Energy recovery from wastewater using microbial reverse-electrodialysis electrolysis cells

The concern about energy demand due to the limited fossil fuels resource and the continuous consumption is a burning environmental issue. It means the emergence of new sustainable energy solutions must be exigent. Microbial reverse-electrodialysis electrolysis cells (MRECs) can bio-electrochemically produce pure hydrogen gas from wastewater by two driving forcesone is directly electric energy extraction from organic matters using exoelectrogenic bacteria on the anode and the other is the salinity gradient energy using reverse electrodialysis (RED) stack, and is therefore a mean of supplying sustainable energy. In most microbial electrochemical technologies (METs) as well as MREC studies, however, high-concentrated buffer solutions have been used to be high conductivity and buffering capacity for high current density by reducing ohmic resistances with pH maintenance in neutral. In particular, the use of phosphate buffer solution (PBS) of them can give rise to not only environmental problems but also excessive costs for wastewater treatment application. In addition, the generation of wastewaters containing phosphorus and nitrogen that can cause the environmental problems is increasing every year. However, although there is an available potential energy in these wastewaters, the additional substantial energy has been actually required to treat them. Several challenges of METs for treating wastewater are still facing same limitations currently exist. Therefore, this study was conducted to overcome the drawback of the employment of buffer solution by pH control and to propose an effective system that can recover wastewater containing nitrogen/phosphorus into useful material. The performances of MRECs were evaluated using hydrogen production rate, yield, Coulombic efficiency, and COD removal rate. First, the aim of this experiment was to stably produce hydrogen gas without buffer solution by controlling the anodic pH under the continuous flow condition (10 cell pairs of RED stacks, HRT = 5, 7.5, and 15 h). The pH of the anolyte was controlled using NaOH solution to maintain in neutral. It was found that it is important to keep the constant amount of organic matter in the anode chamber, indicating that the decreasing in the HRT of anolyte (increasing in the organic matter in the anode chamber) makes the cell current stable and increased. As a result, the MREC without buffer solution accomplished stable hydrogen gas production at a rate of 0.61 m3-H2/m3-Van/d although the conductivity of anolyte was significantly low. Consequently, this study elucidates that it is possible for MREC to produce hydrogen gas without buffer solution in the anolyte, which will be an important step for further research or application to wastewater treatment. Second, this experiment was conducted to recover from the wastewater containing nitrogen/phosphorus into useful material in the form of struvite (MgNH4PO4??6H2O) and hydrogen gas at the cathode of MREC simultaneously, which was named microbial reverse-electrodialysis electrolysis struvite cell (MRESC). To evaluate the hydrogen gas production, the MRESC was compared a MREC as a control under the fed-batch cycles. A 10 mM synthetic struvite solution (Mg2+, NH4+, and PO43-) was added into the catholyte for describing wastewater. The MRESC successfully achieved the phosphate recovery and the hydrogen gas production concurrently, resulting in the struvite crystallization rate of 7.62 g/m2/h and the hydrogen gas production rate of 0.71 m3-H2/m3-Van/d, respectively. The Coulombic efficiency was close to or above 100% with a COD removal of 84 ?? 6%. The morphology and structure of the main component of accumulated crystal at the cathode were verified by a scanning electron microscope with energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction. These results showed that the MRESC system could be used as an effective bioelectrochemical method for simultaneous hydrogen production and phosphate recovery via struvite precipitation.Docto