Ecological Risk Assessment of Heavy Metals from Sewage Sludge, Wastewater, and Adjacent River of Thimphu Sewage Treatment Plant, Bhutan

Résumé fourni par la source

Domestic wastewater is a crucial source of contamination in freshwater ecosystems, with an estimated global production of 127.13 billion tons annually. Various sewage treatment facilities put in place for treatment of municipal wastewater generate sewage sludge, which accumulates contaminants from wastewater, posing risks to waterbodies upon discharge. Although biological, and chemical treatment methods are employed for treatment, they have limitations, and environmental concerns. Despite the recommendation for sludge reuse in agriculture, it may contain harmful heavy metals like Zn, Cd, and Pb, threatening ecosystems, and human health. The discharge of wastewater in the river poses substantial risk due to the heavy metal concentration as well as physicochemical contaminants. Index approach for assessing ecological risks, and extent of contamination of sludge, and water from heavy metals is recommended. A decommissioned sewage treatment plant in Thimphu, Bhutan poses risks due to wastewater leakage into rivers, necessitating the need for a study to evaluate heavy metal concentrations, and physicochemical contaminants, and determine the ecological risks of sludge, and wastewater to the river water. The study was conducted at Babesa Sewage Treatment Plant in Thimphu city, sampling sludge, and wastewater from the plant based on different biological treatment methods, and river water upstream, and downstream. The physicochemical parameter and heavy metals concentration were analyzed, and water quality indices were used to assess pollution, and ecological risk. Statistical analysis of the data for heavy metals and physicochemical parameters was conducted using ANOVA, and Pearson's correlation. Results indicated that the physicochemical parameters were within permissible limits for sludge, and wastewater, thereby affirming agricultural application. Heavy metal concentrations for sludge also differed among ponds, and correlation analysis revealed positive associations. Zn, and Cd had moderate contamination but Cd, and Cu posed more ecological risk in sludge. The wastewater had higher contamination of Pb, and the river water upstream as well as downstream showed the same. River water downstream showed elevated levels of heavy metals compared to upstream establishing significant deterioration in quality. Positive correlations among heavy metals suggest a common pollution source, likely effluents from the sewage treatment plant.

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