Toxic effects of ship exhaust gas closed-loop scrubber wash water

To meet the strict requirements of reducing sulfur emissions, an increasing number of commercial ships have installed exhaust gas cleaning systems (EGCSs). However, wash water produced during the cleaning process is discharged back to the marine environment. We investigated the effects of closed-loop scrubber (natrium-alkali method) wash water on three trophic species. Severe toxic effects were found when Dunaliella salina, Mysidopsis bahia, and Mugilogobius chulae were exposed to 0.63-6.25, 0.63-10, and 1.25-20% concentrations of wash water, respectively. The 50% effective concentration in 96 h (EC₅₀-96 h) for D. salina was 2.48%, and the corresponding total polycyclic aromatic hydrocarbons (PAHs) and heavy metals were 22.81 and 23.67 μg L^-1. The 50% lethal concentration in 7 d (LC₅₀-7 d) values for M. bahia and M. chulae were 3.57% and 20.50%, respectively. The lowest observed effect concentration (LOEC) values for M. bahia and M. chulae were 1.25% and 2.5%, respectively, and the corresponding total PAHs and heavy metals were 11.50 and 11.93 and 22.99 and 23.86 μg L^-1. M. bahia's body weight was negatively correlated with the amount of wash water. Low concentrations of wash water (0-5%) had no significant effect on the reproduction of M. bahia. Although concentrations of 16 PAHs and 8 heavy metals are known, different compounds might react with each other and form more unknown toxic substances, and the measured toxicity comes from synergistic effects between various pollutants. Therefore, future work is needed to clarify other more toxic contaminants in wash water. We highly recommend that wash water be treated before being discharged to the marine environment.