Green roofs have many benefits, but in countries with semiarid climates the amount of water needed for irrigation is a limiting factor for their maintenance. The use of drought-tolerant plants such as Sedum species, reduces the water requirements in the dry season, but, even so, in semiarid environments these can reach up to 60 L m−2 per day. Continuous substrate/soil water content monitoring would facilitate the efficient use of this critical resource. In this context, the use of plant microbial fuel cells (PMFCs) emerges as a suitable and more sustainable alternative for monitoring water content in green roofs in semiarid climates. In this study, bench and pilot-scale experiments using seven Sedum species showed a positive relationship between current generation and water content in the substrate. PMFC reactors with higher water content (around 27% vs. 17.5% v/v) showed larger power density (114.6 and 82.3 μW m−2 vs. 32.5 μW m−2). Moreover, a correlation coefficient of 0.95 (±0.01) between current density and water content was observed. The results of this research represent the first effort of using PMFCs as low-cost water content biosensors for green roofs.

Greywater reuse through decentralized and low-cost treatment systems emerges as an opportunity to tackle the existing demand for water. In recent years, constructed wetlands (CW) systems and microbial fuel cells (MFCs) have emerged as attractive technologies for sustainable wastewater treatment. In this study, constructed wetland microbial fuel cells (CW-MFCs) planted with Phragmites australis were tested to evaluate the potential of combining these two systems for synthetic greywater treatment and energy recovery. Open (CW) and closed circuit (CW-MFCs) reactors were operated for 152 days to evaluate the effect of energy recovery on the removal of soluble chemical oxygen demand (sCOD), nutrients and total suspended solids (TSS). Results indicate no significant differences for sCOD and phosphate removal efficiencies. CW-MFCs and CW reactors presented sCOD removal efficiency of 91.7 ± 5.1% and 90 ± 10% and phosphate removal efficiencies of 56.3 ± 4.4% and 61.5 ± 3.5%, respectively. Nitrate removal efficiencies were higher in CW: 99.5 ± 1% versus 86.5 ± 7.1% in CW-MFCs, respectively. Energy generation reached a maximum power density of 33.52 ± 7.87 mW m −3 and 719.57 ± 67.67 mW m −3 at a poised anode potential of −150 mV vs. Ag/AgCl. Thus, our results suggest that the incorporation of MFC systems into constructed wetlands does allow energy recovery while providing effective greywater treatment.

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