The purpose of this study is to evaluate GHG emission reduction and economic potential of wastewater treatment system by installing an anaerobic digester in Southeast Asia. Then the optimal investment to get GHG emission credit is obtained. In the advanced scenarios, the anaerobic digester is installed in the wastewater treatment system, and methane (biogas) is recovered and combusted. The biogas can be utilized for electricity generation and heat production, and can substitute fossil fuels. The objective of the study is to define the relationship between CER (certified emission reduction) credit and investment costs, and the optimal system by the calculation Internal Rate of Return (IRR). The result shows that the methane recovery and destruction reduces 63 % of GHG emissions from BAU scenario. The biogas utilization for energy has small effect on the GHG reduction. However, the income by energy production has large effect on the IRR. Thus, the IRR in the case that all biogas is utilized for electricity generation is 24%, and the IRR in the case that all biogas is utilized for heat production is 8%. The results of sensitivity analysis also show that the dominant parameter to increase IRR is capital cost. If the capital cost for heat production is half, the IRR is more than 40%.