Unlike nitric oxide emissions, sulphur oxides cannot be reduced by modifying the combustion process inside the engine. All of the sulphur contained in the fuel is output in the exhaust gas. A dramatic reduction in sulphur emissions can be achieved, however, by switching from heavy fuel oil to fuels with a lower sulphur content – such as marine diesel oil or natural gas. This solution could initially be used in coastal ECAs or in harbours, while ships on the high seas can continue to be powered by conventional fuel. The dual tank arrangement required for this, however, is costly and space-consuming. The fuels mentioned above are also significantly more expensive than conventional heavy fuel oil. It must be kept in mind that the operating costs of a ship or diesel power plant are largely made up of fuel costs.
 
The exhaust gas scrubber, known as the open loop scrubber, reduces the sulphur oxide content of the exhaust gases by 90 to 95 per cent. Spray jets similar to the design of shower heads drench the exhaust gas with sea water just before the flue. Water and sulphur react to form sulphuric acid, which is neutralised with alkaline components in the sea water. Filters separate particles and oil from the mixture before the cleaned water is given back into the sea. The disadvantage of scrubber technology is its relatively large space requirements on board. Its operation requires a capacity of 40 to 50 cubic metres of sea water per Megawatt hour of engine power.
 
Our engineers are nevertheless already working on a version known as the closed loop scrubber that uses fresh water in combination with caustic soda as the neutralising additive. The scrubber then requires less space and its water requirements drop to 0.1 cubic metre per Megawatt hour output, and virtually no wash-water is produced that would have to be lead into the sea.

Also in development is a dry scrubber, in which the exhaust gas flows through granulated limestone. This combines with the sulphur to form gypsum, which can then be disposed of on land. The advantage: the sulphur is locked in, meaning it cannot burden the biosphere any more. The disadvantage: a storage room has to be created on board for the granulate, which means sacrificing cargo capacity.