NOVELTY - Universal double-walled drying system for drying, preferably sewage sludge from public, municipal and industrial wastewater treatment plants, from biomass comprising wood chips, sawdust, green cuts, spent grains, fruit pulp, digestate, buffering agents, and kitchens and vegetable waste residues, from production residues of all types, processing waste of all types, and from wet granules of all types, is claimed. The drying utilizes a medium air, as a process air (1), which is extracted from the environment and warmed by one or combination of energy sources, or their heat.    USE - The double-walled drying system is useful for drying sewage sludge.    ADVANTAGE - The drying system has a simple structure, is durable and economic, and carries out the method in an environmental friendly manner.    DETAILED DESCRIPTION - Universal double-walled drying system for drying, preferably sewage sludge from public, municipal and industrial wastewater treatment plants, from biomass comprising wood chips, sawdust, green cuts, spent grains, fruit pulp, digestate, buffering agents, and kitchens and vegetable waste residues, from production residues of all types, processing waste of all types, and from wet granules of all types, is claimed. The drying utilizes a medium air, as a process air (1), which is extracted from the environment and warmed by one or combination of energy sources, which include (i) blower air energy in wastewater treatment plants, (ii) wastewater heat utilization from the wastewater treated in wastewater treatment plants, (iii) power plants, which comprise cogeneration plant and micro gas turbine (3), (iv) heating water (4), (v) gas burner (6), (vi) solar collector (5), and (vii) process waste heat from production or thermal processes in incineration plants, cement plants, pyrolysis plants and bio-gas plants, or their heat.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the assembly with the possible usable energy sources and air currents.    Process air (1)    Micro gas turbine (3)    Heating water (4)    Solar collector (5)    Gas burner (6)