Wastewater can be purified in a device, which includes a cup-shaped vessel, having a wastewater inlet tube for the water at a certain level and an outlet tube at a lower level than the inlet tube, and a horizontal distribution disc, which vertically divides the vessel, has at least one opening and is provided with a substantially vertical distribution tube for receiving the water from the inlet tube. Water above the distribution disc is pumped through a filter head with a filter. Filtered water is let out through the outlet tube, whereas unfiltered water is transferred back to the distribution tube. Flocculation agent is added to the unfiltered water before reaching the distribution tube.