Multivariate prediction of scrubber water toxicity

Report, 2023

The daily operations of maritime shipping give rise to several different waste streams, each contributing to the total load of chemical substances to the marine environment. Risk can be assessed by calculating the risk characterisation ratio (RCR) as the relation between the predicted (or measured) environmental concentration (P(M)EC) and the predicted no effect concentration (PNEC), which is a measure of the environment sensitivity of a specific substance derived from ecotoxicological tests. Since all liquid waste streams from shipping, e.g., scrubber water, grey water and sewage, contains more than one substance, a PEC/PNEC summation approach can be applied as a conservative first step methodology to predict the risks for adverse effects from mixture exposure. The summation approach is based on the concept of concentration addition which means that all substances present in a mixture contribute to a cumulative effect. However, if PNEC values are not available for substances identified in a liquid waste stream, these will not be included in the risk assessment. For substances where PNEC values are not available, Quantitative Structure-Activity Relationship (QSAR) models, can be applied to predict the toxicity based on the chemical structure and/or properties of a substance. In this report, QSAR model output were used as complement to the experimentally derived ecotoxicological PNEC values, to allow for more substances to be included in the risk assessment of waste streams. Cumulative risk characterization ratios (RCRsum), summarising the substance specific risks, were calculated for open and closed loop scrubber water and compared to the ecotoxicological response from EMERGE D2.3, where whole effluent tests of scrubber water were conducted on different marine organisms. The results showed that the inclusion of alkylated PAHs in the risk assessment improves the prediction of scrubber water toxicity. The results suggestt that alkylated PAHs contribute to >85% of RCRsum of open loop scrubber water, yet the ecotoxicological response can still not be fully explained. The variability of the ecotoxicological responses, both within species and between species, indicates there are several unknowns related to scrubber water mixture toxicity.
Scrubbers have the highest RCRsum of all liquid waste streams included in the assessment (grey water, bilge water, sewage, ballast water, open and closed-loop scrubber water). Also, when comparing the volume weighted RCRsum for each waste stream for a model ship operating with an open or a closed loop scrubber for one year, it is apparent that the scrubbers have the highest contribution to risk of all waste streams and that open loop scrubbers contribute to more than 99% of the total volume weighted RCRsum.
The output from this deliverable can be used in the future work within EMERGE where work packages 6 and 7 will synthesise the output for the different case study regions and to further assess the environmental impact from shipping in general, with the use of scrubbers in particular.