In order to achieve improved collection of sludge in a settling basin, without employing moving parts, a collecting trough is provided, this trough being open at the top and bounded on the sides by horizontal overflow-weirs and being intended to rest on the bottom of the basin. A line, located level with the bottom of the collecting trough, issues from the trough and runs to a suction pump. The trough is covered by a hood which widens towards the bottom and leaves an unobstructed interspace between itself and the overflow-weirs, this hood possessing openings which lie at a lower level than the overflow-weirs.