Simultaneous hydrogen production and struvite recovery within a microbial reverse-electrodialysis electrolysis cell

Abstract In this research, a novel Microbial reverse-electrodialysis electrolysis struvite-precipitation cell (MRESC) was developed for energy recovery through struvite (MgNH4PO4·6H2O) crystallization and hydrogen production concurrently in a single process without any electrical-grid energy consumption. This hybrid system can effectively transfer the salinity gradient energy to electrical energy as a driving force to produce hydrogen gas coupled with struvite recovery and organic wastewater degradation. A MRESC containing 10 pairs of RED cells, supplied solutions typical of high concentration (600 mM NaCl) and low concentration (12 mM NaCl) at 1.0 mL/min, was operated in the fed-batch mode. The rates of hydrogen production and struvite crystallization were determined to be 0.71 m3-H2/m3-Van/d and 7.62 g/m2/h, respectively. The gas produced was > 92% H2. The Coulombic efficiency was close to or above 100% with a COD removal of 84 ± 6%, and an overall energy efficiency of 28%. 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 energy recovery in the form of pure hydrogen gas and struvite simultaneously.