Quantification of microplastics in ship-generated greywater and their contribution to Baltic marine pollution

The Baltic Sea remains one of the most contaminated marine seas globally, receiving diverse pollutant inputs from land-based and maritime sources. This study quantifies the concentrations and loads of microplastic (MP) in ship-generated greywater (GW) and evaluates their potential contribution to Baltic marine MP pollution. Eight GW streams from five vessels were sampled, and fifteen MP polymer types were identified and characterized. MP concentrations ranged from ≈ 38,000 MP/m 3 in mixed accommodation-laundry-galley (ALG) stream to ≈ 602,000 MP/m 3 in laundry GW. Polyethylene terephthalate (PET) was the dominant polymer (58%), detected in all GW samples, while polypropylene (PP, 16%) appeared in only three streams from two vessels. Estimated annual MP loads from the studied Roll on - Roll off - Passenger (RoPax) vessels ranged between ≈ 1.24 and 7.59 billion particles, which are typically delivered to municipal wastewater treatment plants (MWTPs) via Port Reception Facilities (PRFs). Considering the total Baltic fleet’s greywater discharge of ≈ 5.4 million m 3 /yr in 2022, up to 1.1 trillion MP/yr could have been released directly to the sea, with ≈ 93% originating from passenger ships. If this volume generated in 2022 were instead delivered to PRFs and treated at MWTPs, ≈ 6 million - 651 billion MP/yr could still enter the Baltic environment, depending on the treatment efficiencies and technology configurations employed at the MWTPs. These results demonstrate that ship-generated GW is a significant yet understudied source of microplastic (MP) to the Baltic Sea. Moreover, while advanced systems with tertiary treatment technologies on board and ashore can remove ≈ 95 – 99.9% of MP, residual emissions remain substantial given the large wastewater volumes generated. Effective mitigation strategies should therefore focus on source identification and prevention within shipboard systems, particularly in laundry, galley and accommodation operations, to minimize MP inputs into GW streams and, ultimately, the marine environment.