Filtration-equipped ballast water management systems as practical infrastructure for marine microplastic mitigation

Microplastics are pervasive contaminants in marine environments and are increasingly recognized as a risk to marine ecosystems and to human health. Because ships take up and discharge large volumes of seawater, ballast water has been identified as a pathway for the transport of microplastics across regions. However, quantitative evidence for microplastic capture by commercially relevant ballast water management systems (BWMSs) during system operation remains limited. In this study, we evaluated the microplastic capture potential of a Filter + UV-type BWMS equipped with a 50 μm screen filter at a land-based test facility of the Korea Institute of Ocean Science and Technology. Influent test water and backwash water discharged during automatic filter cleaning were sampled and analyzed using micro-Fourier transform infrared spectroscopy. The influent test water contained 1,760 particles m-3 of microplastics. At the time of the sampled backwashing event, 84.0 ± 0.96 m³ of seawater had passed through the filter, corresponding to an estimated 147,769 particles entering the filtration unit. Analysis of the entire 200 L backwash sample recovered 123,337 particles, equivalent to approximately 1,469 particles m-3 of treated seawater and an apparent retention efficiency of 83.5% relative to the influent concentration. Although polymer composition differed between influent and backwash samples, the results demonstrate that commercially relevant BWMS filtration can retain and concentrate microplastics within the backwash stream. These findings highlight the potential of existing BWMS infrastructure to function as a practical interception point for marine microplastics during ballast-water treatment.