Syngas fermentation via gasification is a two-stage process, which contains gasification of feedstock into syngas and syngas bio-methanation by anaerobic microorganisms. This project is a study on syngas fermentation. The gasification feedstock can be difficult-to-degrade solid waste so; waste volumes are reduced while green energy is produced. The main target of this thesis was to study novel configurations of reverse membrane bioreactor (RMB) in order to retain microbial cells inside the digester and thereafter increase methane production. In the first experiment, microbial cells encased in PVDF sachets were proved to perform efficiently in batch mode in comparison to free cells at optimum temperature, 55 oC. Moreover, encased cells in co-digestion of syngas and organic waste exhibited higher methane amounts compared to pure syngas treatment. Encased cells were then tested in thermophilic semi-continuous process and showed better performance compared to the free cell reactor. The RMB containing encased cells retained successfully the cells during the 154 days of the experiment, while free cells were washed-out. The highest amounts of methane from RMB and the free cell reactor were produced during the 126th - 130th day (6 and 1.5 mmol/day, respectively). In the last experiment, a RMB containing 13 membrane layers of enclosed cells was studied and compared to a conventional reactor of free cells. The RMB performed successfully in syngas bio-methanation under semi-continuous conditions during 49 days. The highest methane amount produced was 10 mmol/day in both RMB and free cell reactor. / Program: Industriell bioteknik