Rethinking the impact of PFAS emissions in biosolids thermal processes through a holistic life cycle assessment

INTRODUCTION
Land application of biosolids is a beneficial way to recycle nutrients and carbon to soil, but Per- and polyfluoroalkyl substances (PFAS) are becoming a major hindrance to this process due to their persistence and potential for toxicity (McNamara, 2023, Holmquist 2020). With new regulations restricting land application and a reduction in potential landfill sites, many wastewater treatment facilities (WRRFs) are turning to thermal processes for biosolids management (Winchell, 2022). These processes not only reduce residual solids loads but may also offer the potential to remove and/or transform PFAS (Winchell, 2022; McNamara, 2023).

PFAS have undoubtedly impacted the world of biosolids management. While our industry strives to adapt to changing regulations, we still have not assessed how important PFAS impacts are when considering the overall impact of biosolids to human and environmental health (i.e. the impact of PFAS on toxicity in tandem with more traditional concerns such as global warming and eutrophication). For this research we combined PFAS fate data from our own experiments and from literature on thermal processes. These results were then included in a life cycle assessment (LCA).

METHODS
Lab-scale pyrolysis and gasification experiments were conducted as part of WRF Project 5211. Targeted analysis of PFAS was completed by Eurofins on influent biosolids and effluent solids (char), liquid (py-liquid), and gas (py-gas or gasification gas). These data were used to assess removal and release of PFAS in the effluent phases. Incineration data from Winchell, 2023 was used for incineration PFAS removal performance as a reference scenario.

LCA Structure
1) Five biosolids treatment scenarios for a 'typical' WRRF were compared, distinguishing between different thermal processes: thermal drying only, pyrolysis at low (500°C) and high temperature (800°C), gasification, and incineration. Table 1 describes each treatment train.
2) Mass and energy balances across each scenario were conducted, as well as a global warming potential (GWP) evaluation, using the BEAM*2022 tool (NEBRA et al., 2022), and a eutrophication potential assessment.
3) PFAS emissions were first characterized in a new impact category called 'PFAS potential,' representing total mass of PFAS potentially emitted as fluoride equivalents.
4) PFAS emissions were then also characterized within the toxicity impact categories using the modified USEtox PFAS framework from Holmquist et. al. 2020 (i.e. for ecotoxicity, human cancerous, & noncancerous toxicity).

RESULTS/DISCUSSION
Global Warming
The incineration scenario had the highest GWP, driven mainly by scope 1 emissions (Figure 1). The scenarios with a fertilizer offset credit applied had negative scope 3 emissions. This was particularly significant for thermal drying only, because the biosolids had the highest nitrogen content, which largely drove the GWP fertilizer credit. The pyrolysis scenarios were determined to have the lowest GWPs, largely driven by lower scope 1 emissions through carbon sequestration credits achieved by the stability of the biochar product.

Eutrophication
The thermal drying only and gasification scenarios had the highest eutrophication impact, driven by the increased hauling of the wet biosolid and the lack of fertilizer offset credit in the case of gasification (Figure 2). All land applied scenarios had a beneficial fertilizer offset credit. This credit was largely similar between these scenarios, as the phosphorus content of the solid drove eutrophication impact for fertilizer credit and was assumed to be similar between scenarios. The lowest eutrophication potential was in pyrolysis.

PFAS Potential & Toxicity
Incineration and gasification had good PFAS removal, which led to lower human noncancerous toxicity potentials (Figure 3). Pyrolysis data indicated generally transformation of PFAS rather than removal. While higher PFAS were measured in the pyrolysis scenarios compared to the thermal drying only scenario, thermal drying only was assumed to have higher or equal PFAS compared to pyrolysis products, as the higher measured pfas in pyrolysis outputs is not a product of PFAS 'generation' but instead, transformation into measurable species.

Little difference was observed in freshwater ecotoxicity and human cancerous toxicity, largely due to the dominance of heavy metals, making up at least 99.5% and 96.2% in these categories across scenarios (while a portion of the heavy metals would likely volatilize, the majority were assumed to stay in the solids residual equally among scenarios). Figure 4 shows the relative contribution of each input to the toxicity impacts, focusing on the thermal drying only scenario for simplicity. PFAS emissions can be seen to only be impactful for human noncancerous toxicity.

CONCLUSION This analysis highlights the relative importance of PFAS when taken into context with other known and regulated toxicants such as heavy metals. It also highlights how pyrolysis is the best alternative in more traditional LCA impact categories like global warming and eutrophication potential, but may not be the best option if complete PFAS destruction is a primary driver for a utility. The key impact of this work is the reframing of PFAS as not the only consideration, but one of many aspects to be included for holistic biosolids management.