Introduction: Sulphonamide antibiotics are common pollutants in aquatic environments due to their persistence and widespread use in human and veterinary medicine. Conventional wastewater treatment processes do not remove them efficiently. Therefore, new sustainable methods for eliminating pharmaceutical contaminants are being investigated. We studied sulfisoxazole (SSX), which is often used for treating urinary tract, respiratory, and ear infections because of its stability and broad-spectrum activity. Objective: The aim of the study was to investigate the efficiency of removing sulfisoxazole from synthetic wastewater using a combined technique of biological treatment with the alga Chlorella vulgaris and photocatalysis with titanium dioxide (TiO₂). Special attention was given to comparing different sequences of the two methods (first algae then photocatalysis, and vice versa). Methods: In laboratory conditions, we conducted experiments comparing different groups of synthetic wastewater samples with added antibiotic. The samples were subjected to treatment with C. vulgaris and photocatalytic treatment using TiO₂ under UVA light. We monitored algal growth, changes in SSX concentration using HPLC analysis, and basic physico-chemical parameters (temperature, pH, conductivity, dissolved oxygen). The results were statistically processed and the efficiency of individual approaches and their sequences was compared. Results: Algal growth was not inhibited by the presence of SSX; in some groups, it was even better than in control samples without the antibiotic. Independent algal degradation of SSX was not effective, whereas photocatalysis achieved significant reduction in antibiotic concentration (up to 64 %). Higher efficiency was achieved by combining photocatalysis first followed by algal treatment, where the final antibiotic concentration decreased to 24 %. The degradation of SSX was more gradual and consistent throughout the treatment. Discussion and Conclusion: The results confirm that the combined use of photocatalysis and algal treatment provides synergistic effects in the removal of pharmaceutical contaminants such as SSX. The sequence of treatments is crucial, as the combination of photocatalysis followed by algae yielded the best results. This study therefore opens possibilities for the development of sustainable wastewater treatment technologies for removing sulphonamide antibiotics using combined techniques.
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