Estimating microplastic concentrations and loads in cruise ship grey waters

Microplastics (MP) are persisting pollutants in the marine environment and pose a significant threat to the world’s oceans. These pollutants are likely to be found in cruise ship grey waters that originate from laundries, showers, baths, sinks, dishwashers, and galleys. Grey water can be discharged untreated from cruise vessels to seas legally. This thesis attempts i) to estimate microplastic concentrations and loads in cruise ship grey waters using a constructive research approach; ii) to create a computational tool for these estimations; iii) to gather relevant input data for the tool and to identify data uncertainties, gaps and needs; iv) to identify ways to reduce MP loads from cruise ship grey waters to seas. Input data on cruise ship characteristics, potential MP sources on ships, MP characteristics, wastewater generation, and ship wastewater treatment technologies was gathered through a thorough literature review mostly on existing scientific and grey literature. MP sources included in this examination were laundry, rinse-off personal care and cosmetic products, and house dust. Results indicate that laundry is the most significant MP source in cruise ship grey waters and that untreated cruise ship grey water contains 0.2–6 mg MP/L or 2000–50000 MP/L. According to the results, discharges of untreated grey water from cruise ships to the marine environment can result in daily microplastic emissions of 30–2000 mg MP/d/person or 2 x 105 – 107 MP/d/person, and annual loads of 10–500 g MP/a/person or 9 x 107 – 4 x 109 MP/a/person. Future research attempts should focus on empirical MP quantification from cruise ship grey waters. Replacing synthetic textiles with natural textile fabrics would prevent the formation of most microplastics on board cruise ships. If synthetic textiles are used and washed, a majority of direct microplastic loads to the marine environment via untreated grey water can be prevented by storing laundry water on board in holding tanks and discharging it to port reception facilities. Should laundry water be treated among other wastewater fractions, then most microplastics end up in sewage sludge. This sludge should either be delivered to port reception facilities or treated onboard using e.g. incineration or hydrothermal carbonization to prevent direct emissions from cruise ships. Ultimately, regulatory measures to ban discharges of untreated grey water or sewage sludge from ships should be required from International Maritime Organisation in order to assure the decrease in microplastic pollution from ships.