As an alternative to the conventional activated sludge (CAS) process, this paper investigates the use of B350M and B350 group microorganisms immobilized on carriers in a pair of Biological Aerated Filter (BAF) reactors to pre-treat oil field wastewater before desalination. By operating the biodegradation system for 142 days with a hydraulic retention time (HRT) of 4 h and volumetric load 1.07 kg COD (m(3) d)(-1) at last, the reactor immobilized with B350M achieved mean degradation efficiencies of 78% for total organic carbon (TOC) and 94% for oil, whereas that with B350 only reached 64% for TOC and 86% for oil. The influent wastewater contains organic substances from C13H28 to C32H66, and a total of 16 polycyclic aromatic hydrocarbons (PAHs). The degradation efficiencies of PAHs in the BAF immobilized with B350M and B350 microorganisms are 90% and 84%, respectively. It is observed that the biological diversity of microorganisms in the reactor containing B350M (seven more strains of bacteria survive) is richer than in that containing B350. A large quantity of filamentous microorganisms developed in both reactors without causing foaming or bulking. (c) 2006 Elsevier Ltd. All rights reserved.