Urine Trap,
Gates Foundation

A key breakthrough came with the application of the “teapot effect”. By leveraging surface tension, a liquid moving at low speed follows an edge and is guided toward the back (as seen when a teapot drips). At higher speeds, the liquid stream separates and flows past the edge. This principle allows slow-moving urine and fast-moving flush water to be separated without the need for valves or chemicals—a simple yet disruptive solution that remains invisible to users.

Conventional urine-diverting toilets typically feature a front bowl with a central outlet—similar to a urinal—to separate urine from flush water. During flushing, a valve must close the urine outlet and redirect the flush water to the rear. In practice, however, issues such as toilet paper residue and blocked valves prevented this design from gaining widespread adoption.