Increased oxygen partial pressure enhances toxicity reduction by membrane aerated biofilm reactor treating oil and gas industrial wastewater

• Increased oxygen partial pressure improved bioreactor performance. • All treatments reduced toxicity from onshore and offshore produced water. • Highest removal rate achieved at 6 h HRT with 30.1 g-COD m −2 d −1 for offshore water. • Biofilm communities were dominated by hydrocarbonoclastic and sulphur cycling bacteria. Wastewater produced during oil and gas extraction processes is one of the largest residual water streams. Also known as produced water (PW), it has a complex composition and is typically toxic to aquatic ecosystems. This study explores the use of membrane aerated biofilm reactors (MABR) for biological removal of dissolved organic compounds and associated toxicity. Three different oxygen partial pressures (0.2, 0.6 and 1 bar), the last two mimicking subsea pressures at 20 and 40 m below the sea level, respectively, were tested at three different hydraulic retention times (HRT of 24, 12 and 6 h). Reactors were fed both with onshore and offshore PW. Removal rates for organic carbon increased at decreasing HRT. Highest oxygen partial pressure led to best performance at 6 h HRT (30.1 ± 3.0 g-COD m −2 d −1 ) when treating onshore PW and at all HRT when treating offshore PW, with highest COD removal rate still at 6 h HRT (9.4 ± 1 g-COD m −2 d −1 ). Removal efficiencies ranged from 78 % when treating onshore PW at 24 h HRT to 36–49 % when treating offshore PW at 6 h HRT. All treatments led to a reduction in toxicity (>92 % reduction for offshore PW), with 0.2 bar oxygen partial pressure showing worst performance, while 0.6 and 1 bar did not show significant differences. The biofilms were dominated by hydrocarbonoclastic, responsible of hydrocarbons biodegradation, and sulfur cycling bacteria. Overall, MABR were effective treating PW and operating at higher oxygen partial pressures yield to better performance, especially at high volumetric loading rates.