Nutrient recovery and energy production from digestate using microbial electrochemical technologies (METs)

Abstract

This work aimed to test both energy production and the recovering of phosphorus from high solid content digestate by coupling microbial electrochemical technologies (MET) with a crystallization process. Digestate coming from an anaerobic digester was used as the feeding substrate for a Microbial Fuel Cell (MFC) and a Microbial Electrolysis cell (MEC) to produce electricity and H2, promoting P removal (as struvite: MgNH4PO4∙6H2O) as consequence of the pH increase at the cathode level. MECs were equipped with low cost stainless steel mesh cathodes to be compared with a benchmark cathode, i.e. Pt electrode, and used to reduce the METs cost. The effluents coming from METs were then treated to further reduce P content by promoting struvite precipitation in a crystallization device, by adding either pure Mg salt (MgCl2) or an Mg by-product (seawater bitterns). MFC tests showed a Coulombic efficiency of 35%, with a volumetric power produced of 14.2 ± 0.15 W m−3. MEC (+1.07 V vs SHE) systems produced 1.90 ± 0.04 LH2 L−1d −1 using a stainless steel mesh (SSM) and 2.02 ± 0.03 LH2 L−1d −1 using a platinum cathode. Both MET treatments led to a phosphate (PO43−) reduction of between 21% and 30% and an organic load reduction (chemical oxygen demand) of between 27% and 44% of the digestate. The addition of MgCl2∗6H2O or seawater bitterns, in the molar ratio of 1.6: 1 (Mg:P) led to a further reduction of phosphate (PO43−) of between 60% and 70% with an overall reduction of phosphate (PO43−) of ≈90%. These results show how the MET systems could be used for COD and nutrients reduction as well as H2 or electricity production from digestate. Moreover, the SSM cathode gave a similar performance to the Pt cathode, while seawater bitterns seem to be useful in substituting MgCl2∗6H2O. The results obtained showed that the combination of the system proposed in this work with a biogas plant could be a low cost solution for the recovery of nutrients and for hydrogen or electricity generation from digestate.