The invention relates to a crystallization apparatus and to a method for crystallizing a liquid from a solution, a mixture of liquids or a suspension. The apparatus comprises a growth zone (4) in a vertically positioned tank (2), which growth zone is positioned below a nucleation zone (5). Via a delivery line (13) the seed crystals are fed from the nucleation zone to the bottom end of the growth zone. In the growth zone the ice slurry slowly moves in a vertical direction at such a low velocity that heavier particles are able to precipitate and to be discharged at the bottom end. Preferentially, the nucleation zone comprises a vacuum apparatus for effecting a reduction in pressure in the nucleation zone to between 10 -4 and 7·10 -3 bar. Using the apparatus according to the present invention it is possible for waste water and effluent to be concentrated in a simple manner, clogging of, for example, filters owing to the settling of solid particles being prevented.