Harmonizing Thermal Hydrolysis Pretreatment: Digestibility, Dewaterability, and Microbial Symphony in PFAS-Laden Sewage Sludge

This research aims to elucidate the impact and efficacy of thermal hydrolysis on anaerobic digestion for mitigating PFAS-laden sewage sludge, addressing a critical knowledge gap resulting from the absence of a definitive standard method to quantify PFAS in biosolids. It is widely recognized that biological treatments, including anaerobic digestion, are generally considered inefficient for the treatment of PFAS-laden sewage sludge. For the first time, our study reveals how thermal hydrolysis pretreatment induces a significant shift in the microbial community, fostering an environment conducive to the effective mitigation of PFAS contaminants. Consequently, this research provides valuable insights into the potential robustness of thermal hydrolysis pretreatment in managing the complexities of PFAS-contaminated sewage sludge, making a substantial contribution to the current scientific landscape. Furthermore, this paper presents both qualitative and quantitative assessments of conventional anaerobic digestion versus thermal hydrolysis pretreatment in terms of COD removal, biodegradability, and dewaterability. It correlates these findings with the changes in the microbial community resulting from both experimental conditions. The paper explores the examination of thermal hydrolysis as an intensification process aligned with the objectives of water resource recovery facilities (WRRF) to achieve sustainability within the circular economy framework. Accordingly, six semi-continuous bioreactors were operated over a sixty-day period with a hydraulic retention time of fifteen days under mesophilic conditions. Three bioreactors represented conventional anaerobic digestion (RAW TWAS bioreactors), while the remaining three represented thermally hydrolyzed anaerobic digestion (HTP TWAS bioreactors). The study demonstrates that pre-treating thickened waste-activated sludge (TWAS) at 170 °C for 30 minutes and a pressure of 3 bars prior to anaerobic digestion enhanced sludge solubilization, biodegradability, and viscosity. The soluble chemical oxygen demand (SCOD) concentration increased from 213 mg/L in conventional anaerobic digestion (Raw TWAS) to approximately 15,000 mg/L in the thermally hydrolyzed TWAS sample (HTP TWAS). The average reduction in volatile suspended solids (VSS) and total suspended solids (TSS) in the HTP TWAS was 51% and 43%, respectively. Thermal hydrolysis significantly influenced particle size distribution, shifting the mean diameter from 206 to 104 µm and drastically reduced TWAS viscosity from 510 cP to 18 cP after thermal treatment. The impact of thermal hydrolysis on pre-treated TWAS was evident in increased methane production and methane percentage in the biogas generated (Figure 1). HTP TWAS bioreactors exhibited a higher average methane production of 551 mL/day compared to 365 mL/day for untreated ones, with corresponding methane yields of 176 mL/g TCODadded versus 116 mL/g TCODadded (Figure 2). Additionally, HTP TWAS digesters demonstrated higher destruction of volatile solids than Raw TWAS. The integration of thermal hydrolysis and anaerobic digestion addressed the excess sludge elimination issue associated with the biological process alone, significantly improving the dewaterability of anaerobically digested sludge two-fold (Figure 3). Moreover, this study, for the first time, examined the microbial communities in conventional anaerobic digestion, thermal hydrolysis anaerobic digestion, and PFAS-laden sewage sludge under both conventional and thermal hydrolysis anaerobic digestion conditions (Figure 4). The impact of thermal hydrolysis on the microbial community reveals increased microbial diversity and richness during the anaerobic digestion process. Thermal hydrolysis exhibited a substantial influence on both microbial community composition and more pronounced effects on archaeal community distribution and methanogenesis pathways and co-occurrence patterns. The relative abundance of Firmicutes, Bacteroidetes, and Proteobacteria was higher in the thermally hydrolyzed digestate samples than in conventional anaerobic digestion digestate. Thermally hydrolyzed TWAS-fed bioreactors showed enrichment in Tissierellaceae and Porphyromonadaceae, positively correlating with high methane production. In contrast, the microbial community of Raw TWAS had a higher relative abundance of hydrolytic microorganisms (Chloroflexi) capable of carbohydrate hydrolysis. Thermal hydrolysis demonstrated the resilience of AD in overcoming potential ammonia and PFAS toxicity attributed to the adaptability of its microbial community to environmental changes.