Nanofiltration is a possible separation technology for phosphorus recycling from sewage sludge or sewage sludge ash. Nanofiltration can be applied for this purpose in combination with a low pressure wet oxidation or acidic hydrolysis of sewage sludge or leaching of sewage sludge ash. The feed of the nanofiltration in this application is in the pH range of 1.5 to 3 and contains phosphoric acid, sulphuric acid as well as metal ions which should be rejected. This paper presents experimental data and multi-ion transport modelling. The model is based on the solution diffusion model with permeability coefficients as the only input parameters. For several simplified synthetic solutions permeability coefficients could be successfully fitted. By interpolation of the parameters as functions of pH and concentration in the nanofiltration feed, retentions could be calculated for additional compositions. A validation with real solutions obtained from sewage sludge was performed. (C) 2014 Elsevier B.V. All rights reserved.