A method for adjusting S/Na -balance of a sulphate pulp mill, the method comprising - providing a pulp mill concentrated non-condensable gas stream (101, 201) containing sulphur compounds into a scrubber (102, 202), the pulp mill concentrated non-condensable gas stream (101, 201) having a sulphur concentration of above 5 g/m 3, - scrubbing the pulp mill concentrated non-condensable gas stream (101, 201) containing sulphur compounds in the scrubber (102, 202) with an aqueous scrubbing solution (109, 209) containing an alkaline agent, whereby at least some of the sulphur compounds react with the alkaline agent, thereby producing a concentrated non-condensable gas stream derivate (110, 210) and an aqueous spent scrubbing solution (104, 204, 204a, 204b) containing sulphides, - adjusting pH of the aqueous scrubbing solution (109, 209) with the alkaline agent, such that the pH of the aqueous scrubbing solution (109, 209) is above 11.5, - introducing the aqueous spent scrubbing solution (104, 204a) containing sulphides into a bioreactor (105, 205) located downstream of the scrubber (102, 202), - aerating the bioreactor (105, 205) with a gas (111, 211) comprising weak malodorous gas from the pulp mill, the weak malodorous gas having a sulphur concentration of less than 0.5 g/m 3, - oxidizing the aqueous spent scrubbing solution (104, 204a) containing sulphides in the bioreactor (105, 205) biologically in an oxidizing reaction by means of sulphur-oxidizing microbes, thereby obtaining an aqueous suspension (106, 206) containing elemental sulphur, and - separating the elemental sulphur from the aqueous suspension (106, 206) in a sulphur separation unit (107, 207) located downstream of the bioreactor, thereby obtaining a residual solution (109a, 109b, 209a, 209b) and a precipitate (108, 208) containing the elemental sulphur. - providing a pulp mill concentrated non-condensable gas stream (101, 201) containing sulphur compounds into a scrubber (102, 202), the pulp mill concentrated non-condensable gas stream (101, 201) having a sulphur concentration of above 5 g/m 3, - scrubbing the pulp mill concentrated non-condensable gas stream (101, 201) containing sulphur compounds in the scrubber (102, 202) with an aqueous scrubbing solution (109, 209) containing an alkaline agent, whereby at least some of the sulphur compounds react with the alkaline agent, thereby producing a concentrated non-condensable gas stream derivate (110, 210) and an aqueous spent scrubbing solution (104, 204, 204a, 204b) containing sulphides, - adjusting pH of the aqueous scrubbing solution (109, 209) with the alkaline agent, such that the pH of the aqueous scrubbing solution (109, 209) is above 11.5, - introducing the aqueous spent scrubbing solution (104, 204a) containing sulphides into a bioreactor (105, 205) located downstream of the scrubber (102, 202), - aerating the bioreactor (105, 205) with a gas (111, 211) comprising weak malodorous gas from the pulp mill, the weak malodorous gas having a sulphur concentration of less than 0.5 g/m 3, - oxidizing the aqueous spent scrubbing solution (104, 204a) containing sulphides in the bioreactor (105, 205) biologically in an oxidizing reaction by means of sulphur-oxidizing microbes, thereby obtaining an aqueous suspension (106, 206) containing elemental sulphur, and - separating the elemental sulphur from the aqueous suspension (106, 206) in a sulphur separation unit (107, 207) located downstream of the bioreactor, thereby obtaining a residual solution (109a, 109b, 209a, 209b) and a precipitate (108, 208) containing the elemental sulphur.