NOVELTY - The system comprises waste processing tanks into which solid organic waste is loaded and in which hydrolysis and composting takes place at separate times during a separate stage after hydrolysis is completed, a spraying system (2), a gas-tight biogas reactor tank into which the percolate liquid from the waste processing tank is pumped for generation of biogas under anaerobic conditions during a gasification stage, a biofilter through which air from the waste processing tank passes prior to discharge to the atmosphere during the composting stage, and a filter.    USE - The system is useful for decomposition of an organic matter including municipal waste, industrial, commercial and institutional wastes, garden waste, bio-wastes, curbside clippings, bio-solids, food waste, household waste, agricultural waste, kitchen waste, lawn cuttings and sewage sludge into biogas and a compost, where the compost is useful for producing high value fertilizer for agriculture and the biogas is useful for heating purposes or a block-type thermal power system such as internal combustion engine, fuel cell and turbine, for production of electricity and heat, and vehicle fuel in a pipeline system.    ADVANTAGE - The system is capable of efficiently decomposing the organic matter into biogas and a compost with reduced solid handling requirement, does not require movement of the solid fraction of the waste between different stages of the process, and individually allows control of the biological processes while avoiding over-engineering of the technical facilities.    DETAILED DESCRIPTION - The system comprises waste processing tanks into which solid organic waste is loaded and in which hydrolysis and composting takes place at separate times during a separate stage after hydrolysis is completed, a spraying system (2), a gas-tight biogas reactor tank into which the percolate liquid from the waste processing tank is pumped for generation of biogas under anaerobic conditions during a gasification stage, a biofilter through which air from the waste processing tank passes prior to discharge to the atmosphere during the composting stage, a filter configured to separate solids from a percolate liquid, a heater for heating the percolate liquid either in the biogas reactor tank or outside the biogas reactor tank, liquid process tanks for every waste processing tanks into which the percolate liquid from either the biogas reactor tank and/or the waste processing tank flows prior to pumping to the spraying system for spraying onto the solid organic waste in the waste processing tank during the hydrolysis stage, and a filter tank (6) configured to separate solids from the percolate liquid before the percolate liquid enters the liquid process tank. The waste processing tanks: comprise a drain at or near its base and an acid-resistant lining or coating; are connected in parallel; and are configured to operate at ambient pressure during the hydrolysis stage and/or the composting stage and under anaerobic conditions during the hydrolysis stage. The drain: allows flow of percolation liquid out of the waste processing tank during the hydrolysis stage; serves as a vent through which air flows into and/or out of the solid organic waste in the waste processing tank during the composting stage; and comprises a dual-purpose drain assembly for transport of the percolation liquid from the waste processing tank during the hydrolysis stage and for ventilation of the waste processing tank during the composting stage. The spraying system is configured to spray the percolate liquid onto the solid organic waste in the waste processing tank during the hydrolysis stage. The percolate liquid comprises fatty acids leached from the solid organic waste, and is recycled from the base of the waste processing tank during the hydrolysis stage. The biogas reactor tank comprises a sump to separate sediment and/or particles from the percolate liquid. The percolate liquid from both the biogas reactor tank and the waste processing tank flows into the liquid process tank prior to being pumped to the spraying system for spraying onto the solid organic waste in the waste processing tank during the hydrolysis stage. The drain assembly comprises rails through which conduits such as pipes are drilled. Each of the conduits has a first end facing the waste processing tank and a second end facing a pipe through which the percolation liquid flows out of the waste processing tank during the hydrolysis stage and through which air flows into the waste processing tank during the composting stage. The first end has a diameter of less than 90% of the diameter of the second end. Each of the rails comprises a notch forming a shelf for supporting a removable plate that prevents the solid organic waste in the waste processing tank from entering the pipe. The conduits are formed in a concrete base below the waste processing tank.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an apparatus for digestion and composting of solid waste.    Process module (1)    Spraying system (2)    Draining bottom (3)    Pump (4)    Filter tank. (6)