Membrane technology has been significantly gaining importance in water treatment applications recently due to their environmentally friendly operation, and less complex and cheaper operating conditions compared to the conventional water treatment studies. To provide sustainable and efficient membrane applications, membrane selection is a crucial factor for both fabrication and operation processes, and it is determined by membrane characterization, which presents critical parameters such as average membrane pore size and pore size distribution (PSD). There are two main characterization methods as the direct and indirect, the latter is based on mathematical equations, more affordable, and it can characterize larger membrane areas compared to the direct techniques. This study reviews the indirect membrane characterization methods according to their backgrounds that are Young-Laplace Equation-based, Kelvin Equation-based, Gibbs-Thomson Equation-based, and spectroscopy-based techniques. After giving the theoretical details of the methods, the measurement details and relevant experimental procedures are presented. Lastly, the remarkable indirect membrane studies, which have been performed from past to present, are explained in detail.
