Dissolved hydrocarbon breakthrough reveals hidden vulnerability in rapid sand filters for SWRO pretreatment during oil spills

Seawater reverse osmosis (SWRO) desalination is vital to global freshwater supply but remains vulnerable to coastal oil spills that introduce dissolved hydrocarbons into intake water. In particular, persistent dissolved mid and short-chain n-alkanes may remain in the water column following weathering of surface slicks. Although rapid sand filters (RSFs) are widely used in SWRO pretreatment, their efficacy in removing dissolved oil-derived hydrocarbons, particularly chain-length–dependent fractions, remains poorly understood. This study presents a controlled laboratory stress test to evaluate the vulnerability of dual-media RSFs (anthracite over sand) under stable dissolved hydrocarbon loading. Artificial seawater, spiked with crude oil, was passed through lab-scale RSF columns for 96 h, simulating post-weathering spill exposure. Dissolved hydrocarbons concentration beneath the oil slick reached 0.13 mg/L. Gas chromatography revealed effective retention of long-chain n-alkanes (C28–C32), while short and mid-chain n-alkanes (C16–C27) exhibited removal efficiencies of 63–82%, with adsorption primarily occurring in the anthracite layer. Lighter fractions of n-alkanes (C8–C15) were volatilized during extraction and therefore not quantified. In addition, SEM-EDS analysis confirmed progressive organic accumulation across filter media. Despite partial removal, residual dissolved hydrocarbons in the effluent could translate to approximately 3 kg/day in a full-scale SWRO plant, representing a measurable hydrocarbon mass load to downstream treatment units. The implications for downstream processes were not evaluated in this study. These findings demonstrate a vulnerability in RSF-based pretreatment and highlight the need for enhanced strategies, such as dissolved air flotation or granular activated carbon, to safeguard SWRO infrastructure in oil-contaminated environments.