NOVELTY - A solid city waste (SCW) material is treated by extruding the material under high pressure, stirring the pulp while circulating air in the environment, sorting the fermented material, and disaggregating and subdividing the dry residue.    USE - For the treatment of SCW materials containing humid organic matter.    ADVANTAGE - The method provides a quick and controlled conversion to directly dumpable of usable materials. It conducts aerobic fermentation without any uncontrolled diffusion of hydrocarbon gas or of other undesired emanations to the atmosphere.    DETAILED DESCRIPTION - Treatment of solid city waste (SCW) material containing humid organic matter, comprises shredding, and partially subjecting the material to biochemical conversion for purposes of stabilization. The material is extruded under high pressure through a die having extrusion holes which are large enough to allow passage of chips of solid, inorganic matter. An organic pulp is produced, incorporating the chips and having been heated by the extrusion, a dry residue is retained in the die. The pulp is stirred incorporating non-decomposable chips in an environment closed to the atmosphere, while circulating air in the environment in order to ferment the pulp until it is stabilized. The fermented material is sorted to separate it into fragments of inorganic matter, and stabilized powder or granulate usable as a soil amendment. The dry residue is disaggregated and subdivided into at least two fractions of materials of different weight by floating it in an airstream.    An INDEPENDENT CLAIM is included for a system for treating SCW material containing humid organic matter, comprising a primary chopper (10) for shredding the SCW material, an extruder (14) having a perforated cylindrical die (11), into which a punch of a cross-section definitely smaller than that of the internal cylindrical cavity of the die is adapted to penetrate, loadable with shredded SCW material coming from the primary chopper, unloading mechanism (18) for expelling the residual material from the die after extrusion and collecting mechanism (16) for collecting the material extruded through the die, a fermentator(s) (20) closed to the atmosphere, feedable with the extruded material, and equipped with air circulation mechanism adapted to introduce air into the fermentator through a first set of apertures and to withdraw air through a second set of apertures, a sieve (76) feedable with the fermented material for subdividing it, a secondary chopper (82) supplied with the residual material in the die by the unloading mechanism, and an airflow grader (84) feedable with the shredded residual material and adapted to separate the residual material into at least two fractions of different weight.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of the system.    Primary chopper (10)    Perforated cylindrical die (11)    Extruder (14)    Collecting mechanism (16)    Unloading mechanism (18)    Fermentator (20)    Sieve (76)    Secondary chopper (82)    Airflow grader (84)    output hoppers (86, 88, 90)    Input hopper (92)    Air blower (98)