Dogbè Clément Adjahouinou^{1, 2,}, Chaim Vivien Doto^{3, 4}, Elias Alexandre Sètondji Adanlokonon⁵, Fouad Zouhir⁶, Mouhamadou Nourou Dine Liady²

Like developing countries, particularly in Sub-Saharan Africa, Benin still faces limited access to basic and improved sanitation services. However, water consumption and, consequently, wastewater production continue to grow in line with population increases, especially in urban areas. In this context, wastewater is often discharged into the environment untreated, threatening public health and compromising ecosystem integrity. This work assessed the physico-chemical and organic pollution of the mixed wastewater discharged by the storm sewers into Cotonou Lagoon, providing data to efficiently manage effluent treatment. Monthly samples were collected at three sewers outlets (S1, S3 and S3) over an annual cycle. Temperature, pH, electric conductivity (EC) and dissolved oxygen were measured in situ, while turbidity, suspended matter (SM), ammoniacal nitrogen, nitrites, nitrates, orthophosphates, sulphates and chemical oxygen demand (COD) were analysed by molecular absorption spectrophotometry standard methods. The Biological oxygen demand (BOD5) was assessed by the oxiTop manometric method. Results showed that the wastewaters were highly mineralised exceeding the recommended limit (EC > 2.5 mS cm^-1), with high turbidity associated with an elevated concentration in SM, reaching 120 mg L^-1. They were also loaded with nutrients, particularly nitrogen and orthophosphates out of standards limits, which could contribute to eutrophication in the receiving. Dissolved oxygen levels are consequently low (< 5 mg L^-1), while BOD5 and COD values indicated moderate organic pollution, which was moderately biodegradable according to the biodegradability index (BI) global mean varying from 3.11 (S3) to 4.24 (S1). The studied wastewater quality varied significantly across seasons, with the most polluted yet readily biodegradable wastewater observed during the LDS (BI = 2.31±0.90). Principal component analysis highlighted two main wastewater classes: those enriched in nitrogenous and organic loads with low DO and low BI, which were associated with the LDS and those dominated by phosphorus loads, low pH, particularly linked to the SRS. These findings demonstrate that untreated wastewaters discharged into Cotonou Lagoon carry substantial nutrient and organic loads, underscoring the urgent need for effective wastewater treatment before discharge to preserve lagoon ecosystem integrity.

Sewers, sanitation, biodegradability, eutrophication, aquatic ecosystem