Assessment of the chemical concentrations and the environmental risk of tank cleaning effluents in the Baltic Sea

The environmental risks of tank cleaning waters were evaluated in this study by conducting a risk assessment of five target chemicals (nonylphenol, phenol, sulphuric acid, styrene and xylenes) following EU risk assessment methodology in which the Predicted Environmental Concentrations (PECs) of the chemicals are determined and compared to their corresponding predicted no effect concentrations (PNECs). The results of the risk assessment demonstrated the importance of prewashing cargo tanks ashore before the main washing. Without prewashing the cargo tanks and discharging the generated prewashing effluents into reception facilities ashore, the PEC/PNEC ratios obtained for further washings exceed accepted levels regardless of the chemical and the quantity of chemical residue (15:50:300 l). In contrast, if the cargo tanks are prewashed and the remaining prewash residues in the tanks are small (≤15 l), the risk arising from tank cleaning effluents stays at accepted levels, and no harm is caused to aquatic organisms. When the prewash residue is around 50 l, the risk arising from further washings mostly remains at accepted levels. Whenever the prewashing is carried out, the tank washings only represent a minor and local risk for water biota. The study showed that prewashing requirements for the most hazardous category of X cargoes and high-viscosity and solidifying Y cargoes are necessary to keep the hazards arising from these substances at accepted levels. Furthermore, MARPOL Y class also includes persistent and toxic chemicals whose release in the sea without prewashing is not recommended, even if the regulations might not necessarily require prewashing.