NOVELTY - Biogas plant comprises a first digestion chamber adapted to at least partially digest a biodegradable organic slurry; a second digestion chamber provided in operative communication with the first digestion chamber for receiving the partially digested organic slurry; a reservoir for preparing and storing high-yield methanogenic bacterial culture; and recirculation means operatively connected between the second digestion chamber and the reservoir.    USE - The biogas plant is useful for producing biogas (claimed); and for preparing an inoculum which provides rich-bacterial culture.    ADVANTAGE - An enhanced efficiency and steady biogas production at a substantially low production cost is provided. The biogas plant provides better quality biogas having higher calorific value at a low production cost. It prevents hydraulic short-circuiting. It is largely operated by gravity thus reducing the pumping costs. The biogas plant or method does not require large amount of a methanogen-containing biomass substrate.    DETAILED DESCRIPTION - Biogas plant comprises a first digestion chamber adapted to at least partially digest a biodegradable organic slurry; a second digestion chamber provided in operative communication with the first digestion chamber for receiving the partially digested organic slurry, the second digestion chamber being adapted to anaerobically digest the partially digested organic slurry to produce biogas and sludge; a reservoir for preparing and storing high-yield methanogenic bacterial culture, the reservoir being adapted to provide bacterial culture for maintaining adequate methanogenic bacterial population in the second digestion chamber; and recirculation means operatively connected between the second digestion chamber and the reservoir, the recirculation means adapted to circulate at least a portion of sludge between the second digestion chamber and the reservoir. An INDEPENDENT CLAIM is a method for producing biogas comprising partially digesting a biodegradable organic slurry in a first digestion chamber by thermophilic reactions, preferably at a temperature in the range of 45-50 degrees C, to obtain a partially digested organic slurry; conveying the partially digested organic slurry to a second digestion chamber; feeding a methanogenic bacterial culture periodically from a reservoir to the second digestion chamber for maintaining adequate methanogenic population; degrading the partially digested organic slurry anaerobically in the second digestion chamber by means of the high-yield methanogenic bacteria to produce biogas and sludge; collecting the sludge in at least one post-chamber; and accumulating the biogas in a dome-structure located on the operative top-end of the second digestion chamber via a gas outlet pipe.