A modified quick, easy, cheap, effective, rugged and safe (QuEChERS) based methodology with a dispersive solid-phase extraction (d-SPE) clean-up followed by liquid chromatography-triple quadrupole mass spectrometry analysis was developed and validated for simultaneous determination of seventeen multiclass antibiotics in sewage sludge. The target antibiotics included four sulfonamides, three fluoroquinolones, three macrolides, two tetracyclines, three β-lactams, trimethoprim and thiamphenicol. Extraction solvents, buffer salts and d-SPE sorbents were optimized. Chromatographic separation of target antibiotics was performed on a Poroshell EC-C18 column while detection and quantification were achieved with a triple quadrupole mass spectrometer using electrospray ionization in both positive and negative modes by fast polarity switching. The method limits of detection ranged from 0.003 ng g⁻¹ dw (trimethoprim) to 120 ng g⁻¹ dw (oxytetracycline). Intra-day precision of less than 18% (% RSD) was achieved. The applied d-SPE clean-up was effective for reducing matrix effects due to co-extracted components. The method was successfully applied for measurements of target antibiotics in sewage sludge samples from three wastewater treatment plants (WWTPs) in Nigeria. Except for β-lactam antibiotics which were not recovered during the method development, most target antibiotics were detected in the sludge samples. The highest concentrations were found for tetracyclines (up to 4689 ng g⁻¹ dw for oxytetracycline) and fluoroquinolones (up to 1201 ng g⁻¹ dw for ciprofloxacin). This work expands our knowledge on the applicability of QuEChERS method for the extraction of antibiotic residues from sewage sludge. The findings revealed the ubiquitous presence of the majority of the target antibiotics in the investigated Nigerian WWTPs.

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.