Excess sludge handling is as important as treating the wastewater in determining the operational performance and costs in modern municipal wastewater treatment plants (WWTPs). In practice, excess sludge handling is widely implemented using anaerobic digestion (AD), in which the organic matter inside the sludge is not only stabilised and partly degraded, but also converted to methane gas as alternative fuel. Thus, optimal AD performance aims for maximising both sludge reduction and energy recovery, which is sometimes difficult to achieve in practice. Troubleshooting and optimisation are challenging to implement, due to the limited accessibility of anaerobic digesters and uncertainties of key influencing factors, including mixing behaviour and feed sludge characteristics. This thesis is focused on the possible enhancement of AD performance, by characterising the sludge rheology and evaluating its impact on sludge mixing inside the digesters. A full-scale digester, applying the gas-mixing mode, was selected for detailed investigations. Rheological properties of waste activated sludge (WAS) and digestate (DGT) from the selected WWTP were characterised, using both rheological measurements and rheological model fitting. DGT showed yield-pseudoplastic behaviour well characterised by the Herschel-Bulkley model. However, WAS demonstrated complex yield-pseudoplastic behaviour, which was better characterised by hybrid model fitting. The rheological instability, characterised by the distinct flow status and transitions, and considerable correlations to solids content, could give more insight in viscoelasticity and thixotropy of concentrated WAS. Moreover, recommendations for developing a proper rheological measurement protocol were also formulated. The thixotropic behaviour was further explored by involving the rheological impacts of shearing duration and temperature. Under the long-term shearing conditions, the complex thixotropic behaviour was well characterised by two limitation states: Initial and Stable, which showed distinct rheological properties for concentrated WAS, while small rheological difference for DGT. These distinct rheological properties of concentrated WAS were clearly reflected in its pipe flow behaviour, which was well assessed using the computational fluid dynamics (CFD) model with effective rheological data integration. Temperature had a striking impact on the sludge rheology, and strongly correlated with the solids content and degree of sludge stabilisation. The discrepancy in impact between long-term shearing and temperature implied different mechanisms responsible for shifting the equilibrium of hydrodynamic and non-hydrodynamic interactions for sludge structural deformation and recovery. Abstract II Moreover, the gas-sludge flow and mixing were characterised in detail, firstly using a refined lab-scale CFD model. Bubble size, phase interaction forces, and liquid rheology significantly impacted predictions of the two-phase flows. A more reliable and complete model validation was obtained by performing a critical comparison. The mixing performance approximated a laminar-flow reactor (LFR) that distinctly deviated from the expected continuously stirred tank reactor (CSTR) design. The results underline the importance of a proper phase-interaction description for reliable flow and mixing characterisation. The developed model setup was further applied to the full-scale digester. Results revealed considerable dependency of the flow and mixing characterisation, with the rheological input data. The predicted dominant shear rate level was out of the effective shear rate range of the Ostwald model. This finding limited the model application, since the apparent viscosity overestimation at low shear rates led to flow and mixing overestimation. However, the Herschel-Bulkley model better fitted low shear rates, and predicted large gradients of apparent viscosity in the poor flow regime. Distinct flow and mixing compartments were obtained based on the gas-sparging height, including a plug-flow compartment above, and a dead-zone below. Although inducing insufficient mixing, the applied gas-sparging may still be useful to mitigate short-circuiting, accumulative sedimentation and effective volume reduction…