Surendra Pradhan, a postdoctoral researcher, started researching the matter and discovered that the best way to utilise wastewater nitrogen was to recover it in ammonium sulphate form, which is used in fertilisers and does not contain residual chemicals and pharmaceuticals that are often present in slurry. As a bonus, the same process can recover another agent of eutrophication, phosphorus, which up to now has, like nitrogen, mostly been a problem that wastewater treatment facilities must remove.
Traditionally, the fertiliser nitrogen needed for ammonium sulphate has been made using the Haber-Bosch method, a process developed by German scientists one hundred years ago that converts atmospheric nitrogen into ammonia. The process requires high temperatures and pressures, and is thus quite a glutton for energy: the manufacture of fertiliser nitrogen accounts for an estimated two percent of global energy consumption.
"Recycling is always commendable, but rarely economically viable. We launched several projects only to discover that it was surprisingly difficult to sell our product. The average farmer did not utilise compost and there were few buyers for slurry containing human waste in particular. I finally came to the conclusion that the most sensible option was to manufacture something for which there already was a market", Postdoctoral researcher Surendra Pradhan had to acknowledge that his wastewater business made little sense