In order to maximise the efficiency of biogas desulphurisation and reduce the oxygen cost during microaerobic digestion, it is essential to know how the process occurs. For this purpose, a reactor with a total volume of 266 L, treating 10 L/d of sewage sludge, was operated with 25.0 L and without headspace. Under anaerobic conditions, the H2S concentration in the biogas varied between 0.21 and 0.38% v/v. Next, O-2 was supplied from the bottom of the reactor. At 0.25-0.30 NLO2/L-fed, the biogas was entirely desulphurised, and its O-2 content remained below 1.03% v/v, when the digester had 25.0 L of gas space. However, with almost no headspace, the H2S content in the biogas fluctuated from 0.08 to 0.21% v/v, while the average O-2 concentration was 1.66% v/v. The removed H2S accumulated in the outlet pipe of the biogas in the form of S-0 due to the insufficient headspace. (C) 2014 Elsevier Ltd. All rights reserved.