Occurrence and Risk Assessment of Antibiotic Residues in Sewage Sludge of Two Nigerian Hospital Wastewater Treatment Plants

Antibiotic residues in sewage sludge can present detrimental environmental effects due to sewage sludge application onto soils for agricultural purposes. Reports on the occurrence of antibiotics in sewage sludge and risk assessment due to the application of sewage sludge onto soils are still limited in Africa. The occurrence of fourteen antibiotic residues in sewage sludge from two Nigerian hospital wastewater treatment plants was investigated. For the first time, the potential environmental risk of target antibiotics associated with the use of sewage sludge for application onto soils in Nigeria was assessed. Risk assessment was carried out using both terrestrial and aquatic toxicity data. All target antibiotics were detected in at least one sludge sample. Ciprofloxacin and ofloxacin (fluoroquinolones) had the highest concentrations, up to 674 ng g(-1) dry weight for ciprofloxacin. All ten antibiotics evaluated for terrestrial ecological risk in sludge-amended soils presented low risk. Only three out of thirteen antibiotics assessed for aquatic ecological risk in sludge-amended soils posed medium risk, while the remaining antibiotics presented low risk. Antibiotic mixtures presented low risk in sludge-amended soils. A more holistic evaluation of the potential risks due to a mixture of a wider scope of chemicals in Nigerian sewage sludge is recommended prior to application onto soils as fertilizers for agricultural purposes.