NOVELTY - Producing biogas, involves (i) culturing anaerobic microbial consortium in a high solids medium for a period of time sufficient within a first reactor, (ii) culturing methanogenic anaerobic culture in a low solids medium for a period of time sufficient in a second reactor, (iii) removing effluent from the low solids phase in the first reactor, (iv) transferring effluent from the first reactor to the second reactor, (v) removing effluent from the second reactor, (vi) transferring effluent from the second reactor to the first reactor and (vii) recovering biogas in the first and second reactors.    USE - The method is useful for producing biogas such as methane (all claimed).    ADVANTAGE - The simplified and efficient method provides biogas in an economical manner.    DETAILED DESCRIPTION - Producing biogas, comprises (i) culturing at least one anaerobic microbial consortium in a high solids medium comprising 10-20% of 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 a first reactor, where the high solids phase is positioned within a top portion of the first reactor and the low solid phase is positioned within a bottom portion of the first reactor, and the high solids phase and the low solids phase are in direct contact with each other, (ii) culturing at least one methanogen-rich anaerobic culture in a low solids medium comprising at most 1% of solids and having a pH of 6.5-8.5 for a period of time sufficient to generate methane in a second reactor, (iii) removing 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, (iv) transferring effluent from the low solids phase in the first reactor to the second reactor, (v) removing effluent from the low solids medium in the second reactor, (vi) transferring effluent from 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 biogas in at least one of the first and second reactors. An INDEPENDENT CLAIM is included for a system for producing biogas from high solids waste, comprising the first reactor, the second reactor, one or more units for transferring liquid from the first reactor to the second reactor, one or more units for transferring liquid from the second reactor to the first reactor and a controller that uses data concerning one or more of (a) conditions in a high solids phase in the first reactor, (b) conditions in a low solids phase in the first reactor, and (c) conditions in a low solids liquid medium in the second reactor, where the controller is capable of adjusting a flow of liquid from the first reactor to the second reactor and from the second reactor to the first reactor in response to the data.