Effect of thermal history on municipal digested sludge rheology: Experimental and modeling

The rheological characteristics of digested sludge are severely temperature dependent. An effect of temperature on rheological properties of digested sludge is partially irreversible. In this paper, the effects of temperature change and thermal history on viscosity of digested sludge were studied using a stress-controlled rheometer which was connected to a temperature controlled water bath. Digested municipal sludge were collected from one of Melbourne wastewater treatment plants and subjected to change of temperature from 20-80°C. In addition, the effect of temperature on the sludge composition was studied by measuring solubilized chemical oxygen demand (COD) in sludge liquor. The Herschel-Bulkley equation was used to model the data. The yield stress and the apparent viscosity were measured and compared at different temperatures. The results showed that by changing the temperature, the sludge becomes more fluid; however, the rheological characteristics of sludge including yield stress and apparent viscosity do not come back to their original values after cooling the heated sludge. The solubilized COD showed a composition change after cooling the heated sludge which confirmed the irreversible change in the rheological characteristics of digested sludge