Is polycyclic aromatic hydrocarbon concentration significantly underestimated in scrubber effluent discharge?

Abstract

With the wide-scale use of exhaust gas cleaning systems (scrubbers) by ships and the associated scrubber washwater discharge, there is growing concern regarding the risk of the marine environment posed by scrubber washwater, especially for the organic contaminants “Polycyclic Aromatic Hydrocarbons (PAHs)” which is a toxic washwater pollutant component. The detection of PAHs in scrubber effluent has been conducted widely by two different methods: the Gas Chromatography-Mass Spectrometry (GC-MS) method of labs and the fluorescent method by online PAHs sensors. Unfortunately, the current lab analysis only focuses on the EPA 16 PAHs rather than any other derivatives, resulting a knowledge gap between accurate estimating the actual concentrations of PAHs in the onboard scrubber effluents to onboard PAHs monitoring. This gap leads to controversial opinion on scrubber effluent environmental impact as well as ambiguity in “sulphur cap” regulation implements. In this study, we conducted a scrubber effluent sampling program on three ships, and the lab analysis indicated that alkylated derivatives of naphthalene, phenanthrene, and fluorene clearly dominate in all PAHs of the scrubber effluent. However, since current PAHs monitoring devices survey/approval solely report the content of the EPA 16 PAHs from lab report, the overlooked alkylated PAHs may lead to a significant underestimation (up to 30 times lower) of total PAHs concentration in scrubber effluent. Consequently, the overall marine environment risk of scrubber effluent could be substantially underestimated considering alkylated PAHs are often reported more toxic than their parent PAHs. This study may trig re-examinations of PAHs monitoring methods and lead to an understanding that the present discharge regulations require an overhaul.