Enhanced pyridine biodegradation under anoxic condition: The key role of nitrate as the electron acceptor

•Pyridine biodegradation was significantly enhanced in the presence of nitrate.•The integrated predenitrification process was effective for pyridine removal.•High recirculation rate was suggested for the operation of ABR–MBBR system.•Paracoccus, Thiobacillus and Paludibacter were dominant in the anoxic system.