Electrolysis of Tertiary Water Effluentsa Pathway to Green Hydrogen

Hydrogen production via water electrolysis has the potential to lead to the decarbonization of the hard-to-abate sectors. The need of high-purity water to operate conventional electrolyzers would still require a purification stage prior to electrolysis. Tertiary effluents from municipal wastewater treatment plants could reduce pressure on existing stressed water resources and enable hydrogen production at scale. In this study, we investigated the feasibility of producing hydrogen with a commercial proton exchange membrane electrolyzer by using tertiary effluents from an operating wastewater treatment plant. Analysis and modeling indicated that chloride (around <1 ppm), a ubiquitous component found in effluents, did not influence the electrolysis conditions. Monovalent cations such as Na⁺ and K⁺ decreased the electrolysis efficiency at lower electrolysis currents, but at high electrolysis currents, their migration to the cathode side led to a recovery in efficiency. Divalent cations have a stronger affinity to the electrolyzer-separating membrane. Their presence forced the use of reverse osmosis (RO) prior to electrolysis. Upon RO treatment, it was possible to electrolyze real tertiary effluents at a maximum efficiency of 59%, i.e., a hydrogen production rate of 1.79 L/min at 35 A of electrolysis current. These results indicate that tertiary effluents can be purified by standard technologies and used to produce green hydrogen. This is also a circular economy opportunity that can produce value-added products from a wastewater stream that is normally discharged to the environment.