Impact of feed properties on membrane performance for ultrafiltration of charged solutes

Four polyethersulfones spiral wound ultrafiltration (UF) membranes with a nominal molecular weight cut-off (MWCO) of 5 kDa and 1 kDa were applied to separate neutral and charged solutes. The effects of the charge properties of solutes on membrane performance (permeate flux and retention) were evaluated. The membranes with the same nominal MWCO did not always provide similar membrane performance for uncharged and charged solutes. Similar permeate fluxes did not correspond to similar membrane retention. In the case of similar nominal MWCO, a higher membrane permeability likely resulted in increasing membrane retention. The charge states of solutes did not influence the permeate flux, but an increase in feed concentration did significantly decrease the flux. Therefore, membrane retention depends on the charge properties of solutes and the feed concentration. The UF membranes were used to fractionate an industrial fish protein hydrolysate at a high concentration. Although the adjustment of the pH and ionic strength changed the charge properties of peptides in the hydrolysate, the charge interactions did not significantly impact the membrane performance. The four membranes produced permeate products with different molecular weight distributions. Despite these differences, the products still had the same amino acid profile. The retention factors of amino acids were slightly dependent on their hydration numbers, with more hydration giving slightly lower retention. This means that the UF of fish protein hydrolysate was mainly based on size exclusion. In general, the membranes retained more peptides containing leucine, phenylalanine and methionine.