The use potential of zero valent scrap iron (ZVSI) for promoting anaerobic production of waste activated sludge (WAS) was assessed. The results indicated that the presence of ZVSI effectively enhanced WAS methane production. Methane yield reached the maximum of 174.9 +/- 1.5 mL/g VSSfed with addition of 1.0 g-ZVSI/g VSS, increasing by 38.3%. The corresponding values for sludge hydrolysis (%H), acidogenesis (%A), and methanogenesis rate (%M) increased into 52.6 +/- 0.4%, 44.5 +/- 2.9% and 40.8 +/- 0.3%, respectively, versus 45.2 +/- 0.5%, 40.9 +/- 2.0% and 29.5 +/- 0.2% in the test without ZVSI. Further investigations confirmed that ZVSI could offer electrons (H-2/[H]), promote H-2 consumption and afford beneficial pH environment for WAS digestion, which stimulated the metabolisms of critical microbes involved in the process and, subsequently accelerated the hydrolysis-acidification-methanation steps of WAS. As a result of this, methane production was upgraded. The superior ability of ZVSI to serve as a source of electron donors will open a new door for WAS treatment. Moreover, ZVSI is less expensive, and convenient to use, providing a cost-efficient alternative for simultaneous energy recovery along with waste iron scrap conservation. (C) 2014 Elsevier B.V. All rights reserved.