To answer the first question, the answer is potentially multifaceted. The authors note in the introduction that there is value in recovering ammonium ions, since they are an important raw material, particularly for fertilizer. Although it is not explicitly stated, water containing only ammonium but not sodium, would be very useful as a fertilizer material. Waste water recovered for irrigation is often problematic owing to the long term accumulation of sodium over periods of evaporation, which in high concentrations, kills plants. Another reason is to save energy. Separations always require energy. Sodium is not problematic when waste water is added to water, ammonium is. It takes more energy to separate ammonium and sodium than ammonium alone. A third possibility is that under the right conditions, one could imagine a similar system being used to oxidize ammonium to nitrogen gas, thus removing its environmental risks. Sodium ions might saturate the system, leaving less capacity for ammonia.
I do not know a definitive answer from a known process where such a consideration applies, but these answers all seem reasonable from my experience. It may be that the practice would depend on this process going commercial, which is a considerable way off I'd guess.
The last two graphs refer to the removal of ammonium in the presence of other ions. Small amounts of the other ions are indeed removed, but the important point is the enrichment of ammonium from the initial ratio in the waste water to a much lower level.
There are in fact many kinds of ion selective membranes and possibly even electrodes. These are often used in analytical chemistry. Many people are familiar with pH which involves a proton selective membrane for detection; there are also pF electrodes for the detection of fluoride, and pCl electrodes to detect chloride, etc, although they are far more esoteric.
The difference with this system opposed to alternatives is the claim that it has lower energy requirements, which is, of course, a good thing.
Thanks for your interest and for asking.
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