The anaerobic conversion of waste to biogas in a biogas digester is influenced by a number of factors including mixing ratios, type of substrate, temperature, organic loading rate, pH, and carbon/nitrogen ratio. The appropriate mixing ratio of water and animal waste leads to effective biogas yield. Different mixing ratios proposed in literature for various animal waste for improving biogas yield are reviewed. Characterization and application of animal slurry, the choice of animal slurry as suitable feedstock for biogas production, the storage of animal slurry, the agitation process of slurry as well as mixing ratios of animal slurry are evaluated.