Occurrence and characteristics of microplastics in greywater from a research vessel

The discharge of greywater from ships, an uncounted sea-based source of microplastics (MPs), is a growing concern. Yet, empirical data on MPs from this source are currently limited. Here, the abundances and characteristics of MPs in greywater from a research vessel were investigated according to water usage type (e.g., galley, cabin, and laundry). The mean abundance of MPs was highest in greywater from the laundry (177,667 n/m³), followed by the cabins (133,833 n/m³) and galley (75,000 n/m³). However, no significant differences were found in the MP abundances among greywater types due to high variability of triplicate samples collected every five days. Fiber-type MPs accounted for 66% of the total MP abundance and fragment-type MPs for 34%. Microplastics in the size range of 100-200 μm exhibited the highest levels among size classes. The dominant polymer identified in all greywater samples was polyester (53%), followed by polypropylene (23%). Marine coating origin MPs (6%) were also observed in all types of greywater. The greywater generation rate during the cruise was 0.15 m³/person∙day. Annual MP emissions per person by the greywater discharge of the research vessel was estimated to be 4.1 × 10⁶ n/person∙year (equivalent to 3.0 g/person∙year).