Characterization, treatment, and reuse of oilfield wastewater and landfill leachates

Abstract

Wastewater poses numerous health and environmental hazards that require management or treatment. In addition, water scarcity continues to intensify, indicating a larger need for water conservation and new water sources. Wastewater reuse could prove to be a beneficial water source for a variety of industries. Agriculture in particular accounts for 70 % of freshwater withdrawals globally. Currently, studies estimate that 1.7 % of municipal wastewater is intentionally reused. This suggests 98% is discharged back into rivers and streams. Industrial wastewater is another form that is either discharged back into streams after treatment, placed in holding ponds, or injected into wells. In this study we investigated the efficacy of membrane-based treatment of oil and gas wastewater (objective 1). Secondly, we investigated the reuse potential of oil and gas wastewater through chemical, toxicological, and economic testing (objective 2). Finally, we developed a biochar-based treatment strategy for removal of remove per- and polyfluoroalkyl substances (PFAS) from landfill leachate (objective 3). Treatment and reuse of unconventional oil and gas (UOG) produced water are important strategies that address the dual challenges of water scarcity and pollution posed by UOG production. Considering the high salinity and complex chemistry of UOG produced water, it is important to comprehensively analyze the water quality and potential ecological risk of treated produced water for reuse applications. To address objective 1, we evaluated and compared the efficacy of pretreatment followed by nanofiltration (NF) and reverse osmosis (RO) using membranes of varied permselectivity in treating produced water from the Niobrara Shale play in Colorado. We determined the efficacy of each technology in removing inorganic and organic constituents as well as reducing toxicity on Daphnia magna. Our results show that the pretreatment step resulted in a minor reduction of chemical constituents and toxicity and that the NF permeates did not meet the water quality criteria for irrigation and livestock drinking water. Despite high removal rates for most contaminants in the produced water by RO, the concentrations of chloride and boron as well as the sodium adsorption rate (SAR) in the RO permeates exceeded irrigation guidelines. We observed the passage of surfactants with molecular weights much higher than the molecular weight cutoff of NF and RO membranes, suggesting that membranes are not an absolute barrier to organic contaminants. Our results demonstrate that thorough chemical and toxicological analyses are needed to understand the feasibility and potential risk of treating UOG produced water for beneficial reuse. Oil and gas produced water (PW), may help alleviate regional water scarcity affecting agriculture, but is often rich in salts and organic compounds that constrain agricultural applications. To address objective 2, we evaluated the reuse potential of conventional PW through a comprehensive assessment of chemistry, toxicity, and economics by investigating PW from 18 conventionally drilled wells from sandstone formations in the Colorado Denver-Julesburg Basin. Ammonium, total dissolved solids, boron, sodium, and chloride were all close to recommended guidelines for livestock and crop irrigation and surface water discharge. Diesel and gasoline range organics and polycyclic aromatic hydrocarbons were detected in low concentrations in evaporation ponds compared to oil water separators, suggesting volatilization or degradation of organic compounds. Radium levels were generally low, but select samples exceeded the regulatory 5 pCi/g threshold, categorizing them as Non-Exempt TENORM (Technologically Enhanced Naturally Occurring Radioactive Material) waste. EC50 with Daphnia magna (D. magna) showed little to no toxicity for PW sampled in evaporation ponds in contrast to EC50 values of 12 % at the oil water separator, indicating that volatile organics controlled toxicity. However, the Aryl Hydrocarbon Receptor (AhR) bioassay illustrated toxicity not captured by the EC50 test. After chemical and toxicological analyses, it is clear that treatment is required, which informed our techno-economic assessment (TEA). Current PW volumes result in a treatment cost of $5.38/m3 ($1.42/barrel) by nanofiltration, but a scenario with increased volumes will result in a lower cost of $3.83/m3 ($0.60/barrel). Our chemical, toxicological, and economic assessment indicates that the PW in this study has potential to be discharged to surface water or reused for cattle and crop irrigation. Landfills generate a wastewater referred to as leachate, which is produced through degradation of waste and rainwater. Landfills have become a point source for PFAS. PFAS are seen in high concentrations in the leachate and even higher concentrations once the compounds concentrate through foam fractionation. It is important that the wastewater is treated in order to avoid groundwater contamination and improve management techniques. Treatment techniques, like membrane distillation or electrochemical techniques, are expensive and energy intensive. Biochars are a waste product that have potential to sorb PFAS, especially once surface modified. In this study, we investigated two weed feedstocks: Knotweed and Golden Rod. Both Knotweed and Golden Rod biochars were able to remove between 42 - 48 % of total anionic PFAS present in foam fractionation foamate. In landfill leachate, the unmodified biochars removed 50 - 70 % of total PFAS. When modified with poly(diallyldimethylammonium chloride), also referred to as PDADMAC, the biochars performance improved by almost 30%. Granular activated carbon (GAC) outperformed biochar in the foamate, but performed similarly with leachate samples. This likely due to high salt concentrations and a higher ratio of nonpurgeable organic carbon (NPOC) to PFAS. There was preferential sorption of longer chained PFAS, which is due to the increase in hydrophobicity. Similarly, there is preferential sorption towards sulfonated head groups over carboxylic head groups due to the stronger electron withdrawing seen in sulfonated groups. Biochars are an effective way to sorb PFAS from both landfill leachate and foam fractionation foamate, but could be better utilized in a treatment train or sequential sorptions.