Antibiotics in slow-rate biofiltration processes: Biosorption kinetics and equilibrium

Recent evidence indicates the widespread occurrence of numerous pharmaceutically-active compounds (PhACs), including antibiotics, in surface waters world-wide. Slow-rate biofiltration (SRBF) systems, such as slow sand filtration and bank filtration, are a promising option for removing PhACs and other trace organic chemicals because these systems employ both sorption and biodegradation simultaneously and are relatively simple to install and operate. Thus, we investigated the kinetics and equilibrium of the sorption of selected antibiotics (erythromycin, ERY; sulfamethoxazole, SMX; and ciprofloxacin, CIP) to biofilm as a first step in characterizing the fate of antibiotics in SRBF systems. Sorption experiments were conducted using a continuous feed rotating annular bioreactor (CFRAB) system and the antibiotics were fed singly or together as a cocktail at environmentally-relevant concentrations of 0.33 and 3.33 μg/L. Sorption rate constant (k) values ranged from 0.048 to 4.465 h¹ with the trend as follows: (SMX >ERY>CIP). The biofilm organic carbon sorption constant (K_oc) ranged from 4,140 to 200,000 L/kg. The relative order of K_oc (CIP>>ERY>SMX) mimics what has been found for sorption to other organic sorbents (typically expressed as K_oc), but does not agree with the trend in reported logK_ow values (ERY>SMX>CIP). This suggests that octanol may not be a good model for biofilm with respect to sorption of antibiotics. In the presence of dissolved NOM, both the rate and extent of antibiotic sorption to biofilm decreased. Results from this work are valuable for modeling antibiotic fate in SRBF processes and other natural and engineered biofilm-mediated systems.