The objective of this study was to assess the ammonium retention, when the transmembrane pressure (TMP) in nano filtration (NF) and reverse osmosis (RO) membranes are changed. Two pH levels were tested: pH 5, where ammonium is as NH4+, and pH 7 which is an average of the most common range in aquaculture (pH 6–8).In this study a model solution of ammonium–water and commercial membranes of polyamide supplied by Alfa Laval(DSSHR98PP) and Dow (NF90, NF200 and NF270) were used, with a membrane area of 0.01018 m2. A critical TMP was determined, where the repulsive electrochemical forces at the active layer were exceeded by the transmembrane pressure, causing an irreversible decrease of the retention values