teacher would be accounted for 1 PE.

The wastewater analysis of Ndiebene Gandiol school, as presented in Table 3, highlights several major challenges that need to be addressed. An average BOD5 of 366 mg/l, which is relatively high, indicates a significant amount of biodegradable organic matter, typically associated with the presence of organic contaminants. This is corroborated by an average COD of 1275 mgO/l, signaling a considerable presence of organic matter. Even more alarming is the high concentration of fecal coliforms, averaging 2.05E+7 (CFU/100 ml), indicative of major bacterial contamination posing a serious public health risk. These analyses, conducted by [31] , attest to the credibility of the results. The range of parameters analyzed provides a comprehensive view of the water quality situation. Several, especially the high concentration of fecal coliforms, alert to the severity of contamination. These data, combined with the school’s deficient sanitary infrastructure, are even more concerning. These findings, placed within the broader context of school sanitation and public health issues, underscore the urgent need for targeted sanitation interventions. Indeed, students and teachers are exposed to considerable health risks, potentially impacting not only their health but also their attendance and academic success. Faced with such a situation, immediate measures are imperative to enhance water quality within Ndiebene Gandiol school, namely:

- Establishing an effective wastewater treatment system.

- Organizing hygiene-focused educational sessions for students and staff.

- Implementing modern sanitary infrastructure that meets current standards.

- Strengthening collaboration with local authorities and specialized entities for regular monitoring and appropriate maintenance.

In summary, the data collected highlight the urgency of undertaking corrective measures to improve water quality in this institution. It is essential to take vigorous steps to ensure a healthy study environment conducive to students’ well-being. The interpretation of the data collected on Ndiebene Gandiol 1 school reveals crucial aspects to consider for a thorough analysis. The limited number of samples could influence the representativeness of the results. Diversifying samples over a longer period would be ideal to gain a more comprehensive overview of potential variations in water quality. Not taking the topography into account could distort the understanding of pollutant spread. Integrating this dimension could refine the recommendations. The absence of analysis of climatic variations may affect the relevance of treatment methods, given local seasonal fluctuations. However, the study offers a complete perspective on water quality thanks to the diversity of analyzed indicators. The results, from [31] , are highly credible. The emphasis on health indicators, such as fecal coliforms, underscores the health issues associated with wastewater.

The combined analysis of the information contained in Table 1 and the pollutant load of the wastewater classifies the study site as a small to medium-sized entity in terms of wastewater sanitation. Various treatment solutions can be considered for a community of this size, as highlighted by [27] . When we look at intensive (or conventional) wastewater treatment sectors, such as activated sludge, trickling filters, rotating biological contactors, and high-performance lagoons, their respective disadvantages (significant energy consumption, significant sludge production requiring proper management, regular monitoring and management required, larger installation space than for activated sludge, sensitivity to toxic shocks, need for mechanical equipment, risk of clogging, among others) make them less suitable. Wastewater treatment through extensive systems, whose advantages and disadvantages are presented in Table 4, mainly refers to systems that require little or no energy and largely rely on natural processes. These appear more suitable for the site in question. According to the analyses performed on the school’s wastewater, BOD5, COD, and fecal coliforms are particularly high. CWs and natural lagoons emerge as the most suitable solutions for effectively treating these high concentrations. Taking into account the site constraints, available resources, and community preferences, the choice falls on constructed wetlands with reeds as recommended by the work of [32] in relation to the hot and dry climate of the area.

According to [33] [34] [35] , the process of sizing a planted filter, also known as a constructed wetland, for wastewater treatment is inherently complex and requires careful consideration of various critical factors. A primary element is the incoming hydraulic flow, defined by the amount of water that needs to be treated daily, which determines the required surface area of the CW from the hydraulic point of view. The organic load, that is, the level and nature of the contaminants present, directly influences the required level of treatment and determines the size and depth of the filter. The choice of vegetation is also strategic: although different plants have varying abilities to assimilate pollutants, reeds (Phragmites australis) are commonly used. The composition and granularity of the filtering media, whether sand or gravel, affect the water retention time and the efficiency of the treatment. CWs can have a horizontal or vertical flow configuration, each with its advantages and disadvantages. Climatic conditions, such as temperature or precipitation patterns, can impact the performance of the filter. It is also essential to define the maintenance frequency to ensure optimal system efficiency, as well as the duration the water is retained within the filter is critical for its effectiveness.

The overall project encompasses various components, ranging from the construction of toilet blocks to the refurbishment of classrooms, the rehabilitation of ancillary structures, the construction of a perimeter wall, and finally, the implementation of a horizontal flow reed bed filter. However, our focus is specifically on the financial monitoring related to the construction of the reed bed filter. The financial summary of the project, breakdown in West African CFA franc (FCFA), is as follows: Initial site setup costs are 100,000, while terrain preparation and excavation work come to a subtotal of 253,000. Concrete and masonry work, including reinforced concrete and block masonry, total 1,031,600. Materials for transition layers and filtration in wet zones cost 343,000. Waterproofing, including preparation and hydraulic testing, comes to 350,075. Plant installation,

specifically Typha, adds 20,000. Sanitary plumbing reaches 188,000. Inspection chambers and connections cost 367,100. The total before tax is 2,652,775, with an 18% VAT of 477,500, leading to a grand total of 3,130,275. Converted to United States dollars (USD), using an approximate exchange rate of 608 FCFA to 1 USD, the amount is approximately $5148.48. Please note that exchange rates fluctuate constantly, so it is advisable to check the current rate for the most accurate conversion.