This study presents a novel process for producing pipeline-quality biomethane by anaerobic digestion (AD) of sludge with in-situ biogas cleanup and upgrading using corn stover biochar. The biochar has high surface area (105 m(2)/g), high ash content (45.2% dry weight) and high concentrations of potassium, calcium and magnesium (14.2% K2O, 3.9% CaO and 4.2% MgO of the ash content, respectively). The biocharamended digesters produced near pipeline-quality biomethane (>90% CH4 and <5 ppb H2S), facilitated CO2 removal by up to 86.3%, boosted average CH4 content in biogas by up to 42.4% compared to the control digester, close to fungibility of natural gas. The biochar addition enhanced the methane yield, biomethanation rate constant and maximum methane production rate by up to 7.0%, 8.1% and 27.6%, respectively. The biochar addition also increased alkalinity and mitigated ammonia inhibition, providing sustainable process stability for thermophilic sludge AD. The biochar-amended digestate is enriched with nutrients such as potassium, nitrogen and phosphorus, and therefore has great potential for soil applications. (C) 2015 Elsevier Ltd. All rights reserved.