When SO2 is dissolved in water, a reaction occurs whereby the sulphur dioxide is ionised to bisulphite and sulphiite, which is then readily oxidized to sulfate in seawater containing oxygen . This process increases the Chemical Oxygen Demand, which could potentially have an adverse impact on aquatic systems.
Process of Oxidation
SO2 + H2O <> “H2SO3” (sulphurous acid) <> H+ + HSO3– (bisulphite)
Note: there is no evidence that sulphurous acid exists in solution, but the molecule has been detected in the gas phase
HSO3– (bisulphite) <> H+ + SO32- (sulphite)
SO32- (sulphite) + ½ O2 <> SO42- (sulphate)
How is it measured?
- Using worse case scenarios Karle and Turner  evaluated how much dilution of wash water would be required to return oxygen levels to within 1% of those of the ambient water.
- Using different waters from full seawater to full freshwater and intermediate alkalinities/salinities, it was found that, other than for full open ocean water above 15oC, no further dilution was required if the pH of the water had already been corrected to within 0.2 of ambient.
- As is known from various in-field tests and modeling of discharge plumes that the pH and oxygen of discharged water very rapidly returns to that of the surrounding water, especially when the vessel is underway, the IMO does not require dissolved oxygen to be monitored 
 Seawater scrubbing – a reduction of SOx emissions from ship exhausts. Karle and Turner. Publisher: The Alliance For Global Sustainability Gothenburg 2007
MEPC 58/23 Annex 16, Report of the Marine Environment Protection Committee on its fifty-eighth session. International Maritime Organization, 2008.
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