Within the European circular economy roadmap, it's important for wastewater treatment plant's (WWTPs) to recover energy and become energy neutral or positive. In the last few years, it has become increasingly interesting to boost energy recovery through the biogas upgrading. The aim of this work is to study a rapid hydrogenotrophic methanogenic culture enrichment strategy capable of limiting the organic degradation unbalance and allowing a fast start-up phase of the in-situ biogas upgrading reactors, at pilot or full-scale. The approach was tested with 2 + 1 control lab-scale CSTRs filled with anaerobic sludge collected from a full-scale WWTP. The experimentation lasted 50 days and was divided into 5 phases: the anaerobic digestion start-up followed by four H2 injection phases (H2/CO2 ranging from 1:1 to 4:1 on molar basis). Despite a temporary slight increase in the total concentration of volatile fatty acids during phase II (2.56 gCH3COOH·L−1), and in phase III a mild pH increment indicating the expected CO2 depletion (anyway below 7.4), the strategy proposed was effective. In the last phase, in the biogas a methane content of about 80% was achieved, thus suggesting that the use of H2/CO2 above the stoichiometric value could further improve the biological biogas upgrading.