Complementary modelling analysis and experimental investigation of performance-limiting factors of electrochemical membrane systems for chemical-free pH regulation

Abstract

pH is critical for optimizing the efficiency during water treatment processes. The electrochemical membrane system (EMS) offers a chemical-free method to adjust pH in situ, where a piece of low-cost filtration membrane is placed between two electrodes and the water electrolysis occurs to generate H⁺ and OH⁻ ions. In contrast to previous studies that have provided a qualitative understanding on performance-limiting factors of pH regulation in the EMS, this study aims to quantitatively analyze these factors. Herein, we integrate the theoretical modelling with the experimental investigation to quantitatively evaluate how performance-limiting factors affect pH changes and the specific energy consumption (SEC) in the EMS. The effluent pH achieved ∼4.5 and ∼10.0 under a low current density (CD) of 0.5 mA/cm² with an extremely low SEC of 0.009–0.011 kWh/m³ for all the membranes tested under the operating surface loading rate (OSLR) of 1200 LMH. When the CD increased and the OSLR decreased further, the effluent pH finally achieved ∼2.0 and ∼12.0, but with a less energy-efficient level of SEC. Membrane properties insignificantly affected pH changes, while the higher electric resistance of either membranes or electrolyte solutions increased the system's SEC. Moreover, the dissolution of CO₂ from the air into the electrolyte solution exhibited a buffering effect on pH changes. These findings provide practical guidance for the EMS design and operation, contributing to enhancing the performance and the energy efficiency of the EMS in broad water treatment industries.