Occurrence, risk assessment and elimination of chemicals of emerging concerns in wastewater treatment plants In Western, Kenya

Abstract

Chemicals of emerging concern (CECs) are a global threat due to their adverse effects on aquatic organisms and human health. Wastewater treatment plants (WWTPs) are a significant source of these compounds into the environment at varying concentrations ranging from ng/L to µg/L. However, data on the removal efficiencies of waste treatment technologies in Africa are limited. Therefore, this study aimed to assess the occurrence, removal efficiency, and potential toxic risk posed by CECs in wastewater treatment plants. The objectives were to: i) identify and quantify CECs from WWTPs, ii) evaluate the removal efficiencies of CECs by the selected WWTPs, and iii) perform ecotoxicological risk assessment of the detected CECs in the effluent of the four selected WWTPs in western Kenya. Solid-phase extraction and high-performance liquid chromatography coupled with high-resolution mass spectrometry were used for sample preparation and analysis, respectively. A target list of 795 compounds, including pesticides and biocides, pharmaceuticals, and industrial compounds, among others, was used. The toxic unit (TU) approach was applied to evaluate the risk posed by the contaminants present in the effluent of the WWPs. A total of 353 compounds were detected across influents and effluents of the sampled WWPs, with the most frequently detected compound classes being pharmaceuticals (102), pesticides (70), and industrial chemicals (72). The highest influent concentrations were detected for caffeine (830 µg/L), deoxycholic acid (719 µg/L), 2-oxindole (43 µg/L), ibuprofen (24 µg/L), and dichlorvos (14 µg/L). Notably, previously undocumented antiviral drugs such as emtricitabine and amantadine were reported for the first time at concentrations ranging from 4 ng/L to 536 ng/L. In the effluent, high concentrations were detected for caffeine (17 µg/L), TMDD (1.3 µg/L), cetirizine (2.3 µg/L), dichlorvos (1.3 µg/L), and sucralose (3.1 µg/L). Removal efficiencies of the compounds varied greatly, with 286 compounds having positive removals and 67 compounds with negative removal efficiencies. Compounds with > 80% removal efficiency included ibuprofen, trimethoprim, TMDD, diclofenac, and diazinon. WWTPs employing a combination of primary and secondary stabilization ponds, activated sludge, and trickling filters performed better in removing CECs. Risk assessment revealed crustaceans had the highest potential risk, with toxic units (TUs) up to 5.5, driven primarily by dichlorvos and diazinon. Algae and fish predominantly experienced chronic toxicity, with dichlorvos being the primary driver of toxicity for algae and didecyldimethylammonium for fish. This study underscores the substantial contribution of WWTPs to contamination of aquatic environments, with pesticides, pharmaceuticals, and industrial chemicals being the most persistent. It provides evidence-based data on the need for technological advancements in CEC removal, chemical use, and disposal, as well as robust monitoring and regulatory measures.