NDS 430 3" & 4" Emitter with Elbow UV Adapter, Green Pop Up Automatic spring action prevents damage from lawnmowers The special patented design allows the emitter to open with a negligible amount of pressure Since the emitter is closed during dry weather, debris and rodents cannot enter the drain pipes The pop-up emitter is opened by the hydrostatic pressure of water flowing through the drain pipe Fits 3-inch and 4-inch hub sewer and drain fittings