This thesis evaluates a novel technique to increase the active biomass inside continuously stirred tank biogas reactors with possible benefits of shorter retention times, higher degree of degradation, higher methane yield and tolerance of higher organic loading rates. The technique includes addition of magnetic biomass carriers to the process which, after adhesion of active microorganisms, can be magnetically separated at reactor outflow and reintroduced to the process.

The evaluation of magnetic biomass carriers included methods such as batch experiments, quantitative real-time polymerase chain reaction and continuous reactor experiments with different organic loading rates and addition of volatile fatty acids. The results show that reintroduction of magnetic biomass carriers does indeed work: an accumulated biomass of microorganisms is achieved inside the reactor during a continuous process. Magnetite was selected as the most promising biomass carrier, microbiological studies of the particles show that microbiological colonization of magnetite is present with preferential adhesion of hydrogenotrophic methanogens, important for the methanogenesis. The anaerobic digestion with magnetite as biomass carrier present increased process stability and elevated degrading potential of volatile fatty acids, as well as leading to higher methane content when subjected to increased organic load. Thus, the total gas production is increased in certain situations when using magnetic biomass carriers, why further studies of appropriate hydraulic retention times, organic loading rates and substrates are warranted.