3. Results and discussion3.1. Anaerobic biodegradability of heavy oil refinery wastewaterBiodegradability in terms of BD% is an integrate index, which canreflect the degree of biodegradation of the wastewater, preferablyunder anaerobic conditions. A batch experiment was performed todetect anaerobic biodegradability of heavy oil refinery wastewa-ter. The experiment was undertaken for 661 h. At the end of theexperiment, cumulative methane production, residual VFA andsoluble COD concentration were determined. According to theabove-mentioned equations, methane conversion (M%), acidifica-tion rate (A%) and biodegradability were calculated, and the resultsare shown in Table 1. The results indicated that such wastewatercan be treated by the anaerobic treatment process because morethan half of organic compounds can be biodegraded into biogas.However, considerable time was spent on removing over 90 % ofthe organic pollutants when the initial COD concentration wasapproximately 2000 mg/L. This result demonstrated that the refin-ery wastewater contains lots of macromolecular and complicatedorganic compounds. Therefore, the UASB reactor should be oper-ated under low OLR and long HRT when treating heavy oil refinerywastewater.