The Danco Complete Air Gap Assembly with Matte Black Air Gap Cap prevents sewage from flowing back into the dishwasher. The dishwasher air gap is mounted above the sink and prevents contaminated water from re-entering the dishwasher from the drain via backflow. It is a simple way to make certain wastewater and debris never re-enter your clean water supply. When you are running your dishes through a wash cycle the last thing you want is for them to emerge streaked with grime pumped in from your clogged garbage disposal. Since these two paths never intersect there is no risk of wastewater creeping back into your clean appliance.