A rotating drum is provided for the biological/mechanical conversion of organic waste material to a valuable biomass. An aerotolerant anaerobic bacteria within the rotating drum promotes the fermentation of the waste material, while the tumbling of the waste material due to the rotation of the drum mechanically shears the waste material. Air carrying volatile organic compounds released by the fermentation process can be collected from the drum and scrubbed to recover the volatile organic compounds. The cleansed air can then be recirculated back into the drum. The waste material can be screened prior to fermentation in the drum to remove a fraction of the smallest particles. The biomass produced by the drum can also be screened.