There are multiple techniques to enhance the calorific value of biogas by removing impurities, especially carbon dioxide (CO 2 ). This study used a T-shaped microchannel to purify biogas by seawater containing 0.1 wt% of Iranian modified clinoptilolite zeolite and several precipitates (i.e., water distillation, phosphogypsum, power plant clarifier unit). These additives increase the alkalinity and CO 2 absorption ability of seawater. Effects of temperature, liquid flow rate, and biogas flow rate on the CO 2 removal efficiency have been investigated. The response surface methodology is also employed to construct a quadratic model to predict the CO 2 removal efficiency as a function of these variables. The P -value for all variables was less than 0.05, indicating that all four models were significant. Moreover, according to R 2 values ranging from 0.9920 to 0.9997, the experimental CO 2 absorption values have acceptable agreement with the model predictions. The maximum CO 2 capture by seawater solutions containing zeolite and precipitates of phosphogypsum waste, plant clarifier, and water distillation at 30 ° C, in a liquid flow rate of 150 ml/h and a gas flow rate of 50 ml/min was 96.85, 96.01, 92.99, and 90.23%, respectively. The results achieved in this study are essential for the appropriate design of a micro-reactor for biogas upgrading and understanding the effect of operating conditions on its CO 2 removal efficiency. • Biogas upgrading (CO 2 removal) by low-cost materials is studied in micro-reactor. • Seawater is used as an absorption solution in the micro-reactor for the CO 2 capture. • The applied materials show high efficiency in a broad range of operating conditions. • All studied absorbents completely remove CO 2 under the optimum conditions. • The suggested method reduces cost and energy required for the biogas upgrading.