A sludge collector arrangement has a settling tank having side walls sloping downwardly and inwardly towards the base of the tank to from a V-base. A sludge collection unit located in the bottom of the tank has one funnel part facing in one direction and another funnel part facing in the other direction so that when the unit is driven backwards and forwards in the V-base, sludge is driven into one or other of the funnel parts. The funnel parts communicate with a central chamber and a port in a wall of the chamber allows sludge to be drawn by a suction pump from the chamber for subsequent disposal.