The invention relates to a device and a method for the purification of effluent. The device comprises a filter container (1A), containing a filter medium (2a, 2b), through which the contaminated effluent (3) is run in a direction (filtration direction), thus depositing dirt particles on the filter medium (2a, 2b). The risk of filter blocking is present in such filtration devices, especially at low filtration speeds. It is known that filter blockages can be avoided by the addition of flocculating agents or by increasing the filtration speed. The addition of flocculating agents makes the filter process more expensive and increasing the filtration speed reduces the quality of the filtrate. According to the invention, a high filtrate quality may be guaranteed without addition of flocculating agents with said device, whereby the cross-sectional area of the filter container (filtration area) running perpendicular to the filtration direction, expands in the filtering direction, whereupon the speed of the effluent being run through (filtration speed) continually reduces along the filtration direction.