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This study evaluates the economic and environmental performance of alternative greywater management strategies for passenger ships operating in the Baltic, using the Port of Trelleborg as a representative case. Greywater (GW), comprising water from showers, sinks, galleys, and laundries, remain unregulated under MARPOL Annex IV, despite its substantial loads of nutrients, organic matter, microplastics and metals. As shipping traffic grows, especially in enclosed and sensitive marine areas such as the Baltic Sea, the lack of international regulation creates uncertainty for ports and shipowners on how GW should be handled. This study provides the first integrated cost-benefit analysis (CBA) of ship-to-port greywater reception, land-based treatment, and reuse in the Baltic Sea context.

Ten management scenarios were examined, ranging from direct discharge at sea (SC1) to advanced land-based treatment with full or partial reuse (SC5B₁ – SC5B₅). The analysis quantifies (1) societal costs, including capital and operating expenditure for shipboard systems, port reception facilities (PRF), and municipal wastewater treatment plants (MWTP/PWTP); (2) stakeholder costs and benefits for ships, ports, and municipalities; and (3) environmental benefits (EB) expressed monetarily using pollutant shadow prices for nutrients, organic matter, suspended solids, metals, and microplastics. Reuse benefits were also estimated based on avoided potable-water production and distribution.