The present invention relates to methods and plants for the treatment of an organic waste material, wherein waste is subjected to anaerobic fermentation in a biogas digester; effluent is mechanically separated from the biogas digester into a concentrated fraction and a liquid fraction; the liquid fraction is heated to a high temperature below the boiling point of the liquid; the heated liquid is introduced to a flash column to partially remove volatile carbon dioxide, the pH of the liquid is elevated and ammonia is removed from the liquid.