PURPOSE: To separate solids and liquid thoroughly and easily by treating solid- liquid mixtures which are difficult to be treated with conventional filters and dehydrators by means of a settling and separating device with inclined plates, a filter device utilizing filter aids and a filtering and dehydrating device.

CONSTITUTION: Solid-liquid mixtures contg. fine solids such as municipal waste water, industrial waste water and the like are introduced into a settling and separating device P with inclined plates constituted of a hollow cylinder 4 having a screw shaft 3 provided with many screw vanes 2. The liquid ascends along the vanes 2 and enters a reservoir chamber R by passing through fine holes 5. The solids descend along the vanes 2 and settle in the lower part of the cylinder 4 with an increase of the concn. The liquid enters the inside of a filter aid chamber Q of river sand or the like where the fine solids are removed with the filter aids. The liquid flows between flow regulating plates 7 from the lower part of the chamber Q and enters the hollow chamber 4 where the liquid together with the previously separated and settled solids are filtered and dehydrated and then discharged.