Syngas is produced by thermal gasification of both non-renewable and renewable sources including biomass and coal,and it consists mainly of CO,H2 and CO2.With the increased demand of natural gas,the conversion of syngas to methane is attracting much attention currently.Catalyzed chemical processes are well established for the syngas methanation.They normally involve high pressure and/or temperature,may be problematic when impurities are present,and tend to have low product specificity.We proposed a novel biological method to convert syngas to CH4.Syngas is injected into the biogas reactor treating organic waste,and simultaneous organic waste treatment and syngas biomethanation in the biogas reactor can be achieved.We have tested the conversion of CO(the main component of syngas)to CH4 in a biogas reactor treating sewage sludge.Batch experiments showed that CO was inhibitory to methanogens,but not to bacteria,at CO partial pressure between 0.25 and 1 atm under thermophilic conditions.