Abstract: A method and a system for treating water on board a vessel before discharging such water to the sea. The water to be treated is pumped through a primary heat exchanger (12) where the water is pre-heated and then through a secondary heat exchanger (13) where the water is further heated, the secondary heat exchanger (13) being heated by thermal fluid from a central heating system primarily heated by heat recovered from different type of heat generating equipment and also possibly by fuel and/or electricity in cases where the heat recovered from the heat generating systems is not sufficient to treat the water in a sufficient manner.