Abstract

The main objective of this study was to evaluate the effects of combined antibiotics on biomass stability and the occurrence of antibiotic-resistant bacteria (ARB) in activated sludge (AS) used for domestic wastewater treatment. Two aerobic lab-scale reactors were operated: one for the experimental approach (R1) and the other as a control (R2). In R1, two exposures were conducted with combined antibiotics of amoxicillin (AMX), ciprofloxacin (CIP) and ceftriaxone (CTX) at concentrations of 32, 4, and 2 ?g/L, respectively, during exposure 1 (E1), and 64, 8, and 4 ?g/L, respectively, during exposure 2 (E2). In these cases, removal efficiencies, oxygen uptake rate (OUR), specific oxygen uptake rate (SOUR) and ARB occurrences were evaluated. Within the results of this study, a significant effect on the AS biomass in terms of performance and kinetic parameters was observed with values below 10 %, 20 mgO2/L·d and 100 mgO2/gVSS·d for organic matter removals, OUR and SOUR, respectively, during E1. However, during E2, AS biomass performances and kinetic parameters improved, with a 50 % increase in these values. Moreover, ARB occurrences were only detected during E2, with abundances between 6.6 and 8.2 × 102 CFU/mL. These findings suggest that the re-expose of AS biomass to combined antibiotic concentrations induced bacterial adaptation to this selective pressure resulting in the recovery of the heterotrophic activity and improved removal efficiency. © 2025 Elsevier Ltd