||Most wastewater treatment plants yield sewage sludge from the processes of sold-liquid separation through sedimentation as a result of chemical or biological reaction. The amount of sewage sludge produced continues to be increased as the environmental regulations become more stringent and the treatment capacity of wastewater treatment plant enlarges. Therefore, treatment and disposal of sewage sludge becomes an important environmental issue. One of the environmentally acceptable treatment methods is the biological digestion to reduce the volume and to recycle the useful resource of the sewage sludge for the benefit of the community. Anaerobic sludge digestion has been commonly adopted because of several advantages such as the productions of methane as an energy resources and solids residue as a soil conditioner, reduction in mass and volume of sludge, and reduction of odor nuisance and pathogens. The anaerobic process is mainly controlled by the bacteria that are sensitive to pH, substrate composition, temperature and other factors. In order to provide a favorable environment for the microorganisms to survive, it is necessary to have efficient mixing in the anaerobic sludge digester. Three-dimensional (3-D) computational fluid dynamics (CFD) simulations are performed to study the mixing in an egg-shaped anaerobic digester. The mixing is provided by a mechanical draft tube mixer and eight nozzles in the digester. The governing equations used in this study are the 3-D Navier-Stokes equations that describe the conservation of mass and momentum, and the mass diffusion equation to describe the change of volatile solids (VS). The initial and boundary conditions of flow and volatile solids (VS) concentration are given according to the daily operations of the digester. The qualitative velocity and VS concentration fields are computed with a commercial CFD code. Numerical results in terms of periodically steady state flow pattern are presented to demonstrate the development of flow pattern, location of vortices and VS concentration field in the digester. The significances of nozzles and fluid viscosities on the flow pattern are explored, and the decay of VS concentration in the digester is also discussed.