Analytical methods

COD, TS, VS, TN and VFA were determined according to the standard methods³⁴. pH was measured daily by a pH meter (FE20, Mettler, Switzerland). Oil content in the oil refinery waste activated sludge was determined by an oil-content analyzer (GG330-OIL-420, ZhongXi, China) according to national standard method in China³⁵. Oil extraction followed Soxhlet extraction method³⁶ and carbon tetrachloride was used as extractant. The organic chemical composition of the extracted oil was characterized using a gas chromatograph-mass spectrometer (GC-MS). The GC-MS conditions followed the method of Wang et al.³¹. The sludge samples were centrifuged at 6000 rpm for 15 min and then filtered through a 0.45 μm disc-filter (JinTeng, China). Obtained supernatant was used to determine SCOD, VFA, protein and polysaccharide. Soluble protein was determined according to the Lowry method³⁷, and polysaccharide was determined using the Anthrone-method³⁸. The composition of VFA including acetic acid, propionate acid, butyric acid, and valeric acid was determined by a Gas chromatograph (SP-2100, Beijing Beifen-Ruili analytical Instrument Co. Ltd.) equipped with a FID detector and a capillary column. The temperatures of oven, injector and detector were 130 °C, 250 °C and 260 °C, respectively. Biogas composition was analyzed by this gas chromatograph equipped with a TCD detector and a Porapak-Q column. Helium gas was used as carrier gas, and temperatures of oven, injector and detector were 80 °C, 100 °C and 150 °C, respectively.