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.
Highlights
The Bill and Melinda Gates Foundation have presented a Reinvent the Toilet Challenge to researchers (Global Development Program, 2012)
Shear rate–shear stress flow curves were produced for faecal samples of various moisture contents, by sweeping through a range of shear rates and producing a corresponding range of shear stresses
Shear rate–apparent viscosity flow curves were produced by calculating apparent viscosity data for each shear rate–shear stress data point on every curve
Summary
The Bill and Melinda Gates Foundation have presented a Reinvent the Toilet Challenge to researchers (Global Development Program, 2012). The objective of the challenge is to provide a sanitation solution for poor unserved households at a user cost of less than USD 0.05 per person per day. A wide range of processes have been proposed. Some of these proposals involve the source separation of urine and faeces; each of these streams is handled separately. This implies that that these streams need to be mechanically handled prior to subsequent processing, e.g., drying, gasification or combustion. Viscosity data are needed for the design of the mechanical handling equipment
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