It can be seen from Fig. 2 that the mean COD of the final effluent decreased to around 90 mg/l, indicating that the COD of the wastewater is reduced by about 74% (mean value) after the treatment by the combined process and meets the criterion specified by Chinese discharge standard for water pollutants from oilfield industry (GB8978-1996). The COD removal effect using the present biosystem is more predominant than other techniques used to treat similar oilfield wastewater. Although the COD removal in the UASB reactor was influenced by the COD change of the influent, the final effluent COD was found to remain stable. This indicates that the combined biosystem has a good anti-shock ability as the carriers in the I-BAF reactor can protect microorganisms immobilized on them.