BIOLOGICAL TREATMENT OF HYPERSALINE PRODUCED WATER USING BIOFILM TECHNOLOGY

Abstract

Gas and oilfield produced water (PW) is associated with severe water quality issues, such as high salinity and high fractions of refractory organic matter. Sustainable management strategies for PW are a major challenge due to the high volume of wastewater generated and the toxicity of aqueous components, including dissolved organics. Biofilm-based biological treatment is a promising technique for removal of the biodegradable organic component of PW due to multiple performance advantages over conventional activated sludge systems, including smaller footprint, high settleability, and higher resistance against salinity and toxic compounds. Biofilm-based treatment also has the potential to degrade a broad range of organic compounds due to the presence of stratified redox zones within the same biofilm structure. In this work, laboratory-scale sequencing batch reactors were constructed and used to assess the performance and viability of different biofilm-based biological treatment systems for produced water. First, the formation of aerobic granular sludge (AGS) under hypersaline conditions using both an enriched halophilic inoculum and an activated sludge culture was studied as a first step toward introducing biofilm technology to the field of hypersaline PW treatment. AGS formation could be achieved at hypersaline conditions up to 85 gL-1 NaCl) and was improved by the use of the enriched halophilic inoculum. Granule performance and stability were linked to the relative in production of extracellular polymeric substances (EPS) and alginate-like exopolysaccharides (ALE), but not to substitution of divalent cations in the EPS. Second, two biofilm-based reactors, one containing AGS and the second containing a biofilm grown on activated carbon support material (Bio-GAC), were used to degrade a high-strength organic load consisting of a mixture of aromatic hydrocarbons (benzyl alcohol, o-cresol, and phenol) in a hypersaline synthetic produced water. The removal efficiency and production of secondary degradation by products were monitored and compared to the results from a conventional activated sludge reactor. The Bio-GAC reactor achieved 98% removal of influent organics with minimal export of catechol, a potentially toxic byproduct. The AGS reactor also showed high levels of organic degradation, but had more difficulty removing phenol. Both reactors outperformed the conventional activated sludge reactor (CAS) in terms of organic load removal. Sorption studies and X-ray photoelectron spectroscopy (XPS) analysis showed that Bio-GAC had rapid sorption and degradation of aromatics, while the AGS accumulated aromatics rapidly then degraded them more slowly. Finally, the long-term performance of the two aerobic biofilm reactors (Bio-GAC and AGS) was compared to that of two anaerobic biofilm reactors with anaerobic biofilms grown 1) on an activated carbon support material (AnGAC) and 2) in an up-flow anaerobic sludge bed reactor (UASB) with respect to removal of influent organics and export of biomass. All four reactor designs achieved 85% removal of influent aromatics after an extended conditioning time, with the aerobic systems outperforming the similar anaerobic designs. Biomass export was greater from the aerobic reactors, but all effluents would require treatment to remove solids. XPS showed differences in the structure of the biofilms formed under aerobic and anaerobic conditions. XPS technique is a powerful tool to monitor the fate of organic compounds within the biofilm and to observe the development of the biofilm itself. The current study framed the application of biofilm-based technologies to remove toxic organic loads from hypersaline wastewater and overcome drawbacks associated with conventional biological treatment systems. Overall, the outcomes of this study can be manipulated beneficially to treat a wide variety of industrial wastewaters.