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The current study presented a novel process of biogas upgrading to biomethane (higher than 97%) based on anaerobic sludge and zero-valent iron (ZVI) system. When ZVI was added into an aquatic system with anaerobic granular sludge (AnGrSl) under anaerobic abiotic conditions, H2 was generated. Then, the H2 and CO2 were converted by the hydrogenotrophic methanogens to CH4. Biogas upgrading to biomethane was achieved in 4 days in the AnGrSl system (50 g L-1 ZVI, initial pH 5 and 20 g L-1 NaHCO3). In this system, when zero-valent scrap iron (ZVSI) was added instead of ZVI, a more extended period (21 days) was required to achieve biogas upgrading. X-ray diffraction (XRD) analysis revealed that the materials in a reactor with CO2 or biogas headspace, exhibited a mixture of ferrite and the iron carbonate phase of siderite (FeCO3), with the latter being the dominant phase. VOCs analysis in raw biogas (in the system of anaerobic sludge and ZVI) highlighted the reduction of low mass straight- and branched-chain alkanes (C6-C10). Also, H2S and NH3 were found to be substantially reduced when the anaerobic sludge was exposed to ZVI compared to the cases where ZVI was not added. This study found that simultaneously with biogas upgrading, VOCs, H2S and NH3 can be removed in a system of ZVI or ZVSI and AnGrSl under aquatic anaerobic conditions.

In the current study, a novel process of ex-situ biogas upgrading to biomethane is presented which is based on a system consisted of anaerobic sludge and zero valent iron (ZVI). The ZVI when is added in an aquatic system with anaerobic granular sludge generates H2 under anaerobic abiotic conditions. Then, the H2 and CO2 are converted by the hydrogenotrophic methanogens to CH4. Biogas upgrading to biomethane was achieved in 4 days in a system of anaerobic granular sludge, 50 g L1 ZVI initial pH 5 and 20 g L1 NaHCO3. In this system when zero valent scrap iron (ZVSI) was added instead of ZVI required longer period (21 days) to achieve biogas upgrading. Volatile organic compounds (VOCs) analysis in raw biogas (in system of anaerobic sludge and ZVI) showed mainly the reduction of low mass straight- and branched-chain alkanes (C6-C10); however, no other particular trend regarding the removal of other VOCs was observed. H2S and NH3 were found to be substantially reduced, when the anaerobic sludge was exposed to ZVI compared to no decrease in serum bottles free of ZVI.

This study investigates a new approach for carbon dioxide (CO2) conversion to methane (CH4) by using Zero Valent Iron (ZVI) and anaerobic granular sludge under aquatic mild conditions. Numerous studies have used ZVI to stimulate the anaerobic digestion of waste and wastewater. Up to date however, little research has been done on the use of ZVI and anaerobic granular sludge for the conversion of CO2 as a sole carbon substrate to CH4. The results showed that 75 g L−1 ZVI, 2 gVS L−1 anaerobic granular sludge at initial pH of 6 under daily pH regulation contributed to substantial CH4 production. It was pointed out that the initial ZVI (50 g L−1), with anaerobic granular sludge, was sufficient for the production of the same amount of CH4 over 2 CO2 re-feedings for a total of 67 days. To identify the microbial pathways for CO2 conversion to CH4, the following inhibitors were independently added: antibiotics for bacteria inhibition, 2-Bromoethanesulphonate (BES) for methanogens inhibition and Trichloromethane (CHCl3) for inhibition of bacteria and methanogens. Results showed that when antibiotics were added, they did not have any negative effect on CO2 conversion to CH4. This demonstrates that CO2 is converted to CH4 mainly through hydrogenotrophic methanogens and the contribution of homoacetogens was minor. Under these conditions of anaerobic granular sludge and ZVI the addition of BES and CHCl3 resulted in a high generation of H2, no production of CH4 and generation of 1110 mg L−1 and 730 mg L−1 in 8 days respectively.