Abstract: A two-phase anaerobic treatment system and method for the treatment of wastewaters containing simple and complex organic constituents is provided wherein the complex organic constituents are broken down into simple organic constituents by acidogenic bacteria in a Phase One reactor and the simple organic constituents from the Phase One reactor are converted into biogas, mainly methane, in a Phase Two reactor by methanogenic bacteria. The method includes the steps of feeding wastewater to the Phase One reactor either in an intermittent batch mode or semi-continuous mode, and withdrawing effluent from the Phase One reactor preferably in a batch mode. Effluent from the Phase One reactor is fed to the Phase Two reactor in an intermittent batch mode while effluent from the Phase Two reactor is withdrawn in a batch mode. The method minimizes the transfer of suspended solids from the Phase One reactor to the Phase Two reactor.