Influence of zero valent scrap iron (ZVSI) supply on methane production from waste activated sludge

• Zero valent scrap iron (ZVSI) was used to stimulate the anaerobic digestion of sludge.
• The highest methane yield was 174.9 ± 1.5 mL/g VSSfed with ZVSI 1.0 g/g VSS, increasing by 38.3%.
• ZVSI enhanced the methanogenesis via acting as electron donors and creating favorable environment.
• Responsible mechanisms for enhancing methane formation in the presence of ZVSI were proposed.

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 (H2/[H]), promote H2 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.

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