Treatment of PFAS impacted wastewater using ozofractionation and sonolysis with treatment vaildation using top assay

Loss of a concentrated aqueous film-forming foam resulted in PFAS impact to nearby surface water and infrastructure including a domestic/industrial sewer and stormwater system which serviced numerous industrial facilities. The sewer and stormwater systems were isolated to prevent further impact to the environment. Approximately 6 million litres (ML) of wastewater and 6 ML of stormwater were collected and contained in 20 kL tanks. The objective of the project was to treat collected water to a concentration of less than 0.25 μg/L Sum of PFASs (28 compounds) as measured by total oxidizable precursor (TOP) assay. This project was the first example of ozofractionation for PFASs treatment with performance validated by TOP assay. Samples of PFASs concentrated in the foam fraction were treated using sonolysis to cause their complete destruction. Efficient removal of PFASs via partitioning with air bubbles has been demonstrated in the peer-reviewed literature (Lee et al. 2017; Meng et al. 2018), and PFASs affinity for the air water interface is central to their functionality in AFFF. OCRA is a patented process (that includes a series of chambers in which ozone micro-nano-bubbles (MNB) are sparged through PFAS-impacted water to facilitate gas-liquid interface partitioning of dissolved PFASs into a foam fraction which is collected as a PFAS concentrate. The concentrate can then be destroyed via destructive processes such as sonolysis, The limiting factor for removal of PFASs is available gas-liquid interfacial surface area. Ozone is the ideal gas for removal of PFASs from water relative to other gases such as air, because of its propensity to form stable bubble emulsions, as a result of bubble repulsion from increased zeta potential (Liu, Wang et al. 2012). This effect maximizes the interfacial surface area and bubble longevity, resulting in very effective collection and concentration of PFASs (Ross et al., 2017). Sparging ozone gas through OCRA columns while PFAS-impacted water passes through creates a PFAS-enriched foam fraction that can be collected. Depending on co-contaminants and initial PFAS levels, this creates a volume of ~0.5-5% PFAS-concentrated foam fraction and 95-99.5% treated water, with >99.9% removal of long-chain PFASs and >99% removal concentrations of short-chain PFASs (Horst et al., 2018). OCRA systems have treated PFASs ranging in concentration from <1 μg/L to > 5,000 μg/L total PFAS, as measured by the Total Oxidizable Precursor (TOP) Assay. Sonolysis (the use of ultrasonic waves to break down a substance) utilises ultrasound's unique quasi-adiabatic bubble collapse that results in localised temperatures >5,000 K and pressures ~1000 atm, within bubbles, whilst the bulk solution can remain at ambient temperature. Sound waves at frequencies generally between 20 kHz to 1,100 kHz are able to generate cavitation in water. Simply defined, cavitation is the creation of microbubbles in a fluid due to negative pressures. As sound waves move through water, there are oscillating cycles of rarefaction and compression, which causes cavitation and allows bubble growth. When the microbubbles are collapsed during compression cycles, significant energy is released in the form of point sources of extreme heat, light and plasma-like conditions. High frequency ultrasound is reported to destroy a wide range of PFASs, including both long-chain and short-chain molecules (Ross et al 2018; Merino et al 2016). (author)