The anaerobic treatment of wastewater including yeast, Pichia pastoris, with a high concentration of 30 g/l was carried out. To digest the high VSS concentration, a two-phase anaerobic digestion process consisting of liquefaction and gasification was used. Performance of thermophilic liquefaction (53°C) was better than mesophilic liquefaction (37°C) with respect to VSS digestion, using a upflow anaerobic filter reactor. The digestion efficiency of VSS was about 80% at an organic matter volumetric loading rate of 1 to 2 g/l/d. The effluent from the liquefaction process had to be diluted prior to the gasification treatment. The allowable maximum TOC loading rate was only 1 g/l/d to achieve a TOC removal efficiency of more than 80%. It was shown that thiourea inhibited 70% of the gasification performance even at a low concentration of 10 mg/l, and the reason for the inhibition of gasification seems to be the presence of a thiourea-like compound in the wastewater.