Modeling the instant acidification effects of open-loop scrubber discharges along shipping routes

Abstract

The widespread use of high-sulfur fuels in maritime shipping has raised serious environmental concerns, prompting the International Maritime Organization (IMO) to mandate low-sulfur fuels or exhaust gas cleaning systems (EGCS). However, open-loop EGCS discharges are highly acidic, posing potential risks to busy shipping routes and coastal ecosystems. This study developed a discrete equation-based model to assess instant seawater acidification from open-loop EGCS discharges under three scenarios. The effects of ship size and ocean currents were also examined. In the single-ship single-discharge scenario, surface pH fell below 7 within 20 m in 60 s. Continuous discharge from one or two ships led to pH levels below 4.2 in the same area and time frame, indicating wider acidification impacts. pH recovery to 7 took 1 h for single discharge, and up to 30 h for two-ship continuous discharge, suggesting prolonged acidification from combined discharges. Sensitivity analysis showed horizontal currents increased pollutant spread, while vertical currents intensified local acidification. Larger vessels caused more severe pH drops, despite similar spatial patterns across ship types. These findings highlight the environmental risks of open-loop EGCS use in coastal waters and underscore the need for stricter regulations, improved discharge strategies, and continuous monitoring to protect marine ecosystems.