Occurrence and Characteristics of Microplastics in in a Ship’s Greywater According to Usage Patterns

Marine microplastics are a global environmental issue. However, there are still large data and knowledge gaps in microplastics from sea-based sources. One of the concerned sea-based microplastic sources is ships' greywater discharge. Greywater generated from galley, laundries, showers, and washbasins in a ship can be discharged directly to the sea without treatment. In this study, we present the sampling approach of greywater and information about the abundance and characteristics of microplastics. To our knowledge, it is the first attempt to determine microplastic abundance and characteristics in ships' greywater according to its use categories (galley, laundry and shower, and cabin washbasin). Greywater samples were collected from three different holding tanks (tanks A, B, and C) at R/V Onnuri of Korea Institute of Ocean Science and Technology (KIOST). To enable the sample collection, the discharge system was converted to manual discharge and an additional pump and valve were installed on the pipeline connected to each tank. Greywater was sampled when the vessel was at anchor (1^st sample) and during the research cruise (2^nd sample). For the 1^st sample, a grab sampling was conducted and for the 2^nd sample, samples were collected at 5-day intervals. Semi-automated FTIR analysis was conducted for microplastic identification. During the analysis, fiber bundles composed of polyester (PES) and polypropylene (PP) were detected. Bundles were counted as one individual particle or composing particles were counted individually, if possible. The highest microplastic abundance was found in tank C from the laundry and shower room in which a large number of microfibers such as PES fibers were detected. The average microplastic abundances were 149,660±77,574 n/m³ (62,000–209,600 n/m³) in 1^st sample and 135,563±87,141 n/m³ (75,000–177,667 n/m³) in 2^nd sample. The microplastic abundances were similar and this can be attributed to the fact that people use the ship's facilities where greywater can be generated even while at anchor. In addition, not only the generation of microplastics but also greywater could be large during navigation. Though the microplastic abundances were similar in 1^st and 2^nd samples, a greater variety of polymers were detected in 2^nd sample (25 types) than in 1^st sample (15 types). Polymers used in paint were also highly detected in the 2^nd sample (8%) than in the 1^st sample (2%). This might be due to more diverse activities took place on the deck and inside the ship during the research activity than when at anchor. Fibers were more dominant in 2^nd sample (66%) than in 1^st sample (25%). This may be because more people use washing machines while sailing than at anchor. In addition, since many cabins are occupied during the research cruise than when at anchor, washing in cabins and fibers detached from fabrics from people’s activity during navigation might have contributed to relatively higher PES fiber composition in 2^nd sample. The results of this study would be useful in understanding the sea-based microplastic pollution through ship’s greywater, and for estimation of the microplastic emission from ships to the marine environment.