Beyond the PFAS treatment Barrier: Key Lessons from an 18-Month Pilot on PFAS removal from Wastewater

At the intersection of water reuse and PFAS regulatory pressure, Western Municipal Water District in southern California has embarked on one of the first full-scale PFAS adsorption installations for municipal wastewater prior to aquifer recharge. For Western, the complexity of PFAS impacts proved to be far beyond those observed in drinking water matrices with PFAS concentrations greater than 1,000 ng/L coupled with concentrations of effluent organic matter approaching 6 mg/L of total organic carbon (TOC). With this challenging water quality coupled with tightening PFAS regulations in wastewater discharges, direct adsorptive treatment from their tertiary treated wastewater was expected to yield rapid changeout intervals and subsequently higher cost. As water reuse treatment applications become more common in the United States, PFAS treatment will become a critical driver of process selection and operating cost. Tasked with developing a robust PFAS treatment scheme for aquifer recharge of wastewater effluent in southern California, this presentation will focus on the outcomes from an 18-month PFAS wastewater treatment investigation with bench and pilot scale results related to the feasibility of granular activated carbon (GAC) and ion exchange resin (IX) coupled with extensive pretreatment to meet newly released PFAS MCLs in wastewater treatment applications.

Bench scale rapid small scale column tests (RSSCTs) were used to screen adsorbents and operational conditions that subsequently informed the development of a pilot study to investigate adsorbent treatment effectiveness. RSSCTs were conducted for GAC, IX, and novel sorbents at a variety of empty bed contact times (EBCT). The best performing PFAS adsorbents from the RSSCTs were initially piloted for 1 month directly on the tertiary treated wastewater before bio-fouling and high concentrations of effluent organic matter quickly exhausted the adsorbents necessitating a pretreatment step to prolong PFAS adsorption performance and sustain operability. In this first phase of PFAS adsorption piloting, PFOA breakthrough occurred in less than 20 days with GAC operated at 20 minutes EBCT while biological fouling contributed to excessive head loss accumulation and inhibited pilot operation. After an unsuccessful direct adsorption pilot, a pretreatment piloting campaign comprised of conventional coagulation, flocculation, sedimentation, and cloth filtration was instituted to reduce concentrations of competitive adsorbates (i.e. TOC) and algal cells that were inhibiting adsorption effectiveness.

This conventional pretreatment train was selected to utilize existing full-scale infrastructure at the WWRF which could be optimized in this application upstream of PFAS adsorption. Other pretreatment configurations were also considered including direct filtration schemes eliminating the sedimentation phase and media filtration schemes omitting the cloth filtration step. Optimization of the piloted pretreatment train was conducted through a series of jar testing experiments to determine adequate pH and coagulant dose as well as piloting trials to modify floc particle sizes for better removal in downstream sedimentation and filtration processes. Pretreatment was able to reduce TOC levels by 30% and turbidity levels by 90% to sustain a paired adsorption train with both GAC and IX in series to treat PFAS concentrations greater than 1,000 ng/L down to non-detect for more than 100 days. Comparison of adsorption longevity of the pretreated wastewater effluent to direct adsorption treatment suggested that adsorption performance improved by a factor of 3 yielding considerable savings associated with adsorbent changeout. Results of the pilot study and coupled cost estimates and facility layouts served as the basis for treatment selection and full-scale design.

The implications of this pilot study highlight the decreased PFAS treatment longevity and subsequently higher changeout costs that are anticipated in water reuse applications. Analysis of PFAS changeout intervals revealed that there was cost incentive to invest in optimizing adsorption pretreatment, as was done in this study, to prolong adsorbent treatment effectiveness. This presentation will summarize bench and piloting results used to optimize TOC removal in adsorption pretreatment that ultimately contributed to improved PFAS treatment longevity for downstream GAC and IX. Piloting nuances associated with PFAS adsorption from treated wastewater including how to mitigate biological fouling and necessary water quality targets for pretreatment will also be shared. As one of the earliest pilot studies evaluating adsorption technologies for PFAS removal from wastewater, this presentation will highlight results that can be used to better inform treatability evaluations in other water reuse applications demonstrating the cost savings and operational benefits of adsorption pretreatment and hybrid adsorbent orientations.