the foregoing objects of the said invention are accomplished and the problems and shortcomings associated with the prior art techniques are overcome by the present invention as described below in the preferred embodiment . embodiments of the invention are discussed below with reference to fig1 . in a preferred embodiment , the process comprises two or more anaerobic reactors or digesters a culture preparation tank , a filtration unit , a manure preparation unit and a gas storage unit . the two stages / phases i . e . solid state methanogenesis and liquid state methanogenesis are carried out separately in the said two types of digesters . the number of digesters varies depending on the retention time of the biomass used . the first and the second or the one and the other phase are carried out in presence of microorganisms . the reactors are provided with facilities for temperature control and stirring mechanism as required . in a preferred embodiment , the process of biomethanation comprises two or more methane digesters ( 2 ), ( 5 ), gas collecting unit ( 7 ), culture preparation tank ( 10 ), filtration unit ( 11 ) and manure preparation unit ( 12 ). the solid state methane digester ( 2 ) comprising a reaction chamber ( 3 ) for conversion of biomass into biogas , a vertical perforated unit probably tube ( 4 ), spray recirculation system ( 13 ), leachate outlet port ( 17 ) is in fluid communication with the liquid state methane digester ( 5 ), digested material outlet port ( 18 ) which is in communication with the culture preparation tank ( 10 ) and a fixed gas collecting chamber . the dry feed / biomass and substrate specific culture is introduced into the solid state methane digesting tank ( 2 ) from the feed storage tank ( 1 ). the feed / biomass preferably used are agro residues , oilcakes like paddy straw ; wheat straw , maize , napier grass , press mud , castor , sal , food waste , biodegradable municipal waste and alike . the total solids of the biomass preferably are in the range of 15 - 20 percent . the organic solid waste is digested by addition of specially developed microbial population capable of producing required enzymes . the microbial consortium is specifically prepared for a particular feed as a target and is enriched with natural microbial mixtures such as cow - dung , sewage and the like , by a process of restricting its nutrition to the subject feed over a period of time . once enriched , this consortium can be propagated and made available for deploying in the reactor . the said solid state methane digester ( 2 ) has a percolation unit / tube ( 4 ) internally connected in parallel to the base of the reaction chamber of said solid state methane digester ( 3 ) to facilitate percolation of lechates from the feed present in the said digester ( 2 ). the process maintains the required temperature for microbial activity for biogas generation due to recirculation of lechates produced by the biomethanation process . the lechates are heated to a desired temperature for optimum performance of the bacterial consortium present in the digesters . the organic solid waste is digested to produce biogas , which generates percolate and solid digestate . the solid digestate is further utilized for feed and manure preparation . a sprinkler and / or spray recirculation system ( 13 ) is introduced into the said solid state digester ( 2 ) which sprinkles digested slurry on top of the reaction mixture from the said liquid state methane digester ( 5 ) and collects percolate and / or lechates in a manifold created at the base of the solid state methane digester ( 2 ). the lechates produced from individual percolation unit are collected through a manifold into the said liquid state methane digester ( 5 ). the sprinkler ( 13 ) is in fluid communication with the said liquid state methane digester ( 5 ) and is suspended internally in the head space of the said solid state methane digester ( 2 ). the solid state methane digester ( 2 ) has a conduit / outlet arranged ( 18 ) at the lower region of the digesting tank for discharging sludge from the digesting tank , which is in communication with the culture preparation tank ( 10 ). the biomass is passed through the reaction chamber ( 3 ) for a period of time ( about 5 to 20 days ) sufficient for the feed mixture to be anaerobically digested . the period for degradation of biomass to biogas is variable , which depends on the retention time of the substrate used . the liquid state methane digester ( 5 ) consists of a reaction chamber ( 6 ) and a flexible biogas collecting vessel preferably a dome shaped ( 7 ) for extraction of biogas generated from solid state methane digester ( 2 ) and liquid state methane digester ( 5 ). the biogas collecting vessel ( 7 )/ dome is mounted in the head region of the liquid state methane digester ( 5 ), which is movable and displaces gas by vertically upward movement . the liquid state methane digester ( 5 ) has provisions for collecting the percolate at the lower region of the digester ( 8 ) and an outlet at another lower end ( 9 ) for recycling the digested slurry into the solid state methane digester ( 2 ) by means of spray pumps and dispensers . the sludge produced is further filtered through a filtration unit ( 11 ) which may be sand filter or any other appropriate filter unit . the filtered sludge is finally dispatched for manure preparation into the manure preparation unit ( 12 ) manually . the number of solid state methane digesters varies with respect to the retention time of the biomass / substrate used for biogas generation . the spray recirculation system ( 13 ) maintains the desired temperature conditions inside the solid state methane digester ( 2 ) which in turn is controlled by solid handling pump ( 14 ). in the said liquid , state methane digester ( 5 ), the conversion into biogas is brought by housing enriched microbial consortia , leachates and / or percolates from said solid state methane digester ( 2 ). the conversion into biogas is brought by addition of enriched microbial consortia , followed by heating it between temperature ranges of about 30 degree centigrade to about 40 degree centigrade or as may be required by the consortia of microbes , with or without occasional stirring . the reactor size is optimized taking into consideration the microbial population and retention time required for digestion . introduction of the liquid state methane digester optimizes biogas generation from the non digested slurry which otherwise is disposed off . valves and regulators ( 15 ) or any other appropriate flow control mechanism are introduced to control the flow of lechates / slurry . the culture preparation tank ( 10 ) is than regularly fed with the substrate and portion of the solid digested material from the solid state methane digester ( 2 ). the gas generated from the digesters is collected in storage vessel . the mixture produced in the said culture preparation tank ( 10 ) is utilized as feed further to obtain biogas . the said non digested material prepared in the said culture preparation unit ( 10 ) is introduced partly in the said solid state methane reactor along with fresh biomass and partly treated for manure preparation . the said non digested material is introduced onto filtration tank ( 10 ) for draining excess water / liquid present in the solid digested material . the said , non digested material further is subjected for composting to manure ( 12 ) in order to achieve the desired quality of carbon to nitrogen ratio . biogas generated from both the digesters ( 2 , 5 ) is collected in a gas collecting assembly which further can be utilized for cooking purposes or generating electricity or other productive uses like vehicle fuel with or without cleaning . biogas can also be converted to purified methane and compressed to replace cng in vehicle . the said process reduces the hydraulic retention time for the biomethanation process to 5 to 20 days depending on the substrate . the said process also reduces water consumption by about 50 percent as to that required by conventional method of biomethanation . the said process introduces dual biomethanation process by introduction of liquid state methane digester ( 5 ). while considerable emphasis has been placed herein on the specific steps of the preferred process and components of the preferred embodiment , and many details have been set forth for purpose of illustration , it will be appreciated that many alterations can be made and that many modifications can be made in the preferred embodiment without departing from the principles of the invention . these and other changes in the preferred embodiment as well as other embodiments of the inventions will be apparent to those skilled in the art from the disclosure herein , whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation . the invention is further described with the help of following non limiting illustrations . fifteen kilograms of fresh napier grass with total solids ( ts ) ranging from 20 to 25 percent , is pulverized upto 3 to 4 mm and allowed to undergo the said process of biomethanation . as the actual biogas output is about 50 percent more than the expected , the biogas to digester volume ratio is improved from 0 . 85 to about 1 . 2 - 1 . 5 . the retention time is reduced to 18 days , whereby the conventional wet biogas digester producing biogas at similar gas potential recovery required over 25 days retention time . thereby confirming the said invention . 2 . 5 kilograms of paddy straw with total solids ( ts ) of about 88 to 90 percent is allowed to undergo the said biomethanation process . the following results are observed : the conventional wet type biogas digester producing biogas at similar gas potential recovery required over 25 days retention time , the route followed by the said process takes 18 days retention time to produce biogas . thus confirming the said process . the process reduces the overall hydraulic retention time required for the biomethanation process , thereby reducing the size of the biogas digester of equivalent capacity . the process describes a solid state anaerobic fermentation , thus the invention overcomes the problem of scum formation , thereby increasing the efficiency of the biomethanation process . the process allows use of mixed and / or multiple solid feeds as substrates for anaerobic digestion to produce biogas and manures . the process describes dual biomethanation , wherein leachate from the solid state methane digester is utilized in liquid state methane digester to maximize biogas production . the process produces less waste water than wet digestion systems and therefore requires less equipment for managing this effluent stream .