NOVELTY - An anaerobic poly-phasic bio-digester for digestion of multiple organic matter comprises a digester phase comprising a trapezoidal vessel with at least two anaerobic fermentation chambers; where the trapezoidal vessel ensures maximum contact area with the earth.    USE - The anaerobic poly-phasic bio-digester and method are used for digestion of multiple organic matter (claimed).    ADVANTAGE - The bio-digester functions efficiently irrespective of the weather conditions while providing higher, natural thermal stability; and the bio-digester has low cost and high scalability (all claimed). The anaerobic digester is designed to achieve high energy yield and high purity sludge with the requirements of thermal stability and structural support. The apparatus is mechanically and biologically reliable. The digester is simple and is not susceptible to clogging and provides for separation, and selective treatment of solid and liquid components of waste streams in a very large capacity biogas digester. The unique design of the biodigester results in higher yields and higher production rates by the anaerobic digestion of organic wastes and plant materials. The versatile anaerobic digester can simultaneously accept multiple organic feedstocks.    DETAILED DESCRIPTION - An anaerobic poly-phasic bio-digester for digestion of multiple organic matter comprises a pre-digester phase and a digester phase, where (i) the pre-digester phase comprises at least two mixing tanks having unit for cutting the matter into small pieces and an interlocking loop enabling continuous recycling of contents from one the tank to the other; and (ii) the digester phase comprises a trapezoidal vessel with at least two anaerobic fermentation chambers; a unit of transferring the contents into and from the chambers; a unit of partially de-watering the contents; a geomembrane lining the vessel; a geomembrane sheet based balloon forming the digestion vessel; a stretchable cover or balloon for the vessel; a system to support the geomembrane roof of the digestion vessel and the stretchable cover; and a unit for draining the spillage from the vessel and runoff rainwater from the roof; where the digester phase lies in the trapezoidal trench at a sufficient depth in soil; the trapezoidal vessel ensures maximum contact area with the earth; the bio-digester functions efficiently irrespective of the weather conditions while providing higher, natural thermal stability; and the bio-digester has low cost and high scalability. An INDEPENDENT CLAIM is a process of anaerobic poly-phasic digestion of multiple organic matter by (a) processing feedstock in interlocking mixing tanks to obtain a homogenous slurry mixture; (b) transferring the slurry obtained in step (a) into the first stage digester; (c) retaining the contents for a period of at least 5 to 10 days to allow commencement of methanogenic bio-reaction in fermentation chamber of the digester; (d) dewatering the contents of step (c) to obtain higher solid digestate; and (e) transferring the digestate of step (d) to second stage digester; where the process results in a higher quantity of biogas produced per unit volume of input organic waste, and a high methane content in thus produced biogas, ranging from 60-70%; and the process results in high quality organic manure.