Abstract
While oilfield produced water (PW) is one of the largest, unclaimed wastewater streams of the oil industry, it could potentially be used as a cultivation medium for microalgae. Microalgae could help with the remediation of this water while also delivering biomass that can be transformed into valuable byproducts such as biofuels. The coupling of these two purposes is expected to cut production costs of biofuels while aiding environmental protection. In this study, we compared the cultivation capacity of the marine model diatom Phaeodactylum tricornutum in media at varying salinities and in media composed of PW from two oilfields in the Central Valley of California that differed drastically in the concentration of inorganic and organic constituents. Specifically, we measured the carrying capacity of these media, the maximum growth rates of P. tricornutum, its cellular lipid accumulation capacity, and its capacity to remediate the most polluted PW source. Our study shows that P. tricornutum can successfully adjust to the tested cultivation media through processes of short-term acclimation and long-term adaptation. Furthermore, the cultivation of P. tricornutum in the most heavily polluted PW source led to significant increases in cell yield and improved photosynthetic capacity during the stationary phase, which could be attributed chiefly to the higher levels of nitrate present in this PW source. Chemical water analyses also demonstrated the capability of P. tricornutum to remediate major nutrient content and potentially harmful elements like fluorine and copper. Because P. tricornutum is amenable to advanced genetic engineering, which could be taken advantage of to improve its cultivation resilience and productivity in an economic setting, we propose this study as a step towards essential follow-up studies that will identify the genetic regulation behind its growth in oilfield PW media and its remediation of the PW constituents.
Highlights
During oil extraction, water trapped with the oil in underground reservoirs is brought to the surface
We consider it very likely that homologous recombination events were responsible for the observed adaptation of the strains to the 0 psu and the McF-produced water (PW) conditions. These results show that prolonged cultivation of P. tricornutum might be a strategic approach to optimize its performance in the specific conditions pertaining to economic mass cultivation in wastewater sources, including oilfield PW
Our analysis shows that diatom cultivation in PW media can achieve substantial biomass that can be used for downstream processing
Summary
Water trapped with the oil in underground reservoirs is brought to the surface. Oil operators commonly dispose of the PW in evaporation ponds, re-inject it into subsurface reservoirs, or transport it to offsite storage facilities [1] These practices do not achieve repurposing of the water for beneficial uses such as land irrigation or groundwater augmentation. Remediation of PW for such beneficial uses relies on costly physicochemical procedures like filtration, reverse osmosis, flotation, adsorption, oxidation, or electrodialysis [2,3] An alternative to these procedures is bioremediation, the use of biological systems to achieve water purification. An alternative biological solution is remediation through microalgal cultivation [7,8] In this case, microalgae either metabolically assimilate the water contaminants [9], or the contaminants adsorb to cell walls or binding sites inside the cell [10]. Because this method permits the concomitant production of biomass through photosynthetic carbon fixation, microalgal-based systems could improve the economic viability of PW bioremediation [11,12,13]
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