Assessment of Microplastic Abundance and Discharge from Greywater of Ships

Greywater drained from showers, washbasins, laundry, and dishwashers can be discharged to the sea without further treatment unless in no discharge zone. In 2021, International Maritime Organization indicated greywater generated from ships as one of the possible sources of sea-based microplastics. However, there are only a few studies on microplastics in greywater, most of which have used the scientific literature for statistical estimation of the amount of microplastics in cruise ships. It is necessary to evaluate the amount of microplastics in real ships’ greywater for accurate calculation of emission. This study aims to measure microplastics in a ship’s greywater by its usage and to estimate the preliminary global emission of microplastics from a ship’s greywater discharge into the ocean.

Greywater was collected in three different holding tanks at 5-day intervals from a research vessel Onnuri of Korea Institute of Ocean Science and Technology (1,370 tons) before and during the cruise for 18 days (April 21–May 9, 2022) with 33 persons on board. A total of 83 m³ of greywater was generated during the cruise. The average microplastic abundance was 135,563±87,141 n/m³ (range: 60,500–322,500 n/m³) in greywater. There were no significant differences in the abundance of microplastics in greywater usage (mainly galley, laundry and shower, and cabin washbasin). The level of microplastics in greywater in this study was several orders of magnitude higher than those in effluents from terrestrial wastewater treatment plants in the literature.

The greywater generation rate during navigation was 0.15 m³/person/day, which was comparable to previous studies with various types of ships. The greywater sub-flows accounted for 51%, 17%, and 32% of tank A (drained from the galley and 5 cabins with showers), tank B (18 cabins, 2 laboratory rooms, and 4 washbasins), and tank C (laundry, shower, and washbasin), respectively. The number of microplastic from total greywater was highest in tank C (44%), followed by tank A (29%) and tank B (18%). However, there was no difference in the number and mass of microplastics. The annual microplastic load in greywater from R/V Onnuri was estimated to be 1.2ⅹ10⁸ n/year or 91 g/year. Based on the greywater microplastic emission factor abundance in this study, it is estimated that about 28ⅹ10¹² particles or 13–29 tons of microplastics could enter the global oceans from greywater of global fleets (>100 gross tonnages) in a year.