From Newtonian to non-Newtonian fluid: Insight into the impact of rheological characteristics on mineral deposition in urine collection and transportation.

The deposition of phosphorus-based mineral solids in urine diversion systems has been one of the main challenges for the large-scale practical applications of urine source separation. Accurate rheological characterization of urine slurry is of high importance for its practical flow performance. The rheological data of urine slurry was obtained using a narrow gap rotating rheometer. Based on current pipe flow theories and the obtained rheological data of urine slurry, the transition velocity was determined. The impacts of solid concentration and temperature on the rheological behavior of urine slurry were investigated in this study. Urine slurry behaved as a Newtonian fluid at low solid concentration. By contrast, urine slurry changed from Newtonian to non-Newtonian fluid with the increase in solid concentration, demonstrating a shearing thinning behavior and yielding stress fluid. The impact of temperature on the apparent viscosity of the urine slurry was described using an Arrhenius-type function. Moreover, the impact of solid concentration and temperature on the transition velocity was quantified, which indicated that the non-Newtonian behavior of the urine slurry in the compression settling region has a significant impact on the pipe flow behavior, leading to the formation of a compressed layer on the bottom of the pipe. The targeting understanding of transition velocity is particularly useful for the practical design and optimization of urine piping system, especially on how to mitigate pipe blockages. Based on the evaluation of different piping systems, this work proposed several potential urine collection and transportation modes.