Advanced treatments for PFAS removal from landfill leachate: evaluating biological and ozone based chemical approaches

In the last decades, the rapid industrialization, the population growth and the urbanization led to the increase in waste generation. The practices of reuse, recycling, energy and matter recovery from waste are not sufficient to cope with this rise. Therefore, landfilling still remains a widely used practice. Within landfills, waste undergoes a number of physical, chemical and biological changes and releases micropollutants within the landfill leachate, making this matrix one of the most difficult to treat (Kumar et al., 2023). Leachate is a complex mixture containing very high concentrations of biodegradable and recalcitrant toxic compounds (Qian et al., 2024). It contains a variety of micropollutants, such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), pharmaceuticals, personal care products, pesticides, microplastics (MPs), per- and polyfluoroalkyl substances (PFASs), and many more. Although micropollutants are present in very low concentrations they have significant impacts on the ecosystems, economy, human health. The subject of this study are per- and polyfluoroalkyl substances, better known as PFAS, substances synthesized since the 1950s and entered the composition of a great many commercial products because of their outstanding hydro-, oleo-repellency and high stability characteristics. PFASare found in a wide range of products, from fertilizers to food packaging, from personal hygiene products to fire-fighting foams. These, at the end of their life cycle, sent to landfills release fluorinated substances into leachate, where the concentrations can reach up to thousands of μg/L (Gallen et al., 2017). This study stems from the need to identify a solution for PFAS removal in landfill leachate and aims to evaluate the removal efficacy of PFAS and other key chemical parameters of two bench-scale treatment schemes: a biological treatment conducted in a Sequencing Batch Biofilter Granula Reactor (SBBGR) -and an ozone-enhanced biological treatment. Given the matrix complexity and the high chemical stability of PFAS, conventional treatments are inadequate for the removal of fluorinated substances. Hence the need to investigate an integrated approach combining biological degradation with chemical oxidation. Biological treatment was conducted in an SBBGR, an advanced biological treatment system, which is an upflow reactor in which leachate was fed, treated, and extracted sequentially. The plant operated in sequential mode with 8-hour treatment cycles. Each cycle featured a fill, reaction, and discharge phase. The chemical upgrading included an additional phase, the integration of biological degradation with chemical oxidation, performed with ozone at two different doses (4.0 g/L and 5.5 g/L). The discontinuity of the SBBGR system allowed oxidative treatment with ozone to be used in a specific and controlled manner. The ozonation step, following biological treatment, was specific for resistant biological degradation compounds. Ozone, dosed in a controlled manner, allowed for the partial oxidation of recalcitrant substances before returning them back to the biomass action. The experimentation consisted of four phases: a start-up phase, a second phase with a steady state biological , a third phase in which biological treatment was enhanced with ozone at two different dosages, and a final phase in which reactor worked in biological mode fed with leachate at high PFAS concentration. During the preliminary start-up phase, an appropriate feeding program was used: gradual dilutions with water at decreasing ratios to acclimate the biomass to high salinity values (approximately 23 mS/cm) and to stimulate the growth of the species involved in the process. The influent and effluent of all treatment schemes were characterized in terms of traditional parameters and PFAS. Per- and polyfluoroalkyl substances were analyzed by a mass spectrometer interfaced with very high-pressure liquid chromatography.