Experimental Investigation And Modeling Of Separation Performance Of Nanofiltration Membrane For Ammonium And Bicarbonate Salts Solutions

Nanofiltration (NF) is defined as the separation progress between Ultrafiltation (UF) and Reverse Osmasis (RO). The separation principle of NF contains sieve effect and charge effect, presenting two typical separation characteristics: one is the intermediate molecular weight cut-offs (MWCO) between RO and UF membranes; the other is the selective permeate of ions of different charge and diffusion property. As a consequence, NF is a promising separation technology for surface and ground water treatment. However, compared with RO, the absence of mature modeling software of NF is to some extent limiting the generalization of large scale application of NF in drinking water production. In the preceding works, a noval model was proposed to evaluate the separation performance of NF for the mixed electrolytes solution based on the observed transmissions of single salts. Further considered, this model still has inadequacies, including the inconsideration of weak electrolyte and pH values.In this work, the separation experiments of three NF membranes (ESNA 1-LF, ESNA 1-K and LES 90) for single and mixed ammonium and bicarbonate salts solutions were carried out. The experimental results indicated that under the original pH value of ammonium and bicarbonate salts solutions, ammonia and carbonic acid dissociated into the form of ammonium and bicarbonate ions almost totally. The rejection progress of ammonium and bicarbonate ions was a combination of sieve and charge effect. As a result the observed transmission of these single salts decreased with flux increasing, while it increased with feed concentration increasing. In conclusion, the rejection properties of ammonium and bicarbonate salts present the same regulation with strong electrolytes. Furthermore, with pH variation, the rejection of ammonium and bicarbonate salts is associated with dynamical equilibrium. The observed transmissions of total ammonia and inorganic carbon were increased obviously with the growth of proportion of uncharged molecular in solutions. The rejection of uncharged molecular by NF membrane was influenced by sieve effect only, and the observed transmission of uncharged molecular through NF membrane is neither influence with flux nor feed concentration, the only influence is membrane pore size. Another important conclusion is that the transbility of uncharged molecular is much higher than ions.Aiming on modeling the rejection of ammonium and bicarbonate salts solutions, the model proposed before is extended by introducing dynamical equilibrium of weak electrolyte to expanding the application of this model. In the extended model, the proportions of charged ions and uncharged moleculars disassociated from ammonium and bicarbonate salts are calculated separately with pH value of feed solution. The permeate performance of charged ions of ammonium and bicarbonate salts is taken into summation model together with strong elelctrolyte ions, and the calculated transmissions of ions are summated to the transmission of uncharged moleculars with their proportions, result in the transmissions of total ammonia and inorganic carbon. In conclusion the transbility of weak electrolyte is directly related with proportions of charged ions and uncharged moleculars, influenced by pH values through dynamic equilibrium. At last the extended model was applied on prediction the observed transmissions of components of complex mixed salts solution containing weak electrolytes such as ammonium and bicarbonate salts. The deviation between model evaluation and experiment results was limited.