Treatment of heavy oil wastewater by a conventional activated sludge process coupled with an immobilized biological filter

A field pilot study had been constructed in the Liaohe oilfield, China to treat heavy oil wastewater enriched with large amounts of dissolved recalcitrant organic compounds and low nutrient of nitrogen and phosphorus by conventional activated sludge process (CAS) coupled with immobilized biological aerated filter (I-BAF). After biological treatment, the chemical oxygen demand (COD) was removed around 64% when the hydraulic retention time (HRT) was 18 h. The average effluent COD reached approximately 75 mg L−1, which met the national discharge standard. Gas chromatography–mass spectrometry (GC–MS) indicated that the CAS could completely remove phenolic, alkenes, aldehydes and organic acid compounds from the wastewater and the alkane components were removed by the I-BAF. Environment scanning electron microscopy (ESEM) disclosed that bacteria flourished in both reactors during the operating period and most of them resemble rods and filaments. The bacterial community structure analysis based on Polymerase Chain Reaction–Denaturing Gradient Gel Electrophoresis (PCR–DGGE) technology revealed that the predominant bacteria in the CAS reactor belonged to the Pseudomonas, Planococcus groups and the Agrococcus, Acinetobacter groups that were major degraders in the I-BAF reactor. Although some high molecular weight n-alkanes (C15–C23) were found to be refractory in our biotreatment systems, it could be improved by optimizing the process.