(Invited) Electrochemical Processes Drive Membrane-Based Recovery of N and P

Abstract

We expend a large amount of energy and money on the treatment of municipal and industrial wastewater, relying on redox reactions to transform contaminants (e.g, ammonium, reduced carbon) into other less/non-harmful species (N2/NO3 -, CO2), or on biological accumulation approaches that sequester contaminants into bacteria (e.g., biomass, PO4 -3). In the worst case, these contaminants are simply released into the environment where they cause harm and are lost. Yet, many of these contaminants are actually valuable resources (e.g., NH3, PO4 -3). In an age where environmental sustainability and climate change mitigation are of utmost importance, it makes little sense to invest large amounts of energy into the conversion and elimination of these valuable resources, only to turn around and use fossil fuels to produce more. In addition, the value of these resources is often higher than the value of the treated water, making their recovery an economically-attractive prospect.Here, we discuss our recent work on membrane-based separations of ammonia, phosphate, and volatile fatty acids (VFAs) from different waste streams. Critically, in all cases we achieve separation by leveraging electrochemical reactions and processes. For ammonia recovery, we utilize an electroactive membrane contactor that is used as a cathode – the application of cathodic potentials to the membrane surface increases the local pH at the membrane/water interface, which transforms ammonium to ammonia and enables its effective volatilization and recovery across the membrane. For VFA recovery, we use a composite electroactive membrane that combines electrothermal heating and an electrochemical pH swing to protonate and extract these volatile species from water. Finally, for phosphate separation, we engineer a phosphate-specific pathway through a cation exchange membrane, and use electrodialysis to pull phosphate ions from a mixed anion solution. Together, these approaches can be used to recover valuable products from complex waste streams, such as municipal and industrial wastewater, without the need to rely on bulk pH and temperature modification.