Mechanisms of increased methane production through re-circulation of magnetic biomass carriers in an experimental continuously stirred tank reactor

Magnetite particles were used in a semi-continuous process as magnetic biomass carriers to separate and reintroduce microorganisms in a CSTR reactor. In comparison to a control reactor the methane content during the semi-continuous process was elevated when magnetite particles were used. The difference was most apparent during the fermentative step directly after feeding and upon direct spiking with volatile fatty acids. Total DNA quantification of the separated magnetite particles revealed high association of microorganisms. Furthermore, quantitative real-time PCR analysis of the associated microbial consortia indicated that the hydrogenotrophic Methanobacteriales was overrepresented at the particle surface. Thus, the increased methane production could be coupled to both the crowding and shorter interspecies distances between the groups involved in anaerobic digestion, as well as a preferential adsorption of hydrogenotrophs. By bringing the hydrogenotrophs closer to the primary fermentative bacteria and increasing their relative number the produced hydrogen during acidogenesis is more effectively utilized and more carbon dioxide is converted to methane. Furthermore, by the same cause, the rate of acetogenesis increased as the hydrogenotrophs more effectively could consume the hydrogen produced and thereby keep the hydrogen partial pressure low. 2 INTRODUCTION One limiting factor in anaerobic digestion (AD) is the long generation time of anaerobic microorganisms and, in relation to this, the contradictive desire to keep a short hydraulic retention time (HRT) which can lead to a " wash-out " of microorganisms (Lalov et al., 2001; Fernández et al., 2008). Furthermore, the biogas production is limited by the amount of cells inside the reactor when substrate is in excess (Fernández et al., 2008; Nicolella et al., 2000).