It might seem like a urinal that doesn't use any water would be complicated, filled with moving parts and pumps and complex valves. But actually, waterless urinals are relatively simple. Most use gravity and a specially designed trap chamber filled with a liquid called sealant, to send waste cleanly down the drain [source: Waterless]. Depending on the manufacturer, the sealant can be formulated differently. But all sealants are lighter than water, don't evaporate and are usually made at least partially from oils (like vegetable oil) [source: Waterless].
If you look at a waterless urinal, you will see a circular dome surrounded by tiny channels for water to move through. This dome is the trap, a removable cartridge that makes waterless draining possible. In the most common design, the waste flows into the trap through the series of circular slits along the top of the cartridge. As it enters the cartridge, the urine filters through the sealant, which stays on top because it is lighter than water [source: Reichardt]. The urine then flows down into a central reservoir. In the center of that reservoir is an open pipe that drains out into the bathroom's waste line, the same waste line that standard urinals use. The urine slowly fills up the reservoir, and when the level of liquid gets higher than the rim of the pipe, the excess drains out [source: Reichardt]. So, at all times, there is a small amount of urine inside the reservoir. As more and more men come and use the urinal, the old waste flows out and the new waste flows in.