Full-Scale Demonstration of Low-Cost Solution for Selenium Removal in a Refinery's Activated Sludge WWTP Via Chemical Coprecipitation

Background
Selenium is a natural occurring element that is an essential trace element in living organisms1 but can be toxic at elevated concentrations2. It is prevalent in surface water at geographies with elevated levels of sedimentary deposits and from anthropogenic activities (e.g., agricultural irrigation, coal and phosphate mining, coal combustion, and oil refining)3. In petroleum refining facilities, selenium is a naturally occurring constituent of crude oil that is transferred to wastewater streams through refining processes. Many refineries currently have NPDES permit discharge limits for selenium. There is regulatory momentum for more consistent and stringent selenium standards that will affect numerous industries including petroleum refineries. On June 30, 2016 USEPA finalized the Aquatic Life Ambient Water Quality Criterion for Selenium4 replacing previous recommended National Water Quality Criterion5 under the Clean Water Act. Sixteen states have or are in the process of adopting all, or variations of the Criterion with more expected near term. Activated sludge wastewater treatment plants (WWTPs) can remove selenium as a micronutrient, colloidal adsorption of biochemically reduced selenium to biomass, and/or chemical adsorption to metals. Previous Brown and Caldwell (BC) work documented how aerobic, anoxic, and anaerobic zones alter the forms of selenium in an activated sludge nutrient removal treatment facility and how to optimize zones for enhanced selenium removal6. Low ORP conditions present in anoxic and anaerobic zones create a biochemical environment to reduce soluble selenium. Conversely, in aerobic conditions with positive ORPs, reduced selenium forms such as soluble selenides (e.g., selenocyanate {SeCN}, selenomethionine {C5H11NO2Se}, etc.) and methylseleninic acid (CH3SeO2H) can serve as electron donors resulting in oxidation of selenium to selenite (SeO3-2) and selenate (SeO4-2). Figures 1 and 2 show various forms of selenium at various ORPs and pHs3,7. Selenium in refinery wastewater is predominantly present in the soluble reduced form of selenide, as selenocyanate from sour water stripping discharges. Commonly, refinery WWTPs have activated sludge treatment that oxidize selenocyanate to selenite and to a lesser amount selenate. Selenite can be removed via chemical adsorption or coprecipitation to aluminum and iron hydroxides formed from addition of aluminum and iron salts. At the correct pH, mixing regime, residence time and settling regime greater than 75% of selenite can be removed through chemical adsorption8.
Given the aquatic toxicity issues/expanding regulatory requirements, industrial dischargers with selenium, like refineries, have and continue to evaluate treatment options to increase selenium removal to meet lower discharge limits. Typically, selenium removal has been targeted at the main source of selenium in the sour water stripper. This paper presents the results of a four-month long full-scale trial at a US refinery that evaluated the effectiveness of an innovative application of iron coprecipitation for improved selenium removal at the refinery's existing 10 MGD activated sludge treatment process. The refinery received selenium discharge limits for the first time during their most recent NPDES permit renewal. The permit specified that the new selenium limits be met within fifty-nine months after the permit effective date. The trial was conducted with minimal additional infrastructure to the treatment facility other than the chemical addition system, which resulted in significant capital savings for the refinery in comparison to the cost of a new source or tertiary treatment system for selenium removal.
Approach
A simplified process flow diagram of the refinery's WWTP is shown in Figure 3. The full-scale trial focused on the two essentially identical parallel, but completely independent, activated sludge system trains. A common influent was split to each parallel train with Train 1 receiving ferric chloride at the secondary clarifier and no addition to Train 2, the control. Ferric chloride dosages to Train 1 were gradually increased then decreased during the trial. During the trial, the average ferric chloride dosage as iron was 14 mg/L with a range of 9 mg/L to 35 mg/L. Selenium analyses were performed from multiple locations in the treatment process at the WWTP as shown in Figure 3. Total selenium analyses were completed three times per week from April 15 to August 10, 2021. Three samples were analyzed for the selenium forms of selenate, selenite, selenocyanate, and unknown selenium species. Clarifier effluent TSS, mixed liquor TSS, aeration basin and clarifier pH, effluent total iron, and SVI data were also collected regularly during the trial. Results Table 1 includes average total selenium concentrations during the trial at each sampling location. Figure 4 shows Train 1 and Train 2 total selenium effluent and Train 1 ferric chloride concentrations during the trial. Ferric chloride to Train 1 resulted in an average clarifier effluent total selenium concentration 39% less than that of Train 2. With ferric chloride dosages greater than 20 mg/L as iron, Train 1's effluent was consistently below the WWTP's daily average total selenium discharge limit of 20 µg/L. Selenium forms quantified are shown in Table 2. Selenocyanate was the predominant selenium form into the WWTP. Selenite dominated selenate in the activated sludge system. Figure 5 shows the clarifier effluent TSS for each train. Table 3 presents the average clarifier effluent TSS and clarifier underflow SVI. This confirms ferric salts as effective coagulants for improving sludge settleability. Train 1 had a clarifier effluent TSS 52% less than Train 2 during the trial. Train 1 had a 13% lower SVI than Train 2. The sludge generation data for both trains were evaluated, and no measurable increase was observed during the trial.
Conclusions
Ferric chloride addition can enhance total selenium removal. The results demonstrate how a simple modification to an existing refinery's WWTP to incorporate iron coprecipitation can improve selenium removal when the selenium is most present as selenite, thereby, offering a simpler and more cost-effective removal method of selenium for NPDES compliance compared to traditional oxidation/reduction/adsorption methods at the sour water stripper and other end of pipe methods. While this meets NPDES compliance objectives, there is ample room for further improvement of selenium removal through operational and chemical optimization of the WWTP based on BC's experience from other similar applications. The approach for removing selenium presented here is a simple/low-cost solution that could be applied to numerous other refinery and non-refinery WWTPs to meet selenium discharge limits. Because of the positive results from the trial, and major cost savings (i.e., millions of dollars) in comparison to other options considered, the refinery is proceeding with design and implementation of a permanent iron coprecipitation system at the existing activated sludge treatment system.