Selectivity of polyamide nanofiltration membranes for cations and phosphoric acid

The results shown in this paper are part of a study to investigate the use of nanofiltration membranes for the recycling of phosphorus from sewage sludge. It especially contains the results of an experimental study about the influencing factors pressure, pH and feed composition on the selectivity of nanofiltration membranes for phosphoric acid and multivalent cations. For the use of pretreated sewage sludge an effective pressure of 11 bar could be identified as an effective operational point. The selectivity turned out to increase with decreasing pH which was studied in a pH-interval of pH 1–4. An increasing concentration of multivalent cations in the feed solution had a negative effect on the permeability of phosphoric acid. Accordingly it could be observed that the nanofiltration worked significantly more effective with sewage sludge ash eluates of the concentration 10 g/l than eluates of the concentration 100 g/l.