Effect of Thermal Hydrolysis on Rheological Behavior of Municipal Sludge

Properly understanding of sludge rheological properties is important for designing of pumping and translating. Effect of thermal hydrolysis on rheological properties of municipal sludge was studied using a rheometer (DHR-2) with concentric cylinder geometry. Test results indicated both raw and thermal treated sludge displayed non-Newtonian rheological behavior with shear thinning, exhibiting thixotropic properties and viscoelasticity. The Herschel-Bulkley model could represent their flow behavior more accurately than other models. However, for the raw sludge, as solid content reached 120 g/L, fluidity disappeared, and all the rheological models could not describe it accurately. After thermal hydrolysis, the consistency index (k) decreased significantly, while the flow index (n) increased, suggesting that thermal treated sludge was much closer to the Newtonian fluid compared to the raw sludge. Both raw and treated sludge exhibited strong dependence on solid content and temperature. Correlations between solid content, temperature, and viscosity were expressed by an exponential equation and an Arrhenius type equation, respectively. Analysis of thixotropic properties illustrated that evolution of viscosity over time could be expressed by a first-order (solid content lower than 100 g/L) and a second-order thixotropic kinetic equation (solid content higher than 100 g/L), respectively, for raw sludge. For treated sludge, it could be simulated by a first-order thixotropic kinetic equation. Furthermore, the dynamic test indicated viscoelasticity of treated sludge decreased remarkably. For treated sludge, as the solid content was larger than 120 g/L, viscoelastic parameters were linearly correlated with logarithm frequency.