Modelling the required membrane selectivity for NO₃⁻ recovery from effluent also containing Cl⁻, while saving water

We present a general outline for the selective recovery of NO₃⁻ from a (waste) water stream also containing Cl⁻. The key element of the technology introduced and simulated here is a membrane unit demonstrating NO₃⁻ over Cl⁻ permeation selectivity. The membrane is hypothesized to be hydrophobic and with that exploiting the difference in dehydration energy between NO₃⁻ and Cl⁻. Apart from NO₃⁻ recovery, the process also aims to reduce water consumption. Based on a generic outline of the process, the key parameters are defined, being the NO₃⁻/Cl⁻ concentration ratio in the (waste) stream, the fraction of NO₃⁻ and water recovered, and the selectivity of the membrane. The sensitivity of the separation process to these four parameters is evaluated. In the second part of the paper, the same principles are applied to a real-life process, i.e., NO₃⁻ recovery from the effluent (waste) water of a fertilizer production plant. The aim was to calculate the membrane NO₃⁻/Cl⁻ permeation selectivity required to recover 90% of NO₃⁻, given a threshold value for the Cl⁻ concentration in the permeate stream and recycle 30% of the water, starting from two different NO₃⁻/Cl⁻ concentration ratios in the effluent (waste) water. With 51 mM Cl⁻ in the effluent (waste) water and a Cl⁻ threshold of 9.9 mM, a membrane selectivity of 3 suffices. The required selectivity increases to 30 when the Cl⁻ in the effluent (waste) water is 200 mM and the Cl⁻ threshold is 4.2 mM. Reported NO₃⁻/Cl⁻ membrane selectivities are still modest, with a maximal selectivity found in literature of 3. Strategies to develop membranes of significant higher selectivity are briefly discussed.