Rheological characterisation of thermally-treated anaerobic digested sludge: Impact of temperature and thermal history

Abstract

This study investigated the partially irreversible effect of thermal treatment on the rheology of digested sludge when it was subjected to temperature change between 20 °C and 80 °C and then cooled down to 20 °C. The yield stress, infinite viscosity and liquor viscosity of sludge were measured at 20 °C for different thermal histories and were compared to the evolution of the solubilised chemical oxygen demand (COD) of sludge liquor. The results showed that thermal history irreversibly affects sludge rheology as the yield stress of sludge which was heated to 80 °C then cooled down to 20 °C was 68% lower than the initial yield stress at 20 °C. This decrease was due to the irreversible solubilisation of solid matter during heating as underlined by soluble COD data which did not reach its original level after thermal treatment. Measured soluble COD of sludge which was heated and cooled down was much higher than the soluble COD of initial sludge. We found a proportionality of the increase of soluble COD with the decrease of the yield stress as well as increase of infinite viscosity.

Ultrasound, thermal and alkali treatments affect extracellular polymeric substances (EPSs) and improve waste activated sludge dewatering

“…1). Similarly, Farno et al [30] found that the increase in the level of solubility induces that some particulate organic matter inside the sludge changed their phase and solubilised into liquid medium (interstitial liquid), thus increasing the viscosity of liquid medium. Thus, the bulk solutions of treated sludges are more complex fluids comprised of EPSs, Table 3 Capillary suction time, Ostwald-de Waele parameters and water removed by centrifugation of untreated and treated sludge samples.…”

The viscoelastic characterisation of thermally-treated waste activated sludge

“…Solubilisation of COD due thermal treatment is also an indication of structural change in sludge after thermal treatment. Farno et al (2014) [8] showed that sludge which was treated at 50-80°C and then cooled down to 20°C contained higher soluble COD compared to the untreated sludge at 20°C. This can be because of irreversible destructing bonds in sludge during the thermal treatment.…”

“…Literature survey suggests that thermal treatment of sludge (digested or WAS) at 50-80°C results in modification of sludge rheological properties such as yield stress and apparent viscosity [7]. Farno et al (2014Farno et al ( , 2015 and 2016) [7][8][9] showed that the yield stress and the apparent viscosity of digested sludge and WAS reduced by increasing thermal treatment temperature and its duration and there is a linear relationship between thermally-induced compositional change of sludge indicated by soluble COD and the decrease in yield stress and infinite viscosity. They proved that the compositional changes of sludge (i.e.…”

Structural model to study the influence of thermal treatment on the thixotropic behaviour of waste activated sludge

“…Similar results were reported by Bougrier et al [35], who found that the CST value increased at low temperatures and decreased at temperatures higher than 130°C. Farno et al [36] found that the increase of soluble chemical oxygen demand (COD) with temperature of thermal history (up to 80°C) was linearly proportional to the increase in viscosity of the liquid medium of the sludge. This could possibly explain the increased CST value at temperatures below 100°C.…”