A simple in-situ biomethane system to upgrade biogas was developed by using differential solubility of biogas which normally contains 35-45% carbon dioxide (CO2) and 55-65% methane (CH4) by volume. The biomethane system consists of mesophilic plug-flow sorghum digester coupled with a leachate recycle loop to an external CO2 stripper. The leachate produced in the mesophilic plug-flow digester flows to the stripper where dissolved CO2 is removed. Then the leachate that CO2 was completely stripped out is recycled back to the plug-flow reactor, resulting in absorbing CO2 and enriched CH4 contents in digester offgas from the mesophilic plug-flow digester. Offgas CH4 contents was correlated well with leachate recycle rates and alkalinity. To maintain a biogas methane content over 95%, 3 volume of leachate recycle per volume of reactor per day(3 v/v-d) and at the reactor alkalinity of 4 g/L as CaCO3 was required. Even at an intermittent stripping ratio up to 3 hours stripping(N2 sweep gas 700 ml/min) and 1 hour no-stripping, the offgas methane content over 95% was achieved. It thus resulted in a 25% reduction in the total energy and sweep gas consumption. The TVS removal efficiency of the biomethane system was 80 percent which corresponded to 96% of the control reactor. The leachate recycle rates directly affected methane productivity that appeared to be 0.71 v/v-d at 3 volume of leachate recycle per volume of reactor per day(3v/v-d) and at the reactor alkalinity of 4 g/L as CaCO3.