NOVELTY - Selective permeation apparatus comprises acidogenic reactor (1) for mixing pulverized biomass with fresh water and acidogenic microorganisms; methanogenic reactor (7) for conversion of volatile fatty acids to methane; filter unit (2) for separating acid solution and slurry; tank for filtering acid received form filter unit; filter having semi-permeable membrane tubes enclosed in chamber, first peristaltic pump for pumping acid solution received from tank; and second peristaltic pump for pumping methanogenic slurry received from the reactor.    USE - For producing biogas for various types of slurry (claimed).    ADVANTAGE - The selective permeation apparatus for biogas production for segregating the acidogenic and methanogenic stages, which improves the efficiency of biogas generation by eliminating the need to continuously replacing the contents of the methanogenic stage; provides much faster conversion of feed into biogas without undue loss of the methanogens. The process speed is improved, thus reduces capital cost, over the conventional digesters, for getting the same output. The process of biogas production is continuous i.e. without any break or requirement of stream reversals.    DETAILED DESCRIPTION - A selective permeation apparatus comprises an acidogenic reactor (1) for mixing pulverized biomass with fresh water and acidogenic microorganisms; a methanogenic reactor (7) containing methanogens for conversion of volatile fatty acids to methane; a filter unit (2) capable of separating acid solution and slurry from the mix received from acidogenic reactor; an acid filtrate tank capable of filtering the acid received form the filter unit; a filter having several of semi-permeable membrane tubes enclosed in a chamber, a first peristaltic pump for pumping the acid solution received from the acid filtrate tank through the semi-permeable membrane tubes of the filter and collecting back to the acid filtrate tank; and a second peristaltic pump for pumping the methanogenic slurry received from the methanogenic reactor around the semi-permeable membrane tubes of the filter and collecting back to the methanogenic reactor, where the acid solution is allowed to pass through the semipermeable membrane tubes and is collected back in the acid filtrate tank, where the methanogenic slurry is allowed to pass around the semi-permeable membrane tubes and collected back in the methanogenic reactor, by enabling passage of volatile fatty acids out of the membrane and permitting the migration of volatile fatty acids from the acid solution to methanogenic reactor without exchanging microbial populations using the principle of concentration equilibrium. The organic matter used as feed is pre-treated using chemical, physical, or enzymatic means to convert it into small units to facilitate acidogenesis. An INDEPENDENT CLAIM is included for a selective permeation method/process for producing biogas for various types of feeds involving feeding acidogenic reactor with biomass, fresh water and acidogenic culture; filtering the acidogenic mix by filter unit for separating acid and slurry; collecting acid solution in tank for circulating; pumping the acid solution by first peristaltic pump from the acid filtrate tank through a filter having the semi-permeable membrane tubes and collecting back to the acid filtrate tank; simultaneously, pumping the methanogenic slurry in the methanogenic reactor by second peristaltic pump from the methanogenic reactor around semi-permeable membrane tubes of the filter and collecting back to the methanogenic reactor for circulation, where the acid solution is allowed to pass through the semipermeable membrane tubes and is collected back in the acid filtrate tank, where the methanogenic slurry is allowed to pass around the semi-permeable membrane tubes and collected back in the methanogenic reactor, by enabling passage of volatile fatty acids out of the membrane and permitting the migration of volatile fatty acids from the solution to methanogenic slurry without exchanging microbial populations using the principle of concentration equilibrium.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of selective permeation apparatus.    Acidogenic reactor (1)    Filtering unit (2)    Acid filtrate tank (3)    Peristaltic pump (4)    Methanogenic reactor (7)