Novel NoMix toilet concept for efficient separation of urine and feces and its design optimization using computational fluid mechanics

Separation of urine from feces in NoMix toilets bears a large potential for mitigating problems associated with the efficient reuse of excreta. A challenge lies in the design of these toilets that should maximize the amount of collected urine while at the same time minimizing its mixing with flush water that partly spills into the urine collector. We present a novel design idea that takes advantage of the so-called tea pot effect, which describes the attachment of liquids to a curved wall due to surface tension. This effect is used to efficiently guide the urine along a curved entrance into the collector. Computational Fluid Dynamics (CFD) simulations of the two phase (water/urine - air) system are carried out on different toilet geometries where we vary the inflow to the urine collector. In particular, we investigate two different radial curvatures and two V-profile grooves whose combinations result in four toilet geometries that are tested. We demonstrate that the urine collection can be dramatically improved by tailoring curvature radius and V-profile grooves, while keeping the amount of flush water in the urine collector at a minimum. A reduction to less than 2.5% of flush water entering the urine collector could be achieved by implementing a curvature radius of 15 mm and a V-profile groove width of 60 mm. The improved geometry's performance makes it an appealing candidate for broad application both in western and developing countries and we anticipate that this technology might replace traditional toilet designs in the future.