A filter unit is disclosed suitable for use in a filter housing. The filter unit comprises superposed and spaced-apart inclined lamellar structures, preferably in the form of hollow truncated cones, each having a lower end side opposite an upper end side and a passage for a flow of liquid between each two of the lamellar structures. Each passage has an inlet for receiving an inflow of liquid to be filtered and an outlet for discharging an outflow of filtered liquid. In each of the passages, an overflow dam wall following a sinuous path is obstructing the flow of liquid and retains particulate matter contained in the liquid. The filter unit is mounted vertically in a filter housing. The invention also concerns a prefilter comprising a filter unit as described above mounted vertically in a filter housing. Such prefilter can advantageously be used at the outlet of a septic tank