Abstract: Apparatus for separating solid particles, such as amalgam waste from the waste water from dentists' surgical units, employs at least one filter body through which the liquid flows and which consists of a filter medium enclosed between two perforated walls, and at least one sedimentation zone. The filter medium consists of balls of alumina having a diameter of 0.05-15 mm and a density such that the balls float in or are carried along by the liquid flowing through the filter body. The balls, which do not fill out the entire volume between the perforated walls (although they fill out the cross-sectional area of the filter body), rub against each other in the flowing liquid, so that the filter body is self-cleaning.