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.