Shear rheological properties of fresh human faeces with different moisture content

Abstract

Dry sanitation requires the handling of faeces, which vary in age and degree of transformation. Rheological data are necessary to support the design of equipment to handle faeces. The rheological properties of fresh human faeces were measured using a variable-speed rotational rheometer. Samples were further tested for moisture content, total solids, volatile content, and ash content. Faecal samples were found to have a yield stress; there was a decrease in apparent viscosity with increasing shear rate. For any given shear rate, higher apparent viscosities are associated with lower moisture contents. Across a range of water contents of 58.5% to 88.7%, apparent viscosities of 27 Pa∙s to 2 014 Pa∙s were measured at a shear rate of 1 s^-1. During constant shear tests, the apparent viscosity of all faeces was found to decrease asymptotically, where the minimum apparent viscosity value increased with decreasing moisture content. A structural recovery test indicates that human faeces are thixotropic in behaviour, where the viscosity permanently decreases to 0.5% of the initial value after a 20 s exposure to a shear rate of 10 s^-1. A linear relationship between viscosity and temperature was found, with a recorded 30.6% decrease in viscosity for a 35.6 °C increase in temperature from 13.4°C.

Keywords: Human stool, mechanical properties, water content, flow curve, viscosity, yield stress