Toluene Removal from Produced Water by Biofilm Granular Activated Carbon System

Abstract

Over the last several decades, new strategies for oil and gas produced water disposal have been explored due to concern over increasing seismicity in areas surrounding deep well injection sites. A large area of research is focused on treating produced water for disposal or beneficial reuse. A major constituent in produced water is aromatic hydrocarbons which have high chemical stability and are resistant to conventional biological treatment. The objective of this study was to investigate the ability of a system utilizing biofilm growth on granular activated carbon (BIO-GAC) to remove toluene, an aromatic hydrocarbon common in produced water. Two lab-scale upflow anaerobic sludge bed (UASB) reactors were operated in three experimental phases. A BIO-GAC reactor was inoculated with granular seed sludge from a UASB reactor at the Cedar Rapids, IA Water Pollution Control Facility and Filtrasorb 400 GAC from Calgon Carbon Co., PA. A biological (BIO) reactor was fed with the same granular seed sludge only with no GAC addition and was used as a control for experimentation. In phase I, the BIO-GAC and BIO reactors were fed with synthetic produced water to simulate real characteristics found in produced water across the United States. Operational conditions were identical for each reactor. The hydraulic residence time was 10 days, and a recirculation pump was used to achieve an upflow velocity of 1.25 m/h at which the sludge bed remained immobile at the bottom of the reactor. After 150 days of acclimation, both reactors had achieved COD removal rates around 80%. During this time, biofilm attachment on GAC particles in the BIO-GAC reactor was confirmed by scanning electron microscopy (SEM) imaging. Phase II involved adding toluene at a level consistent with average concentrations in produced water, 10 mg/L, to the feed water. Analysis by solid phase micro-extraction (SPME) and gas chromatography (GC) found 99.9% toluene removal in all BIO-GAC effluent samples and an average 73.2±8.1% removal in the BIO reactor through 60 days. These results show significant difference between the two systems’ toluene removal abilities, with BIO-GAC clearly superior. The objective of phase III was to observe the effects of salinity on the performance of both reactors. Salinity started at 1% (10 g/L) in the influent feed and was subsequently increased by 1% every 7 days until a final level of 3%. Toluene removal rates in the BIO-GAC reactor remained steady at 99.9% throughout this phase. The BIO reactor, on the other hand, saw toluene removal of 85.5±2.8%, 64.2±7.0%, and 35.1±25.4% at 1%, 2%, and 3% salinity, respectively, displaying a clear decrease in performance. These results indicate salinity affected toluene removal performance in the BIO reactor, but also indicate the BIO-GAC reactor had a resistance to saline shock. The findings of this study demonstrate BIO-GAC’s ability to effectively treat produced water with high levels of toluene, even in hypersaline conditions. Moreover, removal rates of chemical oxygen demand (COD) remained steady at 80% throughout experimentation, indicating BIO-GAC systems have the ability to remove a wide range of constituents from produced water that would not be possible by BIO or GAC alone. Overall, the hybrid BIO-GAC system may be a solution to the produced water disposal problem by presenting a treatment process that can be easily adopted by professionals in the industry.