Abstract: A waste water treatment apparatus of the type in which a carriage support rail extends across the top of the tank intermediate the side walls of the tank and a carriage is mounted on the rail and driven by a carriage drive including an endless chain which reciprocates the carriage along the rail, and a tank cleaning head is mounted on the carriage for movement along a path adjacent the bottom of the tank. The carriage support rail is in the form of a box-shaped beam having inwardly directed flanges at its underside and the chain is mounted inside the box-shaped beam to be protectively enclosed in the beam and the carriage has wheel mounting portions extending upwardly into the beam at opposite sides of the chain and carriage support wheels mounted on the wheel mounting portions inside the beam and in rolling engagement with the flanges on the beam to support the carriage for movement therealong.