Treatment of Produced Water with a Membrane Bioreactor

The oil and gas industry in the United States generates more than three billion tons of wastewater annually. This ‘produced water’ is characterized by saline water containing a variety of pollutants, including water soluble and immiscible organics as well as many inorganic species. Several of the dissolved contaminants, namely benzene, toluene, ethylbenzene, and xylenes (BTEX), are known to be hazardous at low concentrations. Although most produced water is disposed of via re-injection, the remaining amounts that are discharged on the surface are significant. In order to reuse produced water, removal of both the inorganic dissolved solids and BTEX is necessary. Reverse osmosis (RO) membrane filtration is a promising treatment option, especially for inorganics, but fouling of the membranes, due to the organic content in produced water could result in marked reductions in the flux rates. Therefore, pre-treatment is generally required for cost-effective operation. The focus of the current research was, therefore, to evaluate the effectiveness of a membrane bioreactor (MBR) to simultaneously remove both organic acids and hazardous BTEX constituents from produced water.The laboratory scale MBR was operated at a 9.6 hr hydraulic residence time with a 10 L/min aeration rate to treat synthetic produced water containing 430 mg/L of organic acids and 10.25 g/L of inorganic ions which are representative of saline produced water. Results indicate that the MBR system degraded 92% of the organic acids (100% acetate and 85% malonate) in synthetic produced water. The biomass levels (MLVSS) that developed in the MBR system treating high-TDS produced water (1.0 g/L) were lower than those that developed in an identical MBR system treating a low-TDS water (4.5 g/L). This result suggests that the produced water may minimize the rate of biofouling that occurs on the membrane. Though inorganic precipitates did clog the membrane system, controlling the pH of the water was found to minimize this problem as well. When BTEX was simultaneously introduced into the MBR system (1800 mg/day) with organic acids, 80% overall removal via biodegradation was observed. During this period, organic acid removal was maintained at the previously high levels and was not affected by the addition of BTEX. These results suggest that simultaneous bio-degradation of both BTEX constituents and organic acids found in produced water may be possible.