Sulphur from ships' exhaust is a significant contributor to ozone depletion. Thus, when the International Maritime Organisation (IMO) adopted the International Convention for the Prevention of Pollution from Ships (Marpol) Annex VI in 2008-this prohibits deliberate emission of ozone-depleting substances such as sulphur oxides and nitrous oxides by ships-it was hoped that more ships will get fitted with closed-loop scrubbers. Exhaust scrubbers bring down sulphur pollutants in ships' exhaust, and closed loop ones retain the emissions for disposal at the port while open loop ones release the pollutants into the sea after turning them into sulphuric acid.
But, it seems the preference for open-loop scrubbers dwarves that for closed-loop ones-recent data shows that of 3,756 vessels with scrubbers, just 65 have closed-loop. While the IMO approves of open-loop scrubbers, it has strict guidelines of discharge of wash water from exhaust cleaning systems, GlobalData, a data analytics company that put out the numbers, reports. The fact is that it is still uncertain if open loop scrubbers will prove to be sustainable in the long run. While IMO's Suplhur 2020 vision estimates that the capping standards for sulphur content in ships's fuel that came into force from January 1 will lead to a 77% drop in overall sulphur oxide emissions from ships-an annual reduction of 8.5 million tonnes of SOx-if discharge of wash-water remains an unaddressed area, the problem isn't really resolved.