Remediation of per and polyfluoroalkyl substances by pilot-scale applications of advanced water treatment technologies

Abstract

The widespread use of per- and polyfluoroalkyl substances (PFASs) and the associated long-range transport and recalcitrance in the environment has led to the contamination of water resources worldwide. Most conventional water treatment methods are ineffective against PFASs and while the removal of PFASs by advanced water treatment technologies including granular activated carbon (GAC), anion exchange resin (AER), nanofiltration (NF), reverse osmosis (RO), and UV-sulfite has been demonstrated in bench-scale laboratory studies, little information is available on the treatment performance of these technologies at the full-scale. The works in this thesis advance the understanding of real-world PFAS treatment by these technologies using pilot-scale assessments operated under conditions and water matrices representative of full-scale treatment and provide guidance on the advantages and disadvantages associated with each technology.The treatment of PFAS impacted groundwater by four GAC products was evaluated at a municipality near Colorado Springs, Colorado using a pilot-scale system operated for 7 months. Breakthrough of PFASs by GAC was influenced by perfluoroalkyl chain length and headgroup with breakthrough of longer chain PFASs and perfluorosulfonic acids (PFSAs) occurring later than shorter chain PFASs and perfluorocarboxylic acids (PFCAs), respectively. Greater adsorption capacity was found for GAC products containing high volumes of transport pores. A subsequent study compared a better performing GAC product evaluated from the prior study against three AER products in a similar groundwater for 13 months. For AERs, breakthrough of PFCAs followed compound chain length but breakthrough of PFSAs occurred at the same time for all AERs, possibly attributed to accumulation of metals from the source water in the AERs. Still, PFSAs exhibited greater adsorption affinity to AERs than PFCAs. AERs adsorbed 6-7 times more PFASs than GAC per unit media at breakthrough; however, based on volume of water treated at breakthrough, AERs and GAC performed similarly for PFCAs but better than GAC for PFSAs. All AERs evaluated performed similarly. When media replacement is dictated by breakthrough of perfluorooctanoic acid (PFOA), similar operations and maintenance costs can be expected if AER media cost is ~3.5 times GAC media cost. The impact of operating conditions, water matrix, and adsorption on rejection of PFASs by spiral-wound NF and RO membrane elements was evaluated. Membrane operating conditions did not have a significant impact on rejection of PFASs in a laboratory electrolyte matrix and was >98% by NF and >99% by RO. Rejection of the same PFASs present in a groundwater matrix by NF was lower, between 92-98%, and was attributed to water matrix effects. Adsorptive losses of longer-chain hydrophobic PFASs to the NF spiral wound membrane elements and the membrane system were observed but did not affect rejection of PFASs by NF. A subsequent study combined NF and UV-sulfite in a treatment train for the sequential removal and destruction, respectively, of PFASs in groundwater. Most PFASs were rejected to >95% by NF when operating at 90% permeate recovery. UV-sulfite treatment of the NF reject resulted in variable destruction of individual PFASs, with rates also being dependent upon pH and the identity and concentration of UV photosensitizer. Rates of PFCA degradation were greater than those measured for PFSAs and polyfluorinated PFASs and were independent of perfluoroalkyl chain length. In contrast, rates of PFSA degradation increased with increasing chain length. Collectively, >75% of the detected PFAS mass in the NF reject was destroyed after 4 h of UV treatment, increasing to 90% after 8 h of treatment. Electrical energy per order magnitude requirements for the NF and UV treatment train were estimated to be <13.1 kWh/m3 for all PFCAs and 14.1 kWh/m3 for PFOS.