Environmental problems caused by oil spills from ships have been taken seriously and given considerable attention. Conscious action and decision-making by authorities has resulted in better vessels and improved operational procedures, and the number of marine oil spills has drastically decreased over the past decades (Statista 2023). Much less attention has been paid to the smaller but chronic spills of oily wastewater from shipping. The International Maritime Organisation (IMO) has accepted that ship-owners can continue to use heavy fuel oil (HFO) and still comply with the new sulphur reduction rules (MARPOL Annex VI) by installing scrubbers on-board the ships. In a scrubber, the combustion gases are washed with seawater or freshwater, depending on the design of the equipment. The contaminated wastewater contains, besides oil-related compounds, also metals, acidifying sulphur, eutrophicating NOx, and a range of combustion particles. It is discharged to the ocean, either without any pre-treatment (open-loop scrubbers) or with some treatment (closed-loop scrubbers). The use of scrubbers is an economically favourable alternative to switching to cleaner fuels, and since the number of ships equipped with scrubbers have increased dramatically over the past decade (from 242 in 2015 to 4737 in 2022), so have the volumes of discharged scrubber water. In a report from the International Council on Clean Transportation (ICCT) in 2021, the discharge of scrubber water from ships on a global basis was estimated at over 10 gigatons (Gt) annually (Osipova et al. 2021). Discharged volumes in the OSPAR region in 2020 were estimated at 622 million tonnes (OSPAR Commission 2022).
Scrubber water is continuously discharged when a ship is in operation and concentrations of the harmful components in the wastewater (scrubber water) will inevitably be chronically increased around shipping lanes. Among the most problematic contaminants in scrubber water are the polycyclic aromatic compounds (PACs). This is a group of chemicals comprising hundreds of compounds, among them the better known polycyclic aromatic hydrocarbons (PAHs), however chemical analysis of environmental samples is often limited to a set of 16 PAHs. The scientific literature on toxic effects of PACs on marine organisms is extensive but again there is a very limited set of PACs included in most of the studies (see Tables 6 and 7). Although knowledge on the mechanisms behind the toxicity of many PACs is still incomplete, we have today a fairly good overview of how effects are mediated in different organisms and which life stages are most sensitive (see section 7 PAC fate and general toxicity). Still, much of these scientific data are ignored in environmental risk assessments of PAC containing mixtures like scrubber water, and when determining threshold values for PAC concentrations in the environment. An example of the latter is the threshold values, or environmental quality standards (EQS) of the EU Water Framework Directive, for which the last update was made in 2013. Many risk assessments today are based on the same standard protocols, using the same endpoints, the same species/life stages of species, the same exposure times, regardless of the contaminants being investigated. With this approach, there is a risk of overlooking the most sensitive endpoints when examining the toxicity of specific contaminants or mixtures of contaminants, and the consequences for marine ecosystems may be underestimated.
This report presents chemical and toxicity data on PACs detected in scrubber water. The chemical analysis includes an extended number of PACs, beyond the standard 16 PAHs, and information on the “fingerprint” of PACs that is indicative of this specific source is presented. Data show that when limiting analysis to the standard set of 16 PAHs, many other compounds are ignored. Some of them, e.g., the alkylated PAHs, occur in much higher concentrations than the “conventional” PAHs. The report presents results from an extensive review of research on PACs (mainly PAHs) toxicity on marine fish, invertebrates and algae show that effects occur at much lower concentrations than what is commonly detected using standardised protocols. All toxicity data are reported as values that can be applied in risk assessments, such as the lowest tested concentration that was significantly different from control (LOEC), the effective concentration inducing an effect in 50%, 20% or 10% of the exposed population (EC50, EC20, or EC10,). The information can be used in future attempts to estimate the impact of scrubber water and other oily wastewaters on the environment.