Polyamide active layers of low pressure RO membranes: Data on spatial performance non-uniformity and degradation by hypochlorite solutions

The spatial performance non-uniformity of polyamide active layers is investigated by taking measurements of key parameters (permeate flux and salt rejection) at two spatial scales, i.e. using membrane samples of area ∼ 14.5 and ∼ 149 cm2. Two unused low pressure Thin Film Composite membranes (designated as “A” and “B1”) are examined, and a used one (“B2”, of the same type as “B1”) before and after chemical cleaning. Samples, from all three membrane elements, exhibit relatively broad distributions of performance parameters, which are indicative of weak spots or deficiencies throughout the membrane sheets. The flux and salt rejection distributions from unused membranes, compared to those from used ones, are narrower; furthermore, chemically cleaned, used membrane samples display greater flux and reduced rejection compared to new membranes. This evidence confirms that used membranes suffer significant degradation. The smaller size samples, compared to larger ones, exhibit a broader distribution of flux and rejection, which suggests a more random spatial distribution of the weak spots or deficiencies at that scale. Results regarding degradation due to exposure to sodium hypochlorite show that acidic solutions of NaOCl cause the most severe damage to active layers, resulting in rejection deterioration and substantial flux decline.