14The lack of tax incentives for biomethane use requires the optimization of both biogas 15 production and upgrading in order to allow the full exploitation of this renewable energy

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In-situ upgrading of biogas in a mesophilic anaerobic digester of sewage sludge by sparging H 2 through a membrane was studied. Large gas recirculation rates were required to facilitate H 2 transfer to the bulk liquid phase; at ∼200 L L reactor −1 d −1 , H 2 utilization efficiency averaged 94% and the specific CH 4 production in-during conventional digestion, to 0.54 L L reactor −1 d −1 . Sludge digestion was not compromised by elevated H 2 partial pressure nor by the associated rise in the pH (8.1) because of CO 2 removal. In this regard, VFA accumulation was not detected and the performance of VS removal was similar to the observed without H 2 supply. Microbial analysis revealed that homoacetogens were outcompeted by hydrogenotrophic methanogens. Methanoculleus sp., Methanospirillum sp., Methanolinea sp. and Methanobacterium sp. were the hydrogenotrophic archaea present over the experiment.

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