Assessment of oil refinery wastewater and effluent integrating bioassays, mechanistic modelling and bioavailability evaluation.

Water is used in petroleum oil refineries in significant volumes for cooling, steam generation and processing of raw materials. Effective water management is required at refineries to ensure their efficient and responsible operation with respect to the water environment. However, ascertaining the potential environmental risks associated with discharge of refinery effluents to receiving waters is challenging because of their compositional complexity. Recent European research and regulatory initiatives propose a more holistic approach including biological effect methods to assess complex effluents and surface water quality. The study presented here investigated potential effects of effluent composition, particularly hydrocarbons, on aquatic toxicity and was a component of a larger study assessing contaminant removal during refinery wastewater treatment (Hjort et al 2021). The evaluation of effects utilised a novel combination of mechanistic toxicity modelling based on the exposure composition, measured bioavailable hydrocarbons using biomimetic solid phase microextraction (BE-SPME), and bioassays. The results indicate that in the refinery effluent assessments measured bioavailable hydrocarbons using BE-SPME was correlated with the responses in standard bioassays. It confirms that bioassays are providing relevant data and that BE-SPME measurement, combined with knowledge of other known non-hydrocarbon toxic constituents, provide key tools for toxicity identification. Overall, the results indicate that oil refinery effluents treated in accordance to the EU Industrial Emissions Directive requirements have low to negligible toxicity to aquatic organisms and their receiving environments. Low-cost, animal-free BE-SPME represents a compelling tool for rapid effluent characterization.

Conventional and high resolution chemical characterization to assess refinery effluent treatment performance.

Refinery effluents represent an emission source of hydrocarbons (HCs) and other constituents to the environment. Thus, characterisation of effluent quality in terms of concentrations of key parameters relative to permitted standards is important and for total petroleum hydrocarbons (TPH), the specific composition of the HC mixture can affect its toxicity to aquatic organisms. Therefore, this study was designed to analyse TPH, benzene, toluene, ethyl benzene, xylenes (BTEX), polycyclic aromatic hydrocarbons (PAHs), (bio) chemical oxygen demand, total nitrogen, total suspended solids and selected metals before, and after, treatment steps to demonstrate removal efficiencies across 13 refineries with variable wastewater treatment systems. Final discharge concentrations of the measured parameters were by 97% within the so called Best Available Technique Associated Emission Levels (BAT-AELs). Further, TPH composition was characterised using high-resolution two-dimensional gas chromatography (GCxGC) analysis to understand the mass distribution by carbon number and specific chemical class. Measurements were compared to SimpleTreat model predictions for validation. SimpleTreat successfully predicted the shape of the effluent composition since it is essentially a removal constant applied to the influent composition. The predictions were of similar magnitude as, or were greater than, the effluent concentrations since SimpleTreat is based on typical performance and is intended to be conservative. This was especially true for aromatic constituents. Reduction in potential HC exposures also coincided with a decrease in predicted toxicity using a mechanistic oil toxicity model, PETROTOX. Overall, the results indicate that EU petroleum refineries are likely to achieve a high performance level regarding effluent treatment.

Application of the Target Lipid Model and Passive Samplers to Characterize the Toxicity of Bioavailable Organics in Oil Sands Process-Affected Water.

Oil sand operations in Alberta, Canada will eventually include returning treated process-affected waters to the environment. Organic constituents in oil sand process-affected water (OSPW) represent complex mixtures of nonionic and ionic (e.g., naphthenic acids) compounds, and compositions can vary spatially and temporally, which has impeded development of water quality benchmarks. To address this challenge, it was hypothesized that solid phase microextraction fibers coated with polydimethylsiloxane (PDMS) could be used as a biomimetic extraction (BE) to measure bioavailable organics in OSPW. Organic constituents of OSPW were assumed to contribute additively to toxicity, and partitioning to PDMS was assumed to be predictive of accumulation in target lipids, which were the presumed site of action. This method was tested using toxicity data for individual model compounds, defined mixtures, and organic mixtures extracted from OSPW. Toxicity was correlated with BE data, which supports the use of this method in hazard assessments of acute lethality to aquatic organisms. A species sensitivity distribution (SSD), based on target lipid model and BE values, was similar to SSDs based on residues in tissues for both nonionic and ionic organics. BE was shown to be an analytical tool that accounts for bioaccumulation of organic compound mixtures from which toxicity can be predicted, with the potential to aid in the development of water quality guidelines.

Technical basis for using passive sampling as a biomimetic extraction procedure to assess bioavailability and predict toxicity of petroleum substances.

Solid-phase microextraction fibers coated with polydimethylsiloxane (PDMS) provide a convenient passive sampling format to characterize bioavailability of petroleum substances. Hydrocarbons absorb onto PDMS in proportion to both freely dissolved concentrations and partitioning properties of the individual constituents, which parallels the mechanistic basis used to predict aquatic toxicity in the PETROTOX model. When deployed in a non-depletive manner, combining SPME with thermal desorption and quantification using gas chromatography-flame ionization creates a biomimetic extraction (BE) procedure that has the potential to simplify aquatic hazard assessments of petroleum substances since the total moles of all hydrocarbons sorbed to the fiber can be related to toxic thresholds in target lipid of aquatic organisms. The objective of this work is to describe the technical basis for applying BE measurements to predict toxicity of petroleum substances. Critical BE-based PDMS concentrations corresponding to adverse effects were empirically derived from toxicity tests on different petroleum substances with multiple test species. The resulting species sensitivity distribution (SSD) of PDMS effect concentrations was then compared and found consistent with the previously reported target lipid-based SSD. Further, BE data collected on samples of aqueous media dosed with a wide range of petroleum substances were highly correlated to predicted toxic units derived using the PETROTOX model. These findings provide justification for applying BE in environmental hazard and risk evaluations of petroleum substances and related mixtures.

Role of entrained droplet oil on the bioavailability of petroleum substances in aqueous exposures.

Bioavailability of petroleum substances is a complex issue that is affected by substance composition, the physicochemical properties of the individual constituents, and the exposure preparation system. The present study applies mechanistic fate and effects models to characterize the role of droplet oil on dissolved exposure and predicted effects from both neat and weathered crude oils, and refined fuel oils. The main effect from droplet oil is input of additional dissolved hydrocarbons to the exposure system following preparation of the initial stock solution. Toxicity was characterized using toxic units (TU) and shows that replenishment of bioavailable hydrocarbons by droplets in toxicity tests with low droplet content (e.g., <1mg/L) is negligible, consistent with typical exposure conditions following open ocean oil spills. Further, the use of volumetric exposure metrics (e.g., mg/L) introduces considerable variability and the bioavailability-based metrics (e.g., TUs) provide a more consistent basis for understanding oil toxicity data.