NOVELTY - A reactor (1) for anaerobic digestion of slurry comprising organic waste, comprises a tank (2) defining a bottom zone (3), separation zone and top zone (4); slurry inlet mechanism (5) for feeding the slurry; gas inlet mechanism (6) for feeding gas so as to mix the slurry at the bottom zone; an transporting mechanism (7, 8); a downer (9) for assisting in a downward movement of the slurry in the absence of the differential pressure; a gas withdrawal mechanism for withdrawing bio-gas produced within the tank; and automatic pressure sensitive valve mechanism (28) to maintain pressure.    USE - As reactor for anaerobic digestion of slurry comprising organic waste (claimed).    ADVANTAGE - The reactor provides reduction of hydraulic retention time (HRT) from 40 to 24 days, which results in less volume of the digester per ton of organic solid waste to be treated compared to the conventional digester; provides increase in solids loading rate from 2.5 to 4.2 kg TS/(m3.day); provides increase in volatile solids loading rate from 1.5 to 3.3 kg VS/(m3.day); provides increase in VS destruction rate of 40-65%; provides increase in biogas production from 0.4 m3/kg VS reduced to 0.6 m3/kg VS reduced; scales up digester to handle wide range of waste volumes, from small volumes (e.g. small communities, coastal communities, small industries, seafood process, etc) to high volumes (e.g. large industries and municipal wastes); provides intermittent mixing of digester slurry by using the biogas pressure; provides de-alienation of scum formation problem, which is common in conventional digesters; provides increase in the biogas production and methane content in biogas; eliminates short-circuit of feed in the digester; eliminates choking and clogging completely; is suitable for a wide variety of organic solid wastes; and provides decrease in ammoniacal nitrogen concentration in the reactor from 2500 mg/l to 200 mg/l which eliminates the toxicity due to high ammoniacal nitrogen concentration for solids having C/N ratio as low as 6.    DETAILED DESCRIPTION - A reactor (1) for anaerobic digestion of slurry comprising organic waste, comprises a tank (2) defining a bottom zone (3), separation zone and top zone (4); slurry inlet mechanism (5) located at the bottom zone of the tank for feeding the slurry; gas inlet mechanism (6) located at the bottom zone of the tank for feeding gas so as to mix the slurry at the bottom zone; an transporting mechanism (7, 8) assisting in an upward movement of the slurry from the bottom zone to the top zone based on a differential pressure between the zones; a downer (9) operably connecting the top zone and the bottom zone for assisting in a downward movement of the slurry in the absence of the differential pressure; a gas withdrawal mechanism for withdrawing a bio-gas produced within the tank, the gas withdrawal mechanism being operably connected to the gas inlet mechanism for recirculating at least a part of the bio-gas withdrawn; and an automatic pressure sensitive valve mechanism (28) being operably connected to the gas withdrawal mechanism and/or the gas inlet mechanism so as to maintain a predetermined pressure inside the tank. An INDEPENDENT CLAIM is included for method for anaerobic digestion of a slurry comprising organic waste in a reactor involving feeding the slurry to a bottom zone of the tank; feeding gas to the bottom zone of the tank so as to mix the slurry at the bottom zone; enabling an upward movement of the slurry from the bottom zone to the top zone based on a differential pressure between the zones by providing transporting mechanism; enabling a downward movement of the slurry in the absence of the differential pressure by providing a downer operably connecting the top zone and the bottom zone; withdrawing a bio-gas produced within the tank and recirculating at least a part of the withdrawn bio-gas using an automatic pressure sensitive valve mechanism so as to maintain a predetermined pressure inside the tank.    DESCRIPTION OF DRAWING(S) - The figure shows sectional view of the reactor.    Reactor (1)    Tank (2)    Bottom zone (3)    Top zone (4)    Slurry inlet mechanism (5)    Gas inlet mechanism (6)    Transporting mechanism (7, 8)    Downer (9)    Pressure sensitive valve mechanism (28)