Systems and methods are provided for converting organic waste materials from a municipal waste stream to useful products. Organic waste materials having a wide range of compositions such as, for example, yard waste, food waste, paper, and the organic fraction of municipal solid waste are converted into a uniform biomass that is suitable for conversion to useful products, such as fuels. Through the use of a biomixer and a hydropulper, as well as through sorting and screening, the organic waste materials are progressively reduced in size and cleaned of contamination. The resulting uniform biomass is suitable for anaerobic digestion to produce biogas and a residual solid that is suitable for producing a high quality compost.