A method for treating impurities contained in exhaust gases of ships in order to reduce sulphur oxide emissions, the method comprising - scrubbing with water the exhaust gases in an exhaust gas scrubber (1, 1', 1") in order to reduce sulphur dioxide emissions of the exhaust gases, - supplying wash water to be purified, i.e. effluent, containing impurities and exiting from the exhaust gas scrubber (1, 1', 1"), to a purification unit (2, 2', 2") onboard a ship, in which the effluent is purified, and - monitoring a pH value of the purified effluent, and if it is less than a predetermined value, the pH value of the effluent is adjusted to a desired value, after which the purified effluent is discharged into a sea or returned to the exhaust gas scrubber (1, 1', 1"), characterized in that - the purification unit (2, 2', 2") comprises a purification device (120, 120', 120") comprising a band filter (121, 121', 121") comprising a moving inclined filter band (122, 122', 122"), whereby an aqueous fluid containing impurities originating from the effluent is together with one or more precipitating agents fed to a first surface of the inclined filter band (122, 122', 122") of the band filter and filtered through the filter band in order to concentrate impurities contained in the aqueous fluid onto the first surface of the band filter as a precipitate to be removed from the first surface of the filter band, and - the wash water to be purified, i.e. the effluent, exiting from the exhaust gas scrubber (1', 1") and containing impurities, is purified in an effluent circuit (3', 3") included in the purification unit (2', 2") prior to said feeding of the aqueous fluid to the inclined filter band (122', 122") of the band filter, the effluent circuit (3', 3") including at least one membrane filter (4', 4"), and that the wash water to be purified, i.e. the effluent, is circulated in the effluent circuit (3', 3") in which the effluent is filtered through the semipermeable membrane (7', 7") of the membrane filter (4', 4") in order to achieve a purified effluent and a residue containing impurities, the purified effluent being removed from the membrane filter and circulation in the effluent circuit while the residue keeps circulating in the effluent circuit (3', 3") in which it concentrates, after which at least some of the concentrated residue in the effluent circuit (3', 3") is fed, highly concentrated with impurities and in the form of an aqueous fluid, to the inclined filter band (122', 122") of the band filter (121', 121"). - scrubbing with water the exhaust gases in an exhaust gas scrubber (1, 1', 1") in order to reduce sulphur dioxide emissions of the exhaust gases, - supplying wash water to be purified, i.e. effluent, containing impurities and exiting from the exhaust gas scrubber (1, 1', 1"), to a purification unit (2, 2', 2") onboard a ship, in which the effluent is purified, and - monitoring a pH value of the purified effluent, and if it is less than a predetermined value, the pH value of the effluent is adjusted to a desired value, after which the purified effluent is discharged into a sea or returned to the exhaust gas scrubber (1, 1', 1"), - the purification unit (2, 2', 2") comprises a purification device (120, 120', 120") comprising a band filter (121, 121', 121") comprising a moving inclined filter band (122, 122', 122"), whereby an aqueous fluid containing impurities originating from the effluent is together with one or more precipitating agents fed to a first surface of the inclined filter band (122, 122', 122") of the band filter and filtered through the filter band in order to concentrate impurities contained in the aqueous fluid onto the first surface of the band filter as a precipitate to be removed from the first surface of the filter band, and - the wash water to be purified, i.e. the effluent, exiting from the exhaust gas scrubber (1', 1") and containing impurities, is purified in an effluent circuit (3', 3") included in the purification unit (2', 2") prior to said feeding of the aqueous fluid to the inclined filter band (122', 122") of the band filter, the effluent circuit (3', 3") including at least one membrane filter (4', 4"), and that the wash water to be purified, i.e. the effluent, is circulated in the effluent circuit (3', 3") in which the effluent is filtered through the semipermeable membrane (7', 7") of the membrane filter (4', 4") in order to achieve a purified effluent and a residue containing impurities, the purified effluent being removed from the membrane filter and circulation in the effluent circuit while the residue keeps circulating in the effluent circuit (3', 3") in which it concentrates, after which at least some of the concentrated residue in the effluent circuit (3', 3") is fed, highly concentrated with impurities and in the form of an aqueous fluid, to the inclined filter band (122', 122") of the band filter (121', 121").