Impact of temperature and duration of thermal treatment on different concentrations of anaerobic digested sludge: Kinetic similarity of organic matter solubilisation and sludge rheology

Municipal digested sludge is a dispersion of agglomerated particles or flocs in a liquid medium which exhibits yield stress, shear thinning behaviour and high irreversible temperature dependence. In this study, the rate of solubilisation of organic matter in digested sludge was investigated when sludge (at different solid concentrations) was exposed to thermal treatment between 50 and 80. °C for 1 to 60. min. The organic matter solubilisation was measured using chemical oxygen demand (COD) analysis on liquor of the thermally treated sludge. The effects of the abovementioned temperature range and heating duration on yield stress and the apparent viscosity of sludge at different solid concentrations have also been investigated. The results showed that the irreversible effect of temperature was much higher at higher concentrations, higher temperatures and longer duration of treatment. It was also observed that for any sludge concentration the kinetic of COD solubilisation, yield stress and apparent viscosity reductions followed a logarithmic correlation with duration of thermal treatment. This similarity was also pronounced in the linear relationship between solubilised COD and yield stress and apparent viscosity reductions in all tested concentrations and the durations of heat treatment. Modified form of Herschel-Bulkley was used to obtain a master curve, independent of temperature, concentration and duration of thermal treatment. Furthermore, a new model was proposed for the impact of temperature and the duration of thermal treatment on yield stress and apparent viscosity reduction