Methanogenesis is the process of converting the products from theacidification reactor to CH4 and CO2. Methanogens are the highly sensitivemicroorganisms that can consume acetates, hydrogen and CO2 toproduce methane. CO2 reduction contributes to about 30% of methaneproduction. . Two-stage anaerobic digestion (TSAD) of palm oil milleffluent using upflow anaerobic sludge blanket (UASB) mesophilicmethanogenic reactor revealed that Clostridium sp, Lactobacillus sp,Methanosarcina sp and Methanoculleus sp were the most predominantmicroorganisms involved in methane production. In TSAD the methanogenicreactor effluent had 90% COD removal efficiency whereas it isonly 74% in case of single stage system [28]. Based on substrate utilizationmethanogens can be classified into three categories namely acetoclasticmethanogens that utilize acetate to give CH4 and CO2,hydrogenotrophic methanogens utilize hydrogen and carbon dioxide toyield methane and methylotrophic methanogens utilize methanol toyield methane and water. Acetoclastic methanogens and hydrogenotrophicmethanogens contribute to about 2/3 and 1/3 of themethane production. There are few methanogens like Methanosarcina spthat can use both pathways for methane production. Methanogens useCO2 as a terminal electron acceptor to produce methane [29]. Themethanogenic phase is usually operated at higher hydraulic retentiontimes and the process is highly sensitive to pH and temperature.