NOVELTY - Decomposition of organic matter into biogas and compost, involves: (a) loading solid organic waste into waste processing tank; (b) spraying percolation liquid over solid organic waste in processing tank during first hydrolysis stage; pumping percolation liquid into biogas reactor tank for generation of biogas during gasification stage; and pumping percolate liquid out of the biogas reactor tank following gasification stage, for spraying over the solid organic waste in waste processing tank during a second and/or subsequent hydrolysis stage; and (c) ventilating the waste processing tank.    USE - For decomposition of organic matter into biogas and compost (claimed), where the organic matter includes municipal waste, industrial, commercial, and institutional waste, garden waste and sewage sludge.    ADVANTAGE - The method provides digestion and composting of solid waste with limited or simply solids handling requirements. The method does not require moving the solid fraction of the waste between different stages of the process, and only initial loading of the waste and final unloading of the solids after sanitization is needed. The method yields a rough compost product that is stable and pathogen free and the rough compost can be further processed and blended to create high value engineered soils. The method does not require handling of raw, post-methane extraction digestate prior to composting. The biogas produced by this method has a high methane content e.g. greater than or equal to 70% compared to 55-60% using other processes; and also has lower concentrations of volatile organic components (VOCs), siloxane, and other contaminants. The processes of hydrolysis, percolation, methane generation, and composting are carried out separately and therefore occur rapidly and efficiently. There is no need for inoculation using earlier processed material to start fresh batches, thereby insuring full traceability of the waste. The method is self-sustaining and there is no need for heating to start the process, allowing for a high energy efficiency. A wide variety of organic waste types can be processed, in the method for e.g. municipal waste, industrial, commercial, and institutional waste, garden waste and sewage sludge.    DETAILED DESCRIPTION - Decomposition of organic matter into biogas and compost, involves: (a) loading solid organic waste into a waste processing tank; (b) during at least one series of hydrolysis and gasification stages: (i) spraying percolation liquid over the solid organic waste in the waste processing tank during a first hydrolysis stage in which bacterial hydrolysis takes place; (ii) optionally recirculating the percolation liquid after it passes through the solid organic waste in the waste processing tank during the first hydrolysis stage; (iii) pumping percolation liquid into a biogas reactor tank for generation of biogas during a first gasification stage; and (iv) pumping percolate liquid out of the biogas reactor tank following the first gasification stage, for spraying over the solid organic waste in the waste processing tank during a second and/or subsequent hydrolysis stage; and (c) following all hydrolysis and gasification stages, ventilating the waste processing tank for composting of the solid organic waste under aerobic conditions.