Biomethane recovery from source-diverted blackwater through anaerobic digestion (AD) offers a sustainable alternative for modern wastewater treatment. Water-wasting conventional toilets consume great amounts of flushing water (9 L per flush), which results in blackwater with chemical oxygen demand (COD) of 1006 (+/- 61) mg/L and COD/sulfate ratio of 12.2 (+/- 0.9). In this study, the conventional toilets collected blackwater was treated through an up-flow anaerobic sludge blanket (UASB) reactor at 35 degrees C with a hydraulic retention time (HRT) of 2.2 days, which achieved a COD removal efficiency >80 %. Inhibition in blackwater methanogenesis was observed, which was found associated with the growth of hydrogen utilizing sulfate reducing bacteria (SRB) that competed with hydrogenotrophic methanogens and suppressed the biomethane recovery efficiency. The sludge specific hydrogenotrophic methanogenic activity (SMA [H-2 &CO2]) increased from 0.37 (+/- 0.02) g CH4-COD/g volatile suspended solids (VSS)/d (treating high sulfate blackwater) to 0.52 (+/- 0.00) g CH4-COD/g VSS/d (treating low sulfate blackwater) when sulfate-free toilet flushing water was adopted (resulting COD/sulfate ratio of 42.9 [ +/- 5.0]). This study underlines the importance of considering the impact of sulfate on blackwater methane production when designing future blackwater treatment processes. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.