The process of using flat-sheet membrane for simultaneous sludge thickening and digestion (MSTD) was employed. The variations of sludge concentration and rheology were characterized and simulated. Based on mass balance analysis, mathematical models were developed and successfully used to predict and evaluate the variations of sludge concentration and the digestion efficiency in the MSTD process. The apparent viscosity of sludge could be modeled as functions of mixed liquor suspended solids and shear rates. The sludge in MSTD process showed both shear-thinning and viscoplastic behaviour, and under various shear rates different rheological models could be chosen to predict their flow behaviour. It was also found that sludge concentration and viscosity had significant correlations with membrane fouling in the MSTD process.