Initial results of District’s Phase 2 PFAS sampling

Through 2022, the District was engaged in phase 2 of its PFAS sampling plan. While we are completing the final analysis of those results as of March 2023, we can provide some initial findings, and the final results will be added to this website once available.

Recapping the phases

The District is following a multistep, multi-year action plan to understand the presence of PFAS in District influent (incoming wastewater), effluent (cleaned, outgoing water), and biosolids and began a PFAS sampling program in early spring 2021. The objective of the first phase was to understand the types and amounts of PFAS in our influent effluent, biosolids, recovered struvite, and the polymers we use in our treatment process. These results were made available in September 2021. Phase 2 of the District’s sampling plan, which started in early 2022, was designed to continue to close the data gaps identified in the sampling and analysis plan and answer key questions raised after the analysis of phase 1 results.

More specifically, Phase 2 committed to monthly and quarterly testing to begin generating a temporal data set to provide a more holistic view of the types and amounts of PFAS in our influent, effluent and biosolids. We also added additional biosolids process sample points in phase 2 to assess whether the types of digestion processes we use for the different products may influence PFAS concentrations. The District took steps toward evaluating how the acceptance of PFAS-containing wastewater could impact District effluent concentrations. This resulted in a technical memorandum and modeling tool for the District’s use in evaluating acceptance of PFAS-containing wastewater to ensure WPDES permit compliance for our pretreatment and waste acceptance programs. This follows the items in the District’s action plan around expanding pollution prevention focus on industrial users.

Initial results

The information presented here is preliminary; we completed sampling at the end of the fourth quarter of 2022 and await the final analysis of those samples. In addition, this update only focuses on PFOA and PFOS, as those are the two compounds that currently have promulgated surface water discharge standards under state law that affect the District’s discharge permit. The forthcoming report will have a final analysis of all 33 PFAS compounds.

The additional sampling completed in 2022 as part of phase 2 focused on two areas:

  1. Expanding Class A and Class B biosolids sampling to involve characterization of the biosolids at sample points in the solids handling process and product endpoints; and
  2. Expanding influent and effluent wastewater sampling beyond the snapshot data generated in phase 1 and measuring concentrations over time to better understand differences in force main influent data and how that may impact the effluent.

Expanded biosolids sampling results

Looking at PFOA and PFOS for both Class A and Class B biosolids, the levels were consistent over time, and the levels were consistent during the different parts of the solids handling processes. The process to create Class A cake (a drier biosolids product) and Class B Metrogro (a liquid product) is the same, except that the Class A cake undergoes an additional digestion and dewatering process. The levels of PFOA and PFOS in Class A cake are similar to those of Class B Metrogro immediately following the treatment process. However, we did find that Class A cake levels begin to differ from Class B Metrogro levels as the Class A cake sits over time. Overall levels of PFAS found in biosolids during phase 2 are very similar to levels found during phase 1.

Influent and effluent results

When looking at a flow-weighted average of the District’s five force mains, which are the large pipes that aggregate wastewater from our service area and bring it into the plant, the levels of PFOA and PFOS in influent were consistent from month to month. In addition, levels of PFOA were consistently lower than PFOS. When looking at the individual force mains, there were differences over time and some differences among the force mains. However, these differences showed the same pattern whether looking at PFOA or PFOS.

Looking at the effluent over time, PFOA and PFOS continue to be below water quality standards each month. However, some months are different than others, and changes in influent levels show a response in effluent levels for the same month. Overall, the levels of PFOA are consistently higher in the effluent than the influent, and levels of PFOS show the opposite pattern (lower in effluent than influent).

Phase 3 sampling

The District’s goals for phase 3 are twofold: The first is to continue the temporal monitoring of influent, effluent, and Class A and Class B biosolids. By doing so, we will continue to add to the existing District data set to better inform pollution prevention efforts and understand how we will be affected by future regulatory drivers. In addition, this will help align the District’s sampling and analysis with the monitoring protocols outlined in the recent administrative rules that identify water quality standards for discharges of PFOA and PFOS in state surface waters. The District anticipates these standards for these compounds will be in our next permit.

The second goal is to build upon the District’s knowledge of PFAS in recycled biosolid products to better understand if and how pollution prevention efforts will impact PFAS in biosolids. This will entail further analysis of the District’s growing data set, additional review of regulatory guidance documents, and other published resources to supplement the data collected to assess the feasibility of creating a predictive modeling tool for estimating PFOS and PFOA concentrations in biosolids.

Phase 3 sampling begins in late March and will run for 12 months. The final results and analysis for phase 3 sampling will be added to the Results page of this website once available in 2024.