Combined hydrolysis acidification and bio-contact oxidation system with air-lift tubes and activated carbon bioreactor for oilfield wastewater treatment

•Three isolated bacterial strains were evaluated and used in pilot plant test.•The pilot plant was started up quickly by adding microbes and nutrients.•Installation of air-lift tubes enhanced COD reduction in aerobic tanks.•Activated carbon bioreactor (ACB) was an intensive COD reduction device.•An existing wastewater treatment plant can be upgraded by adding an ACB.

This paper investigated the enhancement of the COD reduction of an oilfield wastewater treatment process by installing air-lift tubes and adding an activated carbon bioreactor (ACB) to form a combined hydrolysis acidification and bio-contact oxidation system with air-lift tubes (HA/air-lift BCO) and an ACB. Three heat-resistant bacterial strains were cultivated and subsequently applied in above pilot plant test. Installing air-lift tubes in aerobic tanks reduced the necessary air to water ratio from 20 to 5. Continuous operation of the HA/air-lift BCO system for 2 months with a hydraulic retention time of 36 h, a volumetric load of 0.14 kg COD/(m3 d) (hydrolysis-acidification or anaerobic tank), and 0.06 kg COD/(m3 d) (aerobic tanks) achieved an average reduction of COD by 60%, oil and grease by 62%, total suspended solids by 75%, and sulfides by 77%. With a COD load of 0.56 kg/(m3 d), the average COD in the ACB effluent was 58 mg/L.