Rethinking The Impact of PFAS Emissions in Biosolids Thermal Processes through a Holistic Life Cycle Assessment

INTRODUCTION/BACKGROUND Land application of biosolids provides benefits via recycling nutrients and carbon to soil. However, Per- and polyfluoroalkyl substances (PFAS) are becoming a main hindrance to land application of biosolids with increasingly stringent regulations (McNamara, 2023). PFAS are found ubiquitously in many industrial and consumer products due to their unique chemical properties. These same properties also make them highly persistent and potentially toxic to ecosystems and humans (Holmquist, 2020). New restrictions around land application coupled with a reduction in potential landfill sites for biosolids has led many WRRFs to investigate thermal processes for biosolids management (Winchell, 2022). Incineration provides the benefit of reducing residual solids loads to be managed. Pyrolysis and gasification are emerging thermal process alternatives that also reduce residual solids loads. Moreover, these two processes offer benefits of lower concerns for exhaust gas emissions and the ability to sequester carbon in the solid residuals (Winchell, 2022). Of particular interest now to WRRFs is that these thermal processes can remove and/or transform PFAS (Winchell, 2022; McNamara, 2023). While PFAS are part of the biosolids management decision tree, they are not the only important thing. Any process implemented today still needs to make sense in two decades even if PFAS are no longer a primary concern. Additionally, the relative importance of PFAS in light of other critical environmental concerns like global warming and eutrophication have not been widely studied. While facing a seemingly pivotal moment in biosolids management history, it is critical to understand management options and the relative importance of PFAS. For this research we combined PFAS fate data from our own experiments and from literature through thermal processes. These results were then included in the LCA as their own impact category and as an input to toxicity. To the knowledge of the authors, this is the first LCA on solids handling alternatives to date that has incorporated PFAS toxicity in this way and compared impacts to conventional LCA components. In short, LCA was used to compare thermal drying, incineration, pyrolysis, and gasification holistically with regards to their impacts on humans and the environment. APPROACH PFAS Fate Through Thermal Processes 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 degrees C) and high temperature (800 degrees C), gasification, and incineration. See Table 1 for a description of each scenario's treatment train. 2)Mass and energy balances across the treatment trains for each scenario were conducted, as well as a global warming potential (GWP) evaluation, using the BEAM*2022 tool (NEBRA et al., 2022). 3)PFAS emissions were first characterized alone in an impact category called 'PFAS potential,' representing total mass of PFAS emitted in each scenario as fluoride equivalents. 4)PFAS emissions were then also characterized within the toxicity impact category using the modified USEtox PFAS framework from Holmquist et. al. 2020. 5)These results were than used as an input to the LCA to compare the five thermal process scenarios with five main impact categories: i) GWP, ii) eutrophication potential, iii) freshwater ecotoxicity potential, iv) human cancerous and noncancerous toxicity potential, v) and PFAS potential. RESULTS/DISCUSSION Global Warming Comparison 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 Comparison The thermal drying only scenario had the highest eutrophication impact, driven by the increased hauling of the wet biosolid compared to other scenarios (Figure 2). All land applied scenarios had a beneficial fertilizer offset credit as well. 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 the pyrolysis scenario. Toxicity Comparison 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 two categories across scenarios. Heavy metal loads in the end product were assumed to be similar across all thermal processes, as only a small portion of the heavy metals would volatize, and the majority would remain in the solid residuals. In terms of PFAS potential, pyrolysis did not appear to remove much PFAS, but instead transform them into different PFAS species. Incineration and gasification were the lowest in PFAS potential, due to their high PFAS removal performance. This led also to lower human noncancerous toxicity potentials (Figure 3). Overall, heavy metals had the highest impact in human noncancerous toxicity for each scenario, but PFAS were still impactful. It should be noted that two effect factors were presented for noncancerous toxicity in Holmquist's modified USETOX model (based on epidemiological vs. rodent data). To be conservative, the higher of the two was used. Relative Impacts of Inputs Figure 4 shows the relative contribution of each input to the relevant impact categories, focusing on the thermal drying only scenario for simplicity. Most notably, it can be seen that the impact of heavy metals makes up between 93.2-99.5% of each toxicity potential category for thermal drying only. PFAS emissions comparatively only made up to 6.7% of the toxicity emissions (for human noncancerous toxicity). Eutrophication is mainly impacted by electricity use, while GWP is mainly impacted by scope 1 emissions. The PFAS potential impact category was not shown in the figure. This is because there is only one input to the PFAS potential broken up by the effluent phase the PFAS are emitted to in each scenario. CONCLUSION/SIGNIFICANCE This analysis highlights the importance of PFAS when taken into context with other known and regulated toxicants in biosolids such as heavy metals. Heavy metals made up at least 99.5% and 96.2% of freshwater ecotoxicity and human cancerous toxicity impacts across all scenarios. Even when a conservative assumption for PFAS toxicity was used for human noncancerous toxicity, heavy metals made up the largest portion of this impact as well. This analysis also highlights two biosolids alternatives, gasification and pyrolysis. Gasification was observed to be a relatively low impact alternative in most of the LCA categories that also safeguards against uncertainty around PFAS toxicity and future regulations in biosolids. This option provides not just transformation, but potentially good removal of PFAS species. Pyrolysis however was observed to be the best option in this analysis when considering the relatively low impact of PFAS on the toxicity potential impact categories, and that pyrolysis had the lowest impact in the other critical LCA impart categories. 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.