Biogas, produced as a result of anaerobic degradation of organic matter, is an alternative source of renewable energy and a sustainable and cost-effective fuel due to its high availability. However, before it can be used as an energy source, biogas must be upgraded by removing impurities such as CO2 and H2S to increase its calorific value. In this study, CO2 was removed and synthetic biogas was upgraded at atmospheric pressure in a microchannel using three absorbents of well water, seawater, and drinking water. The effects of operating variables, including absorbent flow rate, biogas flow rate, and system temperature, were investigated. As a function of independent variables, RSM analysis proposed a quadratic model for the absorption process by each absorbent to predict the response (CO2 removal efficiency). Moreover, the experimental values obtained for CO2 absorption were found to satisfactorily match the model values (R2=0.9991-0.9997). The maximum CO2 absorption in well water, seawater, and drinking water at 30 ℃, liquid flow rate of 150ml·h-1, and gas flow rate of 50ml·min-1 was 90.22, 84.95, and 79.66, respectively.