NOVELTY - The system comprises landfill (10) containing waste mass (WM) (12), leachate recirculation system (LRS), air injection system (AIS), and control system for regulating LRS and AIS, such that temperature of WM is maintained below 140degreesF and waste is decomposed aerobically.    USE - For decomposing WM such as municipal solid waste.    ADVANTAGE - The method enables safe landfill operation and bioreactors in aerobic manner and provides cost-effective conversion of landfills from anaerobic to aerobic operations. The device increases the rate of waste stabilization, decreases the rate of production of methane gas, reduces the levels of toxic organics, and decreases the volume of leachate. A significantly reduced threat to the environment and a significant reduction in the cost associated with operation, closure and post-closure of the landfill is achieved. The recirculation of leachate provides, not only an ample supply of moisture, but sufficient oxygen, food and nutrients for organisms in WM. Hence, additional nutrients, such as those from sewer sludge or nitrogen, are not required for this process to operate. The use of a combination of air injection through the vertical wells inserted into WM and horizontal leachate collection system piping, allows flexibility in the application of air and leachate to the proper proportions to achieve a safe WM temperature (120-140degreesF, preferably close to 135degreesF). Maintaining temperatures below 150degreesF is essential to preclude killing the microorganisms needed for the landfill to operate aerobically, to stabilize waste and to reduce landfill operating costs. After the waste is stabilized and rendered harmless, the landfill may be covered and re-developed for other uses, or re-used. Aerobic landfill microbe (ALB) achieves optimal aerobic degradation of organic matter at safe WM temperatures and minimizes exposure of leachate or WM to human health and the environment. ALB system demonstrated over a nine month period that municipal sanitary landfills can cost-effectively be converted from anaerobic to aerobic degradation processes. It was proven through actual experience that the injection of air into WM combined with the recirculation of leachate aerobically degrades municipal solid waste while WM temperatures remain stable. Aerobic landfill operation improves leachate quality, and especially the levels of Biochemical Oxygen Demand (BOD) and concentrations of Volatile Organic Compounds (VOC) in the leachate. The landfill showed reductions in BOD and VOCs. BOD was reduced by at least 70%, (where organics are methyl ethyl ketone). The volume of leachate generated by the landfill is reduced. This is caused by the evaporative effects of the higher WM temperature coupled with the field capacity of WM to absorb moisture.    DETAILED DESCRIPTION - The system comprises landfill (10) containing waste mass (WM) (12), leachate recirculation system (LRS), air injection system (AIS), and control system for regulating LRS and AIS, such that temperature of WM is maintained below 140degreesF and waste is decomposed aerobically. LRS has leachate collector (16) for collecting leachate from WM, and separate units for storing and dripping collected leachate into WM.    AIS has air injector-I for injecting air through the leachate collector up through WM, and air injector-II for injecting air into WM from the top of WM.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic sectional view of the landfill.    Landfill (10)    Waste mass (12)    Cap (14)    Vertical wells (18)    Leachate collector (16)    Blowers (20)