Effect of sludge recirculation on removal of antibiotics in two-stage membrane bioreactor (MBR) treating livestock wastewater.

Two-stage MBR consisting of anaerobic and aerobic reactors was operated at total hydraulic retention time (HRT) of 48 h for the treatment of livestock wastewater containing antibiotics, i.e. amoxicillin (AMX), tiamulin (TIA), and chlortetracycline (CTC), under the (1st) absence and (2nd) presence of sludge recirculation between the reactors. During the operation with sludge recirculation, the removals of organic and nitrogen were enhanced. Meanwhile, the removals of TIA and CTC were found to decrease by 9% and 20% in the aerobic reactor but increased by 5% to 7% in the anaerobic reactor due to the relocation of biomass from the aerobic to the anaerobic reactor. A high degree of AMX biodegradation under both anaerobic and aerobic conditions and partial biodegradation of TIA and CTC under aerobic conditions were confirmed in batch experiments. Moreover, the effect of sludge recirculation on biomass and pollutant removal efficiencies in the 2-stage MBR was revealed using microbial community analyses. Membrane filtration also helped to retain the adsorbed antibiotics associated with small colloidal particles in the system.