An apparatus (100) for purifying fluid, wherein the apparatus (100) comprises an osmosis unit (102) configured for pre-purifying the fluid to be purified by forward osmosis of the fluid, which is to be purified, through an osmosis membrane (104) into a chamber (106) comprising dissolved first ions, in particular first cations and first anions, an ion exchange unit (108) configured for exchanging at least part of the first ions, in particular at least part of at least one of the first cations and the first anions, by second ions, in particular at least one of second cations and second anions, and a re-concentration unit (110) configured for separating the pre-purified fluid after the ion exchange into purified fluid and into a re-concentrate enriched with the respective ions, in particular anions and cations.