Occurrence, distribution, and risk assessment of selected pharmaceuticals in municipal biosolids from the four wastewater treatment plants in Gauteng Province, South Africa

The presence of pharmaceutical residues in municipal biosolids is an emerging environmental concern due to their potential accumulation in agricultural soils following land application. While parent compounds like sulfamethoxazole and carbamazepine are frequently studied, data on their transformation products in biosolids remain limited. This study assessed the occurrence, distribution, and environmental risks of sulfamethoxazole, carbamazepine, and two of their selected metabolites in biosolids collected from four wastewater treatment plants (WWTPs) in Gauteng Province, South Africa. A single biosolid sampling campaign was conducted across the four WWTPs, each characterized by varying operational parameters. Sulfamethoxazole and its two metabolites were not detected in any of the samples. In contrast, carbamazepine and its metabolites-carbamazepine-10,11-epoxide and carbamazepine diol-were detected, with maximum concentrations of 150.67, 601.95, and 89.18 ng/g dry weight, respectively. Carbamazepine and carbamazepine-10,11-epoxide occurred in all samples, while carbamazepine diol appeared in 50% of the samples. The concentration patterns varied across WWTPs and were significantly correlated with biosolid pH, organic carbon content, and oxidation-reduction potential. Risk quotient analysis indicated that the predicted soil concentrations of carbamazepine (0.50 ng/g dry weight) pose negligible ecological risk (RQ = 0.003). However, due to a lack of ecotoxicological data, environmental risks associated with carbamazepine-10,11-epoxide and carbamazepine diol could not be fully assessed. Overall, the findings highlight the persistence of carbamazepine and its metabolites in biosolids and underscore the importance of optimizing sludge treatment processes and generating toxicity data for pharmaceutical metabolites to support safe land application practices.