The photographs above were taken during home inspections by my company, Magpie Property Inspections, in Parker Colorado and Denver, Colorado. The thinner white flexible plastic tube pictured is the waste water discharge line from the dishwasher to the drain, waste and vent system under the kitchen sink. Because of the possibility of water water backing up from the main drain line under the sink into the dishwasher, most jurisdictions require that the dishwasher drain either be installed with an air gap, or a high loop to prevent a back up from occurring.
In the photo on the left, no air gap device or high loop was noted. The photo on the right shows a drain line installed into a drain pipe with an air gap. While some dishwashers have an internal back flow device, normally it is not possible for an inspector to determine if an internal back flow device is installed in a dishwasher unit. It is good practice to inspect to see if either a high loop or air gap has been installed for safer operation of the dishwasher and report if none is found. In some cases a dishwasher may have been installed when this practice was not required, but it we recommend having this checked anytime a plumber is out to your home and to have this addressed, as needed, by a qualified technician to meet current standard.