Evaluation of the release of particles and metals during the in-water hull cleaning process and its environmental risk

Abstract

Ship in-water cleaning is encouraged by certain countries before entering local ports to clear off foulants from ship hulls for preventing the introduction of non-indigenous species to the local marine environment. This cleaning process is expected to be done with a capturing system on board to prevent the release of foulants and paint particles into the water column. However, not all in-water cleaning and capture (IWCC) systems can effectively treat the effluents before releasing back to the water, thus posing risks to the environment. In this study, effluents were collected during in-water cleaning (manual cleaning by divers and automated cleaning by ROV) and analyzed for TSS, particle size distribution, and metal concentrations. A high abundance of particles was observed in 8 to 10 µm size fraction, with a range of 23 to 80% of total particles. It also suggests that it is possible to remove up to 93.7% of particles from effluents by using 8 µm of filter size. Manual cleaning was observed to generate a similar amount of TSS compared to ROV cleaning, with average concentrations of 173.4 and 229.9 mg/L, respectively. After treating the effluent by filtering system, the average concentration of TSS was reduced to 64.4 mg/L. The reduction was also observed in metals concentration, where average Cu concentration reduced from 161.8 to 24.5 µg/L, and average Zn concentration reduced from 1140.2 to 776.5 µg/L. Manual cleaning, on the other hand, produced average Cu concentration of 209.2 µg/L and average Zn concentration of 1513 µg/L. The metal concentrations were converted to release rate and used for predicting the risk in a local port. The predicted concentrations of Cu and Zn showed manual cleaning by divers has higher impact on the local marine environment compared to ROV cleaning. Despite that these concentrations were subjected to vary with the surface area of ships cleaned and cleaning location, in-water cleaning should only be encouraged when capture systems are available to prevent the release of contaminants back to the local marine environment.