Nutrient Recovery by Bio-Electroconcentration is Limited by Wastewater Conductivity

Abstract

Removal and recovery of nutrients from waste streams is essential to avoid depletion of finite resources and further disruption of the nutrient cycles. Bioelectrochemical systems (BESs) are gaining interest because of their ability to recover nutrients through ion migration across membranes at a low energy demand. This work assesses the feasibility of the concept of nutrient bio-electroconcentration from domestic wastewater, which is a widely available source of nutrients in ionic form, collected via sewer networks and easily accessible at centralized wastewater treatment plants. Here, we demonstrate the limits of a three-chamber BES for the recovery of nutrients from domestic wastewater. Because of low ionic conductivity, the measured current densities did not exceed 2 A m-2, with corresponding limited nutrient ion recoveries. Moreover, in a 3D electrode, forcing higher current densities through potentiostatic control leads to higher Ohmic losses, resulting in anode potential profiles and runaway currents and potentials, with consequent unwanted water oxidation and disintegration of the graphite electrode. At the current density of 1.9 A m-2, N removal efficiency of 48.1% was obtained at the anode. However, calcium and magnesium salts precipitated on the anion-exchange membrane, putatively lowering its permselectivity and allowing for migration of cations through it. This phenomenon resulted in low N and K recovery efficiencies (12.0 and 11.5%, respectively), whereas P was not recovered because of precipitation of salts in the concentrate chamber.