Heavy oil is an important part of energy sources, but the refining wastewater is difficult to treat by the
conventional anaerobic process, which has low efficiency and poor ability to resist impact load. In this
study, an up-flow anaerobic sludge bed (UASB) reactor was applied to treat heavy oil refinery wastewater
containing large amounts of polar organics. Through a progressive increase of hydraulic conditions,
the average removal efficiencies of COD and total oil reached 70% and 72%, respectively, at an organic
loading rate (OLR) of 3.44 kg COD/m3 d. GC–MS analysis revealed that more biodegradable organic acids
and alcohols were generated and macromolecular polar organics were degraded into small molecular
intermediates after UASB treatment. The morphology observation of the sludge demonstrated that granular
sludge with an average particle size of 1 mm was formed. Moreover, the predominant species and
microbial community shift could reflect the performance of the reactor. The long-term operation of UASB
exhibited excellent polar organic removal efficiency. The study demonstrated the potential of UASB as
an alternative for high-efficiency anaerobic treatment of heavy oil refinery wastewater.