An improved greywater disposal system is provided which substantially eliminates onboard standing greywater. The system includes one or more onboard plumbing fixtures fluidly connected to a greywater sump. The bottom of the greywater sump is sloped toward a drain therein at a low end. The prongs of a sensor may be suspended in the sump drain and/or in an inlet chamber defined at an inlet end of the greywater sump. The sensor is associated with a controller and sump or bilge pump. The pump is turned on when the prong tips of the sensor become immersed in greywater which occurs substantially simultaneously with the production of the greywater. The onboard plumbing fixtures include one or more sinks, showers, bathtubs, washing machines and air conditioning units. The system may be used on boats, ships, RVs, campers or the like.