Rheology of thermally-treated sewage sludge (anaerobic digested sludge &amp; waste activated sludge)
Creators: name: Farno, Ehsan
Publisher: name: RMIT University
Year: 2024

description: There is an increase in the solid residue of wastewater treatment plants due to population growth and tightening environmental regulation. Anaerobic digestion process in the wastewater treatment plants is a time consuming process which takes several days to be completed. Thermal pre-treatment of sludge before anaerobic digestion can increase the access of microorganisms to organic matter which will result in improving the digestion process. This treatment, however, causes rheological and compositional changes in sludge which needs to be understood. To optimise the hydrodynamics of the anaerobic digesters, determination of rheological properties of thermally treated sludge is required.<br><br>This study investigates the effect of temperature and thermal history in the range of 50-80°C for 1 h on the rheology and composition of digested sludge and waste activated sludge (WAS). The range of temperature and time were selected 50-80°C and 1-60 min, firstly, because it ensures that the compositional changes in sludge are as a result of thermal treatment and solubilisation phenomenon and not digestion by microorganisms. Secondly, the thermal treatment of sludge for several days may not be economical in the large scale.<br><br>First, the flow curve, yield stress and composition (indicated by sCOD) of 2% TS digested sludge were measured before, after and during the thermal treatment. The results showed a significant irreversible decrease in apparent viscosity and yield stress with an increase in temperature and treatment time. Also, the result showed an increase in soluble COD with an increase in temperature and treatment time. It was hypothesised that the rheological changes induced by thermal treatment occur as a result of organic matter solubilisation indicated by COD analysis. An irreversible increase in sludge liquor viscosity after thermal treatment confirmed this hypothesis. <br><br>Second, the sludge flow curves at different concentrations, temperatures and duration of thermal treatment were scalable which resulted in developing a master curve to encapsulate all variables. The release of soluble COD (rsCOD) followed a power-logarithmic function of thermal treatment time in the range of temperature studied. The dimensionless yield stress and infinite viscosity were shown to have the same power-logarithmic relationship with treatment time too. A linear relationship was then found between the rsCOD and dimensionless yield stress as well as the rsCOD and dimensionless infinite viscosity.<br><br>Third, the hypothesis was tested and confirmed for WAS within 3-8.3% TS thermally treated at 50-80°C. It was confirmed that the rsCOD followed power-logarithmic relationship with treatment time regardless of sludge type. Similarly, linear relationships were found between the rsCOD and dimensionless yield stress as well as rsCOD and dimensionless infinite viscosity for thermally-treated WAS. It was suggested that the thermal treatment of sludge breaks down the secondary bonds in sludge flocs and releases polysaccharides, proteins and bound water which modify the rheology. <br><br>Fourth, a linear relationship was also found between rsCOD and dimensionless moduli of sludge in the linear viscoelastic regime. It was also shown that a real-time rheological measurement of complex moduli during the thermal treatment could be used for the estimation of changes in rsCOD.