The study was conducted at the SSTP located in the Koubia district of Niamey Commune I (Figure 1). Built in 2018, the SSTP aims to treat wastewater from households in Niamey, primarily from latrines and septic tanks [10]. Financed by the World Bank through the “Urban Water and Sanitation Project” (PEAMU), it remains the only sewage sludge treatment plant in Niamey.

Figure 1. Location of the study area.

Figure 2. Sewage sludge treatment plant.

The SSTP (Figure 2) consists of following compartments including screening and sand removal basins, sedimentation basin, drying beds, phytopurification basin and filtration basin. The screening and sand removal basins remove large suspended objects.

The Koubia SSTP has a nominal treatment capacity ranging from 300 m³ of fecal sludge per day [11].

Septic tank emptying can be carried out using two primary methods: manual emptying and mechanical emptying. The latter relies on the use of vacuum trucks equipped with vacuum pumps and storage tanks for the suction, transport, and discharge of fecal sludge and wastewater. The capacity of these vehicles generally ranges from 3000 to 10,000 liters. However, in the course of their operations, mechanical emptying operators face various logistical, technical, and infrastructural constraints that can hinder the efficiency of their interventions.

Data collection

The survey targeted three stakeholder groups directly concerned by the activities of the Koubia Sewage Sludge Treatment Plant (SSTP): the plant personnel, the mechanical waste collectors responsible for sludge collection and transport, and the local residents likely to be affected by its operation. The selection of these actors was based on their level of involvement in the fecal sludge management chain and their degree of exposure to the potential impacts of the plant. The collected data encompassed the sociodemographic characteristics of the respondents, as well as their perception of environmental impacts (odor nuisances, insect proliferation, and the degradation of soil, water, and access roads). Additionally, it evaluated the socioeconomic effects associated with the plant’s activities (job creation, reduction of illegal dumping, and perceived nuisances) alongside perceived or observed health risks (diarrheal diseases, skin conditions, respiratory disorders, and other symptoms linked to sludge exposure). For the SSTP operators and collectors, complementary data were gathered regarding working conditions, the use of personal protective equipment (PPE), and professional occupational exposure to sewage sludge. The study sample comprised 75 plant operators and mechanical waste collectors, along with 94 household heads residing near the station.

Field observations

The observations focused on monitoring the sewage sludge treatment process and touring the different sections of the SSTP to identify potential impacts and confirm the perceptions of those surveyed.

Perception of the environmental impacts ofSSTP’s activities

Analysis of the perception survey results reveals that SSTP’s activities generate various environmental and social impacts on the population living in the surrounding area (Figure 3).

Figure 3. Perception of environmental and social impacts.

In fact, around 20% of respondents report that the most noticeable impact is the unpleasant odors caused by stagnant sewage sludge (Figure 4). Septic tank emptier leave the valves open after emptying to discharge the remaining sludge around the SSTP, which stagnate on the road, spoil the appearance of the street, and affect the quality of the ambient air.

Figure 4. Puddles of mud stagnating in the street.

Additionally, 10% of respondents associate this stagnation with the potential risk of groundwater contamination. Twenty percent of respondents believe that the constant passage of sewage trucks contributes to road deterioration.

Other risks associated with SSTP activities include insect proliferation, domestic animal contamination (Figure 5), respiratory diseases, and noise pollution from truck traffic.

Figure 5. Domestic animals are drinking polluted water from sewage sludge.

Impacts socio-économiques de l’activité des vidangeurs

According to septic tank cleaners, manual emptying is more common than mechanical emptying. Eighty-eight percent (88%) of households use manual emptying to empty their septic tanks, compared to twelve percent (12%) who use mechanical emptying. Fourteen percent (14%) of households believe that mechanical emptying operators are better protected because they use personal protective equipment in accordance with SSTP requirements (Figure 6).

Figure 6. Positive impacts of the creation of the SSTP.

Twenty-six percent (26%) of septic tank cleaners report that they remove sludge from two (2) to six (6) tanks daily, at rates ranging from 20,000 to 30,000 XOF (F CFA) per tank.

The creation of the SSTP has benefited the environment and the population’s living conditions. Fifteen percent of households report that septic tank cleaners no longer dump sludge directly into fields or streets because the SSTP site is developed, fenced, monitored, and under the responsibility of the central town hall. This has limited the spread of pollution, according to 26% of those surveyed. Other positive impacts include producing natural fertilizer, creating jobs for young people, and generating funds for the city hall. However, it was noted in the field that sewage collectors lack safety equipment, such as gloves, boots, coveralls, and protective masks. This exposes them to the risk of disease when in contact with fecal matter. Overuse of the station was also observed, which could damage its infrastructure.

Households in the city use two methods of sewage disposal: manual and mechanical emptying. According to the emptier, mechanical emptying is the more common method. This can be explained by the fact that the surveyed households belong to the middle class. Defo et al. (2015) [12] reported that, in Bafoussam, high-income individuals rarely use manual emptying services. According to Gning et al. (2017) [13], most poor households in developing countries cannot afford mechanical emptying services due to their high cost.

The SSTP receives mechanical sludge from 66 removal companies in the city of Niamey [14]. The choice of technique depends on the characteristics and volume of the sludge to be treated as well as a number of criteria that the designer must take into account. These include simplicity, effectiveness, social acceptance, implementation and operating costs, technology used, space occupied, and associated contamination risks. However, the technique adopted by the SSTP has health impacts and risks. According to Goumar et al. (2022) [15], this plant’s purification efficiency reveals the beds’ inefficiency in treating sludge. They report that the elimination of elements such as MS, MES, BOD₅, COD, and NTK never exceeds 50%, and the average COD/BOD₅ ratio of this biodegradable sludge is 1.9. Residents living near the SSTP experience the consequences of this situation in various ways, including foul odors, insect proliferation, and the risk of disease, contamination, and poisoning of animals grazing near the site. The population has identified several other impacts associated with SSTP activities, including noise pollution from the constant movement of trucks, road damage caused by truck drivers, and flooding during the rainy season. For instance, this site has received several complaints from local residents due to the nuisance it causes. Defo et al. (2015) [12] reported similar perceptions of impacts in Bafoussam, Cameroon, where complaints have also been filed. Thus, the leachate and sludge from the SSTP were not suitable for other uses and posed environmental and health risks. Magoumbala (2007) [16] reported that wastewater byproducts can carry certain diseases, including waterborne diseases. He also stated that sewage sludge management is generally problematic in less developed countries due to poor-quality leachate and dry sludge. According to Goumar et al. (2022) [15], co-composting is necessary prior to any use to prevent environmental and human health contamination. Additionally, there is a potential risk of soil and groundwater contamination from recalcitrant organic and inorganic pollutants in these effluents.

Nevertheless, the SSTP’s activities have had a positive impact. It has significantly reduced the illegal dumping of untreated sewage sludge in fields and streets, limiting the spread of environmental impacts and health risks reported by local residents. There are also opportunities to produce natural fertilizer for farmers, provided they comply with pollution standards. According to Yahaya (2014) [14], dried sludge should be properly treated before being sold to farmers and market gardeners as fertilizer and to livestock farmers as fodder, which would add value to the treatment byproducts. Indeed, when valorized, the by-products provide substantial income [16].

However, public perception surveys undermine the income generation impact through job creation for young people and the town hall, which generates an average daily revenue of 109,000 XOF (FCFA). Tagba (2019) [17] reported similar results. Yahaya (2014) [14] stated that this activity reduces unemployment in peri-urban areas.