NOVELTY - Producing biogas, comprises: (i) culturing, in a first reactor, at least one anaerobic microbial consortium in a high solid medium to produce biogas containing methane and to form a high solids phase and a low solids phase within the first reactor; (ii) culturing, in a second reactor, at least one methanogen-rich anaerobic culture in a low solids medium to generate methane; (iii) removing an effluent containing volatile fatty acids from the low solids phase in the first reactor; and (iv) transferring the effluent removed from the low solids phase in the first reactor to the second reactor.    USE - The method is useful for producing biogas comprising methane (all claimed), which is useful for providing heat to digesters or to generate electricity.    ADVANTAGE - The method is capable of simply and economically producing the biogas with high yield and without using expensive equipment in an environmental-friendly and efficient manner.    DETAILED DESCRIPTION - Producing biogas, comprises: (i) culturing, in a first reactor, at least one anaerobic microbial consortium in a high solid medium comprising 10-20% solids and having a pH of 6-7.5, for a period of time sufficient to produce biogas containing methane and to form a high solids phase and a low solids phase within the first reactor; (ii) culturing, in a second reactor, at least one methanogen-rich anaerobic culture in a low solids medium comprising 1% solids and having a pH of 6.5-8.5, for a period of time sufficient to generate methane; (iii) removing an effluent containing volatile fatty acids from the low solids phase in the first reactor at a rate that is sufficient to at least partially offset a decrease in pH within the high solids phase, during the steps of (i) and (ii); (iv) transferring the effluent removed from the low solids phase in the first reactor to the second reactor; (v) removing the effluent from the low solids medium in the second reactor; (vi) transferring the effluent removed the second reactor to the first reactor at a rate that is sufficient to at least partially offset the decrease in pH within the high solids phase; and (vii) recovering a biogas produced in at least one of the first and second reactors. The high solids phase is positioned within a top portion of the first reactor. The low solids phase is positioned within a bottom portion of the first reactor. The high solids phase and the low solids phase are in direct contact with each other.