A Perspective on Managing WWTP Influent Inputs to Reduce PFAS Mass Loads to Biosolids.

Dr. Matthew Askeland, Victorian Environment Lead - ADE Consulting Group

The widespread distribution of per- and poly- fluoroalkyl substances (PFAS) in the environment has raised significant concerns regarding their impact on human health and the environment. This is particularly pertinent in the context of biosolids, the treated residuals from wastewater treatment plants that are often recycled for land application towards increasing soil nutrients and adding organic carbon to improve soil structure. This article aims to outline the challenges posed by PFAS contamination in biosolids, shedding light on the complex interplay of influent sources, PFAS congener composition, and how these impose limitations on management strategies.

PFAS in Biosolids: An Overarching Challenge

While biosolids offer a sustainable means of enriching soils and promoting sustainable agriculture, the seemingly pervasive nature of PFAS compounds has cast a shadow over their beneficial reuse. As a result, unraveling the dynamics of PFAS contamination in biosolids has become imperative for common-sense and holistic waste management strategies in the wastewater management sector. A communication released earlier this year detailed how the organics recycling industry need to look wider, not closer at PFAS in organics waste streams to understand and manage PFAS effectively (https://landandgroundwater.com/ecronicle/pfas-environmental-trends-require-us-to-look-wider-rather-than-closer). However, understanding the types of PFAS species present in these end-of-pipe waste streams, as the article suggests, is only a first step and needs to be accompanied by an improved understanding of where they originate and how their behavior in wastewater treatment processes results in the concentration of some PFAS in biosolids.

Modern studies have frequently demonstrated that wastewater treatment plants do not remove PFAS, and in most cases precursor loads can result in the enrichment of wastewater treatment plant (WWTP) PFAS outputs 19 – 3500 times the influent concentration (Vo et al., 2022; Helmer et al., 2022). Perfluorocarboxylic acids (PFCAs) such as perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA), perfluoropentanoic acid (PFPeA), and perfluorobutanoic acid (PFBA) are frequently encountered, and typically increase in concentration across treatment process. However, other species such as perfluorooctane sulfonate (PFOS) are removed via partitioning into the biosolids fraction (Helmer et al., 2022). Moodie et al. (2022) in part supported this, but demonstrated that the dominant PFAS class partitioning to the solid fraction (43% of mean Σ44 PFAS1 mass) can include the di-substituted phosphate esters as well as the PFCAs. Coggan et al. (2019) found that partitioning to solids within WWTP processes increased with increasing fluoroalkyl chain length. However, Ebrahimi et al, (2021) demonstrated that PFAS sorption to biosolids is also based on the qualities of the residuals themselves, where sorption increased based on protein fraction > organic matter fraction > lipid fraction, and PFAS Kd increased with ionic strengths and decreased with increasing pH values in biosolids. As such, it is clear that the mechanisms which enrich PFAS in biosolids during the WWTP process are complex, and largely governed by inputs into the system such as PFAS species distribution and nature of the suspended solid fractions. Overall, studies show that biosolids can embody significant PFAS mass loads, and if not adequately managed these pose potential risks to land and groundwater when beneficially reused in land application operations (Johnson et al., 2022).

Figure 1 - Biosolid windrow management as generated by DALL-E AI

Mechanisms for Managing PFAS in Biosolids:

Mitigating PFAS risk in biosolids typically involves three principal approaches: prevention, treatment, and risk management through controlled reuse. Prevention seeks to intercept PFAS before they enter biosolids, while treatment methods attempt to eliminate PFAS from the finished biosolids product either through separation-concentration or direct destruction (Vo et al., 2020; Garg et al. 2023). Risk management emphasizes judicious biosolids reuse practices. Among these, behavioral changes regarding biosolids reuse emerge as an attractive, low-capital-cost solution that hinges on effective management practices that promote reuse in a risk proportionate manner, with lower risk (less contaminated biosolids) used for sensitive uses such as agriculture, and higher risk (likely more contaminated) biosolids used for less sensitive end uses (HEPA 2023) such as landscaping and mine rehabilitation. This is contextual, and managing risk in this manner has its limits, where exceeded, more costly risk mitigation mechanisms such as turning biosolids into biochar, energy or disposal are relied upon.

However, stopping PFAS from getting into biosolids in the first place is naturally bandied as the perfect solution, and in some ways, it is. It solves the PFAS problem not only for biosolids, but also for recycled water and effluent. However, this cannot be achieved simply through banning PFAS. Literature appears to suggest that widespread bans and tighter trade waste restrictions would see the legacy issue of managing PFAS already entering the system from various sources with us for decades (Gallen et al., 2018). Further, a change in the types of species present in biosolids shifting from PFOS, PFOA and PFHxS to shorter chain perfluorocarboxylic acid species (Gallen et al., 2022; Helmer et al., 2022) shown that PFAS challenges will extend well beyond PFOS, PFOA, and PFHxS, which are still present in wastewater globally despite phaseouts.

Logically, the next item that is often raised is the potential to treat influent to prevent PFAS from sorbing to biosolids. This presents several key challenges around the complex influent matrix but may be feasible in some cases based on treatment plant scale. Irrespective of whether reducing inputs to biosolids is achieved by reducing PFAS inputs to influent, or treatment of influent, industry needs to know more about PFAS concentrations in influent and the relevant sources to adequately manage PFAS inputs to WWTPs, and hence into biosolids.

Figure 1 - Biosolid windrow management as generated by DALL-E AI

Deciphering Influent Complexity
An operationalizable understanding of PFAS entry routes into biosolids hinges on characterizing influent sources and their PFAS profiles. Influent, comprising trade waste, domestic sewage, and diverse inputs like stormwater and groundwater, and necessitates detailed analysis for effective management. The composition and distribution of PFAS compounds within influent sources assume pivotal roles in shaping biosolids contamination profiles. Szabo et al. (2023) identified PFAS concentrations in influent ranged not only between PFAS species distribution, where perfluoroalkyl carboxylic acids (PFCAs), disubstituted phosphate esters (diPAPs), and perfluoroalkyl sulfonic acids (PFSAs) contributed weekly mass loads of 37%, 30%, and 17% respectively, but also temporally, with different mean sum PFAS detected on different days across a range of PFAS species over a week long monitoring period. Other studies have quantified the per capita background release of PFAS to WWTPs in Australia, with Nguyen et al. (2022) estimating this to be 8.1–24 μg/d/per person for the 12 PFAS species studied. However, noting the variety of species present in wastewater, this is likely an underestimation of total PFAS mass flux, particularly considering that there is limited data for novel short-chain, ultra-short chain, and PFAS precursor species available globally (O’Connor et al., 2022). This is supported by Gallen et al. (2022), where a change from longer chain PFAS species to short chain perfluorocarboxylic acids was highlighted as a key consideration or the future. Effectively managing PFAS contamination in biosolids demands insights into influent PFAS species and their origins, where questions arise if PFAS detected in biosolids are namely transformation products, those PFAS species in influent that have a higher affinity for sorbing to solids, and whether each of these were originating in trade waste, domestic sewage, or other inputs. Irrespective, the influent PFAS mass flow is a factor of PFAS species distribution based on input sources, their relative concentrations, and the relative volumes of each of these inputs. Developing a better understanding of these will assist in evaluating the efficacy of strategies such as trade waste regulations at reducing PFAS mass flows to WWTPs. Striking a balance is crucial, as high PFAS concentrations in a specific source might not necessarily correlate with high mass flows to biosolids due to the prevalence of species that prefer remaining in solution or undergo transformative processes by biotic or abiotic means (O’Connor et al., 2022).

Effectively, literature is somewhat still exploring this notion, with studies such as Feng et al. (2012) presenting an early position that population-related emissions cannot wholly explain the occurrence and concentrations of PFAS in WWTPs and that higher PFAS influent concentrations are typically observed in WWTPs located in predominantly industrial areas. More recently, Gallen et al. (2022) suggested that on days of industrial input to influent, industrial discharges would represent >50% of average annual loads to WWTPs for eleven studied PFAS. The forensic approach employed by Szabo et al. (2023) detected simultaneous pulse events lasting up to 3 hours for PFOS and PFHpS, and considering these were above the baseline mass flow, indicated significant industrial or commercial waste discharge. An earlier study by Coggan et al. (2019) used principal component analysis to show strong associations between odd chained PFCAs, PFHxA, PFSAs, and 6:2 FTS with daily inflow volume and the proportion of trade waste accepted by WWTPs. This is supported by a much larger meta-analysis conducted later by Cookson and Detwiler (2022), where over 460 influent samples across a variety of international studies found effluents from WWTPs that include a significant industrial component to their influent have higher PFAS concentrations.

Conversely, while Nguyen et al., 2022 acknowledged the number of industrial sites positively correlated with the per capita mass load of Σ12 PFAS (r = 0.5–0.63, p < 0.01), it was posited that this only supplemented the background mass flow with more than half of the PFAS (∼59%) mass attributed to background release (domestic inputs) and the remaining (∼41%) to catchment specific point sources and industry. In a review by O’Connor et al., (2022), it was noted that PFOA, PFOS, PFHxS, PFBA, and PFHxA are the most dominant PFAS compounds found in domestic effluents, however concentration and species distributions varied greatly. Vo et al. (2020) noted a broader range of PFAS detected across studies and estimated a domestic background of total PFAS typically less than 100 ng/L. Moodie et al. (2021) suggested based on a population normalised concentration regardless of WWTP, region or capacity, that the domestic inputs constitute the baseline PFAS loading, estimated at 6 mg PFAS per person per year. Interestingly, the recent work published by Thompson et al. (2023) corroborated some of the findings in Moodie et al. (2021), namely surrounding the dominant DiPAP influent load, and the potential for PFAS mass loads to be largely associated with domestic baseline mass flows. In the study, Thompson et al., 2023 found that PFAS loading into wastewater systems from toilet paper included significant inputs of 6:2 fluorotelomer phosphate diester (6:2 diPAP) and contribute from 6.4 to 80 μg/person-year of 6:2 diPAP to wastewater–water systems.

Overall, while literature characterises PFAS mass flows through WWTP influent, effluent and biosolids, these seldom closely explore a sufficiently broad spectrum of PFAS, precursors, and the variety of inputs that constitute influent. Influent is hard to characterize in this sense, as rigorous sampling programs with careful experimental design are required to assist in disambiguating inputs, by sampling the system based on available information on where inputs are still largely or completely segregated. Often the assumptions in such studies pose significant accuracy and uncertainty limitations. Secondly, an understanding of PFAS partitioning behavior and precursor transformation in the WWTP process is required to understand if the inputs targeted for PFAS mass flow reduction would in fact have the desired effect on reducing biosolids PFAS burdens. Noting that reductions of total PFAS mass might not equate to reduction in PFAS mass loads accumulating in biosolids if they have not removed or reduced the specific PFAS species that accumulate in biosolids. As such, where the prevalence of the accumulating species is associated with a specific input (ie. the domestic baseline mass load), controlling other inputs such as trade waste may be misplaced or ineffective.

Figure 3 - Influent as generated by DALL-E AI

Towards Sustainable Management and Resource Recovery

In the pursuit of heightened resource recovery in alignment with national waste policies, addressing PFAS contamination takes center stage. If we can’t measure it, we can’t manage it, and it is clear that outside of identifying key risks, there is still some work to be done to better understand how we can not only reduce PFAS mass loads reaching WWTPs but target the key inputs that result in PFAS enrichment in biosolids. To do so there is a need to manage those species that partition into biosolids in influent, to manage the risk of the resultant residuals. Ignoring this challenge could compromise waste treatment efficiency, biosolids resource recovery potential, and environmental integrity. A profound grasp of influent sources and PFAS characteristics is imperative to achieve the ambitious goal of 80% resource recovery by 2030, while also managing PFAS risks. Only through a comprehensive understanding of the complex factors that see PFAS accumulating in biosolids can we engineer effective and sustainable PFAS management strategies that would ensure biosolids reuse remains a resilient, environmentally sound practice.

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