Low profile wash pads, usable alone or with other like pads in a module array, are disclosed whereby one or more motor driven or motorless vehicles may be wheeled onto or across (or other object placed upon) a full drainage impervious washing platform such that substantially all spent wash liquid and removed debris are caused to flow off from the washing platform to an edge collector or gutter. Debris accumulation at the washing platform is avoided and processing of gutter-contained spent wash liquid and removed debris can occur in offset relation to and without interfering with ongoing washing on the washing platform. Several modes for processing the drainage comprising spent wash liquid and removed debris contained in the gutter are disclosed, one mode comprising automated processing using a screw conveyor.