3.1. Total methane emission from the university central wastewater treatment systems that used CDM methodology Results are presented in Table 1. Sewage handling of Mahidol University generated methane at maximum quantity in 2009, followed by years 2010, 2012, and 2011. Important variables affecting methane emission calculated by means of the CDM were CODinflow and COD removal efficiency, which were significantly higher in 2009 than in the other years, while the amount of wastewater entering the system had similar quantities.