Membranes for water treatments always have a glycerol/water post-treatment before air-drying to maintain their microporous structure during and after air-drying. However, the presence of trace amounts of impurities in glycerol/ water mixtures may have big effects on both pore size and pure water permeability (PWP) of the treated membranes, especially when the membranes have a pore size near or in the nanofiltration (NF) range. By investigating the effects of different contaminants to membranes, this article may provide new guidance for membrane scientists and engineers to study the science and engineering on how to properly post-treat membranes before and after the air-drying process.