Biogas produced in an industrial-pilot scale sewage sludge reactor (5 m(3)) was desulphurised by imposing microaerobic conditions. The H2S concentration removal efficiency was evaluated under various configurations: different mixing methods and O-2 injection points. Biogas was entirely desulphurised under all the configurations set, while the O-2 demand of the digester decreased over time. Although the H2S removal seemed to occur in the headspace, S-0 (which was found to be the main oxidation product) was scarcely deposited there in the headspace. O-2 did not have a significant impact on the digestion performance; the VS removal remained around 47%. Conversely, DGGE revealed that the higher O-2 transfer rate to the sludge maintained by biogas recirculation increased the microbial richness and evenness, and caused an important shift in the structure of the bacterial and the archaeal communities in the long term. All the archaeal genera identified (Methanosaeta, Methanospirillum and Methanoculleus) were present under both anaerobic and microaerobic conditions. (C) 2014 Elsevier Ltd. All rights reserved.