Bursting the fractionation bubble, can an inexpensive surfactant remove PFAS

Introduction The field of per- and polyfluoroalkyl substance (PFAS) treatment technologies is exploding; yet, practical, cost-effective solutions for removing PFAS from municipal wastewater (WW) at full-scale are still deficient. Treating municipal WW for PFAS is challenging due to analytical complications, competing co-contaminants (i.e. TOC), multiple treatment streams, and large volumes of flow. The ideal treatment solution is a balance between removal efficiency and cost. In this study, a cost-effective approach to an established PFAS treatment technology, foam fractionation (FF), is explored. The City of Cedar Rapids commissioned a study to explore treating PFAS at the municipal scale. Waste streams within the Water Pollution Control Facility (WPCF) were analyzed to identify where within the plant PFAS occur and select candidate locations for further testing. Primary influent (PI), primary effluent (PE), and final effluent (FE) were identified for their detectable concentrations of PFAS. Treating PI and PE could remove PFAS early in the treatment process and reduce PFAS loading in primary sludge and biosolids. Alternatively, treating FE catches PFAS before final discharge. FF, selected for being a proven PFAS removal technology for liquids streams, is a process that concentrates PFAS into foamate utilizing bubbles and the amphiphilic nature of PFAS. The foamate, typically 0.1% to 5% of the treated stream, can be further treated. Surfactant addition is optional if the waste stream does not produce foam. Another driver to explore FF was the natural formation of foam at the WPCF in a collection box. Analysis of the foam indicated that PFAS was being concentrated. This also inspired exploring a non-conventional, economical surfactant: dish soap. Dish soap contains the surfactant sodium lauryl sulfate (SLS). A commonly used surfactant for FF is cetyltrimethylammonium bromide (CTAB). Bench scale FF was performed on samples collected from the PI, PE and FE. The objective was to evaluate which stream and surfactant combination had the highest PFAS removal. Methods Three rounds of testing were performed in the Brown and Caldwell Treatability Lab on samples collected of PI, PE, and FE. The objective of round one testing was to measure the foaming potential with and without surfactant (CTAB and dish soap). Samples were tested to determine if foam was produced and if foam collapsed back to liquid. A second round of tests performed bench-scale FF tests on PI, PE, and FE, each with CTAB or dish soap, to determine PFAS removal. Each reaction was aerated for 30 minutes. The volume of foamate and PFAS concentrations in the effluent and foamate were measured. PFAS was analyzed using EPA Method 1633A. To replicate and confirm the results, the tests were repeated for PE with CTAB and dish soap and FE with CTAB was tested in triplicate. The PFAS analysis of the foamate was modified to process the solids fraction and liquids fraction separately. Results and Discussion Round one demonstrated that samples did not foam without surfactant. The foam produced with CTAB and dish soap collapsed back into liquid, indicating compatibility with FF. The second round tested FF on a bench scale. Tests with CTAB produced a larger foamate volume for all samples tested. CTAB also produced foam for a longer duration than dish soap, which stopped after 10 minutes. Comparing CTAB to dish soap, PFAS removal results were mixed: dish soap and CTAB performed similarly on PE, dish soap had higher rates of PFAS removal from PI, and CTAB had higher rates for removal from FE. The test that had the highest rate of PFAS removal overall, was CTAB in the FE. PE was tested again in the third round. Samples with CTAB performed slightly better than samples with dish soap with a lower PFAS in the reactor effluent. Dish soap, however, produced a larger volume of foamate and contained more PFAS compared to CTAB foamate. Additionally, there were nine PFAS compounds detected in the foamate that were not detected in the initial PE sample. Some PFAS were detected in the solids fraction of the foamate samples, however, most PFAS still occurred in the liquid fraction. Even though the dish soap removed more PFAS than CTAB, the foamate volume produced is undesirable. Lower foamate volumes leads to lower additional treatment costs, such as second stage FF, ion exchange, or PFAS destruction. For FF testing on FE with CTAB in triplicate, the highest rate of PFAS removal occurred at the end of the 30-minute test, though some PFAS were completely removed after only 10 minutes (PFOA, PFBS, and PFHxA). Two compounds (6:2 FTS and PFBA) were not statistically removed. PFBA and PFPeA had measurable concentrations in the effluent, though, PFPeA did decrease with reaction time. Both are short chain and have been documented as difficult to remove from WW. Roughly 30% of the PFAS in the foamate was in the solids fraction, indicating PFAS is removed on the solids. The volume of foamate was 3% of the reaction volume. Overall, FF with CTAB performed. Takeaways FF had high rates of PFAS removal for all streams tested, but the highest rates were seen in the FE with CTAB. Foamate volume is an important consideration for additional costs and should be balanced with removal rates. Even though CTAB outperformed dish soap in most tests, dish soap still performed well and it is worth considering depending on treatment goals and budget.