BOOK OF ABSTRACTS 75 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Tuesday, 25 August 2026 / Hall B+C Research Frontiers SL31 Evaluating the Air-Water Interface as the “Fourth Phase” in the Transport of Humic Substances and Organic Contaminants in Wastewater Treatment Kirin Emlet Furst1,2, Mrudula Meghana Kuppa2, Ethan Aaron Gasper2 1 Virginia Tech, Department of Civil and Environmental Engineering, Occoquan Watershed Monitoring Laboratory, 9408 Prince William St, Manassas, VA 20110, kfurst@vt.edu 2 George Mason University, Department of Civil, Environmental and Infrastructure Engineering, 4511 Patriot Circle, Fairfax, Virginia 22030 Organic contaminant transport is typically conceptualized as a threephase system consisting of the aqueous phase, suspended solids, and a third phase composed of dissolved or colloidal organic matter (DOM). In wastewater treatment, however, there is often a fourth phase, an organic-rich layer at the air-water interface known as scum. Depending on influent quality and treatment process, scum contains large buoyant particles, fats, oils and grease (FOG), natural and synthetic surfactants, and other hydrophobic and amphiphilic material. This study demonstrates that wastewater scum could mediate transport of phthalate esters (PAEs), and organic matter more broadly, in ways not captured by conventional partitioning frameworks [1]. Scum samples were collected from primary and secondary unit processes in a conventional activated sludge plant and characterized by fluorescence excitation-emission matrix spectroscopy (FEEMs). Indicators of humic matter, proteinaceous material, plastics, and optical brighteners persisted in secondary scum at 3 to 10 times the intensities measured in raw sewage. Size fractionation indicated the presence of large organic particle assemblies >0.7 µm, possibly including microplastics and detergent micelles. When samples were filtered prior to 24 h tumbling followed by solid phase extraction, all fifteen PAEs were undetectable. When filtered after tumbling, seven PAEs were detected, six at concentrations >1 ppb. This is consistent with large particles, potentially microplastics and/or micellar assemblies, serving as vehicles for PAE transport. These findings suggest that scum could contribute to persistence of multiple PAEs in the aqueous phase through secondary treatment in tandem with the third phase effects identified by previous authors [2]. To investigate sorption mechanisms in scum, isotherm experiments were performed with fifteen PAEs in diluted, size-fractionated scum samples, and the results were fit with the Freundlich model. Quantitative structure-activity relationship (QSAR) models were developed and compared with partitioning models for conventional organic phases where available. For straight-chain PAEs, sorption affinity
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