This made me go down a rabbit hole of techniques for separating sodium and chloride from urea/ammonium/ammonia solutions. I wonder if it could ever be made viable when compared to the Haber process for ammonia production.

Also, phosphorus is the more scarce fertilizer, I think. I remember reading a paper from a Chinese lab that was trying to figure out how to cheaply isolate phosphorus fertilizer from commercial pig farm feces. They decided it wouldn't be cost effective.

Filtration tchnology is evolving by virtue of nanotech evolving. What was cost prohibitive yesterday shouldn't necessarily remain so tomorrow. Someone does have to chase it though for it to get there.