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Several renewable and (claimed) sustainable energy sources have been introduced into the market during the last century in an attempt to battle pollution from fossil fuels. Especially biomass energy technologies have been under debate for their sustainability. A new biomass energy technology was introduced in 2008: the plant-microbial fuel cell (P-MFC). In this system, electricity can be generated with living plants and thus bioelectricity and biomass production can be combined on the same surface. A green roof producing electricity with a P-MFC could be an interesting combination. P-MFC technology is nearing implementation in the market and therefore we assessed the environmental performance of the system with an early stage life cycle assessment (LCA). The environmental performance of the P-MFC is currently worse than that of conventional electricity production technologies. This is mainly due to the limited power output of the P-MFC and the materials presently used in the P-MFC. Granular activated carbon (anode material), gold wires (current collectors), and Teflon-coated copper wires (connecting anode and cathode) have the largest impact on environmental performance. Use of these materials needs to be reduced or avoided and alternatives need to be sought. Increasing power output and deriving co-products from the P-MFC will increase environmental performance of the P-MFC. At this stage it is too early to compare the P-MFC with other (renewable) energy technologies since the P-MFC is still under development., QC 20150505

This study investigates the oxidation of labeled methane (CH4) and the CH4 dependence of sulfate reduction in three types of anaerobic granular sludge. In all samples, 13C-labeled CH4 was anaerobically oxidized to 13C-labeled CO2, while net endogenous CH4 production was observed. Labeled-CH4 oxidation rates followed CH4 production rates, and the presence of sulfate hampered both labeled-CH4 oxidation and methanogenesis. Labeled-CH4 oxidation was therefore linked to methanogenesis. This process is referred to as trace CH4 oxidation and has been demonstrated in methanogenic pure cultures. This study shows that the ratio between labeled-CH4 oxidation and methanogenesis is positively affected by the CH4 partial pressure and that this ratio is in methanogenic granular sludge more than 40 times higher than that in pure cultures of methanogens. The CH4 partial pressure also positively affected sulfate reduction and negatively affected methanogenesis: a repression ofmethanogenesis at elevated CH4 partial pressures confers an advantage to sulfate reducers that compete with methanogens for common substrates, formed from endogenous material. The oxidation of labeled CH4 and the CH4 dependence of sulfate reduction are thus not necessarily evidence of anaerobic oxidation ofCH4 coupled to sulfate reduction.

This study investigates the oxidation of labeled methane (CH4) and the CH4 dependence of sulfate reduction in three types of anaerobic granular sludge. In all samples, 13C-labeled CH4 was anaerobically oxidized to 13C-labeled CO2, while net endogenous CH4 production was observed. Labeled-CH4 oxidation rates followed CH4 production rates, and the presence of sulfate hampered both labeled-CH4 oxidation and methanogenesis. Labeled-CH4 oxidation was therefore linked to methanogenesis. This process is referred to as trace CH4 oxidation and has been demonstrated in methanogenic pure cultures. This study shows that the ratio between labeled-CH4 oxidation and methanogenesis is positively affected by the CH4 partial pressure and that this ratio is in methanogenic granular sludge more than 40 times higher than that in pure cultures of methanogens. The CH4 partial pressure also positively affected sulfate reduction and negatively affected methanogenesis: a repression ofmethanogenesis at elevated CH4 partial pressures confers an advantage to sulfate reducers that compete with methanogens for common substrates, formed from endogenous material. The oxidation of labeled CH4 and the CH4 dependence of sulfate reduction are thus not necessarily evidence of anaerobic oxidation ofCH4 coupled to sulfate reduction.