Selenium in Petroleum Refineries: Pervasiveness, Treatment Approaches, and Regulatory Status

Introduction In petroleum refining facilities, selenium is a naturally occurring constituent of crude oil that is transferred to wastewater streams through refining processes, largely through sour water streams from control of overhead gases from the crude distillation, fluidized catalytic cracking, hydrotreating, and reforming processes. Other smaller sources of selenium in refineries are waters from crude preparation (e.g., washing, desalting, and dewatering) operations. Based on the firm's research and experience, this paper presents an overview of selenium pervasiveness, regulatory status, and treatment approaches in US petroleum refineries. Selenium Presence in Refineries In 2019 there were 143 US refineries. Of those 143, 90 refineries discharge their wastewater directly to waterways or through off-site industrial wastewater treatment plants that later discharge to waterways1. Most of the remaining refineries send their process wastewater to a municipal wastewater treatment plant, and a handful inject their wastewater underground into waste disposal wells or into oil and gas formations. The US EPA identified 39 refinery discharge permits with monitoring requirements and/or numeric limits for selenium1. A report from the Environmental Integrity Project (EIP) recorded selenium discharge concentrations and flows from 81 refineries that were operating in 2021 and considered direct discharges of process wastewater or discharges through an off-site industrial wastewater treatment plant that primarily treated refinery wastewater. EIP reported that average selenium discharge concentration from the refineries was 52 µg/L with a range of average concentrations from non-detect to 394 µg/L. The average daily selenium loading rate from the refineries was 2.1 lb/d with the highest refinery average discharge rate being 14 lb/d2. A complete list of all the identified direct discharge refineries along with their corresponding operable capacity, flow, selenium concentration, and selenium loading will be presented in the full-length paper. Regulatory Status EPA sets the minimum requirements for each industry to reduce pollutants in wastewater. States can raise this bar by setting stricter limits or limiting additional pollutants, but many choose not to. Instead, many states rely entirely on EPA's minimum standards when writing permits. EPA first established effluent limitations guidelines and standards (ELGs) for the Petroleum Refining Point Source Category in 1974 and last updated them in 1985. Selenium is not one of the pollutants currently restricted in these guidelines. To keep pace with improvements in treatment technology, EPA is required to review the effluent limits for industries at least every five years and update effluent guidelines if appropriate. EPA most recently decided not to revise the refinery effluent guidelines on January 20, 2023. There is regulatory momentum for more stringent selenium standards that will affect numerous industries including petroleum refineries based on the 2016 final EPA Aquatic Life Ambient Water Quality Criterion for selenium3 replacing previous recommended National Water Quality Criterion4 under the Clean Water Act. EPA's recommended water quality criteria are not rules, nor do they automatically become part of a state's water quality standards. States must adopt into their standards water quality criteria that protect the designated uses of the water bodies within their area. The new Criterion, summarized in Table 1, provides recommendations to states and tribes authorized to establish water quality standards under the Clean Water Act and includes both fish tissue elements and water column elements. EPA recommends that when implementing the criterion, the fish tissue elements take precedence over the water column, elements, except in certain circumstances. As shown in Figure 1, as of November 2023, 26 states have, or are in the process of adopting all, or variations of the Criterion with more expected near term5. Selenium Treatment in Petroleum Refining Table 2 presents a summary of known full-scale selenium treatment technologies installed at US refineries, based on the firms knowledge. Table 3 describes some more commonly used selenium treatment technologies for refineries. Other refineries have avoided needing to install a selenium treatment system at their refinery by making modifications to eliminate or reduce the applicable selenium discharge requirements (e.g., indirect discharge to POTW in lieu of direct discharge, relocation of outfall to a larger receiving water body, and creation of a mixing zone). Conclusions Historically, many refineries have not had selenium discharge limits because the Petroleum Refining Point Source Category ELGs do not include selenium. However, as more states begin to adopt the 2016 EPA Aquatic Life Ambient Water Quality Criterion for selenium, more refineries are expected to begin to receive selenium discharge limits. Treatment for selenium reduction can become complex and there are many treatment options to consider for a refinery. Most refineries are either treating the stripped sour water by oxidation followed by precipitation or treating at the wastewater treatment system with iron co-precipitation. A few refineries have avoided installing selenium treatment through different regulatory approaches.