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TOPIC: Rheology of faecal sludges

Rheology of faecal sludges 06 Apr 2014 11:16 #8120

  • ChrisBuckley
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  • Chris Buckley is a chemical engineer and has been a member of the Pollution Research Group for over 40 years.
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A Short communication entitled "Shear rheological properties of fresh human faeces
with different moisture content" by SM Woolley, RS Cottingham, J Pocock and CA Buckley has just been published in Water SA Vol. 40 No. 2 April 2014


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.

This data will be of use to designers wishing to design faecal transport systems for dry sanitation systems. The shear thinning nature of the material should be of no surprise (and of some relief)to readers.

Further work in this area is currently underway. Studies include

  • rheology of sludge at different depths of dry VIP latrines (individual, communal and schools)
  • rheology of sludge from pour-flush leach pits

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Chris Buckley
Pollution Research Group
University of KwaZulu-Natal
South Africa
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